DIY PH measurement and CO2 Injection System- Project Summary

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A pH of 8.2 to 8.3 might be a good choice as long as you can keep it there accurately and maintain a good CSI.

pH....................%CO2
7.0........................18.292
7.1........................15.097
7.2........................12.377
7.3........................10.088
7.4........................8.183
7.5........................6.611
7.6........................5.324
7.7........................4.275
7.8........................3.426
7.9........................2.741
8.0........................2.189
8.1........................1.747
8.2........................1.392
8.3........................1.109
8.3456.................1.000

Percent Carbon Dioxide.

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y = 100/(10^(6.35 - x) + 1) and y = 100 - 100/(10^(6.35 - x) + 1), x = 4.5 to 8.3 - Wolfram|Alpha

Wolfram|Alpha brings expert-level knowledge and capabilities to the broadest possible range of people—spanning all professions and education levels.
www.wolframalpha.com www.wolframalpha.com

y = 100-(100÷(1+10^(6.35 – x))), from 7.9 to 8.8 - Wolfram|Alpha

Wolfram|Alpha brings expert-level knowledge and capabilities to the broadest possible range of people—spanning all professions and education levels.
www.wolframalpha.com www.wolframalpha.com

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____________________________________________

Percent Bicarbonate vs. pH.

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JamesW said:
Pool CO2 Injection System Status
Current pH: 8.65
Target pH: 7.40
Injection State: ON
Time Remaining for Injection: 1 hours, 35 minutes
Current Time (UTC-5): 09:14:58 AM
Turn Injection On (Manual Override)
Set Target pH: 7.40
___________________________________________________
Pool CO2 Injection System Status
Current pH: 7.00
Target pH: 7.40
Injection State: OFF
Mixing Delay: 23.000 hours
Choked Flow Differential Pressure: 136.3 kPa (19.8 psi)
Current Time (UTC-5): 06:49:46 AM
Estimated CO2 Added per Day: 2.95 lbs/day
Manual Control
Mode: Manual
Manual Pulse Length: 1.0 minutes
Start New Injection Pulse
Set Manual Pulse Length (min): 1.0
Set Target pH: 7.40
Set Mixing Delay (hrs): 23.000
Firmware Version: 1.0.0
___________________________________________________
Current pH: 8.65
Target pH: 7.40
How sure are you about the pH at 8.65?

Are you verifying with a separate meter?

A target pH of 7.4 to way too low and it will use way too much CO2.

A target pH of 8.0 to 8.3 will be much more efficient.
___________________________________________________
Current pH: 7.00
Target pH: 7.40

How did the pH get down to 7.0?
___________________________________________________
3.67 lbs CO2 is equal to adding 1 gallon of acid as far as lowering the pH.

Estimated CO2 Added per Day: 2.95 lbs/day

That is about equal to adding 103 oz of muriatic acid per day.

That seems like a lot.

View attachment 629091
Click to expand...
That was the page without the pH probe attached in my house when I was finalizing the software. The readings are meaningless, I was just trying to show the page I had written. I've made a lot of changes. My pH target has been 7.8 and I control to that with acid pretty well. I think I can get to 8.0 but CSI is .3+ at pH 8.3 and I'd prefer to stay a little negative to keep the swg life as long as I can get. With my new plaster I'm consuming about 1/3 gal 31.45% each day.
 
JamesW said:
A pH of 8.2 to 8.3 might be a good choice as long as you can keep it there accurately and maintain a good CSI.

pH....................%CO2
7.0........................18.292
7.1........................15.097
7.2........................12.377
7.3........................10.088
7.4........................8.183
7.5........................6.611
7.6........................5.324
7.7........................4.275
7.8........................3.426
7.9........................2.741
8.0........................2.189
8.1........................1.747
8.2........................1.392
8.3........................1.109
8.3456.................1.000

Percent Carbon Dioxide.

