frequently asked questions - alliantech · pressure • load • torque • acceleration •...

Pressure • Load • Torque • Accelerat ion • Displacement • Instrumentat ion

Frequently Asked Questions Honeywell Sensotec (800 ) 867 -3892 2080 Arl ingate Lane Columbus, Ohio 43228 USA Tel: 614-850-6000 Fax: 614-850-1111 www.honeywell .com/sensotec www.sensotec.com

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Honeywel l Sensotec | www.sensotec.com | (800) 867-3892 Copyr igh t 2003 Honeywe l l I n te rna t iona l Inc .

Table of Contents

TABLE OF CONTENTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

ACCELEROMETERS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

HOW D OE S A P I E ZO-E LE C TR I C A C C EL E RO M E TE R W ORK? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 WH A T A R E THE D I F F E RE N T T Y P E S O F A CC E L E R O ME TER? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 WH A T I S A S I N G L E E N DE D C O M P RE SSI ON A C C EL E ROM E T E R? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 WH A T I S A N I S OL A TE D C OM P R E S S I O N A C C E L E R O ME TE R? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 WHA T IS A S HE A R TYPE A CCE LE RO ME TER? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 WHA T IS A P IE Z O-R ESI S T I VE A C CE LE R OM E T E R? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 WHA T IS A S TRAI N GA GE BA SE D A C CE LERO ME TE R? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 WHA T IS THE US EA BL E F RE Q U E N C Y R A NG E? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 WH A T I S A N IEPE A C CE LER O M E TE R?. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10 WH A T I S A N ICP A C C E L E RO M E TE R? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11 WH A T I S A CH A R G E O U TPU T A C C E L E ROM E T E R?. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11 WH A T I S T H E N A T U R A L F R E Q U E N C Y O F A N A C C E L E ROM E T E R? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11 WH A T I S T H E M O U N TE D N A T U R A L F RE Q UE N C Y? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12 WH A T I S B A S E S T RAI N S E NS I T I V I T Y? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13 WH A T I S C R OS S SE NSI T I V I T Y O R T RA N SV E RS E S E NSI T I V I T Y? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13 WH A T I S D Y N A MI C R A NG E? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14 WH A T I S A MP L I TU D E L I N E A R I T Y? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14 WH A T A R E T H E D I F F E R E N C E S B E T W E E N Q U A R T Z C R YS T A L B A S E D A N D C E R A M I C C R YS T A L B A S E D A C C E L E RO MET E R S? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15 HOW DO I M OU N T A N A C CEL E RO M E TE R? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15 WH A T I S A QU I CK FI T M O UN T? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .17 WH E N S HO U L D I US E A VE LO CI T Y O U T P UT A C C EL E ROM E T E R? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .17 WH A T S I G NA L CO N DI T I O NI NG D O I N E E D F O R M Y A C C E L E RO M E TE R? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .18 WH A T A R E G R O U N D I SO L A T E D A C CE L E RO M E TE R S? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .19 WH A T I S A N I S OL A TE D S T UD?. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .22 HOW DO I I NS T AL L A CHARG E A M PL I F I ER? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .22 WH A T I S T H E T R I B O-EL E C TR I C E F F E C T? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .23 HOW DO I CHO OS E THE SENSI T I V I TY O F A N A C CE LE ROME TE R? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .24 WH A T I S T H E O U T P U T O F A N IEPE A C C E L E RO M E TE R? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .24 WH A T I S A N FFT? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .24 WH A T I S C O ND I T I O N M O NI TO RI N G? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .25 WH A T FR EQ UE N C Y RE SP ON S E DO I W AN T F RO M M Y A C C E L E RO MET E R? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .25 WHA T TYPE OF A CCEL E ROME TE R B ES T S UI T S MY AP PL I CA TIO N? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .28

CALIBRATION. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .29

WH Y S H O U L D I CA L I B R A TE? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .30 WH A T I S NIST? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .30 WH A T I S A NIST T R A CE ABL E CA L I B RA TI O N? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .30 CA N A LOA D C E LL B E M A DE T R A CE AB LE TO NIST? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .31 CA N A N O R GA NI Z A TI O N BE NIST T RA CEA B LE? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .31 WH A T I S A2LA O R NVLAP A C C RE DI TA TI O N? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .32 WH A T I S U N CE R T A I N T Y? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .32 WH E N I S U N CE R T A I N T Y I M P O R T A N T? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .32 HO W I S U N CE R T A I N T Y ME A S U R E D? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .33

CALIBRATION CLASS LOAD CELLS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .34

WH A T DOE S A L OA D CE LL C A L I B R A TI O N C O N SI S T O F? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .35 WH A T L O A D C E L L C A L I B RA T I O N S TA N D A R D S H O UL D I A D OP T? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .35 WH A T A R E I MP O R TA N T PA RA M E TE R S FOR A C AL I B RA TI O N C LA SS LOA D C EL L? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .35 WH A T A R E THE C H A RA C T E RI ST I C S O F A L O W U N CE R T AI N T Y C AL I B RAT I O N C LA SS LO A D CE LL? . . . . . . . . . .36 WHY DO ES A CA L I BR A TI O N CLA SS LO AD C E LL H AV E A B AS E P LA T E AN D A C AL I B RAT I O N A DA P TER? . . . .36 WH A T I S T H E U N C E R T A I NT Y O F A SE N SOT E C LO A D CEL L? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .37 WH A T UN C E RT AI N T Y S H O UL D M Y CA L I B RA TI O N RE F E RE N C E L OA D CE L L HA VE? . . . . . . . . . . . . . . . . . . . . . . . . . . . . .37

3


DO ES A LOA D CE L L HA VE TO BE CAL I B RATE D W I TH I T S D I SP LA Y? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .38 WH A T D O E S T H E ASTM E74 S T A N D A R D S P E C I F I C A L L Y S A Y A B O U T C A L I B R A T I O N C L A S S L O A D C E L L S?. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .38

GENERAL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .40

HOW DO I K NO W W HA T A CC U R A C Y CL A S S T O US E FO R M Y S E N S O R? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .41 WH A T I S SE NS I T I V I T Y? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .41 WH A T I S NO N-L I NE A RI T Y? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .43 WH A T I S HYSTE R ESI S? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .46 WH A T I S STAT I C ER RO R BA N D? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .46 WH A T I S CAL I BRA T IO N FA C T O R? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .47 WH E N SH O ULD I F I T A CO N N E C T O R O R IN T E G RA L CAB L E? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .48 WH A T I S T H E DI F FE R E N CE BE TW EE N SUB M E RSI BL E AN D WA TE RP RO O F? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .50 WH A T A R E NEMA A N D IP DE FI N I T I O NS F O R ENV I R O NM E N T AL PROT E C TI O N?. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .51

LOAD CELLS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .53

WH A T I S OV ER L OA D PRO TE C TI O N O N A LO A D CE L L? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .54 WH A T I S A CO M P RE SSI ON ON L Y LOA D CE L L?. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .55 WH A T I S A TE N SI O N ONL Y LOA D CEL L? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .56 WH A T I S A TE N SI O N A N D CO M P RE SSI ON ON L Y LOA D CEL L? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .58 WH A T I S A RO D EN D BE AR I NG? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .58 WH A T I S A LO A D BUT T O N? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .59 WH A T I S LOAD CE L L SY M M E T R Y? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .60 WH A T I S Z E RO B A L A N C E FO R A LO A D CE L L?. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .61 WH A T I S Z E RO B A L A N C E TE M PE R A T UR E EF F E C T? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .62 WH A T I S OUTP U T SPA N TEM P E RA T U RE E F F E C T? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .63 WH E N SH O ULD I HA VE ZE RO A N D SP AN A DJ U S T ME N TS O N M Y LO AD CE LL? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .65 WH A T A R E T H E A D V A N T A G E S A N D D I S A D V A N T A G E S O F H A V I N G A S E N S O R I N T E R N A L A M P L I F I E R O N A L OA D C EL L? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .66 WH Y I S T H E O U T P U T O F MY P RE SS U R E D E T E C TO R Q U O T E D I N MV/V? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .69 HOW DO I K NO W W HA T A CC U R A C Y CL A S S T O US E FO R M Y S E N S O R? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .70 HOW DOE S TE M P E R A T UR E A F F E C T A LOA D CEL L? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .71 HO W D O Y O U C O M P E N S A TE FO R T E MP E RA T U R E I N A LO A D CE LL? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .71 WH A T I S T H E T E M PE R A TU RE C O MP E NS AT I O N R A NG E? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .71 WH A T I S T H E T E M PE R A TU RE OP E RA TI N G RA N GE? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .71 HOW DO I P ICK THE R IG H T FUL L SCA LE O U TP UT FO R A L OA D CE LL? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .72 WH A T I S T H E E F F E C T O F DYN A M I C L OA DS O N A LO A D C E LL? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .72 HO W D O E S A L O A D C E L L W O R K? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .73 WH A T L O A D R A N G E S HO UL D I CHO SE FO R A LOA D CE L L? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .73 WH A T TH I N G S D O I NE E D T O C O NSI D E R W HE N M O U NT I NG A LO A D CE L L? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .74

PRESSURE SENSORS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .76

HOW DOE S A BO N DE D FOI L STRAI N GAGE-BA SE D PRES S U RE SENSO R WO RK? . . . . . . . . . . . . . . . . . . . . . . . . . . .77 HOW DOE S A S I L I C O N-BA SE D PR E S S U RE SE NS OR WO R K? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .78 WH A T A R E AD V A N T A G E S BE T W E E N BO N D E D FO I L ST R A I N GA G E-BA S E D A N D S I L I C O N-BA S E D PR E S S U RE SE N SO R S? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .79 WH A T I S A GA GE PRESSUR E SE NSO R? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .79 WH A T I S A TR U E GA GE PR E S S U R E SENS O R? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .80 WH A T I S A N AB SO L UT E PR E S S U R E SENS O R? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .81 WH E N S HO U L D I US E A N AB SO L U TE P RES S U RE SE N SO R R A T HE R TH A N A G AG E P R ES S U RE SEN S O R? . .82 WH A T I S A DI F F E R E N TI A L PR E S S U RE SE N SO R? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .83 WH A T I S A VA C U U M PRE S S U R E SE NS OR? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .83 WH A T I S A BA R O ME T RI C PR E S S U R E SENS O R? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .84 WHI C H PRE S S U R E RE F E RE N C E S H O ULD I US E FO R MY AP PL I C A TIO N? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .85 WH A T I S OV ER L OA D PRO TE C TI O N O N A PR E S S U RE SE N SO R? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .86 WH A T CO NSI D E R A TI O N S S H O U L D I M AKE W H E N MO UN T I NG A P R ESS U R E S E NS OR? . . . . . . . . . . . . . . . . . . . . . . . .87 HOW CA N I PR O T E C T A GAI N S T W A TE R HA M M E R? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .88 WH A T I S Z E RO BA LA N C E FO R A P R ES S UR E T R A NS D UC E R? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .89 WH A T I S Z E RO B A L A N C E TE M PE R A T UR E EF F E C T? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .90 WH A T I S OUTP U T SPA N TEM P E RA T U RE E F F E C T? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .92 WH E N SH O ULD I HA VE ZE RO A N D SP AN A DJ U S T ME N TS O N M Y P RES S U RE D E TE CT O R? . . . . . . . . . . . . . . . . . . .94 WH A T A R E T H E A D V A N T A G E S A N D D I S A D V A N T A G E S O F H A V I N G A S E N S O R I N T E R N A L A M P L I F I E R O N A P R ES S U RE D ET E C T O R?. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .95

4


WH Y I S T H E O U T P U T O F MY P RE SS U R E D E T E C TO R Q U O T E D I N MV/V? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .97

TORQUE SENSORS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100

WH A T I S T O RQ U E? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101 WH A T I S R E A C T I O N T O RQ UE? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101 WH A T I S R O TA R Y TO R Q UE? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102 HO W D O Y O U M E A S U R E ROT A R Y TO R Q UE? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102 HO W D O RO TA R Y TO R Q UE S E NS O RS W OR K? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103 WH E N S HO U L D I CHOO SE AN I N-L I N E S E N S O R? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105 WH E N S HO U L D I CHOO SE A CLA MP O N CO L LA R? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105 WH E N S HO U L D I S T RA I N GA GE M Y S H A FT A S M Y T O RQ U E T RA N S DU C E R? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106 WH A T A R E THE M AJ O R DI F FE R E N CE S B E TW EE N T HE VAR I O U S FO R MS O F T O RQ U E M E AS U R EM E NT? . . 107

5


ACCELEROMETERS

6


HOW DOES A P IEZO-ELECTRIC ACCELEROMETER WORK?

Piezo-electr ic crystals are man-made or natural ly occurr ing crystals that produce a charge output when they are compressed, f lexed or subjected to shear forces. The word piezo is a corrupt ion of the Greek word for squeeze. In a piezo-electr ic accelerometer a mass is attached to a piezo-electr ic crystal , which is in turn mounted to the case of the accelerometer. When the body of the accelerometer is subjected to vibrat ion the mass mounted on the crystal wants to stay st i l l in space due to inert ia and so compresses and stretches the piezo electr ic crystal . This force causes a charge to be generated and due to Newton law F=ma this force is in turn proport ional to accelerat ion. The charge output is ei ther is converted to a low impedance voltage output by the use of integral electronics (example: in an IEPE accelerometer) or made avai lable as a charge output (Pico-coulombs /g) in a charge output piezo-electr ic accelerometer.

WHAT ARE THE DIFFERENT TYPES OF ACCELEROMETER?

There are many dif ferent type of accelerometers and each has unique character ist ics, advantages and disadvantages. The dif ferent types include: Dif ferent technologies Piezo-electr ic accelerometers Piezo-resist ive accelerometers Strain gage based accelerometers Dif ferent output accelerometers Charge output IEPE output (2-wire voltage) Voltage output (3 wire) 4-20mA output Velocity output accelerometers Dif ferent designs of accelerometer Shear type design

7


Single ended compression design Isolated compression Inverted compression Flexural design

WHAT IS A S INGLE ENDED COMPRESSION ACCELEROMETER?

