Specifications

Internal Half Bridge Voltage
The Internal Half Bridge Voltage diagnostic accessed through internal
channels 10 and 11 measures the voltage across the 1kΩ internal half bridge
resistors. These measurements check the ratio of upper leg to lower leg
bridge resistance. The equations in the instrument firmware which calculate
the actual strain measured, allow for a certain amount of bridge imbalance
due to differences in these resistances. A voltage (resistance) ratio outside
the tolerance causes a bridge imbalance not accounted for in the equations.
This results in inaccurate strain measurements. Figure C-3 represents the
internal half bridge resistors and where the measurements occur.
Specification The ratio of voltages on the upper and lower bridge legs should be
1% ±0.1%.
Set Up To perform the internal half bridge voltage 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. If desired, connect the bridge arrangement to the bridge completion
channels. Strain gages do not have to be connected for this
diagnostic.
CAUTION Use care when externally supplying the bridge excitation
voltage. Voltages greater than 5.4V can damage the bridge
completion resistors.
Figure C-3. Internal Half Bridge Measurements
Appendix C Strain Gage Diagnostics 111
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