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I came across this forum and found that though it has a lot of good information, there are some things about pool water chemistry that are not correct.  Much of what I describe below is summarized (less technically) in the [url=http://www.troublefreepool.com/category/pool-school/]Pool School[/url] at [url=http://www.troublefreepool.com/]Trouble Free Pool[/url].

Free Chlorine (FC) / Cyanuric Acid (CYA) Relationship

Free Chlorine (FC) does not measure the amount of "active" chlorine that participates in disinfection nor in oxidation of ammonia and organics.  The "active" form of chlorine is called hypochlorous acid.  There are several relatively inactive forms including hypochlorite ion, and a series of 6 chemical species with chlorine bound to cyanuric acid known as chlorinated isocyanurates (if there is CYA in the water).  At a normal pH of pool water near 7.5 and with an FC of 3.5 ppm and a CYA of 30 ppm, only 1.5% of the measured FC is in the form of hypochlorous acid, another 1.5% is in the form of hypochlorite ion, and 97% is chlorine bound to CYA as chlorinated isocyanurates.  These relative amounts as a function of pH are shown in a comparison of the traditional industry graph and the real graph when CYA is present as shown in [url=http://www.troublefreepool.com/post4366.html#p4366]this post[/url].

Free Chlorine (FC) is more accurately described as measuring the total chlorine capacity or reserve or reservoir.  As hypochlorous acid gets used up by disinfection or oxidizing an organic or from breakdown by sunlight, it gets replenished quickly from hypochlorite ion and fairly quickly from the chlorine bound to CYA.  Even in the FC test, the chlorine bound to CYA gets released in a matter of seconds (the half-life of one species for releasing chlorine is 0.25 seconds while for another it's 4 seconds).  However, the rate of killing bacteria and inactivating viruses or protozoan cysts and of killing algae and oxidizing organics or corroding metal, etc. is almost completely related solely to the amount of "active" chlorine, namely hypochlorous acid, and not to the amount in reserve -- that is, not to the FC measurement alone.  It is not true that the chlorine bound to CYA gets measured as CC; it gets measured as FC.  Combined Chlorine (CC) is usually monochloramine which is chlorine bound to ammonia and it can also be chlorine bound to some organics, but not to CYA as the chlorine gets released from CYA during the FC test (one can consider it to be much more "loosely" bound to CYA).

The relationship between chlorine and CYA has been known since at least 1974 when the equilibrium constants for the relevant reactions were definitively determined.  Technical details about this are described in [url=http://www.troublefreepool.com/post50548.html#p50548]this post[/url].  It turns out from the chemistry that, at typical pool pH, the amount of "active" chlorine (hypochlorous acid) is proportional to the ratio of FC to CYA.  So to get the same level of disinfection, oxidation, and prevention of algae growth at higher CYA levels, proportionately higher FC levels are required.  There is no such thing as "chlorine lock".  There is no point where CYA locks onto chlorine and no longer releases it.  Chemical equilibria continue to occur, but at higher CYA levels a higher FC level is required to have the same hypochlorous acid concentration or else algae will grow faster than chlorine kills it.  At that point, it will appear as if there is an unusual chlorine demand even before the algae becomes visible.  If the CYA level continues to climb or if the FC is allowed to drop, then the algae growth will accelerate and become visible, usually making the water look dull, then cloudy, and then green (though with some types of green algae it can go more directly to a dark clumpy green).

It was written on this forum that CYA enhances the sanitizing ability of chlorine, but that is absolutely not true unless one is talking about CYA's ability to protect chlorine from degradation from sunlight (i.e. to prevent the FC from dropping to zero quickly).  CYA significantly reduces chlorine's disinfection ability by orders of magnitude.  30 ppm CYA roughly reduces chlorine's disinfection (and oxidation) rate by a factor of about 30 (it just turns out that way near a pH of 7.5).  In fact, 3 ppm FC with 30 ppm CYA is technically equivalent to a pool with 0.1 ppm FC and no CYA in terms of disinfection rates assuming that the 0.1 ppm FC pool is kept at a constant FC (hard to do at such a low FC).  Fortunately, bacteria are very easy to kill with typical 2-log CT values (chlorine concentration in ppm times time in minutes) of 0.04 to 0.1 meaning 99% of bacteria get killed in about 1 minute or less when the FC level is around 10% of the CYA level (and 99.99% get killed after 2 minutes, etc.).  Unfortunately, algae requires a much higher level of active chlorine to kill it faster than it reproduces.  Indoor pools and other pools with no CYA in them are actually over-chlorinated by orders of magnitude.  My wife experiences this effect every winter season when her swimsuits degrade (elasticity gets shot) and her hair is more frizzy and skin flaky from the indoor pool with 2 ppm FC and no CYA compared to our own outdoor pool during the summer with 3-4 ppm FC and 30 ppm CYA where none of these problems occur even after 5 swim seasons.  There are also implciations regarding the rate of production and absolute amounts of disinfection by-products due to the higher concentration of "active" chlorine in indoor pools, but I won't get into that here.

[url=http://www.troublefreepool.com/category/pool-school/chlorine_cya_chart_shock]This chart[/url] in the Pool School shows rough FC minimums for each CYA level though the rough simple rule is that with manually dosed pools you need to keep the FC at a minimum of 7.5% of the CYA level.  For SWG pools (and possibly for automatically dosed chemical injection systems), the minimum FC is 5% of the CYA level.  Of course, lower FC levels don't mean you will definitely have algae in the pool since other factors such as nutrient (phosophate and nitrate) levels as well as degree of sunlight make a difference.  However, even pools high in algal nutrients can be kept free of algae by chlorine alone (my own pool has 2000-3000 ppb phosphates, for example, and my fill water has 300-500 ppb phosphates which are apparantly used for corrosion control).

One can certainly use stabilized chlorine products and let their CYA level build up without increasing the FC level, but if one wants to prevent algae growth, then a supplemental algaecide (at extra cost) is required with weekly maintenance.  PolyQuat 60 can be used as can a phosphate remover.  One could use copper ions to prevent algae, but it is hard to regulate the amount in pools and the copper can stain (especially plaster pools) or have blond hair get a greenish tint and is especially prone to such problems if the pH rises or at higher TA levels.  One can also use 50 ppm Borates in the pool (more on that later).  The main point is that the use of algaecides is not necessary if one maintains the proper FC level relative to the CYA level.

Stabilized Chlorine Builds Up Cyanuric Acid (CYA); Cal-Hypo Builds Up Calcium Hardness (CH)

Though it is well-known that stabilized chlorine, namely Dichlor powder/granular and Trichlor pucks/tabs (aka Chlor Choc), increase CYA levels, there are the following chemical facts that are not normally publicized and that are independent of concentration:

For every 10 ppm Free Chlorine (FC) added by Trichor, it also increases Cyanuric Acid (CYA) by 6 ppm.

For every 10 ppm FC added by Dichlor, it also increases CYA by 9 ppm.

For every 10 ppm FC added by Calcium Hypochlorite (Cal-Hypo), it also increases Calcium Hardness (CH) by 7 ppm.

