Conflicting phosphate test results

[Joe31415]

I had a weird reading on the Hanna once when I clicked the button too fast and went right from inserting to reading. The vial had a ton of tiny microbubbles I guess from the reaction of the reagent and the shaking

I've had that happen as well. Hit the button instead of holding it and I got a reading that was, I think, way too high.
Along those same lines, however, is that you make sure you shake it for the correct amount of time. I (like a lot of others, I'm sure) just shook mine until it seemed like it had been long enough. A few days ago, I started timing it and it turns out that what 'seemed like it had been long enough' was actually more like 30 seconds. Nowhere near the two minutes stated in the directions. Now I set a timer for two minutes and shake it (and include wiping off the vial as part of that) for the full two minutes. My phosphate level magically dropped from .15 to .06. Now, it's important to note that I recently added some PhosGuard, but only about half the recommended amount and for the first three days it was in the tank nothing changed. So, was it the PhosGuard or shaking the vial for a full two minutes?