about weighing

7/28/2019 About Weighing

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Technical contact in Americas: [email protected],

Europe: [email protected], China: [email protected],

Taiwan: [email protected]

www.weighingsolutions.com

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Document No.: 12223

Revision: 15-Jun-2011

Handbook TC0010

BLH NOBEL WEigHiNg SyStEmS

Brands of VPg Process Wehn

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Soluons for Process Wehn and Force measureen

Allen-Bradley Automation Fair St. Louis, MOPresented by Ron Burke BLH

Table o Contents

Overview .................................................................................................3

MV Simulation ........................................................................................8

mV/V Simulation .....................................................................................9

Pushbutton or PROM Calibration ........................................................ 10

Hydraulic or Mechanical Force Application ........................................ 11

Mass or Volumetric Flow Calibration .................................................. 12

Partial Applied Deadweight .................................................................. 13

Full Scale Build-up or Material Substitution ........................................ 14

Full Scale Deadweight .......................................................................... 15

Case Histories ....................................................................................... 16

Document No.: 12223


An Overview of Calibration Methods and Proceduresfor Process and Inventory Weigh Systems


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Document No.: 12223


An Overview of Calibration Methods and

Procedures for Process and Inventory Weigh Systems

Handbook TC0010

BLH Nobel Wehn Sses

Overview

Electronic weigh systems are oten used in the processindustries because they oer a non-intrusive, highlyaccurate, and reliable measurement o mass within aprocess vessel or inventory silo. Properly installed andcalibrated systems routinely achieve accuracies o 0.02%with a measurement precision, or resolution, o one partin 50,000. These perormance speciications comparevery avorably against the best level (0.5%) and lowmeasurement (0.1%) technologies.

The changing industrial climate is placing increasinglygreater emphasis on quality o product. Many o the mostprogressive manuacturers are pursuing ocial recognitiono quality systems through ISO 9000 registration and theimplementation o total quality programs within theirenterprises.

These changes are resulting in a greater awareness oweight as a preerred process variable and the importanceo properly installing, servicing, and calibrating electronicweighing systems. It is now becoming more and moreimportant to not only accurately and properly calibrate,but also to document the cal ibration.

There are several ways to calibrate an electronic weighsystem that range rom a simple electronic simulation toa multi-point applied deadload calibration. The proper

method to use is largely a unction o the required accuracy,traceability, and perhaps most importantly, the methodthat is most practical, given time, budgets, and physicalconguration o the system.

For example, calibration o a reestanding inventory silocontaining a low-cost material can be cost-eectivelycalibrated using an electronic simulation method. However,at the other extreme, a pharmaceutical process reactorwith connected piping and subjected to validation reviewby the FDA may need to be calibrated using deadweightsto ull scale capacity.

The range o calibration options available, where andhow to apply them, the expected results, and casehistories o actual results will be discussed. Whereappropriate, structural issues and system specicationswill be addressed. Finally, a chart will be developed thatsummarizes the methods, results, and applicability.

System Descriptions

A traditional system uses several load cells to ully supportthe vessel or structure being measured. The analog mVsignal rom each o the transducers is connected in parallelwithin a summing circuit that provides a single mV signaloutput corresponding to the average o the multiple loadcell signals. This averaged signal is usually connected tothe input o a weight transmitter/indicator device whereit is conditioned, digitized, scaled, and displayed and/orretransmitted (Figure 1).

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More recent technological advancements have resultedin digitally summed systems that operate somewhatdierently (Figure 2). In a digitally summed system, theindividual load cell output is digitized, thereore providing

a known or calibrated measurement o each vessel supportpoint. This digitization can take place in either the loadcell itsel, or in a separate transmitter device. The outputo each transducer is communicated on a simple local areanetwork to a display or re-transmission device where thedigital values are added or summed together.

