Thanks for all the input!

PoolDoc: I would really like to see that news report when you find it.

ChemGeek: The pools I was testing were not previously shocked with MPS. The Palintest photometer we use does have a reagent to offset the effects of MPS on FAC though; it's called OxyStop and we do use it when some of our pools have been recently shocked.

The 0.1 CC reading I got was a very faint pink titration reading. The photometer read 0.3 CC so the difference was about 0.2 CC.

If there is that 0.2 margin of error, it does make sense and I guess that means our photometer is just a bit more accurate. I am concerned about the 0.2 ppm difference because most of my operators still use the titration kit, and I don't want to hold them to eliminating the CC to 0 if they can't even read it on their kits. We advise our operators to eliminate all CC even if it's below legal threshold just so they are aware of it and the CC levels don't get out of hand.