UNITED STATES DEPARTMENT OF THE INTERIORBUREAU OF RECLAMATION

PROCEDURE FORUSBR 1045-89

CALIBRATING FORCE TRANSDUCERS (LOAD CELLS)

INTRODUCTION

This procedure is under the jurisdiction of the Geotechnical Services Branch, code D-3760, Research and Laboratory ServicesDivision, Denver Office, Denver, Colorado. The procedure is issued under the fixed designation USBR 1045. The number immediatelyfollowing the designation indicates the year of acceptance or the year of last revision.

1. Scope

1.1 This designation outlines the procedure forcalibrating force transducers (load cells) for laboratory use.Two calibration methods are described. Method A outlinesa calibration procedure using a platform scale load frameand a set of calibration masses for applying compressiveforces or banging a set of calibration masses to apply tensileforces. Method B describes a calibration procedure usinga universal testing machine. Force transducers (load cells)can be calibrated in tension or compression using eithermethod.

2. Auxiliary Tests

2.1 The universal testing machine used in thisprocedure must be calibrated in accordance with ASTME 4. The platform scale load frame used in this proceduremust be calibrated in accordance with USBR I012 priorto performing this calibration procedure.

3. Applicable Documents

3.1 USBR Procedure:USBR 1012 Calibrating Balances or Scales

3.2 ASTM Standard:E 4 Load Verification of Testing Machines

4. Summary of Method

4.1 A force transducer (load cell) is loaded in eithertension or compression using either a platform scale loadframe and mass standards (method A, compression) andhanging standard masses (method A, tension), or auniversal testing machine (method B). Forces are appliedto the force transducer in increments over its rated capacity.The electrical output of the force transducer is comparedto the known force applied at each force increment; andthe linearity, hysteresis, repeatability, and creep of the forcetransducer are determined.

5. Significance and Use

5.1 A calibrated force transducer (load cell) must beused in the laboratory to ensure reliable test results.

5.2 This calibration procedure is to be performed uponreceipt of the force transducer and annually thereafter.

6. Terminology

6.1 Definitions are in accordance with USBR 3900.

6.2 Terms not included in USBR 3900 specific to thisdesignation are:

6.2.1 Creep.-Time-dependent strain or deformation;for example, continuing strain with sustained stress (ASTMdefinition).

6.2.2 Hermedcallj sealed-Sealed and guaranteed tobe airtight.

6.2.3 UniversaL-In the text of this procedure, theterm universal implies that the device can apply eithercompression or tension loads.

6.2.4 Linearicy.-The variation of force transduceroutput values from a straight line. The measurements areobtained using a series of standard loads applied over thetotal rated capacity of the force transducer.

6.2.5 Repeacabilicy.-The maximum differencebetween force transducer output readings for repeatedloadings under identical loading and environmentalconditions.

6.2.6 Hysceresis.-The maximum difference betweenforce transducer output for the same applied load; onereading obtained by increasing the load from zero to theforce transducer rated capacity, and the other by decreasingthe load from the force transducer rated capacity to zero.

6.2.7 Raced Capacity (maximum capacity).-Themaximum axial load the force transducer is designed tomeasure within its specifications.

7. Apparatus

7.1 Force Transducer (load cell).-A transducer (fig. 1)which converts changes in force into an electrical output.

7.1.1 Components of the force transducer shouldpossess the following characteristics:

7.1.1.1 The force transducer should be hermet-ically sealed to prevent damage to the internal electricalcomponents in the event of exposure to moisture.

74

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USBR 1045

7-23s8 (8-86), TBureauof Reclamation FORCE TRANSDUCER (LOAD CELL) CALIBRATION 1 Designation USBR 1045-_89-

MANUFACTURER ExampleCALIBRATION METHOD: E] A - PLATFORM SCALERC (RATED CAPACITY) (a) 200 Ibf

VOLTAGE OUTPUT (b) *l 0.000 V

"CALIBRATION PERFORMED BY

176 2001MODEL NO. SERIAL NO.