Since CYA doesn't get reduced except by dilution from splash-out, backwashing (and a slow oxidation from chlorine as well as breakdown from bacteria if a pool is "let go" over the winter), the levels can build fairly quickly.  If chlorine usage is 2 ppm FC per day, then this increases CYA from Trichlor by 36 ppm CYA per month.  Shocking with Dichlor would only increase this even faster.  Pools with cartridge filters where there is no backwashing typically build up CYA the fastest.

Lowering Total Alkalinity (TA)

Adding a pure acid or a pure base makes the pH and TA fall or rise together so adding acid to lower the TA and then adding a base to adjust for the pH will not work.  The only way to efficiently lower the Total Alkalinity (TA) level is through a combination of acid addition with aeration, preferably at lower pH as this accelerates the outgassing of carbon dioxide.  Pools are intentionally over-carbonated.  The equilibrium concentration of carbon dioxide in water at pool pH (near 7.5) exposed to air would have a TA of only 10 ppm (with no CYA).  Fortunately, the rate of outgassing is fairly low until the TA gets higher or the pH gets lower.  If a net acidic source of chlorine is used, such as Trichlor or Dichlor (more on that later), then a higher TA level of 120 ppm or more makes sense to help balance the tendency for the pH to drop.  However, if net neutral sources of chlorine are used (again, more on this later), such as the hypochlorite sources (bleach, chlorinating liquid, Cal-Hypo, lithium hypochlorite), then having a lower TA no higher than 80 ppm helps to keep the pH more stable.  It is an ironic and confusing fact of pool water chemistry that a higher TA actually causes the pH to rise in spite of providing more pH buffering.  This is because TA is not only a measure of pH buffering, but a measure of the carbonate level in the water (if TA is suitably adjusted for the CYA level -- aka "adjusted TA" or "carbonate alkalinity") and it turns out that the outgassing rate varies as the square of the carbonate alkalinity (the adjusted TA level) while the buffering effect is more linear.

The "slug" or "acid column" methods of lowering TA were formally debunked in [url=http://www.poolhelp.com/JSPSI_V1N2_16-30_AcidColumn.pdf]this technical paper[/url].  The reason that the method described above works is that acid lowers both pH and TA while aeration at low pH (which accelerates outgassing of carbon dioxide) raises the pH with no change in TA.  So the net combination of the two is a lowering of TA.  The procedure simply accelerates a process that would happen over a longer period of time anyway.  It takes 3.2 fluid ounces of full-strength Muriatic Acid (31.45% Hydrochloric Acid) to lower the TA by 10 ppm in 10,000 gallons.

It has been posted that a rather wide TA range of 80-300 ppm is acceptable.  The higher TA will make the pH rise unless a lot of acidic chlorine is being used.  Again, with Trichlor the high TA levels could be OK, but certainly not with hypochlorite sources of chlorine.  Also, the difference in the saturation index between a TA of 80 and a TA of 300 is about 0.6 which can be enough to cause cloudiness or scaling if other water parameters, particularly Calcium Hardness (CH), are not adjusted accordingly.  For plaster pools, it is important to have the saturation index be closer to zero.  This is not necessary for vinyl pools and is partly needed for fiberglass pools (to protect the gelcoat).  If you have very hard water, such as well water, with a high CH, then having a high TA is not a good idea.  The saturation index, as well as chemical dosage amounts, may be calculated by [url=http://www.thepoolcalculator.com/]The Pool Calculator[/url].  There are also technical reasons why for plaster pools a higher CH and lower TA are better, even with both situations adjusted to a zero saturation index.

Muriatic Acid vs. Sulfuric Acid

It has been stated that use of Sulfuric Acid is preferred because Muriatic Acid has fuming.  It is true that full-strength Muriatic Acid fumes, but there are often half-strength (15-16%) versions available that fume less.  Also, both sulfuric acid and dry acid (sodium bisulfate) increase the level of sulfates in the water.  A higher level of sulfates can be damaging to softer natural stones and to cement as magnesium sulfate is particularly destructive from splash-out and evaporation salt crystallization pressure.  Occassional use of sulfuric acid may not be a problem, but frequent use could be.  One cup of 38.5% sulfuric acid in 10,000 gallons increases sulfate levels by 2.9 ppm.

In the U.S., some pools have not only chlorine injectors, but acid injectors for automated pH balance and I haven't heard of fouling issues when using Muriatic Acid.  It seems to be reasonably pure here, but I don't know what it's like in France.

Hypochlorite Sources of Chlorine are Net pH Neutral

One of the myths in the pool/spa industry is that hypochlorite sources of chlorine, such as bleach, chlorinating liquid, Cal-Hypo and lithium hypochlorite, make a pool rise in pH over time.  Though it is true that these products raise the pH of water upon initial addition, what is not taken into account is the fact that the usage (consumption) of chlorine is an acidic process that exactly compensates for the initial rise in pH.  As the chlorine gets used up, the pH drops back down.  This is described technically in [url=http://www.troublefreepool.com/post4367.html#p4367]this post[/url].  Dichlor is touted as being nearly pH neutral, but in fact it's continued use lowers the pH over time.  Of course, pools WILL see an increase in pH over time if their TA level is higher due to the carbon dioxide outgassing effect I described earlier in this post.  Also, there is a small amount of excess lye in bleach and chlorinating liquid, though the amount is small.  For 6% bleach that is at a pH near 11.5, such as Clorox Regular, the amount of lye is negligible (around 0.025%) and does not contribute to any net pH rise.  For 6% bleaches that are at a pH near 12.5, this is around 0.25% "excess lye" and would cause a pH rise of 0.2 per month if the rate of chlorine usage were 2 ppm FC per day.  For 12.5% chlorinating liquid that is typically at a pH near 12.5, this is also around 0.25% "excess lye" but causes a pH rise of only 0.1 per month with 2 ppm FC per day daily usage (it's half since the chlorine is twice as strong as bleach).  These pH rise rates are for pools with a TA near 80 ppm and no Borates.

Saltwater Chlorine Generator (SWG) Pools are pH Neutral

Similar to the above analysis for hypochlorite sources of chlorine, the generation of chlorine in an SWG is identical to adding hypochlorite to the pool (without the excess lye).  When accounting for the consumption of chlorine, it is pH neutral.  Nevertheless, most SWG pools experience a rise in pH, sometimes quite significant.  This is caused by two factors.  One is the increased amount of aeration and disturbance of the water surface from the SWG bubbles (mostly hydrogen gas).  As noted earlier, lowering the TA level helps reduce the rate of carbon dioxide outgassing thus reducing the rate of pH rise (as counterintuitive as that may seem).  Another factor is if any chlorine gas that is generated does not fully dissolve in the water and instead outgasses.  Having returns pointed downward might help, but may not be good for circulation depending on the pool.

SWG pools should generally have higher CYA levels, around 60-80 ppm (70-80 ppm is even better) as this significantly reduces the amount of chlorine loss due to sunlight.  Another industry myth is regarding the diminishing returns of higher CYA levels, but experiments from pool users have shown that the protection of chlorine from sunlight occurs not only from chlorine binding with CYA which then does not break down from sunlight, but also from CYA itself absorbing UV and shielding lower depths of water.  This latter effect is non-linear.  The higher CYA levels mean lower FC consumption so the SWG on-time can be turned down and this reduces the rate of pH rise.  The higher CYA levels require higher FC levels so 60 ppm CYA should have 3 ppm FC and 80 ppm CYA should have 4 ppm FC to prevent algae growth.  The 1-3 ppm FC recommendations from the SWG manufacturers are not sufficient.

