This is my response as a continuation from this thread.
I was wrong (assuming the industry's "CYA protection of chlorine graph" was correct) and Mark's (mas985) experiment proved that. There is a proportionate loss per time -- that part is still true -- but the rate of loss as a function of CYA level isn't as predicted based on how much chlorine is attached to CYA. There's an additional effect such that higher CYA levels protect chlorine a lot more as you move from around 50 to 80 ppm, even if you raise the FC to keep the FC/CYA constant. We don't know why for certain, but I suspect that it has something to do with CYA's direct shielding of chlorine below through its direct absorption of UV plus less than perfect circulation near the surface. So I suspect you are remembering older info and yes, we are learning more as we go along.Originally Posted by duraleigh
As for piku's original question and the truetex guy, I wrote to him a while ago (in 2004 and 2006) and exchanged E-mails about this. He keeps a relatively low FC level that is somewhat constant but there are two problems with that. First is that local demand can readily consume all the FC in an area, especially if the pool is smaller. Second is that even trying to maintain 0.5 ppm FC, this is still overchlorinating the water since the normal recommendation of an FC around 11.5% of the CYA level is equivalent to 0.1 ppm FC with no CYA. In fact, this is around the 650 mV ORP he talks about, though ORP varies by manufacturer and device (it's not nearly as "absolute" as that industry claims). He says things on his site that are just plain wrong such as the following:
The cyanuric acid levels suggested are so high (30 ppm) as to make the chlorine useless as an algaecide and almost useless as a sanitizer.
There are of course equilibria involved, but the effect of the cyanurics is to make all the chlorine ineffective all of the time. "How ineffective" depends partly on the cyanuric concentration, but it doesn't take much to ruin the chlorine. As little as 10 ppm is a problem, but even at that level you get scant UV benefit. At 40ppm cyanurics, chlorine is hobbled to being a weak bactericide, and won't touch anything larger like algae, and won't oxidize amines.
Chlorine is like a powerful vacuum, and adding cyanurics is like restricting the vacuum hose. It doesn't matter that you have unlimited time to sweep; big particles won't get picked up no matter how long you try.
This is just pure bunk and he obviously did not actually look at the equilibrium constants. I've always said that the rough rule of thumb (when the CYA is at least 5 times higher than the FC) is that CYA reduces the amount of disinfecting chlorine by a factor equal to the CYA level. So 30 ppm CYA reduces chlorine's effectiveness by a factor of 30. The question is whether this is a problem or not also accounting for the FC level -- in general, it's not only not a problem but it's actually better since it prevents over-chlorinating. As for the statement that it won't touch anything larger like algae and won't oxidize amines, that's not true and we've got hundreds if not thousands of pool owners proving him wrong every day on The Pool Forum and Trouble Free Pool. It's only when you start talking about things like Giardia that are harder to kill where it might make sense to have strong chlorine levels, but with Crypto even chlorine with no CYA won't inactivate it except at very high levels over many hours so it might be better to handle these cases with other means (this thread gets into that discussion).
Most heterotrophic bacteria have very low CT values of 0.08 or lower such that one only needs the equivalent of 0.1 ppm FC with no CYA to kill 99% of them in one minute or less. So even lower levels of chlorine will kill bacteria faster than they can reproduce, but it takes a higher level of chlorine to inhibit algae growth. It's still relatively low, however, at around 0.06 ppm FC with no CYA to prevent algae growth (up to a phosphate level of around 3000-4000 ppb which is very high). This translates to an FC that is 7.5% of the CYA or the Minimum column in Ben's original chart. The "big particle" analogy is completely wrong -- a low effective concentration of chlorine only affects kill rate so as long as that rate is faster than the reproduction rate of the thing you're trying to kill, then you prevent its growth. The idea that somehow the CYA or the chlorine bound to the CYA somehow makes the remaining unbound chlorine completely ineffective is just plain wrong. This link is one example of a scientific paper that shows that the unbound chlorine, specifically hypochlorous acid, is what matters in inactivation of this particular protozoan cyst. The CYA and chlorinated cyanurates (chlorine bound to CYA) are not relevant and don't prevent the unbound chlorine from doing its job.
In my opinion, there is no need to use more chlorine than you have to for disinfection and prevention of algae. Higher disinfecting chlorine levels will just oxidize skin, hair and swimsuits faster (and is more corrosive to equipment, but they are pretty resistant to begin with) and will produce disinfection by-products faster and for ammonia breakdown will do so in greater quantities. My wife has to replace her swimsuits she uses in an indoor pool (with 2 ppm FC and no CYA) in the winter every single season while the ones she uses in our outdoor pool (with 3.5 ppm FC and 30 ppm CYA equivalent to 0.1 ppm FC with no CYA) last for multiple seasons with far less wear due to the factor of 20 difference in effective chlorine concentration. CYA can be seen as not only protecting chlorine from breakdown by UV in sunlight, but also as a chlorine (hypochlorous acid) buffer holding extra chlorine in reserve as needed and moderating the strength of chlorine. This is not a bad thing. It just means one needs to adjust the FC level to be high enough relative to CYA. His system will only lose a comparable (or maybe even smaller) amount of chlorine if he maintains a very low FC level such as 0.2 ppm but then that isn't enough to not get used up locally by excess sweat/urine, etc. If 0.2 ppm FC with no CYA is what he believes is good, then he can accomplish that by using an FC level that is around 20% of the CYA level and have EXACTLY the same disinfecting chlorine (and ORP reading), but with the advantage of having plenty of extra chlorine in reserve as well as protected from sunlight (to a degree you can choose).