Re: Adding Muriatic Acid to bring TA down
It really depends a lot on the specific pool and how much aeration or churning of the water there is, but having a higher TA and using hypochlorite sources of chlorine (bleach, chlorinating liquid, Cal-Hypo, lithium hypochlorite) will most certainly tend to make the pH rise faster. Having the TA lower, usually at around 80 ppm for pools, works much better. This benefit is most especially seen in saltwater chlorine generator (SWG) pools.
There is clear science that explains this effect, but I won't get into that here. Pools are essentially over-carbonated and that's what TA is mostly measuring. Just as when you stir up a carbonated beverage, the carbon dioxide outgasses, the same thing happens in a pool and this process makes the pH rise with no change in TA. You need to think of bicarbonate (baking soda, Alkalinity Up) as having TWO effects -- one which increases pH buffering and another which increases the tendency for the pH to rise. The latter effect is more dominant at higher TA levels.
If you are using acidic sources of chlorine, such as Trichlor pucks, then a higher TA is definitely recommended and helps to stabilize the pH in two ways. First, the higher TA does buffer the pH more, but also the greater outgassing of carbon dioxide makes the pH rise which counteracts the pH drop from the Trichlor. In theory, you can set a TA high enough to have a perfectly stable pH with the only effect being a TA that gets lower over time (so needs baking soda or Alkalinity Up to raise it).
Richard