understanding strain using NI9236

[Kaltal]

Hi,

I am using a cRIO 9073 with a NI 9236 to measure strain gauges. I am using a modified version of NI9236 Getting Started project found in the examples folder of Labview, to do my data collection. I then use another vi to do my offset, averaging and converting to micro strain.

When I apply tension to the Aluminum bar the gauge is mounted on I get a negative strain and when I compress it I get a positive value. My understanding was that it should be the other way around. Another thing is that even when I use the value regardless of the sign I seem to get garbage results.

When I add a weight of 311.05g to a 19mmx3mm bar of Al, I should be getting a strain of 0.79 micro strain, not -19. I see the -19 right on the front panel of the NI 9236 Getting Started (host).vi, while it is running, and it is not part of the vi I have edited, rather something NI wrote. I am sure it is not a programming error, I cannot understand the discrepancy. My math cannot be that off can it?

Any help to clarify this would be appreciated.

[Kaltal · Accepted solution]

OK...maybe a little slow here, it might help if I was in the correct range for the strain gauges...311g is too little weight to be doing any tests with...sigh.

My main question about the sign convention is still valid though, why is it the opposite to the way I have always thought strain should be?

I will add some propper weight onto this thing to see what values I get.

Steve C

[USBarnes]

Kaltal,

Could you give more information about the strain gauge you are using and how you have it wired to the NI 9236? More specifically, which terminals of the strain gauge are connected to which terminals/analog input channels? Including a datasheet for the strain gauge would be helpful as well.

[glskinner]

Sounds like your wires are crossed, inverting the polarity. Barnes is right though, we would need the specs of the gauge and details on how your wiring the gauge. 😄

[Kaltal]

Well I am a fool, thanks for trying to help but I do tend to get in my own way somethimes

I tested today by applying larger force values to the sample, and got the numbers to agree with my calculations...sigh.

I am not sure how I got the inverse to show up that one time, all I can think is I had it on a slow data rate and so it was lagging behind what I was actually doing

Next time I will have a good nights sleep before comming on the forums and asking daft questions.

Steve C