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)

With either circuit, one lead leg, L

or L

, is in one of the two opposing arms of

the Wheatstone bridge. It is important that the gage be wired such, and that
these two leads be the same length, diameter and wire type. It is preferable to
use a twisted pair for these two wires so that they will undergo the same
temperature and electromagnetic field variations. With this configuration,
changes in wire resistance due to temperature occur equally in both arms of the
bridge with negligible effect on the output from the bridge.

4.3.2 Quarter Bridge Strain with Dummy Gauge Calculations

The calculations for this bridge setup are the same as for the 3-Wire Quarter
Bridge circuit. See Section 4.1.2 Quarter Bridge Strain with3 Wire Element
Calculations for details.

4.3.3 Quarter Bridge Strain with Dummy Gauge Example Programs

The programming for this bridge setup is the same as for the 3-Wire Quarter
Bridge circuit. See Section 4.1.3 Quarter Bridge Strain with3 Wire Program
Examples for details.

4.4 Quarter Bridge Strain Lead Resistance Compensation

When using quarter bridge strain (full bridge with one active element) with
long lead lengths, errors can be introduced due to the resistance of the leads.
This section covers both mathematical and Shunt Calibration methods used to
rectify these errors. The techniques covered in the section can be used with
circuits using a 4WFBS’s completion resistor or a dummy gauge for the
resistive element in the third arm of the Wheatstone Bridge (arm opposite of
active gauge). The only difference is that when using a dummy gauge, the
4WFBS module’s gold shunt receptacles cannot be used. These receptacles are
connected to the dummy resistor supplied by the 4WFBS module.

One potential error with long leads is due to the leads' resistance change from
temperature fluctuations. When using a three wire strain gauge, wired as
depicted in Figure 4.1-2 3-Wire ¼ Bridge Strain Wiring, with the three leads
all the same length and laid out together (all three experience the same
temperature swings), the leads' resistance changes are self compensating. It is
preferable to use a twisted pair for the two wires (L and G) carrying the current
so that they definitely undergo the same temperature and electromagnetic field
variations. With this configuration, changes in wire resistance due to
temperature occur equally in both arms of the bridge with negligible effect on
the output from the bridge.

Another error that is introduced when using long leads, is a sensitivity
reduction of the system. There are two methods to rectify this error. The first
is mathematical. The second is to perform a shunt calibration. Sections 4.4.1
and 4.4.2 cover these methods for ¼ Bridge Strain circuits.

4.4.1 Mathematical Lead Compensation for 3-Wire, ¼ Bridge Strain

The same equations pertain whether a completion (dummy) resistor or a
dummy gauge is used to complete the third arm of the Wheatstone Bridge. So
the material in this section is relevant for wiring setups shown in Figures 4.1-2,
4.3-2, and 4.3-3. The math and the programs used would be identical for all
three of these circuits.