Load cell question for the experts

[vic92]

Hello, I must first say that I think this is an excellent forum, I have found some usefull info in the past.(thanks)

The question I have today is about loadcells, I am working in my companys R&D department I am trying to set up a pair of load cells to measure tension. The load cells I am using are brand new. They are rated at 100 lbs each , 10vdc exc. and the are 3mv/v.

I have hooked up 1 l/c to a data aquisiton system I have which is currently meassuring the excitation voltage (10.0vdc) and the output (m/v). I am sure all my readings are correct, because I am using a calibrated multimeter for verification.

I do understand that @ 30mv I should have 100lbs of tension and @ 0 lbs I should have 0 mv or close to. I was told with no load there should be no more that .3 mv positive bias ( which is fine I am reading .08 - .15mv). One question is: does this change the scaling? is it not now 0-30mv? also when I hang a weight (calibrated 5.005lbs +/- .001 ) I get about 1.95 mv which doesn't seem right I think I should be getting about 1.5 mv.
I also tried hanging a 25 lb weight and got a reading of 8.59 mv. which should been closer to about 7.5 mv (according to my math) I am trying to figgure out why my numbers arent meshing, Does any 1 have any ideas?

 

[waht]

I'm not really familiar with this sensor, but you could try a few things. Plot the voltages vs weight, up to 100 lb if possible. See how the graph looks like, if it's a straight line that means the whole thing is offset by some constant voltage.

Also, you might want to vary the bias voltage, +/- 0.5 volts, see if the readings change.

It's also the possible the multimeter has a too high impedance, so you could try loading the sensor with maybe a 1K resistor, see if it affect the data. Or vice-versa, it's possible the multimeter represents a too low impedance and sucks out the bias voltage from the sensor. In that case you would need a to check it with a high impedance probe, maybe 10 Megaohm using an oscilloscope.

These are the things I would try first based on limited knowledge of the sensor.

hope that helps.