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Shock - How Often - Swimming Pool Help

Shock - How Often

Chlorinating, maintaining the right chlorine levels, chlorine problems. Dichlor, trichlor, cal hypo, bleach, granules, chlorine pucks or sticks.

Shock - How Often

Postby duane575 » Wed 08 Apr, 2009 04:49

I have heard from a number of pool supply stores that I should shock my pool every week to eliminate the problem of combined chlorine. I have gone more than 3 weeks without shocking my pool and I still have no combined chlorine reading. Is it necessary to shock if I don't have combined chlorine?
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Shock - How Often

Postby Pool User » Wed 08 Apr, 2009 06:09

I still have no combined chlorine reading. Is it necessary to shock if I don't have combined chlorine?

You only shock to get rid of algae, accumulated organic wastes or combined chlorine. You don't need to do a periodic shock. :thumbup:
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Shock - How Often

Postby Me... » Wed 08 Apr, 2009 08:23

Shock = 10ppm and is for oxidation of organics etc.

Super Chlorination = 10 x the Combined Chlorine reading and is to get rid of Chloramines.

You can measure combined chlorine but you can't measure organics. A weekly shock may be needed but a periodic one for the heck of it should be done or you stand the chance of a buildup. Once it starts your chlorine consumption will go thru the roof and your chemistry possibly with it. A shock also stands the chance of removing chloramines but only if the organic load is minimal enough.
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Shock - How Often

Postby chem geek » Wed 08 Apr, 2009 11:48

If you properly maintain the Free Chlorine (FC) level in your pool sufficiently high relative to the Cyanuric Acid (CYA) level, then you do not generally need to shock your pool. I didn't have to shock my pool even once last year except I did so upon startup just for the heck of it (I keep the pool chlorinated over the winter as it doesn't freeze).

The primary component of sweat and urine is urea followed by ammonia. Urea does take longer to oxidize, but will still do so over days and as it builds up it oxidizes faster as it gets more concentrated. You get up to a steady state after a short time. In a residential pool with a low bather load, the organic buildup is low so the regular chlorine addition is able to keep up with it. In an outdoor commercial/public pool with CYA and a much higher bather load, then shocking can help combat the higher bather load if it looks like the daily FC consumption is getting too high.

If one introduces harder-to-oxidize organics into the pool and notices a buildup, then shocking can speed up the oxidation of such organics. It's partly a personal choice, however. My wife's suntan lotion would create a visible film on the water after her daily swimming. I used to shock to get rid of it, but then the FC level would be high and take a while to drop (I also tried using non-chlorine shock as an alternative). Though the higher FC did clear the organics faster, we ended up not shocking and seeing what would happen. The film did not build up and if there was no swimming for 2-3 days it would completely go away. So it was something we just live with since it's getting reintroduced every day anyway. The film breaks up when swimming and dissipates easily -- it's only when first opening the cover that it's at all noticeable in some places. I've suggested her using a more evenly spread thinner layer of suntan lotion, but that didn't get very far. :shock:

Most people on The Pool Forum and Trouble Free Pool do not shock their pools and they don't have any problems with organic buildup or increasing chlorine demand. You are right that if you notice the chlorine demand (rate of FC usage) climbing, then a shock can help knock that down, but usually this is because the FC/CYA ratio is too low so algae growth is starting (it's not visible at first) and the shocking kills off that algae. If one maintains sufficient FC relative to CYA, then the algae growth doesn't get started in the first place.

The 10x rule is technically incorrect, because it is for chlorine relative to ammonia, not Combined Chlorine (CC) measured in the chlorine tests, but I won't get into that here.

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Shock - How Often

Postby czechmate » Wed 20 May, 2009 12:31

Richard,
thanks for clearing up the chlorine shock myth.
I was about to ask the very same question. I have a new, 3 weeks old Diamond Brite plaster.
The pool is 22000 gal ,treated with a 50lb of boric acid.
TA = 100
PH = 7.5
CYA = 45
CH = 260
Phosphates= 100ppb
TDS = 600
Since the pool is sparkling clean and I brush twice daily (retired guy), when my FC dropped to 2ppm I decided not to waist 2 pounds of high grade Cal-hypo and added just one pound instead.
The FC is now about 5-6 which along with low phosphates should protect the pool.
At least that was my theory. Your preceeding article pretty much supports my decision.
Did I miss anything?
P.S.
I had a pool since 1977 and nobody ever mentioned it to me, that shocking is an option.
If this really works, it is priceless discovery! Not only the cost , but the lost swimtime as well.
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Shock - How Often