I suspect that the reason for the "diminishing returns" of CYA myth is that in commercial pools it is true that anything above around 30 ppm CYA doesn't seem to make any difference, but this is due to the higher bather loads in such pools where much of the chlorine demand is due to oxidation of ammonia/urea from sweat/urine (and suntan lotion, etc.)  In a residential pool, the bather load is typically very light so the loss of chlorine from sunlight dominates.

50 ppm Borates

An effective method for helping to control the rate of pH rise and for

other benefits is to use 50 ppm Borates in the pool and is especially useful in SWG pools.  The borates

(mostly boric acid at the pH of pool water) acts as an additional pH

buffer particularly suited to buffer against a rise in pH and it also

acts as an algaecide that may let one lower the SWG on-time as less

chlorine gets consumed fighting algae.  Borates may be added by a combination of 20 Mule Team Borax (sodium tetraborate decahydrate) and Muriatic Acid added alternately and separately.  It may also be added, at extra cost, in a pH neutral product such as Proteam Supreme Plus (which is mostly boric acid).

Test Kits

There has been some discussion of good test kits.  Though the DPD test has been mentioned, it has some significant limitations.  It bleaches out at higher (> 10 ppm FC, usually) chlorine levels and it can also be hard to read accurately if using a visual comparison to a standard.  A far better (though more expensive) chlorine test is FAS-DPD as it can measure accurately to 0.2 ppm or 0.5 ppm (depending on sample size) for Free Chlorine (FC) as well as Combined Chlorine (CC) separately and also can measure up to 50 ppm without bleaching out (or if a flash of pink is seen, more DPD powder can be added).  It is a titration (count the drops) method so even someone who is color blind can use this test.  You can see a demo of the test [url=http://www.taylortechnologies.com/ChemistryTopicsCM.ASP?ContentID=11]here[/url].

A good test kit will not only contain a FAS-DPD chlorine test, but will also test for pH, Total Alkalinity (TA), Calcium Hardness (CH) and Cyanuric Acid (CYA).  These tests use drops of reagents and should not be test strips which are generally not as accurate.  A good test kit is the Taylor K-2006 which (in the U.S.) can be obtained at a good online price [url=https://asp5.secure-shopping.com/spspools/details.asp?prodid=1259&cat=1200&path=]here[/url] or the TF100 test kit from tftestkits.com [url=http://www.tftestkits.com/]here[/url] with the latter kit having 36% more volume of reagents so is comparably priced "per test".  Unfortunately, I do not know which tests are available in France.

Chemical Equivalents at the Grocery Store

Sometimes pool stores charge much more for the identical chemical product as found in grocery stores.  The following are chemical equivalents using brands I know are sold in the U.S.  Unfortunately, I do not know the brands sold in France, but I list the ingredients which might be helpful.

pH Up -- sodium carbonate; same as Arm & Hammer Super Washing Soda (careful: NOT the laundry detergent)

Alkalinity Up -- sodium bicarbonate aka sodium hydrogen carbonate; same as Arm & Hammer Baking Soda

Calcium Hardness Increaser -- calcium chloride (usually anhydrous); same as DowFlake and Dow Peladow

Chlorinating Liquid -- sodium hypochlorite; 6% unscented standard bleach (lower in strength, but otherwise the same), but do not use specialized versions with extra additives

For raising the pH, one can also use Borax instead (20 Mule Team Borax) with the advantage of the TA only rising half as much as with pH Up since there are no carbonates added with Borax.  Borax is sodium tetraborate decahydrate though if you find a product with sodium tetraborate pentahydrate that is also fine to use (it just has less bound water content).

Total Dissolved Solids (TDS)

Another myth is that high TDS causes problems with pool water chemistry or chlorine effectiveness, etc.  It isn't TDS itself that is the issue, but rather what specific components of TDS are out of balance.  Historically, CYA would build up in pools and since this took time, the TDS would rise as well mostly due to the choride resulting from continued chlorine usage.  So TDS was a "proxy" for old water that often had higher CYA and therefore had problems since the FC was not increased proportionately.  An example of why TDS by itself doesn't matter, consider SWG pools that have a high TDS of around 3200 ppm with 3000 ppm salt (chloride measured as ppm sodium chloride).  It is true that eventually the water may become somewhat saturated with certain organic compounds that do not get fully oxidized and are dissolved so do not get filtered out and these can potentially make the water dull looking, but this is rather rare unless the water is very old or the bather load is much higher.

Combined Chlorine and Shocking

It is not necessary to shock a pool weekly if one maintains a proper FC level relative to the CYA level.   Ammonia and organics in the pool are continually oxidized by chlorine so if the chlorine level is maintained, then there is no need to shock.  In a properly maintained residential pool, the Combined Chlorine (CC) level is always near zero -- sometimes measures at 0.2 and rarely measures at 0.4 ppm unless there is high bather load such as right after a swim party.  You do want to shock the pool at higher FC levels if you know the bather load was very high, or if someone had an accident (urinary or fecal), or if a dead animal was in the pool, or if you are fighting algae, or if you measure significant (>= 0.5 ppm) Combined Chlorine (CC).

The rule that it takes 10x the CC level to shock a pool with CC is technically incorrect.  The 10x rule comes from getting rid of ammonia with chlorine, but combined chlorine is not ammonia but rather monochloramine that already has one chlorine attached to it.  Monochloramine is measured as ppm Cl2 as are all chlorine measurements while ammonia is measured as ppm Nitrogen.  Whereas the stochiometric quantity of chlorine needed to oxidize ammonia is an FC (ppm Cl2) that is 7.5x the ammonia level (ppm N), the amount for CC is an FC that is 0.5x the CC level (ppm Cl2).  Nevertheless, due to the presence of CYA slowing down the breakpoint of ammonia and oxidation of monochloramine, the 10x rule is reasonable -- or the shock level of chlorine may be used instead.  This makes the reaction go faster even though it's more chlorine than needed to reach reaction completion.

For those that do not believe that a pool with significant algae can be cleared using chlorine (bleach or chlorinating liquid) alone, see [url=http://richardfalk.home.comcast.net/~richardfalk/algae/algae.html]this thread[/url] for a pool opened during the spring with significant algae.

Richard

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Richard - much of your post goes over my head in terms of chemical descriptions.  However, I am sure you are v knowledgeable and it was interesting.

We only have algae at the start of the year when we open tje pool up and never any problem getting rid of it.  Once the pool is clear and working well we use a basic tester kit and add chlorine tablets and PH moins whenever indicated (often).  It has never caused us a problem and the gite guests often say how 'silky' and clear the water is.

