Confused about CYA need

[tcm]

I know I read the purpose for CYA a 100 times or more here. But I am still confused about it. I have a couple of questions I need explained to help me better understand the need for it.

What does it actually do for my Chlorine level?

I understand that it protects from chlorine loss due to UV sunlight, but why then is it the higher my cya is the higher my FC levels have to be?

In otherwords it seems to me that cya protects a certain amount of chlorine from sunlight consumption, but then I need more chlorine because the chlorine bound to the cya is usless??????

I know I'm missing something here, please shed some light on this for us more simple minded people.

 

[JasonLion]

You almost have it. Essentially, CYA does two main things. It protects FC from being lost to sunlight, and it "buffers" the chlorine, holding some percentage in reserve (ineffective but available in case the FC level goes down). Both effects increase as the CYA level increases. To compensate for the chlorine held in reserve, you need to raise the FC level as the CYA level goes up to keep the same active chlorine level. Despite this, you still lose less chlorine to sunlight as CYA goes up.

 

[tcm]

So does this mean that if and or when the useable FC drops then the cya bound or buffered chlorine is then released as back up?
Thanks Jason,
I guess the reason this doesn't make sense to me is in the case of a SWG pool. The CYA recommended levels are higher then a manual dosed pool. If SWG is producing chlorine at basically a constant level, as long as the pump is running, then why would I need higher CYA to hold chlorine in reserve when it is constantly dosing the pool with new chlorine? And if this is true the higher the CYA the harder my SWG has to work to keep levels high enough to compensate for the cya levels? It makes sense for a manual dosed pool where chlorine is only replaced when put in??

 

[jblizzle]

You missed the point that at a higher CYA there is LESS FC consumption everyday. So the SWG actually does not have to work as hard to maintain a level. It would have to work harder to raise FC from zero, which is why we say to add bleach to initially raise the FC up and then allow the SWG to maintain it.

Posted with Tapatalk ... sorry if I sound short ... hate typing on phone

 

[JasonLion]

So does this mean that if and or when the useable FC drops then the cya bound or buffered chlorine is then released as back up?

Yes, exactly. As the active chlorine gets used up, the CYA releases some of the bound chlorine to become active again. The amount bound up by CYA is always some percentage of the total FC level, with the specific percentage depending on the CYA level.

This has a couple of advantages. For example, without any CYA, high FC levels can damage hair. But with CYA you can have much higher total FC in the pool available for sanitizing, but the active level is lower so no hair damage.

The ideal CYA level is a balancing act. Higher CYA means less chlorine lost to sunlight. But higher CYA also means problems fighting algae (because shock level goes up and the total amount of chlorine involved becomes expensive and annoying). With a SWG the balance shifts to higher levels because higher CYA levels mean less total chlorine used so longer cell life, and SWG pools are less likely to get algae so not as important what happens when you do get algae.

 

[tcm]

Thanks Jason,

The water in my head is a little less cloudy now. Need to shock the brain every now and then.