Should I get the Phosphurous Checker?

[asylumdown]

Originally Posted by tytown

Just started measuring with Hanna ULR unit but have trouble getting the same reading twice. I've almost gone through a pack of reagent performing multiple measurements between my fresh, mix, and tank water. For example, tank readings were 57, 107, 35 (0.17, 0.32, 0.10 ppm), so I'm pretty much calling BS on this unit unless I'm doing something completely wrong... It's supposed to accurate to +-5% of its range so that's +-10 PPb or 0.03 ppm.
Does anyone get consistent readings close to the units rated accuracy???

nope. Never once. Thought it was me, then I looked in to it. Testing at that resolution outside of an analytical lab is about as accurate as guessing your level by throwing darts at chart.

To be consistent at that sensitivity, you'd need to be measuring the volume of your sample with a micro-pipette, using a fresh pipette every time you collected the sample. You'd also need to be getting the exact same amount of reagent out of the powder pillow every single time (which you never are). You'd also need to be washing your sample cuvette in hydrochloric acid every couple of tests, only ever rinsing the cuvette out in the purest of DI/distilled water, and doing dummy tests with DI/distilled water before each test, as tiny amounts of phosphate can bind to the glass.

You'd also need to be collecting your samples from the exact same spot, at the exact same time, after following the exact same routine. Even dipping your hand in to the water at the time you collect the sample could throw off the results enough to make the number meaningless.

It's why I prefer the regular PO4 kit. It's not as sensitive, and the numbers still need to be viewed as relative rather than absolutes, but at least it's more consistent from reading to reading and gives you a better sense of the actual trend in your tank. When one result is 0.10 and the next one 5 minutes later is 0.32, you have spent 15 minutes obtaining no useful information.