However, this year we have had an unusual problem  We have a toddler pool, and the big pool water overflows from a weir into it and then gets re-circulated into the main pool  In the Spring we emptied the toddler pool and I took off most of the tiles which were falling off in places, and resurfaced the sides of the pool with the quick drying cement mixture which indicates on the bag that it is for pools and wet places.  I then painted the whole toddler pool with pool paint and it looked really great - immaculate in fact.  However within about 4 to 6 weeks a green stain appeared around the water line of the toddler pool and this is now about 4 inches deep and looks v ugly.  It does mostly come off with vigorous brushing but quickly re-appears.  I am certain it is not algae.  There are no similar marks on the main pool - which is still tiled - yet both pools have the same water.  I would be grateful for any ideas as I will be re-painting the toddler pool again next year - out of necessity.  Thanks

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A smaller pool (especially as small as a toddler pool) can have its water chemistry vary rather quickly since the volume of water is so much smaller.  The pH may have risen, for example.  If you had copper in the water, high pH could precipitate it out and could look green.  The chlorine could get used up faster as well leading to algae, though you say you're pretty sure it's not algae.  That's about all I know; perhaps others can help you out more.  You mention there is circulation through the toddler pool, but if it's not fast enough then the water chemistry may be different in that toddler pool -- something you could easily check.  Being very shallow, the chlorine level may be lower as there is less Cyanuric Acid "depth" to protect it.

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Well I certainly don't really see the benefit of posting this article.

There is certainly some science there, but there are also many ‘claims’, which have no substantiation whatsoever - serves only to confuse the issue I think, as their validity is questionable.

Pool maintenance and habits in the US do not bear much resemblance to the like in Europe or Exemplar countries such as Australia as the base conditions particularly the nature of the water are very different. The principles of chemistry may be the same but the application is no.

We do not recommend for example that pool be run at chlorine (FC) levels up to 4ppm -that is a bit excessive. It has been clearly established and to date not disputed that at a Level of 1.5ppm (FC), the bacterium E.Coli has a life span of no more than 0.75 seconds (so long as CYN 30ppm, TA 80ppm pH 7.3) So why would any one raise the bar - to do what exactly? It is inconceivable to me that anyone managing these parameters manually could maintain such standards throughout a season? I suspect it’s a bit of 'Yankee Overkill' to make sure but entirely unnecessary in my opinion. In addition, the difficulty in managing chloramines is tied up with combined ammonia so the approach here is to get rid of ammonia -using zeolite filtration (better) rather than ZAP it with more chlorine (logic).

Environmentally, it’s smarter to reduce levels of Chlorine consumption to 'just enough' levels and employ mechanism such as Auto dosing to keep it there. Overdoing it is a backward step.

For the other posters question about the toddler pool- I would be interested to know the composition of the deposit on the walls.

One factor that is often a problem, is that Mums put extra sunscreen on toddlers and are reluctant to wash it off before they have a dip and it could be this that is impregnating into the pool walls, certainly it is a devil of a stuff to get out of liners as it soaks right through in experiments I have conducted.

Anyway, test is the only way to tell for sure.

If you want any help then let me know.

Andrew

 

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[quote]We do not recommend for example that

pool be run at chlorine (FC) levels up to 4ppm -that is a bit

excessive. It has been clearly established and to date not disputed

that at a Level of 1.5ppm (FC), the bacterium E.Coli has a life span of

no more than 0.75 seconds (so long as CYN 30ppm, TA 80ppm pH 7.3) So

why would any one raise the bar - to do what exactly? It is

inconceivable to me that anyone managing these parameters manually

could maintain such standards throughout a season? I suspect it’s a bit

of 'Yankee Overkill' to make sure but entirely unnecessary in my

opinion. In addition, the difficulty in managing chloramines is tied up

with combined ammonia so the approach here is to get rid of ammonia

-using zeolite filtration (better) rather than ZAP it with more

chlorine (logic).

Environmentally, it’s smarter to

reduce levels of Chlorine consumption to 'just enough' levels and

employ mechanism such as Auto dosing to keep it there. Overdoing it is

a backward step.[/quote]

It takes a far higher level of chlorine to kill algae than it does to kill bacteria.  The higher FC level is to prevent algae growth without the need for supplemental algaecides, but is relative to CYA level and I was not recommending 4 ppm FC independent of CYA level (my pool isn't running at the minimum FC/CYA ratio).  You also completely missed the point that the higher FC level when the CYA level is higher is not any stronger.  If you've got any indoor pools or other pools in Europe that don't have any CYA in them yet have 1 ppm FC, then they are over-chlorinating the pools by at least a factor of 10.  FC does not measure the amount of active chlorine (hypochlorous acid).  Obviously, it is not possible to chlorinate such non-CYA pools with only 0.1 ppm FC so why don't you use CYA to moderate chlorine strength/effectiveness?  If you are so concerned about minimum chlorine levels, then why expose people to 10+ times as much chlorine as necessary in such pools?  Virtually all of the respiratory and ocular problems are associated with indoor pools with no CYA and the difference is not just the lack of sunlight and poor air circulation, but also the creation of disinfection by-products 10+ times faster and at 10+ times higher end-point concentrations, at least for trichloramine when oxidizing ammonia (urea is still an open question).  The Jafvert & Valentine breakpoint chlorination model predicts this (as did earlier Wei & Morris and Selleck & Saunier models).

E.Coli is one of the easiest bacteria to kill and has a 99% CT value of 0.04 at pH 7.5 so hypochlorous acid concentration of 0.02 while an FC of 1.5 ppm with 30 ppm CYA and pH 7.3 is a hypochlorous acid concentration of 0.022 so that's a kill time of just under 1 minute, not 0.75 seconds.  If the pool had no CYA in it, then the hypochlorous acid concentration would be about 0.87 and the kill time would be 1.4 seconds which sounds like what you are quoting.  The difference with and without CYA is roughly a factor of 30.  The effects of CYA on chlorine kill times is well known.  [url=http://www.pubmedcentral.nih.gov/picrender.fcgi?artid=546955&blobtype=pdf]This link[/url] shows the kill time of Streptococcus faecalis being less than 0.25 minutes with 0.5 ppm FC and no CYA at a pH of 7.4 while with 25, 50 and 100 ppm CYA the time increases to 4, 6, and 12 minutes.  From no CYA to 25 ppm CYA that's at least a factor of 16 slowdown in kill time.  Figure 4 in that paper shows the typical HUGE difference in kill times when CYA is present.  [url=http://www.pubmedcentral.nih.gov/picrender.fcgi?artid=1775396&blobtype=pdf]This link[/url] shows the kill time in Table 3 of E.coli for 99.999% inactivation in 30 seconds needing < 0.11 ppm FC with no CYA at pH 7.2 but requiring at least 0.8 ppm FC with 50 ppm CYA.  This translates to roughly a 10 second kill time for your 1.5 ppm FC with 30 ppm CYA at pH 7.3.  The fact that it is the hypochlorous acid concentration that is the killing agent is most apparent in the careful study shown [url=http://www.pubmedcentral.nih.gov/picrender.fcgi?artid=239534&blobtype=pdf]here[/url] which states "The results strongly suggest that HOCl is the predominant cysticide with no measurable cysticidal effect of the chlorinated cyanurate species."

