Mark - use tank water IMO, I mean, this test is not telling you that much apart from, it dosent cost anything (no sachets), and it lets you know if your technique or vials are bad. You can do it many times to check your technique and practice it too. If you fill the 2 vials with tank water, add no sachets, and do the test as if you were using sachets (including the shaking), then if you get a reading thats not zero, you KNOW youre doing something wrong, or your vials are bad etc. It could be too much shaking making microbubbles, or finger prints on the vials etc, but something is wrong because the sachets produce a blue color in the presence of phosphate that the meter can 'see' (it shines green light through the blue liquid and so the blue liquid reduces the transmission of green light, the amount of 'reduction' is the ppm phosphate). If theres no sachet, theres no blue, and if the water in test C1 and C2 is the same (whether its tank water, or RO), but you have a reading, then something about the technique or equipment is wrong.
I don't want to suggest I am an expert with this test, but I did tinker for weeks when I first got it, until I was happy with the result. Recently I've been doing several tests a day for various reasons. I've got through over a hundred sachets since I got the meter, need to order more now (I'm actually spending too much so I am trying to use my own chemicals to save money). I also read up on how it works, and that helped. If you understand what the meter is looking for, you will easily see what can interfere with that result. I also found that practice helped getting really consistent results. When you are very clean with your technique and do things exactly the same each time, consistency improves. This test is at the edge of the technical ability of phosphate testing without getting far more complex, but you have to be a bit professional in how you use it if youre measuring very low levels.
A few things that might mess tests up:
The water in the vials should never touch your fingers, or anything apart from the vials (never put your finger over the top of the vial for instance).
The rim of the vial should never touch your fingers (fingers might have phosphates absorbed on the skin, or dust etc).
The sample of water should be taken from under water, so that you don't 'skim' the surface of the tank water when you take the sample.
Personally I take the sample in an area of flow where the water that flows past my fingers does not go into the tube (ie take the sample with the vial underwater in the direct flow of a pump, pointing towards the pump).
The vials should be capped immediately, and then (still capped) rinsed under the tap IMO (to get rid of salt, prevent it getting onto the checker light diodes, and make sure the glass is clean).
The vials must be clean and the caps too. I store them with the caps tight on so they stay wet inside to prevent deposits forming, I rinse them with tap water after use several times with a final RO rinse, and I rinse them several times before using them (last rinse in RO). I do the same with the caps. It does take me a good few minutes to do a test from start to end.
I think that rinsing with tank water is good too - when ready to take a sample, I fill the vials, then empty them back to the tank. Do the same again. Last time I do this, I dont empty them back to the tank - thats my sample water.
Once the vials are full, washed with tap water and dried, I remove one cap, pour a sachet in it, cap it again, rinse it under the tap again, and dry it again. Then I start to shake it. I shake it for 2 mins by the clock. Then I put it down on its base, take the other vial (with no powder in it), clean it very carefully until its perfectly shiny (dry buffing it, I use a tissue but maybe a clean, lint free cloth would be better), put it in the checker without touching the glass, holding it by the cap, and close the checker, zero it (C1), take that (blank) vial out, clean the other vial in the same way, put it in the machine and close it, hold the button down so the countdown starts, and sit back for 3 minutes. A the end of that 3 mins, the reading is ready.
I also visually check before putting either vial in the machine, that the liquid, whatever color it is, is perfectly clear. Any turbidity at all will mess the test right up. Its got to look like diamond or sapphire - really sparkling clear. If your tank water is not clear enough for that, I don't know if the test will work well for you. The professional version of this test requires filtering the sample if its at all turbid.
My hanna 713 ritual is almost superstitious, but I also got screwy results at first, and this ritual is the result of my experiences. Much of it is probably unnecessary but I don't think any of it will detract from accuracy, just waste your time, so you can always try missing steps out. I had very low phosphates at the time which magnifies slight differences in technique. I retested the same vials multiple times and got 0.03, 0.00 etc (no new sachet, just putting vials back in machine etc) which is where I learn about clean vials. With ultra clean everything I pretty much don't get this variation.
These 3 links are lab instructions, and the hach instructions, for phosphate testing using a similar or identical chemical technique. They are interesting also to get a handle on the aspects of the procedure that the professionals use.
http://water.epa.gov/scitech/methods/cwa/bioindicators/upload/2007_07_10_methods_method_365_3.pdf
https://www.uvm.edu/bwrl/lab_docs/protocols/OrthoP_Lachat.pdf
www.[B]hach[/B].com/asset-get.download.jsa?id=7639982670
