Residual Chlorine Value?

[chem geek]

Saturday I got over 3 inches of rain and that was a real problem especially after I had added fill water just that morning. PH was off the colorameter, alk was low, cya had dropped some, and the chlorine was almost non-existent.

Should I be expecting to see these lower cya levels due to dilution or is it because I am at the very bottom of the range of my testing capabilities for cya? Everyone says you add cya once a season generally!!

If you were adding fill water to make up for water that had evaporated from the pool, then you haven't had any net change in pool water chemical amounts [EDIT]except for adding whatever is in your fill water[END-EDIT]. When pool water evaporates, everything in the pool gets more concentrated, but when you add make up water you dilute it back to where it was [EDIT] except for adding what was in your make-up water[END-EDIT].

It is only the rains that are of a sufficient amount to raise the pool water level so that it spills over (into a drain, or otherwise) that cause a true net dilution of the chemicals in your pool. And yes, this is a problem you will have to deal with by adding chemicals (including CYA, sodium bicarb for alkalinity and even calcium chloride) at least once during your rainy season (depending on how much rain you get -- my guess is that once mid-season might be enough).

As for the CYA test, yes it is hard to measure since most tests only go down to 30 (I've seen some go down to 20) so you are really guessing when you still see the black dot and you've completely filled the tube all the way to the top during the test. The good news is that if you have less CYA than you think you have, you end up with more disinfecting chlorine than the minimum needed (assuming you follow Ben's chart for total chlorine levels); the bad news being that less CYA will have you lose chlorine at a faster rate [EDIT]from sunlight[END-EDIT].

Richard