chapter 6 sensing, conditioning and calibratingutsc.utoronto.ca/~quick/pscb01h3s/manual/ch06.pdf ·...

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Chapter 6Sensing, Conditioning and Calibrating

At the “front end” of any system for acquiring data is a sensor or transducer of some kind. In thischapter we survey a number of sensors that the student of science might encounter in alaboratory. We describe the output of the sensor, how the output is conditioned, the calibration ofthe data produced by the sensor and conditioner and the issues of consistency and accuracy.1 Weconclude the chapter with a description of how the devices come together in a larger collection,say in a weather station.

GeneralAn argument can be made for regarding a transducer as a device that stimulates a system (e.g., aloudspeaker vibrating the air) and a sensor as a device that responds to that stimulus (e.g., thehuman ear). But in practice, the difference between the two is blurred enough not to warrant thecontinued use of different terms except in special circumstances. In this chapter we shalltherefore treat the word sensor and transducer more-or-less synonymously.

BeginningsMuch of what data acquisition is about begins withthe measurement of a change in some physicalquantity. That quantity might be temperature, atmos-pheric pressure, relative humidity, wind speed anddirection, the amount of rain that has fallen in someperiod of time, magnetic field strength, light intensityand sound intensity to name a few. To detect thesechanges and to deal with them electrically, a sensorand signal conditioner are required (Figure 6-1).

sensor (IC)

very smallvoltage, current

or AC signal

conditioningcircuitryamplifieror filter

measurable voltage

Figure 6-1. A packaged “sensor” often includes a sensor ICand a signal conditioner.

The sensor itself is most often an integrated circuit(IC) that outputs a very small DC voltage, DC currentor AC signal in response to changes in the physicalquantity. This signal, being usually very small, mustoften be amplified, filtered, or otherwise conditionedbefore being applied to a measuring instrument of

practical sensitivity. Sensors are sometimes sold as apackage that includes a signal conditioning circuit.

Of course, the end result, the voltage, needs to beexpressed in the same unit as the sensed quantity in aprocess called calibration. It is often the signalconditioning circuit itself that determines the ultimatecalibration factors for the device. If the device is alinear one then the output variable y will depend onthe originating variable x in a relationship of the form

y = a0 + a1x , …[6-1]

where a0 and a1 are constants to be determined fromthe calibration. If the relationship is non-linear then amore complex function must be found using curve-fitting techniques (to be discussed in Appendix E).But far and away most sensors and signalconditioners are designed to produce a linear output.

SourcesOver the last thirty years an extensive market in sen-sors has evolved from the rising interest in weathermonitoring, mineral exploration, medicine, and theenvironmental sciences. Sensors are now purchase-able “off the shelf”. At the high end in quality are thesensors used in medicine and those approved by“official” bodies like Environment Canada and theNational Oceanic and Atmospheric Administration(NOAA). Companies like Campbell Scientific, Vaisala,

Sensing, Conditioning and Calibrating

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Kipp and Zonen, R. H. Young and others specialize inproducing research grade instruments for this market.In the middle range of cost are the sensors acceptableto organizations requiring non-critical monitoring. Atthe low end in cost are the sensors used in scienceteaching: physics, chemistry and biology. These areavailable from suppliers of general science equipmentsuch as PASCO and Vernier Software to name two.

The Role of the DataloggerIn this chapter, we shall refer from time to time to a“datalogger”. A datalogger is a dedicated system thatlogs, collects or stores data for subsequent usage. Adatalogger might be an in-lab computer to which the

sensors are connected directly, but usually is a dedica-ted black-box microcontroller placed close to thesensors in the field or in an enclosure on the roof of abuilding. The datalogger, if equipped with a tele-phone or radio modem, can be commanded to down-load data to a host computer. For example, thedatalogger in the weather station of the University ofToronto at Scarborough (UTSC), which we describe ina little detail at the end of this chapter, downloadsdata on the hour via a telephone modem to acomputer in the physics lab. This data is stored on thecomputer’s harddrive, and by means of serversoftware, made available for subsequent distributionover the internet (Chapter 7).

Sensors and ConditionersIn this section we describe a number of sensor types and their signal conditioning circuits. Thesedevices vary widely in accuracy, robustness and reliability. For guidance, we label them as ofresearch quality or the type supplied by Vernier Software for educational use.

PositionPosition and motion are arguably the most basic ofphysical quantities to be measured. They are especial-ly important in robotics, industrial applications andsecurity.

1 ActuatorIn robotics, the position of the limb that simulates thehuman arm or leg must be set or read electrically. Theangular position can be tracked with the assistance ofa potentiometer actuator (Figure 6-2a).

carbonwafer

ø

R

arm

Figure 6-2a.A potentiometer actuator. The resistance R isproportional to the angle φ.

The robot limb is attached to a pivot simulating thelimb joint and also to the wiper of a potentiometerresistor (Chapter 2). As the limb rotates, the resistance

between the wiper and one of the two end terminalsof the potentiometer changes. The resistance (whichmay be linearly or non-linearly dependent on theangle depending on the potentiometer’s construction)is then converted to a position in degrees measuredrelative to one of the end terminals. This conversioncan be done with a prepared calibration.

An actuator can be made to produce a voltage withthe circuit drawn in Figure 6-2b. If the resistors havethe same nominal value R then the range of theoutput voltage (on the right) will be 0 to one-half theapplied voltage V.

V

R

R

Figure 6-2b. A circuit for obtaining a voltage from therotation of a potentiometer.

2 Hall Effect Motion DetectorTransducers based on the Hall effect (Chapter 1) arebuilt into thousands of industrial devices to measure


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micro-displacements. Put simply, if a semiconductorcarrying an electric current is immersed in a magneticfield then a potential difference develops across theconductor in a direction perpendicular to the fieldand to the direction of current flow. The magnitude ofthe potential difference is directly proportional to themagnetic field strength and to the magnitude of thecurrent. The sign of the potential difference can aid inidentifying the type of charge carrier making up thecurrent flow.

One sensor type very popular in industry andeducation is the Honeywell SS94A1 Hall effect trans-ducer. (We describe it more fully in the section onmagnetic field below.) This device utilizes a Halleffect IC in conjunction with a magnet mounted onthe object to be moved. How this sensor is employedin displacement measurement is illustrated in Figure6-3.

Figure 6-3. A Hall effect sensor used in unipolar head-ondisplacement detection (from documentation made availableby Honeywell Corporation).

Documentation made available by Honeywell des-cribes the motion illustrated in the figure as unipolarhead-on displacement. As the magnet, on the movingobject, moves away from the detector, the magneticfield strength (and thus the Hall voltage) detecteddecreases. In general, this decrease in field strength

depends non-linearly on displacement. But if thedisplacement is sufficiently small then the depend-ence is approximately linear and a function like eq[6-1] describes the displacement to high precision. Thereare many other examples such as this one that youcan read for yourself on the Honeywell web site.

3 Ultrasonic Range FinderThe ultrasonic range finder is a device developed by

Polaroid Corporation for theautofocus camera market. It isnow to be found in most under-

graduate physics labs for the purpose of tracking theposition in real time of a moving object.2 This sensoris usable over much larger distances than is a Halleffect sensor—of the order of meters. The transducerworks like a loudspeaker producing a burst of pulsesat the ultrasonic frequency of 40 kHz. The burstmoves away from the transducer through the air atthe speed of sound and reflects from the object. Thedifference in time between a burst sent and received iscarefully measured. Since the speed of ultrasound inambient air is known, the time can be converted to adistance in meters. Subsequently, if the position ismeasured at regular clocktimes then the velocity andacceleration of the object tracked can be calculatedwith the appropriate software on the controllingcomputer. Though this device is an especiallyinteresting one, you will not likely be using it in thiscourse as it requires special processing and computercontrol.3

4 Magnetic Contact SwitchFor security reasons, it is often necessary to determineif two objects, normally together, are in fact togetheror apart. Radio Shack sells a number of contact orproximity switches that enable this kind of sensing tobe performed; they consist of two parts, a magnet partand a sensor part (Figure 6-4).

magnetsensor

Figure 6-4. A magnetic contact switch.


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The sensor part consists of two contacts that are eitherattracted together (closed) or repelled apart (opened)by the magnet. For the switch to work as intended themagnet needs to be very close to the sensor, prefer-ably touching. The resistance between the terminals ofthe sensor thus depends on whether the magnet istouching the sensor or not, and can range from amegohm or higher (open circuit) to less than an ohm(closed circuit). When the magnet and sensor aretouching, the circuit can be open or closed, dependingon the switch type. Typical characteristics of thesetwo switch types are listed in Table 6-1.

Table 6-1. Characteristics of two types of magnetic contactswitch sold by Radio Shack.

Normally RS type # Closed R Open R

Open 4900533 < 1 Ω > 1 MΩClosed 4900532 > 1 MΩ < 1 Ω

Max Ratings: 130 VDC 50 mA

Like any mechanical switch, this sensor can be inter-faced with a computer equipped to detect logicchanges. A likely circuit is drawn in Figure 6-5. Theseries resistor can have any value provided the max-imum current rating of the sensor is not exceeded.

