High PH
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What is your TA? What is your CYA? ("in range" is not a test result reading. Posting a full set of test results NOT done with strips would be very useful to pinpoint exactly what is going on in your pool.) These are the two most important parameters in slowing pH rise in a salt pool. Make sure the CYA is at the manufacturer's recommended MAXIMUM (usually around 80 to 100 ppm, depending on the manufacturer of your unit). The rationale is that higher CYA will allow a shorter cell run time to maintain the desired FC level. This translated into less out-gassing of CO2 (caused by the production of hydrogen gas bubbles in the salt cell when it is on) which directly translates into slower pH rise. Which brings us to TA.
Your pool is over carbonated (this is what TA really is, a measure of bicarbonate in the water). By lowering the TA (to between 60-80 ppm) and raising the Calcium if needed to keep the saturation index in range (Important for plaster pools, possibly important for fiberglass pools, not important for vinyl pools) you will also slow the out-gassing of CO2 and therefore slow pH rise.
Out-gassing of CO2 is the MAIN cause of pH rise in swimming pools. Lower the TA (and therefore slow the out-gassing of CO2) and slow the pH rise.
Also, adding borates to 50 ppm (either by using a commercial product like Supreme or Optimizer) or by using borax and muriatic acid will introduce a secondary pH buffer system that effectively "locks" the pH at around 7.6 to 7.7 for extended periods of time.
Finally, what is the surface of your pool? Plaster pools (including aggregate finishes) often require more acid than other pool surfaces. This is the nature of the beast and if the plaster is less than a year old it is still curing and will require even more acid. All you can do is keep feeding it acid until the plaster is fully cured (normally about a year).
pH in any pool is never completely stable and will change. Aeration (out-gassing of CO2) causes pH to rise. The lower you put the pH the faster it rises (because you have converted more bicarbonate into carbonic acid--CO2 in water) so try and keep you pH no lower than 7.6 and lower it when it rises to 7.8 (once you drop the TA of course). You will get more pH stability than if you lower the pH more!
So, in a nutshell, to minimize pH rise in a salt pool:
1. raise CYA to the maximum, normally 80 ppm or 100 ppm depending on the manufacturer of your salt system.
2. Lower TA to 60-80 ppm (and adjusted calcium hardness upward if needed to maintain water balance.
3. Maintain pH between 7.6 to 7.8
4. (optional but highly recommended) add borates to 50 ppm.
It's really pretty easy but I will caution you that just doing one of these steps by itself probably won't have much effect. You really need to do all of them (at least the first 3) to really slow down the pH rise.
(Now don't go getting any ideas!)
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