Nevertheless, an FC of 1.5 ppm with 30 ppm CYA would still have 99% kill times on the order of a minute or two for most bacteria so should be fine to prevent most person-to-person transmission which is the main concern in commercial/public pools.  It's certainly far more than needed to prevent uncontrolled bacteria growth since bacteria double in population (i.e. their generation time) in around 15-60 minutes.  Also, 1.5 ppm FC with 30 ppm CYA is a 5% ratio which is about the limit to prevent algae growth.  I only said 7.5% for manually dosed pools (and 5% for SWG pools) which at a CYA of 30 ppm would be 2.2 ppm FC so we're not so far off.  Again, you are looking at 4 ppm FC in absolute terms when by itself this means nothing -- if there were 80 ppm CYA in the water then the active chlorine (hypochlorous acid) concentration would be the same as 1.5 ppm FC with 30 ppm CYA.  I never said you need to maintain a 4 ppm FC.  I said you have to maintain an FC/CYA RATIO.

Chloramines form almost immediately when chlorine combines with ammonia.  It takes very little more chlorine to oxidize it to nitrogen gas and it doesn't take that long, but if you have a lower FC with high CYA then it could take hours.  Other organics such as urea take longer so specialized filtration makes some sense there, but this isn't a problem in residential pools anyway -- it's higher bather load pools that would be more the issue.  So specialized filtration takes care of combined chlorines, but how is algae growth prevented?  With 1.5 pm FC and 30 ppm CYA that should be OK in most pools to prevent algae, but those with higher algal nutrient (phosphate, nitrate) levels might need a bit more -- closer to 2.2 ppm FC.

So I'm not in disagreement with you regarding 1.5 ppm FC with 30 ppm CYA, but if you continue to use ChlorChoc or other stabilized chlorine products, then the CYA level will often climb (especially if the pool is not small or if it has a cartridge filter) and then the 1.5 ppm FC will not be sufficient to prevent algae growth in some pools without use of a supplemental algaecide (or using a higher FC level to keep the FC/CYA ratio constant).  The number one reason for pools getting algae even when maintaining an industry-recommended FC level is that the continued use of stabilized chlorine increases the CYA level making that fixed FC level less effective and allowing algae to grow faster than hypochlorous acid can kill it.  Your automated chlorine dosing system ([url=http://www.poolguy.fr/products/product-sanitization.htm]here[/url]) is a great thing as it maintains an FC level without increasing CYA levels, but most people do not have such a system -- many use inline chlorinators or floating feeders using ChlorChoc Trichlor stabilized chlorine so that's where the relevance of the FC/CYA ratio comes in -- you have choices: 1) dilute the water to keep the CYA level lower, but then may need to add back CH in plaster pools, 2) increase the FC level to keep the FC/CYA ratio constant, but if this is done with the inline chlorinator than this just adds CYA to the water faster as well, 3) keep the original FC level at higher CYA levels but use a supplemental algaecide to prevent algae growth or 4) switch to using unstabilized chlorine to stop increasing CYA levels.

Can you be specific with the "claims without substantiation"?  When I was referring to inaccuracies in posts on this forum, those were "claims without substantiation" such as that low levels of CYA enhances chlorine's effectiveness by making the molecules heavier or that there is such a thing as "chlorine lock" neither of which are true or are at best misleading (see your post in [url=http://www.completefrance.com/cs/forums/1256643/ShowPost.aspx]this thread[/url]).  You recommend a TA range of 80-300 ppm in several posts and do not recommend to people to lower their TA level but never ask them if their pools are experiencing a rise in pH nor whether they have plaster pools or their other water parameters to determine the saturation index (or whether they have gas heaters since that is where scale can develop first if the saturation index is too high).  I linked to the chemistry and sources behind the chlorine/CYA relationship and its chemical equilibrium and you see additional sources above of the roughly linear FC/CYA ratio relationship to chlorine kill times.

Richard

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I hope not; I've never been a very good shot. [8-)]

I agree with using the minimal amount of chemicals possible, but if one wants to truly use the absolute minimum chlorine level, then one could have 1-2 ppm FC with 100 ppm CYA for roughly 3 times lower "active" chlorine levels.  99% kill times rise to 3-6 minutes, but for residential pools where person-to-person transmission risk is low anyway, this is a choice one can make.  To prevent algae, one could use a weekly maintenance dose of PolyQuat 60 algaecide or could use a phosphate remover -- either one is around $2-3 per week for a typical residential pool.  Or one could use 50 ppm Borates, but I don't know if that will be enough by itself to prevent algae at those levels of FC/CYA.  The main downside with this approach is that IF you get algae by letting the FC get lower or not using enough algaecide, then it's hard to fight the algae bloom at such a high CYA level.

Like I said in the original post, there's a lot of good information on this forum and SOME things about pool water chemistry that are not correct.

I'm still trying to figure out what isn't substantiated and has questionable validity.  Though one can perform the calculations based on the equilibrium chemistry known since 1974, one can also use my spreadsheet [url=http://richardfalk.home.comcast.net/~richardfalk/pool/PoolEquations.xls]here[/url], but this is not for novice users.  [url=http://www.hc-sc.gc.ca/ewh-semt/pubs/water-eau/escherichia_coli/treatment-traitement-eng.php]This link[/url] gives a table of CT values which is where the standard 0.04 for E.coli comes from, but that is at 5C so at higher temperatures the CT value would be lower which would be more consistent with the 0.75 seconds that PoolGuy quoted.  [url=http://aem.asm.org/cgi/reprint/66/4/1702.pdf]This link[/url] has a 99.9% CT value for E.coli at a pH of 7.0 and 23C of 0.09 as shown in Table 2 which would be equivalent to around 0.03 at a pH of 7.5 and 99% kill rate.  However, these are CT values with NO CYA in the water and the point was that CYA significantly reduces chlorine's effectiveness and that this is predicted by the equilibrium calculation of the hypochlorous acid level.  There is no enhancement of chlorine effectiveness at low CYA levels and there is no chlorine lock at high levels.  It's all just equilibrium chemistry with predictable results.

Richard

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I want to emphasize again that MOST of what is posted on this forum, most especially from Poolguy, is excellent information.  I am only questioning a relatively small subset.

For reference, the Poolguy post in the thread [url=http://www.completefrance.com/cs/forums/1192638/ShowPost.aspx]here[/url] states the following referring to Cyanuric Acid: "... it not only

protects the chlorine from UV but also enhances the disinfection qualities of

chlorine."  It was that latter part that I was disputing -- CYA does not enhance the disinfection qualities of chlorine; quite the opposite as I described in the earlier post on the effect of CYA on chlorine kill times.