View attachment 629163

View attachment 629161

y = 100/(10^(6.35 - x) + 1) and y = 100 - 100/(10^(6.35 - x) + 1), x = 4.5 to 8.3 - Wolfram|Alpha

Wolfram|Alpha brings expert-level knowledge and capabilities to the broadest possible range of people—spanning all professions and education levels.
www.wolframalpha.com www.wolframalpha.com

y = 100-(100÷(1+10^(6.35 – x))), from 7.9 to 8.8 - Wolfram|Alpha

Wolfram|Alpha brings expert-level knowledge and capabilities to the broadest possible range of people—spanning all professions and education levels.
www.wolframalpha.com www.wolframalpha.com

View attachment 629167

View attachment 629168

____________________________________________

Percent Bicarbonate vs. pH.

View attachment 629169
Click to expand...
Very helpful James, thank you much. I'm pretty sure I can do 8.0 but a little concerned about .3+ csi at 8.3 pH. My TA is about 80 and CH is 250 constant (well water make up is 250).
 
JamesW said:
View attachment 629205
Click to expand...
Yep, the challenge for me long term is going to be keeping TA that low. Especially if I switch to CO2. I've been driving it down with acid additions and finally got it to 60 today. At TA 80 those csi numbers don't look so good.
 
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The main concern with a low TA is not to drop the carbonate alkalinity to 0.

With a CYA of 60, you can go down to about 40 TA.

With a CYA of 80, then you should stay at TA = 50.

If you have borates, then you have to go higher.

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​

Borates and Adjusted Alkalinity.​

Adjusted TA = TA – (CYA X CYA C.F) – (Borate x Borate CF)

Borate C.F (correction factor) based on pH.

pH.......CF

7.2.....0.051

7.4.......0.0786

7.6......0.1248

7.8......0.1989

Cyanuric Acid correction factor based on pH.

pH........CF

7.1.......0.24

7.2.......0.26

7.3.......0.28

7.4.......0.30

7.5.......0.32

7.6.......0.33

7.7.......0.34

7.8.......0.35

7.9.......0.36

For example, if the pH = 7.6, TA = 90, Borate = 50 and CYA = 70, the adjusted alkalinity is 90 - (70 x 0.33) – (50 x 0.1248) = 60.66.
 
I don't add any borates unless I have more trouble with pH control. Even if I have to add 1/3 of a gallon of acid per day (or CO2 equivalent) don't have the problems I was having a few years ago. Borates definitely helped but eventually the plaster cured and I didn't need it anymore. Never saw any other reason to use borates than to help with pH control.
 
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suzook said:
I owned an in ground pool up north for 10 years. I never lost a drastic amount of ph in 18 days. What am I missing here? Not being a wiseass, but a serious question.
Click to expand...
@setsailsoon is using CO2 to lower pH...not raise it.
 
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suzook said:
I owned an in ground pool up north for 10 years. I never lost a drastic amount of ph in 18 days. What am I missing here? Not being a wiseass, but a serious question.
Click to expand...
Most pools gradually have their pH increase so they need acid added. PH increases with aeration so more spillovers, surface area increase the pH gain. I'm using a system to add CO2 instead.
 
