When do you add Muriatic Acid?

[BowserB]

As a micro manager of pool water, I tend to get carried away, but I'm trying to do better and reduce pool maintenance to the level of only a part time job. Now that the IC40 has FC under control, I've turned to pH. I used to shoot for the middle of the 7.2-7.8 range, but 7.5 seems elusive, so I'm just trying to stay within the recommended range and keep CSI barely under -0-. When do you consider it time to add MA? At 7.9? Or do you do a preemptive strike at 7.8?

 

[JoyfulNoise]

Wait until the pH reaches 7.9/8.0 and then only add enough acid to reduce the pH to 7.6. If your TA is low enough, then that cycle will yield the longest stretch between acid additions. Going lower than 7.6 forces more of the dissolved bicarbonate ion to form aqueous CO2 which then drives up the outgassing rate and the pH rise. pH rise is not linear - it is faster when the pH is in the 7.0 to 7.5 range and then slows appreciably once the pH is over 7.6. The pH scale is logarithmic in nature and so small changes in pH values result in large changes in concentrations.

 

[BowserB]

Wait until the pH reaches 7.9/8.0 and then only add enough acid to reduce the pH to 7.6. If your TA is low enough, then that cycle will yield the longest stretch between acid additions. Going lower than 7.6 forces more of the dissolved bicarbonate ion to form aqueous CO2 which then drives up the outgassing rate and the pH rise. pH rise is not linear - it is faster when the pH is in the 7.0 to 7.5 range and then slows appreciably once the pH is over 7.6. The pH scale is logarithmic in nature and so small changes in pH values result in large changes in concentrations.

Thanks, Matt. That is consistent with my non-expert observations. If I try to maintain 7.5, I'm adding acid daily, but once it's at 7.7 it hangs there a while, and will hold at 7.8 for several days.