PO4 test kits - Hanna VS Salifert

[GoFish]

Makes sense. Just wondering why the Hanna says .221 ppm at the same time the Salifert has barely even a hint of blue in the range of 0.00-0.03 where it's should be showing a darker colour. I'm not a fan of the colour guessing game with Salifert but positive the shade is no darker than 0.03, let alone in the .1 - .25 range

I'll carry on using the ULR, I just thought the 2 were similar in accuracy