pH meter drift measuring H2O but not wort

[philipCT]

I recently replaced the probe on my Hanna pHep 5. This fixed some strange readings I was getting during calibration and measurement. Now I see a different problem:

When I measure the pH of my tap water, which I do while setting up for every brew, or my sparge water to check on the effectiveness of the salts, the meter will get to a seemingly stable reading after a minute or two, but then it will drift upward very slowly over the next 5-10 minutes. So I'm talking about an initial reading of something like 5.8 that drifts up to 6.4 over 10 minutes.

Normally, I would think it's time to replace the probe, but (1) I just did that, and (2) I don't have this same problem when measuring wort.

Anyone have any idea why this might be happening with water but not with wort?

Answers to the most obvious questions first:
-yes I calibrate before every brewday and the calibration is behaving exactly the way it should according the the Hanna pHep video, and displays that little message indicating calibration success
-yes, I carefully store the probe in storage solution after every brewday
-no, I never put the probe into hot stuff. I cool all samples to room temp first
-yes, the readings I get on the wort are very much in line with predictions made with my Brunwater profiles, so all seems well except for the wacky water readings

 

[mabrungard]

Tap water under pressure can have excess CO2 that can come out of solution when exposed to atmospheric pressure. Maybe that is the reason the pH is changing. I suggest letting the water sit and equilibriate for a while, then check how your meter performs.

 

[ajdelange]

I think Martin's suggestion an excellent one particularly if you are on a well. In dissolved CO2 is responsible the pH should drift upwards slowly over time. You can speed CO2 exit by shaking the sample or pouring it back and forth between two tumblers so you might want to try that.

Very low in mineral content in the water can also be a cause of unstable pH readings. The pH measurement depends on current flow, tiny though it be, and pure water is a pretty good insulator. Adding some table salt to the water should overcome this effect.

 

[philipCT]

Thanks, Martin & AJ. I'll try those things and post back results.