Maybe I'm overdoing it.

[TimS]

Instead of my usual long, drawn out explanations, I'll ask a simple question. There's some background for this, but...

Does UV burn chlorine faster at higher levels than at lower levels? Without having done any controlled tests yet, it looks like it does. When I add bleach to my pool, I typically aim for a value between the "Target" on chem geek's chart and the "Maximum" from Jason's Pool Calculator. With my CYA of 70, that's between 8 and 10. The chlorine loss to sunlight seems to be greatest from 10 to 9, and lowest from 6 to 5 (the minimum.) Is that right? It seems logical, but has anyone actually tested it?

If so, I've probably been wasting bleach trying to make sure I never fall below the minimum. (After last year's algae problems, I don't want to deal with that again, so tend to err on the high side. I should probably just trust that if I do get an algae outbreak, it's easy enough to fix.)

 

[duraleigh]

YOu are overthinking it just a little but you ARE NOT wasting bleach by keeping your FC up. Letting it drop too low just once and getting a little algae then requires a shock process which will MORE than consume what little you consume by having your FC up just a little.

In short......do not let your FC dip too low....ever! :lol: Warding off the need to shock is a real bleach saver!!

 

[waste]

Tim, how ya doing, why aren't you swimming?

It's my understanding that the higher the concentration of chlorine, the more vulnerable it is to degradation from heat and UV.

But... as Dave so sagely points out, you've gotta keep the fc high enough to do it's job or risk 'nasties' in the water :|

The molecular reactions are way too far over my head to try and explain, but if it takes a few more bottles of Chlorox to keep the pool clean and swimable - I'd just go ahead and add the bleach

 

[TimS]

OK, here's a bit of background on this. When I opened this year (mid-March) my CYA was 40. I held it there until the beginning of May when I noticed that my FC was dropping faster than I thought it should. As it started getting warmer and the cover was off more, I noticed that in a full day of sun, I'd lose about 3-4ppm FC. In my case, that's 3-4 quarts per day. I decided to try raising my CYA to reduce this, so I brought my CYA up to 70, thinking that the higher concentration would better protect the chlorine, thus reducing my daily use. As the days got hotter, the sun more intense, and the pool open longer, my chlorine use remained right around 3-4ppm/day. Over the past few days, I've been testing two or three times per day to try to come up with a better grip on what's really going on. I've noticed that if I bring the FC to 10 ("Maximum" on the Pool Calculator) it seems to drop to 8 within a few hours, but the drop from 8 to 6 takes substantially longer.

So I've been trying to determine if I can get away with only raising it closer to Richard's "Target" of 8 and still have more than the minimum 5 left after 24 hours. So far, it's been working, but being the realist (DW calls it paranoid) that I am, I don't like letting it drop to within .5 of the minimum. What if I don't get chlorine in the pool for two days? I'd definitely be below the minimum, and at risk of an algae outbreak.

This is what prompted the question in the first place. It doesn't look like it makes any difference whether I bring FC to 9 or 10. After 24 hours, I'm going to be around 7, depending on bather load. So I was wondering if the drop from 10 to 9 happens so rapidly that the extra quart isn't really doing anything in the way of sanitation and organic control, and is just UV fodder. On a couple of occasions I've started with an FC of 7 and NOT added any chlorine to see how long it takes to hit 5.5. In both cases, that 1.5ppm drop took another 24 hours. So from 10 to 7 is 24 hours and from 7 to 5.5 is 24 hours. The second 1.5ppm seems to last as long as the first 3ppm.

I haven't tested often enough and with detailed enough records to prove any of this yet, but that's sure what it looks like is happening.

If I'm losing 3-4/day with a CYA of 70, I wonder what I'd be losing at CYA 40. It should be quite a bit more, so it should be saving me money on bleach in the long run. Of course, I still need to save enough bleach to pay for the CYA itself.

My bleach is cheap enough that it's not a really big deal, but if it's a waste, then it's a waste. (No offense, Ted ) Why burn chlorine I don't need to burn? So far, I've been looking at it as a bit of insurance, but like I said, it appears that the extra 1ppm doesn't really matter very much, if at all.

Tim.

 

[chem geek]

Generally speaking, the loss of chlorine from sunlight is proportional to the amount of chlorine in the water so it loses a % of chlorine per unit time rather than an absolute amount. So with all else equal, you'd expect to lose twice as much chlorine at 10 ppm FC than at 5 ppm FC.

So you might think that at higher CYA levels and proportionately higher FC levels that you might lose more chlorine. There are two reasons this doesn't happen. First, is that at the same FC/CYA ratio there is the same amount of chlorine unbound to CYA so that loss isn't going to be any higher. Second, even though some of the loss is from chlorine bound to CYA which is higher in concentration, there is apparently a CYA shielding effect that protects chlorine below the surface. Mark did some experiments that showed this effect in this post (and the rest of that thread).

