Specifications

Section 11. Programming Resource Library
11-11
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 CR1000.
4) The zero function of FieldCalStrain() allows the user to set a particular
strain as an arbitrary zero, if desired. Zeroing is normally done after the
shunt cal.
Zero and shunt options can be combined through a single CR1000 program.
The following program is provided to demonstrate use of FieldCalStrain()
features. If a strain gage configured as shown in FIGURE 11.1-1 is not
av
ailable, strain signals can be simulated by building the simple circuit shown
in FIGURE 11.1-1, substituting a 1000 Ω
potentiometer for the strain gage. To
reset calibration tests, go to LoggerNet | Connect | Tools | File Control and
delete .cal files, then send the demonstration program again to the CR1000.
Case: A 1000 Ω strain gage is placed into a Wheatstone bridge at position R1
as shown in FIGURE 11.1-1. 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.
FIGURE 11.1-1. Quarter bridge strain gage schematic with
RC resistor shunt locations shown.
Send Program EXAMPLE 11.1-5 to a CR1000 datalogger.