CO2 out gassing question

[eqbob]

I was reading some threads about TA (currently bringing mine back down from 90 to where I want it at 60)(was gone for 9 days) and following some links and came across an outgassing chart ChemGeek made which was referenced in some posts.

https://www.troublefreepool.com/~richardfalk/pool/CO2.htm

I confess to being completely clueless as to what this chart really means and how to read it? There are three colors in the chart and they're not referred to in the notes at all.

Luckily I have plenty of aeration capabilities in the pool between my sconces and the swim jets so aerating back up after a an acid addition is a relatively simple process.

Right now, the pool is in full sun all day, so the FC demand is higher, so salt cell is on at a higher percentage, so pH is rising more rapidly, so keeping TA down lower helps moderate the pH rise which is inevitable in SWG pools.

Pool water temp is a balmy 91. Too bad I can't swim in it for 10 days due to a bad ear infection I got while scuba diving in Cozumel.

 

[chem geek]

The color in the chart is somewhat arbitrary where I used green for < 10, orange for 10-20, and red for > 20. The numbers reflect how over-carbonated the pool is with respect to air. 0.0 would be in balance with as much carbon dioxide in the water as would be in equilibrium with air.

Since the actual rate of outgassing depends on other factors besides TA and pH, you can't go by the color coding to know how fast your pH will rise. In spas with aeration jets, you'll get a lot of outgassing even at values far lower than 10 while in pools with covers even values above 20 won't have significant outgassing.

 

[eqbob]

Ok. So if I have a current TA of 90 and a pH of 7.6, my read value is 6.2 which is some large multiplier on the equilibrium amount. (I don't understand the 1.0 is 2x, 2.0 is 3x...does that means 6.0 is 7x or is it a log scale?).

The more I look at the table and try to decipher what it means, and /or what it indicates to do in terms of pool care. The more confused I get.

 

[chem geek]

It's not a log scale. It's just offset by 1. It's the ratio of the actual carbon dioxide concentration in the water divided by the concentration that should be there at equilibrium and then subtract one from this (if I had time, I'd change the chart adding 1 to everything and having 1 be equilibrium).

The only thing the table tells you is that there the carbon dioxide is more out of balance (more over-carbonated) at lower pH or higher TA so means there will usually be more pH rise and more acid addition to control pH when the pH is lower and/or the TA higher. That's all it means.

Many people don't understand that a higher TA leads to a faster pH rise when using hypochlorite sources of chlorine. TA is a source of rising pH due to over-carbonation and that's all the table shows. The table also shows that lower pH is worse so it's better to not only lower the TA but also target a higher pH where it may settle in and the pH not rise as quickly. Some people think they should keep adding acid to get to a lower pH like 7.2 since then they don't have to add acid as frequently when it rises to 7.8, but that's not how it works -- you end up adding a lot more acid if you do that compared to just going from 7.8 down to 7.5, for example. Of course, if one is intentionally trying to lower the TA, then the lower pH makes the outgassing faster so speeds up that process and you use more acid in a shorter period of time for that process (the lowering TA process just accelerates what would otherwise go on over a longer period of time with spread out acid addition).

 

[eqbob]

Ok. Kewl and thanks! That's exactly the explanation I was looking for. That also provides scientific basis for two of the TFP principles of the target TA and pH levels.

 

[chem geek]

Yup. It also provides scientific basis for the Lower Total Alkalinity procedure that keeps the pH low to accelerate the carbon dioxide outgassing (the aeration is not part of the table since its effects on the rate of carbon dioxide outgassing are much more difficult to quantify). That chart also explains why when lowering the TA it gets harder (slower) to lower it as the TA gets lower.