If you get consistent readings with the Hanna checker using the 3 minute timer as you describe then I'd feel fairly confident in its numbers. Assuming of course you have newish reagents - if memory serves there was a time period that there were bad batches of reagents (fairly easy to confirm if you had one of the bad batches - just do 2 tests in a row and see if the #'s match well enough or not).
Not sure how this applies to Salifert's PO4 test kit but I noticed that with the Mg test kit (which is a titration based test kit rather than a colour comparison, so this might be an apples-to-oranges comparison) that the reagents would expire and then you wouldn't get good readings anymore. So if the test kit is more than say a year old, I'd consider replacing it.
If in doubt though, I think repeatability is the real pathway to figuring out which is the more trustworthy result, if you know someone who has a third test kit that you could ask for a test, see if you can get a value that corroborates one for you over the other..
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-- Tony
My next hobby will be flooding my basement while repeatedly banging my head against a brick wall and tearing up $100 bills. Whee!
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