I don't want to start a bit to do about the accuracy of the DPD test, I am quite sure it is very accurate. However, I slammed my pool this weekend and went to test the water this morning for the final test of the OCLT test and took two readings with the same water. The first time I measured FC I didn't feel I used the correct amount of powder and just didn't feel good about the test. I measured 13.5 FC which would have caused me to fail the OCLT so I measured it again and measured 15 FC. Any idea what I might have done wrong on one of the two tests?
I have probably done 10 or so tests over the last few days and tried to do them all exactly the same but this morning I got two different results.
Also I had seen something about the water and the meniscus when measuring FC with the DPD powder but after searching again I couldn't find the post. Should the top or bottom of the meniscus be at the 10ml mark or does that small amount of water really make that much of a difference?