Leadwire Resistance
The Leadwire Resistance diagnostic accessed through internal channels 8
and 9 enables you to determine the resistance of the leadwires from the
strain gage to the bridge completion channel terminals. The bridge
completion channels on which this diagnostic is performed are channels
0 and 1. The leadwire resistance diagnostic is used for
1
⁄
4
bridge
arrangements only.
Leadwire
Desensitization
Corrections
When the leadwire resistance is known, leadwire desensitization corrections
can be made to the measured strain values. Leadwire desensitization is the
measurement error caused by the leadwire resistance.
Correction for leadwire desensitization is achieved by multiplying the
measured strain value by the quantity (1 + Rl/Rg). In equation form, this
appears as:
e(actual) =
ε
(measured) x (1 + Rl/Rg)
where
ε
(measured) is the measured (
1
⁄
4
bridge) strain, Rl is the leadwire
resistance, and Rg is the resistance of the strain gage (120
Ω
or 350
Ω
).
Specification
There is no specification for leadwire resistance; however, typical values
can range from a few tenths of an ohm to several ohms. Among the factors
that will affect leadwire resistance are wire gage, wire length, and
temperature.
Set Up
To perform the leadwire resistance diagnostic, your system should be
configured as follows:
1. If the bridge excitation voltage is externally supplied, connect the
supply to the terminal module as shown in Figure C-1. Set the
voltage to 5.0V.
2. Connect the
1
⁄
4
bridge arrangement to bridge completion channel 0
(Figure 2-4 on
page 28
.
3. Place the specimen in a constant (preferably unstrained) state
throughout the procedure.
CAUTION
Use care when externally supplying the bridge excitation
voltage. Voltages greater than 5.4V may damage the bridge
completion resistors.
108 Strain Gage Diagnostics
Appendix C
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