topic breakoff: low fc=cc?

[reebok]

With CYA at 40 you never want to allow FC to get below 3, even for a moment. Low FC levels can result in small amounts of CC. If the CC is the kind that is volatile, ie the kind that you smell in the air, it might not show up on the test.

I'm starting a new thread here based on this quote taken from the "Chlorine odor at start up of filter pump" thread in Just Getting Started.
I am testing daily and keeping my chlorine around 8-9 based on cya of 70. I missed 2 days because I worked and then got an overtime shift. I work 24 hour shifts, so I was gone for 48 hours. my last test before I left was fc=6.5 (first test with fas-ddp) so I added enough chlorine to give me around 8ppm. when I returned today, chlorine was 5.5 and I had .5 (well probably less because it wasn't really pink...) CC's. it definitely changed the clear fas-ddp test color, and it definitely got clearer when I added one drop of the CC test reagent, but it wasn't bright pink either. anyway, this worried me a little because I thought maybe I was getting algae. I added ~98oz of chlorine which should have put me a little above 9ppm. I tested about an hour later and I was right at 9.5 or 10ppm, kind of hard to tell if it took 19 or 20 drops. the CC test was now 0.
so that was a long story but what piqued my interest was this quote that says low FC can result in small amounts of CC. is there any reference or explanation for this? just curious. thanks.

 

[JasonLion]

The process of chlorine breaking down organic contaminates takes two steps. In the first step, the chlorine combines with the contaminate and forms combined chlorine (CC). In the second step an additional chlorine reacts with the CC and breaks it down further.

The first step proceeds more quickly than the second step. Sometimes when FC is a little too low, you run out of chlorine in some local patch of water after the first step has mostly finished, leaving CC and no more chlorine to react with it. If the CC happens to be the right kind, it can then outgas from the pool at that point, before more FC shows up to complete the second step.

If you are maintaining the recommended FC level, there will almost always be enough FC to complete the breakdown through both steps, and you never notice the CC that exists for only a moment between the two steps. But when you start running out of FC, the first step can use up all of the chlorine quickly, leaving excess CC lying around.

Keep in mind that local patches of water can run out of FC while there is still a measurable FC level in the pool as a whole. The contaminate level varies from one patch of water to another, and it takes time for FC from a "clean" patch of water to migrate into the patch that has run out of FC because of a high contaminate level.

 

[reebok]

thanks for the info.