A single ended compression accelerometer is where the crystal is mounted to the base of the accelerometer and the mass is mounted to the crystal by a setscrew, bolt or fastener.

A single ended compression accelerometer

WHAT IS AN ISOLATED COMPRESSION ACCELEROMETER?

Single ended compression accelerometers can be susceptible to base strain and so to al leviate this problem the crystal is isolated from the base by mounting on an isolat ion washer or by reducing the mounting area by which the crystal is mounted to the base.

8


Isolated compression accelerometers

WHAT IS A SHEAR TYPE ACCELEROMETER?

A shear type accelerometer is where the seismic mass is attached to the crystal so that i t exerts a shear load on the crystal rather than a compressive load. Shear type accelerometers are designed for appl icat ions that are l ikely to encounter signif icant base distort ion from thermal transients or where they are mounted onto f lexible structures.

Shear type piezo-electr ic accelerometer

WHAT IS A P IEZO-RESISTIVE ACCELEROMETER?

A piezo-resist ive accelerometer is an accelerometer that uses a piezo-resist ive substrate in place of the piezo electr ic crystal and the force exerted by the seismic mass changes the resistance of the etched bridge network and a whetstone bridge network detects this. Piezo-resist ive accelerometers have the advantage over piezo-electr ic accelerometers in that they can measure accelerat ions down to zero Hertz.

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WHAT IS A STRAIN GAGE BASED ACCELEROMETER?

A strain gauged based accelerometer is based on detect ing the def lect ion of a seismic mass by using a si l icon or foi l strain gauged element. A whetstone bridge network detects the def lect ion. The def lect ion is direct ly proport ional to the accelerat ion appl ied to the sensor. Like the piezo-resist ive accelerometer i t has a frequency response down to zero Hz.

WHAT IS THE USEABLE FREQUENCY RANGE?

For an accelerometer to be useful the output needs to be direct ly proport ional to the accelerat ion that i t is measuring. This f ixed rat io of output to input is only true for a range of frequencies as described by the frequency response curve.

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Typical Piezo-electr ic frequency response curve.

The usable frequency response is the f lat area of the frequency response curve and extends to approximately 1/3 to ½ of the natural frequency. The def ini t ion of f lat also needs to be qual i f ied and is done so by quoting the rol l of f of the curve in ei ther percentage terms (typical ly 5% or 10%) or in dB terms ( typical ly +/- 3db)

WHAT IS AN IEPE ACCELEROMETER?

IEPE stands for Integrated Electronics Piezo Electr ic and def ines a class of accelerometer that has bui l t in electronics. Specif ical ly i t def ines a class of accelerometer that has low impedance output electronics that works on a two wire constant current supply with an voltage output on a DC voltage bias. IEPE two wire accelerometers are easy to instal l , have a wide frequency response, can run over long cable lengths and are relat ively cheap to purchase. The IEPE technology has general ly replaced most 3 wire accelerometers and is broadly used for most appl icat ions except for special ist appl icat ions such as zero Hz

11


accelerometers, high temperature appl icat ions or 4-20mA accelerometers used in the process industr ies.

WHAT IS AN ICP ACCELEROMETER?

ICP is the trademarked PCB name for IEPE accelerometers. I t stands for ‘ Integrated circuit-piezo electr ic ’

WHAT IS A CHARGE OUTPUT ACCELEROMETER?

All piezo-electr ic accelerometers work by measuring the charge generated by a crystal that is being compressed or shear loaded by a mass inf luenced by accelerat ion. In most appl icat ions this high impedance charge output is converted to a low impedance voltage output by the use of integral electronics. However in some appl icat ions integral electronics are not appropriate such as high temperature or high radiat ion appl icat ions. Charge output accelerometers are self-generat ing and would typical ly have ampli fy ing electronics mounted several feet away from the local heat or local radiat ion source.

WHAT IS THE NATURAL FREQUENCY OF AN ACCELEROMETER?

The natural frequency of an accelerometer is the frequency where the rat io of output is at i t highest. The natural frequency of an accelerometer is def ined by the equation

From a frequency roughly 1/3 to ½ of the natural frequency the rat io of output to input becomes non-l inear and therefore makes measurements from this region dif f icul t to interpret. Therefore the higher the natural frequency of an accelerometer the higher frequencies where the output to input is l inear and the higher the frequencies that can be measured.

12


I t can be seen from the formula for natural frequency that to increase the natural frequency the mass needs to be as small as possible and the st i f fness needs to be as high as possible. A small mass usual ly means a lower sensit iv i ty and this is true of most high frequency accelerometers.

WHAT IS THE MOUNTED NATURAL FREQUENCY?

An accelerometer has a di f ferent natural frequency when i t is in free space to that when i t is mounted. The only frequency that is of interest to the user is of course the mounted natural frequency and is often the one quoted in the specif icat ions. The mounted natural frequency is of course dependent on the st i f fness of the mounting structure to which i t is attached and is therefore quoted as the natural frequency of the accelerometer as instal led according to manufacturers instruct ions. Gluing, magnetical ly mounting or loose bolt ing down to a surface wi l l s ignif icant ly reduce the mounted natural frequency.

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WHAT IS BASE STRAIN SENSIT IV ITY?

Base strain sensit iv i ty is the erroneous signal that is generated by an accelerometer when the base is subjected to bending, torque or distort ion ei ther by mechanical movement or thermal stressing. The relat ive movement of the base of the accelerometer squeezes the crystal in an accelerometer and the seismic mass mounted on the crystal . Base strain is where the base distorts the mass whi le accelerat ion causes the seismic mass to distort the crystal . These two forces on the crystal are indist inguishable and so reduct ion of the base strain is vi tal for good signals only to be generated. The more indirect ly that a crystal is mounted to the base under strain the less sensit ive the accelerometer is to base strain. Single ended compression sensors are the most prone to base strain sensit iv i ty and shear type accelerometers the least. Isolated compression accelerometers are a good compromise between have good base strain immunity and the disadvantages that shear type accelerometers br ing in terms of sensit iv i ty and robustness.

Base Strain Sensit iv i ty

WHAT IS CROSS SENSIT IV ITY OR TRANSVERSE SENSIT IV ITY?

An accelerometer produces a charge output when the crystal is compressed. That same crystal also produces a charge, albeit a much smaller one, when a shear load is exerted on the crystal . The accelerometer therefore produces a charge when i t is vibrated in the axis 90 degrees to the main axis of measurement, which is indist inguishable from accelerat ion in the main axis. Conversely shear type accelerometers produce an erroneous signal when they experience cross axis accelerat ion only this t ime i t loads the crystal in compressive mode.

14


Cross axis sensit iv i ty

The sensit iv i ty of the accelerometer to a transverse vibrat ion is known as the transverse sensit iv i ty and is typical ly less than 5% of the sensit iv i ty to an ‘on axis’ accelerat ion.

WHAT IS DYNAMIC RANGE?

The dynamic range of an accelerometer is the range between the smallest accelerat ion detectable by the accelerometer to the largest. A piezo-electr ic accelerometer produces a charge proport ional the force appl ied to the crystal , which due to the seismic mass on the crystal is proport ional to accelerat ion appl ied. The piezo electr ic effect can be detected for very small forces or accelerat ions al l the way through to very large accelerat ions. In most cases the smallest accelerat ion is dictated by the ampli fy ing electronics noise f loor and for high g levels to the voltage rai l used by the power supply. The design of the accelerometer wi l l also play a part in what shock g levels an accelerometer can withstand before the crystal is i rreparably damaged or the structure holding the crystal is distorted. Compression accelerometers are the most shock resistant design of accelerometer. Accelerometers with integral electronics have a maximum output voltage determined by the circuit design and the input voltage. The maximum output for an IEPE accelerometer is typical ly 4-8 volts. An accelerometer with a sensit iv i ty of 100mV/g with electronics that has a maximum output of 5V wi l l obviously have a dynamic range of +/- 50g whi le an accelerometer of sensit iv i ty of 10mV/g wi l l have a dynamic range of +/- 500g

WHAT IS AMPLITUDE L INEARITY?

The ampli tude l ineari ty of an accelerometer is the degree of accuracy that an accelerometer reports the output in voltage terms as i t moves from being excited at the smallest detectable accelerat ion levels to the highest. This

15


accuracy is qual i f ied by the l ineari ty. Typical ly the ampli tude l ineari ty is 1%. The dynamic range describes the minimum to maximum accelerat ions that can be detected. The output of an IEPE accelerometer can typical ly go from 100 micro g to 500g. This dynamic range is dependent on the electronics used with the accelerometer ei ther internal or external, as is the output l ineari ty over the dynamic range.

WHAT ARE THE DIFFERENCES BETWEEN QUARTZ CRYSTAL BASED AND CERAMIC CRYSTAL BASED ACCELEROMETERS?

Ceramic Crystals

Quartz Crystals

Man made piezo electr ic crystals Natural piezo electr ic crystals Higher output sensit iv i ty Lower output sensit iv i ty Less expensive More expensive Higher pyro-electr ic effect at elevated temperatures

Lower pyro-electr ic effect at elevated temperatures

Higher crystal decay rates at elevated temperatures

No crystal decay rates with t ime or temperature

Lower temperature of operat ion Higher temperature operat ion

HOW DO I MOUNT AN ACCELEROMETER?

The mounting of an accelerometer affects i ts frequency response. The mounted natural frequency is dependent direct ly on the st i f fness of the mounting. The higher the st i f fness the more the mounted natural frequency approaches i ts maximum. The least st i f f mounting of an accelerometer is magnetic mounting and the highest st i f fness is using a high tensi le setscrew t ightened to the correct torque mounted on a hard f lat surface. Other mounting methods come in between these two extremes.

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I t is important to ensure that the si te chosen for the accelerometer is ground f lat for at least an area larger than the base of the accelerometer. A sl ight smear of Si l icone grease wi l l ensure a st i f f bond between accelerometer and structure.

Surface preparat ion and set screw instal lat ion

When using a mounting stud i t is important to ensure that the stud does not bottom out in ei ther the base of the accelerometer or the dr i l led locat ion hole. High tensi le strength set screws that have a shoulder wi l l prevent this eventual i ty from happening.

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Incorrect ly mounted accelerometer

WHAT IS A QUICKFIT MOUNT?

A quickf i t mount is used in instal lat ions where the accelerometer wi l l be removed between monitor ing the accelerat ion or velocity vibrat ion yet wi l l be repeatedly place back in the same locat ion. Such instal lat ions include machine health monitor ing using data col lectors.

Quickf i t mounting for an accelerometer

WHEN SHOULD I USE A VELOCITY OUTPUT ACCELEROMETER?

Velocity output accelerometers are usual ly used in condit ion monitor ing appl icat ions where velocity is a much better parameter for judging the health of a machine. Doubl ing of velocity vibrat ion equates to a doubl ing of the deter iorat ion of the health of the machine. Velocity can also be used in lower frequency appl icat ions where the accelerat ion ampli tude of vibrat ion is too small to measure and the velocity vibrat ion maybe of a higher and more

18


meaningful value. Velocity vibrat ion accelerometers are only real ly effect ive i f the frequency of vibrat ion is higher than 2Hz but more ideal ly 5 Hz.

WHAT SIGNAL CONDIT IONING DO I NEED FOR MY ACCELEROMETER?

All internal ly ampli f ied accelerometers need a power supply be i t a constant current IEPE supply, a 4-20mA loop, a 10V bridge excitat ion or a bipolar +/- 15V supply for a three wire accelerometer. The output of the accelerometer is now condit ioned to an AC voltage whose ampli tude is proport ional to the ampli tude of vibrat ion with a frequency the same as the frequency of the vibrat ion. An AC voltage signal needs further signal condit ioning to retr ieve any useful data. This signal condit ioning takes three main forms: a) Overal l voltage levels in ei ther RMS or peak to peak b) Spectral content analysis c) Snap shot t ime domain analysis

Overal l accelerat ion levels in RMS terms

Overal l accelerat ion levels in Peak-Peak terms

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Breaking the accelerat ion signal into i ts frequency components

Viewing the accelerat ion signal on a storage scope or transient recorder

WHAT ARE GROUND ISOLATED ACCELEROMETERS?

Ground loops can be a signif icant problem to al l type of sensors where the signal is un-ampli f ied or the signal levels are low. Ground loops occur when dif ferent parts of the structure lab or bui lding have dif ferent electr ical grounds. These grounds may only di f fer by a few mil l ivol ts or less. When areas with di f ferent grounds are connected by sensor cables then unless measures are taken to prevent i t a ground loop are set up in the cable that can be signif icant when compared to low level voltage signals that come from the sensor.

20


Ground loops are often very di f f icul t to detect so i t is prudent to take

precautions to prevent their effects. There are a number of ways that ground loops can be prevented. The f i rst is to hard wire di f ferent parts of the structure to ensure that each area has exact ly the same ground.

Preventing ground loops by ensuring al l parts of structure have same ground

Ensuring dif ferent parts of a plant have the same ground may not be so easy part icular ly when long distances are involved or structures carry noise generat ing machinery. In these cases i t may be better not to el iminate ground loops but to prevent their effects inf luencing the sensor output. This can be achieved by mounting the accelerometer on an electr ical ly isolated mounting stud. In this way the accelerometer si ts on a local ly constructed instrument

21


ground and ensures that now ground loop exists between this and the measuring instrument.

Isolated mounting bases el iminate problems with ground loops

The same effect as mounting the accelerometer on an electr ical ly isolated mounting base can be achieved by isolat ing the accelerometer internals from the outer case of the accelerometer. This is done by the manufacturer. Mounting the accelerometer on an isolat ing base or internal ly isolat ing the accelerometer does reduce the st i f fness of the accelerometer and therefore reduces the mounted natural frequency. I t is for this reason that not al l accelerometers come automatical ly with internal isolat ion.

