Poolsmith CO2 injection for pH control

[JoyfulNoise]

I did a quick check in my area ... only one outfit refills tanks as far as I could find (the rest only exchange) and the price is roughly $2.50/lb to fill a tank.

I did some quick calculations for my pool and 1 lbs of CO2 gas injected is the equivalent of adding about 35 fl oz of MA. That much MA lowers my TA by ~ 8.5ppm which is what I typically will add every 7 days or so. I think the last time I bought MA, I spent about $5/gallon. So each acid additions roughly costs me about $1.40. If I use an equivalent amount of CO2, about 1 lbs, it costs $2.50. So it would cost me about $1 more using gas injections.

Of course, there is the convenience factor of using a dosing system and CO2 injection doesn't affect TA so the CSI will remain more stable. Also, no chloride is added with CO2 injection.

I would say that, in general, the cost of using gas versus liquid is more for just about anyone but it's not prohibitively so. And, if it gets you the convenience factor you’re looking for, then the reduction in hassle is likely well-worth to additional cost ... most people that own pools already factor in the "luxury cost" to their monthly budgets so haggling over a few extra dollars here or there during a swim season is not really all that important. And you can always switch back to using liquid in the winter when the acid/pH demand is a lot lower and save the gas for the swim season.

 

[cptkirk]

I did a drop test on pH this morning (before running the Poolsmith system), it was around 7.8 (I had added a quart of acid the day before). I ordered an Apera PH-60 meter that came in this afternoon, and after calibration it measured 7.56 after today's set of injection cycles (drop test was around 7.6 so looks like they agree). I'm planning to test the pH each day in the morning (before injection) and in the afternoon (after injection) to see how the pH changes over time. Poolsmith recommends to run the controller in "standard" mode (cycles run every day) for the first two weeks, then after the pH stabilizes at the desired level, switch to "saver" mode (cycles run every other day).

 

[cptkirk]

Matt, I'm interested in your calculations on the equivalence between CO2 and MA. Can you show your work? Thanks.

 

[cptkirk]

So far performance has been pretty consistent. I'm currently running the SWG at 20% from 8am to 5pm, running the pump at 50% 24hrs. The CO2 injection cycles start at 8am, and I think they are running for about 3 to 3.5 hours (2 minutes on, 2 minutes off during that interval). When I measure the pH in the morning before chlorination/injection starts, the pH is around 7.8. When I measure the pH in the late afternoon (after chlorination ends but before my wife goes for her daily swim), it is around 7.6. I am currently sourcing C02 from just one of the tanks so I can get a better idea of consumption.

 

[JamesW]

You should track your alkalinity over time to see if it's increasing or staying the same.

Keeping the CSI as close to zero as possible will significantly reduce the amount of carbon dioxide that you need to use.

Keeping the pH at 7.8 to 7.9 and the TA as low as necessary to keep the CSI and pH stable will also help.

 

[Flying Tivo]

The tanks are ganged together and run through a single tube which I threaded through some flexible conduit to the controller.

I would not Gang all three together, just gang 2 of them and leave the third one as backup. When the first 2 run out and you need a fill you can run on the third without loosing control and so on.

 

[rockaway99]

Looking into installing a Co2 system as well - so this is a very timely thread!
Did you purchase your tanks and regulators on your own or through Poolsmith?
I'm looking forward to your updates - I've had it with the acid game, regardless of the cost.
I've tried the Stenner route and had a pretty expensive Ph controller for awhile before ripping it out due to chronic probe failures.
The Co2 system seems quite practical to me.

 

[cptkirk]

Alk is currently 80 ppm. Will test weekly to see how it changes. Calcium is currently around 290, will bring it up to about 350 in the next day or so. CYA is currently about 30, probably will bring it up to 40. I don't really seem to need to bring it up higher, my CL is well controlled at this level. If I keep my salt around 3000, alk 80, CYA 40, CH 350, pool around 82 degrees, then a pH range from 7.6 to 7.8 will have everything in pretty good balance

 

[cptkirk]

I would not Gang all three together, just gang 2 of them and leave the third one as backup.

As currently configured, I can't disconnect a tank from the regulator assembly while the other tanks are open, the gas will leak out of the open tanks. Would have to get a plug to take the place of the tank that has been removed.

 

[cptkirk]

Did you purchase your tanks and regulators on your own or through Poolsmith?

