As an offshoot of another thread (on chlorine true costs), I bring out this
discussion, mostly by trader4 and me, on what the *actual* effect is of CYA
on chlorine sanitization for an *outdoor* home (uncovered) pool.
To repeat the starting point, this chart, which is what "I" follow, shows
that the target free chlorine levels are (far) higher than most people think,
for a decently high stabilizer level (my pool has CYA of around 50ppm):
Page 9 of this article discusses how CYA increases kill time:
which, in the end, concludes (verbatim) "The EPA Swimming Pool Water Disinfectants
standard, DIS/TSS‐12, needs to be revised to reflect the effect of CYA.
While the "chart" is intended to be a *pragmatic* tool for the outdoor
pool homeowner, the science is what we're after:
Anyone with experience with the interaction of iso-cyanuric acid (stabilizer)
and desired free chlorine levels is welcome to add value here, as I simply
follow the recommendations from the "pool school":
Article I read says that chlorine is lost from hypoclorite by photolysis
which generates chloride ion and oxygen. Stabilizer slows this down.
Chemistry is outlined:
DannyD. wrote, on Thu, 26 Jun 2014 18:37:21 +0000:
As trader4 has noted elsewhere, the "government" agencies see to
propose a "single" minimum standard for free chlorine:
The government agencies don't seem to fully take into account the
optimal ratio of cyanuric acid to free chlorine, which is described
in the homeowners chart described in the OP.
For example, here is a governmental PDF which clearly discusses that
CYA reduces the effectiveness of chlorine sanitization; yet, they
still persist in outlining only minimum chlorine standards given
maximum iso-cyanuric acid levels:
Can we "assume" the reason for this single-number min:max approach might
be that it's "easier" for the government to propose single-number minimum
chlorine for maximum stabilizer standards as opposed to cl:cya ratios as
shown in the pragmatic charts?
I don't know the answer.
That "is" the answer to the subject line, but unfortunately I didn't
formulate the subject line as well as I should have, since, the part
we're trying to figure out is how much chlorine do you need for a given
cyanuric acid level.
For example, this chart shows that, at 50ppm CYA, you want to target a
free chlorine level of 6 ppm (and up to 30ppm for shock):
Given that 6ppm is rather high as a target level, the question mainly
ends up being *why* public agencies don't recommend higher chlorine levels
for public outdoor uncovered pools.
The answer (whatever it turns out to be) has an implication for homewowners
whose pools are outdoor and uncovered.
Can't tell you.
I'm just a retired chemist who was curious about the chemistry.
I remember as a kid going to public pools that chlorine level was very
high and pools reeked of chlorine. Today going to a private swim club
there is no hint of a chlorine smell.
DannyD. wrote, on Thu, 26 Jun 2014 18:49:38 +0000:
Basically, this thread is all about whether we homeowners should aim for
min-max fc-cya numbers, as outlined by government agencies, or if we should
target fc:cya ratios, as proposed in the paper below:
which says the minimum free chlorine should be a ratio of at least
7.5% of the cyanuric acid stabilizer level.
Minimum FC === 2ppm, Maximum CYA === 100ppm
Minimum FC >= 7.5% of CYA
That's the fundamental question.
On Thursday, June 26, 2014 3:03:08 PM UTC-4, DannyD. wrote:
As I explained in the other thread, the reference you found
has a graph of what's going on:
Figure one shows what happens. Yes the more CYA you have the more
it reduces the sterilization effectiveness of whatever amount of
chlorine you have. But it also shows that a low levels of CYA, eg 20ppm
having chlorine of 1.5ppm is still high enough for effective sanitation.
And if you look at the other end of the curve, eg 60ppm CYA, then while
it further reduces the effectiveness of the chlorine, adding even more
chlorine doesn't do anything. You could have chlorine at 1.5 or chlorines
a 4 and it's almost the same. If that graph is right, then you're fighting
a losing battle trying to take chlorine up to try to make it sanitize
In short, at low CYA levels, 2 ppm of chlorine is apparently an
At higher levels, 2 ppm is still capable of sterilizing and going higher
to 4 ppm, 8 ppm isn't going to make it better anyway. There is a whole lot
of pool history that apparently agrees, because the vast majority of people
are keeping chlorine at 1 to 3 and they don't even know what their CYA
level is. Pools aren't green, people aren't getting sick.
