Campbell Scientific 4WFBS120, 4WFBS350, 4WFBS1K 4 Wire Full Bridge Terminal Input Modules - Manual (Page 25)

4WFBS120, 4WFBS350, 4WFBS1K 4 Wire Full Bridge Terminal Input Modules (TIM)
Another temperature induced error in a quarter bridge strain circuit is due to
the Temperature Coefficient of Resistance (TCR) of the completion resistor in
the arm opposite the strain gauge. The 4WFBS TIMs use a high quality
resistor having a TCR of 0.8ppm/°C to minimize these errors. For our example
above, this could lead to an error in the reading of approximately 10 μStrain,
assuming that the datalogger experiences the same level of temperature
variation. This error could be additive or subtractive to the other errors as the
resistor manufacturer does not specify the polarity of the change in resistance,
only the absolute magnitude.
These errors, with exception to the completion resistor’s TCR, can be
mathematically compensated for to some degree. It should be remembered that
the curves and equations given are the average for the given batch of gages and
are only applicable when mounting on the specified material. An alternative
approach to eliminate the errors is to either use a dummy gage, from the same
batch mounted on identical material, or to use a half or full bridge strain
circuit.
Dummy gauges can be used to compensate for these false apparent strain
readings. A strain gauge that is mounted on a coupon that is not undergoing
mechanical stress and is used as the resistive element for the Wheatstone
bridge arm opposite the active gage is referred to as a “Dummy Gauge”. This
non-active gauge in the other arm of the Wheatstone bridge is referred to as a
“dummy gauge” because it is not subjected to “load induced” strains.
With the two opposing gauges experiencing the same temperature conditions,
the temperature effects on the active gage will be nullified by the equivalent
temperature effects on the dummy gauge. Figure 4.3.1 depicts a Quarter Bridge
Strain circuit with a Dummy gauge.
L
3
R
2
=1 K Ω
R =1 K
1
Ω
- +
L
2
L
3
Dummy Gauge
L
1
Active Gauge
FIGURE 4.3-1. Quarter bridge strain circuit with dummy gauge
It should be noted that the coupon on which the dummy gauge is mounted can
still be subjected to temperature induced strains. This can be used to null
temperature induced strains in the monitored member if the dummy gauge is
mounted to a coupon made up of material having the same Temperature
Coefficient of Resistance (TCR) as the member that the active gauge is
mounted to. Conversely, the dummy gage could be mounted to a coupon with
a negligible TCR allowing for the monitoring of temperature induced stresses.
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