Internal ly isolated accelerometers can prevent ground loops but have a reduced

frequency response as a result

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WHAT IS AN ISOLATED STUD?

An accelerometer isolated stud is used in appl icat ion where the possibi l i ty for ground loops exists which can corrupt the output of the sensor. Isolated studs do reduce the frequency response of the accelerometer somewhat so caut ion should be taken i f high frequency data needs to be measured.

HOW DO I INSTALL A CHARGE AMPLIF IER?

Charge output accelerometers are used in appl icat ions where: High temperatures environments are encountered High radiat ion environments are encountered Very high frequency accelerometers are used where no room exists for internal electronics Charge output accelerometers are self-generat ing and so no excitat ion is required but a local charge ampli f ier is used to convert the charge output to a voltage. The charge output accelerometers do however have high output impedance. This high output impedance makes them susceptible to noise, cable movement ( tr ibo-electr ic effect) and low insulat ion resistance. To minimize these effects i t is important to have; a charge ampli f ier- impedance converter mounted as close to the accelerometer as possible, prevent cable movement, use low noise co-axial cable and ensure al l surfaces are kept clean and dry.

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Instal l ing a charge output accelerometer and clamping low noise co-axial cable

Charge ampli f ier is located as close to accelerometer as possible but away from

the host i le environment

WHAT IS THE TRIBO-ELECTRIC EFFECT?

Tribo-electr ic effect is when a spurious signal is generated by a charge output accelerometer by the movement of the co-axial cable. To prevent the tr ibo-electr ic effect the low noise cable needs to be clamped down as close to the accelerometer as possible. See How do I instal l a charge ampli f ier?

Tr ibo-electr ic effect

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HOW DO I CHOOSE THE SENSIT IV ITY OF AN ACCELEROMETER?

Accelerometers with integral electronics have a maximum output voltage determined by the circuit design and the input voltage. The maximum output for an IEPE accelerometer is typical ly 4-8 volts. An accelerometer with a sensit iv i ty of 100mV/g with electronics that has a maximum output of 5V wi l l obviously have a dynamic range of +/- 50g whi le an accelerometer of sensit iv i ty of 10mV/g wi l l have a dynamic range of +/- 500g I f the maximum g levels l ikely to be experienced is known then dividing this number by 5 volts wi l l give the maximum sensit iv i ty that should be used to get this dynamic range Example; Vibrat ion expected to be seen is 300g. Sensit iv i ty wi l l be 5 divided by 200, which equals 16.6 mV/g. The nearest sensit iv i ty would be a 10mV/g accelerometer.

WHAT IS THE OUTPUT OF AN IEPE ACCELEROMETER?

An IEPE accelerometer is a two-wire sensor that requires a constant current supply and outputs an AC voltage output on a DC voltage bias. The DC bias is often removed by the use of a decoupl ing capacitor.

WHAT IS AN FFT?

An FFT is short for Fast Fourier Transform and is an algori thm that is used to obtain frequency content data from t ime domain signal. Spectral analysis, frequency analysis are terms also used to describe obtaining frequency content data from t ime domain signals.

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WHAT IS CONDIT ION MONITORING?

Condit ion monitor ing is where the health of a rotat ing machine is monitored using vibrat ion levels. As the health of a machine (example becomes unbalanced, fan blades corrode, bearing surfaces degrade) deter iorates so the ampli tude of the vibrat ion the machine generates increases. By monitor ing the vibrat ion levels over a long period of t ime this gradual deter iorat ion of the health of the machine can be assessed unt i l the vibrat ion levels get to a point where the machine needs to be taken out of service and overhauled. Analysis of the frequency content of the machine vibrat ion signal wi l l indicate not only that the health of the machine has deter iorated but also root causes can be attr ibuted to the problem. Example: An 8 bladed pump running at 6000 rpm (100Hz) wi l l produce a vibrat ion signal with 100 Hz frequency i f i t becomes unbalanced, 200 Hz i f i t becomes misal igned 800 Hz i f the blades become corroded and 43-47 Hz i f the bearings start to go into oi l whir l .

WHAT FREQUENCY RESPONSE DO I WANT FROM MY ACCELEROMETER?

The frequency response of the accelerometer needed for test ing depends on what frequencies of vibrat ion are required to be measured. An accelerometer should have a high enough natural frequency as to capture al l the frequencies required to be measured.

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Natural frequency suff ic ient ly high to capture al l f requencies in signal

Problems start to ar ise however when the vibrat ion content of the accelerat ion to be measured gets close to the natural frequency of the accelerometer.

Frequencies to be measured approach the natural frequency of the

accelerometer In these instances distort ion of the accelerat ion by the high gains seen near the natural frequency can give a false picture of the reported accelerat ion ampli tudes at high frequencies.

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Accelerat ion signal is misrepresented by non-unity gain of the higher

frequencies To overcome this problem one of two things needs to happen: a higher frequency accelerometer needs to be used

A higher natural frequency accelerometer solves the problem of measuring high frequency accelerat ions I f the higher frequencies are not required to be measured then using a low pass f i l ter should f i l ter them out.

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A low pass f i l ter removes high frequency components of the measured signal

WHAT TYPE OF ACCELEROMETER BEST SUITS MY APPLICATION?

Accelerometer Type Advantages Disadvantages Single ended compression

Robust Highest natural frequency High shock resistance

Poor base strain character ist ics

Isolated base compression

Robust High natural frequency

Better base strain performance

Shear

Best base strain performance Best temperature transients immunity Smallest size

Less robust Lower shock resistance

Charge output

High temperature operat ion Suitable for radiat ion environments Small s ize

Requires local charge ampli f ier Susceptible to tr ibo-electr ic effect

Piezo-resist ive Measures down to zero Hz

Limited high frequency response

Strain Gage based

Measures down to zero Hz High shock resistance

Limited high frequency response

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CALIBRATION

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WHY SHOULD I CALIBRATE?

Any measurement is subject to degradation due to use, abuse, dr i f t or ageing. To understand this degradation cal ibrat ion at regular intervals needs to be carr ied out to character ize the instrument after degradation, to restore the instrument to an ‘as new’ condit ion as regards i ts measurement performance and to reference the measurement to National Standards. ISO9000 and many other standards specify the maximum period between re-cal ibrat ion as once every two years and more frequently i f the instrument degradation is signif icant during that period. (Typical ly 1% degradation) Many users adopt an annual cal ibrat ion as the standard interval between cal ibrat ions. Sensotec Note Many customers adopt the annual cal ibrat ion but very few do a comparison between the current cal ibrat ion and the previous cal ibrat ion to ascertain the degree of degradation and determine a suitable re-cal ibrat ion t ime period.

WHAT IS NIST?

NIST is the National Inst i tute of Standards and Technology, which is the US federal government agency responsible for the maintenance of National Standards.

WHAT IS A NIST TRACEABLE CALIBRATION?

A NIST traceable cal ibrat ion means that the cal ibrat ion can be traced by an unbroken chain of documented steps, comparisons and stated uncertaint ies r ight back to the nat ional standards.

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CAN A LOAD CELL BE MADE TRACEABLE TO NIST?

No. Only the results produced by that load cel l are traceable to NIST provided that the condit ions under which the results are obtained are clearly understood and under control . For example i f a load cel l and signal condit ioning are cert i f ied by NIST the readings taken by the load cel l are not NIST traceable unless the way the measurements are taken is clearly understood and the condit ions under which they are taken is under control .

CAN AN ORGANIZATION BE NIST TRACEABLE?

No an organizat ion cannot be NIST traceable. Only the results of the organizat ion can be traceable

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WHAT IS A2LA OR NVLAP ACCREDITATION?

In order to reduce the uncertainty of a load cel ls measurements or results the cal ibrat ion needs to be carr ied out by a competent person using appropriate cal ibrat ion equipment and adopting good cal ibrat ion pract ices A2LA and NVLAP are two cert i fy ing bodies that audit companies against ISO17025, which is a standard that ensures competent people, carry out good cal ibrat ion pract ices using good cal ibrat ion equipment

WHAT IS UNCERTAINTY?

Uncertainty is a tolerance band around any measurement result that indicates the range of results that would be reported i f the test were carr ied out on inf ini tely accurate equipment using standards held at NIST or any other internat ional ly recognized standards. Uncertainty is used to qual i fy measurements and their absolute accuracy and is expressed as a tolerance foe example: 422 lbf +/- 0.28 lbs. (+/- 0.28 is the uncertainty) That means that i f NIST standards were on inf ini tely accurate equipment that the results that would be reported would l ie somewhere in the range of 400.28 lbf and 399.72 lbf. Uncertainty est imations are developed by very detai led mathematical analysis and by observat ions

WHEN IS UNCERTAINTY IMPORTANT?

Uncertainty is important when carrying out cr i t ical absolute measurements and is not important when making relat ive measurements. When measuring breaking forces for seat belts i t might wel l be important to know i f i t breaks at 740 lbs or 760lbs. I f however measurements are being made on the force required to press f i t gearbox bearings on an automotive product ion l ine the absolute value of the load may not be as important as to ensure that the same load is appl ied every t ime. When absolute measurements are stated they should always be qual i f ied by adding the uncertainty example 28 lbf +/- 0.16 lbf or somewhere a qual i f icat ion statement should be stated such as “al l readings have an uncertainty of 0.07%. Uncertainty is very important when cal ibrat ing load cel ls, as cal ibrat ion is the t ime that a cel l is checked on how i t measures absolute loads and checked against nat ional standards or loads that are traceable to nat ional standards. Obviously a load cel l that has an uncertainty of +/- 0.15lbf is better than a load cel l that has an uncertainty of +/- 1.2 lbf. This might be because i t is a better

33


load cel l or al ternat ively the load cel l with the better uncertainty might just have been cal ibrated to a higher standard.

HOW IS UNCERTAINTY MEASURED?

Uncertainty is not measured but is est imated. Uncertainty est imations are developed by very detai led mathematical analysis and by observat ions. Uncertainty and i ts determinat ion is a science al l of i ts own and is often very di f f icul t , complicated and t ime consuming. Many cal ibrat ion labs employ people ful l t ime just to determine uncertainty of measurement results obtained within their faci l i ty. NIST recognizes two main methods of obtaining uncertainty. Method A Determined by analysis, measurements and observat ions of results. Al l the contr ibut ing elements that give r ise to any error in measurement are measured and the standard deviat ion obtained. The measurement results of al l the contr ibut ing components are then added together using the square root of the sum of the squares and an overal l standard deviat ion determined. The overal l uncertainty is then stated as ei ther 2 t imes the standard deviat ion or 3 t imes the standard deviat ion. Method B Determinat ion of the uncertainty by a theoret ical and mathematical analysis rather than by observat ion. Sensotec Note Al l uncertainty measurements carr ied out at Sensotec are carr ied out using Method A

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CALIBRATION CLASS LOAD CELLS

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WHAT DOES A LOAD CELL CALIBRATION CONSIST OF?

There is no specif ic standard for load cel l cal ibrat ion except ASTM E74, which is targeted toward the force test ing machines. A number of large companies and organizat ions such as Boeing and the USAF also have standards for load cel l cal ibrat ion. Standards such as ISO 17025 and Z54 lay out guidel ines for cal ibrat ion but do not specif ical ly address load cel ls. As a result each manufacturer has their own standard of load cel l cal ibrat ion. However a typical cal ibrat ion wi l l indicate sensit iv i ty whi le more comprehensive cal ibrat ion might indicate l ineari ty, error, best f i t straight l ine hysteresis. A more elaborate cal ibrat ion might cal ibrate the load cel l in tension and compression and might use more data points. Al l cal ibrat ions should be traceable to NIST standards.

WHAT LOAD CELL CALIBRATION STANDARD SHOULD I ADOPT?

Calibrat ion costs t ime and money so i t is important to adopt a standard that is comprehensive enough to cover the needs of the appl icat ion but not so comprehensive that t ime and money is spent needlessly. The cal ibrat ion should be comprehensive enough to ensure that the uncertainty is four t imes better than the system that is to be cal ibrated. I f measuring the uncertainty is too cumbersome, too complex or insuff ic ient t ime can be invested in the project then at least an appreciat ion of the uncertainty should be attempted. For example: I f the test r ig on which the load cel l is being used can only consistent ly reproduce loads to an accuracy of 5lbs then a load cel l cal ibrat ion that ensures better than 1.25 lbs is l ikely to be suff ic ient.

WHAT ARE IMPORTANT PARAMETERS FOR A CALIBRATION CLASS LOAD CELL?

Any load cel l can be a cal ibrat ion class load cel l provided a) I t ’s uncertainty is known b) I t is used to cal ibrate load cel ls whose uncertainty only needs to be 4 x more (the general ly accepted rat io) than this cal ibrat ion standard c) I t is only used over part of i ts measurement range (say 20% to 100%) which is dictated by the uncertainty of the cel l For example A 1000 lb load cel l with an uncertainty of 0.5 lbs (which means that i f i t measures a load and i t indicates 760 lbs i t could actual ly be anywhere between 759.5 lbs to 760.5 lbs.) Is used as a reference standard to cal ibrate other load cel ls provided that these cel ls under test are not required to report loads with a greater uncertainty of +/- 2 lbs. In addit ion this reference cel l would not be able to carry out cal ibrat ions with loads less than 200 lbs. This lower l imit is

36


determined by mult iplying the uncertainty by 400 for class A load cel ls and 2000 for class AA load cel ls.

WHAT ARE THE CHARACTERISTICS OF A LOW UNCERTAINTY CALIBRATION CLASS LOAD CELL?