[] S - UNIVERSAL MACHINEN FORCETRANSDUCERFAC'•OR(c)=(@)/(b) 20,000 •llbf/V [--]N/V

DATE__ CALIBRATION CHECKED BY DATE__

CALIBRATION MASS REQUIREMENTS

MASS REQUIREDFORCE INCREMENTS Ibm" N.2x (RC} = 40.0 4'0.0 i.4 x {RC) = 80.0. 80.0-6x (RC)= 120.0 120.0.8x (RC)= 160.0 160.01.0x (RC) = 200.0 200.0"IMPLIES Ibm = Ibf

ACTUAL MASS USEDIbm" N40.080.0

120.0160.0200.0

FORCE TRANSDUCER CALIBRATION: [] COMPRESSION [] TENSION

INCREASING FORCE INCREMENTS

FORCE INCREMENT

[]Ibm" [] N(1)

T/'ial No. I

80.0 Jt•

FORCE TRANSDUCERVOLTAGE OUTPUT

(V)(2)

0.001

FORCE TRANSDUCEROUTPUT (2)x(c)•] Ibf ON

(3)

O. 021.986 39.723.978 79.56

6.00(2__7.98710.000

Trial No. 2

0 0.001

40.080.0120.0160.0

1.9893.9865.9938.001

200.0 10.009

120.00159.74

200.00

DECREASING FORCE INCREMENTS

FORCE INCREMENT FORCE TRANSDUCER FORCETRANSDUCERVOLTAGE OUTPUT

(V)(5)

LINEARITY% ERROR

Trial No.I

HYSTERESIS% ERROR

OUTPUT%:,[](6)

0.005 0. 102. 000 40.003. 994 79.88

[] Ibm" [] N(4)

o4(3.0

,6oo---r 2-5 .o5 994

7. 993

119.88m159 86200. O0I0.000

O. 02 0.004 O. 08

39.78

79.72

119.86

160.02

2.000 40.004.002 80.O46.002 120.O48.006 160.12

0.040.080.0

120.0160.0

200.0200. 18 10.009 200. 18

PERCENT ERROR CALCULATIONS

REPEATABILITY%.E RROR

,oo

0.01 0.04 O,.Op

0. 14 0. 14 0.03

0.16 0.08

0.06 0.07

0.06 0.14

0.00 0.09

FORCE APPLIED [•] Ibf [] N

TRANSDUCER OUTPUTAT ZERO MINUTES

TRANSDUCER OUTPUTAT 35 MINUTE

% ERROR =•x 100 =

ACCEPT I'•REJECT []

REMARKS:

CREEP%ERROR

(a) 200.0

(b) 200.3

E;.15

[] Ibf [] N

•Ibf ON

0.03

0.1l

0.16

O. 09

O. 0.5

0.00

0.22

0.00

0.13

Triel No.2

0.010.11

0.140.070.010 09

Figure 5. - Force transducer (load cell) calibration -- example.

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ll.3.7 Carefully remove the scale beam pin; and, ifnecessary, turn the wheel of the platform scale load frameto level the scale beam.

11.3.8 The output of the signal conditioner shouldread +10.000 volts d-c. If the output is not _10.000 voltsd-c, adjust the gain control on the signal conditioner toobtain +10.000 volts d-c. Polarity of the output signaldepends on the wiring of the force transducer and thedesire of the user. Record the Voltage output (fig. 5).

11.3.9 Place the scale beam pin under the scale beamand lower the beam onto the scale beam pin by turningthe wheel of the platform scale load frame until the forcetransducer is completely unloaded. Calculate and recordthe "Force transducer factor" (fig. 5).

11.3.10 If necessary, readjust the signal conditionerso the output is equal to 0.000 volt d-c. If it is necessaryto again zero the signal conditioner, repeat subparagraphs11.3.5 through 11.3.9.

CAUTION.-Once the appropriate gain has been set do notreadjust. If it is readjusted, the calibration values are incorrectand the readings are unreliable.

11.3.11 Remove all tile mass from the scale beamand calculate calibration mass requirements as shown onfigure 5.

11.3.12 Place sufficient mass on the scale beam soa force equal to 20 percent of the rated capacity of theforce transducer is applied.

11.3.13 Lift the scale beam off of the pin by turningthe wheel and carefully remove the scale beam pin. Ifnecessary, turn the wheel of the platform scale load frameto level the scale beam. Read and record the Force incrementapplied and the Force transducer voltage output as shownin the Increasing Force Increments section of figure 5.

11.3.14 Repeat subparagraphs 11.3.12 through11.3.13 for force increments of 40, 60, 80, and 100 percentof the rated capacity of the force transducer.

11.3.15 Unload the force transducer by removingmass from the scale beam to correspond to 80, 60, 40,20, and 0 percent of the rated capacity of the forcetransducer. Level the scale beam after each decrement ofmass is removed. Read and record the Force incrementapplied and Force transducer voltage output for each forceas shown in the Decreasing Force Increments section offigure 5.

