Total alkalinity and why your controller cares about it

Total alkalinity (TA) measures the alkaline substances dissolved in the water, chiefly bicarbonate in a swimming pool. New York's Sanitary Code sets a range of 80 to 120 mg/L for public pools (10 NYCRR 6-1.19). Within that range, alkalinity acts as a buffer: it resists rapid pH swings and gives the pH controller something stable to work against.

When alkalinity is too low

pH becomes unstable and bounces as sanitizer and acid are fed. Plaster and grout can etch, metals can corrode, and staining is more likely. For an automated controller the effect is direct: an ORP or pH controller reacts to swings it did not cause, feeds to correct them, and overshoots. Operators often see unusually high chlorine readings in the morning after a night of unstable pH on a low-alkalinity pool.

When alkalinity is too high

pH becomes difficult to lower. The controller calls for acid or carbon dioxide repeatedly with little visible result, and high pH reduces the effectiveness of free chlorine as a disinfectant. Water may also cloud. Note that carbon dioxide feed, while it lowers pH, adds to alkalinity over time, so pools on CO2 need particular attention to the TA trend.

What to do

• Test total alkalinity at least weekly with a fresh reagent kit, more often after large water additions or chemical adjustments.
• Trend it. A controller's data log shows whether pH swings correlate with alkalinity drifting out of range.
• Adjust in stages and re-test; large single corrections create new swings.
• If a controller seems to be fighting the water, check alkalinity before assuming a probe or feed problem.

FreyTech service programs include water testing and chemistry review on scheduled visits, and BECSys controllers with the optional alkalinity meter can monitor and control TA directly. Either way, alkalinity is the foundation the rest of the chemistry sits on.