Normal "burn" rate for chlorine in a day

[chem geek]

Well, this is controversial because we haven't yet sorted out whether CYA gives better protection than shown in this graph and this is getting discussed in this thread. If CYA only protects chlorine via a roughly 6-7 hour half-life in direct noontime sun, then I would expect worst-case burn-off in places like Arizona, southern Texas and south Florida in peak summer to be up to half the FC in a day (the sun isn't directly overhead the entire time and that roughly compensates for the 14 hours or so of sunlight). Technically, the average sun intensity at the spring and vernal equinox (i.e. 12 hour days) is around 32% of the peak intensity (integral from 0 to 2*pi of the sine of the angle all divided by 2*pi) but summer days are around 14 hours so (approximately) 14*0.32 = 4.5 hours of equivalent direct noontime sun so an expected loss if chlorine not maintained constant (at a CYA of around 30 ppm so a half-life of around 6 hours) of around 1-exp((ln(0.5)/6) * 4.5) = 41%. I just use 50% to be conservative.

That is not the same as the burn rate if the chlorine level is kept constant. In that case, the burn rate is 1 - ( (ln(0.5)/6) * 4.5 ) = 52%. That is, maintaining the constant chlorine level results in a greater loss which makes sense. "Half-Life" numbers refer to declining values of chlorine over time (i.e. the time it takes to get to "half" the original chlorine level). So your pool is burning chlorine at around 30% / 52% = 58% of maximum. That seems very good for this time of year and your being in Pennsylvania -- and your number could be lower than expected due to not having complete sun exposure from sunrise to sunset (plus not all of the pool water is exposed to the sun when it's lower, etc. etc.... many many factors).

This site gives the altitude of the sun which today (at peak near noon) in Philadelphia is 70 degrees instead of 90 directly overhead so you get sin(70) = 94% of peak which really isn't so bad. By comparison, today in Los Angeles it's 76 degrees, in Phoenix it's 77 degrees, in Houston it's 80 degrees, and in Miami it's 84 degrees.

Richard