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Kern, I read somewhere on this forum (probably at least a year or more ago) that you should never use distilled water with an Extech pH meter (or maybe with any pH meter, but definitely not an Extech). I don't recall the exact details of why ... but I have followed that information, with good results. I just clean with a bit of soapy water and then rinse in tap water, then dry with a paper towel. I do this just before using the meter. Then I clean again, dry again, and put it back in the cap with the 4.0 buffer.

Valley, I commented on your other thread - no way did your milk start at 7.9 pH; it should be more like 6.7. So if your meter is off by 1.2, that would make your 6.5 reading actually reflect 5.3 -- perfect pH for stretching!

I cannot think of a single scientific reason that using distilled water in the manner I described would be harmful to the instrument.  Some distilled water has ozone added to it for "freshness" and that might possibly be harmful as ozone is an oxidizer.  Outside of this though, nothing.  Tap water coming from rainfall is naturally distilled and likely has only picked up trace minerals before it reaches your tap.  I doubt that it really matters if you use distilled or tap water.

I couldn't think of any reason why it would be a problem, either ... but since, at the time, I had just gotten my meter, I decided to be safe rather than sorry! Sounds like the method each of us is using is working.