Understanding TA / pH Changes

[Stoopalini]

I think I understand the basic principals for controlling pH, and the impact on TA:

• Lowering pH:
  • Use Muriatic Acid - Will also lower TA
• Raising pH:
  • Airation - Will not raise TA
  • 20 mule team Borax - Will raise TA slightly
  • Baking Soda - Will raise TA more than 20 mule team Borax

Since my Pebblesheen pool is not even a week old, I assume I'll be frequently lowering pH for a while. My TA started at 160, so it's been pretty easy to just add MA to lower pH and TA, then wait for the pH to rise to 7.8/8.0, and add more MA.

But now that my TA is down to 90, and I expect it to continue to drop as I add MA to lower pH ... so I'm wondering what my MO should be going forward.

For example: Once my TA gets down to the 60 or even 50 mark, but my pH is still rising over time ... do I continue to use MA to lower the pH, but then have to do something to raise the TA back up since the MA will continue to lower TA? Or ... once the TA is down in the 60-70 range, will it become less affected by MA additions, and it'll remain stable in that range even though I'll be adding MA to lower pH?

I realize I'm not there yet, but I expect I will be within a week or so, and just want to be ahead of the curve for my plan of attack.

 

[mknauss]

You will find your pH will not rise much with a lower TA. You also do not have as much TA drop with each acid addition as the TA lowers.
You should never get into the hamster wheel of chemistry of adding acid then adding baking soda. Do not over correct the pH.

Baking Soda = big TA change, small pH change
Borax = Big pH change, small TA change
Washing Soda = big pH change, big TA change. Probably more TA change than you want.

 

[Stoopalini]

You will find your pH will not rise much with a lower TA. You also do not have as much TA drop with each acid addition as the TA lowers.
You should never get into the hamster wheel of chemistry of adding acid then adding baking soda. Do not over correct the pH.

Baking Soda = big TA change, small pH change
Borax = Big pH change, small TA change
Washing Soda = big pH change, big TA change. Probably more TA change than you want.

Nice, thanks Marty. Up to this point, I've been dropping the pH to 7.2, then aerating a lot with my waterfall, to intentionally raise the pH up to ~8, so I can add MA to drop pH back to 7.2 .... this is just to get my TA in line though.

If I understand what you're saying ... the high TA is a contributing factor to how fast my pH is rising with the new plaster, as well as with the aeration. So when my TA is at a more reasonable level (say 70), it won't rise as fast anymore, and of course, I won't run the waterfall for 6-8 hours a day anymore either.

At this point, with a current TA of 90, I was going to change my MO to only lower pH to 7.6 (instead of 7.2) going forward, but still make the MA additions around ~8.0 pH level.

According to PoolMath, lowering the pH from 8 to 7.6:

• At 90 TA, requires 13oz of 31.45%, and will lower TA by 3.6
  • Close to 3 doses before TA drops to 80
• At 80 TA, requires 12oz of 31.45%, and will lower TA by 3.3
  • 3 doses before TA drops to 70
• At 70 TA, requires 11oz of 31.45%, and will lower TA by 3.1
  • Roughly 3.25 doses before TA drops to 60
• At 60 TA, requires 9.4oz of 31.45%, and will lower TA by 2.6
  • Close to 4 doses before TA drops to 50

I do see the pattern, and how the lower TA values result in less MA needed to adjust the same pH change ... But these changes will still accumulate over time, won't they? Or will the TA recover from these small changes naturally, and I won't need to intervene and intentionally raise the TA?

Part of the underlying reason for my question has to do with the waterfall. My wife really likes it running, but I understand I won't want to run it as much once my TA is below 80. I think I'll end up scheduling the waterfall with the EasyTouch, and it'll be some trial and error until I find a good balance between pH and TA impacts from the waterfall's aeration and MA additions.

 

[mknauss]

If you run the waterfall all the time, your pH will rise, and you will fight with TA. Best to only run it 30 minutes a day or so unless you are enjoying it poolside.

The pH rise due to new plaster will slowly decrease. It will be taken over by the rise in TA due to your fill water. You will balance with an occassional acid addition, more in summer when you have evaporation, and your TA will stabilize. Unless you run the water fall incessantly.

 

[Stoopalini]

Ahhh, the auto-fill water ... makes sense. My fill water measures TA 160, so I can certainly see how that will help keep things in check.

Thanks again Marty!

 

[Stoopalini]

So in an ideal world, the auto-fill would add enough water to raise the TA in line with respect to how much the pH rises from running the waterfall. So when I add MA, it reduces the pH to compensate for the aeration, and also lowers TA to compensate for the 160 TA make-up water coming from the auto-fill.

Of course, i realize I'm far from the ideal world scenario ... but it helps me to understand the system as a whole to think of it like this.