And last (so far) is my (chem geek's) response:
cliff_s,
Believe me, I'm completely with you on basing information on scientific facts, careful experiments and studies. However, I also believe that the science has to explain what goes on in real pools and if there's an inconsistency, then the science needs to be examined to explain what really goes on. As waterbear points out, there has been a lot of feedback from users of several pool forums and that is where the basis comes from regarding lower levels of Free Chlorine working well in most SWG pools.
If you haven't already seen my posts regarding the chemistry of pool water, take a look at
this thread where I talk about (and calculate) the chlorine/CYA relationship. At the end of the first post is a link to a spreadsheet that calculates the pool water chemical species including the chlorine/CYA compounds (chlorinated isocyanurate species). At post #45 in
this long thread I calculated the chlorine concentration buildup based on CYA concentration and assumed water flow through the salt cell. It may very well be the case that the SWG manufacturers have mostly simply decided on 60-80 ppm CYA just to protect chlorine a little bit longer (but not that much longer -- the half-life for chlorine in direct noontime sun is 6 hours at 30 ppm CYA and 7 hours at 60 ppm CYA so the returns are definitely diminishing). However, more and more users have been reporting that the higher CYA levels have lead to dramatic improvements in salt cell efficiency in that they can turn down their salt cell "on" time far below what would be explained by the breakdown of chlorine from sunlight at different CYA levels.
You can also look at
this thread where I look at disinfection rates and show that a very, very small amount of disinfecting chlorine kills most bacteria and inactivates viruses (but protozoa such as Giardia and Cryptoporidium are very hard to kill using chlorine alone). The intermediate exception is the bacteria, Pseudomonas aeruginosa, that causes "hot tub itch" which requires higher levels of chlorine than we recommend for pools, but this bacteria seems to thrive much more in the hotter water found in hot tubs so that's where the higher disinfecting chlorine levels really need to be used. The issue with needing chlorine in pools is mostly to keep away algae since it takes higher levels of chlorine to kill algae than it does to kill the "easy-to-kill" bacteria. Ben's
Best Guess CYA chart was created from his experience with many commercial pools plus residential pool users' experience on the Pool Forum. The chart's "Min" and "Max" columns very much track actual disinfecting chlorine levels of 0.03 and 0.07 ppm (of HOCl, measured as ppm Cl2) except for very low CYA levels where the minimum chlorine is more determined by not having it run out (i.e. it is hard to keep 0.1 ppm FC in the pool, not only due to sunlight, but due to localized consumption).
You are right about salt pools possibly influencing different species of algae, but remember that a regular pool starts out with about 350 ppm salt (chloride as ppm sodium chloride) due to the initial addition of sodium bicarbonate with calcium chloride. Over time with the addition of chlorine, this salt level rises to around 500-1000 ppm in most pools, especially those using a liquid form of chlorine (bleach or chlorinating liquid) which contains salt as well as hypochlorite. The level of salt in modern SWG pools is 3000 ppm which is not that high especially compared with the ocean at around 35,000 ppm. So though there is a difference, I doubt that the species of algae in 3000 ppm pools is significantly different than in non-salt (350-1000 ppm) pools, though only real testing can determine that for certain.
A vigorous discussion of the Langelier Saturation Index may be found at
this thread.
This thread shows a large study of commercial pools and is the main study the pool chemicals and distribution industry uses to claim that CYA levels do not matter (only Free Chlorine matters). And just to show that I am not biased towards salt pools,
this thread talks about possible downsides to the extra salt and the SWG (I'm not biased against, either -- I just feel that full disclosure of information is useful to make wise decisions).
Over the past couple of years I believe that several of us on the Pool Forum have added some scientific backing and explanations to the experiences that pool users have been seeing. I absolutely agree that we have NOT done rigorous experiments, but unfortunately there have been very few such studies and some of those that have been done were done in ways to bias the data through incorrect methodologies (in my opinion) while others have not. In some cases we have tried to "experiment" using real pools and that's how we determined the amount of disinfecting chlorine needed to keep away mustard/yellow algae -- a couple of users actually varied their pool's chlorine levels to see the algae go away, then come back, by raising and lowering the chlorine level. There have also been "experiments" with many pools regarding the "rising pH" problem, especially in SWG pools and I would say that we have definitively determined that lowering the TA level reduces the rise in pH and especially the amount of acid needed to keep the pH stable. This is a strong indicator that our "theory" that the cause of the pH rise is due to the greater outgassing of carbon dioxide from the aeration from the hydrogen gas bubbles generated in the SWG is correct. If the conventional wisdom that the SWG produces alkalinity (from hydroxide) were the true cause, then lowering the TA would not have helped the problem -- it would have made it worse as far as pH rise and would have had no effect on the total amount of acid needed to maintain pH (
this post describes how the use of bleach, chlorinating liquid, and an SWG are pH neutral in terms of chlorine addition AND usage combined). We also believe that the use of borates helps reduce the rising pH problem though we are less certain as to why (we think it's due to the borates inhibiting algae so that less chlorine is needed to do so) and more info on this can be found at
this thread.
Anyway, if you have information on scientific studies or suggestions for experiments or anything else that can be helpful in our quest to understand what goes on in pools so that we can directly address problems and, most importantly, prevent them at low cost and with ease-of-use, please let us know. This has been a group effort and we encourage and appreciate all feedback.
Richard