A low uncertainty cal ibrat ion class load cel l has the fol lowing attr ibutes: a) The cel l has a high repeatabi l i ty (or a low repeatabi l i ty error) b) The l ineari ty curve is wel l known. ( I t does not have to be highly l inear. I t just has to be wel l known either as a polynomial curve or a look up table) c) The cel l should have a low hysteresis d) The cel l should have very low creep e) The cel l should have low dri f t f ) The cel l should be cal ibrated with a low uncertainty i .e. with good cal ibrat ion pract ices on good cal ibrat ion equipment. g) The read out should be highly accurate and have a good uncertainty h) The f ixtur ing used should be careful ly designed that ensures accurate cal ibrat ion with high repeatabi l i ty

WHY DOES A CALIBRATION CLASS LOAD CELL HAVE A BASE PLATE AND A CALIBRATION ADAPTER?

In order to get high repeatabi l i ty ( low repeatabi l i ty error) and low creep from a cal ibrat ion class load cel l Then the fol lowing condit ions need to be met:

a) The cel l sensing element must be mounted on a f lat surface b) The load cel l sensing element must be r igidly f ixed to the structure.

Rigidly f ixed usual ly means bolted down c) The load cel l element should be torqued down evenly to avoid distort ion d) Any threads used in the loading path should be pre-tensioned to avoid

thread creep during the load cycle e) Al l compressive forces should be appl ied absolutely perpendicular to the

sensing element and any side loading should be avoided. f ) Al l tensi le forces should be appl ied to the load cel l absolutely

perpendicular to the sensing element. The cal ibrat ion adapter and base plate help achieve al l of these goals as shown in the diagram.

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WHAT IS THE UNCERTAINTY OF A SENSOTEC LOAD CELL?

The uncertainty of a Sensotec load cel l is determined by a number of factors a) The type of load cel l b) The cal ibrat ion procedure used in i ts cal ibrat ion c) The range of the load cel l d) The cal ibrat ion test stand that was used to do the cal ibrat ion e) I f the low uncertainty opt ion was specif ied at t ime of manufacture f) I f the load cel l includes pul l plate and cal ibrat ion adaptor. An 10,000 lb imperial c lass cal ibrat ion load cel l with pul l plate and cal ibrat ion adapter and SC2000 cal ibrat ion class signal condit ioning may have an uncertainty of 0.75 lbf or 0.0075% A 100 lbf model 41 would have an uncertainty of 0.05 lbf or 0.05%

WHAT UNCERTAINTY SHOULD MY CALIBRATION REFERENCE LOAD CELL HAVE?

The simple answer is that a cal ibrat ion reference load cel l should have an uncertainty that is 4 t imes better that the load cel l i t is going to be used in cal ibrat ing. The more complete answer is however that i t is not str ict ly the load cel l that has the uncertainty but the results obtained by that load cel l . In order to get results from a cal ibrat ion reference load cel l the load cel l needs a display, i t needs signal condit ioning, i t needs a cal ibrat ion stand or means of loading and i t needs a cal ibrat ion procedure. I f you look at the contr ibut ing

38


uncertaint ies in the results obtained from a cal ibrat ion load cel l the largest contr ibutors to error in descending order are l ikely to be:

• Fixtur ing • Calibrat ion r ig or means of loading • Reference load cel l display and signal condit ioning • Reference cal ibrat ion load cel l cal ibrat ion • Reference Load cel l creep • Reference Load cel l hysteresis • Reference Load cel l l ineari ty

I f a low uncertainty cal ibrat ion result is desired then i t makes sense to reduce individual contr ibutors to the overal l uncertainty budget. I f the f ixtur ing, cal ibrat ion r ig, display and signal condit ioning are poor i t would seem point less to spend t ime and money employing a low (good) uncertainty load cel l . I f the f ixtur ing, cal ibrat ion r ig, display and signal condit ioning are good then i t would be worthwhi le spending t ime and money gett ing a low uncertainty cal ibrat ion carr ied out on a low creep, low hysteresis, high l ineari ty load cel l . .

DOES A LOAD CELL HAVE TO BE CALIBRATED WITH ITS DISPLAY?

Str ict ly speaking the answer is no. However when using the load cel l the uncertainty of the signal condit ioning cabl ing and the display unit need to be determined and added to that of the load cel l . In addit ion the signal condit ioning/display unit needs to be within i ts annual cal ibrat ion. I t is for these reasons that the signal condit ioning/display unit is often included in the cal ibrat ion of the load cel ls as i t a) determines the uncertainty of the load cel l and signal condit ioning unit combined and b) ensures that the signal condit ioning unit gets i ts annual or biannual cal ibrat ion.

WHAT DOES THE ASTM E74 STANDARD SPECIF ICALLY SAY ABOUT CALIBRATION CLASS LOAD CELLS?

a) I t out l ines a procedure on how load cel ls should be cal ibrated and the terminology used

b) Load cel ls should be tested by deadweight test ing machines and hydraul ic test machines and specif ies the uncertainty of these weights and how local gravity needs to be determined.

c) I t def ines how the l ineari ty curve should be def ined as a polynomial curve with a 2n d order f i t but that up to 5t h order can be used.

d) I t states that a cal ibrat ion should be carr ied out at 10%, 20%, 30%, 40% 50%, 60%, 70%, 80%, 90%, 100% in ascending loading only. I t also states that i f increments in deadweights cannot be obtained to carry out these percentages that al ternat ives can be used but need to be specif ied in the cal ibrat ion cert i f icate

e) A cal ibrated load cel l cannot be used below 10% unless weights were appl ied below 10% and cal ibrat ion results obtained.

39


f ) A load cel l can never be used below 2000 t imes the uncertainty of the cel l for a class AA load cel l and 400x the uncertainty

g) A load cel l should never be used below 2% of i ts ful l scale output. h) I t specif ies good cal ibrat ion pract ices to adopt l ike temperature control

and appl icat ion of the weights etc. i ) I t specif ies that after the cel l is taken through one 19 point cal ibrat ion

cycle that the cel l should be rotated by 120 degrees and a 19 point cal ibrat ion carr ied out again and then rotated again through 120 degrees for a third 19 point cal ibrat ion carr ied out.

j ) The uncertainty of the cel l is reported as 2.4 x the standard deviat ion of the results obtained during the cal ibrat ion process.

k) The uncertainty for a class A standard load cel l should never exceed 0.25% and 0.05% for a class AA load cel l

l ) The temperature error over the stated temperature range should not exceed 0.01% for Class AA load cel ls and 0.05% for Class A load cel ls

m) Load cel ls need to be re-cal ibrated 1 year after manufacture and then every 2 years provided the cal ibrat ion has not changed by more than 0.1%. I f the cal ibrat ion has changed then the cel ls need to be re-cal ibrated more frequently unt i l a new t ime interval is establ ished.

n) I t specif ies a format for the report or cal ibrat ion cert i f icate ASTM E74 does not specify the required accuracy of a load cel l nor does i t specify l ineari ty or hysteresis

40


GENERAL

41


HOW DO I KNOW WHAT ACCURACY CLASS TO USE FOR MY SENSOR?

I t depends on the appl icat ion. How much does error in your project matter? I f you’re f i l l ing a tub with water then i t may not matter at al l . I f you’re f i l l ing of a tub with chemicals, the consequence of error could be severe. I f the consequences of error are signif icant, you should ident i fy al l sources of error and their contr ibut ion to total error. You can then “snap” pressure sensors with di f ferent levels of accuracy into your evaluat ion and see the impact of sensor error on each. I f you have other error factors that are much larger than the transducer then upgrading the transducer accuracy may not matter. Remember to translate the accuracy into hard numbers to get a better perspect ive. Is a 1000 PSI pressure transducer with .1% accuracy good enough? The real quest ion is whether an accuracy of +/- 1 PSI is accurate enough. Is detect ing between 999 and 1001 PSI at a true pressure of 1000 PSI acceptable? I t depends on the appl icat ion. You can always opt for the highest accuracy, but i t can be more beneficial to analyze accuracy in the context of the appl icat ion’s needs, transducer costs and transducer lead t imes.

WHAT IS SENSIT IV ITY?

The rat io of change between a transducer’s output and input is known as i ts sensit iv i ty. For example, a transducer that produces 1 mV for every 100 psi has a sensit iv i ty value of .01 mV/psi. Under ideal condit ions, a transducer’s sensit iv i ty value does not change between zero and ful l scale. A transducer that produces 1 mV for every 100 psi would then, under ideal condit ions, also produce 2 mV for an appl ied pressure of 200 psi, 3 mV for an appl ied pressure of 300 psi, and so. A transducer’s ideal sensit iv i ty can therefore be mapped as a straight l ine, and the transducer’s sensit iv i ty value, expressed as the rat io of output to input, then equates to the slope of that l ine

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Ideal Sensit iv i ty is Represented as a Straight Line

Notice also that under ideal condit ions, there is zero output when there is zero input. However, the actual sensit iv i ty of a transducer f luctuates sl ight ly between zero balance and ful l scale. Some reasons for this might be due to manufactur ing and materials imperfect ions, electr ical interference, and even the age of the transducer. In addit ion, a transducer usual ly produces some amount of output even at zero balance. Thus, true sensit iv i ty actual ly equates to a non-l inear funct ion with a zero offset.

True Sensit iv i ty is Represented as a Curve

Because true sensit iv i ty is non-l inear, the true sensit iv i ty value of a transducer ( the rat io of output to input) wi l l not always be the same at any point between zero balance and ful l scale. In order for a sensit iv i ty value to be constant, the sensit iv i ty must be expressed l inearly. Most manufacturers use a best f i t straight l ine to represent sensit iv i ty.

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Example of a Best Fit Straight Line

The sensit iv i ty value can then be expressed as the slope of the best f i t straight l ine, which becomes the value quoted on the transducer’s cal ibrat ion cert i f icate.

Sensit iv i ty as the Slope of the Best Fit Straight Line

Sensotec Note In some cases, Sensotec uses the slope of a best f i t straight l ine as a transducer’s quoted sensit iv i ty on i ts cal ibrat ion cert i f icate. In other cases, Sensotec uses the slope of a terminal point straight l ine. (See What is Non-Lineari ty?)

WHAT IS NON-LINEARITY?

In i ts broadest sense, non-l ineari ty refers simply to a departure from something that is l inear.

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In the world of transducers, non-l ineari ty is the maximum deviat ion in output between a transducer’s sensit iv i ty curve and a l inear representat ion of i ts true sensit iv i ty curve drawn between nominal zero and ful l scale. Non-l ineari ty is measured on increasing input only, and is expressed as a percent of ful l scale output. An example of non-l ineari ty for a transducer is ±0.15% F.S. Determining non-l ineari ty for a transducer raises a quest ion of how to create the l inear representat ion of a transducer’s true sensit iv i ty. Often a best f i t straight l ine, which is based on the least squares method, is employed.

Best Fit Straight Line Compared to Ideal Sensit iv i ty

When a best f i t straight l ine is used, transducer non-l ineari ty is simply the greatest deviat ion between the transducer’s sensit iv i ty curve and the best f i t straight l ine obtained mathematical ly using the least squares f i t method

Non-Lineari ty Based on Best Fit Straight Line

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In other cases, a terminal point straight l ine is used to determine transducer non-l ineari ty. The terminal point straight l ine is drawn between nominal zero and output at ful l scale.

Terminal Point Straight Line

Terminal point straight l ine is often a more pract ical best straight l ine, as i t is easy to understand and implement. The user simply takes the output at zero and the output at ful l scale and assumes a straight l ine relat ionship. Using a terminal point straight l ine results in a greater (worse) value for non-l ineari ty than using a best f i t straight l ine obtained mathematical ly.

Non-Lineari ty Based on Terminal Point Straight Line

Sensotec Note Sensotec uses the terms l ineari ty and non-l ineari ty interchangeably. Sensotec uses the terminal point straight l ine method and least squares f i t best straight l ine to determine i ts transducer’s non-l ineari ty. The datasheets indicate the method used when quoting specif icat ions.

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WHAT IS HYSTERESIS?

Hysteresis refers to the behavior of a transducer to produce dif ferent output values for a common input point depending on whether appl ied input is increasing or decreasing. Hysteresis is due to the behavioral patterns of metal crystals, which expand and contract di f ferent ly. As appl ied pressure on a transducer increases, the non-l inear representat ion of the transducer’s output traces i ts true sensit iv i ty curve. But as appl ied pressure on a transducer decreases, the non-l inear representat ion of transducer output results in a di f ferent sensit iv i ty curve. Hysteresis is then the greatest di f ference between output readings for a common input point, one reading obtained whi le increasing from zero input, and the other whi le decreasing from ful l scale output. The deviat ion is expressed as a percent of ful l scale. An example of hysteresis for a transducer is ±0.10% F.S.

Hysteresis as the Deviat ion between Increasing and Decreasing Values

WHAT IS STATIC ERROR BAND?

Stat ic error band is a performance specif icat ion that takes into account the effects of transducer non-l ineari ty and hysteresis. The stat ic error band is an error envelope that is determined by drawing two l ines paral lel to the best f i t straight l ine (going through normalized zero point) with a width that is determined by the hysteresis curve. An example of stat ic error band for a transducer is ±0.04% F.S. Notice that a best f i t straight l ine, rather than a terminal point straight l ine, is used to calculate transducer error band. The best f i t straight l ine must take into account both curves. Once the best f i t straight l ine has been determined, two l ines, which are both paral lel to the best f i t straight l ine, are then drawn through the points of maximum deviat ion. The ent ire region between these outer l ines is known as the stat ic error band.

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Stat ic Error Band and the Best Fit Straight Line

WHAT IS CALIBRATION FACTOR?