2.2 kΩ

Open HIGH

Closed LOW5 Vmagnetic

switch

to digital input lineof DAQ card

digital gnd

Figure 6-5. A simple circuit employing a magnetic contactswitch.

This sensor was developed for use in securitysystems. The magnet can be fixed to the moulding ofa window frame and the magnet to the glass. If thewindow is opened the two parts are separated, thenormal logic state changes and an alarm can be madeto sound. Other uses of this device are in animaldetectors. An advantage of a magnetic switch overconventional mechanical switches is that it is less

susceptable to “contact bounce” (though someamount of bounce will still exist). It was intended atthe time of writing that this kind of switch would beavailable for student use in Lab #4.

ForceForce (and indirectly mass) is commonly measuredwith a strain gauge sensor in conjunction with aWheatstone bridge. This kind of circuit is at the heartof the electronic balance (Appendix A). Another kindof force sensor is used in the Vernier Software DualRange Force Sensor described next.

Vernier Software Dual Range Force SensorThe Vernier Software Dual Range Force Sensor uses

strain gauge technology to meas-ure force based on the bending ofa beam. The resistance of strain

gauges attached to both sides of the beam undergoesa change as the beam bends. The strain gauges usedin conjunction with a bridge circuit then produce achange in the output voltage. The signal conditioningcircuit ensures that this voltage change is directlyproportional to the change in force. In the VernierSoftware sensor a switch allows the user to choosebetween two ranges: ±10 N or ±50 N. (Recall that thegravitational force on a 1 kg mass is about 10 N.)Resolution and range of the sensor and calibrationdata are given below.

Resolution and Range:±10 N range: 0.01 N (or 1 g)±50 N range: 0.05 N (or 5 g)

Vernier’s Calibration Data (see below for the meaning ofthese numbers):±10 N range: a0 = 12.25 N a1 = –4.90 N/volt±50 N range: a0 = 53 N a1 = –21.0 N/voltThe signs on these numbers assume pulling forces arepositive, pushing forces negative.

A Few Words About CalibrationWhen a signal conditioning circuit is inserted bet-ween the sensor itself and the measuring instrument,calibration is necessary. Vernier Software ensures thattheir signal conditioning circuits provide a linear out-put. The company supplies the calibration data asvalues of an intercept a0 and a slope a1. This meansthat if a voltage x is output by the conditioning circuitthen the quantity desired, say y, can be calculated


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from a linear function, eq[6-1]. Our description of theVernier Software sensors below will include the con-stants a0 and a1. In addition, Vernier software hasproduced a LabVIEW sub-VI that provides thecalibration data for most of their sensors. Though youdon’t need LabVIEW to use these sensors, we explainhow to use this VI at the end of this chapter.

TemperatureMost of us measure a temperature every day with adevice like a mercury or alcohol thermometer. But athermometer made of glass is just not suited for com-puter interfacing. Devices that produce a voltage andtherefore can be used in computer inter-facing are thethermocouple, thermistor and the solid state temp-erature sensor. We describe an example of each thatyou will likely have a chance to use in this course. Anunderstanding of the thermocouple will prove usefullater when we come to describe the thermopile andpyranometer, both of which involve thermocouples intheir construction.

1 Thermocouple 4

Thermocouples have been used to measure tempera-ture for many years. An industry exists to support thehardware and software requirements of thermo-couple use. The advantages of a thermocouple overother temperature sensors are its low cost, wide temp-erature range and robustness. We have discussed thebasic principles of the thermocouple in Chapter 2. Ifyou connect a thermocouple junction to a measuringdevice (Figure 6-6) you get a small, but measureable,voltage.

To measuring instrument

J3 (+)

J2 (–)

J1

Iron

Constantan

Copper

sample

Figure 6-6. A J-type thermocouple connected directly to ameasuring instrument.

But a problem arises with this setup: the very act ofconnecting thermocouple wires to the inputs of ameasuring instrument has the effect of creating twoadditional thermoelectric junctions.

The circuit in the figure shows a J-type thermo-couple placed at a sample whose temperature isrequired. Let us suppose that the two thermocouplewires are connected to the input sockets of a measur-ing instrument and the sockets are made of copper.The circuit now contains three dissimilar metal junc-tions: J1, J2 and J3. J1, the thermocouple junction at thesample, generates a Seebeck voltage proportional tothe temperature of the sample. J2 and J3 each havetheir own Seebeck coefficient and generate their ownthermoelectric voltage proportional to the temp-erature at the instrument sockets. To determine thevoltage contribution from J1 correctly, you wouldhave to know the temperatures of junctions J2 and J3as well as the voltage-to-temperature relationships forthese junctions. You would then have to subtract thecontributions of the parasitic thermocouples at J2 andJ3 from the measured voltage. This procedure wouldbe tedious and prone to error.

Temperature Reference

A way of avoiding the need of this procedure is toprovide some kind of temperature reference to com-pensate for these unwanted parasitic thermocouples.The temperature reference traditionally chosen is anice bath (temperature 0 ˚C). In fact, the NationalInstitute of Standards and Technology (NIST) thermo-couple reference tables were created with this setup(Figure 6-7). A selection from the table for a K-typethermocouple (–100 ˚C to 1000 ˚C approximately) isreproduced in Table 6A-1 in the Addendum to thischapter.

In Figure 6-7, the measured voltage depends on thedifference in temperatures T1 and Tref. Here Tref is 0 ˚C.If the instrument lead connections are made on anisothermal block then the leads are held rigorously atthe same temperature and the voltages generated atthe two connection points are equal and opposing.The net voltage error added by these connections istherefore zero.

Finding Temperature From NIST Tables

Having measured the voltage produced by a certainthermocouple type with an ice-bath reference with anaccuracy acceptable to you (more on this below) themost obvious way of getting the temperature is by


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looking it up from a NIST lookup table. An exampleof how to do this is given in Example Problem 6-1.

J1

Metal 1

Metal 2

Copper

sample

To measuringinstrument

Ice BathTref = 0 C

IsothermalRegion

Figure 6-7. A thermocouple with an ice bath reference.

Example Problem 6-1Looking Up a Temperature from a NIST Table

To get the temperature of a sample suppose you use aK-type thermocouple with an ice-bath. Using a high-quality DMM you measure the voltage to be (0.480±0.001) mV. What do you conclude is the temperatureof the sample in ˚C?

Solution:From Table 6A-1 the entry closest to 0.480 mV is 0.477mV corresponding to a temperature of 12 ˚C. Thus weconclude that the sample temperature is 12 ˚C to anaccuracy of ±1˚C (assuming all other uncertainties arenegligible).

Of course, this kind of lookup can be done by com-puter. For an example see TCLookup.vi in the LabVIEWDemos for Chapter 6.

Finding Temperature From a NIST Function

Polynomial functions have been fitted to the standardthermocouple data, over limited ranges of tempera-ture, and the coefficients of the functions publishedfor use much like the data itself. Thus the temperaturecan be calculated directly from the polynomial func-tion. Table 6-2 lists polynomial coefficients for J- and

K-type thermocouples that have been published byNIST. Example Problem 6-2 shows how to use them.

Table 6-2. NIST Polynomial Coefficients for thepolynomial T = a0 + a1v + a2v2 … + anv n where T is in ˚Cand v is the thermocouple voltage in microvolts.

Thermocouple Type

J KRange 0 ˚C to 760 ˚C 0 ˚C to 500 ˚Ca0 0.0 0.0a1 1.978425E-2 2.508355E-2a2 -2.001204E-7 7.860106E-8a3 1.036969E-11 -2.503131E-10a4 -2.549687E-16 8.315270E-14a5 3.585153E-21 -1.228034E-17a6 -5.344285E-26 9.804036E-22a7 5.099890E-31 -4.413030E-26a8 1.057734E-30a9 -1.052755E-35

Example Problem 6-2Calculating a Temperature from a NIST Polynomial

Calculate the temperature in Example Problem 6-1from a NIST polynomial.

Solution:The temperature of the sample is known to be in therange 0 ˚C to 500 ˚C so the coefficients as listed inTable 6-2 apply. Thus we have to third order (remem-bering to write the voltage in microvolts):

T = a0 + a1v + a2v2 + a3v3

= 0 + (2.508355*10 –2)*480

+(7.860106*10 –8)*(480)2

−(2.503131*10 –10 )*(480)3…

= 12.0 ˚C,

which agrees with the value found from the lookuptable.


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For a demonstration of how to use LabVIEW tocalculate a temperature from a NIST function seeTCPolyCalc.vi in the LabVIEW demos for Chapter 6.

A Few Words About Precision

We have seen that a typical thermocouple produces avoltage, but a very small one. It should therefore beevident that if you use a thermocouple with aninstrument directly, the instrument must be capable ofmeasuring a few hundreds of microvolts with anacceptable accuracy to give a value of temperaturewith reasonable precision. To give an example, digitalmultimeters with 6-1/2 and 3-1/2 digits of precisionmight yield the following results under identicalconditions with a K-type thermocouple:

0.3914 mV (Agilent 34401A DMM)0.4 mV (RS DMM on the 200 mV range)

The Agilent DMM gives 4 digits of precision whereasthe RS DMM gives only 1! The Agilent DMM gives anacceptable measurement, but the RS DMM does not.In the case of the latter an uncertainty of ±0.1 mV (atleast) translates to an uncertainty of ±3 ˚C beforeuncertainties from any other sources are factored in.(This applies as well to modest-cost DAQ boards likethe PCI-1200 described in Appendix A and Lab #4.)Because of cost, it is impractical to use an Agilent34401A DMM with a thermocouple. This exampleillustrates the desireability of amplifying or condition-ing the signal before applying it to a measuringinstrument. We consider an example in the nextsection.

