"Unsafe" FC for a given CYA

[Rangeball]

Is it possible to maintain "unsafe" free chlorine levels at a given CYA ppm?

Excluding jumping in right after one shocks, of course

For example, let's say a pool has a CYA of 20 ppm. At night the owner shocks the pool to 12 ppm, per the BG chart.

The next day, after the sun has been up for several hours, the owner jumps in.

I've been under the assumption that the pool should be safe, as the CYA will protect and hold chlorine up to the max level in the chart, and the excess will have been lost to the sun. Right? Or can the CYA actually protect and hold a higher chlorine level than what is indicated in the max column, potentially creating an unsafe condition?

 

[JasonLion]

Chlorine levels drop any time the sun is out, much faster in direct sunlight than indirect. The rate at which this happens depends on your CYA level. FC levels will also drop for other reasons, typically when organics get into the pool (people, pollen, dirt) There isn't anything which would cause the FC level to "snap" to the level of the Max column on Ben's best guess chart. It will drift down over time from where ever you put it, cross the max column and keep right on going down.

Just how "unsafe" a given level of chlorine is is under debate. Some people believe that it is safe at any plausible level you would find in a pool. Others don't like to swim when FC is above the max column in Ben's best guess chart.

 

[Rangeball]

Jason, I realize that, and should have clarified saying no organics left in the pool.

Basically, I was under the assumption that in direct sunlight FC not protected by CYA burns off pretty quickly, like within the hour. I also assumed that a given CYA can only bind too and protect so much of the FC in a pool, and the balance is susceptible to sun loss.

If this is the case, and one doesn't jump in right after shocking after sundown, and assuming they aren't using a cover and getting in right afte the cover is removed, excess unprotected FC would be very hard to maintain in a pool after exposure to direct sunlight for a short period of time, right?

Is the max in the BG chart the max that the CYA can protect at a given level?

 

[JasonLion]

That is not the way it works. CYA will bind to some percentage of the chlorine at any given time. If the chlorine that is not bound to CYA goes away the CYA will release some of the bound chlorine to be active again. The percentage of the chlorine that gets bound up mostly depends on the CYA level. All of this is something of an oversimplification, but this describes the primary things going on in the typical pool.

Regular chlorine drops by about 50% every half hour in direct sunlight. Chlorine bound to CYA drops by 50% after about 8 hours of direct sunlight. At higher CYA levels there are additional factors that protect things more, but the basic principals still hold. So over half an hour you will lose half of the active chlorine, but at the same time the CYA will release some of the bound chlorine, keeping your active chlorine level about constant, while the total chlorine level drops steadily.

 

[KurtV]

rb,
According to this post by Richard (aka chemgeek) on poolforum, swimming even at shock levels probably isn't a problem in a pool with a decent level of CYA (so long as you're not drinking too much of the water).

 

[chem geek]

Yes, I know it's a controversial statement to make, but given what I know about the chemistry, even getting into a pool at "shock" levels of around 0.3 ppm disinfecting chlorine level (see the "0.3" column in this chart) is technically equivalent to getting into an indoor pool with around 0.6 ppm FC and no CYA. It WILL degrade swimsuits faster (over months), but isn't going to harm you as far as reaction rates are concerned. Drinking the water is technically probably safe if not done frequently according to some toxicity documents I've seen, but I wouldn't take that chance (rats given 100 ppm FC water showed no problems, but if there are disinfection by-products in the water then they might cause cancer and that wouldn't show up for quite some time, though the risk is small).