Ph Is Finally Where It Needs To Be.

[jasontjames]

After 6 months of head scratching and adding copious amounts of acid, my pH is finally in the ideal range. I have a 10k gallon gunite salt water pool that was built this past summer.

How did I finally get the pH to come down? I turned off the waterfall for a week. Don't know if this is just coincidence or not. I read somewhere on the vast expanse of knowledge that is the internet that aeration by a waterfall could cause the acid that I was adding to be ineffectual. Does this theory hold water?

I am curious to find out from the experts on here whether or not that is what allowed me to get my pH under control.

If it is what allowed me to get in down, how long should have I be running the waterfall at a time? I am new to the pool world and would like to know if I am doing something wrong.

Thanks,

Jason

[chem geek]

What you experienced is completely explained by the chemistry/physics. Aeration increases the rate of outgassing of carbon dioxide and that causes the pH to rise. However, decreasing aeration by turning off a waterfall is not the only way to handle this. Fundamentally, the Total Alkalinity (TA) is too high so whenever you find that your pH tends to rise and you aren't explicitly adding chemicals that would cause such a rise, you should lower your TA level. This is especially true in saltwater chlorine generator (SWG) pool since the aeration and chlorine outgassing tend to make the pH rise anyway, even without a waterfall.

If your CYA level is high at 70-80 ppm, which helps lower chlorine consumption thereby letting you lower your SWG on-time (which reduces outgassing which reduces the rate of pH rise), then your TA should be at around 70 ppm -- you might need to increase your Calcium Hardness (CH) somewhat, especially if you go any lower in TA. Also, you can set a target pH of 7.7 instead of 7.5 as that will also help.

I suspect that you weren't just adding acid because if you were, then over time the TA would drop and the rate of pH rise would slow down. I suspect you were also seeing the TA get lower and added baking soda or Alkalinity Up to raise it -- that was part of the problem. Higher TA levels have the pH rise faster.

You can learn much more about this in the Water Balance for SWGs article in the Pool School at Trouble Free Pool.

Richard

[jasontjames]

What you experienced is completely explained by the chemistry/physics. Aeration increases the rate of outgassing of carbon dioxide and that causes the pH to rise. However, decreasing aeration by turning off a waterfall is not the only way to handle this. Fundamentally, the Total Alkalinity (TA) is too high so whenever you find that your pH tends to rise and you aren't explicitly adding chemicals that would cause such a rise, you should lower your TA level. This is especially true in saltwater chlorine generator (SWG) pool since the aeration and chlorine outgassing tend to make the pH rise anyway, even without a waterfall.

If your CYA level is high at 70-80 ppm, which helps lower chlorine consumption thereby letting you lower your SWG on-time (which reduces outgassing which reduces the rate of pH rise), then your TA should be at around 70 ppm -- you might need to increase your Calcium Hardness (CH) somewhat, especially if you go any lower in TA. Also, you can set a target pH of 7.7 instead of 7.5 as that will also help.

I suspect that you weren't just adding acid because if you were, then over time the TA would drop and the rate of pH rise would slow down. I suspect you were also seeing the TA get lower and added baking soda or Alkalinity Up to raise it -- that was part of the problem. Higher TA levels have the pH rise faster.

You can learn much more about this in the Water Balance for SWGs article in the Pool School at Trouble Free Pool.

Richard

I did not add any baking soda. The TA was running at 170ppm and it is now at 70. I will monitor the TA over the next week and see if it stays low. Should I try to move it up to about 80-90ppm or should I try to maintain it at the 70ppm level? Thanks for the reply.

[chem geek]

Well, with the TA starting out at 170 ppm, no wonder you were fighting rapidly rising pH. Did you find that over time the rate of rise seemed to slow down, though was still unacceptable? As for what to do now, that depends on whether you plan on running your waterfall. If you do and if this still causes the pH to rise, then you can continue to lower the TA via acid addition, but as I said above, if you lower it much more you'll want to compensate via a higher pH target and/or higher Calcium Hardness (CH). The Pool Calculator can be used to calculate the saturation index -- don't forget to include the salt level since that's usually around 3000 ppm for a saltwater chlorine generator (SWG) pool. You'll want the saturation index to be near zero or slightly negative (perhaps -0.1 or -0.2 but not much below that) to avoid excessive scaling in the SWG cell. I wouldn't raise your TA at this point -- if the saturation index is low, then raise the pH a little (up to 7.7) and/or the CH (assuming you've got a plaster pool).

Richard