Vernier Software Thermocouple

One example of a K-type thermocouple sold with asignal conditioning circuit is theVernier Software ThermocoupleSensor (TCA-DIN). Its signal

conditioner is reproduced in Figure 6-8. This circuit isjust a non-inverting amplifier with gain (the basicprototype is shown in Chapter 2, Figure 2-47).

The two leads of the thermocouple (one junction isplaced in an ice bath) go to the Black and Red inputs.The calibration quoted by Vernier Software wascarried out with the Red input positive with respect tothe Black. Thus to measure a temperature greater than0 ˚C using Vernier’s calibration, you should place thereference junction (Black input) in the ice bath, and tomeasure a temperature less than 0 ˚C you should do

the reverse—place the probe junction (Red input) inthe ice bath. The 0.1 µF capacitor in the conditioningcircuit reduces electrical noise that might be picked upon the thermocouple wires, an important considera-tion in a noisy environment like a science lab.

+

–

TLC271

2.2 k

2.2 k

2

3

100 k

0.1 µF

1

54

10 k

6

Black

Red

1 µF

7

Figure 6-8. Conditioning circuit in the Vernier SoftwareThermocouple Sensor.

Specifications Claimed by Vernier:Range: 0 to 1400 ˚C when used with

reference junction in ice bathTypical Accuracy: ± 5 ˚C.Range: –200 to 0 ˚C when used with

probe junction in ice bath.Resolution: 0.7 ˚C (10-bit measurement)Current Req’d: 0.05 mA @ 5V DC

Vernier’s Calibration Data:a0 = –27.632 ˚C a1 = 552.632 ˚C/volt

Hardware Solution

If an ice bath is not convenient or unavailable a hard-ware solution is at hand for providing cold junctionthermocouple compensation—in the form of an IC,the Analog Devices AD595 (Figure 6-9).5 This devicegives a voltage output two orders of magnitude largerthan given by a thermocouple. It is also less suscep-table to ground loop problems and noise pickup.

This IC is both thermometer and thermocoupleamplifier. It is precalibrated by laser trimming tomatch the characteristics of a K-type thermocouple,and in principle, is useable over the full temperaturerange of the thermocouple. (Other AD59x devicesmatch the other thermocouple types.) The TC leads


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Research

are soldered directly to input pins 1 and 14 on the IC.Inside the IC, an AD590 solid state temperature sensor(to be described below) measures the ambient temp-erature and enables the appropriate compensationvoltage to be subtracted from the TC voltage toelectronically reference 0 ˚C. The output is then scaledto produce a signal level of 10 mV/˚C using thestandard calibration tables. This 14-pin IC packagerequires only a supply voltage of about +5 V and athermocouple to measure temperature to an accuracyof ±1 ˚C.

14

1

AD595

+ 5V 10 mV/˚Cchromel-alumel

Figure 6-9. The AD595 showing it connected to a thermo-couple.

An IC like the AD595 makes the task of measuring atemperature more convenient than with a barethermocouple with ice bath. You will almost certainlyhave the opportunity of using one of these devices inthis course as each workstation is supplied with one.Our campus-built design enables it to be used just asconveniently with the DAQ card as with a DMM.

Calibration Data:a0 = 0 ˚C a1 = 100 ˚C/volt

Example Problem 6-3Measuring a Temperature with an AD595 Device

An AD595 device plugged into a Radio Shack DMMreads 0.202 volts on the 2 volt range. What is thetemperature in ˚C?

Solution:According to the above calibration:

temperature(˚C) = 0.202(V)x100(˚C / V)

= 20.2 ˚C.To calculate the uncertainty in this measurement youwould use the specification sheet for the 2V range ofthe DMM given in Appendix A.

Error

The main sources of errors in using a thermocouplewith a signal conditioner are the quality of the icebath and the calibration of the signal conditioneritself. Some error can arise from the thermocouplewire. Wire error is caused by inhomogeneities in thethermocouple manufacturing process. These errorsvary widely depending on the thermocouple type andeven the gauge of wire used, but a value of about ±2˚C is typical. For the very best accuracy a thermo-couple sensor plus signal conditioner should berecalibrated whenever thermocouple wire is replaced.Care should be taken to ensure the ice bath consists ofa mixture of crushed ice and water throughout thevolume of the bath.

2 ThermistorAs was seen in Chapter 2, the resistance of a ther-

mistor changes markedly with temp-erature. In principle, a thermistor can

be very simply used: you place it in contact with theobject whose temperature you require and then youmeasure its resistance. From a calibration curve (anexample is given in Chapter 2, Figure 2-10) or a fittedfunction you interpolate (or look up) the temperature.An ice-bath reference is unnecessary.

But problems can arise. The mass of most thermis-tors is small, and the instrument or method you em-ploy to measure the resistance may cause self-heatingof the thermistor if too much current is driventhrough it. (In some cases, this can even happen witha DMM in ohmmeter mode.) Self-heating is reduced ifthe thermistor is placed in good thermal contact witha sample of some bulk, or one having a high thermalconductivity (like a fluid or metal object). To reduceself-heating a thermistor can be used with the circuitdrawn in Figure 6-10. The resistance R is chosen toensure that over temperatures of interest, the currentthrough the thermistor, RTH, does not exceed 100 µA.The voltage drop across the thermistor is then meas-ured and the thermistor’s resistance is calculated. Thefinal step is to determine the temperature from theresistance as we have already described. Thermistorsare widely used in research-quality sensors (for an


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Research

example see the HMP35C below). You will almostcertainly use a thermistor in this course (and probablythe Radio Shack type 271-110A) as we have an amplesupply of them.

A thermistor is usually characterized by its resis-tance at 25 ˚C and whether it is an NTC (NegativeTemperature Coefficient) or PTC (Positive Tempera-ture Coefficient) type. The Radio Shack 271-110Athermistor is an NTC type with a resistance of 50 kΩ±1% at 25 ˚C.

1.5 V

R

Rth

Vref

Vout

I

Figure 6-10. Using a thermistor with a simple resistance-divider circuit can minimize thermistor self-heating.6

CALTERM 66486 Multimeter Temperature ProbeThere are a number of thermistor probes on the mar-ket designed to be used with specific types of multi-meters. One that we have purchased in quantity fromsurplus is the so-called CALTERM 66486 MultimeterTemperature Probe. This probe is an NTC type in apackage resembling a soldering iron with fingerguard. Thermistors thus packaged are used conven-iently and safely and are relatively robust makingthem practical in harsh environments.

Claimed Specifications:Range: –30˚C to +150 ˚CResistance: approx 12 kΩ @25 ˚C

You will likely use this probe for practice incalibration.

3 Platinum Resistance ThermometerHigh-end temperature sensors often feature a

Platinum Resistance Thermometer Detector(PRTD). (For more information on

PRTDs see Chapter 2.) The actual sensor in research-quality PRTDs is a coil of thin platinum wire mounted

on a strain-free quartz support and sealed inside aceramic case. Other models use a thick film of platin-um on a substrate of alumina (Al 2O3). The standardplatinum thermometer is carefully trimmed to have aresistance of 100.0 Ω at 0 ˚C. The value of α, thetemperature coefficient of resistance, of a standardsample of platinum is about 0.392 Ω.˚C–1. A PRTD’stemperature vs resistance characteristic is highlylinear as is evident in Figure 6-11 (a graph of the filePRTD.dat stored in the Data folder). The calibration ofa PRTD is therefore relatively simple, requires littlesignal conditioning, and can be made very accurate.Though you will not likely use a PRTD in this course,it is worthwhile knowing of its existence.7

0 40 80 120-200

-100

0

100

Resistance (Ω)

Te

mp

˚C

Platinum Resistance Thermometer

Figure 6-11. The temperature-resistance characteristic of a

PRTD is highly linear.

4 Solid State DevicesA number of IC temperature-measuring devices onthe market are designed to output a voltage directlyproportional to temperature, thereby minimizing theamount of signal conditioning required. We focushere on one of them: the Analog Devices AD590. Wefirst describe the IC itself and then proceed todescribe a commercial sensor that features it.

AD590

The AD590 is designed as a two-terminal currentsource. Its internal structure is comprised of theequivalent of 9 transistors, though its structure willnot concern us here.8 Its temperature range is –55 ˚Cto +150 ˚C over which it is linear to ±0.3%. Thus itsrange is more limited than is a thermocouple or aPRTD, and especially at high temperatures (for exam-


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ple, it cannot be used at liquid nitrogen temperatureor at the temperature of a candle flame). The AD590requires a voltage source of anywhere between +4 Vand +30 V. Its current output is directly proportionalto absolute temperature with a proportionality con-stant of 1.0 µA/K. If this current is then made to flowthrough a precision 1 kΩ resistor (Figure 6-12) theproportionality constant in terms of voltage becomes1 mV/K.