Calibrat ion is the process of standardizing an instrument by determining i ts deviat ion from a desired standard. I t is through the cal ibrat ion process that one obtains the proper correct ion factors for the transducers deviat ion. Cal ibrat ion is essential ly the comparison of transducer outputs when compared to a reference standard. Every transducer is shipped with a sensit iv i ty on i ts cal ibrat ion cert i f icate so that the electronic equipment associated with the transducer can be set up correct ly. Sensotec Note Sensotec expresses the sensit iv i ty of the transducer by stat ing a cal ibrat ion factor rather than a sensit iv i ty. The cal ibrat ion factor for a transducer is the transducer’s output value at ful l scale when the output has been normalized ( i .e. zeroed). The l ine drawn through normalized zero and a transducer’s cal ibrat ion factor equates to the best f i t straight l ine of the transducer output. Thus, the transducer’s cal ibrat ion factor in effect establ ishes the transducer’s sensit iv i ty.

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Cal ibrat ion Factor Quoted on the Cal ibrat ion Cert i f icate

WHEN SHOULD I FIT A CONNECTOR OR INTEGRAL CABLE?

Using a connector makes i t easy to disconnect (or reconnect) a sensor’s cabl ing which also makes i t easier to remove or replace the sensor too. Thus, using connectors is an excel lent choice for temporary sensor appl icat ions. However, in addit ion to sometimes being more expensive than integral cable opt ions, connectors introduce a point of vulnerabi l i ty for potent ial water, moisture, and mechanical damage in the connect ion i tsel f .

Connector version of sensor Because of size constraints, i t is often impract ical or impossible to f i t connectors to small sensors; thus, integral cable technology is often the only opt ion for small sensors. As can be imagined, integral cables are often used for more permanent instal lat ions, where connect ing and disconnect ing a sensor’s cabl ing is not planned. Because the cable is integrated with the sensor, the points of vulnerabi l i ty with respect to water, moisture, and mechanical damage that can occur with connector technology, are el iminated; on the other hand, integral cables require strain rel ief protect ion to prevent the cable from gett ing sheared off or r ipped out. I f an integral cable is ever damaged, the ent ire sensor must be repaired or replaced.

Miniature sensor with integral cable

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Whi le i t is possible to f i t a connector to a submersible sensor (using a submersible connector), using an integrated cable is much more cost effect ive. Because a submersible sensor (Fig 24c) is typical ly part of a permanent instal lat ion, an integrated cable becomes a much more pract ical choice. In some cases, though, an appl icat ion might not require true submersibi l i ty, but only a degree of water protect ion; thus one must be able to ident i fy the true need.

Submersible Sensor The fol lowing table summarizes the advantages and concerns for connectors and integral cables:

Connector - Cable Comparison

Connector

Integral Cable

Submersible

Easy to d isconnect cab l ing f rom the sensor Easy to rep lace the sensor Idea l for temporary ins ta l la t ions More expensive Poin t o f vu lnerab i l i ty Subject to water , mois ture, mechanica l damage

Idea l for permanent ins ta l la t ions Of ten the on ly opt ion on smal l sensors Cable must be protected to avo id damage St ra in re l ie f o f ten requi red Cable damage means rep lac ing or repai r ing sensor

Submers ib le connectors are very expensive In tegra l cab le is a more cost e f fec t ive so lu t ion More permanent ins ta l la t ion (by def in i t ion) Ensure of def in i t ion between waterproof and submers ib le

Connector - Cable Comparison Table

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WHAT IS THE DIFFERENCE BETWEEN SUBMERSIBLE AND WATERPROOF?

There are many categories of environmental protect ion against water. For example, a product might be designed for water protect ion in various forms, such as protect ion from dripping; spraying; splashing; jet spray; immersion; submersion; and so on. A product with an environmental protect ion rat ing against dr ipping water might not have suff ic ient protect ion against splashing water or jet spray. Similar ly, a product protected against spraying or splashing might not have suff ic ient protect ion against submersion.

Three Environmental Protect ions for Enclosures Waterproof is a general term with respect to environmental protect ion. A waterproof sensor might actual ly be rated only against a specif ic type of water ingress, such as splashing, dr ipping, spraying, and so on. A sensor that is rated against submersion, on the other hand, is probably also protected against al l other forms of water ingress. Submersible sensors, which are rated by depth of submersion, enjoy the highest environmental protect ion against water.

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WHAT ARE NEMA AND IP DEFINIT IONS FOR ENVIRONMENTAL PROTECTION?

The National Electr ical Manufacturers Associat ion (NEMA) has been developing standards for the electr ical manufactur ing industry for more than 70 years. NEMA’s environmental protect ion standards, which are used in America, are expressed numerical ly as fol lows:

NEMA Number Defini t ions: NEMA-#

# Meaning

1 General Purpose (Indoor)

2 Water Drip Proof ( Indoor)

3R Dust Tight, Rain Tight, and Ice Resistant (Outdoor)

4 Water Tight and Dust Tight ( Indoor/Outdoor)

4X Water Tight, Dust Tight, and Corrosion Resistant ( Indoor/Outdoor)

9 Indoor Hazardous Locations (Not Appl icable to EMS Equipment)

12 Industr ial Use - Dust Tight and Drip Tight ( Indoor)

13 Oi l Tight and Dust Tight ( Indoor) NEMA Number Defini t ions Table

Thus, an enclosure that is rated as NEMA-4 is both water t ight and dust t ight, whether indoors and outdoors. Europe uses a di f ferent system (IP) to express environmental protect ion for enclosures. Protect ion categories are expressed by two numbers. Each number def ines the protect ion level. The f i rst number refers to a part ic les protect ion; the second number refers to water protect ion.

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IP Number Defini t ions: IP##

1st # Meaning 2nd # Meaning

0 No Special Protect ion 0 No Special Protect ion

1 Protected Against Sol id Objects > 50 mm in Diameter

1 Protected Against Dripping Water

2 Protected Against Sol id Objects <12 mm in Diameter

2

Protected Against Dripping Water When Ti l ted Up to 15D C From Normal Posit ion

3 Protected Against Sol id Objects <2.5 mm in Diameter

3 Protected Against Spraying Water

4 Protected Against Sol id Objects <1 mm in Diameter

4 Protected Against Splashing Water

5 Dust Protected 5 Protected Against Water Jet Spray

6 Dust Tight 6 Protected Against Heavy Jet Spray

7 Protected Against the Effects of Immersion

8 Protected Against Submersion

IP Number Defini t ions Table For example, IP54 is considered both dust protected and splash protected. Note that there is a di f ference between the effects of immersion and submersion. Immersion protect ion means that an ingress of water wi l l not cause harmful effects when the enclosure is temporari ly immersed in one meter of water for standard condit ions of pressure and t ime. Submersion, on the other hand, means that an ingress of water wi l l not cause harmful effects when the enclosure is cont inuously immersed in water under more severe condit ions, which are agreed upon by the manufacturer and user.

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LOAD CELLS

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WHAT IS OVERLOAD PROTECTION ON A LOAD CELL?

Overload protect ion in a general sense refers to how a system or device is protected from damage that can result f rom an input that exceeds a designed l imit . For example, overload protect ion in an electr ical appl icat ion might involve using a fuse or circuit breaker to protect the system from a current overload. Overload protect ion on a load cel l refers specif ical ly to the means used to prevent the cel l ’s diaphragm from deflect ing beyond i ts designed elast ic l imit . Without overload protect ion, a load cel l ’s diaphragm experiences irreversible damage under too much appl ied input. To achieve overload protect ion in a load cel l , a mechanical stop is inserted to prevent the diaphragm from deflect ing beyond i ts elast ic l imit The mechanical stop bottoms out when excessive load is appl ied.

Overload Protect ion on a load cel l ( in this case mounted between base plate and load cel l ) Load cel ls that do not have mechanical overload protect ion enjoy an overload capacity by default typical ly 50%. This means that a 100 lbf load cel l without mechanical overload protect ion can sustain a 150 lbf load without incurr ing damage. Notice that when a load cel l ’s maximum designed input l imit is exceeded, the load cel l ’s output does not increase further ( the output becomes asymtopic) Mechanical overload protect ion lends i tsel f more easi ly to compression load cel ls than to tension load cel ls. Because of the internal mechanical design of

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tension load cel ls, i t is di f f icul t to insert a mechanical stop that provides suff ic ient overload protect ion. Sensotec Note: Sensotec sensors operate in an elast ic range of .002 - .003 inch. Because of the typical design of most load cel ls, Sensotec can f i t overload stops on most compression load cel ls, but on only a few tension load cel ls. There are also some load cel ls that, because of their unique shape, are unable to have overload stops in ei ther compression or tension. At Sensotec, every load cel l has a sl ight ly di f ferent def lect ion. Because tolerances on a load cel l ’s internal dimensions are not t ight enough to permit a generic overload protect ion stop, any protect ion stop that is inserted must be custom made, custom f i t ted, and then custom tested. This process increases the cost signif icant ly.

WHAT IS A COMPRESSION ONLY LOAD CELL?

A compression only load cel l is a load cel l that has been designed specif ical ly to measure only compression. Most load cel ls work in both compression and tension to some degree. However, some load cel ls, by vir tue of their physical construct ion, are better suited to ei ther compression or tension. Sometimes i t is preferable to use a load cel l that measures both compression and tension without enabl ing i ts tension measuring capabi l i ty. Rather than design a load cel l that measures only compression, a load cel l capable of measuring compression and tension can be shipped with cal ibrat ion simply carr ied out only for compression. This type of load cel l might be considered a compression-only load cel l , al though i t is technical ly a compression and tension load cel l being used only for compression. Compression-only load cel ls are usual ly f i t ted with a load button to minimize side loading.

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The physical design of the load cel l is typical ly better suited for ei ther compression or tension. Compression-only load cel ls tend to have a high diameter to height rat io. Tension-only load cel ls, on the other hand, tend to have a high height to diameter rat io. When a compression and tension load cel l is used only for compression, cal ibrat ion is provided only for compression loading. I t is more expensive to provide cal ibrat ion in both direct ions for a load cel l . I t is less expensive, on the other hand, to provide cal ibrat ion in just one direct ion (tension only or compression only). I t is also important to consider which physical attachments wi l l come into play with a load cel l ’s appl icat ion. Some load cel ls wi l l be better suited for specif ic attachments.

WHAT IS A TENSION ONLY LOAD CELL?

A tension only load cel l is a load cel l that has been designed specif ical ly to measure only compression. Most load cel ls work in both compression and tension to some degree. However, some load cel ls, by vir tue of their physical construct ion, are better suited to ei ther compression or tension.

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Physical design of a tension load cel l Sometimes i t is preferable to use a load cel l that measures both compression and tension without enabl ing i ts compression measuring capabi l i ty. Rather than design a load cel l that measures only tension, a load cel l capable of measuring compression and tension can be shipped with cal ibrat ion simply carr ied out only for tension. This type of load cel l might be considered a tension-only load cel l , al though i t is technical ly a compression and tension load cel l being used only for tension.

Tension Only Appl icat ion and load cel l f i t ted with rod end bearings Tension-only load cel ls tend to be long in shape, and are often f i t ted with rod-end bearings, which minimize side loading.

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WHAT IS A TENSION AND COMPRESSION ONLY LOAD CELL?

A tension and compression load cel l is a load cel l that has been designed specif ical ly to measure both tension and compression. Most load cel ls work in both compression and tension to some degree. However, some load cel ls, by vir tue of their physical construct ion, are better suited to ei ther compression or tension. Thus, a tension and compression load cel l is one that has been physical ly designed in such a way to be able to measure both compression and tension effect ively. Note that a tension and compression load cel l should come with two cal ibrat ion factors: one for compression measurement, and one for tension measurement.

Sensotec Note: Load cel ls are suppl ied with tension-only cal ibrat ion unless the tension/compression cal ibrat ion opt ion is ordered.

WHAT IS A ROD END BEARING?

Rod end bearings are self-al igning spherical bearings used in tension appl icat ions to prevent side loading. Using male or female attachments, rod end bearings attach to a stat ic rod. The bearing i tsel f swivels in order to accommodate the rod’s varying misal ignment. Ratings exist for both load capacity and lubricat ion requirements. Some rod end bearings can actual ly be created with integrated threaded studs in the bearing. Common appl icat ions for

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rod end bearings include l inkages, shif t control rods, and use with tension load cel ls.

Male Rod End Bearing Female Rod End Bearing

WHAT IS A LOAD BUTTON?

A load button is a physical feature of a compression load cel l . A load button is a domed shaped loading point that is welded or threaded onto the side of the load cel l that has been designated to receive the appl ied load. Load buttons are used to measure compression loading correct ly by el iminat ing the potent ial for side loading. A load button is rounded so that the load being measured always rests on the highest point of the button. A load button ensures that the ent ire appl ied load issues a force in a direct ion that is perpendicular to the load cel l .

Load Button Ensures a Force Perpendicular to the Load Cel l

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WHAT IS LOAD CELL SYMMETRY?

Load cel l symmetry refers to whether a load cel l exhibi ts the same sensit iv i ty for both compression and tension usage. Al l load cel ls have a non-symmetry of a greater or lesser extent. The l inear representat ion for compression in a load cel l is not, by default , symmetr ical to the l inear representat ion for tension. Thus, when both sensit iv i t ies are mapped, the sensit iv i ty curves for compression and tension are di f ferent and have a di f ferent slope. Most tension and compression load cel ls are used in only one direct ion for an appl icat ion so the quest ion of symmetry is not an issue. On some occasions, though, appl icat ions might cal l for true symmetry in a load cel l . An example might be an appl icat ion, which measures both compression and tension in a hydraul ic l ine. In this case, working with a single cal ibrat ion factor for both direct ions might be preferred to having to balance two cal ibrat ion factors. This convenience can be achieved i f the load cel l exhibi ts near true symmetry. I t is possible to achieve near true symmetry in a load cel l . The process, however, is expensive.

Sensotec Note: Symmetry can be calculated and minimized for customers where this phenomenon is an issue. Cal ibrat ion class load cel ls have very low symmetry character ist ics.

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WHAT IS ZERO BALANCE FOR A LOAD CELL?

