Buffering % of CYA

[jcowart]

First off I do have a question but a little back story first. My daughter had some friends over and since we like to entertain the parents "pool owners themselves" stayed. Both made comments on how clear the pool was. I told them that I use bleach to chlorinate and what I target my FC at. The wife was like "that's too high and it will eat your skin off" and did not swim. Personally I did not care and am not one to argue a losing battle. He and his daughter did swim and his first comment was that he could read a book under water if he could hold his breath long enough. So after that the questions started flying "away from his wife". He told me that his maintenance was tabs in the floater and something called no mor problems. I think the light bulb lit when he said the pool store sold him some TA up for $12.99 and that he could have used 49 cent per pound walmart baking soda. I think I may have a convert to the clear side.

Question: I maybe wrong but I think I have seen over the years that the buffering is something like 7.2% per 10ppm of CYA. Is that right or wrong and is there a linear % that is used. I know it might not be that simple and without diving too deep. Is there a simple layman's way of explaining the FC/CYA relationship?

 

[jblizzle]

I think this is what you are asking about? The chart shows the active level of chlorine for combinations of FC and CYA.
https://www.troublefreepool.com/~richardfalk/pool/HOCl.htm

Or maybe that is not what you are asking. Please try to clarify.

Or maybe you mean what is the minimum required FC level given a CYA level? That is around 7.5% I think.

 

[JasonLion]

I suspect you are thinking of: The recommended minimum FC level is 7.5% of the current CYA level.

 

[jcowart]

Ok let me try to reword this. Is there an easy way of trying to explain the FC/CYA relationship without just saying here is the chart. Something along the line of, if your FC is X and your CYA is X your pool, body, and equipment is only really seeing a lower FC of X because of the buffering at any one given time.

 

[JasonLion]

If you maintain FC at 7.5% of the CYA level, the actual active disinfecting chlorine level will be constant, even as CYA changes (and you change FC to match). The remaining chlorine is "held in reserve" by the CYA.

 

[chem geek]

Ok let me try to reword this. Is there an easy way of trying to explain the FC/CYA relationship without just saying here is the chart. Something along the line of, if your FC is X and your CYA is X your pool, body, and equipment is only really seeing a lower FC of X because of the buffering at any one given time.

This link that Jason linked to in his post earlier shows the active chlorine (HOCl) level at various FC and CYA levels, but the equivalent FC at pH 7.5 is roughly double these numbers. An easy rule-of-thumb that is close enough is that the equivalent FC with no CYA is equal to the FC/CYA ratio. So an FC that is 7.5% of the CYA level has the same active chlorine as 0.075 ppm FC with no CYA. The equivalence is in disinfection rates and also oxidation rates of your swimsuit, skin and hair, so basically the effect on your body. It's also equivalence for outgassing rates for chlorine (though not resulting chloramines). So the amount of effective chlorine in the water in a pool with CYA is much, much, much lower than a pool with no CYA.

As for chlorine bound to CYA, this paper shows that CYA itself has minimal skin absorption so the same should be true of chlorine bound to CYA. As for the significantly reduced effects of CYA on pathogens as well as chemical reactions, look at the "Chlorine/CYA Relationship" section in the first post of the thread Certified Pool Operator training -- What is not taught. Roughly speaking, the chlorine bound to CYA is 1/150th the strength of unbound chlorine so we generally ignore it.

The problem is that if they don't understand chemistry they won't believe you. What Jason (the other one -- JasonLion) wrote is the best way to explain it in terms of most of the chlorine (95%+) being held in reserve and effectively inactive, but again they simply may not believe you.