Poolsmith CO2 injection for pH control

[cptkirk]

I built a little shelter to keep them out of direct sun. There's a picture in post #16 on this thread (dated April 3). It's open in the back and on one side that doesn't get sun. The tanks don't necessarily have to be close to the controller, just need to make sure you order enough tubing. Kit comes with 30 feet, they charge $1/foot for any extra you need. Mine are about 25 feet away. My understanding is the tanks have a relief valve that will pop to release the gas if the pressure gets too high.

I'm still experimenting with the injection duration and flow volume to keep the pH in the range where I want it. Also there are two modes, one where it injects every day, and another where it injects every other day. Initially, I was running injection every day for about 4 hours at 2.5 CF/H (note, injection only occurs half the time during the injection cycle, so that corresponds to 5 cubic feet per day). At that rate, a 20 lb tank (175 cubic feet) would last about 35 days. I'm currently running at the same duration but every other day and cut the flow back to 2.0 CF/H to see how that goes. Will update this thread when I settle on my parameters.

 

[cptkirk]

When I filled the tanks, they were at about 500 psi. They are rated to 3000 psi, so there is quite a bit of margin. If the worst happens, there is a safety valve that will blow and I'll lose that tank of gas, but there shouldn't be a violent explosion. The company is based in Phoenix and many of their customers are in that area, and the heat is far more brutal there than where I am in the FL panhandle. We'll see what happens over the summer.

 

[Prickly]

I decided to cancel my Hayward CO2 injection order and go with this system as well as I prefer manual adjustments versus relying on ORP.

I have a concern about injecting CO2 into the pump though as the water traveling to the heater will be quite acidic I assume. I have no way to divert water from my heater. Do you think I could hook it up to the pipe after the heater or can it only go in the pump port?

 

[rockaway99]

When I filled the tanks, they were at about 500 psi. They are rated to 3000 psi, so there is quite a bit of margin. If the worst happens, there is a safety valve that will blow and I'll lose that tank of gas, but there shouldn't be a violent explosion. The company is based in Phoenix and many of their customers are in that area, and the heat is far more brutal there than where I am in the FL panhandle. We'll see what happens over the summer.

Thank you - good luck with this project!

 

[JoyfulNoise]

When I filled the tanks, they were at about 500 psi. They are rated to 3000 psi, so there is quite a bit of margin. If the worst happens, there is a safety valve that will blow and I'll lose that tank of gas, but there shouldn't be a violent explosion. The company is based in Phoenix and many of their customers are in that area, and the heat is far more brutal there than where I am in the FL panhandle. We'll see what happens over the summer.

CO2 at normal outdoor temperatures and high pressure forms a dense liquid inside the tank similar to how pressurized propane is mostly a liquid. The total amount of gas in the head space of the tank is fairly low and catastrophic blow outs are highly unlikely. Any fracture in the vessel or break in the valve would simply cause the liquid CO2 to boil off and the tank would ice up and cool off considerably from the large amounts of heat absorb. It’s usually only tanks that are mishandled and damaged that have issues. This is why so many stations will only do tank swaps rather than refills because they don’t want the liability associated with someone that may hand them a damaged pressure vessel. Industrial gas delivery companies always swap tanks out and inspect them as part of the sale and delivery of gas to commercial customers because of the need to verify that tanks are in good condition. If you keep your tanks in good condition and inspect them regularly they should have very long service lives.

 

[cptkirk]

I have a concern about injecting CO2 into the pump though as the water traveling to the heater will be quite acidic I assume. I have no way to divert water from my heater. Do you think I could hook it up to the pipe after the heater or can it only go in the pump port?

I asked about that. Poolsmith could not tell me what the pH of the water post-injection was, but claimed that from their testing and field experience they have no reports of damage to heaters. My experience is that during 4 hours of injection cycles, the pH in my 20k gallon pool drops about two tenths of a point (say from 7.8 to 7.6). I think during that time I'm turning over about 9k gallons through my pump. Someone more knowledgeable than me could probably estimate how much the lower the pH is pre-injection vs post-injection, but it seems to me the water is not dramatically more acidic and would still be in the safe range for the heater unless you were maintaining a pH on the lower end of the spectrum.

The system as designed is intended to inject only at the pump. I guess it would be technically possible to rig up some other injection point but my guess is that system would lose a lot of efficiency (you would use more gas to get the same pH drop).