Zero Balance is the load cel l output when no load and proper supply voltage are appl ied. Zero balance is expressed as a percentage of ful l scale. Ideal ly, the output would be exact ly zero, but i t is not due to several factors. The electr ical and mechanical components of transducers have inherent stresses and offsets during the assembly process. These offset the zero balance.

Zero offset present during manufacture We add resistance to ensure sat isfactory zero balance for new transducers.

Zero balance resistor added to br idge network Operat ing environment and transducer history can change the zero balance. Factors include temperature, transducer age, and transducer overload. I f customers not ice a change in their zero balance, they may want to adjust their balance using ei ther external instrumentat ion or local zero adjustments.

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Annual test ing and cert i f icat ion can ensure precise output throughout the l i fe of the transducer.

WHAT IS ZERO BALANCE TEMPERATURE EFFECT?

Zero balance is the load cel l output at no load. This balance changes as temperature changes

We can minimize the zero balance change due to temperature by insert ing compensating resistors. When a temperature changes drive the transducer output higher, the changes are also dr iving the compensation resistor to lower output.

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The zero balance offset is described as a percentage of ful l scale change per degree Fahrenheit (% F.S. / F) A 10000 lb sensor with .002% F.S. / F wi l l have a zero shif t of 0.2 lbs for every degree change from the cal ibrat ion reference temperature.

WHAT IS OUTPUT SPAN TEMPERATURE EFFECT?

Output Span refers to the output between zero and ful l scale. The output span is the output value of the sensor at ful l load (Cal ibrat ion factor), expressed as mV/V. A sensor with a cal ibrat ion factor of 3 mV/V wi l l have an output of 30 mV at ful l load i f i t is being suppl ied with 10V power.

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Output span varies with temperature, and we insert span compensating resistors to minimize this variat ion.

10V

Given outputfor a given load

at reference temperature

75 F

800 lb load

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The temperature effect on span is measured as a percentage change in rated output per degree of temperature change. A 10000 lb load cel l with .002 Rdg/F wi l l exhibi t 0.2 lb span shif t for each degree of temperature change. Customers can adjust the zero and span of their outputs by adjust ing the zero and span adjustments on their transducer ( I f they have internal ampli f icat ion), or on the instrumentat ion that reads the transducers.

WHEN SHOULD I HAVE ZERO AND SPAN ADJUSTMENTS ON MY LOAD CELL?

Zero and span may shif t due to temperature, repeated loading, or sensor aging. The preferred method to adjust zero and span is through the use of external instrumentat ion. This al lows users to track the changes they’ve made and revert back to previous values i f needed.

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Customers may purchase ampli f ied output load cel ls with zero/span adjustments direct ly on the unit . This also al lows for zero and span adjustment, but does not al low the user to track changes or revert to previous values. Once adjusted, the sensor is no longer cal ibrated as i t was from the factory. Customers wi l l also want to consider the operat ing environment of the transducer when select ing whether or not to use sensors with zero/span adjustments. I f the sensor were going to be inaccessible, then i t would be better to not have adjustments direct ly on the sensor. A signif icant advantage of having the adjustment screws is that users can get cal ibrated, precise output by use of annual test ing and cert i f icat ion, even as the transducer wears or ages in service.

WHAT ARE THE ADVANTAGES AND DISADVANTAGES OF HAVING A SENSOR INTERNAL AMPLIF IER ON A LOAD CELL?

Systems without internal ampli f icat ion must receive supply voltage from an external source, and must send small s ignals ( i .e. 30mV) back to the ampli fy ing source.

A good sensor output may become distorted by the electr ical noise. These errors can be large and give signal to noise rat ios of less than 20.

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Internal ampli f icat ion is a good way of reducing the effects of noise. The internal ampli f iers are housed in the same unit as the sensor. This ensures the signal ampli f icat ion is accomplished inside the transducer.

This makes the system less vulnerable to electr ical noise and creates a higher signal to noise rat io. The larger output also al lows A/D converters to create a higher resolut ion output. Because the internal ampli f iers are so close to the sensor, l ine drops in excitat ion are el iminated.

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The ampli f ier outputs are low impedance, and internal ampli f iers, al though they have some r ipple associated with them don’t contr ibute not iceably to system inaccuracy and signal to noise rat ios of 10,000 are not uncommon.

Internal ampli f iers may not be feasible under certain condit ions. Specif ical ly, the circuitry in the amp cannot be subjected to extreme temperatures. I f sensor placed in a locat ion inaccessible to users (hazardous environment, smal l space, long distance), zero and span adjustments may not be able to be tweaked when needed. Internal ampli f iers increase the overal l s ize of the unit , which may be concern in some appl icat ions.

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WHY IS THE OUTPUT OF MY PRESSURE DETECTOR QUOTED IN MV/V?

mV/V output al lows you to el iminate much of the error due to power supply voltage change. A mV/V output impl ies that di f ferent levels of excitat ion may be provided to the transducer. The ful l -scale output of the transducer varies direct ly with the excitat ion. A sensor with a cal ibrat ion factor of 3 mV/V wi l l exhibi t 30 mV at ful l pressure i f i t is being suppl ied with 10V power, but only 15 mV at ful l pressure i f i t is being suppl ied with 5 V.

Output varies with supply voltage. I f we don’t know how much the change in supply voltage affected our output, then we cannot possibly know how much our change in output was due to an actual change in pressure

Many users monitor transducer output AND power supply excitat ion. Changes in output are compared to the supply voltage to discount effects from voltage

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shif ts. Using the mV/V relat ionship, users can tel l how much of their output change was due to an actual change in pressure

This approach is known as a rat io metr ic approach because i t rel ies on the rat io of voltage output to the Cal ibrat ion Factor (mV/V) to determine pressure. For example, i f we have a 3 mV/V, 100 lb load cel l : Supply Voltage Load Output 10 Volts 100 lbs 30 mV 5 volts 100 lbs 15 mV

HOW DO I KNOW WHAT ACCURACY CLASS TO USE FOR MY SENSOR?

I t depends on the appl icat ion. How much does error in your project matter? I f you’re f i l l ing a tub with water then i t may not matter at al l . I f you’re f i l l ing of a tub with chemicals, the consequence of error could be severe. I f the consequences of error are signif icant, you should ident i fy al l sources of error and their contr ibut ion to total error. You can then “snap” load cel ls with di f ferent levels of accuracy into your evaluat ion and see the impact of sensor error on each. I f you have other error factors that are much larger than the transducer then upgrading the transducer accuracy may not matter. Remember to translate the accuracy into hard numbers to get a better perspect ive. Is a 1000 lb load cel l with 0.1% accuracy good enough? The real quest ion is whether an accuracy of +/- 1 lb is accurate enough. Is detect ing

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between 999 and 1001 lbs at a true load of 1000 lbs acceptable? I t depends on the appl icat ion. You can always opt for the highest accuracy, but i t can be more beneficial to analyze accuracy in the context of the appl icat ion’s needs, transducer costs and transducer lead t imes.

HOW DOES TEMPERATURE AFFECT A LOAD CELL?

Temperature causes changes in zero and span shif t for load cel ls.

HOW DO YOU COMPENSATE FOR TEMPERATURE IN A LOAD CELL?

The character ist ics of al l t ransducers vary with temperature. We instal l addit ional components that act opposite of the transducer’s inherent temperature character ist ics. I f the transducer’s components increase output with temperature, the compensation components decrease output, and vice-versa. To select the proper compensation components, we conduct a thorough test. We vary the pressure and temperature whi le measuring the output. We use this data to select components that best compensate for temperature in each specif ic load cel l .

WHAT IS THE TEMPERATURE COMPENSATION RANGE?

The temperature compensation range is the temperature range in which our sensor can be operated up to ful l scale whi le st i l l maintaining our accuracy specif icat ions. This is di f ferent than the temperature operat ing range, which is the temperature range in which the sensor may be operated safely but the accuracy specif icat ions may not be met.

WHAT IS THE TEMPERATURE OPERATING RANGE?

The temperature operat ing range is di f ferent than the temperature compensation range, and i t is cr i t ical to understand the di f ference between the two. The temperature operat ion range is the range of temperatures in which our sensor can be safely operated up to ful l scale (accuracy specif icat ions MAY NOT be met). The temperature compensation range is the range in which our sensor can be operated up to ful l scale whi le st i l l meeting our accuracy specif icat ions.

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HOW DO I P ICK THE RIGHT FULL SCALE OUTPUT FOR A LOAD CELL?

Several factors should be considered when select ing the r ight ful l scale output for a load cel l . What is the maximum transient load your system wil l see? Transients may degrade sensor performance i f they are too extreme. To evaluate i f your transient is acceptable, compare i t to the safe overload rat ing. The safe overload rat ing of the transducer is the maximum load that can be appl ied without permanently degrading the sensor’s character ist ics. How often wi l l the load in your system cycle? Transducers, l ike al l mechanical components, wear over the course of many cycles. This wear can change your transducer character ist ics. One way to extend your transducer’s l i fe over many cycles is to pick a larger range load cel l .

Dynamic loading can be a very signif icant factor when considering the load range to choose for your appl icat ion. See Dynamic loading of load cel ls.

WHAT IS THE EFFECT OF DYNAMIC LOADS ON A LOAD CELL?

Dynamic loads on a load cel l can have a dramatic effect on your load cel l and in many cases destroy your load cel l without you being aware of i t . Dynamic loads are fast act ing loads that require high frequency signal condit ioning to detect i t because often they happen so fast. I f the system is designed correct ly unknown dynamic loads are not a problem. Dynamic loads often occur during instal lat ion when the cel l is at i ts most venerable. Consider a 100lb load cel l s i t t ing on the f loor. I f a 3/8inch bal l bearing weighing less than 2 oz’s is dropped from a height of 18 inches the load cel l experiences a 161 lb load. This innocuous effect permanently damages the load cel l . Any signal condit ioning attached to the load cel l would have to have an update rate of 300 updates per second to detect this high-speed event. Thus a dropped wrench on the load cel l or the dropping of the load cel l on the f loor from just a few inches can damage the cel l . Damage is dramatical ly reduced when the forces appl ied on the load cel l are not generated by hard incompressible surfaces. I f the f loor is wood or hammer blow was inf l icted by a brass head instead of steel, then forces generated and damage done is considerably less.

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HOW DOES A LOAD CELL WORK?

At the heart of a load cel l is a strain gage. A strain gage is a device that changes resistance when i t is deformed or stressed. The precise posit ioning of the gage, the mounting procedure, and the materials used al l have a measurable effect on overal l performance of the load cel l . A strain gage is then cemented to the surface of a beam, diaphragm or column within a load cel l . As the surface to which the gage is attached becomes strained, the f ine wires of the strain gage wires expand or compress changing their resistance proport ional to the appl ied load. In most strain gages four gages (or sometimes eight gages are used in the making of a load cel l . Mult iple strain gages are connected to create the four legs of a Whetstone-bridge conf igurat ion. When an input voltage is appl ied to the br idge, the output becomes a voltage proport ional to the load on the cel l . The more load that is appl ied to the cel l the more the br idge becomes unbalance and the larger the output. This output can be ampli f ied and processed by signal condit ioning or data acquisi t ion equipment. In order to increase sensit iv i ty of the whetstone bridge al l the arms are act ive and the four strain gages are arranged so that two arms of the br idge is in compression whi le the other two arms are in tension.

WHAT LOAD RANGE SHOULD I CHOSE FOR A LOAD CELL?

The load range for load cel l should obviously be a l i t t le more than the maximum load that the cel l wi l l encounter during normal use. Bui l t into al l Sensotec load cel ls is a 50% overload capabi l i ty. That means that i f a cel l is rated to 100lbs that i t can endure 150 lbs before permanent damage is done to the cel l . The

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load cel l wi l l not hold specif icat ion between 100 and 150lbs. Above the overload range is a loading region where the cel l becomes progressively damaged. The further into this region, the more damage that is done, unt i l the cel l eventual ly breaks. Cel ls that have been loaded beyond 150% are not always damaged so that they cannot be used but they wi l l permanently have an electr ical offset due to the ‘set ’ that the load cel l has now taken. This offset can be easi ly zeroed out by the load cel l electronics. However because of this permanent damage the cel l wi l l have a lower future overload capabi l i ty dependent on how much progressive damage was sustained. I t is also important to take into considerat ion the dynamic loading on a load cel l when specifying the loading range of a load cel l . Dynamic loading can be catastrophic to a load cel l . In this demonstrat ion a 3/8 inch bal l bearing weighing 2 ozs is dropped from 1ft onto a 100 load cel l . The load cel l experiences 140 lbs of load. I f the bal l bearing were to have been dropped from any higher the cel l would have suffered irreparable damage.

WHAT THINGS DO I NEED TO CONSIDER WHEN MOUNTING A LOAD CELL?

A load cel l wi l l only perform to i ts specif icat ion i f i t is mounted correct ly. Many t imes the load cel l is blamed for poor measurement performance when in fact the mounting arrangement is the source of the inaccuracy. In order of for the load cel l to perform correct ly the strain gauged element must be uniformly stressed when under load. In order for this to happen the fol lowing condit ions should be met: Flat surface The mounting surface for the load cel l should be f lat , preferably a ground surface. This ensures that the load cel l is strained evenly and that high spots do not induce an uneven loading on the load cel l and therefore uneven stress levels. Hard, r igid surface The mounting surface for the load cel l should be hard and r igid, This ensures that the surface does not distort or bend and twist under loading condit ions. The loading stresses on the surface can be very severe part icular ly when miniature load cel ls are being used. Loads of 40,000 lbs per square inch are not uncommon. Once again i f the surface is hard and r igid i t wi l l not distort and this ensures that the load cel l is strained evenly and therefore experiences even stress levels. Level surface The surface should be level so that al l the load is appl ied paral lel to the main axis of the load cel l . This ensures no cosine error is induced but also ensures that side loading does not create problems for the load cel l , which wi l l only have some degree of immunity. Loading appl ied paral lel to the main axis. To ensure that the load cel l only sees loads that are paral lel to the main axis. In tension load cel ls rod end bearings can ensure that the load is always

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appl ied paral lel to the main axis and in compression load cel ls load buttons ful f i l l a simi lar funct ion.

