Magmeters flow errors

plcsoftvsd

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Sep 2007
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Greetings,


Has anybody has experience with milk flow meters magmeters (24vdc) with 4-20 ma output with errors in flow rate.

We have some magmeters which start showing 20% higher flowrate for some time (couple of hours) and then stabilise.

I cannot reveal the meter manufacturer's name for obvious reasons,..but in general what can cause such variations of flow.

I have to mention here that there are 25 flowmeters connected to one power supply and there are 6 power supplies in total for 150 milk flowmeters.

All ideas are appreciated.
 
Greetings,


Has anybody has experience with milk flow meters magmeters (24vdc) with 4-20 ma output with errors in flow rate.

We have some magmeters which start showing 20% higher flowrate for some time (couple of hours) and then stabilise.

I cannot reveal the meter manufacturer's name for obvious reasons,..but in general what can cause such variations of flow.

I have to mention here that there are 25 flowmeters connected to one power supply and there are 6 power supplies in total for 150 milk flowmeters.

All ideas are appreciated.

Sure you can mention the manufacturer's name.
I know in the early '90s E&H Promag meters had issues with conductivity of the liquid it was measuring. On water meter would display one flowrate, then when high brix sugar products were sent through meter, another flowrate would be displayed. This has since been resolved.

MassFlow and MagFlow are sensitivity to foam and turbulance of the product being measured.

If these are existing meters that have worked for years, has anything else changes?

Such as new pumps installed, new control loops added, new type of product being processed?
 
I cannot reveal the meter manufacturer's name for obvious reasons
I don't understand this statement. You sure can. Not that it makes any difference though.

Any possibility of air (bubbles) getting into the system?
I agree mag meters don't like air, the meter needs to be in a low spot, is it?
 
1) Have the errant magmeters ever worked properly, without errant high readings?

2) How do you know that the readings (when errantly high) are not correct?

3)
I have to mention here that there are 25 flowmeters connected to one power supply and there are 6 power supplies in total for 150 milk flowmeters.quote]You have 6 'groups' of mags grouped by power supply. Are the errant magmeters all in one power supply group?

4a) Is the faulty/errant high reading a step change or a gradual increase in reading?

4b) When the reading stabilizes (corrects itself), is the correction a step change or a decrease in reading?

5) Are the errant mags all of the same model/series?
 
Best way to avoid bubbles is to install flowmeter upward, the fluid must travel from bottom to top, any disturbance either downstream or upstream must be at least 10 diameter away from the flowmeter.
 
Danw,

1. Yes they have worked properly in the past.

2. By actually measuring it.

3. They are in one supply group, but this group keeps changing and few meters from adjacent group also show igher readings. ( here i would like to mention, the readings are always higher, not lower).

4a. The reading is step change

4b. The reading is a step correction.

5. All are same meters.

The meters are at an 45 deg angle and they have the milk flowing down.

cheers.
 
One problem was found, one UPS was installed for the all 24 dc power supply and during that process phase and neutral wires got interchanged.


So the 24 vdc power supply had the AC side L and N changed. Will this make any difference?

And should the earth terminals of all these 24 dc power supplies should be connected to ground or left open( there are two earth terminals of the power supply one on the AC side and one on the DC side) as shown below.

230 vac in.

L N E



+ - E

24 VDc out


Should both sides E terminal be grounded or only the AC side needs grounding?

What is the recommended way for earthing to avoid noise?

all ideas appreciated.
 
Hi,

I don't think the polarity change L-N is the cause of your flowmeter problem. The DC side of the power supply doesn't need to be earthed if it is only being used for analogue circuits but the AC side should be.

Do the analogue input cards on the PLC have isolated channels?
If the channels are not isolated, noise on one channel can affect the signals on the other channels.

Do the bad flowmeter signals all occur on the same analogue input card?

Is the value displayed on the flowmeter itself correct and the analogue signal incorrect? Or is it both values?
 
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Hi plcsoftvsd

You most be sure you have grounded your meters. What sort of pipe are you using before and after the meters. If it is PVC pipe ground is very importend.

Banker
 
4a. The reading is step change


The meters are at an 45 deg angle and they have the milk flowing down.

.

These 2 statements to me leave me thinking that air bubbles are moving past the flow meter and I know from practice that air bubbles cause step changes.

You need to look at the installation instructions but I feel 45 deg angle mounting of mag meters is not recommended
 
These 2 statements to me leave me thinking that air bubbles are moving past the flow meter and I know from practice that air bubbles cause step changes.

You need to look at the installation instructions but I feel 45 deg angle mounting of mag meters is not recommended

Correct, the flowtube must always be flooded.

If mounted horizontally must be at a low point in the pipe, paying attention to the recommended distance from pipe elbows.

And if mounted vertically must be mounted in pipe section where flow is rising.
 
You say the flow is down, I think that's your problem
I have installed a mag with flow in the downward direction (vertical pipe)where it was the only choice however the signal from it is eratic. I suspect what is happening is air trapped in the line is causing the milk to travel in only part of the pipe resulting in a higher velocity at the electrodes. As you know, mag meters measure velocity.
After a while the system finally gets rid of the air and so the reading drops to normal.
I have tried mag meters in vertical down situations where I was certain no air existed however the signal is erratic. I will never do that again.
Roy
 
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Air bubbles will effect but these meters have worked well in the same setup for a year.

The milk is not flowing continuously and is released in batches.

The probes and at the bottom end of the pipe and hence the probes are always dipped in milk.

The meters have plastic pipes on both sides.The meters are not grounded.

If they need to be grounded will they require grounding on both sides or only on one side of the meter.
 
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