Regarding the "active" chlorine vs. the chlorine combined with Cyanuric Acid, Poolguy says in the post in the thread [url=http://www.completefrance.com/cs/forums/703049/ShowPost.aspx]here[/url] that "
The reagent liquid does have a short life and surprisingly its

measurement is of no interest because it measures Total chlorine,

including the chlorine 'locked' by Stabliser (cynuric acid)." and regarding the type of testing kit in the post in the thread [url=http://www.completefrance.com/cs/forums/577433/ShowPost.aspx]here[/url] that "... don't bother with the reagent kits go for the tablets DPD1. they are

much more accurate and give you the information you what to know -

active chlorine."  Both of these statements imply that the chlorine bound to CYA is measured as Combined Chlorine or only as Total Chlorine (and not Free Chlorine) which is the sum of Free Chlorine and Combined Chlorine.  This is not true.  The 1973 paper (presented at a conference in 1974) that definitively determined the chlorinated isocyanurate equilibrium constants defined "free chlorine" as the sum of hypochlorous acid and hypochlorite ion while "reservoir chlorine" was that plus the chlorinated isocyanurates so that may be where some confusion came from as that "free chlorine" is not the same as that measured by the Free Chlorine tests.  Some quotes from that document are [url=http://www.troublefreepool.com/post52499.html#p52499]here[/url].  The EPA document I link to in that post ([url=http://www.epa.gov/hpv/pubs/summaries/tricltrz/c14659rr.pdf]here[/url]) says the following (bold emphasis mine):

Half Life: See table above. The half life of the hydrolysis reaction is equal to 0.693/kfor. Hydrolysis of the monochlorinated

compounds HClCY– (t1/2 = 4.08 sec) and ClCY–2 (t1/2 = 0.25 sec) occurs in seconds. The hydrolysis rates for trichloro-s-triazinetrione

or the sodium dichloro-s-triazinetriones have not been determined but would be even faster than for the monochloro-s-triazinetriones

since the second and third chlorines are held less strongly than the first chlorine. Therefore, all of the chlorinated isocyanurates

hydrolyze in less than a few seconds.

The hydrolysis reactions they are referring to are the following (unfortunately, this forum does not seem to support superscripting or subscripting):

HClCY– + H2O ↔ H2CY– + HOCl

ClCY–2 + H2O ↔ HCY–2 + HOCl

The first compound in each equation is one of the 6 forms of chlorine attached to CYA (the "CY" represents a cyanuric acid core).  The first compound on the right is one of the 4 forms of cyanuric acid (and its dissociated ionic forms).  The HOCl is hypochlorous acid which is the "active" chlorine.  There is an equilibrium between the left-hand side and the right-hand-side though in normal pool water the equilibrium is way over to the left which is why kill times are so adversely affected.  During the DPD or FAS-DPD Free Chlorine (FC) test, the hypochlorous acid gets used up oxidizing the dye (as described in [url=http://www.hach.com/fmmimghach?/CODE%3AL70191473%7C1]this document[/url] from Hach) so the equilibrium in the above equations gets shifted to the right via LeChatlier's Principle.  This happens quickly in a matter of seconds using up all of the chlorinated isocyanurate compounds so that the Free Chlorine (FC) test is measuring the chlorine capacity/reserve/reservoir, NOT the instantaneous "active" chlorine or hypochlorous acid concentration.

The Combined Chlorine (CC) test measures those compounds that do not release chlorine in the time of the FC test (so not the chlorinated isocyanurates since they already released their chlorine during the FC part of the test) and instead require the addition of iodide to release the chlorine to form iodine that then reacts with the dye in the test.  The most common CC is monochloramine which is chlorine combined with ammonia; this chlorine does not get released during the time of the FC test though if you read the document from Hach I linked to above, the section on "Monochloramine Interference in the Free Chlorine Test" alludes to the relative slowness of chlorine being released from monochloramine to interfere with the test: "The reaction of DPD with free chlorine is rapid. If the color is measured within one minute, the monochloramine breakthrough will be minimal."  The free chlorine being referred to is hypochlorous acid and/or hypochlorite ion (though it's denoted as Cl2 in their diagram) as they are the strong oxidizers since "The DPD amine is oxidized by chlorine to two oxidation products."

You might say that the reserve chlorine should get released from being bound to CYA and would then replenish any active chlorine that gets used up fighting bacteria and algae and that would be true.  This is why the FC is an important measurement to know whether you have enough chlorine to not "run out" or get used up by chlorine demand.  However, it does not tell you anything about the RATE of chemical reactions involving the active form of chlorine, hypochlorous acid, since that is dependent on the instantaneous concentration, not the amount in reserve.  It is analogous to having an army with active duty soldiers on the front line with guns that are killing the enemy while the soldiers in reserve do not help in the kill rate, but do replenish those active soldiers that get killed.

Another way to see this is via Oxidation-Reduction Potential (ORP).  The following graphs show 620 samples from 194 pools and spas with the

data collected by Jeff Luedeman in Bloomington and Richfield, Minnesota.  [url=http://richardfalk.home.comcast.net/~richardfalk/pool/FC-ORP.gif]This first graph[/url] shows Free Chlorine (FC) vs. ORP and is not highly correlated while [url=http://richardfalk.home.comcast.net/~richardfalk/pool/HOCl-ORP.gif]this second graph[/url] shows calculated hypochlorous acid (HOCl) concentration (from the FC, CYA and pH parameters) and is much more highly corrleated (though there is still a lot of noise as is typical in portable ORP sensors and in dynamic pool environnemts).  In the second graph, the unstabilized pools (i.e. no CYA) are the diamonds on the upper-right, the stabilized pools (>= 30 ppm CYA) are the diamonds on the lower-left, and the red-bounded squares in the middle are stabilized pools with < 30 ppm CYA so that 15 ppm CYA was assumed for those data points in the graphs.  There are also sensors with specific membranes (including amperometric sensors) that measure hypochlorous acid and they are consistent with the equilibrium chemistry demonstrating that FC measured by chlorine tests means nothing by itself without also knowing the CYA level (in terms of the "active" chlorine level) and that even a small amount of CYA significantly reduces the "active" chlorine level.

Richard

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Chem Geek ( my good fellow)

 

I must say that I (we) are most flattered that you have taken time out (and a considerable amount of that it seems to be) to visit us in our little forum over the ‘pond’. Surely as there must be only around 500 + visitors to this Forum, it should pale when compared to the mighty and extensive audience you are used too.

 

It does show that power of the Internet and that even communities such as ours can be international and benefit from all that that brings, such as your learned self to these shores. I am certainly no match for your zeal, no do I have time at my disposal to research, rebut or even type to volumes you have so generously offered - I have to work for a living.  However, I shall convey your remarks to my colleagues, who will not only have more time but the Academic prowess to engage your propositions and research, and I will certainly post their responses for your consideration.

 

I am sure that you took some offence to my remark that the propositions were of ‘questionable validity’, and for that I an sorry. Its that you see this audience is composed primarily of ordinary folk, many of whom have retired from business life and have dreamed of having a pool for some part or most of their life. England, Scotland and Ireland being what they are it’s not favourable nor practical to have one – they have no clue what your talking about, or why you are saying it. I on the other hand, grew up in Australia, where a pool is a necessity, certainly as important as a television for infantile development, so these matters are more familiar to me. Now all that is very chatty and ‘steady as we go’, but its this point to wit I pose my question to you.

 

What you have written to date may very well conform to accepted science, on that I do not wish to debate as yet (until I have consulted further), it may also be true that I have take some scientific ‘liberties’, personally I do not think that they are so grave. But I would hazard a guess that all but a very few of this audience, would have no clue what you have taken a considerable amount of your time to write about. Hence, my challenge you should you accept it, is:

 To write in no more than 2 sentences and no more than 4 lines in a style which is intelligible, digestible and scientifically correct to our audience.(say for example a retired couple in their early 60’s with no chemistry education what so ever and who have never owed a pool before)

 

What is the nature, relationship and effect of Cyanuric acid v chlorine, and Total alkalinity v pH in a swimming pool/spa, and why should owners care about this.

 

NOW if you can do that, then I will be very impressed, and I will say so. Because it’s this task I have been struggling with, and to wit you have raise objection to my projections.

