I think that is why most people recommend the Hanna ULR phosphorus checker. It is +/- 5ppb phosphorus which is 0.015ppm PO4. The Red Sea may be +/- 0.02ppm but that is using a non-included photo sensor to determine color. it doesn't take into account interpretation error by the user.
Yes that’s very true, my eye is now the photo sensor with a wide error margin!
The Red Sea phosphate test arrived today, it’s better than the original one I used.
The test came out at about 0.06ppm, Hanna was showing 0.08 so a slight difference I guess. I’ve got both again now anyway, I just wanted to check if the Hanna was out or not.