I needed to register on the forum, but after doing that I downloaded the attachment and it is the same as
this paper. I believe the poster may be referring to Figure 1 which is Monsanto data showing that most of the degradation of chlorine is prevented with 25 ppm and that 50 and 100 do not show much benefit beyond 25 ppm. Wojtowicz notes that "The large initial drop of 12-20% over the first half hour is inconsistent with the slow linear loss of about 10% over the next 3 hours." He also notes that other data with 50 and 150 ppm CYA show losses of 35% and 19% respectively, which is noticeably different. Experiments in a Phoenix pool showed very little degradation of chlorine when there is no bather load. Wojtowicz attributes most chlorine loss to thermal decomposition where the chlorine is reacting to break down CYA.
For whatever reason, this data is inconsistent with what we see in pools on this and other forums. Even at the same active chlorine level, so FC/CYA ratio, where one would expect the same rate of chlorine loss if it were due to only unbound chlorine getting broken down by sunlight, we actually see a lower loss rate as the FC and CYA rise proportionately together and I speculate it is due to a direct CYA shielding effect of lower depths from UV. Also, Mark did experiments described in
this post showing a drop of 50% in FC level when the CYA was 45 ppm while the drop was only 15% at 80 ppm. After looking at other reports, I came up with a table in
this post that shows that higher CYA levels even with proportionately higher FC levels have lower absolute FC loss though the effect is not nearly as strong as Mark saw because he used the same FC level and different CYA levels whereas my table is showing a constant FC/CYA ratio.
The protection effect of CYA is the main reason that typically undersized saltwater chlorine generators recommend using 60-80 ppm CYA because they would not be able to keep up with daily chlorine loss otherwise.
In my own pool, I can say with certainty that the claim from Wojtowicz that there is hardly any chlorine loss from sunlight when even modest amounts of CYA are present and that it is nearly all from thermal degradation is wrong. Yes, the losses from thermal degradation and from chlorine reacting with a pool cover are substantial, especially at higher temperatures, and in my pool at 88ºF this is around .7 ppm FC with around 4 ppm FC with 40 ppm CYA. When the pool cover is open and the pool is exposed to sunlight, the chlorine loss rate is substantially higher where even a couple of hours increases the loss from 0.7 to 1.0 ppm and a full day of exposure would lose over 50% (over 2ppm). Even with the pool not used (i.e. no bathers) there's no way the chlorine loss is due solely from the breakdown of unbound chlorine alone. I speculate that chlorine does in fact break down from chlorine bound to CYA, but at a slower rate, but since most of the chlorine is bound to CYA this loss cannot be ignored.
