adding muriatic acid to covered swg pool

[chem geek]

You are correct that as the water gets colder and the sun is lower in the sky, then the SWG on-time can generally get reduced and this will reduce the rate of pH rise (due to less chlorine and carbon dioxide outgassing). If your TA is high, however, the pH can rise even without an SWG. Lowering the TA level (to around 80 ppm) is one way to reduce the rate of pH rise in general.

[PatL34]

If you have a new pool, or had it relined recently, this could be the cause of the pH rise as well as what Chemgeek mentioned. You will have to keep adding muriatic acid until the pool or lining stops leaching out calcium salts.

Hope this helps.

Pat

[waterbear]

Main reason the pH rises is because of outgassing of CO2. There are several ways to minimize this:

Make sure the CYA level is high enough (usually around 80ppm) minimize cell on time and the generation of hydrogen bubbles that out gas CO2

Keep the TA on the low side (perhaps as low as 60 or 70 ppm) to keep the carbonation level of the pool down so there is less outgassing of CO2

Keep aeration to a minimum. If you have water features that aerate the water such as waterfalls, spillovers, deck jets, or fountains do not run them all the time but only when you are entertaining or using the pool.

Do NOT drop the pH too low. Try not to go below about 7.5-6-7.6 when you lower pH. Remember, the lower the pH the faster the outgassing of CO2.

Keeping a cover on the pool can help minimize CO2 outgassing and keep the pH from rising as fast but remember that the pool does need to be undcovered from time to time so chlorination byproducts can gas off.

Add borates to 50 ppm to introduce a secondary borate buffer. This, along with a TA on the low side as noted above, can help stabilize ph at around 7.7 for long periods of time.