Today I used 2 cuvettes for all the tests to minimize any variable caused by the use of the second cuvette on the retests. Care is taken to use the same alignment when inserting into the checker (10ML symbol facing front) and the checker was turned off for ~10 seconds between retests. The results were

4/25/12 7:00AM Initial - 18 ppb. Immediate follow up tests - 2, 0, 0, 0, 4, 3

*I conducted the retest two additional time as I was getting results of 0 ppb.

For comparison, here are are the daily results side-by-side

4/25/12 7:00AM Initial - 18 ppb. Immediate follow up tests - 2, 0, 0, 0, 4, 3
4/24/12 7:00AM Initial - 29 ppb. Immediate follow up tests - 11, 9, 8, 12
4/23/12 7:00AM Initial - 43 ppb. Immediate follow up tests - 5, 8, 9, 13, 8

I believe the trend of phosphates being lowered is correct as the GFO reactor was recently changed, but I am not confident in which value range to trust. I am puzzled by the difference and the fact that the retests always produce much lower results than the initial tests - seemingly well beyond the stated error rate for the device. Also curious is that all the retests provide very consistent results within the stated accuracy range.

I wonder if any other hobbyists with a 736 could replicate these tests to compare results? It only takes a few minutes as the retests are almost instant and require no additional reagents. I'd be happy to PM or speak with anyone interested to make sure the processes are identical.

I'm still hopeful Hanna will respond with some insight.