The following table shows rough FC loss percentages at various CYA levels and shows proportionally scaled FC levels to keep the FC/CYA at a 5% ratio (SWG pools). These obviously will vary a lot depending on the amount of sun, so this is just an example. You can see that even at 100 ppm CYA there is still enough loss of chlorine that an SWG should be able to be set at a lower percentage on-time (i.e. it's not below some limit like a 10% on-time unless one seriously over-sized the cell).
CYA ... FC ... % Loss/day ... % Loss/hr ... FC loss/hr .... % ontime (0.5 ppm FC/hr = 100%)
... 0 ... 0.1 ....... 99.6% ......... 50.0% ........... 0.07 ............ 14% (but can't realistically maintain 0.1 ppm FC everywhere)
... 0 ... 1.0 ....... 99.6% ......... 50.0% ........... 0.69 .......... 138% (can't keep up)
. 30 ... 1.5 ........ 67% ........... 12.9% ........... 0.19 ............ 38% (say, 40%)
. 50 ... 2.5 ........ 40% ............. 6.2% ........... 0.16 ............ 32% (say, 30%)
. 80 ... 4.0 ........ 22% ............. 3.1% ........... 0.12 ............ 24% (say, 25%)
100 ... 5.0 ........ 15% ............. 2.0% ........... 0.10 ............ 20%
160 ... 8.0 .......... 7.7% .......... 1.0% ........... 0.08 ............ 16%
0.5 ppm FC per hour would be 1.5 pounds chlorine per day (24 hours) in a 15,000 gallon pool. The "FC loss/hr" is calculated assuming continuous maintenance so is not a simple multiplication ( FC loss/hr = FC * (-ln(1-(Loss/hr))/1hr ). The "% Loss/day" is without any maintenance of the FC level so the absolute loss slows down as the FC drops. The % Loss per hour assumes about 8 hours of "noontime equivalent" sun per day in the summer ( (Loss/hr) = 1-(1-(Loss/day))1/8 ). The above assumes full sun in southern latitudes.
The example with 0 ppm CYA requires a minimum FC to not run out locally, but I also show the theoretical equivalent FC of 0.1 that would give the same disinfection/oxidation rates if somehow it could be maintained throughout the water. The active chlorine (hypochlorous level) is the same for all of the above as is the hypochlorite ion level. All but about 0.05 ppm of the FC is bound to CYA. Most of the chlorine loss during the day is from breakdown of the chlorine bound to CYA. The higher CYA levels shield lower depths from UV to give a protection from breakdown that more than makes up for the higher required FC level. The 160 ppm CYA is a guess and in practice other chlorine loss factors will start to dominate such as the slow chlorine oxidation of CYA itself which may be 0.2 to 0.3 ppm FC per day (depending on temperature) so 0.03 or so per hour. The above table assumes no bather load nor extraordinary chlorine demand (say from pollen, etc.) and cooler temps (closer to 80ºF). I know that in our own pool if covered with no sun and no bather load that at 88ºF the daily chlorine loss is around 0.6 to 0.7 ppm FC per day (perhaps from oxidation of the pool cover? and for uncovered pools there is a small amount of hypochlorous gas outgassing), so the low implied losses at higher CYA levels may not get as low as shown.
So you can see that going from 30 ppm CYA to 80 ppm CYA can cut the required SWG on-time percentage roughly in half even maintaining the same FC/CYA ratio for consistent disinfection/oxidation rates. In practice, people don't see quite this amount of dramatic change, but they do see a large improvement and the table may not properly account for some non-linear effects (some have reported more change seen going from 60 to 80 ppm CYA than from 30 to 60 ppm CYA).
