phosphate testing

[Johnny Reefer]

I just got a Hanna HI 93713 Phosphate low range photometer and I am confused by the readings I got from it.

For my reef tank I got 1.82mg/L.
For my FOWLR I got 2.70mg/L.
WTF? My PO4 has never been above 1.00. My FOWLR tested at that for a time in ’06.

The powder was not completely dissolved when I put the sample in the photometer. They say to shake it gently. But they also say to time it for 3 minutes once the sample is in the photometer. But I’m assuming time is of the essence, given the 3 minutes timeframe? I can’t be standing there shaking the sample hard for 10 minutes until the powder is dissolved, can I? They say not to let it stand for too long. They say nothing of ensuring the powder is fully dissolved. But they do talk about debris and bubbles throwing the reading off.
Oy vey! Yet another test kit with crappy instructions.

The leaflet says it measures PO4³ˉ. I see on Wikipedia that that’s inorganic phosphate. Okay.

The leaflet has conversions. To convert PO4³ˉ to P2O5 multiply reading by 1.49.
To convert PO4³ˉ to P multiply reading by 0.33.

I have been using the Elos PO4 kit for testing up to now. I should state that I do not read this test exactly as directed by Elos. They say to look down at the sample through the top of the vial. I have always looked from the side, as this is how other test kits are.

Latest readings with Elos was yesterday.
Reef: < 0.1
FOWLR: 0.1

If I convert my Hanna readings to P, I get.
Reef: 1.82 x .33 = 0.6006 mg/L. This is more like it!
FOWLR: 2.7 x .33 = 0.891. Again, more like it!

I don’t get what I’m missing here.

I’m assuming P is organic phosphate? Maybe I should just be happy with the converted readings I get of that? I have read/heard that organic phosphate is the more important phosphate to keep tabs on anyway?

Sorry. This chemistry business is one of my weaker aspects of the hobby.

Anyone with any experience or light that could be shed on this would be greatly appreciated.

Thanx much,