PH transmitter problems

MrQ · Oct 29, 2010

Hello,

I am having some poblems with a pH transmitter. MAybe someone here can help.

We have a pH probe mounted in a drinking water pipe with flowing water. It is a flat face probe designed to be in a flow.

The pH of the water in the pipe is about 7.5.

Al of the sudden the value displayed on the transmitter starts to drop slowly following a straight line. Maybe from 7.5 to 4.5 in two hours. We have a handheld meter that verifies that the actual value is around 7.5 Both the display and the 4-20ma signal drops.

The cable to the probe is screened.

The transmitter is mounted in front of a cabinet. The cabinet contains a small PLC, display, powersupplies, some small relays. It is supplied by 230V AC 50Hz. No threephase or motor control.

Any ideas? EMC? If it was electrical
disturbance I would say that it would be more noise than a decreasing signal.

sthompson · Oct 29, 2010

MrQ said:
Hello,

I am having some poblems with a pH transmitter. MAybe someone here can help.

We have a pH probe mounted in a drinking water pipe with flowing water. It is a flat face probe designed to be in a flow.

The pH of the water in the pipe is about 7.5.

Al of the sudden the value displayed on the transmitter starts to drop slowly following a straight line. Maybe from 7.5 to 4.5 in two hours. We have a handheld meter that verifies that the actual value is around 7.5 Both the display and the 4-20ma signal drops.

The cable to the probe is screened.

The transmitter is mounted in front of a cabinet. The cabinet contains a small PLC, display, powersupplies, some small relays. It is supplied by 230V AC 50Hz. No threephase or motor control.

Any ideas? EMC? If it was electrical
disturbance I would say that it would be more noise than a decreasing signal.

Make and Model of the PH Probe?

Stu.....

leitmotif · Oct 29, 2010

pH is (was?) commonly measured using conductivity. IF the conductivity of water increased suddenly then that may affect pH reading.

One possible cause is sudden flow in water mains ie fire department fighting a fire. High flow in mains shakes loose all the rust and sediment in pipes.

Did you check probe in a standard pH solution? That is the first thing I would do.

Dan Bentler

MrQ · Oct 29, 2010

Hi,

It is FLS/ChemX3

http://www.flsnet.it/page/chemx3.php

Gil47 · Oct 29, 2010

I have seen a similar situation.

My cure was to make sure I had an earth near or around the probe but not touching the probe itself .

My feeling was that the material of the pipe and near a bend was allowing a skin charge to develop, which was affecting the PH probe and causing the creepage.
I never resolved the issue I just lived with the earth till we shifted the probe when we installed new PH controls

leitmotif · Oct 29, 2010

If this could happen (and why not?) then it would be worsened with plastic pipe - uh would it not?

Dan Bentler

MrQ · Oct 29, 2010

Gil47 said:
I have seen a similar situation.

My cure was to make sure I had an earth near or around the probe but not touching the probe itself .

Tried this today I connected a wire from the earth in the cabinet to the pipe. My problem was that today I could not force the problem to show at all. Hate this type of problems

MrQ · Oct 29, 2010

If my problem is not due to grounding but instead of interference. Could a ferrite core like the one used on the cord from a laptop powersupply be a solution??

Gil47 · Oct 29, 2010

I will throw another suggestion

If in the cable used from the PH probe to the Meter you have wire cores that are not used tie them down to earth at one end as they possibly could start picking up a capacitive charge that might affect the signal

My feeling is that I would not expect the ferrite to make a difference unless there was some high frequency noise source nearby

MrQ · Oct 30, 2010

After the pH Probe(1m) on the pipe some water is diverted to go in to a electrolyze equipment(2 meters hose). The flow in the main pipe can be 10-100 lpm and less than a liter per minute is diverted.

Can electrical charge from this equipment affect the conductivity in the water and affect the pH probes reading.

danw · Oct 30, 2010

>Can the process/piping affect a pH reading?

Yes. The pH probe's electronics are measuring picoamps of current. The circuit can be affected by external potentials in its installed environment.

A grab sample in a glass beaker on a lab bench is well isolated electrically. A process might or might not be.

There are some installations where I could find a solution. Once, I discovered the AC powered analyzer wad not earth grounded. Another time I removed the earth ground ( the 24Vdc power supply had its negative side earth grounded.

Before you chase stray voltage or grounding issues, have you calibrated the probe to check its response?

Is the process changing temperature over the time that you see this downward drift in pH? (is the temp comp circuit functional?)

leitmotif · Oct 30, 2010

danw said:
>Can the process/piping affect a pH reading?

Before you chase stray voltage or grounding issues, have you calibrated the probe to check its response?

Is the process changing temperature over the time that you see this downward drift in pH? (is the temp comp circuit functional?)

I said it before and I say it again
HAVE YOU CHECKED THE PROBE IN A KNOWN SOLUTION?
That is two of us suggesting this.

Uhh have you even checked the pH in your system?

Dan Bentler

MrQ · Nov 2, 2010

leitmotif said:
I said it before and I say it again
HAVE YOU CHECKED THE PROBE IN A KNOWN SOLUTION?
That is two of us suggesting this.

Uhh have you even checked the pH in your system?

Dan Bentler

Sorry that I did not reply to above but yes of course I did check in a known buffer solution.

However today I resolved the problem. The Display/Transmitter had a PT100 input for temperature compensation. The probe that is used has no PT100 and the temperature is quite steady.

When there was no PT100 connected the terminals for the PT100 where supposed to be connected together. Did so and all of the sudden it worked.

leitmotif · Nov 2, 2010

Darn it - doncha just hate it when they work better after all bits n pieces are connected?

Dan Bentler

TimWilborne · Nov 2, 2010

MrQ said:
When there was no PT100 connected the terminals for the PT100 where supposed to be connected together. Did so and all of the sudden it worked.

Are you sure you are suppose to just connect them together? Usually there will be a chart or formula to figure out what size resistor to put across the terminals for a given water temperature. If it requires a resistor you will not get a linear value with a jumper

PH transmitter problems

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