Typical Component and System Accuracies

Strain gage technology based load cells consist o a straingage sensor mounted on a metallic structure that deormsunder load. The metallic structure is design to operate asa very linear and repeatable spring. Additional passivecomponents within the load cell sensing circuit compensate

or temperature eects over a wide range o operatingconditions. The perormance o these transducers isnormally states as a percentage o rated load, or ull scaleoutput. Typical perormance specications are:

KIS Load Beam Perormance Specifcations

Combined error 0.02% rated output

Repeatabi lity 0.01% rated output

Temperature eects 0.0008% / C

Creep 0.02% rated ouput (5 min)

Instrumentation or load cell systems typically providevery good perormance with resolution capabilities obetter than on part in 50,000 and non-repeatability o lessthan 0.01%. The highest quality devices have temperature

eects as low as 2 ppm/C. Other components o thesystem include cabling, etc., which through the use oremote sensing, have negligible eects.

The tried and true method to calculate expected systemaccuracies is to assume that all errors are random andto use the RMS method to determine the maximumprobable error:

ProbableAccuracy

(Combined Error)2 (Temp. Effects)2 (Repeatability)2

Total Number of Load Cells

This approach has been applied to weigh systems orover 30 years, and has proven to be a reasonable, butconservative, predictor o actual installed perormance.The ollowing printout is a spreadsheet programzdeveloped by BLH to automatically perorm theRMS calculation and tabulate system accuracies in severalormats (Figure 3).

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Applications ReviewProcess and inventory vessel weigh systems bear very littlein common with platorm and truck scales. Vessels tendto be supported on three or our support points and arealmost always connected into the process with pipes orconveyors. Depending on the material being processedthey can also be subjected to continuously changing loaddistributions.

Attached Piping

Piping connected to a vessel being weighed can,i improperly designed or installed, create three basic

contributors to system errors:1. Shunting o load resulting in non-repeatabilities and

non-linearity.

2. Horizontal orces resulting in load distribution changesthat appear as drit.

3. Vertical thrust orces that cause signicant changes inthe measurement under pressurized process conditions.

Well designed systems will use horizontally connectedpipes with either suiciently unsupported lengths tominimize vertical orces, or in non-pressurized system,fexible sections to allow ree vertical movement. Systemsexpected to encounter large thermal changes mayalso need to incorporate expansion loops to minimizehorizontal thrust loads. Typical load cells combined withsupport system structure defection is less than 0.25 in.Consequently, a piping design that allows or that amounto vertical movement, with minimized piping reaction, willproduce a linear and repeatable measurement (Figure 4).

The ollowing ormula (Figure 5) has been used to estimatethe negative eects o connected piping given the pipe Kactor. Vertical piping, with or without fexes or bellows-type ttings should be avoided.

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Structural ConsiderationsThree point support systems are desirable rom a start-upand calibration point o view because they tend to be sel-leveling three points dene a horizontal plane. In systemsthat use our or more points o support, each point must bemeasured independently during start-up and calibrationto make sure that the load is evenly distributed. I it is notwell distributed or reasons other than a truly unbalancedload, the system will need to be shimmed to achieve aneven distribution. A general rule o thumb is to achievebalance within 20-30% (dierence between highest andlowest reading).

Non-uniorm or excessive support delection can also

cause changes in load distribution as the vessel is loaded.It is always desirable to have uniorm support defection.Maximum overall defection should not exceed 0.5 orresult in an out o plumb condition o more than 0.5 deg.

It is also important to take into account the relationshipbetween total vessel and support defection and defectiono attached piping support. It is good practice to anchorpiping supports to the same structure that the tank is on sothat the defection will match (Figure 7).

Data Communications

Weigh systems are capable o repeatabilities o 0.01%

and measurement resolutions o 50,000 counts. (Somesystems provide up to 4 million counts.) Analog datacommunication is commonly limited to 12 bit or 4096counts o resolution in order or a host computer basedcontrol system to benet rom all o the perormance aweigh system can oer. It is necessary to use either a highresolution (16 bit) analog output, or to incorporate a digitalcommunication technique.

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Calibration MethodsBasic Procedures: Beore beginning an actual calibration,it is always necessary to inspect the vessel/weigh systemor structural, piping, or other mechanical deciencies.It is also important to independently measure the outputrom each load point to determine balance and to shim irequired. These independent measurements can be mademanually with a high resolution DVM, or automaticallywith some o the newer digitally summed instrumentationsystems.