Postby chem geek » Wed 20 May, 2009 13:18

You didn't miss anything. You can keep algae from growing simply by maintaining an FC that is at least 7.5% of the CYA level for manually dosed pools. I have 2000-3000 ppb phosphates in my pool by simply maintaining an appropriate FC/CYA level. You can learn much more by reading the Pool School.
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Shock - How Often

Postby Me... » Wed 20 May, 2009 13:30

Shocking and Super Chlorination are not options, they are requirements. But, only if needed. The problem is many people, in fact most are so busy skimping on chlorine and proper chemical maintenance that they create the problems that require the Shocking. Also many pools are built with inadequate filtration systems and/or have higher bather loads than the pool should be expected to handle. Maybe they are built outside in an area where the pool is exposed to tons of pollen, needles/leaves and other organics blown in by the wind.

If you have a nicely built indoor pool that only your and/or your wife do some laps in a few times a week you are going to have to do next to nothing. Many people reading these forums will have smaller aboveground pools with a skimmer/return combo and a very small filter system. Combine that with the fact that these pools will probably be the neighbourhood pool and you have a recipe for LOTS of shock requirements.

I don't believe leading people to generally believe that Shocking is an option is a very good idea.
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Shock - How Often

Postby chem geek » Wed 20 May, 2009 21:14

People using Dichlor-then-bleach in their spas (too keep CYA in check) and that use sufficient chlorine to always measure FC before their next soak also have not needed to shock and that has been rather high bather load. You are right that most people don't use enough chlorine -- especially in spas. Roughly speaking, it takes around 3-1/2 teaspoons of Dichlor or 5 fluid ounces of 6% unscented bleach or 7 teaspoons of MPS to oxidize the bather waste from one person-hour of soaking at high temps (100F-104F).

Yes, you need to use a sufficiently high FC/CYA ratio to prevent algae growth and a sufficiently high FC to accommodate the bather load. It is true that shocking will cover up the sins of poor pool maintenance (to some degree), but I'd rather teach people what to do properly. It's similar to using an algaecide. You have to use one if you don't watch your CYA level, but if you manage your pool properly you don't need to use it at all.

As for above-ground pools, including Intex pools with small filters, they still need to maintain the chlorine levels properly and I totally agree with you that anyone not doing that is going to run into trouble and that shocking helps to "make up" for too low chlorine during the week. There have been many Intex pool users at The PoolForum and Trouble Free Pool who have managed their water without weekly shocking and without getting algae or other issues.

There are, of course, very legitimate times when one should shock including fecal accidents, dead animals, vomit, or lots of other organics including pollen that get into a pool or any other time one notices the overnight chlorine loss > 1 ppm or a significant CC > 0.5 ppm. The best way to deal with pollen and other organics that tend to consume a lot of chlorine is to get a skimmer sock as they will collect a lot that you can then regularly remove rather than get caught in the filter and consume more chlorine (until the filter is cleaned).

So we're not disagreeing technically. You're just wanting to give safer advice assuming people won't be responsible and I'd rather give advice for those who can be responsible and caveat it saying that if you aren't, then there are fallback activities such as weekly shocking that can be done.

Richard
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Shock - How Often

Postby czechmate » Wed 20 May, 2009 22:19

Mea culpa, mea culpa.

Richard,

I completely understood your theory, where proper maintenance of sufficient FC would make a shocking an option.
I also understand, where the other guy becomes concerned about giving people wrong ideas.
It was my wrong choice of word that brought it up.

It is little like promoting a standard shift for a small car.
If you willing to learn a stick shift, you will use less gas, drive with more spunk and have a lot more fun.
On the other hand, stick shift is not, for obvious reasons for everyone.

As for me, I am glad I have read your article, because I have the time and ability to drive this stick shift!
Thanks,

Ivan
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Shock - How Often

Postby chem geek » Thu 21 May, 2009 13:58

Ivan,

No need to apologize; it's not your fault at all. Me... and I disagree on some things, but usually it's more a practical matter since he's got experience in the industry whereas I do not and I have more knowledge of the chemistry and see how things are working with users at pool forums. Some of the time, there's not an absolute "right" answer as to an approach so the best thing is to get all the information out there so that people can choose what to do. So caveat emptor, and do what makes sense for you.

Richard
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