When using the HI 736 ULR Phophate checker, I note considerable variations between the initial test done using the 3 minute timed method and subsequent, immediate retests made using the same sample via the instant method.
Tests are made according to the manual, all cuvettes are wiped clean with a microfiber cloth prior to each test, each cuvette is rinsed with tank water prior to testing. Care is take to make sure all the powder agent is introduced into the cuvette. The instant re-tests are done using the second cuvette for the C1 standard sample.
Over the last two days I have obtained the following results...
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 understand the use of a second cuvette for the C1 tank water standard for the instant tests might introduce some variation, but I have alternated the the cuvettes used and still obtain similar variations.
Has anyone else experienced this issue? I've posted this in Hanna's forum with the hope they can explain why initial testing might produce results much higher than all subsequent testing? FWIW, based on my tank trends, I believe the instant retest values to be more accurate.