Industry understanding of FC/CYA relationship
- Thread starter wayner
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Evaporation and refill does not lower any chemical levels because only water evaporates; CYA and CH do not.
You probably just had enough summer rain overflow or splash-out and carry-out over time plus the CYA does slowly get oxidized by chlorine.
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It's not a Canadian thing. There are a smaller percentage of posts on this forum that talk about very decent pool stores that don't try and sell extras and that are honest and reasonably well informed. Even so, hardly any of those understand the chlorine/CYA relationship except they know that the buildup of CYA from stabilized chlorine can be a problem. They don't know the FC/CYA ratio that prevents algae growth, but they know that using chlorinating liquid or bleach is OK.
My pool store sells a LOT of chlorinating liquid, but they also sell Trichlor tabs. They also service around 2000 pools (out of two locations with retail stores) and this pool service generally uses Trichlor tabs in floating feeders or inline chlorinators, but they try and maintain 4.5 ppm FC and if the water is dull or algae visible then they shock with chlorinating liquid weekly. If the CYA hits 100 ppm, they do a partial drain/refill to lower it or they use a phosphate remover (especially for yellow/mustard algae). So they have some visceral understanding of the relationship, but not a precise one.
That is likely the cause if my cya drop. Kids being wild men and the fact that I have not added stabilizer all year. Before my guy at the pool store retired I never did any testing other than sticks. Went in, like the cow being led to slaughter, with my water offering and wallet open and did what I was told when it got cloudy. The couple times I had big issues with cloudiness were low cya so SWG could not keep up and the "phosphates". When I was forced to take my pool maintenance into my own hands I was a bit leery. Then I saw someone with the intelligence of a walnut use a Taylor 2006 at the pool store. He tried to charge me $125 for one, I left. Spent $60 on amazon and have been free ever since.
Great post Svenpup!! Very well said indeed.
Chemgeek, thank you for your continued unwavering efforts to inform and educate us all.
Chemgeek, thank you for your continued unwavering efforts to inform and educate us all.
Chem Geek,
I read the paper you referenced by Rubin and not being a chemist but rather an engineer (so I do read tons of papers like this) I have aquestion that maybe you could answer. It seems to me that pH was just as important as concentrations during this study. As i stated, I am not a chemist, but most of the charts and formulas rely on pH as one of the factors. The author even states "At pH values below 7 free chlorine exists primaily in the form of hypochlorus acid (HOCL) which is 80 times more effective as a bactericide than hypochlorite (OCL-)." Seems to me that keeping pH low would be just as important as concentrations. Maybe even more important.
I read the paper you referenced by Rubin and not being a chemist but rather an engineer (so I do read tons of papers like this) I have aquestion that maybe you could answer. It seems to me that pH was just as important as concentrations during this study. As i stated, I am not a chemist, but most of the charts and formulas rely on pH as one of the factors. The author even states "At pH values below 7 free chlorine exists primaily in the form of hypochlorus acid (HOCL) which is 80 times more effective as a bactericide than hypochlorite (OCL-)." Seems to me that keeping pH low would be just as important as concentrations. Maybe even more important.
I also agree that there are some good shops out there who don't try to rake you over the coals every time you walk in the door. But, even some of the good ones have their head in the sand about CYA/FC. I have a friend in Florida who has been going toe the same mom & pop shop for years. They don't push stuff on him and seem fairly competent at water testing. He is not a TFP follower, but his primary source of chlorine is 12.5% pool store chlorinating liquid. Seldom does he use solids of any sort.
When he was in a while ago he was explaining my (then) CYA 120 and that I kept my FC at around 15. The pool store owner just couldn't get over that I was constantly "shocking" my pool and that it was not safe to swim in..... He wanted to see FC no higher than 5.
So, there are "nice" stores out there but even they can give out bad information.
I agree that we will never change the industry except as we are doing it, one pool at a time.
I'll let chem geek answer later but my take on this is that yes, at a pH less than 7, free chlorine would be even more effective at killing bacteria. However, people swim in pools and pools are made of stone materials. At pH less than 7, pool water is irritating to the eyes. Tears have a pH of about 7.4 and at pH levels of 7.0 or less, you'll notice with irritated eyes and perhaps even skin. Also, gunite, plaster and any pool surface except vinyl are sensitive to scaling (due to high pH) and corrosion (due to low pH). Many times at pH levels lower than 7, water is corrosive to pool lining surfaces, decking materials and even pump station internal plumbing. How corrosive depends on total alkalinity, calcium hardness and all the other factors that go into calculating CSI or Calcite Saturation Index. In a lab beaker, low pH is great for battling pesky bacteria, not so much in a pool.
I usually refer to the paper by the lead author, O'Brien, and not by the editor of the book, Rubin, that has a collection of papers (presentations at a conference) from various authors.
Though there is a dependency on pH, one can easily overcome that and tune the active chlorine level via the FC/CYA ratio. Also note that the hypochlorous acid (HOCl) concentration varies a LOT less from pH when CYA is present. The statement you quote is referring to free chlorine in their definition which is unbound chlorine, so the sum of hypochlorous acid (HOCl) and hypochlorite ion (OCl-) where their Figure 14.7 shows free chlorine rising somewhat slowly going to pH 7 and somewhat faster going to pH 8, but at pH 7 it's HOCl that is rising while at pH 8 it is OCl- that is rising (also that Figure looks at a much broader range of pH than just 7-8). Look instead at Figure 14.5 and look at the dotted line labeled "HOCl". Notice how relatively flat it is from pH 7 to 8. This is the same as I show in this post where you can see that while the HOCl drops by 50% when going from pH 7.5 to 8.0 when no CYA is present, it only drops by around 15% when CYA is present. Similarly, going from pH 7.5 to 7.0 with no CYA increases HOCl by around 50% but with CYA it only increases by around 35%. This is because CYA is a hypochlorous acid buffer.
As Joel noted, there are other reasons you wouldn't want the pH at 7.0 so instead it's much easier just to have a somewhat higher FC/CYA ratio target at pH 7.5 and we pretty much have this set to prevent algae growth since providing adequate disinfection is much easier. So the entire discussion about "germicidal responsibility" in the paper isn't relevant to us since we are already beyond fast kill times (though not a high as the EPA DIS/TSS-12 which is overkill) and are instead set at a level to prevent algae growth. A list of kill times when the FC/CYA ratio is roughly 10% is shown in this post. The roughly 5% FC/CYA ratio in SWG pools would have kill times roughly double listed in that post (that shows the times for 3-log reductions or 99.9% kill).