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PRESSURE SENSORS

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HOW DOES A BONDED FOIL STRAIN GAGE-BASED PRESSURE SENSOR WORK?

A bonded foil strain gage-based pressure sensor measures an applied pressure in one of two ways. In some models, such as miniature pressure sensors, foil strain gages are bonded to a diaphragm

Gauged diaphragm on a miniature pressure sensor

But in other models, foil strain gages are bonded to an element that is mechanically connected to a diaphragm, such as by a bolt the strain gages are strained as applied pressure is transmitted from the diaphragm to the gauged element.

Gauged element and mechanical transmitter

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HOW DOES A SIL ICON-BASED PRESSURE SENSOR WORK?

A silicon-based pressure sensor measures an applied pressure by detecting the effect of the pressure against a silicon substrate. Strain gages in a silicon-based pressure sensor are not bonded to, but etched onto a silicon substrate. Notice that the silicon substrate is not connected to the diaphragm via any mechanical transmitter; rather, a transmission fluid (oil) is used to transmit the pressure from the diaphragm to the silicon substrate.

Silicon based pressure sensor

As an applied pressure pushes against the diaphragm, the transmission fluid presses against the silicon substrate. As the silicon substrate experiences deformation, the strain gages, which are etched onto the silicon substrate, register this change. As with bonded foil strain gage-based pressure sensors, strain gages in a silicon-based pressure sensor are arranged in a whetstone bridge circuit formation. The bridge circuit detects any change in electrical resistance, which occurs once a strain gage has been bent due to deformation of the silicon substrate, as a deflection from the initial zero voltage reading. The deflection means the applied pressure to the sensor has changed. Sensotec Note: Sensotec uses bonded foil strain gages, bonded semiconductor strain gages, gauged silicon substrates, and sputtered technologies in its pressure sensors depending on which is most suitable for the application.

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WHAT ARE ADVANTAGES BETWEEN BONDED FOIL STRAIN GAGE-BASED AND SIL ICON-BASED PRESSURE SENSORS?

Bonded foil strain gauge based pressure sensors are excellent choices for high pressure and high temperature applications. Strain gage based sensors are also very flexible in the number of designs they can be used for. This makes them ideal for short run specials or unique, application specific solutions. Silicon-based pressure sensors offer a higher frequency response with a higher overload capability. Silicon-based pressure sensors are often used in low pressure applications due to their higher sensitivity. Sensotec Note Sensotec-specific pressure sensor information is summarized in the following chart.

Si l icon vs. bonded foi l sensor comparison chart

WHAT IS A GAGE PRESSURE SENSOR?

A gage pressure sensor is a bonded foil strain gage-based pressure sensor designed to measure applied pressure referenced to sealed atmospheric pressure. The atmospheric pressure in a gage pressure sensor, which is sealed to prevent moisture and other air particles from entering the sensor, always reflects the atmospheric pressure on the day of manufacture, as opposed to the current or other date.

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Gage Pressure Sensor

A gage pressure sensor operates in the same way as a bonded foil strain gage-based pressure sensor. As pressure is applied to a diaphragm, pressure is transmitted to a gauged element via a mechanical connection, such as a bolt. As the gauged element is deflected, strain gages, which are bonded to the element, reflect this as a change in resistance. The bridge circuit identif ies the resulting change in electrical resistance as a change in applied pressure to the sensor. (Consult the Pressure Reference Chart to compare how the various pressure sensors reference pressure.)

WHAT IS A TRUE GAGE PRESSURE SENSOR?

A True gage pressure sensor is a bonded foil strain gage-based pressure sensor designed to measure applied pressure referenced to current atmospheric pressure. A true gage pressure sensor employs a dual diaphragm. One side of the diaphragm has the pressure to be measured applied while to second diaphragm, required to prevent moisture and other air particles from entering the sensor is vented to reference the current atmospheric pressure.

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True Gage Pressure Sensor

A true gage pressure sensor operates in the same way as a bonded foi l strain gage-based pressure sensor. As pressure is appl ied to a diaphragm, pressure is transmitted to a gauged element via a mechanical connect ion, such as a bolt . As the gauged element is def lected, strain gages, which are bonded to the element, ref lect this as a change in resistance. The bridge circuit ident i f ies the result ing change in electr ical resistance as a change in appl ied pressure to the sensor. (Consult the Pressure Reference Chart to compare how the various pressure sensors reference pressure.) Sensotec Note Sensotec is one of only a few companies offer ing true gage pressure transducers. I t also unique in offer ing a second diaphragm that protects the sensor from corrosion and damage by venting to air with humidity and dir t .

WHAT IS AN ABSOLUTE PRESSURE SENSOR?

An absolute pressure sensor is a bonded foil strain age-based pressure sensor designed to measure applied pressure referenced to sealed vacuum, or absolute zero pressure.

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Absolute Pressure Sensor

An absolute pressure sensor operates in the same way as a bonded foil strain gage-based pressure sensor. As pressure is applied to a diaphragm, pressure is transmitted to a gauged element via a mechanical connection, such as a bolt. As the gauged element is deflected, strain gages, which are bonded to the element, reflect this as a change in resistance. The bridge circuit identif ies the resulting change in electrical resistance as a change in applied pressure to the sensor. (Consult the Pressure Reference Chart to compare how the various pressure sensors reference pressure.)

WHEN SHOULD I USE AN ABSOLUTE PRESSURE SENSOR RATHER THAN A GAGE PRESSURE SENSOR?

The primary considerat ion concerns whether the atmosphere plays a role in the appl icat ion for which the pressure sensor is being used. I f any part of the appl icat ion is exposed to the atmosphere, such as an automotive emission system or in level measurement in an open tank, a gage pressure sensor should be used. But i f the atmosphere has no effect on the appl icat ion, ei ther type of pressure sensor can be used. However, when measuring extremely high pressures, such as those found in hydraul ic appl icat ions, an absolute pressure sensor is typical ly used. (Consult the Pressure Reference Chart to compare how the various pressure sensors reference pressure.)

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WHAT IS A DIFFERENTIAL PRESSURE SENSOR?

A di f ferent ial pressure sensor designed to detect a change in pressure as the di f ference between two appl ied pressures. A di f ferent ial pressure sensor employs a dual diaphragm with each diaphragm l inked to a common central gauged element ( foi l based sensors) or si l icon substrate (si l icon based sensors). I f both appl ied pressures are equal, both diaphragms are balanced, and the reading shows zero; but i f ei ther diaphragm see a di f ferent pressure from the other this di f ferent ial pressure is transmitted to the gauged element. As the gauged element or si l icon substrate is def lected so the gages register a change in resistance. The bridge circuit ident i f ies the result ing change in electr ical resistance as a change in appl ied pressure to the sensor.

Dif ferent ial Pressure Sensor

Differential pressure sensors are commonly used to measure the rate of flow in a pipe. By monitoring the pressure drop on either side of an orifice plate, the mass flow rate of the fluid in the pipe can be determined. (Consult the Pressure Reference Chart to compare how the various pressure sensors reference pressure.)

WHAT IS A VACUUM PRESSURE SENSOR?

Vacuum can be measured in two ways. By a) a Gage Pressure Sensor measuring pressure below atmospheric pressure (i.e. referenced to atmospheric pressure) or b) by an Absolute Pressure Sensor measuring pressure greater than absolute zero but less than atmospheric pressure. For user convenience the Gage Pressure Sensor designed for vacuum applications is usually scaled to report a decrease in pressure below atmospheric pressure as an increase in positive voltage. Thus, at

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current atmospheric pressure, a vacuum pressure sensor actually reports 0 psi; but at absolute zero pressure, a vacuum pressure sensor reports the value of the current atmospheric pressure as psi of vacuum.

Pressure Reference for Vacuum Pressure Sensor

Because vacuum pressure sensors are measuring small pressure changes (max of 15 psi) the output can be scaled inches of water, inches of mercury or in psi. (Consult the Pressure Reference Chart to compare how the various pressure sensors reference pressure.) Sensotec Note: Sensotec uses the term vacuum gages to define gage pressure sensors scaled in psi of vacuum or inches of water of vacuum. Absolute gages that are scaled to 15 psi are just considered absolute gages.

WHAT IS A BAROMETRIC PRESSURE SENSOR?

A barometric pressure sensor is an absolute pressure sensor that is used to measure barometric pressure. The full scale output can be calibrated in a number of different ways but is typically 16 to 32 or 0-30 inches of mercury. (Consult the Pressure Reference Chart to compare how the various pressure sensors reference pressure.)

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Pressure Reference for Barometr ic Pressure Sensor

WHICH PRESSURE REFERENCE SHOULD I USE FOR MY APPLICATION?

It really depends on what the application is. However the best way to answer to this question is to show all of the various options so that an informed decision can be made. The Pressure Reference Chart below (See Figure 8a) shows the different points of reference against which pressure is measured by the various pressure sensors. Absolute pressure sensors reference pressure above absolute zero pressure; thus, absolute pressure sensors are capable of measuring pressure at any level. Gage pressure sensors reference pressure above a sealed atmospheric pressure; the atmospheric pressure used is the atmospheric pressure that existed on the day the sensor was sealed.

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Pressure Reference Chart

True gage pressure sensors reference pressure above the current atmospheric pressure; because air can enter and exit through a vent in the sensor, the atmospheric pressure referenced is today’s atmospheric pressure. Vacuum pressure sensors reference pressure of a vacuum below atmospheric pressure; vacuum pressure sensors are scaled to report decreasing negative pressure as increasing posit ive voltage.

WHAT IS OVERLOAD PROTECTION ON A PRESSURE SENSOR?

Overload protect ion on a pressure sensor is simi lar to overload protect ion on a load cel l . A mechanical stop is inserted to prevent the sensor’s diaphragm from def lect ing beyond i ts elast ic l imit .

Overload Protect ion as a Mechanical Stop

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Mechanical overload protect ion stops are used less often in pressure sensors than in load cel ls. Bonded foi l strain gage based pressure sensors can have mechanical overload protect ion easi ly f i t ted; si l icon-based pressure sensors, because of their minute size and unique def lect ion/deformation process, have mechanical overload protect ion f i t ted less often. Notice that al though si l icon-based pressure sensors rarely have mechanical overload protect ion that comes with a mechanical stop, they exhibi t a higher overload capacity than non-mechanical ly protected foi l strain gage based pressure sensors. Si l icon-based pressure sensors can sustain a 400% overload capacity. Bonded foi l strain gage based pressure sensors without mechanical stops can sustain only a 50% (typical) overload capacity. Thus, a 100 psi si l icon-based pressure sensor can sustain a 400 psi appl ied pressure without incurr ing damage. A 50,000 psi bonded foi l strain gage pressure sensor, on the other hand, without a physical overload protect ion stop, can sustain a 75,000 psi appl ied pressure before experiencing irreversible damage.

WHAT CONSIDERATIONS SHOULD I MAKE WHEN MOUNTING A PRESSURE SENSOR?

A Sensotec pressure sensor can be mounted to a pressure l ine or vessel in a number of di f ferent ways: Male thread, female thread, tapered thread, ‘c lean in place’ connect ion, f lush mount or through hole. The connect ion wi l l depend on the appl icat ion, the f luid of which the pressure is being measured and the pressure rat ing of that f luid. Select ion needs to be made careful ly part icular ly i f the pressures are high. Male or female threads are used extensively whi le tapered threads are used when the thread is rel ied upon to perform the seal ing. Clean in place connect ions ensure that when the f luid is not present in the system that no crevices exist in the system where f luid can be trapped and become contaminated when the next lot of f luid is passed through the system. Typical appl icat ions include the food and beverage industry and the medical industry.

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Flush mount connect ions are made when bleed off paths are not pract ical or inf luence the measurement.

HOW CAN I PROTECT AGAINST WATER HAMMER?

Water hammer is one of the most common reasons for fai lure in a pressure sensor. Water hammer is the phenomenon where a fast moving f luid is suddenly stopped by the closing of a valve. The f luid has momentum that is suddenly arrested causes the incompressible f luid to minutely stretch the vessel in which i t is constrained. A large bang is heard large pressure spike is generated. Any weak part of the system is subject to distort ion. The pressure sensor sees this pressure spike and can easi ly be and is very often damaged by i t . The effects

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of ‘water hammer’ can be dramatical ly reduced with the insert ion of a snubber in the pressure l ine just before the pressure sensor. The snubber is usual ly a mesh f i l ter or sintered material that al lows pressurized f luid through but does not al low large volumes of f luid through and therefore pressure spikes though in the event of water hammer.

WHAT IS ZERO BALANCE FOR A PRESSURE TRANSDUCER?

Zero Balance is the pressure sensor output when no pressure and proper supply voltage are appl ied. Zero balance is expressed as a percentage of ful l scale. For Gage pressure sensors, zero balance is measured at atmospheric pressure (0 PSIG). For absolute pressure sensors, i t is measured at ful l vacuum (0 PSIA). Ideal ly, the output would be exact ly zero, but i t is not due to several factors. The electr ical and mechanical components of transducers have inherent stresses and offsets during the assembly process. These offset the zero balance.

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Zero offset present during manufacture We add resistance to ensure sat isfactory zero balance for new transducers.

Zero balance resistor added to br idge network Operat ing environment and transducer history can change the zero balance. Factors include temperature, transducer age, and transducer overload. I f customers not ice a change in their zero balance, they may want to adjust their balance using ei ther external instrumentat ion or local zero adjustments. Annual test ing and cert i f icat ion can ensure precise output throughout the l i fe of the transducer.

WHAT IS ZERO BALANCE TEMPERATURE EFFECT?

