CYA, Chlorine and a question

[Winger 03]

Need a little help in filling the knowledge gap I have.

I get that you need some measure of CYA to keep from losing your chlorine to the sun. Having that measure of CYA requires you to have a higher level of chlorine then you would usually have if you did not have that measure of CYA.

What is escaping me is what happens to the chlorine that is being "protected" by the CYA? Does it eventually get released and therefore able to be used to sanitize the pool? Or for a given CYA level, is a certain amount of chlorine needed before it is effective. In other words, with 0 CYA and 5 ppm of chlorine, it looks like all 5 ppm of chlorine is available for sanitizing. So then theoretically (and I ma making up numbers here) with a CYA of 30, and the same 5 ppm of chlorine, some part (lets call it 3 ppm) of that 5 ppm is being "protected" by the CYA leaving less for sanitization (which is why you need more chlorine when you have CYA - right?) resulting in 2 ppm or chlorine being avaialble for immediate use. What happens to the 3 ppm that is being protected, and when (if ever) does it get used?

I am going to convince a relative to go the BBB route or at least leave the pool store, but I know this question will come up and I want to be prepared. I pretty much understand and can explain the rest of what we talk about here, but there is a gap in my knowledge that I can not seem to close on my own.

 

[JasonLion]

Think about it this way: The CYA is always holding some fixed percentage of the chlorine in reserve. If the active chlorine gets used up by something, some of the chlorine held in reserve will be released to restore the correct percentage held in reserve. The percentage held in reserve depends on the CYA level. At higher CYA levels more chlorine is held in reserve.

Imagine that your CYA level is such that 90% of your chlorine is being held in reserve, and your FC level starts at 10. That means that 1 ppm of chlorine is available for sanitation, and 9 ppm are held in reserve. Then imagine that enough algae falls in the pool to use up 1.5 ppm of chlorine. The 1 ppm of chlorine that is active will get used up very quickly. That will leave 9 ppm of total chlorine, so 0.9 ppm of chlorine will be released. Of that 0.9, another 0.5 will get used up, leaving 0.4 active. Again active is less than 10% of the total chlorine, so more chlorine will be released from the reserve. In the end you will have 8.5 ppm of total chlorine, with 10% or 0.85 ppm active and the remaining 7.65 held in reserve.

 

[Winger 03]

Well, that makes perfect sense! I never really hear folks take about the reserved chlorine getting released for use, and I do not recall seeing that mentioned anywhere (though I am sure is has been noted several places).

Thanks for the simple explaination, I appreciate it.

 

[chem geek]

Another thing to note is that the Free Chlorine (FC) test isn't just measuring the active chlorine, but the entire reserve. This is because the active chlorine reacts with the dye in the test and/or the FAS-DPD titrating drops until all of the chlorine is used up from the reserve at which point the sample turns from pink to clear. This is why one needs to measure both the FC and the CYA levels to get the true picture of how much active chlorine there really is -- looking at only FC alone doesn't give the full story.

The idea of a chlorine reserve has been discussed here before such as in this post (and posts before it in that thread) and a more technical discussion in this post and this post.