calibrating a load cell

sir,
i joined a company as a trainee. pls clear the doubts i am having

1. we have calibrated the load cell 200t with 20t as 10% is recommended. we got 1% error what we have to do if we got more than that i.e 5% or 10% error if again we have to calibrate means how?

2. we are having 4 load cells of each 200t capacity i.e is 800t total and each loadcell is having rated output as 1mV. what is the total
output of loadcells is it 1mV or it becomes 4mV

Thanks
abbu
 
dear abubaker
first of all, make it confirm before calibration that the weighing platform is not touching anywhere with fixed part, i mean check for any mechanical abnormality. If everything is fine then i think there should not be any problem and the error should be within range.

Which weighing system are you using?

second when we connect the loadcells the output is summation of all the loadcells, so it should be 4mV.
 
Hi Abbu

If you have calibrated at 20t and get 1% error at 200t , you could recalibrate at 20t with an offest (Save 20t as 19.8 or 20.2t). This will reduce the error at 200t.
Many indicators have linearising facility. You can try this.
You can also enter the mV/V into indicator by "direct span".

The rated output of your load cell is 1mV/V (not 1mV).
So if your load cell excitation is 10V , you will get 10mV (10*1mV/V).
If you connect 4 load cells with 10mV signal in parallel , your total output will be 10mV. (This is similar to connecting 4 batteries/ cells of 1.5V output in parallel. Your total output will be 1.5V. )

Regards
Sam
 
Hi Sam,

Yes you are right. I wrote it wrong. If we are connecting the load cell signal with 1mV/V in parallel, the output signal will be 1 mV. But how come the signals of all four loadcells are summed? I mean depending upon the load on all four load cells, the mV signal will be different. Then how they are summed?
 
Hello sir,

Thanx for the valuable information you have given. Actually it is 1mv/v you are right. I again checked that one. Thanx a lot. I will recalibrate that thing with the value you have given.

Regards,
abbu
 
hello sir,
i want to know the formula in order to calibrate weigh feeder i.e for gravimetric mode what is the formula and what r those parameters needed. can u take some parameters and can u explain me with an a example.


thanks
abbu
 
I want to offer a simple solution. Before that, you should try & understand whether the load cells are connected in averaging fashion / summation fashion. In other terms, what is your total weigh bridge capacity, is it 200 T / 800 T?

Secondly, there is a simple method of calibration for this. Ask your vendor & he will be able to supply you a variable potentiometer (with a few resistors connected & soldered to it). It comes in a sealed pack with calibration certificate. Using this, connect the potentiometer to the indicator & calibrate for zero. Once the zero is complete, turn the potentiometer fully clockwise & then calibrate for span. Your indicator is calibrated now.

Coming to the loadcells: first empty the platform. As our colleague said, the platform should not touch anywhere to any ground / any other mechanical structure. Now, in the junction box, remove the jumpers. Removing the jumpers means, you have isolated the individual load cells. Now, check the output of each loadcell. If all all normal, you should get approximately same level of voltage as the output of loadcells. If there is a deviation, change the worst affected loadcell with a new one. But most of the times, since you are averaging the output (if the capacity is 200T) it should not make a huge difference for a slight variation in the individual voltage outputs.

Good luck & update us with your findings.
 
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