PH rising, TA falling, CH down --what's going on?

[JPlunket]

I am mystified.

5000 gal, vinyl-lined, "Endless" pool, using plain Clorox to chlorinate, maintaining at about 6ppm (CYA is about 40).

pH was down to 7.2 a week ago, based on measurement taken several hours after I added Muruatic acid to bring it down from 7.8+. At that same point, TA was 80, CH was 130.

The only other thing I've added during the week since is about 32 oz of 6.5% bleach every other day --about 3ppm. The pool has been covered by a "bubble" enclosure. My wife and I have both been swimming hard for 30 minutes daily --no other use, no direct sunlight, the only dynamic being pool water condensing on the bubble, some of which drips back into the pool, which really shouldn't matter, since the bubble surface is very clean.

Now, a week later, TA is down to 70, CH still at 130, but pH is up to 7.8 again. I say "again" because pH has kept rising inexplicably for the last months or so.

This seems completely implausible to me, based on the expert insights I've picked up here in the forum.

Is there some explanation possible, or is this just anolamous, perhaps due to faulty readings?

[mas985]

PH rise is fairly typical with a pool that is used a lot due to the aeration. Rain can also raise PH some due to the aeration.

As for the TA, TA can only be lowered with acid so if you didn't add any and unless you got a lot of acid rain recently, I suspect the first TA measurement could have been off some. If you didn't allow the acid to fully mix before measuring, this could make the measurement a bit high.

[JPlunket]

Thanks, MAS. I had been adding acid over the last few weeks, trying to offset simple pH rise. But why would that ultimately both lower TA AND raise pH? That's what I can't figure out.

I'm going back to basic steps.

1. Get TA up with sodium bicarb, maybe to 120 or a bit higher initially.

2. lower pH with muriatic acid.

3. get CH up to 180.

4. lather, rinse, repeat --see where this gets me.

[mas985]

Acid lowers both TA and PH. Aeration increases PH due to CO2 outgassing.

Also, if you raise TA, PH will tend to increase faster with aeration so having TA at the lower end will help keep PH in line as well so I wouldn't raise TA.

[oboe1960]

chlorine at 6 ppm is way high for an Endless Pool; it's supposed to be maintained between .5 and 1. Maybe let the chlorine subside to a normal level before attacking the pH and TA?

[chem geek]

The 6 ppm FC with 40 ppm CYA is equivalent in active chlorine (hypochlorous acid) concentration to 0.14 ppm FC with no CYA so far lower than most indoor pools. Nevertheless, it should not be necessary to keep it that high and could instead target a minimum FC of 3 ppm.

0.5 to 1 ppm FC would only be for a pool not exposed to sunlight (such as an indoor pool) with no Cyanuric Acid (CYA) in it, though even then it's over-chlorinated compared to an indoor pool with 4 ppm FC and 20 ppm CYA, for example. FC alone does not indicate the active chlorine level. It only gives the chlorine capacity. This has been known definitively since at least 1974 as described in this paper. You can also see how the FC/CYA ratio is a better proxy for the equivalent FC with no CYA by reading the "Chlorine / CYA Relationship" section in this post.

Unless you use a supplemental algaecide or phosphate remover, you need to keep the FC level in a manually dosed pool at least 7.5% of the CYA level or higher. In a saltwater chlorine generator (SWG) pool, the FC needs to be around 4.5% (so say 5%) of the CYA level. These are the minimums needed to prevent algae growth in virtually all pools regardless of nutrient level (i.e. phosphate and nitrate levels).