Campbell Scientific CR6 CR6 Measurement and Control System - Page 226
FieldCalStrain, StrainCalc
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Section 7. Installation 1. Shunt calibration does not calibrate the strain gage itself. 2. Shunt calibration does compensate for long leads and non-linearity in the resistive bridge. Long leads reduce sensitivity because of voltage drop. FieldCalStrain() uses the known value of the shunt resistor to adjust the gain (multiplier / span) to compensate. The gain adjustment (S) is incorporated by FieldCalStrain() with the manufacturer's gage factor (GF), becoming the adjusted gage factor (GFadj), which is then used as the gage factor in StrainCalc(). GF is stored in the CAL file and continues to be used in subsequent calibrations. Non-linearity of the bridge is compensated for by selecting a shunt resistor with a value that best simulates a measurement near the range of measurements to be made. Strain-gage manufacturers typically specify and supply a range of resistors available for shunt calibration. 3. Shunt calibration verifies the function of the CR6. 4. The zero function of FieldCalStrain() allows a particular strain to be set as an arbitrary zero, if desired. Zeroing is normally done after the shunt calibration. Zero and shunt options can be combined through a single CR6 program. CRBasic example FieldCalStrain() Calibration (p. 226) is provided to demonstrate use of FieldCalStrain() features. If a strain gage configured as shown in figure Quarter-Bridge Strain-Gage with RC Resistor Shunt (p. 226) is not available, strain signals can be simulated by building the simple circuit, substituting a 1000 Ω potentiometer for the strain gage. To reset calibration tests, use the support software File Control (p. 496) menu to delete .cal files, and then send the demonstration program again to the CR6. Case: A 1000 Ω strain gage is placed into a resistive bridge at position R1. The resulting circuit is a quarter-bridge strain gage with alternate shunt-resistor (Rc) positions shown. Gage specifications indicate that the gage factor is 2.0 and that with a 249 kΩ shunt, measurement should be about 2000 microstrain. Send CRBasic example FieldCalStrain() Calibration (p. 226) as a program to a CR6 datalogger. Figure 60. Quarter-Bridge Strain-Gage with RC Resistor Shunt 226
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