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Loadcell Strange Behavior after Calibration
The loadcell keeps showing decreasing values in the output. The calibration of the loadcell has been done with very small weight compared to tank capacity.

We have a 300 Ton Liquid Chlorine Tank. There is a strange behavior in the loadcell output. The device can calibrated by the zero calibration, and by the load calibration. I believe both were done correctly during loadcell installation(I wasn't there that time). I suppose that due to pressure changes inside the tank, the zero had shown non-zero values when it was empty,so multiple zero calibrations has been done with time. At some point, I believe the load calibration has been done by mistake instead of zero calibration, thus overriding the initial load calibration.

Due to the fact that it is chlorine tank, it is not safe to fill water due to possible damage to tank. I don't have much experience with external weight/pressure method, but due to the fact that the tank has isolation, and cracking risk, I am unsure that it may be the right solution(perhaps pressure on the legs of the tank may work). So, until now the load calibration has been done with a man standing on the tank leg and entering his weight as a load.

Now the behavior that we observe is following:
The devices measures small weigths correctly right after the calibration with human weight. But after several minutes, the nominal output starts to decrease. Example:

1) I do the zero calibration once the tank is empty.

2) I stand on the tank leg(my weight is 60kg).

3) I enter 60kg as the load during load calibration

4) Then another person stands on the tank leg (lets say 100kg weight)

5) The device measures this weight correctly.

6) After few minutes the device value starts to fall down. The zero goes to negative values. It decreases by few kgs per minute.

7) Even when the zero is negative, the tank measures human weight correctly again.

I want to understand the following:

a) What might be reason of the constant decrease with time?A leakage inside the piping is possible, but I doub't it. Can cabling electrical problems lead to this kind of behavior?

b) Given that the small weights are measured correctly, can we state that the measurements would be correct at 50+ tons too?

What do you think? I would appreciate your help.

Sounds like drift in your transmitter/converter electronics.

1. device age (in service) as it may be a warranty issue

2. unless you are flowing Cl, the (tank+contained) measured mass
does not change

3. pressure will not affect the reading, only mass flows

4. temperature drift unlikely, that requires large delta-T swings

5. connected piping must be properly designed to avoid loading
if not you could have apparent mass changes as the piping drains
(free draining pipe connections required), and so on...

By FATIH TURAN on 30 May, 2018 - 12:27 pm

You would have to apply minimum 10% of the maximum weight to test the calibration. In your case it would be 5 tons.

regards
MFT

Dear All,

Thanks a lot for your feedback.

It has been clarified that the decrease in the loadcell output is due to the leakage of one of the valves which in result transports the chlorine to another tank.

Again thank you very much for the valuable information.