Inaccurate strain readings from two seperate SCXI units. Correct readings using cDAQ.

[OKors]

I'm using a SCXI 1000 with 1520 bridge module and 1314 terminal block to acquire strain data from a purpose-built "custom" full bridge device (aka custom load cell from now on). This strain data is used to calculate a slope, then scaled in software so it can be read as load cell which outputs Lbf.

Problem I had is after applying the new calculated scaling values based on the uE slope created perviously, the output of the custom load cell did not match the output of the load cell "standard" being used to apply the force. The custom load cell was reading 140lbs less than the standard.
Strain gage wizard showed the calibrations for the custom load cell were sucessful multiple times over and over.

Different SCXI unit (identical in hardware configuration) from another workstation connected to my system had the exact same results; 140lbs off after calcuation and scaling.

As a hail mary I connected a cDAQ with NI9237 full-bridge module to the system. Followed the same setup and calibration routine I did with the SCXI units. Generated another force vs strain curve but this time the output of the slope was much different than what the SCXI's produced. Low and behold after scaling and conversion the output from the custom load cell and the load value from the standard agree.

How is it that two different SCXI units can output the same (both wrong) strain values and the cDAQ  is spot on?

At the end of all this, the only thing that makes sense is something in the hardware of the two SCXI's are throwing the strain gage shunt calibration wizard off.

Needless to say this creates a huge problem for me as I had no way of knowing my strain measurements were off unless I checked it against a known good standard.

Please help. This needs to be resolved asap.

[Henrik_Volkers]

The output of a straingage is usually mV/V   ...  mV(measured bridge output) / V (per V bridge exitation)

My first guess is a wrong configured exitation voltage 🙂

[OKors]

Excitation is always 10v.

This was even measured with a DVM.

9.99Vdc to be exact.

[PezDeSpencer]

Hello OKors,

Can you include screenshots of both your SCXI Task/global channel within MAX as well as the same for your cDAQ task. I am wondering if there is some setting/gain that is different between the two. 

I would have suggested calibration of the SCXI card, but since you have already tested two with the same result this seems highly unlikely. 

Also please refer to this document if you have not already done so and attempt the troubleshooting steps

LINK:

Troubleshoot Inaccurate Strain Gauge Measurements with the SCXI 1314

http://digital.ni.com/public.nsf/allkb/625E1FE5C6DD2A0C862579600073009E

Spencer R | NI