Calibration Standards: Traceability o calibration standardsis an important issue beore beginning any calibration.Traceability is in essence an unbroken chain linking the

measurement to a recognized standard. In the case ostandards or calibrating weigh systems, the recognizedstandards organization is NIST (National Institute orStandards and Technology).

In simpliied terms, there are our levels o standardstraceability:

1. NIST Standards The physical standards that are inplace at NIST.

2. Primary Standards Deadweight liting machinesin a controlled environment and precision voltagestandards.

3. Secondary Standards Portable deadweights, masterbridges (load cell simulators), and voltage measurementstandards.

4. Working standards Transer standards such ascalibrated load cells, digital volt meters, and portableload cell calibrators.

Total system uncertainty is calculated using the RMS othe uncertainties o each standard in the traceability chain:

Summation o Uncertainties (Probable Error)

UT= (U1)

2 + (U2)2...(U

n)2

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MV Simulation

The output rom a load cell is an analog mV signalproportional to applied orce. I we assume that the loadcell calibrations are correct, either because they are new orhave recently been calibrated as a component, it is possibleto calibrate the indicator/transmitter device by applyinga known millivolt signal that would correspond to theapplied orce (Figure 8).

Applications: Uniormly loaded vesselswith minimal piping restrictions.

Accuracy: 0.25 to 1.0% depending on calibrationaccuracy o mV source and accuracy o load cell

calibration specications. Benefts: Low cost, ready availability o

mV source.

Defciencies: Does not prove mechanicalcharacteristics o entire system or calibrationo the load cells.

Equipment: mV source, DVM, load cellcalibration specications.

Procedure (Analog Summed Systems)1. Disconnect one load cell rom summing unit. Install

in its place one dummy Wheatstone bridge with thesame impedance o the load cell that was removed.

2. Connect mV source in parallel with signal leads on thedummy bridge.

3. On system indicator/transmitter, acquire a zerocalibration point.

4. With a DVM, measure the excitation voltage. Calculatea known mV span point by multiplying the excitationvoltage by the desired orce vs. mV/V span point on theload cell calibration sheet:

Live Load or

Calibrated Range

Total SystemCapacity

x x Excitation xRated Outputof Load Cells

MillivoltValue

5. Dial in an mV signal that corresponds to a knownorce span point. Acquire/adjust this span point in thesystem indicator/transmitter.

6. Reconnect the load cell and acquire a new zero point.Calibration is now complete. Check calibration byapplying a known weight.

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mV/V Simulation

Load cell outputs are normally stated as an mV/V signaloutput vs. applied load. This unit o measure simple meansthat there will be X mV o output at ull capacity or everyvolt o applied excitation. Load cell manuacturers otenoer special calibrators that electrically simulate theWheatstone bridge circuit and are adjustable in discretesegments to provide a range o mV/V outputs. Since manyload cell instrumentation systems use a ratiometric gaincircuit, the use o an mV/V calibrator is preerred over anmV signal source (Figure 9).

Applications: Uniormly loaded vessels withminimal piping restrictions.

Accuracy: 0.10 to 1% depending upon calibrationaccuracy o mV/V source and accuracy o load cellcalibration specications.

Benefts: Low cost.


Equipment: mV/V calibrator load cellcalibration specications.

Procedure: (Analog Summed Systems)1. Disconnect one load cell rom summing unit.

2. Connect mV/V simulator in its place. Set the output atzero mV/V.

3. On system indicator/transmitt er, acquire/adjust azero calibration point.

4. Calculate the required mV/V value to use or a spancalibration point using the ollowing ormulas:

a. 4 Load Cell System, 2000 lb. Capacity Each

b. 2 mV/V = 2000 lb on each load cell

c. In analog summed system, 2 mV/V = 8000 lb.

d. Calibrate or max. 4000 lb. live load

e. (4000/8000) X 2 mV/V X 4 = 4 mV/V or 4000 lb.

5. Dial in the mV/V signal that corresponds to a knownorce span point. Acquire/adjust this span point in thesystem indicator/transmitter.

6. Reconnect the load cell and acquire/adjust a new zeropoint. Calibration is now complete. Check calibrationby applying a known weight.

*An interpolation procedure may be required i the actualmV/V setting is not selectable on the calibrator.