Ah. As a chemist, you might be interested in this recent Time Magazine
news article about kids' pee in a pool creating weapons of war! :)
No, It’s Not Safe to Pee in the Pool, Says Science
"Uric acid from human urine mixes with chlorine to create the cyanogen
chloride (CNCI) and trichloramine (NCl3). CNCI is a toxic compound that
can harm organs like the lungs, heart, and central nervous system."
Heh heh ... of course, the levels of CNCI are low ...
How much pee in a pool would kill you?
What Happens When You Pee in the Pool?
But, back to the desired FC-to-CYA ratio of 7.5%, it seems that every
public health document that I can find lists a minimum FC and a maximum
CYA, but, they don't seem to specify the desired ratio.
It seems surprising to me, since they all recognize that CYA lowers FC
efficiency (i.e., killing time); so, maybe the ratio matters ... maybe it
I follow the 7.5% FC-to-CYA recommendations here for my pool:
But, I'm willing to listen to alternate arguments.
trader_4 wrote, on Thu, 26 Jun 2014 15:44:40 -0700:
While that paper is hard for me to read:
The "figure 1" you speak of does seem to say what you say:
which is the standards of 2ppm FC and, say, 60ppm CYA still has a
redox potential (ORP) sufficient to kill microbes.
And, certainly, the CDC is not specifying a FC-to-CYA ratio:
So, now I'm confused *why* the pool experts I've been following insist
that the target FC-to-CYA ratio should be something like 10%?
Luckily, it works (I don't have algae problems when I keep the
ratio high, for example), but, certainly there are a *lot* of people
with algae blooms who complain that their FC levels are adequate.
So, I don't know whom to believe now.
Thanks for opening my eyes - but - I'm confused more than ever now.
I wish I knew more about pool chemistry.
DannyD. wrote, on Fri, 27 Jun 2014 03:05:03 +0000:
Even harder to understand, this paper seems to say the opposite!
"For each CYA concentration, there is an [FC]/[CYA] ratio such that
hypochlorous acid is present sufficiently to control algae under
all possible conditions."
So, one says that the FC level is what matters, while the other
just as clearly says it's the FC/CYA ratio that matters.
So, I'm really confused.
trader_4 wrote, on Thu, 26 Jun 2014 15:44:40 -0700:
I've been reading (and re-reading) that paper, since it basically
concludes that CYA levels of greater than about 20 ppm are not
necessary in an outdoor uncovered pool (see additional graph below):
And, at the same time, it concludes that anything greater than around
20ppm CYA is counter productive in oxidizing and sanitizing:
As I understand your argument, it's that the redox potential is
fine (at around 700mV) at even 1.5 ppm FC for a reasonable level
of CYA (of about 20 ppm) as shown in Figure 1 we've been discussing:
Yet, even after having shown that figure, the paper later admits
in Part II, that:
"Even when the free chlorine was as high as 4 ppm, a significant
[bacterial] plate count existed because the ORP [redox potential]
in that spa was 537 mV. The pH was 6.9, so why was the ORP so
low in that spa’s water? Excessive cyanuric acid was the culprit,
as was the case in all but two of the thirteen spas exhibiting ORP
levels at or below 630 mV... ”
So, in part I, the paper argues that 1.5ppm FC is fine, in and of
itself; while in part II, the paper tacitly agrees that the CYA
level has to be taken into account, even at 4 ppm FC.
While the paper does *not* say the ratio of FC to CYA is important,
it tacitly implies that the *ratio* of FC to CYA *is* important.
Still reading (and trying to understand) though ...
DannyD. wrote, on Fri, 27 Jun 2014 05:02:07 +0000:
Still trying to understand, the paper clearly says that a redox
potential (aka ORP) was the *only* factor that predicted sanitation.
Specifically, the paper said a safe dividing line was about 650mV
Yet, the paper distinguishes between sanitation & *oxidation*, so
that might be one source of my confusion.
For example, the paper concluded that pools (unlike mine) with
high organic loads, would do better with less cyanuric acid, because
of the debilitating effect CYA has on *oxidation* of organics.
So, I'm beginning to see that perhaps my confusion is that most
of the papers on the FC:CYA ratio are discussing *sanitation*
while this paper seems to concentrate more on *oxidation*, both
of which are (differently) affected by the FC:CYA ratio.