AD 590

1 kΩ Vout = 1 mV/K

+ 4 - + 30 V

Figure 6-12. A circuit for the measurement of temperatureusing the AD590.

Recall from Chapter 2 that a current source drives aconstant current through a load regardless of changesthat might occur in the load resistance. Thus at anambient temperature of 0 ˚C (273 K) the AD590 drivesa current of 273 µA through the 1 kΩ resistor, therebyproducing a voltage across that resistor of 273 mV. Ofcourse, in a commercial sensor this voltage may beconditioned further, resulting in a calibration some-what different from this. Indeed, in the event that anoutput voltage of 1 mV/˚C is required then someform of signal conditioning will be necessary. Thealternative is to perform the conversion in software.

Vernier Software Standard Temperature Probe

The Vernier Software Standard Temperature Probe(TPA-DIN) is a general purposelaboratory temperature sensorwhich features the AD590JF

temperature sensor. The signal conditioner (Figure 6-13) produces an output voltage that is linear withtemperature over the range of –50 ˚C to +150 ˚C.

The AD590 is placed at the end of a brass tube. Thetube is covered with teflon ©FEP heat shrink tubingto protect the probe from damage in most environ-

ments encountered in teaching labs. The narrowtemperature range of this sensor limits its use to ex-periments involving heated fluids like water. Thoughthis sensor is used in the first year physics lab youwill not likely use it in this course.

+

–

LF356N

2

3

4

6

AD590input

7 47 output

1 µF 50 V

17.2 k 1%

5 V

10 µF25V

85

MAX 1044ICL 7660

3

LM385or

ECG 7080

VoltageReference

5.5 k 1%

10k 1%

17.2k 1%

24

10 µF 25V

shortingswitchonjack

10 µF 25V

Figure 6-13. Signal conditioning circuit in the VernierStandard Temperature Probe.

Specifications Claimed by Vernier:

Range: –50 to +150 ˚CAccuracy: ±0.2 ˚C after calibrationResolution: 0.073 ˚C with 10-bit measurementCurrent Req’d: 7.4 mA @5V DCResponse Time (time for a 90% change in reading):

in Water (with stirring) 8 to 10 seconds (typical)in Moving Air 100 seconds (typical)

Vernier’s Calibration Data:a0 = –53.073 ˚C a1 = 58.341 ˚C/volt

Relative HumidityRelative humidity (RH) is traditionally measuredwith a glass-enclosed instrument called a dry-bulb/wet-bulb hygrometer. But today, RH iscommonly measured electrically with an IC equippeda special built-in capacitor whose dielectric constant(and hence capacitance) is affected by RH. There are anumber of these devices on the market produced


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Research

mostly by start-up companies specializing in productsfor the environmental sciences. One of the newerlesser-expensive devices is the HY-Cal Engineering“Integrated Circuit Humidity Sensor” IH-3602-L.9 Wedescribe this IC in a little detail and then move on to acommercial sensor featuring it.

IH-3602-LThe IH-3602-L (Figure 6-14) is a CMOS device with 3active terminals. The DC output voltage Vout varieslinearly with RH levels from 0 to 100%. With propercalibration the device is accurate to within ±2%. Theoutput (measured relative to the negative powerinput terminal) is ratiometric and varies linearly from0.8 V at 0% RH to 3.65 V at 100% RH when poweredby a 5 volt regulated source. The output is fedthrough resistors R1 and R2 which form a voltagedivider that reduces the output voltage of the sensorto a range of between 35 and 158 mV. The change involtage is typically 28.5 mV/% RH. This output isthen applied to the signal conditioning circuit ormeasuring instrument.

IH-3602-L

3

4 5

R12.21 MΩ

R2100 kΩ

5 volts regulated

1.3 VVout

Figure 6-14. The Hy-Cal IH-3602-L humidity sensor and afew supporting elements.

Vernier Software Relative Humidity SensorThe Vernier Software relative humidity sensor (RH-

DIN) employs the IH-3602-L IC ina circuit essentially the same asshown in Figure 6-14. The IC is

housed inside a small black plastic box. The box notonly protects the sensor, but shields it from light. Thisis necessary since the sensor is slightly light sensitiveif the light strikes it in the right way. Holes in the boxprovide the necessary air circulation. Response timeof the unit in still air is 30 minutes. Response time isshorter if there is good air movement.

Vernier’s Calibration Data:a0 = (1.02 ± 0.02) %RHa1 = (0.0280± 0.0003) %RH/volt

Vaisala HMP35CIn the UTSC weather station, temperature and RH are

measured by a rugged, accurate two-sensor device, the HMP35C manufac-

tured by Vaisala Inc. (Figure 6-15). The sensor usesVaisala’s patented capacitive polymer H chip for RHmeasurement (described in Chapter 2) and a thermis-tor to measure temperature. This device is compatiblewith many dataloggers sold by Campbell Scientificand certainly with the CR10X datalogger, the data-logger used in the UTSC weather station. Specifica-tions for the sensor are listed below the figure.

Figure 6-15. The Vaisala Model HMP35C RH & Tempera-ture probe.

Selected Specifications:RHHumidity Range: 0.8 to 100% non-condensingOutput Signal Range: 0.008 to 1 VDCAccuracy (@20 ˚C): ±2% RH, 0 to 90% or ±3% RH,

90 to 100%.


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TemperatureRange: –36 ˚C to +50 ˚C

The documentation supplied by Campbell Scientificexplains how to calibrate the device should it provenecessary. The datalogger calculates temperatureusing the built-in thermistor in a manner much likeone would do so in the lab. The datalogger firstelectrically determines the resistance of the thermistorand then calculates the temperature from the fifth-order polynomial function appropriate for thethermistor type.

The HMP35C is designed to measure the tempera-ture of ambient air, and therefore should be kept fromexposure to direct sunlight. For this reason the devicein the UTSC weather station is mounted inside anaccessory especially designed for this purpose, a GillMulti-Plate Radiation Shield (Figure 6-16). The shieldis constructed like a series of inverted pie plates,allowing for the free movement of ambient air overthe sensor but at the same time preventing the sensorfrom being dampened by rain or heated by directexposure to the sun.

Figure 6-16. In the UTSC weather station, the RH & Temp-erature probe of Figure 6-16 is mounted inside a GillMulti-Plate Radiation Shield. The HMP45C is indicated inthis photograph though the HMP35C has the sameappearance.

Atmospheric PressureThe most obvious device to use for measuring atmos-pheric pressure is a mercury barometer. The readingyou obtain at zero altitude or sea level under averageweather conditions is 101.32 kilopascals (kPa). (For adiscussion of pressure and conversions between thevarious units see Chapter 2.) However, a mercurybarometer, like a glass thermometer, is unsuited forcomputer interfacing. As a first example we describe avery convenient family of IC chips, two commercialsensors that employ them, and then move on to thepressure sensor that forms part of the UTSC weatherstation.10

SenSym SCXxxxANC FamilyThe SenSym SCXxxxANC family of pressure sensorchips (Figure 6-17) are designed to produce an outputvoltage that varies linearly with pressure. They have amembrane that flexes as pressure changes. These sen-sors are set up for absolute pressure measurement, soone side of the membrane is a vacuum. Specialcircuitry minimizes the errors that might be caused bychanges in temperature. The “xxx” in the part numberindicates that the range of the sensor is 0-xxx poundsper square inch (psi), xxx = 015 and xxx = 100 beingthe most common. The “A” in the part numberindicates that they measure absolute pressure. Specifi-cations for the SCX15ANC and the SCX100ANC aregiven below.

Port B(not usedon SCX15ANC)

Port A

Figure 6-17. A sketch of the Sensym SCXxxxANC family ofpressure sensor ICs. Four of the six pins are active.

Internally, the pressure sensors have two ports. Onlyport A, the one nearest the pins (Figure 6-17) is used;port B is sealed off with a vacuum inside. On theVernier Software Barometer sensor and Pressuresensor (to be described below) a hole provides accessto Port A.


6-13

Specifications:

SCX15ANC:Range: 0 to 15 psi or 0 to 30.54 in. Hg. or 1.02 atm

(narrower with signal conditioning circuitry).Maximum pressure that the unit can tolerate withoutpermanent damage: 30 psi or 61 in. Hg.Sensitivity (before amplification):1.25 mV/psi or 0.614mV/in. Hg.Long term stability:± 0.1% full scale readingResponse Time:100 µs

The SCX100ANC is similar to the SCX15ANC exceptfor a wider range and greater sustainable pressure. Itis less sensitive than the SCX15ANC.

