# Load Cell Calibration Without Weights?

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Machine has 4 load cells (30klb each, 2mV/V) into a Summing Box, then the single output from the summing box going into LC2 on an I/O LC3.

The Load Cells are measuring clamping force of a bolted joint and, as such, there is no real way to put a reasonable known weight onto the LC's and hit "Calibrate."  Eventually we'll have a calibrator come out with a through-hole Load Cell to dial it in, but we need to get it in the ballpark to get started.  In this application, relative values are more critical than accuracy so if we're off 5% no big deal.

I used some simple math to determine that it should take 111,000 or so pounds to reach 80mV/V output which is the top of the scale.  Question is, if I use Edit Calibration Point and fake in a Zero and a Full-Scale Output point, will that get me reasonably close to accurate?  I thought it seemed workable - theoretically 0.8mV (80mV/V * 10V Excitation) should end up at a Raw value of 8,388,607.  But, when I was looking at data the Raw values were about 1/8th of the pounds I was expecting to see, so that kind of shot my theory up a bit.

Anyway, hopefully this makes some sense and someone can shed some light!

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HiMikahB,

You refer to 10V exitation. It is not relevant to LC1/LC3.

You have to use internal LC1/LC3, which is 5V DC or AC.

External exitation not allowed.

Calibration should be done with a real weight/force to have reliable working (and safe) system.

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HiMikahB,

You refer to 10V exitation. It is not relevant to LC1/LC3.

You have to use internal LC1/LC3, which is 5V DC or AC.

External exitation not allowed.

Calibration should be done with a real weight/force to have reliable working (and safe) system.

Wow, not sure why I had it in my head that it was 10V excitation - thanks for setting me straight.  Regardless, my question remains the same.

While I appreciate your concern, there is no safety issue related to the accuracy of force measurement on this machine.  The load plate is 4" square and sits vertically, so as I said previously simply piling some known weight on it is not really feasible (or, indeed, safe).

The load I was clamping yesterday was something like 20,000 pounds (very roughly).  At that load, I was seeing Raw values of around 3150 which doesn't make sense to me even at 5V excitation.  I was also seeing Raw values of around -200 with no load which also doesn't make sense to me (it is not possible for these button-type load cells to be under tension).

I guess I will just experiment with it today.  Some help would be much appreciated, though.

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We ended up using an estimate by applying a fastener with a known torque and referencing existing torque/tension data - so that should be close.

Only issue I have now is that the output from the load cells is oscillating strangely - both at zero and under load, I'm getting a very smooth, consistent sinusoidal-looking oscillation of about 0.1% of FSO.

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