Understanding chlorine loss

[nutria]

So, I am trying to understand how the loss occurs. From what I have read here, the CYA protects from loss from UV exposure by binding to some fraction of the chlorine, the "excess" so to say is available for the killing of nasties and oxidizing organics. But I have also read that loss will happen due to evaporation, etc and that will be a certain proportion of the total chlorine in the pool rather than a particular umber of ppms, which is why you don't want to overshoot much the target level. I suppose all of this is also dependent on the surface area exposed (which explains why covers should help) and temperature, etc

I would like to figure out a reasonably close to optimal plan for my cya levels and the dosing given that I can really only test or add things at most once per day. If someone could explain the loss mechanism in some detail it would be very helpful. I want to figure out whether a cover makes sense and by how much to overshoot if I am going to be away for a day or two. Or maybe whether I should just go with a SWG ...

And I haven't done any chemistry since high school, Maybe I shouldn't say it, but I pretty much hated chemistry and now I am paying the consequences.

 

[pooldv]

I cannot explain all of those loss questions. But, I think I can make it simpler for you. Recommended CYA level is 30-50. Since you live in hot, sunny texas I would go for 50.

With 50 CYA go look at the Chlorine CYA Chart and see that min is 4, target is 6 and shock is 20.

It is safe to swim in your pool when chlorine is anywhere between 4 and 20. So, there is no real need to really try to dial in whether you want 6 or 8. Put it at 8 and see how it goes. Test and add daily when you can, write it down and look for a pattern. Then when you know you can't add tomorrow add twice as much as normal. When you can't add for a few days take it up to 20. The only important things are to never let it drop below minimum and no reason to go over shock.

We had a minor algae issue this weekend, ooops. We spent 5 or 6 hours in the pool with FC at 19 and it was great. No chlorine stink, no discomfort, nothing. Really not any different than when it is 8.

 

[Scuba_Steve]

In general, from what I have seen around here, 3-5ppm of FC is about an average loss per day. I lose 2-3 at my CYA level of 70 with my SWG turned off.

If you bump up to shock level you should be fine for a weekend.

 

[nutria]

I cannot explain all of those loss questions. But, I think I can make it simpler for you. Recommended CYA level is 30-50. Since you live in hot, sunny texas I would go for 50.

With 50 CYA go look at the Chlorine CYA Chart and see that min is 4, target is 6 and shock is 20.

It is safe to swim in your pool when chlorine is anywhere between 4 and 20. So, there is no real need to really try to dial in whether you want 6 or 8. Put it at 8 and see how it goes. Test and add daily when you can, write it down and look for a pattern.

Thanks for the explanations and recommendations. I know what you says makes sense, but I can't help wanting to understand a little the reasons behind the recommendations. I didn't like chemistry which is why I became a physicist ... and I want to be able to explain my pool, not just enjoy swimming in it. Weird, I know

 

[pooldv]

Not weird. Well, maybe a little. There are plenty of other weird folks around here who will come along and tell you everything you could ever want to know about chemistry.

 

[Scuba_Steve]

You need a good lesson from chem geek then. I just had to say his name. I think he searches for his name to be mentioned, because he always seems to show up shortly after someone does lol.

 

[pooldv]

Right!

 

[chem geek]

Yes, I do search for my name so that forum sections other than The Deep End and some Chemistry 201 don't end up filled with a lot of chemistry.

I'm not sure where you read about chlorine evaporating, but in pools with CYA that is pretty negligible. Basically, most of the chlorine in a pool with CYA is bound to CYA -- roughly 97% to 99% depending on the specific "normal" (not SLAM) FC and CYA levels. The chlorine bound to CYA is not very reactive -- it has less than 1/150th the oxidation power of hypochlorous acid and almost no disinfecting power. It is not volatile and it may break down in sunlight but much more slowly than unbound chlorine. You need to think of the chlorine bound to CYA as chlorine held in reserve that is released (via chemical equilibrium) rather quickly as any unbound chlorine gets depleted.

The main loss of chlorine in an uncovered outdoor residential pool is from the UV in sunlight. In direct noontime sun the unbound hypochlorous acid has a half-life of 2 hours and 10 minutes while the unbound hypochlorite ion has a half-life of 20 minutes. At a pH of 7.5 with roughly 50/50 of each, the half-life is 35 minutes. In a pool with no CYA, the chlorine at the surface absorbs enough of the UV when it breaks down that there is less in lower depths so in an average 4.5 foot pool the overall half-life at pH 7.5 is roughly one-hour. So without CYA, figure that half the chlorine is broken down every hour.

With CYA in the water, this loss is cut down considerably, but what is interesting is that at higher CYA levels, even with proportionally higher FC levels to keep the same amount of unbound chlorine, the absolute chlorine loss is less. This may be from some shielding effect from CYA or from the chlorine bound to CYA. Note that the rate of loss from sunlight is fairly independent of the temperature of the water.

So the net of this is that we generally recommend a CYA of around 50 ppm unless the pool is in a very sunny area in which case the CYA can be higher up to 80 ppm. We also recommend a CYA of 80 ppm for saltwater chlorine generator pools to minimize losses to sunlight so that the generator % ontime can be lowered.

The chlorine loss from bather load is very minor in a residential pool unless you have a large pool party or a bunch of urinating kids. A rough rule-of-thumb is that every person-hour in a swimming pool requires around 4 grams of chlorine (in Cl₂ units which is similar to the units of mg/L that is how FC is measured). So one person-hour in a 20,000 gallon pool requires (1 person-hour)*(4 g/person-hour)*(1000 mg/g)/(75708 liters) = 0.053 ppm FC to oxidize the bather waste.

Ironically if you use a pool cover that is mostly opaque to UV, then that cuts down chlorine loss from sunlight, but increases chlorine loss from oxidizing the cover itself. There is a net benefit of reduced chlorine loss, but it doesn't drop as much as the very low loss overnight with no cover and no sun. The rate of chlorine loss from oxidation of bather waste, a pool cover, algae, pollen, etc. is temperature dependent.

Most pools operating at the recommended levels at TFP lose around 2-3 ppm FC per day. Some lose more, some a little less. If you want to minimize chlorine loss, then a cover opaque to UV would help. I have a mostly opaque electric safety cover and my chlorine loss if I don't use the pool is around 0.7 ppm FC over 24 hours but with the water fairly warm at 88ºF. Because the pool gets used almost every day for an hour or so, the usual loss is closer to 1 ppm FC per day. That's at an average FC/CYA level of around 10% -- roughly going from 6 ppm FC to 3 ppm FC with 40 ppm CYA where I dose around twice a week. You can do that if you use a cover. Without a cover, you pretty much need to dose every day (certainly every other day) to not have the FC swing too much.

 

[nutria]

Thanks!!! That was an a great explanation! Now I need to think about it

Sent from my Nexus 5 using Tapatalk

 

[vcarnett]

Chem Geek, thank you! I've been trying to wrap my head around cya/fc. My grapple on this subject was almost complete from my reading of posts from other experts --thanks all-- and yours drove it home.