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Pushbutton or PROM Calibration

Systems that use technologies that individually digitizeand then sum each load cell value oten are equipped withan mV/V reerence within the instrumentation device. Thisembedded reerence is used to establish a relationshipbetween the mV/V output o each load cell and individualorce. Depending upon the manuacturer o the system itmay be possible to establish this relationship by readingcalibration data rom a PROM. In the load cell, or byentering the load cell calibration data through a keypad(Figure 10).

Applications: Vessels with minimalpiping restrictions.

Accuracy: 0.05 to 0.5% depending oncalibration accuracy o mV/V reerenceand accuracy o load cell calibration data.

Benefts: Low cost, no special cal ibratorsrequired.


Equipment: Load cell calibrationdata sheets or load cells with datastored in PROM.

Procedure

1. Either automatically (PROM based) or manually enterindividual load cell cal ibration data.

2. One system indicator/transmitter, acquire adjust zero.

3. Calibration is complete. Check with known deadweight.

Special note: This method and procedure is only valid onsystems that digitize each load cell independently. Analogsummed systems with single channel instrumentationthat is equipped with mV/V reerence will have reducedaccuracies, particularly when load distributions change.

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Hydraulic or Mechanical Force Application

Pulling down, or liting up on a vessel with a knownorce will change the orce on each load cell (Figure 11).I the application o the applied orce accurately modelsthe load distribution o the weigh system, it is possibleto demonstrate the calibration reliability o the entiresystem, inclusive o the load cells, piping, and structuralinfuences. Some systems are designed rom the outsetto incorporate either individual liting lugs or hooks onthe vessel structure to apply the orce. Specialized singleand multi-point hydraulic orce calibration systems canbe abricated or purchased and equipped with load celltranser standards. Specialized instrumentation systems

are also available to apply and precisely measure the orceapplication at each load cell point simultaneously.

Application: Systems equipped with a orce applicationattachment point.

Accuracy: 0.25 to 1.0% depending on accuracy otranser standard and the ability to properly model theactual orce application and distribution.

Benefts: Less expensive than deadweight methods.Proves mechanical and electrical characteristics oentire system.

Defciencies: Requires design orethought to includeorce application provisions and specialized orce

application and measurement equipment.

Equipment: Force transer standard consisting o atleast one load cell and readout device, hydraulic ormechanical orce application equipment.

Procedure

1. Instal l the transer standard in series with the orceapplication equipment. In some cases, this will be atension type load cell installed on a hydraulic ram topull up or down on a central attachment point on thevessel. In other scenarios, individual compression typeload cells mounted on top o liting rams will be used tolit up simultaneously on each load cell support point.

2. Remote all material rom the vessel and acquire/adjusta new zero point in the system instrumentation.

3. Apply a known orce, through the liting or pullingsystem with the transer standard, and acquire/adjust aspan point in the system instrumentation. It is usuallydesirable to perorm a 5 point calibration in order tocompensate or any non linearities that occur.

4. Remove the orce applicat ion equipment, thecalibration is complete.

Special notes: I a multipoint orce application is used, it isvitally important that the orces be applied and measuredevenly and simultaneously. Uneven application will causeload shits that can signiicantly aect the calibration

accuracy.

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Mass or Volumetric Flow Calibration

Even though electronic weigh systems have accuracypotentials that exceed most fow measurement technologies,it is not uncommon to use fow measurement as a transerstandard to calibrate a weigh system (Figure 12). Thisis usually done on systems that cannot be practicallycalibrated in any other way because o mechanical/structural issues. It is possible or a volumetric lowcalibration technique to achieve good accuracy results itemperature and pressure variables are held constant andmonitored and corrected or. Thereore the equipmentrequired must include not only a premium fow meter, butalso temperature and pressure measuring devices. When

using mass low meters, it is not necessary to monitortemperature/pressure variables.

Application: Vessel congurations that prohibit the useo weights or orce application techniques.

Accuracy: 0.25 to 1% depending upon the fow meteraccuracy.

Benefts: Convenience, especially where fow metersare already instal led in the piping. Accurately modelsmechanical characteristics o the system.

Defciencies: possible accumulation o errors, turn-down errors possible.

