4-20mA query

grousedogtom

Hi, I have a 4-20mA hydrostatic level transmitter fitted in a tank of water, it is connected back to a 24volt dc power supply, if I break the loop to take a mA reading with my multimeter I get a current reading of 4.25 mA when the water level is 750 above the transmitter . The scale on the transmitter is 0-30 meters, I don't think this calculates out right, could someone give me there advice please.

ted1

grousedogtom wrote: »

Hi, I have a 4-20mA hydrostatic level transmitter fitted in a tank of water, it is connected back to a 24volt dc power supply, if I break the loop to take a mA reading with my multimeter I get a current reading of 4.25 mA when the water level is 750 above the transmitter . The scale on the transmitter is 0-30 meters, I don't think this calculates out right, could someone give me there advice please.

It should read .486m
Where are you getting 750? That sounds like a 12 bit number. Are you using an offset?

grousedogtom

ted1 wrote: »

It should read .486m
Where are you getting 750? That sounds like a 12 bit number. Are you using an offset?

I measured 750mm from the transmitter to the top of water level.

2011

For these problems use the equation of a line, y=mx + c

y corresponds with the 4 to 20mA output and x corresponds with the level of water in the tank.

c = the offset, which is 4.

m = the slope of the line. See formula here:

http://www.purplemath.com/modules/slope.htm

Where:
x1 = 0
x2 = 30
y1 = 4
y2 = 20

From this, m = 16/30

So we now have the following equation:

y = 16/30(x) + 4

Insert 4.25 for y and solve for x.

y = 16/30(x) + 4
4.25 = 16/30x + 4

So x = 0.46m

What does this tell us?
An output of 4.25mA should correspond with a tank level of 0.46m

Does this make sense?

What is the tolerance on the level transmitter?
Where is the transmitter mounted on the tank? - Frequently when using this method the transmitter does not provide a reading for 0m.
Are you using a differential pressure transmitter? i.e is there a cell top and bottom of the tank?

ted1

grousedogtom wrote: »

I measured 750mm from the transmitter to the top of water level.

What are you to record or display the reading? You might be to adjust the scaling.

I often see poor scaling 4-20 devices, we deal in energy managment so tend to record the data in datalogger sand get get the software to correct the signal

2011

ted1 wrote: »

What are you to record or display the reading? You might be to adjust the scaling.

Good question.
I assume that you are using a calibrated test instrument for this?

Any displays on the loop should not be relied upon unless they has been calibrated/checked already.

I often see poor scaling 4-20 devices

Do you mean that the analog signal is correctly calibrated but does not have the correct scaling factor applied?

ted1

2011 wrote: »

Good question.
I assume that you are using a calibrated test instrument for this?

Any displays on the loop should not be relied upon unless they has been calibrated/checked already.



Do you mean that the analog signal is correctly calibrated but does not have the correct scaling factor applied?

I often see 3.7A when it should be 4A or 19.5A / 2.5A when it should be 20A

We usually deal with flow meters, weight, temp probes, moisture meters etc.

The temperature probes are usually ok. But the like if moisture probes tend to be poor

2011

ted1 wrote: »

I often see 3.7A when it should be 4A or 19.5A / 2.5A when it should be 20A

I assume you mean mA
Why do you do this?
I'm just curious.

We usually deal with flow meters, weight, temp probes, moisture meters etc.

The temperature probes are usually ok. But the like if moisture probes tend to be poor

PT100s for temperature I take it? They are very robust.

What instruments are you using that you have issues with the ingress of moisture?

ted1

2011 wrote: »

I assume you mean mA
Why do you do this?
I'm just curious.



PT100s for temperature I take it? They are very robust.

What instruments are you using that you have issues with the ingress of moisture?

Your correct I do indeed mean mA.
With regards moisture, its actual moisture probes themselves we monitor the moisture of raw materials and determine if its more economical to dry the product before running it through the machines.

Yes we use PT100 converted to give a 4-20mA signal. And generally have no problem with these


We use a A/D data logger and rather than getting a third party to fix there gear which they are pretty hopeless at. We can adjust our settings. As long as we scale 0 and full signal we get accurate results

2011

ted1 wrote: »

I often see 3.7A when it should be 4A or 19.5A / 2.5A when it should be 20A

So you use a range of 3.7 to 19.5mA, just wondering why?

