CYA and Lifetime of Chlorine

[chem geek]

The curve is based on half-life numbers that are for direct noontime directly overhead sun (i.e. summer in L.A. and Florida latitudes). However, there are some inconsistencies depending on the source. For unbound chlorine (i.e. no CYA) the half-life is usually given as 35 minutes based on average pool depths but I found one source that found 11.6 minutes but that was for water in a quartz (transparent to UV) tube. For the bound chlorine (i.e. chlorine attached to CYA), this was calculated from the pool study curves but again I found two such sets of curves, one implying 8.4 hours and the other implying 6 hours. I used 35 minutes and 8.4 hours for the graphs I made. However, changing these numbers even by factors of 2 or so doesn't get even close to Janet's results.

It is this latter assumption that the protection of chlorine from sunlight is primarily due to the bound chlorine having a longer half-life that I am questioning. Not that this may not be happening, but that there may be an additional factor from CYA by itself acting as a "shield" of sorts protecting lower depths.

It is true that Janet's reports may have been influenced by other factors such as those that you listed, especially those that could have lowered chlorine demand after adding more CYA such as lighter bather load or fewer organics or time of year, etc., but it sounded like there wasn't much change in these other parameters. In any event, having a net chlorine loss go from 5 ppm FC per day to around 1.2 ppm FC per day is huge and this latter loss at FC levels between 5 and 9 ppm FC is far lower than predicted by direct sunlight which hits her pool. So even if there were other factors partially contributing, the only known change was the increase in CYA so I thought it worth questioning the prevailing theory.

Richard