Re: How long for muriatic to take effect?
The acid demand test is only a guideline to how much acid you need to add. It sometimes works and it sometimes doesn't becuase it is only an approximation of what will happen in the pool and is based on 31% muratic acid. Some muriatic acid is more dilute. This is why the best way to adjust pH is to add small amounts of the pH adjusting chemical and restest. Is your plaster new or fully cured? New plaster can require more acid to shft the pH. You can get an idea of how much the pH is moving after an hour or two of circulation but it can take as long as 24 hours to see where it really is. Brushing after adding the acid (or any chemical) will help to distribute it through the pool. If the pH didn't move a few hours after adding a quart try adding mabe a cup or two more and see what happens until you get the pH where you want it.
Retired pool store and commercial pool maintenance guy.
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