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Swimming Pools 2018

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It is time to update some of the information on swimming pools, over the years I have tried with other pool technicians around the world to get to the bottom of some pressing pool questions.  Some of those questions get researched and new data published.  I am to pass this on to owners so they can better understand and look after their pools.

   1.  Chlorine is the best anti algae, it works faster and better than most other things.

  2.  Packaged tablets of chlorine (choc, multi action and Lent) are nearly always stabilised chlorine so the more you add the higher the stabiliser level goes.

  3.   Stabiliser AKA Cyanuric acid is more in control of your pool water than the pH is, people check the pH levels but don’t check the stabiliser level. Most green and problematic pools are down to too higher level of stabiliser and you cant accurately check stabiliser with dip strips.

4.    Stabiliser level once it gets too high needs to be brought down by emptying some water and replacing with fresh.

5.    Hydrogen peroxide is a waste of money as it is a stronger oxidiser than chlorine (but a worse sanitiser) it burns out the chlorine in your pool so you cant get a chlorine reading until it’s gone. It cost twice as much as chlorine so why not buy twice as much chlorine and look after your pool properly?

6.    You can look after your pool very easily and cheaply just using Eau de javel (for a source of liquid chlorine) and acid (pH-) from a Brico store. If you need to increase the alkalinity (for a tiled or plaster finished pool) bicarbonate of soda from the super market, Most vinyl liner pools the alkalinity doesn’t matter unless you have a specific problem like pH drifting so ask for specific information.

7.    choc’ng a pool is NOT about buying a product called choc, it’s about raising the free chlorine to a level and holding that level until everything is oxidised out of the water (algae, chloramines etc) and you may need to hold this high level of chlorine for a few days. A one off dose of chlor choc pastilles is not going to achieve very much except raise the stabiliser level some more.

 8.  Anti algae unless it’s copper based doesn’t actually work if you have a green pool already, it’s an industry con! More of your valuable chlorine is used up burning the anti algae out of the pool so making the situation worse!

That is a refresher from what I posted a few years back.

**Going back to point 3**, I said that; "Cyanuric acid is more in control of your pool water than the pH is, people check the pH levels but don’t check the stabiliser level".  (October 2016)  This has now been proven to be correct.

Eye irritation is felt more when the pH is at 7 or below than when it goes higher up to pH9.  Cyanuric acid (stabiliser) is used in outdoor pools, its reduces the rate at which the sunshine burns off the chlorine.  The chemical binds to the chlorine and acts like a sun screen.  What we now have is a compound that is far less dependent on the pH so there is very little difference in the sanitising ability of the chlorine from pH7 to pH9.

What we do know is the reduction in the sanitiser strength due to the bonding of cyanuric acid to the chlorine needs to be countered by an increase in the free chlorine amount. The old idea of free chlorine at 1-1.5ppm is not enough in most cases when you have 30-50ppm of stabiliser in the pool water.  Certainly if you are having trouble with your pool increasing the chlorine level is necessary.  You can help yourself by lowering available nutrient levels in the pool, (no food for algae, no algae) so using a phosphate remover can really help but increasing your chlorine level to 7.5% of the cyanuric acid level.

(CYA @ 30ppm x 7.5% = 2.25ppm chlorine)

(CYA 50ppm x 7.5% = 3.75ppm chlorine)

Whilst these levels are higher, the actual disinfection level is less than an indoor pool at 1ppm of chlorine and no cyanuric acid stabiliser.   The fact that within pH 7 to pH9 there is barely any difference in chlorine's ability to sanitise means testing and tightly controlling the pH is far less important than testing for cyanuric acid stabiliser.  Some areas  have naturally hard water and high alkalinity, spending more time and money getting the level down to the previous recommended pool levels (pH 7.2-7.6), can now stop.  If the higher pH is easier to maintain and doesn't cause undue eye irritation then all is good but you **must** maintain the chlorine level relative to the cyanuric acid stabiliser level.