11.3.16 Repeat subparagraphs 11.3.12 through11.3.15 to obtain two sets of readings.

11.3.17 Using the "Force transducer factor," calculateand record the Force transducer output (fig. 5).

H.4 Method A -- Calibration Using Mass Standards(tension, fig. 3):

11.4.1 Hang the force transducer using appropriateadaptors.

11.4.2 Adjust the signal conditioner so the outputof the force transducer is 0.000 volt d-c.

11.4.3 Attach the hanger to the bottom of the forcetransducer using appropriate adapters. Place sufficient masson the hanger so the force transducer is loaded in tensionto 100 percent of its rated capacity. Record the force appliedas the RC (Rated capacity) (fig. 5).

11.4.4 The output of the signal conditioner shouldread +10.000 volts d-c. If the output is not +10.000 voltsd-c, adjust the gain control on the signal conditioner toobtain _10.000 volts d-c. Polarity of the output signaldepends on the wiring of the force transducer and thedesire of the user. Record the Voltage output, and calculateand record the "Force transducer factor" (fig. 5).

11.4.5 Remove the hanger and all the mass fromthe force transducer so the force transducer is completelyunloaded and calculate Calibration mass requirements asshown on figure 5.

11.4.6 If necessary, readjust the signal conditionerso the output is equal to 0.000 volt d-c. If it is necessaryto again zero the signal conditioner, repeat subparagraphs11.4.3 through 11.4.5.

CAUTION.-Once the appropriate gain has been set, do notreadjust. If it is readjusted, the calibration values are incorrectand the readings are unreliable.

11.4.7 Reattach the hanger to the force transducerand place sufficient mass on the hanger to produce tensileforces equal to 20, 40, 60, 80, and 100 percent of the ratedcapacity of the force transducer.

11.4.8 Record the Force increment applied and theForce transducer voltage output for each load increment.

11.4.9 Unload the force transducer by removing massfrom the hanger attached to the force transducer tocorrespond to 80, 60, 40, 20, and 0 percent of the ratedcapacity of the force transducer. Read and record the Forceincrement applied and the Force transducer voltage outputfor each force increment as shown in the Decreasing ForceIncrements section of figure 5.

11.4.10 Repeat subparagraphs 11.4.7 through 11.4.9to obtain two sets of readings.

11.4.11 Using the "Force transducer factor," calculateand record the Force transducer output (fig. 5).

11.5 Method B- Calibration Using a UniversalTesting Machine (compression, fig. 6a):

11.5.1' Adjust the signal conditioner so the outputof the force transducer is 0.000 volt d-c with zero forceon the force transducer.

11.5.2 Adjust all ranges on the universal testingmachine to zero.

11.5.3 Position the force transducer directly beneaththe loading plate of the testing machine; ensure that theforce transducer loading pins are in the vertical position.(For some force transducers, it may be necessary to turna nut onto each loading pin to hold the load cell vertical,level, and in compliance with the requirements for axialalinement as specified by the manufacturer.)

11.5.4 Carefully adjust the controls on the universaltesting machine until the top load plate just makes contactwith the force transducer.

11.5.5 Compute the force required for 20, 40, 60,80, and 100 percent of the rated capacity of the forcetransducer; select the appropriate load range on the testingmachine to ensure that readings are obtained in its mostsensitive range. Record the RC (Rated capacity) of theforce transducer as shown on figure 5.

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11.5.6 Slowly load the force transducer until a forceequal to 100 percent of the rated capacity of the forcetransducer has been applied to the transducer.

11.5.7 Adjust the output of the signal conditionerso it registers +10.000 volt d-c. The polarity of the signaldepends on the wiring of the transducer and the desireof the user. Record the Voltage output (fig. 5).

11.5.8 Remove the force from the force transducerby adjusting the controls on the testing machine so thetotal force on the force transducer is equal to 0.00 lbf(0.00 N). Calculate and record the "Force TransducerFactor" (fig. 5).

11.5.9 If necessary, readjust the signal conditionerso the output is equal to 0.000 volt d-c. If it is necessaryto again zero the signal conditioner, repeat subparagraphs11.5.4 through 11.5.8.

CAUTION.-Once the appropriate gain has been set do notreadjust. If it is readjusted, the calibration values are incorrectand the readings are unreliable.

11.5.10 Carefully adjust the controls on the testingmachine to produce forces equal to 20, 40, 60, 80, and100 percent of the rated capacity of the force transducer.

11.5.11 Read and record the Force incremencappliedand the Force transducer voltage output for each forceincrement.

11.5.[2 Remove the force from the force transducerto correspond to 80, 60, 40, 20, and 0 percent of the ratedcapacity of the force transducer. Read and record the Forceincrement applied and Force transducer voltage output foreach force as shown in the Decreasing Force Incrementssection of figure 5.

