Question about CYA and FC levels

[Melt In The Sun]

Here's what I think I understand:

From what I've read in pool school and elsewhere on the forum, higher CYA = higher FC required for effective sanitation. Additionally, the CYA level has little effect on the FC loss due to sunlight.

Following this logic, it seems that a high CYA level would be just fine, as long as the FC level was maintained high enough. Bleach use would be the same, since the daily FC loss would be the same. Any necessary shocks would require more bleach than with low CYA, but normal chemistry maintenance would be comparable.

This forum seems like a very logical place, and the first recommendation to one with high CYA is a partial drain. It seems like it would be perfectly manageable, as long as one fully understands the situation. The only issue I see is the really high shock levels.

I must be missing something here, right? :?

 

[anonapersona]

I think that at high CYA levels you are actually still losing a bit more chlorine daily, since you must be starting with a higher level since FC declines as a percentage, not as a ppm loss.

So (making up numbers on decline rate) if you have to maintain FC between 12 and 7 at CYA of 100, per CYA chart, and you are losing 30% a day, then first day you lost 3.6 ppm and end at 8.4, next day you are at 5.9 which is too low and so you needed to add chlorine after the first day. And you need to add 3.6 ppm.

If your CYA is lower, like 40 (again making up numbers on percent decline) then you have to keep FC between 5 and 3 and lest say (since I don't know) you lose more to sunlight like 40%. Then first day you drop 2 and you still need to add chlorine after the first day but you only have to add 2.0 ppm not 3.6 ppm so in one day you are consuming less chlorine.

Now, please understand that the decline rates were made up but I recall here that rates will vary around 30 to 50%, and at about 100% if CYA = 0.

My own experience was that, as above, I was still needing to add chlorine on the same days, just less volume, as I got the CYA down.

 

[duraleigh]

Additionally, the CYA level has little effect on the FC loss due to sunlight.

No. The more CYA, the more blockage of UV rays.

It seems like it would be perfectly manageable, as long as one fully understands the situation.

Yeah, it may seem that way at first glance but practical experience and years of info provided here tells us that too high CYA is perhaps the single most common cause of unmanageable pools.

Sooner or later, the CYA builds to a level so high that previous concentrations of chlorine are simply rendered useless. Then, the huge amounts fo chlorine to fix the pool are so staggering most folks don't want to have anything to do with using that much chlorine......and the pool quickly becomes non- sanitized.

As you said, if you are aware, the pool can be managed but it is far easier to control your CYA level in the first place....resulting in predictable levels of Chlorine and predictable, successful results from your pool.

 

[chem geek]

In theory, if one had a very high CYA level AND didn't already have a nascent algae bloom that creates a higher chlorine demand, then one could just maintain a higher FC level such as 12 ppm FC with 150 ppm CYA. However, this is only a stopgap measure since if anything goes wrong and one starts developing algae, fighting the algae by shocking with chlorine becomes pretty unreasonable. One could use other algae killing techniques, but then that's down a whole other path for pool maintenance.

As for chlorine loss at different CYA levels, at the same FC/CYA ratio then there is roughly the same amount of active chlorine (hypochlorous acid) so its loss isn't going to increase. In fact, because CYA partially protects chlorine from breakdown via shielding lower depths from the UV rays of sunlight, the higher FC and CYA levels result in a lower absolute chlorine loss, at least to 80 ppm CYA where we have tested this in real pools. Much above 100 ppm CYA, however, we don't really know what happens exactly and there are other factors that could begin to dominate such as chlorine oxidation of CYA itself which we know happens, but is usually too slow to worry about.

Richard

 

[Melt In The Sun]

Thanks for the responses. I'm not saying I would want to try it, just seeing if I understand things correctly .

So, based on the comments above, it sounds like a pool loses a set percentage of its FC in a day, rather than losing a set number of ppms per day (that was my misunderstanding). More CYA means a lower percentage loss, but that still may add up to a greater loss in ppm. So, even if the high FC required were perfectly maintained, it still would be burning through more bleach $ than a pool with reasonable CYA levels, and would require ridiculous FC concentrations to shock. Does that all sound right (or close enough)?

Off to drain I go...

 

[duraleigh]

More CYA means a lower percentage loss, but that still may add up to a greater loss in ppm. So, even if the high FC required were perfectly maintained, it still would be burning through more bleach $ than a pool with reasonable CYA levels, and would require ridiculous FC concentrations to shock. Does that all sound right (or close enough)?

Not quite. Reread Richards post. From our anecdotal experience here on the forum, it seems like a properly balanced pool will lose less FC as the CYA level goes up. (20ppmCYA = 3-4ppmFC loss daily............50-80ppmCYA = 2ppm loss daily) I'm making those numbers up but they are reasonably close to our real-world reporting and experience on the forum.

CYA over 100 may, as Richard says, result in less loss but everything becomes so difficult at that level that we think it's unreasonable to attempt to get your CYA there.

 

[anonapersona]

So, that is to say that what I posted is not correct. All those guys are way more experienced than I am so what they say goes. If I have experienced a difference in FC loss it must be due to other things; keeping the skimmer and polaris clean, pool usage, sun and shade and rain.

So, try to forget what I said earlier and go with the pros on this.