SCXI-1520 Zero Offset Changes with Variable Loads

[dj143]

We are using the SCXI-1520 to calibrate sensors based on strain gauge bridges.  The problem I'm having is that the offset changes when changing the bridge resistance.  For example, the output reads 0.0000 mV/V for 350 ohm bridges.  It reads 0.0112 mV/V for 700 ohm bridges.  It reads 0.0135 mV/V for 1000 ohm bridges.  The signal input is a constant zero for all three of these examples.  The span (gain) is very consistent.  In other words, changing the bridge resistance does not affect the gain.   I know the excitation sense lines are working and they are applied very near the bridge.  Enabling or disabling the autozero mode makes no difference.  What can be done to eliminate the variable offset when changing the bridge resistance?

[Marni]

Dear Dj143,

Are you calibrating your strain gauge lines in the Measurement and Automation Explorer (MAX) or in your LabVIEW program?

In MAX you can create a task and use the calibrate tab in the task to calibrate your line. You would then call the task in LabVIEW. If you are using this method, make sure that you are specifying the correct Gage Resistance (gauges in an unstrained position). The default is 350 so it would make sense that you are getting 0 V with this resistance.

In LabVIEW you can use DAQmx Perform Bridge Offset Nulling Calibration.vi to calibrate your lines. There is an example in the Example Finder that shows how to use this VI. (LabVIEW Help menu>>Find Examples. Search calibration. Cont Acq Strain Samples (with Calibration).vi). In this case make sure that you are specifying the correct Nominal Gage Resistance (gauges in an unstrained position). Once again the default is 350.

If you still get incorrect voltages when specifying the correct resistance, let me know how you are calibrating your lines and I will see what I can think of. Have a great day!

Sincerely,

Marni S.