I have a SWCG, why doesn't my pH creep up this year? Lots of rain?

[wayner]

My pool is about 16 years old and I have had a Jandy Aquapure 1400 SWCG since day one. Normally I would have to reduce the pH by adding acid once per month of so through the swimming season. I have had to replace my salt cell every few years and I am currently using a non-Jandy salt cell that was a lot cheaper - I have had this cell for over two years.

This year my pH has stayed pretty much constant at 7.5 +/- 0.1 and I have not added acid once since opening the pool in early May. My TA is also constant at 80.

Why would this be?

We have had a lot of rain this year and I haven't had to add any water, in fact I have had to pump out water. Is it possible that the rain is reducing the pH? It was raining yesterday so I collected some rain water and the pH of the rain that I collected was 7.2. But googling the pH of rainwater gives an answer that rain typically has a pH of around 5.6 due to carbonic acid. Is it possible that there has been enough rain to keep the pH down?

I think my test set is accurate as I also use it to test my spa and the spa pH goes above 8 on a regular basis and I have to add a bit of acid to the spa every week or two.

The other difference in my system is that I have a solar cover and most weeks I keep the solar cover on from Monday-Friday, although for several hot weeks I kept the cover off and it didn't make a difference on the pH level.

 

[mknauss]

The SWCG has no appreciable effect on pH. Adding fill water that is alkaline does. So the pH is stable as you are not adding alkalinity to the pool water.

 

[wayner]

Thanks, I did test my tap water and it seems to have a pH of about 7.8. So using rain as "fill water" rather than tap makes that difference.

But I thought that the aeration of a SWCG, bubbles that come from the electrolysis, causes pH to rise - you're saying that's not the case?

Here is one thread here from over a decade ago on the issue https://www.troublefreepool.com/threads/why-does-ph-rise-with-salt-systems.47539/f

 

[mknauss]

The SWCG process is pH neutral. The hydrogen bubbles do not aerate the water and thus do not provide for CO2 out gassing.

 

[JamesW]

Anode 4Cl^- -> 2Cl₂.

Cathode 4H₂O -> 2H₂ + 4OH^-.

2Cl₂ +2H₂O -> 3H⁺ + HOCl + OCl^- + 2Cl^-

HOCl + OCl^- + uv light -> O₂ + H⁺ + 2Cl^-.

Following the process, we can see that there are 4H⁺ and 4OH^- created, which nets out to pH neutral.

The chlorine gas generated is very acidic and creates 3 hydrogen ions for every 4 hydroxide ions created.

As the hypochlorous acid is broken down by UV, 1 more hydrogen ion is created for a net neutral result.

Assuming that chlorine gain and loss are equal, there's no pH rise.

 

[reggiehammond]

Thanks, I did test my tap water and it seems to have a pH of about 7.8. So using rain as "fill water" rather than tap makes that difference.

Test your fill water TA. I would have expected with TA 80 that you still would have some pH rise until it's down to 50-60 or so.
I'm on a high TA well, so when I add water to my pool during summer evaporation, I also have to knock pH down to lower TA constantly.

 

[wayner]

Test your fill water TA. I would have expected with TA 80 that you still would have some pH rise until it's down to 50-60 or so.
I'm on a high TA well, so when I add water to my pool during summer evaporation, I also have to knock pH down to lower TA constantly.

I tested the water from my tap and it was 7.8, I didn't test the TA. My water comes from the city of Toronto. Googling shows that according to local beermakers the pH ranges from 7.3 - 7.8 and TA is typically around 90. They should know!

 

[Bperry]

I tested the water from my tap and it was 7.8, I didn't test the TA. My water comes from the city of Toronto. Googling shows that according to local beermakers the pH ranges from 7.3 - 7.8 and TA is typically around 90. They should know!

Be careful trusting google/etc. Just cause lots of people are saying something, doesn’t make them knowledgeable. If I trusted them, I’d just stick a bag of copper fittings in my skimmer and then be concerned about chlorine or any of that other stuff.

 

[wayner]

Fair enough, but these folks seem to have a vested interest. When you make beer the characteristics of your water are very important. Toronto Water Profile | GTA Brews
And the city releases an analysis: https://www.toronto.ca/wp-content/u...S-24-0074DrinkingWaterAnalysisSummaryAODA.pdf

 

[JoyfulNoise]

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The other difference in my system is that I have a solar cover and most weeks I keep the solar cover on from Monday-Friday, although for several hot weeks I kept the cover off and it didn't make a difference on the pH level.

Covers make a big difference to pH. If you cover the pool, CO2 cannot outgas from the water and pH will not rise. You don’t need aeration for out gassing to occur - you have a large surface area of water in contact with air, the CO2 will outgas with or without aeration. That’s just the nature of dissolved inorganic carbon in water … happens in every body of water and ocean on the planet. Your pool is no different. So if you cover it, the pH will stay wherever it is. When you remove the cover, outgassing will occur and if there’s enough of it you’ll get a pH rise.

 

[wayner]

Then it is likely the cover - that is the biggest change in the last year or so, other than the amount of rain.

 

[AUSpool]

Then it is likely the cover - that is the biggest change in the last year or so, other than the amount of rain.

More likely that is a combination of the cover and swapping rain water for tap water. Plus a bit of luck that it all worked in your favor.