Phosphate test - Hanna Vs Salifert

[Aqua Monster]

Dear seniors did a phosphate test with Hanna n salifert test kit but both shows different result. Pls advice

Did Hana test few times using same sample, every time is deferent result range btwn 1.4 to 1.8

 

 

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[R0B]

First off kudos for having two test kits, that is simply awesome but agreed this is perplexing having a difference if the tests are both accurate...

There are many reasons a test may vary especially with $20+ hobby kits vs. a lab. Remember this is testing for ultra low readings so anything could have thrown either test off. Some people say the Salifert is not accurate for phosphate unless you are treating very high levels. (I personally use it). Also, if you want consistent results you need to treat this like a lab: Keeping the test kits in perfect storage conditions; making sure all reagents are not out of date nor that they contaminated; throughly cleaning all the viles with acid so no trace elements exist; performing the test accurately and to the letter, so swirling for exactly 30 seconds; making sure the powder reagent is exactly level; that no air bubbles were in the test syringe; and that water levels were exact; Plus remember you are using your eyes to spot colour differences against a graduated card, all of which could be impacted by the intensity level and kelvin of light you reviewed the test with not to mention any reflections. There is so much that could impact a result I am surprised we get any accuracy at all which is where you need to set your expectations. For me it is more about getting in a useable ball park with consistent results over time.

Which leads us to exactly why digital testers are popular, they 'should' be better at measuring the results if the test is performed accurately. However my experience is that they can vary rather widely and I think you also have similar experience. I once tested four times the same water and got significant different results each time. Basically these tests are as accurate as you make them. so read the instructions multiple times and prepare everything up front. Even then the checkers are not infallible. Test viles with finger prints can give off wildly inaccurate results, as can air bubbles in the cuvette. It has even been reported that which way round the cuvette is placed into the checker can impact results. Are the corvettes new or old? I once read about people who store them full of RODI to prevent long term staining. Further, those Hanna reagent sachets are notoriously difficult to extract all the reagent without either spilling it everywhere or trapping some of the reagent in the corner of the pack. If it takes a long time to empty the pack does this impact the mixing time? Many things to get right if you want consistent results.

Furthermore, you have to understand even if you do this correctly the checkers have a limited accuracy built in. You have the Hanna low range phosphate checker (not the new ULR). The model you have has a large test range from 0 to 2.5ppm so isn't aimed at the Ultra Low Ranges we tend to be shooting for in our tanks. Further this model has a built in accuracy of +-0.04ppm+-4%. Hence a reading of 1.18 could actually be anywhere between 1.09 to 1.27 and still be considered 'accurate'. (Note there is a new ULR phosphate checker as well as an older phosphorus ppb checker, whereby both checkers focus on a tighter ULR window, and with much improved accuracy levels).

Either way it can be difficult to be accurate and I would focus on consistency over time. If you really need to get to the bottom of this I would run the Salifert 3 times and the Hanna 3 times and see which is most consistent. I would either get a phosphate calibration fluid or go and get an ICP test. I would then discover which is more accurate. Or at least how much my testing skills needed improving haha.

Sadly I don't have a specific answer for you, maybe another reefer can jump in. Best of luck with your future tests and happy reefing.

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