There have been 2 (if I count them correctly) reports of this varying pH problem (one on the TroubleFreePool site) with the first test being the one that is different from subsequent tests. I don't know what is going on, but will describe what I do and I'll have to test multiple times to see if I get any variation (I think I've done that a few times and don't recall any variation). Whenever I take a pool water sample, I flip the comparator tube upside down and put my hand about a foot or so (probably 18") below the water surface and then flip the tube so it becomes upright and fills with water. I then take the comparator out (now filled with water), then invert it and dump the water back into the pool. Then I go down again (inverted) and repeat to get a water sample. This sample is the one I then spill out excess into the pool (to get to the appropriate line, which for pH is the 44 ml line near the top). I cap the tube (which I rinse in the pool water first) and then take it back for measurement, adding the 5 drops, capping, inverting a few times, then looking at the color. After I'm done with the test, I dump the sample into the sink and rinse out both the tube and the cap with filtered water (though tap water should be fine -- we just happen to have a filtered water faucet at the kitchen sink where I do my tests so I use it).

If the sample is shaken instead of inverted, then it's possible for the aeration to make the pH rise a little. That's one possibility, though doesn't explain why only the first sample is off. It could be that though the tube is cleaned, the cap is not. That would tend to make the first sample bad if the previous test was not a pH test.

Richard