SCX100ANC:Range:0 to 100 psi, 0 to 203 in. Hg. or 0 to 6.8 atm.Maximum pressure that the unit can tolerate withoutpermanent damage: 150 psi or 10.2 atm.Sensitivity (before amplification);0.21 mV/psiLong term stability:±0.1% full scale readingResponse time:100 µs

Vernier Software Barometer SensorThe Vernier Software Barometer sensor (BAR-DIN)employs the SenSym SCX15ANC, being designed forweather studies. The signal conditioning circuit isdrawn in Figure 6-18. A fair amount of signal con-ditioning is required. In fact, the circuit involves fourop-amps of a Texas Instruments quad TLC27.

signal

10 µF

+

–6

54

7

SENSYMSCX15ANC

+

–

12

134

14

+

–

10

94

8

+

–2

34

17

100 k 100 k

100 k

100 k100 k

470 k

5 kgain

2.2 k

100 kOFFSET

100 k

100 k

1/4TLC27

1/4TLC27

1/4TLC27

100 k

1/4TLC27

3

4

5

2

1 k

+ 5 V + 5 V

LM385-2.5voltagereferencediode

A1

A2

A3

A4

Figure 6-18. Signal conditioner circuit for the Vernier Software Barometer Sensor (with Sensym SCX15ANC) and VernierSoftware Pressure Sensor (with Sensym SCX100ANC). All resistors are ±1%.

The 5V regulated voltage required by the IC isprovided by the zener diode LM385. Amplifiers A1and A3 generate an offset voltage that sets the inputvoltage to amplifier A4. A1 is a non-inverting unity-gain amplifier. A4 is in essence a difference amplifier(described in Chapter 2, Figure 2-50). The signal out-put is therefore the difference between the voltages atthe input (V10 – V9).

There are two adjustable potentiometers. To adjust

these potentiometers you need to open the box andremove the circuit board. The small round poten-tiometer is the gain control. This unit is shipped withthis potentiometer in the fully counterclockwiseposition, ensuring that the gain is at its highest value.With this setting, the signal output from the condit-ioner will change by 524 mV for each in. of Hg.pressure change.

The other (rectangular) potentiometer sets the offset


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Research

voltage. Turning it will raise or lower the voltageproduced by the circuit. When you adjust the gainpotentiometer, you often need to adjust the offsetpotentiometer to keep the output from going outsideits limits (0 to 3.4 volts).

Vernier’s Calibration Data:a0 = 24.215 in Hg a1 = 2.292 in Hg/volta0 = 0.809 atm a1 = 0.077 atm/volt

Vernier Software Pressure SensorThe Vernier Software Pressure Sensor (PS-DIN) em-ploys the SCX100ANC chip and is designed to beused at pressures much higher than atmosphere (forexample in the study of Boyle’s Law). The signalconditioning circuit is the same as drawn in Figure 6-18, except that the SCX100ANC chip is used instead ofthe SCX15ANC.

On the assembled Pressure Sensor, a plastic tuberuns from port A inside the box to a three way valveon the outside of the box (Figure 6-19). The twoopenings (or stems) on the three-way valve have asmall threaded end called a luer lock . You may attachplastic or rubber tubing to one of the ports using thesupplied adapter (already mounted on extra tubing).The adapter accepts tubing with an inside diameter of3.2 mm and commonly available 3/16-inch plastictubing can be connected to it. You can also attach the20-mL plastic syringe included with the PressureSensor to this stem. The other port (pointing upwardin the figure) of the three way valve opens to theatmosphere and can serve as a pressure release. Asyou set up your experiments, you can always returnthe pressure to atmosphere by opening this side stem.When the blue control (or “Off”) handle is alignedwith one of the stems, it closes off this stem. Note:Since the side stem also has a luer lock, it is alsopossible to connect the syringe or the adapter withtubing in this side position.

Figure 6-19. Valve assembly on the pressure sensor.

Vernier’s Calibration Data:a0 = 0.000 atm a1 = 2.203 atm/volt

a0 = 0.000 psi a1 = 32.370 psi/volt

Vaisala Model CS105 Barometric Pressure SensorIn the UTSC weather station, barometric pressure is

measured by a Vaisala Barometric Pres-sure Sensor Model CS105 (Figure 6-20).

The sensor uses Vaisala’s patented silicon capacitivesensor to measure barometric pressure over a 600 to1060 millibar range. The CS105 outputs a linear signalof 0 to 2.5 V DC allowing it to be connected directly toa Campbell Scientific datalogger. An integral circuitswitches 12 volts from the datalogger to the barom-eter only during measurement, thereby reducingpower requirements. The sensor is housed in ananodized aluminum case fitted with an intake valvefor pressure equilibration. Specifications for thedevice are as follows:

Figure 6-20. The Vaisala Model CS105 barometric pressuresensor.

Manufacturer’s Specifications:Total Accuracy: ± 0.5 mb @ +20˚C

± 2 mb @ 0˚ to 40 ˚C± 4 mb @ –20˚ to +45 ˚C± 6 mb @ –40˚ to +60 ˚C

Operating Temp: –40˚ to +60 ˚C


6-15

Light DetectionThe sun emits radiation over a wide range of wave-lengths, from the far-infrared to x-rays and beyond.Most radiation sensors have a rather limited range ofresponse (with the exception of the thermopile orpyranometer to be discussed below), so there areinfrared sensors, visible light sensors, and so on. Weshall assume here that you are familiar with the termsresponsivity and with the light intensity units footcandleand lux. If you need to review these definitions seeChapters 1 and 2.

VisibleA number of sensors are available for detecting visiblelight. We have described the construction of some ofthem in Chapter 2. We discuss here a standaloneinstrument and examples of sensors and circuits usedin education and research.

The CdS PhotoresistorAmong detectors of light, the CdS photoresistor is oneof the simplest to employ. It is usually specified as toits dark resistance, its resistance when no light isfalling on it. Its resistance can change from megohmswhen dark to hundreds of ohms under maximumillumination. It is primarily sensitive to green light,but has a response curve that closely follows that ofthe human eye. It exhibits a “memory effect” in that itmay require a second or more to return to its high-resistance state after a light source is removed.Though this slows its response time, it is verysensitive and easy to use.

The resistance of a CdS photoresistor could bemeasured directly with a DMM, much like any otherresistance, and the light intensity determined from acalibration curve (such as shown in Chapter 2, Figure2-13) or found from a lookup table. But self-heatingcan occur. A circuit that avoids the effect of self-heating is drawn in Figure 6-21. This circuit allows forcalibration in terms of current I (0 in the dark to 1/2mA in bright light) or in terms of the voltage acrossthe resistor (0 – 5 volts, approx). Knowing the currentand the voltage the resistance of the photoresistor canbe calculated.

You will likely have access to the CdS detector soldby Radio Shack (#276-1657). Its dark resistance isgreater than 100 MΩ (giving an OVLD reading on theAgilent 34401A DMM).

I

5 V

0 - 1/2 mA

10 kΩ

Figure 6-21. A simple circuit for using a photoresistor.

Extech Foot Candle/Lux MeterA number of inexpensive handheld instruments onthe market are designed to measure a single quantity:temperature, sound level, light intensity and emf toname a few. These instruments are effectively stand-alone sensors. Some of them have an RS-232 port,others an analog port that can be read by a multi-meter or a DAQ card. Some are even designed to beplugged into an existing multimeter. We describe anexample here, the Extech Foot Candle/Lux Meter.Since we have only two of these instruments in thephysics lab, you will likely be using this instrument inthis course, if at all, for the calibration of otherdevices.

The Extech Foot Candle/Lux Meter Model Number401025 (Figure 6-22) displays illumination in units ofFootcandle (Fc) or Lux over three ranges: Fc (0-200, 0-2000 and 0-5000) and Lux (0-2000, 0-20000 and 0-50000). Resolution is 0.1 Fc or 1 Lux with 5% +2daccuracy on all ranges. It has a selectable fast responsetime (1 second) and a slow response time (2 seconds).These functions are selected by two slide switches onthe panel. In addition there is a data hold switch.

The sensor is a Selenium photovoltaic cell (as des-cribed in Chapter 2) fitted with a color correctionfilter. The response of the sensor, shown in Figure 6-23, approximates that of the human eye.

The instrument is shipped with a 10-pin connectororiginally designed for driving an external RS-232datalogger (no longer supported by Extech) and ananalog output that can be read by a data acquisitiondevice (DMM). Analog output is 0.1 mV per count.Maximum output is 200 mV. The core of this device isalso sold as a plugin for the 200 mV range of amultimeter.


6-16

Figure 6-22. The Extech Foot Candle/Lux Meter. Thisinstrument is used mainly in this course for calibrationpurposes.

Figure 6-23. The response of the Extech light meter approx-imates that of the human eye.

Vernier Light SensorThe Vernier Software Light Sensor (LS-DIN) employs

a Hamamatsu S1133 siliconphotodiode. The spectral respon-se of this diode also approxim-

ates that of the human eye (Figure 6-24). A photo-diode is a current source, and so the signal condition-ing circuit drawn in Figure 6-25 is in essence acurrent-to-voltage converter (described in Chapter 2in the section on opamps and shown in Figure 2-49).Thus the sensor as a whole produces a voltageproportional to light intensity.

Figure 6-24. Spectral Response of the photodiode.

+

–CA3140AE

47

0-150000 lux

2

3

100 k 1%

signal

46

10 µF

7

4.02 k 1%

1 M 1%

0-600 lux

0-6000 lux

Vcc

GND

Figure 6-25. The signal conditioning circuit of the VernierSoftware Light Sensor is a current-to-voltage converter.

