Part of me wonders if it's the units themselves then, because people who can't seem to get consistent results never do, while the other camp never seems to have problems. I actually have training in analytical methods and have spent an unfortunate number of hours in a soil and water lab, and I still can't get mine to work properly.
I had a period where I thought my tank was melting down because every week I'd test 0.03-0.08 ppm higher than the week before - even with fresh GFO, and even while dosing lanthanum. Converted to ppm, I was reading in the 0.3 range.
Then i did a sequence of tests back to back in one sitting. The levels rose by 0.03 to 0.08 with each sequential test! the reading was predicted by the total number of tests I had run in that vial, not the level of phosphate in my tank.
I did the acid wash (which melted the max volume marker right off), then did a dummy test with distilled water and still got a reading of like 16 ppb or something.
I promptly drove to Wai's and bought a regular PO4 kit. That appears to have solved my phosphate problem, as my readings have been 0.00 ever since
