I'm starting a new post on this because one of my previous posts concerned discrepancies in the FAS-DPD FC test, using two different bottles of R-0871 reagent, one of which was probably too old. This post concerns discrepancies in the test, using the same bottle of R-0871, which has an expiration date of 9/2017. I'm wondering what mistakes I may be making in my procedures*.
I do all my tests at about the same time in the morning, around 8 AM, using a 10 ml water sample for the FAS-DPD test
On 8/14, I got FC=10.5/CC=0
On 8/15, FC=8.5/CC=0
On 8/16, FC=7.5/CC=0.5. I then did a second test and got FC=8.0/CC=0
On 8/17, FC=6.0/CC=0.5. I then did three additional tests and got FC=8.0/CC=0, FC=8.5/CC=0, FC=7.5/CC=0
I have added no chlorine during this period, because I was waiting for the FC level to come down from the 10.5 level on 8/14. There has been a good amount of sunshine all of these days. From 8/14 to 8/15, I lost 2 ppm of FC, which seems reasonable given past experience. From 8/15 to 8/16, I only lost 0.5 ppm. From 8/16 to 8/17, depending on which test I believe, I either lost 0.5 ppm or none.
Each day, I rinse out the test tube at the end of my testing and put it back in the test kit box, close the lid, and put the box in an outside storage cabinet. However, the next day when I do the first FC test, I do not pre-rinse the tube, since I rinsed it out the day before. So I started thinking that the first reading may be false, and giving me some CC due to microscopic organisms left in the tube after the previous days rinse. But a sequence of tests using the same pool water sample, and rinsing thoroughly between tests, still gives me FC readings that can vary as much as 1 or 1.5. (I rinse with tap water which has no FC or CC since I have a water filter in the house).
So I started wondering if my drop size is consistent. I hold the bottle vertically, and squeeze so that each drop comes out slowly (just a few seconds between drops).
I wondered if I should release the pressure on the bottle after each drop, to let air get back into the bottle before I squeeze it again for the next drop? Would that affect the drop size? I am adding two dippers of R-0870 for each test, which should be enough, given that I am using 10 ml water samples.
* In my college freshman chemistry class, I loved the theoretical part and got A's on all the exams. However, my lab work was apparently sloppy, and I got inconsistent and incorrect results on a lot of the experiments. My lab work actually dropped my overall grade to a B despite my grades on the exams. Maybe its my old curse, reappearing when I test my pool water.