How low can pH safely be if CSI is balanced?

[BrandonWhite]

How low can I safely lower my pH assuming my CSI stays in the balanced range?

By safely, I mean lack of damage to pool plaster and possibly other equipment or stone, tile, concrete.

What I'm getting at, is it the CSI that really matters for corrosion vs scale but we instead focus on a "normal" pH range since that achieves balance for typical water profiles?

 

[ewkearns]

CSI is an index that essentially measures the aggressiveness of the water. pH is maintained, I think, for bather comfort.... perhaps in concert with NaCl.

Excessively low pH can be troubling to the eyes....

 

[Richard320]

7.2 is the lowest you should go for swimmers' comfort.

And there's more than just the plaster to worry about. Even if CSI is good with super low pH, the metal guts of the heater won't like acidic water. Here's an extreme example of what can happen. Maintain your chemicals correctly

 

[chem geek]

DIN 19643 used in Germany and some other countries in Europe as the commercial/public pool standard uses a pH range of 6.5 to 7.2 when alum salts are used for coagulation and 6.5 to 7.5 when iron salts are used for coagulation. However, they design their pools to handle the increased metal corrosion from such lower pH levels. Water utilities typically raise the pH of their water to 7.8 or more to help reduce metal corrosion. So setting 7.2 as a low end is reasonable, especially because it is much harder to maintain 7.0 than 7.2 due to the strength of the carbonate buffer system (i.e. much more carbon dioxide outgassing).

See also Recommended Levels. The reason Recommended Levels is higher is because it is intended to be used without reference to CSI. If you are having a much higher CH or TA then you can have a lower pH, but note that it will be difficult to maintain the low pH if the TA is higher.

 

[chiefwej]

So it sounds like the short answer to the OP's question is 7.2.

 

[BrandonWhite]

I appreciate all of the answers. Indeed I'm learning that 6.8-7.0 doesn't last very long at all. It is always back to 7.2 within several hours.

I rely on my kids' telling me if the water stings their eyes. They'll complain if the water get's too alkaline (7.8 is where I recall the complaints), but so far no matter how acidic I get it, they say "It's fine". I've taken it down to 6.8 a few times.

The reason I'm wondering about all of this, is due to my high CH&TA fill water. The past few months I've been in this cycle where I fill up with ~800 gallons every weekend, then try to lower the TA. The "add acid and aerate to lower TA" is a regular weekly thing.

I don't have a heater or anything metallic in the plumbing. There is no stair or handrail hardware. The only thing I can think of is the stainless ring on the light. So it's had me wondering if adding acid to 6.8 pH once or twice a week is going to really hurt anything, given that my CSI is still Balanced at this pH.

 

[chem geek]

You can get to a point where you don't have to aerate. Instead, the TA addition from the fill water equates to your acid addition to lower it and the pH goes back down to where you started. As you have seen, there is more pH rise at lower pH so if you were instead to try and go from 7.8 to 7.5 then there may be a lower TA where everything balances out. In other words, instead of trying to have things balance at a lower pH, try and have them balance out at a lower TA instead. You should find that you use less acid that way.

 

[chiefwej]

I sure wish I could find that balance, but my fill water is high TA, and I have very high evaporation. I can spend days lowering the TA, but within a week or two it's right back up. Added borates and that helped a bit with pH rise, but there seems to be no balance point.

 

[chem geek]

I see, so you actually need to target a lower pH to have more outgassing so that you are forced to add more acid to lower the pH and TA so that the TA is kept in check because it rises so much from evaporation and refill. Bummer.

 

[chiefwej]

I see, so you actually need to target a lower pH to have more outgassing so that you are forced to add more acid to lower the pH and TA so that the TA is kept in check because it rises so much from evaporation and refill. Bummer.

Yep, it's a vicious circle. And very high calcium makes it worse. I'm always having to watch the CSI. I bought a little help, plumbing my fill to the house's water softner.