Recommended values for a SWG pool

[waterbear]

Thanks Evan. BTW, my water temp is 86°F. I wouldn't mind knowing why those values will work well when you have the time.

I've been battling the pH rise since I have a SWG pool and a spa spill over. In one week, it can go from 7.4 to 8.

OK, I will explain. These values will help minimise the pH rise and give you better pH stability:

FC 4-5 ppm
This one is easy. It has been found that keeping the FC at about 5% of the CYA in a salt pool avoids problems with algae blooms and the need to shock the pool so if the CYA is at 80-100 ppm (and I will explain the reason for that below) then adjusting the cell output to maintain 4-5 ppm FC keeps the FC high enough for the CYA level.

The next three are linked to pH rise from outgassing of CO2, which is the main cause of pH rise in a salt pool (assuming there is no new curing plaster). The recommended levels done together will slow pH rise from outgassing AND keep the calcium saturation index in acceptable range for plaster pools. Since you have a spillover spa the additional aeration it creates speeds up the outgassing of CO2 and the pH rise it causes so these recommendations are very beneficial for your pool to help slow pH rise.

pH 7.6 (and not lower) to 7.8 (when pH hits 7.8 lower it to 7.6)
The lower we put the pH the faster it rises because we convert more bicarbonate in the water (which is what TA really is) into carbonic acid (which, for our purposes, is the same as carbon dioxide dissolved in water. The operative word here is acid, so keep that in mind.) As the pH drops there is more carbonic acid in the water. Carbonic acid is essentially seltzer. If we let it go flat (CO2 outgasses) then the pH rises (less carbonic ACID). the more carbonic acid (carbonation) the faster it will go flat to reach equilibrium. So, the lower we put pH the more carbonic acid we form and the faster it leaves the water and therefore the faster the pH rises. Keeping the pH no lower than 7.6 and not letting it reach 8.0 slows the pH rise AND also keeps the calcium saturation index in an acceptable range for a plaster pool along with the TA and CH numbers recommended.
TA 70 ppm
Lower TA means less bicarbonate ions in the water to be converted into carbonic acid which means slower outgassing of CO2 which means slower pH rise.
CYA 75, 80 or 100 ppm (whichever is the manufacturers maximum recommendation for your SWCG)
Higher CYA means less cell on time is needed to maintain the FC at target level (lower output percentage/power level) which means less formation of hydrogen in the cell which means less aeration of the water from hydrogen gas generation which means less outgassing of CO2 which means less pH rise.
CH 350 to 450
This will keep the calcium saturation index in a balanced range for the above parameters.
Salt, depends on the make of your SWCG. some are 3000, some 3200, some as high as 6000 ppm. It is generally better to run the salt a bit high (by maybe 200 pm rather than a bit low in terms of cell life (but not so high that the high salt shutoff kicks on if so equipped.)
Borates 50 ppm (optional but highly recommended. If you don't add borates then either keep the calcium at the lower end of the range, run the salt a few hundered PPM lower or keep the pH slightly higher to keep the water balanced)
Borates, besides having a beneficial effect on water balance if the calcium is high, also add a secondary pH buffer system to the water that works in conjuction with the bicarbonate/carbonic acid buffer we call TA and works in the opposite directions. While the bicarbonate buffer wants to move the pH upward toward 8.2 ( because of outgassing of CO2) the boric acid/borate buffer want to move the pH down and these together tend to "lock" the pH at around 7.6-7.8 for an extended period of time. For example, when I applied these parameters to my pool my acid use went from about 24 oz. (3 cups) about every 10 days to 2 weeks down to about 6-8 oz every 6 weeks to 2 months! I have a fiberglass pool that is chemically inert so my results are more dramatic than in a plaster pool but even a plaster pool will have better pH stability follwing this guidelines than without.

I DID warn you it gets a bit technical.

I do want to add that since you have a plaster pool, you might have to tweak a few of the parameters to compensate for such factors as whether you have borates, your temperature (which often changes with the seasons if the pool is open all year or most of it), and your salt level. The easiest way to do this is by making small changes to where you put the pH.
For example, in YOUR pool, with a temp of 86 deg and a CH of 425 and salt of 3000, by dropping the TA to 70 ppm and maintaining the pH between 7.6 and 7.9 your SI stays between -.17 and +.11 (essentially perfect)
By adding borates to 50 ppm your pH range increases from 7.6 to 8.2 with a SI of -.25 at 7.6 to -.04 (essentially 0 or perfectly balanced) at 8.2 pH. At pH of 7.7 (where the pH tends to sit for an extended period of time) the SI is -.18 and as the pH rises to 8.0 the SI approaches ) so you will not have scaling conditions, which is excellent for SWCG pools and the very slightly negative SI is still well within acceptable parameters for plaster pools. Even with a + or - change in pool temperature the SI is still WELL within acceptable parameters for a plaster pool (+/-.3) so the water stays balanced.