Campbell Scientific CR9000X Measurement and Control System - Manual (Page 367)

Section 9. Program Control Instructions
ZERO CALIBRATION STEPS (Function = 10)
1. If the Reps and Index parameters are constants, go to Step 2.
If
the Reps and Index parameters are variables then either:
A. Individual Sensor Cal: Set the Reps parameter to 1 and select the
individual sensor to be zeroed by setting the Index parameter or;
B. Complete Array Cal: Set the Index parameter to 1 and the Reps
parameter to the number of elements in the MeasVar variable array.
This will zero all of the elements of the array together.
2. Change the Mode value to 1 while the sensor(s) are at their Zero state. The
current mV per volt output from the Bridge measurement will be used for the
Zero argument of the StrainCalc instruction. After the calibration is
complete, the logger will change the Mode value to 6.
SHUNT CALIBRATION STEPS (Function = 13, 33, or 34)
1. Set the Index parameter, if a variable, to point to the element of the
MeasureVar array on which to perform the calibration. Make sure that the
Reps parameter's value is set to 1.
2. Change the value of the correct element of the KnownR array to the
resistance, in ohms, of the strain gauge that will be shunted. At the un-
shunted condition, change the Mode value to 1. The logger will record this
first point's micro-strain value and then change the Mode value to 3.
3. While the Mode value is 3, apply the shunt resistor across one of the arms of
the wheatstone bridge.
Load the shunt resistance value (ohms) into the KnownR parameter as a
positive number if shunting across:
The arm that holds strain gauge for Function 13
The arm that holds gauge that is parallel to +ε for Function 33
An arm that holds gauge that is parallel to
+ε
for Function 43
Load the shunt resistance value (ohms) into the KnownR parameter as a
negative number if shunting across:
The arm that holds completion resistor for Function 13
The arm that holds gauge that is parallel to ε − for Function 33
An arm that holds gauge that is parallel to ε − for Function 43
Using the correct sign notation on the input resistance of the shunt insures
that the correct polarity is returned (positive strain for tension and negative
for compression). A gauge parallel to +ε is a gauge that experiences tension
when the element that it is mounted on experiences positive strain. A gauge
parallel to ε − is a gauge that experiences compression when the element that
it is mounted on experiences positive strain. See the Function parameter for
code definitions.
When performing a shunt calibration on a bridge with 1 active element
(Function 13: Quarter Bridge Strain), if possible, it is preferable to remotely
shunt across the arm containing the strain gauge as shown with shunt resistor
R1, used with one of our TIMs, in Figure 1A. With this setup, the shunt
resistor value would be entered as a positive value.
If it is not possible to shunt across the gauge, due to accessibility problems, it
is possible to shunt across the bridge arm containing the dummy resistor right
at the datalogger. This shunt setup is depicted with shunt resistor R2 in
Figure 1B. The shunt resistor value would be entered as a negative value.
When performing a shunt calibration on a bridge with 2 active elements
(Function 23: Half Bridge Strain), or with 4 active elements (Function 33:
Full Bridge Strain), the shunt must be done directly across one of the active
gauges.
4. After the shunt is in place, with the shunt ohm resistance value loaded in
KnownR, change the mode value to 4. The datalogger will do the required
calculations, adjust the gauge factor, and change the Mode value to 6.
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