What is the actual effect of iso-cyanuric acid (CYA) stabilizer on chlorine in an uncovered outdoor home pool

[DannyD.]

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):
http://www.troublefreepool.com/content/128-chlorine-cya-chart-slam-shock

Page 9 of this article discusses how CYA increases kill time:
http://standards.nsf.org/apps/group_public/download.php/5891/Chlorine-CYA.pdf
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.
http://www.epa.gov/oppad001/sciencepolicy.htm
http://www.epa.gov/oppad001/dis_tss_docs/dis-12.htm

While the "chart" is intended to be a *pragmatic* tool for the outdoor
pool homeowner, the science is what we're after:
http://www.troublefreepool.com/threads/2177-Chlorine-CYA-Chart

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":
http://www.troublefreepool.com/content/165-getting-started

[Frank]

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:

https://www.azwestern.edu/downloads/NSF%20Grants/pool%20chem-UV-cyanuric%20acid.pdf

[DannyD.]

DannyD. wrote, on Thu, 26 Jun 2014 18:37:21 +0000:

> While the "chart" is intended to be a *pragmatic* tool for the outdoor
> pool homeowner, the science is what we're after:
> http://www.troublefreepool.com/threads/2177-Chlorine-CYA-Chart

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:
http://www.lincoln.ne.gov/city/health/environ/pollu/pdf/Cyanuric.pdf

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.

[DannyD.]

Frank wrote, on Thu, 26 Jun 2014 14:48:59 -0400:

> 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:

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):
http://www.anotherperfectpoolnews.com/wp-content/CPODocs/CYA-Cl-Ratio.pdf

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.

[DannyD.]

DannyD. wrote, on Thu, 26 Jun 2014 18:49:38 +0000:

> 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?

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:
http://www.lowrycg.com/wp-content/uploads/2013/05/Cyanuric-Acid-It-Controls-Your-Pool-rev03.pdf
which says the minimum free chlorine should be a ratio of at least
7.5% of the cyanuric acid stabilizer level.

Government:
Minimum FC === 2ppm, Maximum CYA === 100ppm

or

Homeowners:
Minimum FC >= 7.5% of CYA

That's the fundamental question.

[Frank]

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.]

Frank wrote, on Thu, 26 Jun 2014 15:31:05 -0400:

> 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.

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
http://time.com/45424/no-its-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?
http://arstechnica.com/science/2014/03/ask-ars-how-much-pee-in-a-pool-would-kill-you/

What Happens When You Pee in the Pool?
http://articles.mercola.com/sites/articles/archive/2014/04/26/urine-chlorinated-pool.aspx

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
doesn't.

I follow the 7.5% FC-to-CYA recommendations here for my pool:
http://www.anotherperfectpoolnews.com/wp-content/CPODocs/CYA-Cl-Ratio.pdf
http://www.lowrycg.com/wp-content/uploads/2013/05/Cyanuric-Acid-It-Controls-Your-Pool-rev03.pdf
etc.

But, I'm willing to listen to alternate arguments.