pH changes from CO2 loss

[red-beard]

Normally, I go from 7.5 to 7.8 pH in about 3 days. I add about 1/2 Gallon of Acid, twice a week. TA is 70.

For the past several weeks, I have had a solar pool cover on my pool. Since I've had the cover on, my pH has been almost unchanged. We have a fairly windy backyard so I'm guessing my pH change is CO2 outgassing. The cover is on 24/7, except when I'm swimming, maybe 4-8 hours per day - 3-4 days a week. Pool is uncovered less than 24 hours per week.

For next year, when the pool is fully open, is there a way to keep the CO2 in the pool? I see some people try to stabilize pH by adding CO2. As a home brewer, I know that CO2 is cheap. If I get it in 20 or 40 lb tanks from a welding supply, it is very cheap. Anyone have directions on how to add CO2? I have a line from a defunct ozone producer, which might be of use. I could also use a CO2 beer stone embedded in the wall of a piece of PVC pipe. Is there some sort of flow regulator for this application? I like the idea of an automatic system. I do not need a pH control, as I can adjust it like my SWG.

 

[Dirk]

Sorry, can't help with the CO2, but I do love your idea to turn your entire pool into a giant beer keg (I think that's what you are asking about)!!



PS. I'm lovin' the signature, but where's my residual check?!? I'd like to avoid the trademark lawyers, if possible.

 

[JamesW]

There are multiple factors that affect the offgassing of carbon dioxide. The first is the amount of carbon dioxide that is dissolved in the water.

The amount dissolved in the water is determined by the carbonate alkalinity and the pH.

The pH determines what percentage of the carbonate alkalinity turns into carbon dioxide.

pH........%CO₂
6.35........50%
6.5..........41.5%
6.75........28.5%
7.0..........18.3%
7.2..........12.4%
7.4............8.2%
7.6............5.3%
7.8............3.4%
8.0............2.2%

So, you can see that the pH makes a big difference in how much carbon dioxide there is.

The other factors are all familiar from soda or beer. Containment by capping or covering helps keep carbon dioxide from offgassing.

Shaking or agitation causes carbon dioxide to offgass.

Heat causes carbon dioxide to offgass.

 

[red-beard]

That is exactly what I'm looking for. So if I'm going from ~6.75-7% CO2 to 3.4%. At a TA of 70 ppm, I'm losing about 3.5%, which would be about 2.5 ppm, or about 2.5 mg/kg. Pool is about 120,000 kg, so my loss is about 300 grams in 3 days or about 100 grams per day. A 20 lb CO2 tank should last about 90 days. I expect my CO2 loss will fall off with temperature at the ratio of increased solubility. Maybe 3 tanks of CO2 per year? With something that doesn't go bad. I just need a regulator for ~2 liters per hour, which is pretty small.

 

[JamesW]

The numbers just give you an idea about how much carbon dioxide is in the water, not how much offgasses.

Just keep the TA down and the pH up and the pH rise will mostly stop.

 

[Rattus Suffocatus]

There is a system out there that does just that...https://www.discountsaltpool.com/circupool-total-balance-ph-control-acid-free not inexpensive at all...

I am a homebrewer too.. Carbonic Acid is a relatively weak acid compared with muriatic but it definitively makes a difference as you can tell in the table. Beer is usually around 4.0-4.5 range when fermented, so bear in mind that using the table above at pool water pH levels will be a different ball game than beer.

I have struggled in the last month or so with my pool water temperature getting below 80F to keep the chlorine levels low enough with the SWCG.. with the cover on, and no one swimming (light weights! I think I am going to try this weekend! 77F at the end of the day is as warm as it ever got in the upper Midwest when I was growing up!) I am only losing about 1-2 PPM of FC a day, and pH changes take forever.. so what you are seeing is exactly I am seeing without the pool being used... I literally have my system running at 20% for only 6 hours a day and it's producing too much chlorine at present!

The CO2 injection system can work, especially at the lower alkalinity levels you see. I probably would go with muriatic acid injection for a system here in super high TA southern Arizona though. I suspect you could easily build and have something like that work, or at least assist greatly in keeping your pH levels a bit lower. I really struggle getting my TA under 100 here... I mean a lot....