pH 7.8, TA 8 - creeping in opposite directions

[jonno]

Hi Ben or Janet or one of the other wizards,

This morning's numbers from my Taylor K-2006 are:
TC: 12, CC: .5, pH: 7.8, TA: 8, CYA: 100++ (dot dissappears when tube is 1/3 full)

Over last 4 weeks, pH has gradually increased and TA has gradually decreased. Both have been in ideal the ideal range since opening the pool in mid-May, but are now creeping to the maximum pH and minimum TA levels. We've had 3 or 4 inches of rain over the past two weeks and that usually lowers the pH, but not this time.

The acid demand test for my IG, VL, 25K gallon pool calls for about 3 lbs of dry muriatic acid to bring pH down to 7.5.

The water is crystal clear and has been all season. I switched to using 12.5% bleach exclusively since June 2011 thanks to this forum! I keep the FC between 8ppm and 15ppm due to high CYA. (The water and vinyl liner are both about 15 years old. We know it is time to replace both, but we are trying to eek out a couple more years since all funds are going to tuition payments.)

Questions:
1) Since both pH and TA are at the borderline levels, should I try to adjust them now or monitor them and act when/if the pH increases and/or the TA decreases?
2) If I adjust now, should I first treat the high pH or the low TA?
3) Since pH and TA are moving in opposite directions, will adjusting one cause the other to get worse?
4) For adjusting the pH, I have about 1/3 gallon of 31% liquid muriactic acid. The table in the Taylor booklet says I need 3 lbs of dry MA. Can I convert this to determine how much liquid MA to add?

Thank you again for rescuing me last season!

[aylad]

I'm assuming (correct me if I'm wrong) that you've been using dichlor or trichlor as your chlorine source prior to switching to bleach? (That assumption comes from the now-rising pH and the high CYA since your switch to bleach). If that's the case, the pH is rising because the bleach is not as acidic as the trichlor/dichlor is. Those chems tend to drive and hold the pH down. Since a switch to bleach, you're not adding all that acid, so it's not surprising that it's creeping up. Also, aeration tends to raise pH, so splashing kids, driving rainstorms that splash, etc will drive the pH up a little. You can leave the pH where it is for now, but if it rises any more, just use some of your MA to bring it back down (please read the link in my sig regarding safe use of MA first, though!). There is no reason to raise the TA in your pool (I'm also assuming you meant 80, instead of 8?) unless the pH keeps swinging and you can't control it.

So,...adjust the pH first (it might drop the TA a little, but that's okay). Then keep an eye on it for several days, and if you find that it's not staying stable, THEN you can increase your TA a bit.

[jonno]

Thank you Janet for the advice and explanations. Yes I used 3" tabs in a chlorinator for many years before switching to bleach in 2011. And yes I meant 80 for the TA, not 8. I'll keep an eye on the pH and lower it with MA (safely) if it gets any higher.
John