Adjusting pH under differing TA

[Aqua Lab Rat]

I like to think I have a moderate level of understanding of pH and TA. Definitely not a beginner, definitely not a chemist. But I do a lot of balancing of bodies of water. I know that the amount of chemical needed to tweak pH is affected by (and affects) TA. A lot of the dosages on product labels seem weak to me, and I assume it is mostly to avoid over-dosing. Anyway, here's my question:

If I wanted a rule of thumb for the change in a carbonate or acid dose for high or low TA (arbitrarily 60 or 140) versus 90-100, what kind of magnitude/percent difference would that be? I don't expect a precise answer, just trying to get a general understanding. I'm not really finding good information about this, maybe for a good reason. Might be time to set up an array of little buckets do some experimenting.

 

[Texas Splash]

Might be time to set up an array of little buckets do some experimenting.

Perhaps. Ha. In a controlled setting you might be able to obtain some sort of standard deviation and/or predictable response, but not necessarily in a pool. Much depends on various factors besides just the pH and TA like amount of swimmers, aeration, fill water parameters, degree of separation between the pH & TA, amount of water replenishing, and I'm sure some other items that someone like @Donldson could elaborate more upon.

To your question specifically about the potential differences of dosing to adjust TA based on where it is (extremely high, low, or middle of the road), I'll defer to JD for a more scientific response.

 

[AUSpool]

With borates or without? You can have a play with PoolMath or chem geeks spread sheet.

https://www.troublefreepool.com/~richardfalk/pool/PoolEquations.xls

 

[Aqua Lab Rat]

I've thought about tweaking around with a calculator like Chem Geek's spreadsheet or similar. And I know there are plenty of variables that can affect it. I like messing around with calculations, too. But really I'm looking more for a general rule that I can apply to the 30 or so bodies of water I deal with every day, which could have vastly different chemistry depending on the test being run. For example, I know that based on my source water, I'll probably need to add at least a pound of dry acid and 1.5 cups of calcium per 250 gallons when starting up a spa. General rules like that, except in this case it might be "If my TA is a little high, add 25% more dry acid when adjusting pH".

Anyway, thank you for the replies so far. I don't think I'll be satisfied until I play around with it. If I get something useful I will share it.