1. ## question about daytime FC % loss

Given that my pool gets a good amount of sun each day (I'm in Southern Calif with a South facing backyard), can someone tell me roughly what I should expect for a percentage FC loss during the day (if nobody used the pool), given a CYA level of 40 ppm?

Better still, if there's a thread, chart, etc. in the Deep End section of the forum that would allow me to estimate for myself, given any particular CYA level, please feel free to point me there.

Thanks,
Greg

If it helps in answering my initial question, here's some current numbers:
FC 5
CC 0
CYA 40
TA 90
pH 7.4
CH 350

2. ## Re: question about daytime FC % loss

I think I have seen somewhere on here in a post by either chemgeek or waterbear that it is not unusual to lose 1 - 2 ppm FC in a pool that has full sun all day possibly even more. If you feel like your FC loss is excessive you could raise your CYA level up to 50 or 60 and see if this helps. I know some folks do this and maintain a higher FC level, but they don't have as much burn off of FC with the higher CYA. It is basically a matter of choice for you to determine what is more cost effective for you. I keep mine in the 30 - 50 range and typically lose 1 - 2 ppm of FC which I adjust every evening or afternoon, but my pool is partially shaded from about 4 - 5 pm until dusk.

3. ## Re: question about daytime FC % loss

I don't recall seeing any specific formula, but it shouldn't be too tough to find out.
Assuming that you have done an overnight FC loss test and verified that you are not losing FC to organics, then you can do a 'during the day' FC loss test (before sunup to after sundown) and determine how much is going away to UV.
Organics and UV are the only things which consume your chlorine.

4. ## Re: question about daytime FC % loss

This is a link to another post regarding the same thing... sort of. fc-loss-curve-t12108.html

If your going to use ohmboys formula to determine FC loss due to UV then you need to pick a day when there will be no bather load at all. Any bather load will introduce organics into the pool water and skew your results. However you may want to know what your FC loss is due to UV and bather load. There is no way to completely eliminate the introduction of organics into you pool (ie. bugs, leaves, etc.), but minimizing bather load is a good start.

5. ## Re: question about daytime FC % loss

I remember that chem geek has said with zero CYA, in full sun the half-life (time in which you lose 50%) of FC is 35 minutes. So if you started with 4ppm, after 35 minutes you have 2ppm, after another 35 minutes you have 1ppm, after another 35 minutes you have 0.5ppm. (That's two hours to burn off 3.5ppm if you started with 4.)

Introducing CYA extends the half-life, and how much it's extended varies with the CYA concentration. I don't remember seeing a chart or curve showing how much this effect is, though.

Because the loss to UV is a rate, the more you put in the more you lose (all else being equal). If you normally lose 2ppm per day, doubling your FC target means you'll lose 4ppm per day, unless you add more CYA.
--paulr

6. ## Re: question about daytime FC % loss

Thanks for the replies. I know it's a percentage thing, in that the more you start out with, the more you'll lose due to sunlight, and that more CYA will reduce this percentage loss. Basically I'm wondering what the expected percentage loss will be for different levels of CYA. Right now I'm at 40 ppm CYA so in the immediate term I could use my FC loss during the day right now to estimate what the percentage loss should be for this level (I'm not losing anything overnight right now, so the daytime loss should be to sunlight only if nobody swims). But I imagine there's information out there that would tell me what % loss I should expect to lose for any given CYA level....or perhaps there isn't, but I'm curious if there is.

paulr, the information you posted about half-life is a good start. If I can find out the FC half-life for different CYA levels I could probably then calculate the total % loss for a given number of hours of sunlight.

Thanks,
Greg

7. ## Re: question about daytime FC % loss

I was wondering about the same issue. FC losses at various CYA levels will be useful. The other option would be to see if covering the pool with a solar cover and a lower CYA is better way to reduce chlorine usage than raising the CYA without a cover.

I read in some post that higher FC and CYA around 60 uses less chlorine than lower Fc and a 40 CYA. (assuming no need to shock often)

RV

8. ## Re: question about daytime FC % loss

The problem with making a chart, is that it depends on how much sunlight your pool gets. There isn't any simple way to say, my pool gets X amount of sunlight, because lots of things affect that (like trees giving partial shade, big city smog, etc).

