Testing chlorine when the pump is not running

[famousdavis]

This morning I awoke a little early and thought I'd test the pool water earlier than when I usually do, which is after 8am and after the pool pump kicks on. I'm using the K-2006 kit for now to do daily chlorine testing, until I get a very good sense of how much chlorine my pool uses, and what setting I need on the SWCG to get the chlorine levels I want.

So I got my water sample in the usual way, taking it from the same location in the pool I always do (opposite the returns), dipping my arm up to my elbow before turning the test tube right side up so it fills with water.

When I ran the chlorine test, though, I was surprised by my results! All week long, my FC level has been 3.5 or 4 first thing in the morning. Today, though, it was 1.5. And my CC tests either cause no change to the sample water, or it's just a mild shade of pink that clears up after adding one drop of reagent -- so, the CC has always been between 0.0 and 0.5. Today, though, the sample water turned a surprising shade of dark pink -- and one drop of reagent didn't clear-up the water, but two drops did, so the CC reading was 1.0.

I wondered, what was wrong? Was I getting an algae bloom or something? The water looked fine.

Then I wondered how much it matters that my test was taken when the pool water wasn't circulating. The pump shuts-off at 4pm, so it had been about 16 hours since the pump was last on (my wife and kids were in the pool after 4pm, so the water wasn't still the whole 16 hours -- but it was still for 10 hours before this morning's test).

After the pump turned on, I ran another test after waiting a few minutes to let the water circulate. This time, my test results were in line with the results I've been getting all week long. FC = 3.5, CC was near zero.

So, here's the question: What's going on in the pool that causes the FC to fall sharply a foot below the surface, and the CC to rise a foot below the surface, when the pool hasn't been circulating for ~ 10 hours? Does FC near the surface get used up quicker than FC at the pool's bottom? Does FC tend to sink, and CC tend to rise?

At least I've learned my lesson: only do pool water testing when your pool pump is running!

[Phillbo]

Does not answer your question but I've always done my test at the end of a pump run. I feel it gives me a better idea of what chlorine levels are being produced by my SWCG.

[famousdavis]

Actually, during this time when I'm still learning about my pool and SWCG, I have been testing twice a day, to find out whether I'm getting a net gain/loss in chlorine based upon the current SWCG setting, and also to see how much chlorine I lose overnight. I'll probably continue doing this for another few weeks, at which time I'll relax a bit. It takes time to know how much chlorine gets used based upon bather load and weather conditions, but I've learned a lot over the last few weeks. I still think I have more to learn, though.

[BigDave]

Is it at all possible that one of your swimmers made a "warm spot" in the still water?

[famousdavis]

BigDave: Nope. Among my wife and daughters, that isn't possible. And my youngest son likes peeing in the grass. ;-)

[BigDave]

Perhaps some other localized contaminant, lotion, perfume, hair product? My point is FC is consumed and CC is created when the chlorine does it's job.