mrcceo,
Thanks for sharing the test results. I assume that during the test the samples were stirred or the probes were agitated. I tend to see drifts too, but only when I don't move the probe. Also, were the samples well buffered? I think the lab grade pH meter really shine when the sample's pH is very weakly buffered. Soft water is a good example. But most solutions we test in brewing are strongly buffered.
David,
Keep in mind that the simple Hanna meters are advertised as replacements for strips. They seem to have a comparable accuracy. Given the precision with which we want to hit the mash pH, 5.3 - 5.6 is good, they should be acceptable. But if you are technically inclined you may look for more accuracy in the future. It is my opinion that a good simple pH meter with buffers and storage solution can be had for under $100. That is the SM101 that I mention in the article.
Kai