pH Ceiling - Henry's Law of solubility of gas - Richard Falk chart of values

[Adam_FL]

In 2021 Richard Falk @chem geek published a chart illustrating pH ceilings for different levels of alkalinity. A copy of the chart can be found here: CO2 and pH: Understanding Henry's Law

Would there be any harm in targeting the following metrics to lower the pH ceiling and and reduce the demand for acid?

pH 7.7
CH 500
TA 20
CYA 10
CSI -0.39

 

[Mdragger88]

You may be reducing the acid demand but you will be risking your plaster.
Also you will need more than 10ppm cya in Florida Or you will be dropping below safe fc levels in a blink when the sun shines as well as fc being quiet harsh on swimmers when it is in range . Increasing cya as well as any decrease in temp will make your water even more aggressive.

Calcium Saturation Index (CSI)

How is CSI calculated? Very simple – use our PoolMath tool. To ensure long plaster life and save money, click here to learn more.

www.troublefreepool.com

The
Recommended Levels are designed to protect swimmers, equipment, & surfaces as well as prevent algae. Going much lower than 40ppm ta isn’t recommended.
I am sure someone will be along shortly to give a more in depth explanation of why recommended levels are what they are.

 

[Adam_FL]

But isn't the risk to plaster and equipment determined by the CSI? For aggregate pools the low end of the recommended CSI is -0.6.

 

[Mdragger88]

If you brought your cya into normal range (30-50ppm) you would be over that threshold
.

 

[Adam_FL]

I don't need CYA so I don't use it.

 

[ajw22]

You are anti-CYA so will not rehash that.

The article you linked to does not recommend TA of 20. It recommends “a slightly lower-than-textbook alkalinity of 60 or 70 ppm.“ And it says “measuring and dosing acid correctly cannot be overstated. It is critically important. Lower alkalinity reduces the buffering capacity of your water, …”

TFP publishes a lower range for TA of 60 but some of us find 50 will work ok. I would not go below that.

 

[Adam_FL]

My buffering is fine, I have automatic commercial chlorine and acid feeders that keep my pH and chlorine pegged to my set point.

 

[Mdragger88]

I don't need CYA so I don't use it.

Does the water not feel harsh at a min fc of 2ppm?

 

[Adam_FL]

Water feels great.

 

[Adam_FL]

@JamesW what am I missing here?

 

[setsailsoon]

Nice pool Adam. I wouldn't see anything wrong here if you don't mind the chlorine loss. Wouldn't be for me since I will always have a salt pool within TFP recommended levels so my preferences are waaay different from yours. I'm on the opposite coast of Florida but even with a cover similar to yours chlorine consumption got pretty high when I got below min TFP CYA levels for about 7 months per year. I also ran pH higher with borate buffer in the 7.7-8.0 and just watched CSI to be sure I was as close to 0 and still stay negative.

Chris

 

[JoyfulNoise]

In 2021 Richard Falk @chem geek published a chart illustrating pH ceilings for different levels of alkalinity. A copy of the chart can be found here: CO2 and pH: Understanding Henry's Law

Would there be any harm in targeting the following metrics to lower the pH ceiling and and reduce the demand for acid?

pH 7.7
CH 500
TA 20
CYA 10
CSI -0.39

Most likely not. You saturation will always be negative which means your pool water will always have enough "room" for more calcium. That will always put your plaster at risk of etching but there's really no way to know a priori what that will look like. Will it cause severe plaster roughening ? Probably not on any timescale you would care about. Might it make your plaster visually less pleasing in 10 or 15 years? Maybe, but there may be so many other things wrong with the pool that who cares what the plaster looks like. Is it going to damage any equipment? Probably not. Most metal surfaces are sensitive to corrosion only when the pH is fairly low; calcium saturation has very little effect on metal corrosion in a pool plumbing application. Just know that once you get your pool water where you want it, your acid dosing is mostly going to be used to condition the fill water you have. At a TA of 20ppm for the pool water, your fill water is going to have much higher TA (unless your filling it with captured rainwater) and so the acid you use is simply going to be the result of of keeping the fill water TA in check. There's really no other major source of alkalinity.

I think it's fair to say that you are operating your pool in way that is far outside the norm of what is taught by TFP, and that's ok. It's up to you to run your pool however you like. Just know that much of what you're doing is synthesizing information and pool chemistry techniques from disparate sources and so it's going to be up to you to maintain and monitor the risks that come with doing that. Feel free to report back how it's working out for you so others that may be interested can learn from the experience.

 

[Adam_FL]

Will do. Thanks.

 

[JoyfulNoise]

Will do. Thanks.

One question though - are you using the AES Pulsar briquettes for chlorination or do you have a different source of chlorine (like a Stenner pump with a liquid chlorine tank)?

 

[Adam_FL]

Stenner with liquid chlorine controlled by the BECSys

 

[JoyfulNoise]

Stenner with liquid chlorine controller be the BECsys

Ok. Just to amend what I said previously then, your acid demand will be a function of the alkalinity added by the pool chlorine source AND any added fill water. Those are your primary alkalinity drivers. Some of the alkalinity associated with the liquid chlorine will be offset by acidic oxidation reactions but not all of it.

 

[onBalance]

"But isn't the risk to plaster and equipment determined by the CSI? For aggregate pools the low end of the recommended CSI is -0.6."

One could maintain a -0.6 CSI for a few weeks and be okay. However I suggest that maintaining a CSI of -0.6 on a regular basis is too low and not good for any plaster-based pool.

Consider this: Maintaining a CSI of -0.4 will dissolve and etch away about 4 to 5 pounds of calcium carbonate (plaster) per year from the surface of a 20,000 gallon plaster-based pool. A -0.6 might be about double that figure.
Keeping the CSI at -0.3 and higher would be much better.

 

[Adam_FL]

Thanks @onBalance that is very helpful information. I recently finished my third sulfamic acid treatment to remove scale from my original installation. There is still some more scale that needs to be removed and I was planning on doing a fourth treatment. Do you have any idea if regular brushing of the pool and keeping the CSI low will have the same effect as the sulfamic acid over time? In other words with a scale go away if I keep the CSI at -0.6?

 

[onBalance]

First, you need to make sure that you are dealing with calcium carbonate scale.

Look closely at a whitish area and make sure the scale is covering the entire surface; not just the cement portion, but also the aggregate, which means mostly a uniform coating covering the plaster finish. If the white is just where the cement is, then that probably means that the cement portion (not aggregate) is becoming porous and turning white. No scale.

If the whitish scale is everywhere, then carefully scrape some of the scale off without removing actual plaster. Drop some of the powdered scale into diluted acid with water (1 to 5 is fine). If the powder fizzes vigorously, then it is calcium carbonate scale. If it doesn't fizz, then it is probably calcium sulfate.

If it is calcium carbonate scale, then a negative CSI will slowly remove it with the help of some stainless steel brushing.
Sanding with 80 or 100 grit wet&dry sandpaper will also remove scale, and keep the plaster surface smooth.

 

[ferretbone]

"If it doesn't fizz, then it is probably calcium sulfate." How are you guys removing calcium sulfate When you think you have found it stuck to the plaster?? Taylor says it shouldn't fall out and cover the surface. Is that true?