 

Over to your learned self…..

 

Andrew

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You are right that I did take offense at your remark on questionable validity (and claims with no substantiation).  Thank you for apologizing.  I am sorry that my post was too technical for this forum.  At the start of my first post, I linked to the [url=http://www.troublefreepool.com/category/pool-school/]Pool School[/url] which is not technical but is based on both the science and experiences known to date.

As for what is important from what I wrote, it is the following (there is more than one point, but each will be kept short):

1) Stabilized chlorine products increase the rate of Cyanuric Acid (CYA) in pools and spas far faster than many believe.  Continued use of Chlor-Chock (Trichlor) and/or shocking with Dichlor increase CYA quickly.  EXAMPLE: In my own 16,000 gallon pool with a cartridge filter and an opaque electric safety cover, I used Trichlor pucks in a floating feeder (6 years ago) at a rate of less than 1 ppm FC per day (about 1 3" 8-ounce puck every 5 days), started out with 30 ppm CYA and ended up 1-1/2 seasons later with 150 ppm CYA.  An unusual chlorine demand started and the water looked dull and the pool stores were of no help except to offer additional chemicals to buy.

2) To prevent algae growth, higher levels of CYA need to be accompanied by proportionately higher Free Chlorine (FC) levels OR use of a supplemental algaecide.  Another alternative is to dilute the water to reduce CYA levels, but this uses a lot of water.  For manually dosed pools, the rule is a minimum FC that is 7.5% of the CYA level; for SWG pools (and possibly other automatically dosed pools) it's a minimum FC that is 5% of the CYA level.

3) The Free Chlorine (FC) test measures the chlorine capacity/reserve/reservoir, not the "active" chlorine level that determines the degree of disinfection or the rate of killing algae, etc.  The FC/CYA ratio is roughly proportional to the "active" chlorine level.  Small levels of CYA do not enhance chlorine's effectiveness (but do protect chlorine from rapid breakdown from sunlight) and high levels of CYA do not produce "chlorine lock".  For outdoor pools, a CYA range of 30-80 ppm is reasonable though the higher end of the CYA range is mostly for automated dosing pools (for France, 30-50 is probably sufficient due to the higher latitude so less direct sunlight).

4) There is nothing wrong with high FC levels by themselves if accompanied by high CYA levels (assuming you are not drinking quarts of pool water).  A pool with 10 ppm FC and 80 ppm CYA is not unsafe.  Even a pool shocked with 20 ppm FC and 50 ppm CYA is safer (less reactive) than a pool with 1 ppm FC and no CYA such as indoor pools.

5) A less expensive approach to pool water maintenance is to use unstabilized chlorine (unscented bleach or chlorinating liquid) but this requires frequent chlorine addition so is less convenient if done with manual dosing.  An automated feed system (such as the one you sell) makes this much easier.

6) Higher Total Alkalinity (TA) increases the rate of pH rise in an uncovered pool unless acid is frequently added.  Higher TA, 120 ppm or higher, is appropriate for acidic sources of chlorine such as Chlor Choc (Trichlor).  With bleach, chlorinating liquid, Cal-Hypo or lithium hypochlorite, a lower TA of around 80 ppm will result in significantly lower rates of rising pH.  A higher pH target also results in a lower rate of rising pH (i.e. it's easier to keep the pH at 7.7 than at 7.3 in pools with higher TA that tend to rise in pH).

7) To lower TA quickly, one does not just add acid.  One should first add acid to lower the pH to the next to lowest reading on their pH test (7.0 or 7.2), then aerate the water with fountains, turning returns up, aeration jets, splashing while adding more acid if the pH rises to keep the pH low.  When the TA level is reached, stop adding acid but keep aerating to raise the pH to the desired number.

8) In pools filled with hard water or pools with plaster/gunite/grout surfaces where the Calcium Hardness (CH) has been raised to near 300 ppm, a higher TA can lead to cloudiness or scaling, especially in a gas heater (when used).  For plaster/gunite/grout pools, the saturation index should be used to properly balance pH, TA, CH, CYA, Temp parameters in order to prevent dissolving/pitting of plaster or scaling.  [url=http://www.thepoolcalculator.com/]The Pool Calculator[/url] is a useful tool for this calculation and for calculating chemical dosages.  Vinyl pools do not need a balanced saturation index, but foaming is reduced if the CH level is at least 100-150 ppm.

9) The use of 50 ppm Borates can stabilize pH and act as a mild algaecide (and gives a sparkle to the water).  This can be especially helpful in saltwater chlorine generator (SWG) pools where the tendency for the pH to rise is often very strong.

10) Pool water testing with a good test kit is absolutely the most important thing you can do and the most important investment you can make.  It should accurately test Free Chlorine (FC), Combined Chlorine (CC), pH, Total Alkalinity (TA), Calcium Hardness (CH) and Cyanuric Acid (CYA).

11) Some standard grocery or hardware store products are identical to what is sold in pool stores with the pool store versions at higher cost.

12) Total Dissolved Solids (TDS) is not a problem and is mostly salt.  Look at the other water parameters for issues first.

13) If you properly maintain FC levels relative to CYA, you do not normally need to shock your pool except for unusual circumstances.

One of the main reasons for the misinformation regarding the relationship of chlorine and CYA is the mantra that the stabilized chlorine industry has touted for decades that "CYA doesn't matter; only FC matters" which is untrue.

If some of your colleagues are chemists or know chemistry well, then I can send you or them a copy of the following book (you can E-mail me at RichardFalk AT comcast DOT net to give me an address):

J. O'Brien, J. Morris and J. Butler, “Equilibria in Aqueous Solutions of Chlorinated Isocyanurate”, Chapter 14 in A. Rubin, ed., Chemistry of Water Supply, Treatment and Distribution, 1973 Symposium, (published 1974), Ann Arbor Science, Ann Arbor, MI, pp. 333-358.

Richard

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Richard

Your generosity abounds, and you continue to add more useful discussion to this topic.

I was rather wondering though is possible to say all of what you said in 1-13 .........in just 2 sentences and 4 lines.

Its that task which occupies me.... can you help.

Andrew

 

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 I have to say this [6]  Have had a pool for 20 years in France and never had a problem as long as we keep the ph level correct. A few years ago we took Poolguys advice re using liquid chlorine (javel 9.2) to start the pool off each year and have saved a fortune. About 25% of water is replaced each year. Our pool sparkles all the time and water 'feels' right. I have 'A' level Chemistry & Physics and OH has a degree in Chemistry but we have forgotten most of it and certainly have not needed it to keep the pool sparkling.
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[quote]I have to say this

 Have had a pool for 20 years in France and never had a problem as long

as we keep the ph level correct. A few years ago we took Poolguys

advice re using liquid chlorine (javel 9.2) to start the pool off each

year and have saved a fortune. About 25% of water is replaced each

year. Our pool sparkles all the time and water 'feels' right. I have

'A' level Chemistry & Physics and OH has a degree in Chemistry but

we have forgotten most of it and certainly have not needed it to keep

the pool sparkling.[/quote]

That's point #5 (use unstabilized chlorine such as chlorinating liquid or unscented bleach for lower costs) and point #2 (dilute the water to keep CYA levels lower if using stabilized chlorine such as Trichlor pucks).  I hope you have a vinyl pool, otherwise your not checking the other water parameters, such as Calcium Hardness, can result in plaster dissolving/pitting (point #8).