Fortunately, this is easy to measure on your pool. Simply measure the FC level in the morning, and again in the evening, on a sunny day, and you know how much your pool loses at the current time of year.

9. ## Re: question about daytime FC % loss

Generally, you will lose a fixed percentage of your FC level each day. The higher the FC, the more that will be lost.

At a CYA of about 50 ppm, I lose close to 25% of the FC each day. This is without any external load on the chlorine such as bathers, pollen, etc.

For a BBB pool, target for a CYA of 40 ppm is about 5 ppm of FC so this means you might lose about 1.25 ppm if your pool behaves as mine does.

Chemgeek has a chart of FC half life located here which gives you an idea of how chlorine behaves but the half life may change in a pool because of the depth of water.

As Jason pointed out, there are a lot of other factors involved so it is best to measure it yourself. Just keep in mind that the loss is a percentage of the total.

10. ## Re: question about daytime FC % loss

And using the chart, it seems like for a 3-5 FC at 50 CYA, the half life is about 7 hours. So, in a 12 hour day, the loss should be 65 to 70%. It matches what I seem to lose during the day: about 67% at a 4 foot deep pool.

And in the chart, it does not seem to improve much at higher CYA values. I hope I am reading the chart correctly.

RV

11. ## Re: question about daytime FC % loss

You really can't make that kind of extrapolation from that chart. The amount of sunlight hitting the pool varies significantly with time of day, air quality, trees, latitude, etc. It is impossible to take all those factors into account in a meaningful way.

The protection CYA provides does increase significantly as the CYA level increases.

With CYA at 50, I never lose more than 50% of my FC, usually less. But someone who lived further south, had fewer trees, had better air quality, etc could lose quite a bit more FC than I do.

12. ## Re: question about daytime FC % loss

Well, I sort of suspected that perhaps there would be too many factors involved to really have a set of "known" usable numbers for percentage loss based on a particular CYA level, but just thought I'd throw the question out there. I have seen some posts here where someone will mention to the OP what % FC loss they should expect each day based on their CYA, so I thought that perhaps some numerical guidelines existed. As many of you have mentioned, I can always determine things empirically for my pool by doing early morning and evening measurements (which I've done actually, I was just trying to figure out if my loss seemed reasonable).

JasonLion, based on your last post, it sounds like my FC loss is pretty reasonable (I'm losing 35-40% of my FC each day, with a CYA of 40-45 ppm).

Thanks,
Greg

13. ## Re: question about daytime FC % loss

Not only that, but that chart has been proven to be wrong. Mark did an experiment described in this post that showed that at 45 ppm CYA, the FC dropped by about 50% while at 80 ppm CYA the FC dropped by around 15%, but these both started at the same FC. This means that even scaling the FC proportionately with the CYA to keep the same disinfecting chlorine level, a higher CYA near 80 ppm will lose less absolute FC than a lower CYA near 45 ppm. This was a surprising result, but one that was seen by other forum members as well. For whatever reason, Mark is now seeing a lower loss of around 25% of the FC per day at 50 ppm CYA.

As Jason points out, there are many factors, but I think that there is some sort of additional protection effect at higher CYA levels that more than makes up for proportionately higher FC levels. So in very sunny areas, running at a higher CYA closer to 80 ppm makes some sense IF you are very careful to not let the FC drop too low since fighting algae at a higher CYA level requires a much higher FC level. This is also the reason why the CYA recommendation is higher for SWG pools since it lets one lower the SWG on-time (since less absolute FC generation is needed) resulting in a slower pH rise over time.

Even with no sunlight, there is a chlorine demand. In my own pool, it seems to be somewhere around 0.7 ppm FC per day or thereabouts and that's at a typical FC of around 4 ppm with 30 ppm CYA. I suspect that this amount is a percentage only of the active chlorine concentration, so roughly the FC/CYA ratio, but that's speculation on my part.

Overall, I would say that roughly speaking at 40-50 ppm CYA one loses 25-50% of the FC per day while at 70-80 ppm CYA one loses 15-30% of the FC per day. At 100 ppm CYA, it may be 10-20% of the FC per day. So Greg, your loss does seem to be in the wild-guess range I've just proposed.

Richard

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