I use a 4 to 20mA signal for 0 to 100% of the range.

If the signal is .8 mA the control system generates a "bad transmitter wire break alarm" and if it is >20.8 mA it generates a "bad transmitter short circuit alarm".

With regards moisture, its actual moisture probes themselves

Makes sense, I misread your post

ted1

2011 wrote: »

So you use a range of 3.7 to 19.5mA, just wondering why?

I use a 4 to 20mA signal for 0 to 100% of the range.

If the signal is .8 mA the control system generates a "bad transmitter wire break alarm" and if it is >20.8 mA it generates a "bad transmitter short circuit alarm".




Makes sense, I misread your post

Ideally we use 4-20mA but often find that the instruments aren't giving a correct signal. So we adjust accordingly

It's more hassle to generate bad signals, especially when we can correct.

2011

ted1 wrote: »

Ideally we use 4-20mA but often find that the instruments aren't giving a correct signal. So we adjust accordingly

It's more hassle to generate bad signals, especially when we can correct.

Do you mean you make changes to the offset and scaling factor in the code rather than recalibrate the instrument due to poor repeatability ?

ted1

2011 wrote: »

Do you mean you make changes to the offset and scaling factor in the code rather than recalibrate the instrument due to poor repeatability ?

Yes, often guys fitting the instrument havn't a clue what there doing and can't calibrate them.

so we'll get them to send us 0% and then 100% and then a few in between. we can adjust the scale and offset in our data logger e.g
our A/D will scale from 0 -3560
4mA should be 710 and 20mA should be 3560
we may to set 4mA to be 610 and 20mA to be 3000. when we measure 0% and 100% they are not exactly 4mA and 20mA.

2011

ted1 wrote: »

Yes, often guys fitting the instrument havn't a clue what there doing and can't calibrate them.

That is not good :eek:

We carry out a hot loop test by injecting the loop at 4, 8, 12, 16 & 20mA with a calibrated test instrument and document same. This proves that the loop, the scaling factor, offset and automation are in order. If it is found that the readings are incorrect we park this problem with the instrumentation technician. The instrument should have a signed bench test calibration sheet anyway. That way you have somone to hold accountable if the instrument has been incorrectly calibrated.

so we'll get them to send us 0% and then 100% and then a few in between. we can adjust the scale and offset in our data logger e.g
our A/D will scale from 0 -3560
4mA should be 710 and 20mA should be 3560
we may to set 4mA to be 610 and 20mA to be 3000. when we measure 0% and 100% they are not exactly 4mA and 20mA.

That would work, although it should be unnecessary if the instrumentation techs knew what they were doing. If the instrument is changed at a later date for an instrument that is correctly calibrated you are in trouble though


I can see why you do it but this solution would not be permitted with any GMP instrumentation.

grousedogtom

I will be connecting it to a simex display unit like this,http://www.omniinstruments.co.uk/products/product/moredetails/simex.id793.html, but at the moment I just connected it to a power supply and took mA readings with my meter. I took similar readings with a 0 to 4 bar pressure transducer and got 4mA at 0 bar pressure, 8mA at 1 bar, 12mA at 2 bar, 16mA at 3 bar and 20mA at 4 bar , all calculating correctly. I just taught i should be able to determine the correct level of water in the tank from the mA reading on meter similar to the pressure transducer results. The meter I'm using is calibrated.

2011

You need to carry out a hot loop test on the instrument loop to see that the loop is good. Check that the loop is providing the correct readings back to the local display (and/or control system and or/HMI) when you inject 4, 8, 12, 16 and 20mA.

The you need to bench test the instrument. Calculate the pressure to be applied to the diaphragm at 0, 25, 50, 75 and 100% of the water hight. Apply the pressures with the aid of a suitable calibrated gauge and record the mA output from the instrument. Adjust span and zero as required.

Reassemble and verify that the readings are correct.
This can be achieved by measuring the water level (dip test) or by batching water in with a calibrated flow meter.