 

[JoyfulNoise]

I have a concern about injecting CO2 into the pump though as the water traveling to the heater will be quite acidic I assume...

The answer is ... not much.

If you think about it, the amount of water flowing through the pump is very large in comparison to the amount of gas added. Some rough numbers - assume that the pump is operating such that the system flow rate is around 40GPM (standard flow rate needed by most heaters). A pump strainer basket doesn't hold much more than about 2 gallons of water which means it is being emptied by the pump in about 3 seconds. The gas is being inject at a rate of 2.5 cubic feet per hour and so the amount of gas added in that 3 secs of time is about 0.0021 cubic feet or about 0.06L of gas. 1 liter of CO2 weighs about 1.964 grams and so you're only adding about 0.118 grams of CO2. If you calculate the weight of 2 gallons of water and divided it by the weight of the added CO2, you're only talking about 15.6 parts per million. In terms of acid, that is a very small addition to water that is highly buffered by carbonate and cyanurate alkalinity. So the instantaneous drop in pH is going to be minuscule and the heat-exchanger should be fine.

 

[Prickly]

The answer is ... not much.

If you think about it, the amount of water flowing through the pump is very large in comparison to the amount of gas added. Some rough numbers - assume that the pump is operating such that the system flow rate is around 40GPM (standard flow rate needed by most heaters). A pump strainer basket doesn't hold much more than about 2 gallons of water which means it is being emptied by the pump in about 3 seconds. The gas is being inject at a rate of 2.5 cubic feet per hour and so the amount of gas added in that 3 secs of time is about 0.0021 cubic feet or about 0.06L of gas. 1 liter of CO2 weighs about 1.964 grams and so you're only adding about 0.118 grams of CO2. If you calculate the weight of 2 gallons of water and divided it by the weight of the added CO2, you're only talking about 15.6 parts per million. In terms of acid, that is a very small addition to water that is highly buffered by carbonate and cyanurate alkalinity. So the instantaneous drop in pH is going to be minuscule and the heat-exchanger should be fine.

That's great. Thank you for that explanation.

 

[Prickly]

My Poolsmith is up and running. I'm impressed with how simple it is and how well it is working. It drops my pH much quicker than I expected. I suspect I will be able to have it on every second day for less than half a day. I'm glad you posted this thread. I'd be fiddling with finicky ORP probes on the Hayward CO2 system if you didn't share this!

 

[cptkirk]

I'm still experimenting with my settings. I'm currently running it every day, but I've cut back the flow rate from 2.5 CFPH (where it was initially) to 1.5 CFPH. I figure running it every day will keep the pH in a narrower range.

 

[cptkirk]

Went out to the equipment pad today and noticed that the pressure sensor connected to the filter pump for the CO2 system had a slow drip leak. Seems to be leaking from the connector, not the threads. Texted Alex (one of the owners), he put a new sensor in the mail today. Not ecstatic about the problem, but at least the customer service is good.

Also, through some experimentation I think I have determined the relationship between the infusion setting and the injection time. For each multiple of 10 on the infusion setting, you get 4 cycles of injection (where a cycle is 2 minutes on, 2 minutes off). So for example, if the infusion is set to 180, that corresponds to 72 cycles. The total cycle time is therefore 288 minutes (4 hours 48 minutes), and it's actively injecting for 144 minutes (2.4 hours). If the flow rate is 1.5 CFPH and I'm injecting every day, that means I'm using 3.6 CF each day, and a 20 lb. tank should last about 175/3.6 = 48.6 days. So with my 3 tank set up, I'll have to refill every 4 to 5 months with those settings.

 

[JoyfulNoise]

You don’t use the pool year-round, right? If so, then in the winter months you can switch to MA as the pH will slow down a lot.

 

[cptkirk]

We haven't been using the pool in the winter, but I don't close it up either. The SWG gets turned down some, but that's the only real difference, and my acid demand doesn't seem to change that much (at least that's been my experience so far). I've been measuring pH daily using a good meter, and I'll continue to do that. If it turns out that my pH is dropping after the season, I'll just tweak the settings to reduce the flow rate and/or number of cycles rather than go back to acid.

 

[JoyfulNoise]

We haven't been using the pool in the winter, but I don't close it up either. The SWG gets turned down some, but that's the only real difference, and my acid demand doesn't seem to change that much (at least that's been my experience so far). I've been measuring pH daily using a good meter, and I'll continue to do that. If it turns out that my pH is dropping after the season, I'll just tweak the settings to reduce the flow rate and/or number of cycles rather than go back to acid.