The Vernier Software Light Sensor has three switchsettings: 0-600 lux, 0-6000 lux and 0-150000 lux. Eachsetting selects a different resistor in the feedbackcircuit and therefore a different gain. As can be seen,


6-17

the response of this device and the Extech instrumentdescribed above are almost identical.

Resolution with a 10-bit instrument:0-600 lux: 0.20-6000 lux: 20-150000 lux: 50

If the voltage from the sensor reaches the 2.8-voltmaximum, you need to switch to a less sensitiverange. If the voltage is very small or 0, you need toselect a more sensitive range.

• The 0-600 lux range is selected when the switch isin the middle position. This is the most sensitiverange, and is useful for low levels of illumination.The voltage from the sensor will change 1volt/200 lux.

• The 0-6000 lux range is selected when the switchis in the up position. This is a good generalpurpose range for indoor light levels. The voltagechanges 1 volt/2219 lux.

• The 0-150000 lux range is selected when theswitch is in the down position. This is usedmainly for measurements in sunlight. The voltagechanges 1 volt/50000 lux.

Vernier’s Calibration Data:0-600 lux: a0 = 0 lux a1 = 154 lux/volt0-6000 lux: a0 = 0 lux a1 = 1692 lux/volt0-150000 lux: a0 = 0 lux a1 = 38424 lux/volt

Solar RadiationA typical weather station designed for meteorologicalstudies monitors radiation in the infrared to ultra-violet and often features a pyranometer. We have afew words to say about pyranometers in general, andespecially about the kind of pyranometer employed inthe UTSC weather station. We begin by describing thekind of pyranometer to be found in the intermediatephysics laboratory. This device is called a thermopile.

CA-2 ThermopileA thermopile is designed to detect radiation over theentire spectral range, from the ultra-violet to the infra-red, that falls through a certain field of view. Thethermopile model CA-2 manufactured by the Kippand Zonen Company of the Netherlands (Figure 6-26)is described as suitable for control (ovens) and for

demonstration purposes in schools. This particulardesign is also known as Moll’s thermopile. In the rangefrom 150 nm to 15 µm, the sensitivity of this device isroughly constant, independent of the wavelength.

Figure 6-26. The CA-2 thermopile shown in cross section.

The overall appearance of the device is a massivebrass cylinder (1) of about 80 mm in length mountedon a stand. This cylinder provides the reference objectfor thermocouples (more on this below). The diameterof the cylinder is about 34 mm.

The detector (6) placed at the rear of the inside is aseries of 16 thermocouples wired in series. These areformed of strips of constantan and manganan sold-ered together with silver. Each strip is about 0.5 mmwide and 5 µm thick. The soldering seams runvertically, the ends of the strips being soldered tothicker copper bars to keep their temperature definedand constant. The thermopile has an internalresistance of about 10 Ω and its response is approx.0.16 mV/mW. The absorber is coated with CarbonBlack to make it spectrally non-selective. The radia-tion falling on the inlet aperture strikes the blackeneddetector surface, partly directly and partly afterdeflection by the conical reflector. The measuringjunctions of the thermocouples are in thermal contactwith this surface. The reference junctions are at thetemperature of the cylindrical housing, i.e., practicallyat room temperature. If no radiation is present(strictly speaking, if all surrounding objects and thehousing of the thermopile itself are at the sametemperature), the detector surface takes on the sametemperature as the housing, so that the output of the


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Research

thermocouples is zero.If a hot object (heat emitter) is brought before the

aperture, the detector surface with the measuringjunctions heats up rapidly because of its small thermalcapacity, while the housing with its reference junc-tions still remains at room temperature. After a fewseconds the measuring surface is again in radiationequilibrium with the surroundings.

Since the thermopile is used particularly for themeasurement of very small radiant powers, the temp-erature difference which is now present between thedetector surface and the housing is very small. As anadequate approximation, this difference is thereforeproportional to the heat radiation power which ispresent. Since the thermoelectric voltage is alsoapproximately proportional to the temperature differ-ence between the measuring and reference junctions,a voltage which is proportional to the incident radiantpower appears at the output sockets (9) of thethermopile.

If, on the other hand, an object which is colder thanroom temperature is brought before the aperture ofthe thermopile, the detector surface takes on atemperature lower than that of the housing. Anoutput voltage of the opposite polarity is thereforeproduced.

Irradiance measurements are easily affected by con-vection and thermal radiation losses to the environ-ment. Therefore the detector can be shielded by aglass window (10). It is intended, however, that formost measurements this shield be removed.

Specifications:• Spectral range- without window: 0.2 – 50 µm- with window: 0.3-3 µm• Sensitivity- S1: homogeneous irradiance on front windowapprox. 20-40 µV/Wm–2

- S2: power falling through the window and aperturestop directly on the absorber approx. 0.1 µV/µW- without window: 1.10 x higher• Response time 95%: 30 sec• Field of view: 10˚• Non-linearity (50mV): 3%• Impedance: approx. 150 Ω• Irradiance: max. 2000 W/m–2.

A pyranometer is essentially a highly-sensitive ther-mopile with other features added andespecially designed to detect solar

radiation. The pyranometer in the UTSC weatherstation is Model CM11 also manufactured by the Kippand Zonen Company (Figure 6-27).11 Selected specifi-cations are listed below the drawing.

Selected Specifications:Sensitivity: 4-6 µV/W.m–2

Impedance: 700-1500 ΩOperating Temperature: –40 to + 80 ˚C

Spectral Range (50% points): 305-2800 nmExpected Signal Output

in Atmospheric Application: 0-10 mVMaximum Irradiance: 4000 W.m–2

Figure 6-27. The actual detector is mounted inside thequartz hemispheric dome on the top.

From the figure you can spot the hemispheric quartzdome in which the active element is placed and theprotective shield to which is attached a temperaturesensor (thermistor to compensate for sensortemperature—more on this below). The whole unitsits on three screw legs with a bubble level for level-ling. Kipp and Zonen describe it as well-suited for themeasurement of incoming global solar radiation (0.3to 2.8 µm spectral range), diffuse sky irradiancemeasurements, and surface reflected solar radiationmeasurement research. It is shipped with a calibrationcertificate.

The active element consists of 100 series-connectedthermocouples imprinted on a ceramic (Al2O3) sub-strate using thick-film techniques. The thermocouplesare arranged in a circle, resulting in a low azimuthalerror. The substrate has high thermal conductivity,meaning that the temperature difference across thesubstrate never exceeds 3 ˚C even at maximumirradiance. Thus there is negligible heat convection in


6-19

the inner dome and when tilting, the sensitivity of thepyranometer does not change more than 0.2% at 1400W.m–2. The temperature coefficient of the receiver isthermistor compensated; so the sensitivity remainsconstant within ±1% over at least a temperature rangeof –10 ˚C to 40 ˚C. A second, not illuminated, substrateis installed to compensate for heat flows in the receiv-ing substrate not caused by radiation. The heating andcooling of the pyranometer housing thus hardly affectthe zero of the instrument.

A number of factors affect the voltage output. Solarradiation varies significantly as one moves away fromthe equator. Season and time of day are major con-siderations but surrounding terrain elevation, man-made obstructions, and surrounding trees can alsocause large variations in locations with a small area.For these reasons a pyranometer must be mounted inthe open and on the level.

In this course, you will likely not have the oppor-tunity to examine this sensor up close (being mountedon the roof of the building as it is). But you shouldhave the chance to work with its data output as wedescribe in Chapter 7.

Magnetic FieldMagnetic field strength is not usually one of themeasurements made in the typical weather station.However, very small variations in the magnetic fieldstrength of the earth do provide information on theoccurrence of solar flares. Some observation stationsspecialize in these studies. However interesting thismay be, our description to follow will deal with themore conventional use of a magnetic field sensor,namely, to measure the magnetic field strength of theearth or in the environs of small magnetic samples,current-carrying wires and the like.

Vernier Software Magnetic Field SensorThe Vernier Software Magnetic Field Sensor (MG-

DIN) is an example of a com-mercial sensor utilizing theSS94A1 Hall effect transducer

described earlier (Figure 6-28). This IC produces avoltage directly proportional to magnetic fieldstrength.12 The sensor responds to the component ofmagnetic field perpendicular to the flat sensorcovered with black heatshrink tubing (which can beseen on close inspection). Maximum output occurswhen the white dot on the sensor points toward a

magnetic north pole. When the sensor is suppliedwith 5V, the sensor should read about 2 volts whenthe magnetic field strength is zero. This is called theoffset voltage. The signal conditioning circuit used inthe Vernier Software Sensor is reprduced in Figure 6-29.

Figure 6-28. The Honeywell SS94A1 Hall effect transducer.

+

–

TLC271CP

1 k 1%2

3

8

4

6input

10 µF

7

47 k 1%

10 k

47 k 1%

10 k 1%

200 k 1%200 x

10 x

Vcc

Vcc47 output

Figure 6-29. The signal conditioning circuit in the VernierSoftware Magnetic Field sensor. This is basically an invert-ing amplifier with two gain settings and a voltage offset.