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Folks,

I've spent a lot of time trouble-shooting my CO2 system and the pH probe. This was all done using an ESP32 chip and can easily be done on others like RPi. Here's some updated cost info:
  • Total cost for the electronics is about $50 including the probe.
  • The CO2 setup was ~$45 plus the CO2 tank. You can get them for $100-400.
  • A CO2 fill is about $45.
I Learned a lot in the process that might be useful to pool owners especially as it relates to DIY pH systems. So I'm going to document it here. First let me discuss findings around pH measurement:
  • There are a lot of pretty cheap probes and they all have a probe that easily fits into a standard 1/2" push to connect fitting. You will find a lot of online articles and videos that claim it fits in a 3/8" fitting. That's incorrect.
  • You have to drill out the thin tube stop flange for this to work and best way to do this is in a vice. Drill carefully so you don't damage the o-ring. I have had the probe in pipe with max pressure in the range of 30 psi with no leaks or failures but I would use an additional way to secure it since it's only friction with a single o-ring that holds it in place. The best longer-term solution is a proper pH probe gland fitting. But they're not as readily available and definitely cost a lot.
  • I found a lot of probes from less than $20 to $100+. They all look the same but they are not. There are little details of construction that matter like the kind of interface junction, quality of the glass bulb and thickness and probably other things I haven't discovered yet. I think it's a good idea to go with at least probes in the $30 range as they seem to all have a ceramic junction and have better ratings. Apera is a reliable brand I've used in the past for portable pH meters so I went with theirs.
  • I don't know why but for some reason the reading jumps around a lot. Could be mine has bubbles in the water flow but it seems that even in a still beaker the reading wanders. I've overcome this by displaying a running average of at least the last 25 readings in my software.
  • Calibration is much simpler than I thought it would be based on my previous experience with portable pH testers. You start out by shorting the center pin to the ground and adjust to mid-point voltage. So what in the world does this mean? The board or module the probe plugs into has a supply value that's usually ~5v but you have to check it. In my case that's only 4.83 v. I'll adjust that later and calibrate again but that's where I am now. There's a great video on YouTube that shows how to do this. After this crude adjustment I've found only a little offset is needed.
  • You can also do a more thorough 2 point calibration using a pH buffer. I use 7.01 and 10 since that captures the readings I use. The software I created has ability to do 2 point calibration and then calculates software calibration slope and offset if you want to do it that way.
  • Here's a printout of a screen that displays pH with my home made software:

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As far as the CO2 injection goes here are some of the learning's so far:
  • Further back in this thread there are some extremely useful posts from @JamesW regarding solubility, saturation, pH/TA/Borate levels that helped a lot for my initial settings. Net, net after a lot of experimentation and learning a few things the hard way I've settled in on targeting pH around 8.0 since this allows me to stay in a slightly negative CSI range to be kind to my swg cell.
  • It's very clear I need to bubble in CO2 slowly so I don't lose it to atmosphere. Right now I'm using a 1/4" tube with about 25 psi above pipe pressure to make sure I have choke flow. Next step will be to make a diffuser that has a much smaller opening. They're hard to find at a decent price so I'm converting a readily available Stenner pump quill but sealing the end tight and drilling a couple 1/16" holes in it.
  • I've set up my software to do this by controlling on-off pulses on a solenoid valve connected to a 20# CO2 cylinder. I control pulse time, time interval between pulses, number of pulses. The software allows me to schedule a pulse series or to manually start a series anytime at press of a button on the web page. Here's the complete display of the current software:


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I hope this is helpful and please let me know if there's anything you need to know about this that I left out. Thanks to the experts here that have weighed in on this topic. Also thanks to ChatGPT and Claude ai environments to help we get the code written.

Chris

PS I'll get some photos later today and add them in then.
 
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JamesW said:
Where is the pH sensor located?

Is it before the cell, heater and SWG?
Click to expand...
Right now it's located in the filter pump circuit after the swg (last item before return goes underground at the pad. That was really an easy location since I was doing some other piping work. I plan to put it in the main pump circuit which is an easy change, runs longer so I can dribble in slower and the piping is longer plus returns mostly flow to the floor.

Chris
 
JamesW said:
The pH sensor might be inaccurate when the cell is on.
Click to expand...
Yes, it definitely is. Will need to make some adjustments to the system after we get back. These locations were convenient just to get started. I really barely had time to get it up and running before I left the country for almost 3 weeks. Another improvement will be to make a better diffuser that you suggested previously. I think I figured out how to do that as well. Keeping the pH close to 8.0 for the week before I left also helped a lot and CSI is slightly negative. Also, I'll need to add an under-counter RO unit on my make up water.
 
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