Complete noob with load cells

[Bobobodopalus]

Hello all,


i will start off by saying i know nothing about load cells,
i want a converter/amplifier to output a 4/20mA signal from a load cell.
for reference i am looking at a SIWAREX WL280 load cell.


will any load cell work with any amplifier?
does the excitation voltage have to match or is higher ok?


i am looking at using a OMEGA TXDIN1600Sand assume it will work with anything but i dont really have a clue.

 

[Peter Nachtwey]

will any load cell work with any amplifier?

Most will. When you used the word any I must qualify my answer.

does the excitation voltage have to match or is higher ok?

The excitation voltage should be 10 volts. There may be other levels that I don't know about which is why I won't say any.

i am looking at using a OMEGA TXDIN1600Sand assume it will work with anything but i dont really have a clue.

It is always best to tell us the application. We have lots of experience in controlling presses and pressure in pressure vessels. Noise and response times can be an issue because many load cell amplifiers will filter a noisy or weak signal and that causes phase delay. Also, keep the distance between the load cell and amplifier short because it is easy to induce noise on very small signals.

 

[JesperMP]

The load cell will be specified to output a certain mV/V up to a certain max excitation voltage.
Something like 4mV/V and 10V excitation is normal.

The amplifier or transmitter that connects to the loadcell should be matched.
Some or most amplifiers can be configured for varying mV/V to suit the loadcell used.
If you happen to have a loadcell that outputs 2mV/V and the amplifier expects 4mV/V and cannot be changed, then the value the amplifier outputs will only be 0.5 times the proper value. If you know this you can offset this by the scaling in the PLC that receives the 4-20mA signal from the transmitter. There will be a loss of precision though because you don't use the full scale of the transmitter input.

 

[JesperMP]

The siwarex specs depend on the rated load.
for Emax​ = 0.06; 0.13; 0.28 t --> 1 mV/V
for Emax​ = 0.5; 1; 2; 3.5; 5; 10; 13; 28; 60 t --> 2 mV/V
And 10V or 15V max excitation.

The omega device specs are:
Input Range -7.6 to +7.6 mV/V max. User Selectable
5 Vdc Bridge Excitation Built-In

So they seem to be compatible.

 

[Bobobodopalus]

ok thank you both,
the idea for this was to calculate the gasket sealing pressure on an aerosol can by comparing the amount of force exerted on to the can and the pressure inside of the can.


i am aware that the load cell i mentioned is way too high of a weight limit for what i am after, but now with what you have told me i should be able to get what we need.


in regards to the connection, the load cells wires are listed as EXC+/- and SIG+/-, so would the sig +/- connect to the bridge +/- terminals 7 and 10 on this amplifier?

 

[Bobobodopalus]

i did see that the distance matters alot and the lead of the load cell should not be changed so i will keep that in mind.

It is always best to tell us the application. We have lots of experience in controlling presses and pressure in pressure vessels. Noise and response times can be an issue because many load cell amplifiers will filter a noisy or weak signal and that causes phase delay. Also, keep the distance between the load cell and amplifier short because it is easy to induce noise on very small signals.

 

[Keystone]

Two other things to remember about load cells.
1. Do not cut the attached lead wire, otherwise when you change out the load cell you will have difference in calibration. This is not as critical with one load cell but if you have several going into a summing box it will affect balancing of the load cells if the lead length changes. It sounds dumb but if you work with a lot of load cells you will run across this.
2. Remember a "Load Cell" is also a "Strain Gauge" if you place a load cell in torsion or side load while you are trying to measure a load your "weight" reading will incorrect and not repeatable.

 

[Ron Beaufort]

i am aware that the load cell i mentioned is way too high of a weight limit for what i am after

that sounds like a potential problem ... just guessing for now - but is there a chance that you might indeed have the RANGE - but not be able to measure down to the RESOLUTION that you'll need for a meaningful test? ...

you should probably tell the forum the actual range of pressures that you need to measure ...

it's been a LONG time since I had to deal with Omega - but in the past, their tech support folks were always very helpful with questions like this ... maybe they have something that would match up better? ...

good luck with your project ...

stay safe - stay well ...

 

[Bobobodopalus]

I don't think our process would generate any torsion, but good to know.


The load cell we actually want would be more in the region of 3kg, i just grabbed a load cell to find out which of all of the numbers plastered over the manual actually mattered.

 

[Bobobodopalus]

So i have just got this in set it all up and it is working somewhat, just want to check if some of these issues are normal or if theres a fault somewhere.


the first thing that catches my eye is that when the load cell is connected to the signal conditioner i am getting a negative mV input, around -0.07mV to be exact


is this a somewhat normal issue that i just need to calibrate for or is this absolutely a fault either with the conditioner or load cell, i assume conditioner more likely?


i was completely wrong about load cell weight limit, we are using a 500kg load cell and are expecting to measure around 300 kg


how good would you expect the resolution to be on something of this range? would you expect if you were to put a 10kg weight on it that you would get a very close reading or would you expect it to be plus or minus a few kg's

 

[dcam]

The exciter voltage is supplied by the indicator. It can be all different voltages. When you buy a load cell ,a little paper in the packaging shows the mV per volt value. For example: If your exciter is 10 VDC and your mV per volt value is 2mV. Multiply 10x.002=.020V or 20mV. So at max capacity of the load cell you will see 20mV. Hope this helps. Look up wheatstone bridge to understand strain gauge wiring. Let me know if I can further help you.

 

[JesperMP]

the first thing that catches my eye is that when the load cell is connected to the signal conditioner i am getting a negative mV input, around -0.07mV to be exact

This is normal. There is always some errors in such a circuit, it will not be perfect. That is why in any weighing sequence you always tare the signal at the beginning of the sequence.

i was completely wrong about load cell weight limit, we are using a 500kg load cell and are expecting to measure around 300 kg

The load-cell must be overdimensioned. Dont chose a loadcell that only goes to close to the expected amount of material.

Most loadcells are very sensitive to overload or shock. Most loadcells can go to 150% before getting damaged. Some only to 120% ! We use loadcells that can sustain 200% load before damage. They are also more expensive.
edit: The WL280 loadcells your are using seems to be quite rugged. Not sure if I interpret the specs correctly, but they seem to be able to go to at least 200% without damage.

You probably have some kind of vessel that you fill the material into. The loadcell(s) must be able to withstand the load of the vessel plus material. So if the vessel is 100 kg, and the material is 300 kg, then you are actually very close to the upper limit of the load cell.

 

[Bobobodopalus]

ok thanks, that is fine then, we are actually using the load cell to measure the force applied onto an object we are expecting this force to be around 300kg.


as far as low end resolution goes would you expect this to accurately weigh 10kg's or would you expect that to be way off?

 

[JesperMP]

Look at the specs of the load cell.
There are absolute accuracy and repeatability. It is the repeatability value that is the important one, because you always tare the signal before measuring.

For a 500 kg loadcell from a reputable brand and type I would expect that it can easily reach a repeatability of 0.1%, i.e. +/- 0.5 kg in your case.

One thing to be aware when measuring small values in a system designed for larger values is that mechanical tensions built into the design may affect the measuring. It is not a problem of the loadcell, but of the other parts of the system. It is sometimes a trick to 'pre-tension' the measuring setup. For example weighing 10 kg in a system designed for 300 kg, you would apply dummy weight of f.ex. 50 kg, tare the scale, and then apply the weight to be measured.

One thing that disturbs my OCD, kg is mass, force is N.

 

[Bobobodopalus]

thank you very much i will have a go with that