I have the TF-100 test kit which has both the OTO Chlorine Test & the FAS-DPD Chlorine Test. I use the OTO test daily to monitor chlorine and PH and the FAS-DPD test about weekly or so. Lately I've noticed the OTO chlorine test reading starts to creep up 1 or 2 ppm after the sample has set a few minutes.
From the notes in the Extended Test Kit Directions this is said to be an indication of the presents of CC. ("This test measures TC, but if you watch closely it is possible to get a sense of the presence of CC. When you first add the drops of R-0600, the sample will immediately show the FC level. Then, over the next minute or so, it will drift up to the TC level. If you see the color changing, CC is present. Using the test in this way requires careful attention and a good ability to distinguish shades of yellow quickly.")
However when I use the FAS-DPD Chlorine Test I get a CC reading of zero. After getting the FC results and Adding 5 drops of R-0003 the sample remains clear indicating the CC level is zero.
I don't think I was seeing this last season with the OTO test. Could it be that my reagents have gone bad? They are the original reagents that came with the test kit purchased around the 1st of August of last year. If not that, any ideas of what may cause the different reading between these two tests, and which one should I believe?