Hanna and Salifert getting different results?

[zaphod42]

I have been trying to lower phospates in my 40g breeder that's been running for about 2.5 years. I have lowered them from about 0.6 ppm a month ago to about 0.15 ppm now, according to my Salifert PO4 test kit. I am running low on reagents, so I bought a Hanna checker for Marine Phosphate ULR. It arrived today, I ran my first test and it said 0.29 ppm. Since I am just looking to trend down, it doesn't really matter whether I'm around 0.15 or twice that, but I am curious as to why there is a difference. Any ideas or suggestions? Thanks!

 

[Randy Holmes-Farley Verified Community Expert]

Testing uncertainty is a fact of life in reefing. Some kits, such as magnesium, seem to give folks so much variability that I do not recommend using them at all.

More folks will say believe the Hanna, and I use a Hanna for phosphate, but, of course, I cannot say whether that is correct in every case. Issues of old or bad reagents can plague some types of testing.

 

[mook1178]

Adding to what Randy said, both kits are colormetric, looking at a color change and intensity. the problem i had with the Salifert is it is hard to match colors on their scale sometimes. I like the Hanna because it uses light absorption measured by a spectrometer. The problem there is to be sure to have a good blank reading, then not touch the glass part of the cuvette when taking the measurement. If you do touch and add oil, the light absorption will be off, making the measurement off.

I use the Hanna and use Kimwipes to clean the cuvette before hand. I then use the Kimwipes to hold the cuvette until I am done.