So this is where the chemistry gets a little bit more complex. Think of your pool water like a can of soda. When the can is closed and there's lots of dissolved CO2, the pH is low. As soon as you open the can, the CO2 wants to leave the soda and, as it does, the pH rises. This is exactly what your pool is doing - you are finding the equilibrium point at which you are injecting as much CO2 as the water is off-gassing at a desired pH. So, once you stop adding CO2, the equilibrium will shift to more CO2 leaving the water and your pH will rise. In the winter, if you were to use muriatic acid to control pH, you would keep adding acid at a rate that gives you the pH level you desire. The carbonate alkalinity (part of the overall TA) will shift until you reach that new equilibrium of adding acid and off-gassing CO2. In the winter, when the pool is not in use and the water is colder, you can easily let the pH settle in at a higher value, say 7.8, and not add acid until it goes over 8.0. Then, only add enough acid to get back to 7.8. What you will find is that your water will reach a TA value that gives you a very stable equilibrium such that you're probably only adding acid once every 10 to 14 days. Then, when you want to open the pool for swim season, just turn on your CO2 injection again and maybe adjust the TA to return to the value you had this season.

A bit more work, but it could save you some expensive cylinder refills.

 

[cptkirk]

I think I follow you, but my question though is how far would my TA have to drop to reach this equilibrium? In looking at my LSI calculator, if the water temp is say 58, pH 7.8, CH 360, CYA 40, salt 3000, then TA of 60 gives me an LSI of -.26. To get closer to zero I'd have to jack up the CH quite a bit higher. Whereas, if leave the TA around 80 (where it is now) and I keep a steady 7.9 via CO2 and leave the other numbers alone then I'll be good. Since the water is colder, there *should* be less outgassing (right?) and I should be able to lower my CO2 consumption compared to swim season (I would think). Personally I would rather do that than go back to adding acid.

BTW, thanks for all your input on this thread. I'm going to try to make your zoom call on Saturday, the wealth of information you provide is astounding.

 

[Prickly]

I think I follow you, but my question though is how far would my TA have to drop to reach this equilibrium? In looking at my LSI calculator, if the water temp is say 58, pH 7.8, CH 360, CYA 40, salt 3000, then TA of 60 gives me an LSI of -.26. To get closer to zero I'd have to jack up the CH quite a bit higher. Whereas, if leave the TA around 80 (where it is now) and I keep a steady 7.9 via CO2 and leave the other numbers alone then I'll be good. Since the water is colder, there *should* be less outgassing (right?) and I should be able to lower my CO2 consumption compared to swim season (I would think). Personally I would rather do that than go back to adding acid.

BTW, thanks for all your input on this thread. I'm going to try to make your zoom call on Saturday, the wealth of information you provide is astounding.

I find when my pool is covered and my pump is off my pH stays quite steady so you may not end up needing much CO2 in the winter if you aren't using the pool or running the pump much.

 

[cptkirk]

Makes sense, but in my case the pool is uncovered and operating conditions are similar in winter (except the water is cold and no one is swimming). If my wife was willing to pay to heat it, it could be used (assuming we invested in a cover to help with the heat loss). Instead, she goes to a neighborhood pool that is heated during the winter to do her daily swim.

 

[JoyfulNoise]

I see no issues with your planned winter operation. It will be a good real-world test of the system to see how low you can go on CO2 use in the winter. Others can learn from that as well.

 

[cptkirk]

Installed the new pressure sensor today, and it leaked in the same way the old one did. Looked at the end of tube that was inserted into the pressure sensor and it looked OK, but I trimmed off about a half inch anyway and that seemed to fix the drip. Unfortunately did not end up with a spare because the old one sheared off when I attempted to remove it and I had to use a screw extractor to get it out.

For clarity, Poolsmith calls this the pressure sensor, but it is really just a connector that screws into the impeller-side drain port. It's a relatively simple piece of plastic with screw threads and an o-ring, and a push-style connector for the tubing that runs to the actual sensor in the controller.

 

[Prickly]

I am finding that my pH is so stable with the CO2 injection. I can turn the system off for 4 to 5 days at a time and my pH barely moves. When I was using MA my pH would rise so quickly it needed very frequent adjustments. Are you finding the same thing? I'm surprised by this and definitely not complaining!