Strain guages reading negative when they're expected to be positive.

[Andrw]

We are running Singalview Express 3.0 with the DAQmx with the NI 9237 module reading strain gauges.

The program with default settings is reading negative strain values, when the values are expected to be positive. The strain guage installation was checked, the wiring was checked, and the jumper cables connecting to the NI 9237 were all checked, and according to the manual and the description on the module itself, all of our connections are correct. Yet, when the strain guage is put in what should return a positive strain, is showing up as negative on the Signal Express data view.

Is there a known discrepancy that we are unaware of with the NI 9237, DAQmx, or Signalview Express that causes the singal to be read negative when we are expecting positive?

[HeatherBaker]

Hi Andrw,

What version of DAQmx are you using?  If you are using DAQmx 8.9, then you may want to try this patch.  Also try verifying this behavior with Test Panels in MAX.

[Andrw]

Actually, we're not using DAQmx, what I meant was to say was the Measurement and Automation Explorer.

Upon further investigation, we've triple checked all of our connections and wiring, and they are all correct. At this point, we took a voltmeter and checked the reading at the connectors when Signal Express was running and compared them to what was on the computer screen. The voltmeter was providing the opposite signal that the LabVIEW programs were displaying and the proper signal (i.e. it would show positive when the program would show negative). As a temporary fix we added a "Custom Scale" on the data acquisition step with a negative slope. 

Obviously while this works for the time being, we still cannot understand why the signal is getting flipped between the connection to the computer, and what the computer is displaying. There must be a reason for it, and to fully understand the issue and to avoid any concerns in the future we would like to know what is happening.

[HeatherBaker]

HI Andrw,

DAQmx is the driver that interfaces with the NI-9237.  You may not be programming explicitly with it; however, the driver is making the low level calls.  You may check your driver version under software in Measure and Automation Explorer.  If you are using a version of DAQmx earlier than 9.0, then you will have only one reference.