Equipment: Volumetric fow meter with temperaturemeasurement or mass fow meter, plumbing/pipingconnections into the vessel, Timing device or fowtotalizer, density inormation o material being added.

Procedure

1. Empty vessel and acquire zero point in systeminstrumentation.

2. Initiate low o material into vessel. Stop low andtotalize at approximately 20% o capacity intervals.

Calculate mass that was added and acquire/adjust vespan points into the system instrumentation.

3. On applications using volumetric low, monitortemperature and pressure and correct the masscalculation as required. To minimize fow turndowneects, use three way valves to turn fow on and owithout fow interruption.

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Partial Applied Deadweight

Load cells are very linear devices and thereore it is otenpossible to set a calibration slope with a small portion othe actual system capacity (Figure 13). On systems withconnected piping that may aect linearity throughoutthe calibration range, it is desirable to recheck the slopeat several points to veriy linearity. Bear in mind that thesmaller the check weight is in proportion to the calibratedrange, the less precise the check o the slope will be. Forexample, in a system with a range o 50,000 lb counting byone pound in checked with a weight o, say, 500 lb, a slopeerror o 0.2% would not be detected.

Applications: Virtually any weigh system.

Accuracy: 0.5 to 2% depending upon the size odeadweight in relation to calibrated range.

Benefts: Low cost, ease o implementation, can detectlarge system non-linearities throughout span.

Defciencies: Relatively low accuracy, will not identiysmall to moderate slope errors.

Equipment: Partial capacity deadweight or transerweight (i.e. person weighed on another scale).

Procedure

1. Remove material rom vessel and acquire zero point insystem instrumentation.

2. Apply partia l deadweight and acquire/adjust a spanpoint in the system instrumentation.

3. Remove dead weight and add material to vessel.Re-apply dead weight and observe change in measured

value. It should be the same as the deadweight value, iit isnt, look or source o system non-linearity.

4. Continue to add material to vessel and check withdeadweight throughout system calibrated range.

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Full Scale Build-up or Material Substitution

While it is not always practical to apply deadweights toull capacity on many vessels, it is possible to achievethe accuracy o a ull deadweight calibration by usinga build-up method (Figure 14). This involves applying aprimary standard deadweight to the vessel, acquiring aspan point, removing the deadweight and lling the vesselto the previously acquired span point, then reapplying thedeadweight and entering a second span point, and thenrepeating the procedure until ull capacity is achieved.This method will or all intents and purposes meet theaccuracy o a ull scale deadweight test.

Applications: Virtually any vessel.

Accuracy: 0.05 to 0.2%

Benefts: High accuracy, proves structuralcharacteristics o system to ull capacity.

Defciencies: Time consuming, possible accuracyproblems i small weights are used.

Equipment: Deadweights (minimum 10% o capacity),supply o water or process material.

Procedure

1. Empty vessel and acquire adjust zero point in systeminstrumentation.

2. Apply deadweight and acquire/adjust initial span pointin system instrumentation.

3. Remove deadweight and ll vessel until the enteredspan point is reached again.

4. Apply the deadweight again and acquire/adjust asecond span point in the system instrumentation.

5. Repeat procedure until ull capacity is reached.

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Full Scale Deadweight

Probably the most accurate, or certain, way to calibratea weigh system is to use deadweights that are primaryor secondary standards traceable to NIST. On systemswhere the conguration allows or the uniorm placement,or hanging o weights, this method is highly desirable(Figure 15).

Applications: Small to medium size vessels withuniorm loading and acilities to hang or placedeadweight.

Accuracy: 0.02 to 0.1%, depending on toleranceo weights.

Benefts: Superior traceability o standards, and iweights can be uniormly applied, the method willaccurately model the mechanical characteristicso the system.

Defciencies: Weights are oten dicult to loadand unload, and placement does not always

properly model the actual loading o the vessel.Expensive and time consuming. Oten times notpossible because o vessel/structure conguration.

Equipment: Deadweights equal to ull capacityo vessel weight attachment or placementpoints on vessel.

