Very high CYA levels, HELP!

[Mindiv]

Ben and Chem Geek you both rock!! Actually makes complete sense but that would be because I was a marine bio major with serious chem background and now I'm a physician assistant and live in the scientific world. That is exactly what I thought and explained to my pool guy. He didn't understand prior to our conversation that the tests measured bound and unbound (I made a darned good guess, but based on the reality of binding to stabilize I figured that had to be the case). He had always just done the usual "Chlorine lock" crap. Referenced this forum so maybe I have created another convert or maybe he was just thinking I'm a lunatic! ;-)

Either way...pleasantly happy with my first swim in a FC of 9 and found it much more comfortable than lower levels at other pools. THANK YOU POOLFORUM!!!!!!!! Now off to read all the referenced materials. I would like to speak with authority when people question my way of dealing with my pool this season until my CYA levels are lower and frankly from here on out.

[chem geek]

In my post above, I added a couple of links to papers talking about how the bound chlorine is measured in FC tests and the reaction rates for release of chlorine from bound CYA.

I also want to note that there are two effects from the higher FC at higher CYA even when keeping the FC/CYA ratio constant. The first is that the chlorine bound to CYA is not completely "inactive" as I had indicated. As noted in the following paper:

Effects of isocyanuric acid on the monochlorodimedone chlorinating rates with free chlorine and ammonia chloramine in water

The reaction rate of chlorine bound to CYA could be as much as 1/150th that of hypochlorous acid. So an FC of 9 ppm with 150 ppm CYA would have 0.024 ppm hypochlorous acid (in ppm Cl2 units) at pH 7.5 and 77ºF temperature, but could have up to 9*(1/150) = 0.06 ppm additional equivalent oxidizing power from the chlorine bound to CYA. Since we normally aren't dealing with such high FC and CYA levels, we normally ignore this effect though it may at least partly explain Ben's experience with seeing more effects from such high levels.

The other effect is due to the higher FC being a higher capacity or reserve that will continue to oxidize when one gets out of the pool and the water starts to evaporate. This is tricky to figure out because while there is more capacity to keep on reacting for longer, it's unclear what the increase in concentration from water evaporation will do. All the chemical components increase in concentration equally and ultimately one would end up with precipitate of CYA and possibly Trichlor, but as the active chlorine level goes up it will react as well as outgas so maybe that occurs fast enough to result in only CYA precipitate and no Trichlor. So maybe there is more risk towards oxidizing skin and swimsuits after getting out of the water with high FC, but it's not at all clear if this effect is large enough to be noticeable (i.e. I'm just speculating here). A decent experiment for this would be to notice any difference in chloramine smell on your skin in the high FC and CYA vs. lower FC and CYA (but both with the same FC/CYA ratio), but of course you'd have to have the same sweat conditions for a decent comparison.

Compare this with what is typically the case in many indoor commercial/public pools that use no CYA. Even with 1-2 ppm FC, they have at least 10-20 times the active chlorine level of the outdoor residential pools following Ben's chart. That's a HUGE difference in oxidation rates of swimsuits, skin and hair as well as in the rate of creation of disinfection by-products including smelly and irritating nitrogen trichloride. My wife experienced this difference every year when she swam for 5 months over the winter in an indoor community center pool. She had to buy new swimsuits every season as their elasticity would get shot and her skin would be flaky and hair frizzy. In our own pool used 7 months over the summer season, her swimsuits would last for 7 years and the effects on her skin and hair were noticeably better.