In my own pool, I only use 12.5% chlorinating liquid, added twice a week (I have an opaque electric safety cover), have very stable pH adding a couple of cups of acid about once a month or two.  No other chemicals.  $15 per month in chlorine and acid cost for a 16,000 gallon pool.  Crystal-clear water.  In the spring, I check CH and CYA and adjust accordingly; I sometimes dilute the pool water using winter rains.

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There is something in the agricultural 'air' in our area which means whatever we do the pool is black at the start of the season. We empty about 25% and shock with chlorine, no other choice. Can't see any difference between javel and expensive chlora choc. It clears within a few days we then balance the ph and from then on just test once a week with standard testing kit. A couple of chlorine tablets in skimmers once a week is the usual dosage, no other chemicals. We do have the filtration on 24hrs from mid June to end of August. As the water 'feels' so good I don't think there is a problem with calcium hardness. Have been in other pools and 'felt' the difference. If you pardon the pun I 'feel' the magic factors are the ph and filtration.
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You probably can't tell your pool's Calcium Hardness (CH) level from it's feel -- at least I've never known anyone who could.  I'm not sure what it is you feel different in other pools -- high salt levels many people can feel, but that's a much larger difference than CH (usually 1500 ppm or more salt is usually noticeable as feeling "silky").  Is your pool a vinyl pool such as an above-ground pool or some in-ground pools?  Or is it fiberglass or is it plaster (many in-ground pools)?

As for dilution, what kind of filter do you have (sand, DE, cartridge) and do you backwash weekly?  Do you know the size of your pool in liters or gallons?

I agree with you that pH is important as is filtration, though in pools with good circulation and a decent filter, one turnover of water per day is usually sufficient.  How long it takes for one turnover of water depends on your flow rate (gallons-per-minute, GPM; probably liters-per-minute or LPM in France) which depends on your pump size, speed, pipe size, etc.  Pumps running at high speed can get pretty expensive to operate.  In my own pool, I used to have a 1 HorsePower (HP) pump plus a 3/4-HP booster pump for a pool sweep and was costing me around $1200 per year in electricity (electricity is very expensive where I live at around 32 cents per kilowatt-hour).  I replaced both pumps with a single IntelliFlo variable flow pump and cut my yearly operating costs in half to $600 per year.  I have a solar system with 12 panels on the roof and that needs around 48 GPM flow rate and the pump is at 1500 Watts, but when the solar is off the flow rate is set to drop to 26 GPM and the pump is at 275 Watts.  So if you have a less hefty solar system or no solar system at all, then you can run your pump at slow speed for a longer period of time and save a lot of money.  Even a 2-speed pump can be helpful in this regard.

You can roughly tell how long you need to run your pump by lowering its runtime and seeing the water clarity.  It will probably remain clear down to some point and then start to get dull; just move back up to a longer time above that point where a clarity change is noticeable.  24 hours is not normally needed unless the flow rate is rather low.

Richard

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Chem geek  for  first 10 years we had an in ground tiled pool, for last ten years we have had an in ground vinyl pool. Yes we definitely know the size of the pool and litrage. The filter is sand and we backwash as necessary [:D]

We can definitely 'feel' whether the water is right and funnily enough so can our 'French ' friends, it is something they comment on. Must be a French thing.

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I posted the same time you did.  Thanks for letting me know it's a vinyl pool.  You don't need to worry about the CH level.  Only if your pH, TA, CH combo got high would scaling be an issue, but that's very unlikely unless your water is very hard (such as well water).  When you had a tile pool, then the exposed grout between the tile should have had properly balanced water exposed to it so you may have just been lucky to have water sufficiently high in TA and/or CH or there might have been a slow breakdown of the grout not yet noticeable.

Between your 25% water replacement upon opening and your backwashing weekly, you are significantly diluting the water and that prevents the CYA level from building up from the use of pucks.  The reason I asked for the pool size was so I could calculate it.  Two 3" 8-ounce (227 grams) Trichlor pucks in 15,000 gallons (about 57,000 liters) would raise the FC by 7.3 ppm and the CYA by 4.4 ppm.  So over 6 months, the CYA would increase by 114 ppm if there were no dilution.  Your weekly backwashing and 25% annual water replacement are diluting the CYA to keep it from rising that quickly.

Every pool situation is different which is why there isn't a 2 sentence, 4 line statement that can be made.  If you had a cartridge filter and didn't do annual water replacement, then you would have been much more likely to get algae after a year or two.

I am curious what "right" water feels like and probably will never know since I'm not French [:(], but I know my wife can definitely tell that our pool is in far better shape than the community center pool she has to use during the winter, but there's lots of different factors there.  We're going to be in southern France on a barge trip in about a month as well as in Paris.  We love France.

Richard

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[quote user="Poolguy"]

 

 To write in no more than 2 sentences and no more than 4 lines in a style which is intelligible, digestible and scientifically correct to our audience.(say for example a retired couple in their early 60’s with no chemistry education what so ever and who have never owed a pool before)

 

What is the nature, relationship and effect of Cyanuric acid v chlorine, and Total alkalinity v pH in a swimming pool/spa, and why should owners care about this.

 

NOW if you can do that, then I will be very impressed, and I will say so. Because it’s this task I have been struggling with, and to wit you have raise objection to my projections.

 

Over to your learned self…..

 

Andrew

[/quote]

I for one would be interested in this! We have had our pool for just over 3 years and I fall into this catogory of being over 6 and retired. I struggle with academics but can fix it if I can understand how it works, or can hit it with a big enough hammer [:-))].

So far I have understood little of your info Richard. I have the auto chlor/PH system fitted and most of the time we enjoy good water conditions, but this can be effected by winds and rain straight off the Sahara with all sorts of  'pony poo' in it. It would be nice to understand what is happening. My bible for our pool is by one of your countrymen  Terry Taimminen and he makes most of the comprehension easy-ish. Perhapse you can help???

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Terry's book is very good, though it's weaker on pool water chemical balancing and better on other parts of pool construction and maintenance.  There are subtleties in saltwater chlorine generator (SWG) pools that he doesn't get into, nor does he discuss the FC/CYA relationship.

I can't explain why junk gets into your pool other than what you've already stated, but I can tell you that it adds to what your chlorine in the pool has to deal with.  Obviously, the easiest solution is to get a pool cover since that will keep things out of the pool, but unless you get an electric cover (at more cost), it will take more work to take a manual cover on and off.  If the stuff in your pool tends to clog your filter, you can use a skimmer sock to catch some of it (this is especially useful for pollen).

If you know that a storm is coming, you can raise the SWG on-time to raise the chlorine level in the pool ahead of time and that can help stay ahead of what gets dumped in, though it will still take physical removal of most of it.  After a storm, if the FC level is low, you can more quickly clear the pool by shocking it with chlorine, like Javel or other chlorinating liquid or unscented bleach.  It's much easier to shock a pool this way -- the "superchlorinate" function on an SWG takes longer and just wears out the SWG cell faster so adding liquid chlorine is usually better for shocking.

Richard

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