Campbell Scientific 4WFBS120, 4WFBS350, 4WFBS1K 4 Wire Full Bridge Terminal Input Modules User Manual

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4WFBS120, 4WFBS350, 4WFBS1K 4 Wire Full Bridge Terminal Input Modules (TIM)

It should be noted that a shunt resistor should not be connected
across the active gauge’s leads back at the completion portion of
the Wheatstone Bridge, as this would not correctly account for
the leads resistances. If performing a shunt back at the
instrumentation location, it must be done across the Dummy
Resistor as shown in figure 4.4-3.

NOTE

The 4WFBS TIM modules include 2 gold plated, shunt pin receptacles to
facilitate easy access to the internal dummy resistor. These receptacles, which
accept 0.015 to 0.025 inch diameter pins, are depicted in figure 4.4-4.

Datalogger

cti

ve G
auge

or G

VX or EX

4WFBSXXX TIM

=1
K

Ω

=1
K

Ω

Shunt Receptacle

FIGURE 4.4-4. Wiring for shunt across dummy resistor

To shunt the dummy resistor, simply connect the resistor across the two gold
plated shunt receptacles so that it is in parallel with the dummy resistor.

4.4.2.2 Math for Shunt Calibration of 3-Wire, ¼ Bridge Strain Circuits

The math in this section is done automatically for the user by the
Datalogger’s Operating System. It is included here mainly for
reference and for users with our older loggers that are not
supported by the Calibration Wizard and higher end instructions.
The Calibration Wizard utility which is installed with CSI’s
software packages greatly simplifies the calibration process.

NOTE

The premise is the same when shunting across either arm. The shunted arm
undergoes a reduction in resistance creating a simulated strain. The change in
resistance of the shunted arm is given by Equation 4.4.9:

−

4.4.9

Variable definitions:

ΔR = Change in arm resistance (ohms)

= Nominal gauge resistance (ohms)

= Shunt resistor resistance (ohms)