The regulator assembly comes from Poolsmith as part of the kit. You can order a one, two, or three tank configuration. You can order the tanks through Poolsmith, but the price is slightly better to buy on your own. I'd check first on whether refills are readily available in your area or whether you will need to do a tank exchange. If the latter, check on whether the supplier will sell you tanks that are already filled and see how the price compares.

 

[rockaway99]

The regulator assembly comes from Poolsmith as part of the kit. You can order a one, two, or three tank configuration. You can order the tanks through Poolsmith, but the price is slightly better to buy on your own. I'd check first on whether refills are readily available in your area or whether you will need to do a tank exchange. If the latter, check on whether the supplier will sell you tanks that are already filled and see how the price compares.

Thank you for your response.
I checked with a local supplier for Co2 - NexAir. They quoted me $154.11 for each tank, and $23.61 for refill/swaps.
I'll take a ride over there to firm up that part.

 

[cptkirk]

So far performance is consistent, with the pH generally being about 7.8 in the morning (before injection and chlorination) and 7.6 in the afternoon (after injection and chlorination have completed). I have the flow rate set to 2.5 cubic feet per hour according to the meter, and the injection cycles run for a bit over 3 hours (with injection occurring half the time during that interval). So it would appear I'm injecting around 4 cubic feet per day (rounding up), which means a 20 lb tank should last about 45 days at current consumption rate. Supposedly after a couple of weeks I should be able to cut the consumption rate in half, we'll see if that's how it works out. So far no measurable increase in TA, but it's only been about a week.

 

[JoyfulNoise]

Injecting CO2 does not change TA. CO2 gas undergoes hydrolysis and forms carbonic acid (H2CO3). Carbonic acid dissociates into a hydrogen ion (H+), which reduces pH, and a bicarbonate anion (HCO3-). Every hydrogen ion added decreases TA by one equivalent but every bicarbonate ion created increases TA by one equivalent. So the net effect of adding carbon dioxide to water is no change in TA but a decrease in pH. If you have any increase in TA at all it will be from the addition of external chemicals (like baking soda) OR from fill water that has a TA higher than your pool water.

Sounds like you've got a good system setup so far. Keep us posted on progress so others can learn about the maintenance and costs involved.

 

[cptkirk]

I tested my fill water before I hooked up the system, CL was 1.2 ppm, ph 7.8, Alk 70, Ca 90. If it turns out I don't have to use acid to knock the TA down every now and then, so much the better. Then I'll only need acid to clean the salt cell.

 

[mknauss]

With those water chemistry parameters, you should never have to clean the SWCG.

CH ( if that is what Ca is) is really low. Especially for a plaster pool.

 

[ajw22]

Then I'll only need acid to clean the salt cell.

Every acid cleaning of a SWG removes some of the rare earths on the plates and decreases the life of the cell. You should only clean it with acid when there is visible scale on the plates that can not be removed by blasts of water and scraping with a Popsicle stick.

 

[cptkirk]

To be clear, those numbers are for the fill water, not for the pool itself. On last test a couple days ago, pool TA was 80, CH 310.

 

[JoyfulNoise]

You have very good fill water, your muriatic acid use should be very minimal. Those calcium values for FL are not at all surprising. Eastern states tend to have very low CH aquifers from which water is drawn. Midwestern states tend to draw water from deep wells in contact with limestone and western states tend to use surface waters (rivers) for water sources. Calcium and magnesium are much more prevalent in those water sources.

 

[cptkirk]

System has been in operation for two weeks now. We've had quite a bit of rain over the last week, so had to add calcium, salt, and CYA (currently 310, 3000, and 40 respectively). TA has not budged measurably. My pH seems to be at a lower baseline, consistently around 7.4 in the morning before injection occurs (previously it was 7.8 at that time), which is really lower than I want it. I switched today to "saver" mode, which is supposed to only run injection cycles every other day. I also decreased the flow rate during injection from 2.5 to 2 CFH. I'll continue to monitor pH daily to see what happens. So far I'm happy with the system. Other than the additions mentioned above due to dilution, I haven't had to add anything else. I kept a gallon of acid around just in case, but returned my surplus to HD.

 

[rockaway99]

How did you address the issue of tank storage? I did see your picture with the container you built, but what about the heat?
My research has indicated a high risk of explosion for tanks stored in hot, humid conditions, such as my neck of the woods in the summer. That is a big issue.
Also, do you have a sense yet of how long a 20lb tank will last before refill?