Two newly installed Juice storage process tanks have been fitted with load cells.

We have set up/Calibrated the load cells by following the manual instuctions and scaled the analogue output to the PLC with success

the Problem.......................

As the tank fills and empties, the load cell weight measurement is accurate (Checked by flowmeter). However if we fill the tank and empty it during a hot clean, We end with an empty tank and negative Measurement value (-80 varying)

In other words the scales start of at zero and end up with a minus value. (We have to reset to Zero by tareing)

Any advice welcome

 

Check for strain on the load cells caused by expansion of the tank.

Regards
cww

 

Thanks for replying

How do we check for strain on The Cells and what can we do to stop it ?

 

It depends on what type of cells you have, benders are more susceptible that hangers. Crushers are in between. When the empty reading has drifted try loosening a mounting bolt and retighten. If there is residual side stress that may relieve it. As to how to fix it, that depends on the geometry. The idea is to orient the cell so any movement is in it's least sensitive plane if possible. For benders it is often so they point straight into the motion vector.

For crushers where it's hard to determine the strain gage orientation, you can try monitoring them and turning them for the least deviation. Hangers benefit from a little compliance in the mounting. Each case is a little different, but if that's what's causing your problem a little head scratching will usually provide a solution. If "hot cleaning" causes a shift that doesn't revers on cooling it may be that a mount is slipping.

Regards
cww

 

I think you need to bring the person who designed the tank mounting system back into the picture (if they're not already). A load cell measures load. Whether the load is due to thermal expansion or gravity makes no difference to the load cell. If there is any possibility of thermal expansion then the load cells need to be mounted in such a way that expansion and contraction of the tank doesn't impose any load on them.

This is a mechanical design problem, not an instrumentation problem. Someone needs to go over the mechanical drawings and figure out where the forces are going and how to isolate thermal expansion forces from the gravitational forces. If this wasn't allowed for in the mechanical drawings then I doubt you can fix it by just turning a few calibration knobs on an instrument. A force is a force. In the end it doesn't matter what's causing it; the load cell is going to be affected by it.

 

Thankyou.... We have arranged a visit from the Load cell service engineer. I will reply with the outcome - details

 

Dear Sir;

Is there any outcome for this problem?
I'm much intrested in solution.
Pl. reply.
the outcome - details