Procedure:

1. Remove all material rom vessel and acquire/adjustzero point in system instrumentation.

2. Apply a quantity o deadload equivalent toapproximately 20% o vessel capacity. Acquire/adjust

a span point in the system instrumentation.3. Repeat procedure until ull capacity is reached.

4. Reverse removal o deadweights and check span pointsor accuracy.

5. Calibration is complete.

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Case Histories

Case History #1 mV/V Calibration oInventory Silos

Application: The application involves the calibration o verylarge, relatively reestanding inventory silos containingsel-leveling plastic pellets.

Considerations: Due to the large capacities involved andthe nature o the material being processed, it was notpossible to use deadweights or a material substitution/build-up calibration. However, since there is minimalconnected piping, there is little concern about mechanical

interaction problems and an electronic calibration methodcan be used.

Method Used: The customer chose to use an mV/Vsimulation method to perorm an electronic calibration.Alternatively, an mV source could be been used, but thecalibration uncertainty is greater. A pushbutton, or PROMtype calibration method would have also been possible ithe technology had been available when the equipment wasmanuactured. Unortunately, it wasnt.

Results: The BLH 625 calibrator has a specied accuracyo 0.02% o range. The load cell calibration data sheetsindicated that the load cells were calibrated against asecondary standard with an uncertainty that doesnt

exceed 0.05%. Interaction, or load shunting by connectedpipes was determined to be negligible. The calibrationaccuracy is thereore conservatively stated as 0.1%.

Case History #2 Combination Build-up andDeadweight Calibration o Reactor

Application: Pharmaceutical process vessel with severalhorizontal connected pipes equipped with Tefon-linedlex connections. The vessel volume and weigh systemcomponents are designed or process material with aspeciic gravity o 1.6. Demonstration o measurementaccuracy o at least 0.1% is required or FDA validation othe process. Class F cast iron deadweights o up to 50% ovessel capacity were available and the vessel was equippedwith a central hook to hang weights.

Considerations: Due to the numerous connected pipesand thereore possible load shunting problems, and the

relatively severe accuracy requirement, an electroniccalibration method would be inappropriate. I a build-upcalibration method is used, the high specic gravity othe process material and the limited volume o the vesselrequire that a high density substitute material, actualprocess material, or combination o substitute materialand a large quantity o deadweights be used.

Method: Actual process material and/or a high densitysubstitute material were not available. Consequently, acombination build-up and deadweight calibration methodwas used. Essentially, a build-up test with water was useduntil the vessel was ull, and then deadweights were appliedto bring the weigh system up to ull operating capacity.

The system instrumentation was initially calibratedelectronically using an mV/V calibrator, and then correctedat several span points i needed.

Results: The Class F cast iron deadweights have aspeciied accuracy (tolerance) o 0.01%. The displayedand transmitted weight data agreed within 0.05% o theapplied deadweight at more than 10 points throughout thespan. Consequently, it is conservative to conclude that thesystem accuracy meets the requirement o 0.1%.


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Case History #3 Mass Flow Calibrationo Mix Tank

Application: Pharmaceutical process vessel similar to thatdescribed in Case #2 but equipped with vertical pipeswith lexes in addition to the horizontal lexed pipes.The requirement is demonstrated accuracy o betterthan 1.5% or FDA validation o the process. A speciallycalibrated mass fow meter with a certied accuracy o0.4% was available.

Considerations: Due to the numerous connected pipesand thereore possible load shunting problems, and therelatively severe accuracy requirement, an electroniccalibration method would be inappropriate. Since the

accuracy requirement is not very severe, a mass lowcalibration may be an economical alternative. However, iwater is used, instead o a high density fuid, the ull systemcapacity will not be attainable.

Method: A certiied mass low calibration method wasused to cal ibrate the system to 66% o capacity. An initialelectronic calibration using an mV/V calibrator had alreadyestablished a ull scale baseline calibration. Based upon thelinear and repeatable results o the partial span calibrationcheck, it was decided that mechanical interaction and/orload shunting was not a problem and that the benet ousing deadweights to load to ull scale was not worth thecost o implementation.

Results: The weigh system span point checks were oundto agree with the mass low meter data within 1%.Combining this result with the 0.4% uncertainty o thefow meter calibration results in a combined accuracy o

better than 1.5%.


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