Loadcells

A little OT! Have 2 loadcells mounted in a Conveyor idler frame, they are rated at 25Kg 2mv/v, they are wired in parallel, do I enter them into the integrater as a 50kg or a 25kg? cannot seem to get a consistant answer from the supplier or our sparkey, one says 25 other says 50! both seem to work!

how are they fitted? serial or parallel ?

or in other words, in line or on seperate legs

Both fitted to the same frame one on the left side the other on the right but wired in parallel, and both rated at 25 kgs each.

My vote goes to summing the values of the parallel connected loadcells.
So that would be 50 kg.

+1 for the summing.

Steve

Hmm
Interesting that you say it works connected either way
If wired in parallel to a single input then the max volts will be the same for 1 or 2 loadcells... like 2 batteries in parallel.
If wired to a summing amplifier then it will be more accurate

JesperMP said:

My vote goes to summing the values of the parallel connected loadcells.
So that would be 50 kg.

Click to expand...

Are we voting?
It is a Wheatstone bridge, where you measure the output voltage.
When you connect two AA batteries in parallel how many volts do you have at the output?

mdim.

When you connect the loadcells to a summing box, the output voltage will be a mean value generated by all the bridges in parallel. In other words, the voltage measured will be about the same (if the loadcells are exposed to the same force), even if there are 2 loadcells instead of 1.
But since each of the 2 loadcells will be exposed to half as much force compared to when 1 loadcell is used, and therefore the output voltage will be halved, you have to double the value that the voltage corresponds to (2x25kg=50kg).

I hope that makes it clear.

I have never heard about connecting loadcells in series.

Yes, since you measure only half the load with each cell, you have to double the result to get the real load.
But with weighing the belt of conveyor this is the smallest problem, since due to mechanics, the speed of the belt and so on you have great oscillation in the reading value. Lot of companies that produce belt weighers use metal chains as test load for calibrating the exact load and the filters for the oscillation.

I dont understand what you are trying to say. Jesper answered the OP question 5 posts ago

Thanks for that Jesper, I have 4 of these recently installed 2 set at 25kg and 2 at 50kgs, and with a manipulation of the Span correction factor have managed to get a reasonably accurate reading! so on the next shutdown for maintenance (Nov 4th) will do a full calibration and set them all for 50kgs. cheers

when parallel used the output voltage is 2mV when both cells are max so when cell is 25 kg 2mV/V it will be 50 kg total.
it is no good practice to use them parallell as the compensation is not as it should be.
every cell should be converted and then summed.

Rayk. I have a bit of experience with belt weighers. They are really sensitive for the belt installation. It must be absolutely straight. You must have at least 2 identical idlers on either side without loading point, without changes to the profile, etc.

Also, the load cells shouldnt be part of the idler frame itself, otherwise longitudional forces may be transferred to the loadcell (the manufacturer may claim that the loadcells are insensitive to longitudional forces, but I have found that to be false on several occasions). The loadcells must be installed so that the idler frame rests on the loadcell "freely" so to speak.

Shooter. I dont disagree, but the problem with compensation is mostly if you have long cables between loadcells and converter.
If using a classical converter I recommend to install it locally right next to the loadcells.

Thanks for those tips Jesper, also with 25kg loadcells its very easy to Bend them! found that out the hard way, one was giving 17mv output with no load, also the weighframe with the loadcells need to be 5mm higher than the leadin and leadout frames.