The process has been automated in "saltwater pool" technology, which
generates its own chlorine from a salt water base (which cannot be
tasted: it is much less salty than sea water.) Pool operators no longer
need to add a bucket of chlorine granules from time to time, because
the chemical monitor creates its own. Adjustments of pool chemistry
(e.g. pH) are more conveniently made by other means.
Heh heh ... I suspect most pool owners (with the exception of trader4,
Steve SMS Scharf, and a few others) would be as confused as I, perhaps
even more so, given the technical merits of this argument.
The good news is that the recommendation to keep cyanuric acid to
something below around 40ppm simplifies things, as it basically makes
the trichlor pucks and dichlor powders useless.
The only thing left is the liquid chlorine (for a normal pool).
trader_4 wrote, on Thu, 26 Jun 2014 15:44:40 -0700:
I did some research, and it's clear that the author of that
paper both you and I have been quoting does *not* come highly
regarded by the folks who propose the FC-to-CYA ratio (as
compared to those who propose separate static FC and CYA values).
For example, look at this post by Ben Powell about Kent Williams'
article we've been quoting ...
Ben says "the "CYA, B or B" article is based on a misunderstanding of CYA chemistry --
but since the purpose of those theories was apparently to protect Stranco
controller sales, why bother with getting the chemistry right?"
Likewise, look at this recent forum article:
Health department rules on FC/CYA ratios
Here's what Richard Falk had to say about the Kent Williams article:
"His chart is completely bogus and you should not take it seriously.
ORP is very inconsistent from sensor to sensor across manufacturers
especially measuring lower ORP readings. It is absolutely positively not
true that the ORP all magically converges at 640 mV at a CYA of 70 ppm
regardless of FC level. Even measuring the same water, two different sensors
differed by 100 mV or more in 23% of the pools that had the additional sensors.
See below where I show a graph of FC vs. ORP and the HOCl vs. ORP (this comes
from this post which describes more details):"
Anyway, one reason it's confusing is that the 'experts' themselves don't
agree; so, it will be hard for me to fully understand the implications.
On Saturday, June 28, 2014 1:52:38 AM UTC-4, DannyD. wrote:
Sounds like he's spreading FUD or worse. He's dragging into it that
there is variation between sensors, between sensors in different pools,
etc. There is no indication that different sensors were used to come
up with that graph. At least the author of that article has a graph
and he cites where the data for the graph came from.
Look, you can continue to analyze this all you want. Use 4 floaters
hundreds of gallons of chlorine and keep the level at 10 PPM if you
like. I'm just going to do what uses a fraction of the chlorine and
is consistent with all the health dept and pool standards bodies that
I know about. I have one floater in a 48,000 gallon pool, keep the
chlorine at 1 to 3 PPM and it works fine for me. Works fine for
all the other folks I know too. And it must work across the country,
because otherwise we'd be hearing about all the folks that got sick
from a pool, or all the pools turning green, etc.
You have an incredible knack for over complicating things and turning
something simple that works into a lot of trouble.
Yes, it's covered when they speak about oxidation (versus sanitation).
Anyway, I think the conclusions are something like the following:
(for a typical homeowner manually maintained outdoor unshaded pool)
1. Nobody really understands this FC/CYA stuff ...
2. Even those who think they do understand it, don't seem to agree ...
3. But, all the health departments specify static numbers ...
4. Even though they all admit there *is* an inverse relationship ...
5. And some of the pool schools suggest a working ratio ...
So, there are, essentially, two schools of thought.
The two schools, are, essentially:
a. Keep both the FC and CYA at low (static) numbers, or,
b. Maintain a ratio (of roughly about 10%) FC to CYA levels.
The problem is that the first is impossible, without draining
(because CYA only goes up); and that the second isn't as much
in agreement as is the first.
For example, Trader subscribes to the static numbers, while I
lean toward maintaining the ratio (although I'm on the fence
as data sways me either way).
But, there is a lot of agreement in between.
For example, everyone agrees that you need *some* cyanuric acid
(most seem to agree on a 20ppm to 30ppm range).
And, everyone agrees that this cyanuric acid interferes with
both oxidation and sanitization of the free chlorine.
As a result, everyone agrees that you can't use the dichlor powder
or trichlor pucks forever; so, you're (eventually) stuck using liquid
chlorine (or dilute bleach) eventually (unless you dilute the pool
Which is why I'm going to pull my four floaters out this week!
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