Campbell Scientific 4WFBS120, 4WFBS350, 4WFBS1K 4 Wire Full Bridge Terminal Input Modules User Manual
Page 39
4WFBS120, 4WFBS350, 4WFBS1K 4 Wire Full Bridge Terminal Input Modules (TIM)
If shunting across the active gauge, the resistance of the active arm will
decrease, reducing the output from the Wheatstone bridge simulating a
compressive or negative strain. If shunting across the dummy resistor, the
resistance of the inactive arm will decrease, increasing the output from the
Wheatstone bridge simulating a tensile or positive strain. A precision resistor
(0.02% or better) with an adequate temperature coefficient of resistance (~4
ppm/
°
C)
should be used for the shunt resistor.
In order to perform a Shunt calibration, first record an initial strain reading,
next contact the leads of the Shunt Resistor to the gold plated Shunt
receptacles, and record a secondary strain reading that will include the
simulated strain. Take the difference between the two readings to get this
Recorded simulated strain (
με
R
) created by the Shunting process.
We will need to compare this recorded strain value with the calculated strain
value. Equation 4.4.10 is the standard equation for calculating the microstrain
from the change in the resistance of the gauge:
με
=
Ч
Ч
ΔR
R
GF
10
6
G
4.4.10
Variable definitions:
με
= micro-strain
ΔR = change in arm resistance (ohms)
R
G
= Nominal gauge resistance (ohms)
GF
= Gauge factor
Combining equations 4.4.9 and 4.4.10 results in Equation 4.4.11 that is used
for calculating the simulated strain that is induced by the shunt resistor:
με
S
G
G
S
R
R
R
GF
=
−
Ч
+
Ч
10
6
(
)
4.4.11
Variable definitions:
με
S
= Simulated micro-strain created by shunt resistor
R
S
= Shunt resistor value (ohms)
R
G
= Nominal gauge resistance (ohms)
GF
= Gauge factor
The calculated strain,
με
S
, is compared to the strain readout,
με
R
, from the
instrumentation. A multiplier is derived from the ratio,
με
R
/
με
S
. The gauge
factor is multiplied by this factor to derive an adjusted gauge factor for the
system, GF
Adj
= GF
Raw
x
με
R
/
με
S
, that is used to correct the output from the
instrumentation
.
When performing recursive shunt calibrations, the original, raw gauge factor
supplied by the gauge manufacturer should always be used for GF
Raw
.
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