A magnetic field will cause the voltage output fromthe sensor to increase or decrease, depending on thedirection of the field. The minimum voltage is 0 voltsand the maximum is 4 volts. If the offset voltage is setincorrectly, or if the magnetic field strength is beyondthe range of the sensor, the voltage will reach one ofthese two limits.

A switch on the box housing the sensor enables theuser to select the level of amplification. The lowamplification is used to measure relatively strongmagnetic fields around permanent magnets and


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Research

electromagnets. Each volt represents 32 gauss (3.2 x10–3 tesla). The range of the sensor is ±64 gauss or ±6.4x 10–3 tesla.

The high amplification is used mainly whenmeasuring the magnetic field of the earth and veryweak fields. It can be used for permanent magnets,too, but the sensor should be kept in one position tominimize the reading being affected by the back-ground field of the earth. It is 20 times more sensitivethan the low amplification. Each volt represents 1.6gauss (1.6 x 10–4 tesla). The range of the sensor is ±3.2gauss or ±3.2 x 10–4 tesla. If the sensor tube is heldvertically and rotated until a maximum voltage isfound, magnetic north will be perpendicular to thesensor in the direction of the green and white side.The magnetic inclination in your area can be found byholding the tube so that the white dot is facing north,and rotating the sensor end of the tube down until thevoltage reaches a maximum. The angle of the tubefrom vertical is the magnetic inclination. A selectionof specifications for the SS94A1 IC are listed below.Vernier’s calibration data follows.

Selected Specifications of the SS94A1Supply Voltage; 6.6 to 12.6 VRange: -500 to +500 gaussSensitivity: (5.0 ± 0.1) mV/gauss @ 25 ˚CVout (0 gauss @ 25 ˚C): (4.00 ± 0.04)V

Vernier’s Calibration Data:Low Amplification:

a0 = –80.625 gauss a1 = 32.25 gauss/volta0 = –8.063 millitesla a1 = 3.225 millitesla/volt

High Amplification:a0 = –3.2 gauss a1 = 1.6 gauss/volta0 = –0.320 millitesla a1 = 0.160 millitesla/volt

The magnetic field strength of the earth variesbetween about 0.3 gauss on the equator to about 0.7gauss near the poles.

Wind Speed and DirectionAny weather station worth the name is equipped for

measuring the speed and direction ofthe wind. Electromechanical devices

called an anemometer and wind vane assembly are com-monly used for this purpose. A photograph of typicalinstruments is reproduced in Figure 6-30. These aresimilar to the instruments employed in the UTSC

weather station. The R. H. Young Company is a majorsupplier of these instruments. These sensors interfacedirectly with Campbell Scientific dataloggers so nosignal conditioning is required. We discuss theanemometer and wind vane assembly separately.

Figure 6-30. The R. M Young Wind Sentry consisting ofanemometer or wind speed detector on the left and winddirection indicator on the right.

AnemometerInside the wind speed detector or anemometer is amagnetic switch that toggles as the wind cups rotate,producing an AC signal whose frequency is linearlyproportional to wind speed. Thus the anemometer isone of the few instruments in this chapter that pro-duces an AC signal as output. The frequency of thesignal is measured by a datalogger pulse countchannel, then converted to engineering units (mph,m/s, knots). The Campbell Scientific version usesshielded bearings that lower the anemometer’s thres-hold, thus enabling very low wind speeds to bedetected accurately.

Typical specifications are as follows:

Range: 0 to 50 m.s–1 (112 mph), gust survival 60m.s–1 (134 mph)

Accuracy: ±0.5 m.s–1 (1.1 mph)Threshold: 0.5 m.s–1 (1.1 mph)Transducer: stationary coil, 1350 Ω nominal

resistanceTransducer Output:AC sinewave signal induced by

rotating magnet on cup wheel shaft, 100


6-21

mV peak-to-peak at 60 rpm, 6V peak-to-peak at 3600 rpm.

Output Frequency:1 cycle per cup wheel rotation, 0.75m.s–1 per Hz.

Wind Vane AssemblyA wind vane assembly makes possible the detectionof the direction of the wind. This involves a poten-tiometer. The datalogger supplies a precisionexitation voltage to the potentiometer element; theoutput signal is an analog voltage that is directlyproportion-al to the azimuth of the wind direction (inmany respects like the potentiometer actuatordescribed at the beginning of this chapter). Typicalspecifications are as follows:

Range: 360˚ mechanical, 355˚ electrical (5˚ open)Accuracy: ± 5˚Transducer: Precision conductive plastic potentiom-

eter; 10 kΩ resistanceTransducer output:Analog DC voltage proportional

to wind direction angle with regulatedexcitation voltage supplied by thedatalogger

Mechanical devices such as these two, with their mov-ing parts, inevitably wear out with time and requirereplacing. As expected, their lifetime is closelycorrelated with cost.

Rain GaugeThe rain that falls in some period of time is commonlymeasured using a rain gauge. The typical rain gauge(Figure 6-31) collects the raindrops that fall through aknown cross-sectional area. The raindrops are chan-nelled into what is essentially a bucket that can tip.When enough water has collected, the bucket tips

over and empties.

Figure 6-31. A typical tipping-bucket rain gauge.

As the bucket tips over it triggers a signal on a logicline. The handle of the bucket is equipped with amagnet. When the bucket tips over, the magnet comesclose to a magnetic switch (like the kind describedearlier in this chapter) producing a change of logic orpulse, which is then counted. Calibration consists ofrelating the number of pulses counted per hour withthe rainfall (in inches or cm). The rain gauge used inthe UTSC weather station is a very sensitive one andcan detect about 0.005 inches of rainfall per tip (200counts per inch). You will not actually use the raingauge in this course, though you will have the oppor-tunity to handle the data produced by it (Chapter 7).

One important difference between research-gradeand hobbyist quality rain gauges lies in the quality ofconstruction, sensitivity, and immunity to erroneousreadings being generated by high winds. Thesefeatures tend to be expensive.


6-22

Using Vernier Calibration VIsIn the previous sections we described how a quality sensor is sold with a calibration certificate ora description of how to calibrate the sensor when it is thought necessary. We have seen that eachsensor sold by Vernier Software has an accompanying calibration in the form of the numbers a0and a1 to be used in a linear conversion function (eq[6-1]). In principle, these numbers are all youneed to know to convert the raw voltage produced by the signal conditioning circuit into thequantity desired: temperature in ˚C, pressure in atm. and so forth. But the calibration differs fromone sensor to another. If you are using LabVIEW, however, you may use a standard sub-VI thatwill perform these conversions for all Vernier sensors. We explain in this section how to use thisVI should you choose to do so.

VernierCal.viTo examine the sub-VI do the following…

¬ Load VernierCal.vi. To find it you will have tonavigate Physics >> PHYB01S >> LabVIEWDemos >> Lab #4 >> Vernier Stuff. Its Panel andDiagram are shown in Figures 6-32. The functionsof the control Raw voltage in and the indicatorCalibrated quantity out should be self-evident.

Figure 6-32a. The Panel of VernierCal.vi.

You can see from the Diagram that the VI uses thesub-VI Conversion.vi. Conversion.vi is a free-ware VIposted on the Vernier Software website; it is com-prised of sub-VIs which you can examine for yourselfif you wish. The key control is Vernier sensor. This is alist control that lists all of the Vernier sensors thatwere available at the time of writing (Figure 6-33). Toselect the sensor you want from this control just placethe pointer over the control, hold the mouse buttondown and select from the list. Please feel free to usethis VI in any program you write for your termproject.

Figure 6-32b. The Diagram of VernierCal.vi.

Figure 6-33. The control Vernier sensor is a special listcontrol showing all Vernier sensors.

Sensors, Circuits and Calibration

6-23

Research

The UTSC Weather Station

Some features of the UTSC weather station are sketch-ed in Figure 6-34.13 We shall describethese features in some detail to support

the activities of Chapter 7. You should be able toidentify seven sensors in the figure. There is athermometer, a barometer, a rain gauge, a humiditysensor, a wind direction indicator, a wind speedindicator and a pyranometer. The sensors, shown atthe right in the figure, connect via cables to aCampbell Scientific CR10X datalogger placed in anenclosure. The sensors are of research-quality, as wehave already described, and are compatible with thedatalogger in the sense that the datalogger is equip-ped to perform the necessary calibration or conver-

sions. The sensors and enclosure are mounted on theroof of the building with the enclosure mounted onthe supporting mast. The datalogger is connected to atelephone modem, which in turn is connected to adedicated telephone line.

The controlling computer in the physics lab, calledWeather, runs a software application also sold byCampbell Scientific. The controller commands thedatalogger to download data every hour on the hour.The controlling computer saves this data in a specialdirectory on its harddrive. In addition, the controllingcomputer runs a server software that enables thearchived data to be accessed over the internet via ftp.There are more details on the station in Chapter 7.

Satellite

Rain Gauge

HumiditySensor&OutsideThermometer

Wind Vane&

Anemometer

internetconnection modem

EnclosureFigure

7-35

Pyranometer

roof

modem

Figure 6-34. A sketch of the UTSC weather station. It consists of two main parts, a controlling computer in the physics lab

and the instruments and datalogger mounted on the roof of the building. Communication is by telephone modem.