So while it is true that at any given CYA level a lower FC will result in lower absolute FC loss since the loss is a % of FC level, 6 ppm FC at 80 ppm CYA should have a lower absolute FC loss than 3 ppm FC at 40 ppm CYA.

If you are able to experiment with this yourself, you can let us know if you see this same effect.

 

[TimS]

Generally speaking, the loss of chlorine from sunlight is proportional to the amount of chlorine in the water so it loses a % of chlorine per unit time rather than an absolute amount. So with all else equal, you'd expect to lose twice as much chlorine at 10 ppm FC than at 5 ppm FC.

Yes, this is what I seem to be seeing. I kind of expected that the loss at higher levels might be faster, if for no other reason than that there was more chlorine not bound to the CYA, thus a smaller percentage of the total was protected. Thank you for the confirmation.

So you might think that at higher CYA levels and proportionately higher FC levels that you might lose more chlorine. There are two reasons this doesn't happen. First, is that at the same FC/CYA ratio there is the same amount of chlorine unbound to CYA so that loss isn't going to be any higher. Second, even though some of the loss is from chlorine bound to CYA which is higher in concentration, there is apparently a CYA shielding effect that protects chlorine below the surface. Mark did some experiments that showed this effect in this post (and the rest of that thread).

No, you, and Jason cleared this up for me in this thread

So while it is true that at any given CYA level a lower FC will result in lower absolute FC loss since the loss is a % of FC level, 6 ppm FC at 80 ppm CYA should have a lower absolute FC loss than 3 ppm FC at 40 ppm CYA.

Based on previous, conversations with you, Jason and others, I assumed that this would be the case. The thing that was bothering me was that even with a high CYA level, the drop from 10ppm to 9ppm seemed significantly faster than the drop from 7ppm to 6ppm, so I was wondering if the additional 1ppm (from 9ppm to 10ppm) was really helping anything, or if it was burning off fast enough that it was basically a waste of chlorine, and wasn't significantly contributing to my residual after 24 hours. This is what I seem to be seeing during my current observations. (I can't really call it testing, because I haven't been keeping tight control over the variables, just observing how much chlorine I'm using over various periods with various bather loads.)

If you are able to experiment with this yourself, you can let us know if you see this same effect.

I'll see what I can come up. Right at the moment, I'm in the middle of shocking. The pool got used a lot this weekend, and after watching the "goings" on, I'm sure that my "ool" is most definitely a "pool." Last night I had a CC of 1.0 for the first time ever, and my FC was right at the minimum. Based on that and the number of beetles I've been digging out of the skimmer over the past two days, I decided it would be prudent to shock it just in case.

 

[TimS]

OK, it appears that my CYA reading has been off all along. If my CYA is 50 instead of 70, would this account for the rapid drop from 10 to 8? Since a lower percentage of the FC is protected, thus a higher percentage is unprotected, the very rapid drop from 10 to 8 would seem to me to make sense. Chem geek said that you'd lose twice as much to sunlight at 10 than at 5. If CYA is at 50 instead of 70, at FC 10 would the drop be even faster, since less total FC is protected? It would seem to make sense.

Yesterday morning, before the sun came up, I started with FC 8. I did not add any chlorine, we had full sun all day, I was the only one in the pool, and only for about 15 minutes, the pool water got to 92 by evening, the fountain ran 24 hours, and this morning my FC was still 6. The drop from 10 to 8 is only a couple of hours, but from 8 to 6 lasts 24?

I'm planning to drop my bleach usage to the levels more in line with CYA 50 than 70, and see if it works. Is there any way to be sure other than waiting to see if I get an algae bloom? (This is assuming that I'm doing the CYA test correctly now. (See my CYA testing thread.)

I haven't had the opportunity to run a comprehensive sequence of tests to really quantify this, but will when I have the chance.

 

[chem geek]

The rapid drop from 10 to 8 and then a very slow drop from 8 to 6 doesn't make sense, regardless of the CYA level. The drop should be roughly proportional to the chlorine level so the same % drop over time, assuming conditions are consistent. If the drop from 10 to 8 were during peak sun hours on the pool or during higher bather load (especially urination) while the 8 to 6 had sun lower in the sky or off the pool, then that would make some sense; otherwise it doesn't. This is why most of these sorts of measurements are done at night with the sun off the pool to remove that factor -- if the chlorine isn't holding overnight, then there are other sources of chlorine demand.

If the chlorine were dropping from 8 to 6 over 2 hours, then the hourly measurement would be the following, if the sun were on the pool the same for every hour (which of course it isn't):

10 ... 8.9 ... 8.0 ... 7.2 ... 6.4 ... 5.7 ... 5.1 ... 4.6 ... 4.1 ... 3.7 ... 3.3 ... 2.9 ... 2.6 ... 2.3 ... 2.1 ... 1.9