I have had two instances where my FC has risen between two FAS-DPD tests with no bleach or other chlorine being added to the pool. The tests have usually been spaced apart by 8 - 12 hours. It happened last night and I am trying to figure out why. I know it has to be operator error of some sort. Last night I tested at 10:30pm and got a reading of FC 4.5 / CC 1.0. I tested again this morning and got FC 6.0 / CC 0.5. Both tests where it increased were OTCL tests and both evening tests showed CC of 1.0. It also has been early in the week after a weekend of swimming. The other time it happened my first test was FC 14.0 / CC 1.0 and the second was FC 16.0 / CC 0.5.
I test with a Speedstir and use the TF100 kit. My kit is less than two months old and stored inside.
I don't understand how there can be an increase overnight so I know I must be doing something wrong. I know one possibility is that in performing the FC test, maybe I did not get my FC test water totally clear and I needed to add one more drop of R-871. As the water clears, I am usually very conservative about adding that extra drop. If I am "on the fence" as to whether think its totally clear or not I don't add it. I would rather have a slightly low reading of FC than a high reading. That would would possibly account for 0.5 of the difference between evening and morning.
The other possibility I am contemplating is the R-870 powder. The instructions call for one heaping scoop. Everyone interprets heaping differently and I can honestly saying that I have difficulty in making my "heaping" exactly the same from one test to the other. I am wondering if slight differences in the R-870 powder could be driving this as well. Is the test very sensitive to the amount of R-870 powder? Post #2 in this thread seems to indicate that not enough R-870 powder could impact the test.