czechmate wrote:Your demand on FC will drop to half of todays (4.5 instead 9 ppm) and save you money daily.
That isn't true if one maintains an FC that is proportional to the CYA level. The FC drop with an FC of 6.8 ppm with 90 ppm CYA is lower than with an FC of 3.4 ppm with 45 ppm CYA (see this post for an experiment that shows this). Though at a given CYA level the amount of chlorine lost from sunlight is proportional to the FC level, higher CYA levels protect chlorine from sunlight more through additional mechanisms such as shielding lower depths from UV (i.e. not just from binding to chlorine itself). So having a higher CYA level often results in somewhat lower chlorine usage in spite of the higher FC level needed to prevent algae growth. This is the main reason why 80 ppm CYA is the recommended level for SWCG pools even though the 4 ppm FC is higher than 2 ppm FC with 40 ppm CYA. It is to lower the absolute chlorine loss (FC drop) to reduce SWCG on-time which reduces hydrogen gas bubble generation and associated carbon dioxide outgassing as well as any undissolved chlorine gas outgassing. This reduces the rate of pH rise in SWCG pools.