On the FAS DPD test for CC that is...

I've had CC of 1.5 one time since I got the TF 100 and this was after being gone a week. Got a touch cloudy and I bumped up the FC for a few days...fine ever since. I have passed the OCLT four times now as of this morning. Still however, I always get the little tinge of pink for the CC and just wondered if it will ever show me 0.0. Don't tell Dave, but I have quit burning that last drop most of the time too. And not to worry.... I test FC with FAS DPD a lot, so I have a very good feel for whats' happening. The pool is used daily, but with a very minimal load most of the time.

Just wondering what others have experienced with theirs. Im not worried about it, just curious.

When you are testing for FC, does your water truly get to sparkling clear before you stop? I realized I was getting that residual "super light pink" when checking for CC too. I then started using my reading glasses and holding the test tube up to the white background on my TC/Ph box. Wala! It was still slightly shaded pink when I thought it was clear.

Well it gets completely colorless yes. Mine has never stayed "crystal clear" throughout the addition of DPD reagent. It gets slightly cloudy with whatever reactions take place but again, it gets absolutely colorless...no pink at all. I push the endpoint all the way to the end on FC.

I use a 25 ml sample so have 0.2 ppm resolution/accuracy, but my CC sometimes shows up as 0 (clear) and sometimes as 0.2 (one drop to clear). Note that depending on exactly where your FC tests ends, that will affect the CC test. So if the last drop in the FC test just barely made the sample clear, then the R-0003 reagent can make the sample turn pink that goes away with one drop if it has anywhere just above 0 to just at 0.2 ppm CC. On the other hand, if the last drop in the FC test was nearly clear before the last drop, then the last drop can push it beyond the clear point such that the CC test shows clear even though there may be CC just up to 0.2 ppm.

This is part of the reason why the test is no more accurate than "one drop". Basically, you should not be at all concerned if you measure CC that goes away with one drop. Even with a 10 ml sample where this represents 0.5 ppm, it's not a problem. If you are wondering if the CC is really as high as 0.5 ppm, you can use a 25 ml sample size to see if it is more like 0.2 vs. 0.4 ppm, though it's not a problem either way.

Thank you Sir...that is about what I figured but I hadnt given a whole lot of thought to how much, or little effect that last drop had. I haven't been wrong on figuring out when one drop will end the CC test, so I haven't bee worried at all. Just curious.

Thanks again