Resolution issue

[Jezz]

Hi All,
We have 5 silos which can be filled with water/ milk each contains 50,000 to 70,000 litres now the volume is measured on load cells giving 4-20mA.The problem is when the silo reads empty there can still be a few 100 litres of liquid in it does any one know of a way of getting a better resolution at the low levels as we can end up with 300+ litres of product going down the drain..

Thanks Jezz

 

[drewcrew6]

are the load cells "calibrated" for empty and full?
or is it a matter of incorrect sizes for the load cells?

Drewcrew6

 

[Steve Etter]

Jezz,

Sounds to me like drewcrew6 is right on it. You either have a calibration issue, a load cell size issue, or a design issue.

First, if you are not already checking the calibration of your load cells, this is where you need to start. When you do your calibration the more known weight that can be applied the better, so apply as much as you can. One technique in your situation might be to fill the tank with a known quantity of clean water and calculate the weight using 8.333 pounds per gallon (or 2.2014 pounds per liter).

Second, check that your system is right for the application. A properly sized load cell system will take into account the dead weight of the empty silo itself and then the full weight based on your heaviest/most dense product. If you know these two factors, any reputable load cell manufacture can quickly tell you what size the load cells should be. Someone had to specify (or at least agree to) the acceptable resolution for your system and it is possible that +/- 0.5% accuracy was deemed acceptable at that time. Anyway, you need to find out what the specs on the load cells are and see what the accuracy and repeatability are.

Last (and least likely), check your system design. There should be no external loads on your silos. Nothing leaning on them, no wind allowed to hit them, and no major temperature fluctuation (although many load cells are temperature compensated, I don't know about yours). Are there multiple load cells for each silo? If so, has each been tested independently? If not, your system probably pivots somewhere and you are really only measuring half the load. Check for binding.

Steve

 

[Jezz]

D'oh.... The system is 30 years old the original spec was not quite so tight on wastage so I think you've hit the nail on the head I think we'll have to look into up dating the load cells. They are calibrated regularly with known quantity's of water so the calibration should be ok..

Thanks Jezz

 

[Gerry]

As you initially stated - it's a resolution issue.
70,000 litres of water = 70,000 kg (milk is very similar)
If you're reading the signal in a PLC with 12-bit analogue inputs, then your best resolution is 70,000/4096 = 17.1 kg. You must also consider any possible non-linearities in the system and absolute accuracies.

Here in NZ (land of dairy factories and 200,000+ litre silos) the typical setup is a DP-cell level indicator with float switches for low and high level. The low level (empty) is installed in the gullet of the silo.

 

[JesperMP]

I would like to comment three things:

1. Like Gerry says, there should be level switches to safeguard the analog signal. The risk is too great, of overfilling (in some cases), or in your case of lost material ("underemptying" ?) when the control is based on the analog signal only. Level switches are so cheap that it really isn't an issue.

2. If your control can get a reliable "empty" indication, you can memorize that the tank is empty in the control program, and the moment that filling starts, automatically tare the analog signal.
That should improve the accuracy of the analog signal at the "empty" level.
I do this in my batching programs all the time.

3. Why dont your control program have an extended time for emptying the silos. It is a simple way of making certain that the silo is really empty.
I do this as well in my batching programs.

 

[shooter]

use a liquid sensor just above the bottomvalve of the silo if it is empty then silo is really empty.
if sensor gets wet the signal of load cells can be put on zero, next time of emptying tanks it will be very close.