Maddie,
The responses to your questions in this thread have been absolutely outstanding and are a testament to the education that this site provides to people with pools and spas. Also, your curiosity and wanting to understand the recommendations in
Water Balance for SWGs is admirable.
The concept of a lower TA actually resulting in more pH stability is one of the most counter-intuitive concepts in all of pool water chemistry. The reason for the confusion is that a higher TA has TWO different effects that compete with each other. A higher TA results in greater pH buffering, meaning that it takes a greater addition of acid or base to move the pH a given amount. However, TA is also a SOURCE of rising pH itself because TA is mostly a measure of the carbonation level in the pool and pools are intentionally over-carbonated. So a higher TA level results in faster carbon dioxide outgassing and that results in a faster rise in pH. This latter effect is the dominant one so lowering the TA level usually results in a slower rate of pH rise and also results in less acid being needed over time.
There are two other factors that can lower the amount of acid you need to use, but I'd lower your TA first since it's best to make one adjustment at a time in your pool so you can isolate, understand, and tweak each effect separately. The rate of pH rise from carbon dioxide outgassing is faster at lower pH, so setting a somewhat higher pH target of 7.6 to 7.7 instead of always trying to hit 7.5 or lower is also helpful and what we recommend. Obviously, the use of a pool cover would virtually eliminate the outgassing and is another approach one can take that also significantly reduces water loss from evaporation and the resulting rise in both TA and CH that can result from these quantities that are present in fill water. Finally, one can add 50 ppm Borates to the pool which acts as an additional pH buffer but without the side effect of carbon dioxide outgassing (i.e. it is not a source of rising pH) and also acts as a mild algaecide that sometimes lowers the chlorine usage letting you turn down the SWG on-time even further.
The chlorine and CYA relationship is known science since at least 1974, but has been virtually ignored by the industry in terms of recommendations for FC levels in pools and spas which should really be dependent on the CYA level. We recommend a high 60-80 ppm CYA level in SWG pools to reduce the amount of chlorine broken down by the UV in sunlight since that lets one turn down the SWG on-time resulting in a longer cell life and also reducing the rate of pH rise since some of that rise is related to the SWG itself. At 80 ppm CYA, one really needs 4 ppm FC to prevent algae growth, even in pools that are rich in algae nutrients. Chlorine alone can prevent algae growth, but the active chlorine level needs to be high enough and that level is roughly determined by the FC/CYA ratio so a higher CYA level needs a higher FC level to have the same amount of active chlorine. A pool with 4 ppm FC and 80 ppm CYA has roughly the same active chlorine level as a pool with only 0.05 ppm FC and no CYA. Fortunately, it takes a very low active chlorine level to kill pathogens and prevent algae growth.
Richard
