Campbell Scientific CR3000 Micrologger - Manual (Page 167)

Section 7. Installation
167
Scan (100,mSec,0,0)
'Simulate measurement by exciting channel VX1/EX1
ExciteV (Vx1,mV,0)
'Make the calibrated measurement
VoltSE (RelH2OContent,1,mV5000,6,1,0,250,Multiplier,Offset)
'Perform a calibration if CalMode = 1
FieldCal (3,RelH2OContent,1,Multiplier,Offset,CalMode,KnownWC,1,30)
'If there was a calibration, store it into a data table
CallTable (CalHist)
NextScan
EndProg
7.8.1.6 FieldCalStrain() Demonstration Program
Strain-gage systems consist of one or more strain gages, a resistive bridge in
which the gage resides, and a measurement device such as the CR3000
datalogger. The FieldCalStrain() instruction facilitates shunt calibration of strain-
gage systems and is designed exclusively for strain applications wherein
microstrain is the unit of measure. The FieldCal() instruction (FieldCal()
Demonstration Programs
(p. 158)
) is typically used in non-microstrain applications.
Shunt calibration of strain-gage systems is common practice. However, the
technique provides many opportunities for misapplication and misinterpretation.
This section is not intended to be a primer on shunt-calibration theory, but only to
introduce use of the technique with the CR3000 datalogger. Campbell Scientific
strongly urges users to study shunt-calibration theory from other sources. A
thorough treatment of strain gages and shunt-calibration theory is available from
Vishay at:
http://www.vishaypg.com/micro-measurements/stress-analysis-strain-
gages/calculator-list/
Campbell Scientific applications engineers also have resources that may assist
users with strain-gage applications.
FieldCalStrain() shunt-calibration concepts:
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 (GF
adj
), 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 CR3000.