pH Measurement Drift

  • Thread starter (TECO) David Bergeron
  • Start date

Thread Starter

(TECO) David Bergeron

We have an application where we are measuring pH in a liquid in a pipe at the exit of a tank. A voltage potential builds up between the liquid and ground and causes the pH measurement to drift with this voltage.

Has anyone experienced this problem with pH measurement, and what solutions exists to deal with this problem?

David Bergeron, P.E.
Thompson Equipment co.

R A Peterson

I have seen this with plastic probes. the solution was too use stainless steel piping, well grounded where the sensor was located.

Bob Peterson
This is not that unusual. Chlorine residual analyzers have battled this for years. Any amperometric cell must be grounded with respect to stray voltages carried by the fluid, especially with a highly conductive fluid. Look for a ground loop and separately ground the pH meter. In addition, you might ground the pipe itself at the point of measurement to a separate earth ground than the pH meter is grounded to.

Walt Boyes

Walt Boyes -- MarketingPractice Consultants
[email protected]
21118 SE 278th Place - Maple Valley, WA 98038
253-709-5046 cell 425-432-8262 home office

Al Pawlowski, PE

I have worked on many pH measurement systems and have not seen a real case of drift due to voltage in the measured liquid. The industrial
measurement sensors always that I have seen always have an electrode to eliminate these kinds of potentials. On the other hand, most lab or
portable units that I have seen do not.

Drifts I have seen have been due to very large temperature swings, reference electrode plugging, measurement electrode age and fouling of both. Perhaps one of these is the problem and more frequent/careful care would solve the problem.

In any case, please post any solution that works.
I have worked on many pH measurement systems and have seen a lot of electro-chemical drift due to voltage generated within the process (chemical reaction in progress). The grounding pin or grounding ring on the pH electrode will eliminate this kind of volatile off-set.Verification of that type of problem can be confirm by using portable eqpt with batteries.They are not affected
because they are isolated somehow...
Hanna Instr probe pH probe #1006 is a good sensor to address this problem ,if no temp corr is required else use the 396 from Rosemount Instr if money is not a restriction

Anthony Kerstens

Caution: pH probes are small batteries themselves, with an impedance on the order of 10MOHM. Impedance matching through a unity gain op-amp is a usual method of isolation because
opamps have input impedances of a similar order of magnitude.

This is also why it's not advisable to connect lower impedance devices such as multimeters, PLC I/O, batteries...... unless you want to immediately damage the probe.

That said, there are probes that come with an interface circuit incorporated into them. Check Omega for 4-20mA ready probes.

As for drift, another problem is that probes have a limited life. They contain a solution that leaks out, generating the mV potential. Osmotic resistance from the mixture being measured limits this flow, hence the measurement.

In short, pH probes are a consumable item, unlike other transducers.

Anthony Kerstens P.Eng.