Temperature Impact on pH Probe Longevity

I’ve got another great example of an email question I received and answer on which I was cc’ed. It needs to be lifted out of the depths of email inboxes and sent items folders into the spotlight in the hopes it helps others who may have similar questions.

The question:

We are working on a project where the water overflow from a hot lime softener needs to be measured for pH (typically about 9.5) and we also need to take a sample. The sample needs to be cooled to 50 degC which we can do with a heat exchanger. Do you have any information as to the longevity of pH probes if the sample were cooled to 50 degC as opposed to cooling it further to 25 degC?

I checked with Emerson’s Dave Joseph, a senior industry manager in the Rosemount Analytical Liquid business. Dave responded:

Your question about pH sensor lifetime is actually quite complicated. pH sensors can fail due to glass breakage, glass coating, glass depletion, reference depletion, reference poisoning, and reference plugging, and that’s just to start. Each of these effects are aggravated by temperature to one degree or another, so depending on the expected failure mode, temperature can be more or less of a factor.

Our general rule of thumb is that for every 25 degree C you can expect to half the ideal lifetime of a pH sensor. For instance, if a sensor lasts 2 years at 25 degree C, it might last 1 year at 50 degree C. Note, however, that getting your sensor really cold also tends to decrease lifetime so don’t cool the sensor down to freezing expecting a 4 year lifetime.

Although we have sensors that can operate at temperatures exceeding 120 degree C, it is generally good practice to cool samples down to around 50-60 degree C to prolong their lifetime (by a rough factor of 4-6). This is especially true for high pH samples (over 12) that deplete the glass very quickly. In your case, I would not expect that further cooling the sample much below 50 degree C would justify the added piping complexity.

Thanks for letting me pass on your wisdom to others, Dave! Also, for those who may have come upon this post looking for answers to their pH control challenges, make sure to visit the pH category on the ModelingAndControl.com blog.

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