pH, Free Chlorine, and CYA

[kangell]

Hi - This is my 3rd season with my pool and things have gone well thanks to advice from this site. I do my own maintenance except opening and closing or any specialty work. I still have to add a lot of acid to keep my pH down even though my plaster should be cured by now. I know the SWG raises the pH as well as the bubbler I run at half capacity for 6 hours a day. I'm still adding about 1/3 a gallon every 3 - 4 days for a 32,000 gal pool.

I've been studying how to run my SWG less to help this which brings me to CYA. I've generally kept it at 50 - 60 partly because I'm paranoid of getting too much plus it's a pain to add. I have my SWG set to 35%. Today's test numbers were: FF: 4.2, CC:0, pH 7.6, TA: 60, CYA: 50, CH: 380, Salt: 3400 (SWG says 2900), CSI: -.19, water temp 93(!).

Any suggestions to quit lugging such muriatic acid around?

 

[JamesW]

Don't add acid unless the CSI goes above 0.0.

 

[kangell]

Don't add acid unless the CSI goes above 0.0.

Could you provide a little more detail and reasoning behind that? Given that my Taylor test kit can't measure above 8.0 pH, I don't trust that it gives me the right CSI when pH is high.

 

[JamesW]

Don't let the pH go over 8.0.

Lower pH causes more carbon dioxide and more pH rise.

Multiply the TA (Carbonate Alkalinity) by the percentage to get the (relative) CO2 level.

pH........% CO2

7.0.........18.3
7.1.........15.1
7.2.........12.4
7.3.........10.1
7.4.........8.2
7.5..........6.6
7.6..........5.3
7.7..........4.3
7.8.........3.4
7.9.........2.7
8.0.........2.2

For example, if the pH is 7.2 and the TA is 120, then the relative CO2 level is .124 x 120 = 14.88

If the pH is 7.8 and the TA is 60, then the CO2 is 0.034 x 60 = 2.04

14.88/2.04 = 7.3 times as much pH rise due to CO2 offgassing and 7.3 times more acid use.

If the pH is 7.0 and the TA is 120, then the relative CO2 level is .183 x 120 = 21.96.

21.96/2.04 = 10.76 times more acid use.

This explains why a low pH and high TA causes so much acid use.

It also requires the constant addition of baking soda to keep raising the TA after the acid has lowered it below the target range.

For a pH of 7.2 and a TA of 120 the best option would be to use CO2 as a pH reducer because you are just adding the same amount of CO2 as was lost to offgassing.

However, this is a huge waste of money.

View attachment 593140

y = 100-(100/(1+10^(6.35– x))) and y =100/(1+10^(6.35– x)), x from 4 to 9 - Wolfram|Alpha

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[kangell]

Don't let the pH go over 8.0.

Lower pH causes more carbon dioxide and more pH rise.

Thanks, I appreciate the details. However, in my case I don't have low pH and high TA. My TA is 60 and I had just got my pH down to 7.6. I try to keep it between 7.6 and 7.8 but I have to keep adding a lot of acid. Hence, my original question.

 

[JJ_Tex]

Wait until it hits 8.0 before you add acid. My pool will rise to 7.8 fairly quickly, but it takes a week or more for it to go from 7.8 to 8.0. After it hits 8.0 it will stay there for weeks if I'm feeling extra lazy.

Keeping it below 8.0 rather than below 7.8 saves me lots of frustration and acid lugging.

 

[JamesW]

I would try to keep the pH between 7.9 and 8.3.

Multiply the TA (Carbonate Alkalinity) by the percentage to get the (relative) CO2 level.

pH........% CO2

7.0.........18.3
7.1.........15.1
7.2.........12.4
7.3.........10.1
7.4.........8.2
7.5..........6.6
7.6..........5.3
7.7..........4.3
7.8.........3.4
7.9.........2.7
8.0.........2.2
8.1.........1.75
8.2.........1.4
8.3.........1.1

60 x 0.011 = 0.66 (pH at 8.3).

60 x 0.053 = 3.18 (pH at 7.6).

3.18/0.66 = 4.8. (Relative amount of acid of acid required to address CO2 loss). 4.8 gallons per period vs. 1 gallon per period.

pH rise due to CO2 loss can be detected by pH rise with no associated TA rise.

In my opinion, a pH up to 8.3 is acceptable as long as you can reliably read up to 8.3 and as long as the CSI is not higher than 0.0.

Keeping the CSI as close as possible to 0.0 can help reduce pH rise from plaster dissolving.

Plaster dissolving can be detected by monitoring the TA and CH rise.

For a 32,000 gallon pool, every pound of plaster (calcium carbonate) dissolved results in 3.73 ppm increase in TA and CH.

Has your CH and TA been increasing?

Note: This is more my opinion than necessarily TFP recommended ranges.

I do not necessarily recommend doing it how I would do it, but in my opinion, it can help address excessive pH additions.

 

[JamesW]

In other words, the best way to reduce how much acid you are adding is to reduce the amount of acid you are using.

AKA - Stop adding so much acid.

Before you add acid, ask yourself if it is really necessary to add the acid you are thinking about adding.

In most cases, the answer is no you do not need to add the acid.

Ask these two questions:
1) Is the CSI over 0.0?
2) Is the pH over 8.2?

If the answer to either question is yes, add the acid.

If the answer to both questions is no, do not add acid.

If you add acid, target a pH of 7.9 and not lower.

Note that you might risk calcium scale in the salt cell or the heater, so you have to decide if the risk is worth the possible benefit.

The Heater and Salt System warranties might require specific ranges and you have to decide if you want to go outside those ranges and possibly scale your equipment and void the warranties.

I am not trying to tell you what to do.

My goal is to try to help you understand your options and make the best most informed choice for your situation based on what matters to you.

 

[JamesW]

Going over a pH of 7.9 is typically not a good choice for most people as they will not be able to reliably read the pH at that level.

Some people can do it and that is their choice.

I would do it under specific circumstances but I do not recommend anyone else doing it.

 

[kangell]

Going over a pH of 7.9 is typically not a good choice for most people as they will not be able to reliably read the pH at that level.

Some people can do it and that is their choice.

I would do it under specific circumstances but I do not recommend anyone else doing it.

That's what I'm worried especially since my Taylor kit doesn't measure above 8.0. I just see bright pink. I can do an acid demand test to ballpark I guess. Most of my equipment manufacturers recommend 7.6 - 7.8.

 

[JamesW]

My goal is to try to help you understand your options and make the best most informed choice for your situation based on what matters to you.