Zero balance is the pressure sensor output at 0 pressure. This balance changes as temperature changes

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We can minimize the zero balance change due to temperature by insert ing compensating resistors. When a temperature changes drive the transducer output higher, the changes are also dr iving the compensation resistor to lower output.

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The zero balance offset is described as a percentage of ful l scale change per degree Fahrenheit (% F.S. / F) A 10000 PSI sensor with .002% F.S. / F wi l l have a zero shif t of .2 PSI for every degree change from the cal ibrat ion reference temperature.

WHAT IS OUTPUT SPAN TEMPERATURE EFFECT?

Output Span refers to the output between zero and ful l scale. The output span is the output value of the sensor at ful l pressure (Cal ibrat ion factor), expressed as mV/V. A sensor with a cal ibrat ion factor of 3 mV/V wi l l exhibi t 30 mV at ful l pressure i f i t is being suppl ied with 10V power.

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Output span varies with temperature, and we insert span compensating resistors to minimize this variat ion.

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The temperature effect on span is measured as a percentage change in rated output per degree of temperature change. A 10000 PSI sensor with .002 Rdg/F wi l l exhibi t .2 PSI span shif t for each degree of temperature change. Customers can adjust the zero and span of their outputs by adjust ing the zero and span adjustments on their transducer ( I f they have internal ampli f icat ion), or on the instrumentat ion that reads the transducers.

WHEN SHOULD I HAVE ZERO AND SPAN ADJUSTMENTS ON MY PRESSURE DETECTOR?

Zero and span may shif t due to temperature, repeated loading, or sensor aging. The preferred method to adjust zero and span is through the use of external instrumentat ion. This al lows users to track the changes they’ve made and revert back to previous values i f needed. Customers may order pressure detectors with zero/span adjustments direct ly on the unit . This also al lows for zero and span adjustment, but does not al low the user to track changes or revert to previous values. Once adjusted, the sensor is no longer cal ibrated as i t was from the factory. Customers wi l l also want to consider the operat ing environment of the transducer when select ing whether or not to use sensors with zero/span adjustments. I f the sensor is going to be inaccessible, then i t would be better to not have adjustments direct ly on the sensor. A signif icant advantage of having the adjustment screws is that users can get cal ibrated, precise output by use of annual test ing and cert i f icat ion, even as the transducer wears or ages in service.

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WHAT ARE THE ADVANTAGES AND DISADVANTAGES OF HAVING A SENSOR INTERNAL AMPLIF IER ON A PRESSURE DETECTOR?

Systems without internal ampli f icat ion must receive supply voltage from an external source, and must send small s ignals ( i .e. 30mV) back to the ampli fy ing source

What may start out as clean power can become degraded because of electr ical noise between the excitat ion and the sensor.

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A good sensor output may become distorted by the electr ical noise as wel l The internal ampli f iers are housed in the same unit as the sensor. This ensures the power to the sensor and the signal ampli f icat ion are accomplished inside the transducer.

This makes the system less vulnerable to electr ical noise and creates a higher signal to noise rat io. The larger output also al lows A/D converters to create a higher resolut ion output. Because the internal ampli f iers are so close to the sensor, l ine drops in excitat ion are el iminated. The ampli f ier outputs are low impedance, and internal ampli f iers don’t contr ibute not iceably to system inaccuracy.

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Internal ampli f iers may not be feasible under certain condit ions. Specif ical ly, the circuitry in the amp cannot be subjected to extreme temperatures. I f sensor placed in a locat ion inaccessible to users (hazardous environment, smal l space, long distance), zero and span adjustments may not be able to be tweaked when needed. Internal ampli f iers increase the overal l s ize of the unit , which may be concern in some appl icat ions.

WHY IS THE OUTPUT OF MY PRESSURE DETECTOR QUOTED IN MV/V?

mV/V output al lows you to el iminate much of the error due to power supply voltage change. A mV/V output impl ies that di f ferent levels of excitat ion may be provided to the transducer. The ful l scale output of the transducer varies direct ly with the excitat ion. A sensor with a cal ibrat ion factor of 3 mV/V wi l l exhibi t 30 mV at ful l pressure i f i t is being suppl ied with 10V power, but only 15 mV at ful l pressure i f i t is being suppl ied with 5 V.

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Output varies with supply voltage. I f we don’t know how much the change in supply voltage affected our output, then we can not possibly know how much our change in output was due to an actual change in pressure

Many users monitor transducer output AND power supply excitat ion. Changes in output are compared to the supply voltage to discount effects from voltage shif ts. Using the mV/V relat ionship, users can tel l how much of their output change was due to an actual change in pressure

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This approach is known as a rat io metr ic approach because i t rel ies on the rat io of voltage output to the Cal ibrat ion Factor (mV/V) to determine pressure. For example, i f we have a 3 mV/V, 100 PSI sensor: Supply Voltage Pressure Output 10 Volts 100 PSI 30 mV 5 volts 100 PSI 15 Mv

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TORQUE SENSORS

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WHAT IS TORQUE?

Torque is a measure of the forces that cause an object to rotate. This measurement is a combinat ion of two concerns: the amount of force appl ied to cause an object to turn on an axis; and the distance between the object ’s axis and the point at which the force is appl ied. Torque is the product of these two measurements -- a force and a length -- expressed in pound-feet or foot-pounds. When torque is very small , i t is expressed in ounce-inches or inch-ounces. Consider a bolt that must be t ightened. I f 20 pounds of force is appl ied to the end of a two-foot wrench, then a torque of 40 foot-pounds has been appl ied. Because torque rel ies on two interrelated measurements -- a force and a length -- a proport ional relat ionship exists when one measurement increases or decreases. For example, i f one doubles the distance between the axis of rotat ion and the point at which an unchanging force is appl ied, the torque is also doubled. Similar ly, i f the distance is halved, the torque is also halved. Torque is sometimes referred to as a moment, and the distance between an object ’s axis of rotat ion and the point at which a perpendicular force is appl ied to turn the object is referred to as the moment arm. Thus, an appl icat ion that requires a torque of 100 foot-pounds also requires a moment of 100 foot-pounds. There are two types of torque: react ion torque and rotary torque. (See What is react ion torque? and What is rotary torque?)

WHAT IS REACTION TORQUE?

Reaction torque is the force required to turn an object that is not free to rotate about an axis. For example, using a screwdriver to dr ive a screw into very hard wood or metal requires react ion torque to turn the bi t as resistance is encountered.

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WHAT IS ROTARY TORQUE?

Rotary torque is the force required to cause something that is free to rotate to do so (rotate) cont inuously. For example, a propel ler shaft or transmission shaft requires rotary torque to rotate cont inuously (360º).

Some other examples of rotat ional torque include industr ial motor dr ives and gear reducers. Rotary torque is di f ferent iated from react ion torque. (See What is react ion torque?)

HOW DO YOU MEASURE ROTARY TORQUE?

Rotary torque is the force required to cause something that is free to rotate to do so cont inuously (360º). Rotary torque can be measured by a rotary torque sensor. A rotary torque sensor, or transducer, measures rotary torque by convert ing torque into an electr ical signal. Select ing a torque sensor for your appl icat ion depends upon a number of considerat ions, such as long-term rel iabi l i ty, physical constraints, portabi l i ty, and budget. One way to measure rotary torque is to strain gage the shaft .

In this process one or more strain gages are bonded direct ly to the shaft that rotates. As the shaft deforms due to appl ied torque, so does the resistance in the bonded foi l strain gage. A Wheatstone bridge converts the resistance change into a cal ibrated output signal. Direct torque sensor measurement is general ly preferred to remote or indirect methods of calculat ing torque.

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Another way to measure rotary torque is to mount a transducer in the machine train as an in- l ine pre-gauged sensor.

Wireless torque cel ls consist of a rotary torque transformer connected in- l ine with standard industry f langes. The rotary torque transformer is coupled to the stat ionary port ion of the assembly by wireless transmission. A stat ionary loop antenna induces power into an embedded antenna on the rotat ing torque cel l . The induced power suppl ies the excitat ion voltage to the strain gages and powers the radio transmitter mounted in the torque sensor. The radio transmitter then modulates the strain signal for transmission to a stat ionary antenna. St i l l another way to measure rotary torque is to use a clamp on col lar.

A clamp-on torque cel l is a pre-cal ibrated bending beam mounted between two col lars that clamp onto the shaft . Appropriately spaced knife-edges provide an accurate, rel iable shaft torque measurement without marr ing or modifying the shaft . A clamp-on torque cel l handles shaft diameters from 3 to 32 inches, and as much as 100,000 hp.

HOW DO ROTARY TORQUE SENSORS WORK?

There are two types of rotary torque sensors. Some rotary torque sensors, such as wireless models, are non-contact ing.

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Wireless rotary torque sensors, which are based on radio telemetry, are rel iable and easy to instal l . Wireless rotary torque sensors are more expensive than contact ing torque sensors; however, because wireless rotary torque sensors are non-contact ing, they do not require support bearings or mechanical contact ing parts. As a result , maintenance is el iminated. A stat ionary antenna induces power in a loop antenna on the rotat ing shaft . The power from the rotat ing shaft antenna is condit ioned and excites the strain gages. A shaft-mounted radio transmitter then sends the measurement signal back to the stat ionary antenna. The telemetry antennas need to be somewhat f lexible for ease of mechanical instal lat ion. Receivers should also have adjustments for peak coupl ing of the antenna for maximum induced power and received signal strength. The radio antenna gap is normally less than 3/4 in. Other rotary torque sensors are contact ing. Sl ip r ings are often used in contact-type torque sensors to apply power to and retr ieve the signal from strain gages mounted on the rotat ing shaft . However, sl ip r ings are susceptible to wear. Maintaining an oi l - f ree sl ip r ing is not always easy in many industr ial appl icat ions. Sl ip r ing brushes, as wel l as the support bearings that are internal to these torque sensors, eventual ly wear out. In- l ine rotary torque transformers are ideal for measuring torque when transducers are mounted in l ine with the rotat ing shaft . These consist of a strain gage torque cel l having a cal ibrated output and induct ively coupled to the stat ionary windings on the assembly by the rotary transformer. The rotary transformer couples the strain gages for power and signal return.

The rotary transformer works on the same principle as any conventional transformer except that ei ther the pr imary or secondary coi ls rotate. The rotary transformer is simple and easy to use, and is usual ly appl ied to smaller

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machine trains. Rotary transformers have some susceptibi l i ty to noise and require bearings for support; thus, maintenance is required. The act of mounting the in- l ine transducer also changes system dynamics and can mean the torque values themselves may change.

WHEN SHOULD I CHOOSE AN IN-L INE SENSOR?

In- l ine torque cel ls maintain system dynamics by offer ing high torsional st i f fness. They are pre-cal ibrated, and most have an internal cal ibrat ion system that suppl ies a cal ibrated output signal to adjust instrument span in the absence of a known stat ic torque.

The wireless telemetry feature also el iminates support bearings and their maintenance. However, in- l ine torque systems require cutt ing the shaft or lengthening the machine train to accommodate the inserted in- l ine transducer. Thus, i f cutt ing or lengthening the shaft is not a feasible opt ion, another measuring approach must be employed. (See What are the major di f ferences between the various forms of torque measurement?)

WHEN SHOULD I CHOOSE A CLAMP ON COLLAR?

Because a clamp-on torque cel l is based on wireless telemetry, i t has the inherent advantages of a non-contact system.

A clamp-on torque cel l is immune to oi l and dir t . However, unl ike in- l ine sensors, clamp-on torque cel ls do not require cutt ing or lengthening the rotat ing shaft . The clamp-on feature also al lows the torque measuring system to be moved to other simi lar instal lat ions easi ly in less than 30 minutes. Thus, a clamp-on torque cel l is ideal when torque or horsepower monitor ing forms part

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of the f inal check-out of mult iple machines. (See What are the major di f ferences between the various forms of torque measurement?)

WHEN SHOULD I STRAIN GAGE MY SHAFT AS MY TORQUE TRANSDUCER?

When strain gages are bonded to the shaft , the shaft becomes the transducer.

The general guidel ine for strain gauging a shaft is that the appl ied torque must induce at least 150 to 175 micro-strain. The shaft must also be cal ibrated, a process that usual ly involves loading the shaft stat ical ly and tabulat ing the results. Cal ibrat ing a shaft is easy to do in small systems, but as loads and shaft s ize increase, i t becomes an onerous task. Other concerns associated with strain gauging a shaft involve select ing a locat ion for the strain gages, mounting the strain gages onto the shaft , and protect ing the strain gages from damage through the appl icat ion i tsel f . Any of these tasks can create problems for users who are inexperienced in such techniques. Therefore, outside contractors are usual ly avai lable through the torque sensor suppl iers for most appl icat ions and locat ions. (See What are the major di f ferences between the various forms of torque measurement?)

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WHAT ARE THE MAJOR DIFFERENCES BETWEEN THE VARIOUS FORMS OF TORQUE MEASUREMENT?

The fol lowing chart compares the various forms of torque measurement:

Comparison of Torque Measurement Techniques

In- l ine Sensor Clamp-on Col lar Strain Gauged Shaft

Based on wireless telemetry No support bearings el iminate maintenance Requires cutt ing the shaft or lengthening the machine train to accommodate the inserted in- l ine transducer Pre-cal ibrated

Based on wireless telemetry Immune to oi l and dir t effects Does not require cutt ing or lengthening the rotat ing shaft Can be moved to other simi lar instal lat ions easi ly in less than 30 minutes Ideal appl icat ion when torque or horsepower monitor ing forms part of the f inal check-out of mult iple machines

Strain gages are bonded direct ly to the shaft The shaft becomes the transducer Must select a locat ion for the strain gages, mount the strain gages, and protect the strain gages The shaft must be cal ibrated General ly preferred to remote or indirect methods of calculat ing torque

frequently asked questions - alliantech · pressure • load • torque • acceleration •...

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