11.5.13 Repeat subparagraphs 11.5.10 through11.5.12 to obtain two sets of reading.

11.5.14 Using the"Force transducer factor," calculateand record the Force transduceroutput (fig. 5).

11.6 MethodB--CalibrationUsing a Universal TestingMachine (tension, fig. 6b):

11.6.1 Adjust the signal conditioner so the outputof the force transducer is 0.000 volt d-c with zero tensileforce on the force transducer.

11.6.2 Adjust all ranges on the universal testingmachine to zero.

11.6.3 Attach a ball joint coupler to each side ofthe force transducer as shown on figure 6b.

ll.6.4 Attach the bottom ball joint coupler to thetable of the testing machine.

11.6.5 Carefully adjust the controls on the testingmachine until the top load plate is low enough so thetop ball joint can be attached to it.

11.6.6 Carefully adjust the controls on the testingmachine to slowly apply a tension force on the forcetransducer equal to 100 percent of the rated capacity ofthe force transducer. Ensure that readings on the testingmachine are obtained in its most sensitive range for theforce being applied.

11.6.7 Adjust the output of the signal conditionerso it registers ___10.000 volt d-c. The polarity of the signal

depends on the wiring of the transducer and the desireof the user. Record the Voltageoutputas shown on figure 5.

11.6.8 Remove the tension force from the forcetransducer by adjusting the controls on the testing machineso the total force on the force transducer is equal to 0.00lbf (0.00 N).

11.6.9 If necessary, readjust the signal conditionerso the output is equal 0.000 volt d-c. If it is necessaryto again zero the signal conditioner, repeat subparagraphs11.6.6 through I 1.6.8.

CAUTION.-Once the appropriate gain has been set, do notreadjust. If it is readjusted, the calibration values are incorrectand the readings are unreliable.

11.6.10 Compute the force required for 20, 40, 60,80, and 100 percent of the rated capacity of the forcetransducer. Select the appropriate load range on the testingmachine to ensure that readings are obtained in its mostsensitive range for the force being applied. Record theRC (Rated capacity) of the force transducer and calculateand record the "Force transducer factor" (fig. 5).

11.6.1l Carefully adjust the controls on the testingmachine to produce tensile forces equal to 20, 40, 60, 80,and 100 percent of the rated capacity of the force transducer.

11.6.12 Read and record the Forceincrementappliedand the Force transducer voltage output for each forceincrement.

11.6.13 Unload the force transducer by carefullyadjusting the controls on the testing machine to produceforces corresponding to 80, 60, 40, 20, and 0 percent ofthe rated capacity of the force transducer.

11.6.14 Read and record the Force increment appliedand Force transducer voltage output for each forceincrement as shown in the Decreasing Force Incrementssection of figure 5.

11.6.15 Repeat subparagraphs 11.6.11 through11.6.14 to obtain two sets of readings.

11.6.16 Using the "Force transducer factor," calculateand record the Force transduceroutput (fig. 5).

11.7 Creep Test:11.7.1 Regardless of the method used to load the

force transducer, the procedure is the same for determiningcreep of the force transducer.

ll.7.2 Apply either a tension or compression forceon the force transducer equal to 100 percent of rated capacityand read and record the Force transducer output as the"Transducer output at zero minutes" (fig. 5).

11.7.3 Maintain the force on the force transducerfor a minilnum of 30 minutes, and read the Force transducervoltage output any time after 30 minutes. Record the timeand the force transducer output as "Transducer output at__ minute" (fig. 5).

11.7.4 Calculate the Creep • error (fig. 5).11.8 Calculate the linearity, hysteresis, and repeatability

of the force transducer at each force increment of 0, 20,40, 60, 80, 100 percent of rated capacity.ll.9 Evaluate the values of error obtained. If the

percent error for linearity, hysteresis, repeatability, or creep

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USBR 1045

does not fall within tolerances specified by the manufac-turer, the appropriate subparagraphs of the calibrationprocedure are to be repeated. If the force transducer stilldoes not meet specified requirements, it is rejected forlaboratory use.

12. Calculations

12.1 Calculations required for determining linearity,hysteresis, repeatability, and creep of the force transducerare as shown on figure 5.

13. Report

13.1 The report is to consist of a completed and checked"Force Transducer (Load Cell) Calibration" form (fig. 5).

13.2 All calculations are to show a checkmark.

14. BackgroundReference

14.1 "Handbook of Measurement and Control,"Handbook HB-76, copyright 1976 by Schaevitz Engineer-ing, Pennsauken, NJ, Library of Congress Catalog No. 76-24971.

81