**Going back to point 6;**  For vinyl or fibreglass pools alkalinity is not as important as it is in tiled/plaster pools.  The previous recommended alkalinity levels vary from 80ppm to 200ppm, however the higher the alkalinity figure the easier the pH will climb due to the loss of C02 from the water.  The figure for alkalinity (bicarbonate of soda dissolved in water is a pH around 8.1-8.3)  this is too high for pool use (pH 7.2-7.6)  stay with me bearing in mind the previous CYA topic above and pH, I know.  A customer in the Dordogne has really soft water with low hardness (vinyl pool) with alkalinity at 29ppm I said we must add bicarbonate of soda! (alkalinity +) but slept on it over night, decided nothing bad had happened in the last two seasons, so we left it and monitored.  I carried out experiments on my pool (alkalinity 240ppm) and his (29ppm) using and aerating rig.  The pH in my water sample climbed to 8.3 withing an hour of aeration.  48 hours of the low alkalinity sample (29ppm) showed and increase in the pH to 7.6 from 7.2.  I prepared another sample with alkalinity set at 40ppm and repeated the same test with aeration and over 48 hours the pH reached 7.8.  I double checked the original test with the samples at alkalinity at 29ppm and 240ppm and again the results were almost identical to the first.  I spoke to a friend about this and they confirmed an alkalinity level of 20ppm means the water will balance out with a pH at 7.75 meaning no pH- would be required.

With low alkalinity the pH will reduce much quicker (less pH-) so if you have that kind of water be aware otherwise you can get into a situation the pool industry incorrectly labels pH bounce.  Nothing bounces, for a give amount of pH- the pH reduces, if the pool is high on alkalinity then the pH will climb back up quickly, these are separate cases. My pool and others I have setup the same way with low alkalinity use far less pH- in a season than ever before (my pool less than 1 litre of acid) and the pH does not climb anywhere near as quickly, again making pools easier to look after.  Water at 40ppm alkalinity still has around 7 times more C02 than the air above the pool  so the C02 loss is slower than a pool with higher alkalinity that may have 20 times more C02 than the air. In short most pools are over carbonated (alkalinity) old tables on the web are more commonly for plaster/tiled pools which could use a higher alkalinity.

*aeration is the process that normally takes place in a pool causing the pH to rise. The test rig was built to examine this and accelerate the process but the findings are still correct.

Some flocculents will not work well with alkalinity below about 80ppm.  

Anyone wishing to read further into the CYA/pH paper, the link is below.


Thanks also to Richard Falk and Kim Skinner for their help.

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  • 2 months later...
Until half way through I was eager to write absolutley right... its what I have done ever since first educated by thiere many years ago. And my pool has always been impecable due to following those rules...

Then, the other half... you completely lost me... but hey... for now I am worryless.

Glad you are still posting great stuff!!
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No problem Osie, if things are working well that's great. I just posted the confusing part for those that like the reasoning behind the statements.

Long and the short of it, you MUST have a way of accurately testing the cyanuric acid stabiliser (CYA) to keep control, a simple and cheap tester will provide long and happy days enjoying the pool and not enduring the pool. [:D]

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We've got a polyester (?) coque pool which has developed a brown waterline about 8 inches wide. We bought a gel which said it would clean it and that we didn't need to lower the water. Although it's maybe a bit cleaner it hasn't gone.

Can you advise how to clean it please?
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Hi Pommier,  you'll need to do a couple of tests first so we can see what effects these have before going further.

You need to get hold of some ascorbic acid or a vitamin C tablet.  The brown could either be iron staining or organic staining.  putting the vitamin C tablet or ascorbic acid directly on to the stain and hold it the for a minute or two and on the spot where the tablet was a lighter or even clear patch should result, if it does then it's iron staining.

Holding a chlorine tablet directly on the stain for a minute or two would lighten the mark if it were organic staining.

Please come back with the results 

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Good luck with that, (getting someone out who knows anything)  The only other thought on this is a pool I worked on last year, that was fibreglass and the blue colour had simply faded over the years and left a sort of opaque light brown/fawn glassfibre colour which now looks unsightly. 

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