PO4 test kits - Hanna VS Salifert

[Delphinus]

Maybe slightly off topic but being that I've been doing a lot of PO4 testing lately, I noticed something. I can tell when I'm about to get a wing nut reading, by when it looks like the colour didn't change at all rather than if it changes more than I expect. Ie. if I see a slight blue tinge to the water then I get a lower reading which tends to be inline with the day to day trend. But if it looks perfectly clear then I'll get a reading that puts me into the "WHOA WHAT HAPPENED IN THE TANK TODAY" territory. (Which of course I repeat the test and usually calm down when the next reading makes more sense).

I am using up a box of older reagents though, so maybe that's part of the issue in my case. I would say I get in the ballpark of 1 in 4, are these wingnut readings that don't make sense.