Algae, TA or what?

[chem geek]

In fact, I'll start lowering TA on my pool either tonight or this weekend--it's about 80, so getting it down to 50 should be a snap. Plus I've got my "splashers" to aerate it!

Carl,

Unless your pool experiences a regular rise in pH over time, then lowering the TA isn't going to show any noticeable difference. Also, there is a slow upward pressure on pH from chlorinating liquid and bleach due to the "excess lye" and this varies by brand with 6% Clorox Regular unscented having the lowest with a pH of 11.9. Some off-brand bleaches have a pH of 12.5 or higher. Chlorinating liquid also varies with better 12.5% brands having a pH of 12.5 while others have 13 or higher. If you tell me the concentration and pH of your chlorine product and how much FC you use per day on average, I can tell you the pH rise (with TA assumptions) and amount of acid you need to compensate. No matter how low you lower the TA, you won't get rid of the need to compensate for this "excess lye".

As for recommended TA levels, when using a hypochlorite source of chlorine you generally don't want to go above 80 ppm, but with covered pools a higher TA isn't going to be much of a problem. As for how low to go, that depends on how much aeration there is in the pool and isn't easily calculated. So one would just have a more generic rule for those pools experiencing a regular rise in pH of letting the TA drop as low as 50 ppm if needed and seeing if this improves the rate of pH rise and then compensate pH and/or CH. For most pools, 70-80 ppm will be OK. It's SWG pools and pools with water aeration features that have more of an issue, partly due to increased aeration from hydrogen gas bubbles (though calculations show this probably isn't the entire effect) but also, I believe, from undissolved chlorine gas outgassing. This latter effect can't be compensated by TA and is probably worse in pools with short pipe runs from the SWG to the pool.

As for the point one needs to get in the carbon dioxide equilibrium chart before pH becomes stable, that is only able to be determined empirically and will be different for every pool since the rate of outgassing depends on the amount of air-water surface interaction so on aeration, wind, etc. Very roughly speaking, nearly everyone (with no pool cover) experiences a problem when they get into the red region with 20 or more times as much carbon dioxide in the water than the equilibrium amount from air, many have a problem in the orange region between 10 and 20, and most seem to do OK with some pH rise that is tolerable in the green region less than 10, but there are definitely those highly aerated pools who see a rise in pH until they get down to nearly 2 in the chart. For spas, the hotter temperature and vastly increased aeration from spa jets almost always has to get below 4 and sometimes down to 2 or 1 before the pH becomes stable. The use of 50 ppm Borates provides additional pH buffering when the TA is low, but without the negative pH rising side effect of the carbonate buffer system.

Thanks,
Richard

[CarlD]

"Covered"?

I use my solar cover (though not the last few days as the heat has been extreme) but do leave it off. It doesn't seal, of course.

Current TA is 80 so I guess I'll leave it there. pH hasn't moved from 7.6 all summer (it's usually 7.5 in past years). TA doesn't usually move much on me either. CYA is just under 50, FC is around 5. CH is pretty low--I don't measure it more than once or twice a season. I THINK it was 110 or 120. I have no real source of cal since I haven't used Cal-Hypo in a few years.

I generally use a higher qual LC that claims 12.5% but always tests at 14% (never thought to test the pH).

Normally, I add about half a gallon every other day, but lately, with the heat, I've been using more. In fact, this season, like almost everyone, I've used far more chlorine than usual. I've just drained the first gallon off my 5th carboy of the summer, and it's only early July. 5 is normally a full summer's worth.

I realize you are not "there" yet, but we are going to need practical guidance regarding TA levels "If you pH is this, and your FC is this, and...., and ...., then do THIS."

Thanks for all your hard work.

Carl

[sturev]

Hi Guys,

I'm willing to help if you want it. I have a unique pool that may help prove/disprove theories???

My pool is fully insulated and covered, it stays at a constant 88 degrees and is subjected to maybe an hour or so of sunlight a month (exercise in the evenings & the sun is off the pool). I have no CYA added to the water. I would think it would be considered a heavily aerated pool, as the counter current alone creates a lot of air bubbles and then put me in there flailing around trying to swim upstream ; the whole pool is filled with air bubbles for about 30-40 mins a day.

I've only used the BBB method on a fresh fill about 2 months ago. I have added a total of one box of Borax, 1/2 gal of Acid and 1.5 gal of bleach since the refill. I run FC between 2 & 4; only have to add 6 oz every 4 or 5 days. Right now my TA is on the high side (150; was 230 on fill and I've been working it down in the last few weeks). PH is 7.8, until I add acid to bring down the TA; it takes 48 hrs to go from 7.0 to 7.8.

Let me know if I can be of any help...

[Furbabyvet]

FWIW, I'm keeping the solar cover on the pool now and the pH seems to be rising more slowly, not to mention the chlorine disappearing more slowly. I just wish it didn't make it feel like you're swimming in a nice, comforting bowl of tomato soup . Gotta get a solid color cover...

[PoolDoc]

@ Sturev: thanks for the offer. I'll try to keep that in mind -- I'm working on some plans, now for research, writing, and PoolForum development.

@ Furbyvet: thanks for the info. Your response represents a confirmation of Chem Geek's analysis of TA and pH rise in pools.

Ben