I want the CC test only

[famousdavis]

This morning's FC/CC reading @ 0.5 measurements showed a barely discernible CC level (meaning that I almost didn't have to add 1 drop of reagent to clarify the water during the CC test it was already so clear -- though adding one drop did seem to improve the clarity just a smidgeon). I've noticed over this week that sometimes the CC level turns a slight shade of pink after adding 5 drops of reagent following the FC test, and sometimes it's a barely discernible shade a pink (meaning less than a "slight shade of pink"). Is it normal for CC levels to bounce around slightly? What causes the CC level to drop if you're not shocking your pool?

[aylad]

It is normal for CC to bounce around slightly. Sunshine will cause the CC level to drop........

[waterbear]

It is not how deep the red color is, it is whether there is ANY red color at all. It one drop changes the color and a 2nd drop has no effect then you have .5 ppm CC with a 10 ml sample. Remember, this is NOT a colormetric test and the color can vary based on just how much DPD powder you put in. IF you start with more DPD powder the pink color is more intense in both the FC and CC tests. If you use less it is more pastel. You want to test until one more drop does not produce any more color change since ALL the ttitation tests we do are based on color CHANGE, not the color!

As far as wanting more precision it is really not necessary BECAUSE the reason for testing is to know when to add something, what to add, and how much to add to get the water in balance and unless you know EXACTLY how much water is in your pool (by either metering the water on filling or determining the volume chemically) then you have your first dosing error. Next you have to take in account the precision of the test you are doing and you have your second dosing error (FC test with 10 ml. sample has a precision of 1 drop which is +/- .5 ppm for example) Since the volume of water in the pool is most likely off by a a greater error this testing error is "close enough for government work" so to speak.
Remember it's not rocket science or a lab experiment--it's a swimming pool and, IMHO, you should spend more time swimming and less time testing if your numbers are in range.

With the 10 ml test one drop on the CC test means you can have 0 ppm CC up to 1 ppm CC. Now, the smallest amount of CC detectable by the human nose (strong chlorine smell) is .4 ppm CC so if you are testing at 1 drop but don't smell chlorine then you don't have any apprecialbe CC. If you do smell chlorine then you might want to shock.

The 25 ml test is really more appropriate for COMMERCIAL spas where some localities limit the CC to .4 ppm (which, with the 20 ml test could be as low as .2 ppm or as high as .6 ppm) since CC tends to be a big problem in spas particularly commercial ones) and the fact that .4 ppm has been found to be the threshold for smelling CC.