Specifications

Appendix C

Strain Gage Diagnostics

About This Appendix

This appendix covers the diagnostics associated with the strain gage

multiplexers. The diagnostics check configuration integrity and increase

measurement accuracy. The diagnostics in this appendix include:

• Leadwire Resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Page 108

• Internal Half Bridge Voltage . . . . . . . . . . . . . . . . . . . . . . . . . Page 111

• Shunt Verification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Page 113

• Guard Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Page 116

• Bridge Excitation Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . Page 118

Diagnostic

Channels

The multiplexer diagnostics are accessed through "internal" channels as

well as the "external" bridge completion channels. Table C-1 identifies the

internal channel and the diagnostic on that channel.

Table C-1. Strain Gage Multiplexer Diagnostic Channels

Channel Diagnostic

8/9 Leadwire Resistance. Channels 8 and 9 measure the voltage drop in the

leadwires from the strain gage to the bridge completion circuit, for bridge

completion channels 0 and 1. From this voltage and the value of the bridge

completion resistor, the leadwire resistance can be determined so that

leadwire desensitization corrections can be made.

10/11 Internal Half Bridge Voltage. Channels 10 and 11 measure the lower and

upper legs of the internal half bridge voltage. This checks the stability of the

1,000Ω bridge completion resistors.

12/13 Shunt Verification. Channel 12 places a resistor in parallel with the bridge

completion channel strain gage to simulate a known value of compressive

strain. Channel 13 places a resistor in parallel with the upper leg internal

bridge resistor to simulate a known value of tensile strain.

14 Guard Voltage. Channel 14 measures the guard voltage (Wagner Ground)

to determine if it is 1/2 of the bridge excitation voltage.

15 Bridge Excitation Voltage. Channel 15 measures the bridge excitation

voltage (Vs).

Appendix C Strain Gage Diagnostics 107

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