We use pressure transducer to calculate volume of mik in silos.

I wish to calibrate the levels.

What is the best method to check the transducer ( 4- 20 ma) apart from measuring current.

chaterpilar

Not quite sure what you are askig for here, or trying to do. More detail??

Best regards,

We have a 90000 litres milk silo which has a digital display for the volume of milk stored.

There is a pressure transducer which sends 4-20ma signal to a PLC which converts it to volume.

It has been noticed by the operator that the displayed volume is always less than the actual volume , and the error increases with the volume of milk stored..( if the error is say 500 litres at 20000 litres, it becomes 4500 litres at 70000 litres).

Now i want to remove this error. ( span and zero pots are available on the transducer ).

What is the best way to remove this error?

Hello Chaterpilar

If you have known conditions, then you can work on adjusting the span and offset, but it is an iterative process and must be done in the right order. If you tweak the wrong pots, you will get further and further away from the correct setting.
Unfortunately, because of the application it may take some time to get it right unless you can put the transducer into a controlled environment.

Essentially you adjust the span when the output of the transducer is at it's maximum, and you adjust the offset when it is at its minimum to give the correct readings.
If you were able to provide a controlled environment, you would set the pressure to what should be full scale and adjust the span for 20mA, then you would adjust the pressure for zero and adjust the offset for 4mA. You need to repeat this procedure for as many times as it takes to get to an acceptable accuracy.

If you are unable to calibrate the transducer in a controlled environment, then you will need to do it in situ. Provided that the level in the silo changes between limits, (hopefully full and empty) you can make a span adjustment when the silo is fullest, and an offset when the level is lowest.
If the lowest level is not zero, I would recommend adjusting the offset at the lowest level to halve the error each time, but the span to eliminate the error at the highest level.

Before making any adjustment, determine what the error is at both the maximum and minimum level.
If the error is the same at minimum and maximum, then start with an offset adjustment at the minimum level. If the error is greatest at the maximum level, then start by adjusting the span at the maximum level.

Good luck,
Best regards,

Hello Chaterpilar

What type of cable you are using from transducer to PLC and what is the length?
The cable must be separated form power cables. May be some external noises are creating error.

Thanks Marke/Ab, for your inputs.

Cable is screened and grounded at one end and is in a separate conduit away from power cables.

Chaterpilar

To get an accurate measure of tank level and hense volume you will need a differential pressure transmitter, and you will also have to take into account the specific gravity of the liquid you are measuring as this affects the density. As your milk silo fills up how much cream is sitting on the top. Cream is more dense than milk and will change the level reading.

cheers niallnz

Hello Chaterpilar

In this type of measurement, the signal goes to PLC, converts into decimal value by using formulas in ladder logic of PLC program.
I am assuming that the same methodology is used in your existing system. Now for proper calibration of transducer, you should prepare a jar in same length as the milk tank is. The diameter of jar is no concern; you may prepare it according to diameter of the rod/s of transducer. Now, fit the transducer in the jar and put the milk at the lowest range of transducer and note the reading then fill the jar at the maximum limit of transducer and note the reading. Now you can easily calibrate it.

Thanks everybody..i used a calibrated flowmeter to fill up the tank and adjusted span to show the exact volume.

i did this at stages of 10,000 litres and continued upto the brim ( 90kl).

works fine now...appreciate all inputs....great forum.