Campbell Scientific CR9000X Measurement and Control System - Manual (Page 368)

Section 9. Program Control Instructions
FIGURE 9-1A. Active gage shunt FIGURE 9-1B. Resistor shunt
When using Campbell Scientific's Terminal Input Modules (TIM) with shunt
posts (e.g. model # 4WFBS350), the R2 resistor shown in Figure 9-1B:
Resistor Shunt can simply be shorted across the gold posts located on the top
of the TIM.
Campbell Scientific recommends that the user record the
calibration constants to a data table and upload them to his PC
for a record. NOTE
When a calibration is complete, the *.CAL file will be updated, and the
NewFieldCal function state will be changed to True. The NewFieldCal
function can be used to trigger a user created Data Table to store the
calibration factors.
The values from the *.CAL file can be loaded back into the calibration
variables using the LoadFieldCal instruction.
Description of the ¼ Bridge calculations performed by the datalogger.
The premise is the same when shunting across either arm. The shunted arm
undergoes a reduction in resistance creating a simulated strain. A precision
resistor should be used for the shunt resistor. The change in resistance of the
shunted arm is given by:
ΔR
R
R
R R
G
G
G S
=
−
+
Variable definitions:
ΔR = Change in arm resistance (ohms)
R
G
= Nominal gauge resistance (ohms)
R
S
= Shunt resistor resistance (ohms)
The standard equation for calculating micro-strain from the change in
resistance of the gauge is:
με =
Ч
Ч
ΔR
R
G
G
10 6
F
Variable definitions:
με = micro-strain
ΔR = Change in arm resistance (ohms)
R = Nominal gauge resistance (ohms)
G
GF = Gauge factor
9-38