Quote Originally Posted by geordie
Solar cover was clean and had not been used so far this year at all until 10 days ago.

TA 180 - right where it has been all year... and in the same neighborhood for about 5 years
The data I quoted above is the answer to this "mystery". A TA of 180 is quite high and will lead to a lot of carbon dioxide outgassing and this causes the pH to rise. However, when you add acid to restore the pH, you should have seen the TA reduce as well as the pH, though it doesn't get reduced by very much. Perhaps your fill water is high in TA so whenever you fill up after evaporation you put back in more TA (that's the most likely scenario).

Now when you put a cover on your pool, you essentially stop the carbon dioxide outgassing and therefore stop the rise in pH as well as your need to add acid.

Fortunately, this is a case that is perfectly explained. If you want to reduce your need to add acid, you can either use the cover as you are now doing or you can lower your TA significantly using Ben's Procedure for Lowering Alkalinity though you may need to also apply this procedure to your fill water before adding it to the pool (if possible) or else your TA will rise again.

You should probably test your fill water to see what the TA level is. If it's high, then that would explain why it never seems to drop in spite of the apparent carbon dioxide outgassing.

Richard