Section 7. Installation
162
'Data Storage Output of Calibration Data ─ stored whenever a calibration occurs
DataTable(CalHist,NewFieldCal,200)
SampleFieldCal
EndTable
BeginProg
Multiplier = 1
Offset = 0
KnownWC = 0
LoadFieldCal(true) 'Load the CAL File, if possible
Scan(100,mSec,0,0)
'Simulate measurement by exciting channel VX1/EX1
ExciteV(Vx1,mV,0)
'Make the calibrated measurement
VoltSE(RelH2OContent,1,mV2500,6,1,0,250,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 CR800 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. 154) ) 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 CR800 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.
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