6-24

Figure 6-35. This is a close up view of the enclosure that houses the electronics that form a part of the UTSC weather stationmounted on the roof of the building. Shown at the top of the interior of the enclosure is the CR10X datalogger. Further downcan be seen the Vaisala Model CS105 barometric pressure sensor (shown above in Figure 6-20). Not shown is the telephonemodem which provides communication with the in-lab computer.


6-25

AddendumTable 6A-1. An extract of NIST calibration data for a K-type thermocouple.14 The original NIST tables list measurements atintervals of 1 ˚C, not the 2 ˚C intervals reproduced here.

˚C 0 -2 -4 -6 -8 ˚C 0 2 4 6 8-90 -3.243 -3.306 -3.368 -3.431 -3.492 440 18.091 18.176 18.261 18.346 18.431-80 -2.92 -2.986 -3.05 -3.115 -3.179 450 18.516 18.601 18.686 18.771 18.856-70 -2.587 -2.654 -2.721 -2.788 -2.854 460 18.941 19.026 19.111 19.196 19.281

-60 -2.243 -2.312 -2.382 -2.45 -2.519 470 19.366 19.451 19.537 19.622 19.707-50 -1.889 -1.961 -2.032 -2.103 -2.173 480 19.792 19.877 19.962 20.048 20.133-40 -1.527 -1.6 -1.673 -1.745 -1.818 490 20.218 20.303 20.389 20.474 20.559-30 -1.156 -1.231 -1.305 -1.38 -1.453 500 20.644 20.73 20.815 20.9 20.985

-20 -0.778 -0.854 -0.93 -1.006 -1.081 510 21.071 21.156 21.241 21.326 21.412-10 -0.392 -0.47 -0.547 -0.624 -0.701 520 21.497 21.582 21.668 21.753 21.838

0 0 -0.079 -0.157 -0.236 -0.314 530 21.924 22.009 22.094 22.179 22.265

540 22.35 22.435 22.521 22.606 22.691˚C 0 2 4 6 8 550 22.776 22.862 22.947 23.032 23.1170 0 0.079 0.158 0.238 0.317 560 23.203 23.288 23.373 23.458 23.54410 0.397 0.477 0.557 0.637 0.718 570 23.629 23.714 23.799 23.884 23.97

20 0.798 0.879 0.96 1.041 1.122 580 24.055 24.14 24.225 24.31 24.39530 1.203 1.285 1.366 1.448 1.53 590 24.48 24.565 24.65 24.735 24.8240 1.612 1.694 1.776 1.858 1.941 600 24.905 24.99 25.075 25.16 25.24550 2.023 2.106 2.188 2.271 2.354 610 25.33 25.415 25.5 25.585 25.67

60 2.436 2.519 2.602 2.685 2.768 620 25.755 25.84 25.924 26.009 26.09470 2.851 2.934 3.017 3.1 3.184 630 26.179 26.263 26.348 26.433 26.51780 3.267 3.35 3.433 3.516 3.599 640 26.602 26.687 26.771 26.856 26.9490 3.682 3.765 3.848 3.931 4.013 650 27.025 27.109 27.194 27.278 27.363

100 4.096 4.179 4.262 4.344 4.427 660 27.447 27.531 27.616 27.7 27.784110 4.509 4.591 4.674 4.756 4.838 670 27.869 27.953 28.037 28.121 28.205120 4.92 5.002 5.084 5.165 5.247 680 28.289 28.374 28.458 28.542 28.626

130 5.328 5.41 5.491 5.572 5.653 690 28.71 28.794 28.877 28.961 29.045140 5.735 5.815 5.896 5.977 6.058 700 29.129 29.213 29.297 29.38 29.464150 6.138 6.219 6.299 6.38 6.46 710 29.548 29.631 29.715 29.798 29.882160 6.54 6.62 6.701 6.781 6.861 720 29.965 30.049 30.132 30.216 30.299

170 6.941 7.021 7.1 7.18 7.26 730 30.382 30.466 30.549 30.632 30.715180 7.34 7.42 7.5 7.579 7.659 740 30.798 30.881 30.964 31.047 31.13190 7.739 7.819 7.899 7.979 8.059 750 31.213 31.296 31.379 31.462 31.545200 8.138 8.218 8.298 8.378 8.458 760 31.628 31.71 31.793 31.876 31.958

210 8.539 8.619 8.699 8.779 8.86 770 32.041 32.124 32.206 32.289 32.371220 8.94 9.02 9.101 9.181 9.262 780 32.453 32.536 32.618 32.7 32.783230 9.343 9.423 9.504 9.585 9.666 790 32.865 32.947 33.029 33.111 33.193240 9.747 9.828 9.909 9.991 10.072 800 33.275 33.357 33.439 33.521 33.603

250 10.153 10.235 10.316 10.398 10.48 810 33.685 33.767 33.848 33.93 34.012260 10.561 10.643 10.725 10.807 10.889 820 34.093 34.175 34.257 34.338 34.42270 10.971 11.053 11.135 11.217 11.3 830 34.501 34.582 34.664 34.745 34.826

280 11.382 11.465 11.547 11.63 11.712 840 34.908 34.989 35.07 35.151 35.232290 11.795 11.877 11.96 12.043 12.126 850 35.313 35.394 35.475 35.556 35.637300 12.209 12.291 12.374 12.457 12.54 860 35.718 35.798 35.879 35.96 36.041310 12.624 12.707 12.79 12.873 12.956 870 36.121 36.202 36.282 36.363 36.443

320 13.04 13.123 13.206 13.29 13.373 880 36.524 36.604 36.685 36.765 36.845330 13.457 13.54 13.624 13.707 13.791 890 36.925 37.006 37.086 37.166 37.246340 13.874 13.958 14.042 14.126 14.209 900 37.326 37.406 37.486 37.566 37.646350 14.293 14.377 14.461 14.545 14.629 910 37.725 37.805 37.885 37.965 38.044

360 14.713 14.797 14.881 14.965 15.049 920 38.124 38.204 38.283 38.363 38.442370 15.133 15.217 15.301 15.385 15.469 930 38.522 38.601 38.68 38.76 38.839380 15.554 15.638 15.722 15.806 15.891 940 38.918 38.997 39.076 39.155 39.235390 15.975 16.059 16.144 16.228 16.313 950 39.314 39.393 39.471 39.55 39.629

400 16.397 16.482 16.566 16.651 16.735 960 39.708 39.787 39.866 39.944 40.023410 16.82 16.904 16.989 17.074 17.158 970 40.101 40.18 40.259 40.337 40.415420 17.243 17.328 17.413 17.497 17.582 980 40.494 40.572 40.651 40.729 40.807

430 17.667 17.752 17.837 17.921 18.006 990 40.885 40.963 41.042 41.12 41.198


6-26

Practice Problems

1. A Vernier Software Dual Range Force sensoroutputs a voltage of 867 mV when used on the±10N range. What is the force in N?

2. Example of temperature sensor using the AD595.

3. Example of K-type thermocouple.

4. Example of barometric pressure sensor.

EndNotes for Chapter 61 This chapter is not meant to be the last word on the subject of sensors. I am indebted to the following sources for much ofthe information I have used here: Mr. John Wheeler, chief engineer of Vernier Software, B. E. Paton, Sensors, Transducers &LabVIEW (Prentice-Hall,1998), the Campbell Scientific website: http://www.campbellsci.com, the Honeywell website:http://www.honeywell.com and others given below.2 The ultrasonic range finder marketed by Vernier Software has been used in the first year physics lab at UTSC for manyyears. The specification sheet for the unit has only recently been posted on the Vernier Software website. See TechnicalSpecifications for 6500 Series Sonar Ranging Module (Part # 615077) by Polaroid OEM Components Group. This document isposted on the PSCB01S website.3 Give here the reference from the Physics Teacher.4 The material in this section borrows heavily from Measuring Temperature with Thermocouples—a Tutorial , National Instru-ments Application Note 043.5 This information on the AD595 is adapted from Application Note AN-369 Thermocouple Signal Conditioning Using theAD594/595 on the Analog Devices website: http://www.analog.com. I was informed of thisa device from reading Paton’sbook, ref 1.6 This figure illustrates how the resistance of the thermistor can be determined by a so-called DC:DC Ratio measurementavailable on high quality DMMs like the Agilent 34401A. The ratio Vout/Vref is determined and then the resistance of thethermistor calculated from (Vout/Vref)*R.7 Vaisala makes two temperature-RH probes, the HMP35C and HMP45C. The former employs a thermistor for temperaturemeasurement, the latter (and more expensive) employs a PRTD. We have the former.8 Paton describes its internal structure in his book [Ref1].9 This device is used in the Vernier Software Universal Laboratory Interface (ULI) boards in the first year physics lab.10 For more information on Vaisala products see their website: http://www.vaisala.com.11 For more information on this company’s products, see their website: http://www.kippzonen.com. For more informationon pyranometers generally see www.geneq.com.12 This transducer type is used in the ULI boards.13 The weather station is a joint Physics and Environmental Sciences project at UTSC funded partly with donations fromMyrianne Lorincz. Thank you Myrianne ª.14 This data was taken from the NIST website: http://srdata.nist.gov/its90/main/.

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