Chlorine / CYA

[Jbonnette]

I've been reading that the proper chlorine levels vary depending on CYA level. *What CYA ppm is ideal? Given that you have to add more chlorine when the cya is higher, would the cost be that different if you ran lower cya and just added low amounts of chlorine more often?

[Watermom]

We typically suggest around 40ish for most pools. Some people, like aylad, one of the other mods, intentionally run a little higher cya than most. She is also in LA and with her climate, she does better with her cya around 70 I think. I'll ask her to take a look at your situation and give you her take on it. It may be that a little higher would work well for your pool, too.

[chem geek]

[EDIT] I posted right after Watermom, but we basically are saying the same thing. [END-EDIT]

For an outdoor pool, it doesn't work the way you described because the CYA protection of chlorine from breakdown from sunlight is non-linear which means that even doubling the FC level when you go from 40 ppm CYA to 80 ppm CYA doesn't mean that you lose more chlorine in absolute amount. You actually lose less. The downside is that a higher CYA level makes it harder to solve any problems via shocking since the FC level has to get so much higher. So one usually trades off this risk by using a CYA level that is high enough for reasonable chlorine loss from sunlight. For many residential pools, that's around 50 ppm CYA or so. However, in very sunny areas or for saltwater chlorine generator pools where minimizing SWG on-time is desired to have pH be more stable, then a higher CYA of up to 80 ppm is often used.

[Jbonnette]

That makes sense :). It sounds like a higher ppm might be the best thing for me being that I live in Louisiana and just recently converted to SWG. My pool is partially shaded by a couple of large trees though and I haven't been losing chlorine too badly so I might see how well the SWG can keep up before using 70+ ppm. It seems like you would want to run lower chlorine levels if possible for swimmer comfort...

[chem geek]

Actually, while in the water itself the main rate of oxidation of swimsuits, skin and hair is proportional to the FC/CYA ratio so there shouldn't be much difference between 3 ppm FC with 40 ppm CYA vs. 6 ppm FC with 80 ppm CYA. However, when you get out of the water, it is the FC level that matters in terms of how long it continues to react with your sweat to form chloramines that can smell so from that point of view you are correct that lower would be better.

Remember that the FC/CYA ratio relates to the rate of the chlorine reactions while the FC by itself relates to the capacity or available reserve of chlorine so how long it will continue to react once you leave the pool. And yes, as the water evaporates when you leave the pool the concentration of chlorine increases which is probably why the higher FC levels are associated with faster fading of swimsuits -- it's not a rate issue (i.e. not FC/CYA related), but a capacity issue once you leave the pool.

[aylad]

I live up near Shreveport, and after a few years of trial and error, I find that a CYA of 80 ppm is ideal for my pool, and I end up using much less chlorine with the added benefit that I don't have to add it every day, like people who run lower CYA levels do. It's not so bad now, but I think you'll find that when late June/early July gets here, you'll have trouble keeping the Cl from going to zero during the day. However, once the CYA is in the water, drain/refill is the only way to reduce it, so you're smart to just raise it as far as you need to in order to have acceptable chlorine loss. You can always raise it later!

Incidentally, you might check the owner's manual for your SWG--many require CYA levels in the 70-80 range for proper operation and to maximize cell life...

Janet