Advice on chlorine production please

[baypond]

Hi

Saltwater pool 10m x 5m, average depth is around 1.5m

I have a Poolsquad (95 I think) salt water chlorinator by pool-technolgie. I bought it new last year. I heat the pool with a pompe a chaleur.

This week the pool water hit 28c. We had 4 adults taking an occasional dip, and 4 children in the pool much of the day. The poolsquad was on boost, PH7 and 4.3kg m3, and yet we failed to get more than a very very faint registration of chlorine when testing. I know the unit is working because we made the mistake of leaving the pool for 3 weeks on 80% production with no heating (so water was circa 21/22c) and pool cover, and for the first couple of days we had to turn production off to bring the chlorine levels down to acceptbale levels. The pool is crystal clear.

My question is 1. is it fair to expect the chlorinator to get chlorine levels back up overnight with pool cover on ,and pool temp at 27/28c and 2. What kind of shock treatment is best for this setup/useage for the next time we are at the house, and what quantities. 3. Do you have any idea what I should leave the Poolsquad production at while we are away to avoid the excessive chlorine buildup whie we are away.

many thanks

[Poolguy]

Hi Baypond

If you check the calculator on their website it surmises that your system is good for 4 swims per day at 20°c as its produces 4grams/hour of chlorine (surprisingly low - I would have thought that it made more than that). Never the less, the situations you have described indicate twice the bather load and 8°C higher temps. The one thing you didn't mention was the cyanuric acid (CYN) level, that in it would have a significant effect on the time that the chlorine stayed in the water.

As you can see its important with this and any of these machines to keep an eye on the performance compared to the requirements of the pool, and if there is a shortfall which is quite often the case you have to manually top up the chlorine level. Be sure to raise your pH a little as a flat 7pH is a little to bland for good performance, 7.2 - 7.6 is a good range so a couple of points higher than that is good.

If you do not what to add chlorine then yes continue to run the pump until the electrolyser has had time to catch up, but be assure that (in my opinion) your pool is not safe for swimming in the mean time, until the chlor level reaches over 1ppm to a required 1.5ppm. When you are not there and not swimming the dropping the remnant chlor level to 1ppm would be acceptable, the only way to discover that would be to try various levels and to test, however this is an in exact science as you don't say if your pool is covered at all. Start-up again I would suggest a 24hour choc does above 5ppm would be appropriate, but check your CYN levels as they might be out of range.

Andrew

[baypond]

Hi Andrew,

Thanks, this is very helpful. The pool is covered when not in use and at night, but I think we have been sold a system that doesn't easily meet the intermittent demands we make on the pool. To compensate for that, I think it will be necessary to add chlorine during hot weather and when people loads are high. I will check the CYN levels next time I am there, and also move the PH range to 7.2/7.6 Is their chart misleading? They say that chlorine production efficiency drops from 80% at PH7 to 20% at PH8 ?

Lastly, I guess the addition of chlorine has no material effect on any part of the pool system other than support the chlorinator to keep chlorine levels up?

Thanks  Giles

[chem geek]

Giles,

The graph you refer to [url=http://www.pool-technologie.com/en/pool-treatment/ph-regulation-2.html]here[/url] is in fact misleading.  It is the same as a middle portion of the red curve in the first graph in my post [url=http://www.troublefreepool.com/post50548.html#p50548]here[/url] which is the traditional industry graph showing the active chlorine (hypochlorous acid) concentration as a function of pH.

This may be one of the reasons they want the pH lower at 7.0, but there are several problems with that approach.  First, is that it is harsher on the eyes since typical tears have a ph of 7.5.  Second, it is somewhat harsher on equipment since lower pH is more corrosive.  Third, lower pH will tend to outgas carbon dioxide faster causing the pH to rise more quickly requiring more acid to keep the pH low (see [url=http://richardfalk.home.comcast.net/~richardfalk/pool/CO2.htm]this chart[/url]).  Fourth, their graph does not apply to chlorine's effectiveness when there is Cyanuric Acid (CYA) in the water.  In my post, I show the correct graph to the right of the traditional industry graph.  I also show a pair of log graphs that show the "flatness" of the red curve without CYA (on the left) and when CYA is present (on the right).  Essentially CYA is a buffer for the active chlorine (hypochlorous acid) so changes in pH don't affect the effectiveness nearly as much as without CYA.

Andrew's point about the CYA level is spot-on.  If your pool is exposed to direct sunlight, then the CYA level can have a very dramatic effect on the rate at which chlorine is broken down by the UV in sunlight and therefore the rate at which chlorine needs to be produced by PoolSquad.  You will make your PoolSquad plates last longer and you will reduce the rate of pH rise in your pool (and use less acid) if you are able to lower the on-time of your PoolSquad.  You can do that by having a higher CYA level in the pool.

Now here's where Andrew and I might disagree.  Some pool owner experiences plus some experiments that were made show that having a much higher CYA level of around 70-80 ppm can significantly lower the amount of chlorine loss even if one proportionately raises the Free Chlorine (FC) target to 4 ppm in order to prevent algae growth (without the need for a supplemental algaecide or phosphate remover) in a saltwater chlorine generator (SWG) pool.  However, if you usually keep a pool cover on the pool, then a CYA level in the 30-50 ppm range should be adequate with a minimum FC level of around 5% of the CYA level.

If you wanted to have some extra algae prevention, you could use 50 ppm Borates in the pool as this is not only an additional pH buffer, but is also a mild algaecide (algistat).  This extra pH buffering will also reduce calcium scaling in the PoolSquad cell (I hope that their product alternates polarity periodically to help keep the plates free of scale).

Richard

[baypond]

Thanks, again very useful. What is the best way of measuring CYA? Where would the CYA come from when using saltwater chlorination? I don't think the pool was shocked when opened up this spring. The poolman just dumped a load of salt in the pool (100kg) and let the filter run on full for 3 days. He used some anti-algae solution but that was it.

[Théière]

Hi baypond, a simple poolcheck dip strip should be good enough to check the cya level. The CYA would come from a pool shop most likely in a tub [;-)]

be careful when adding it as if you over dose you will have to remove and replace some pool water to dilute it.

[chem geek]

Test strips are OK for some measurements, but for others they aren't very good and from what I've seen from posts of those who have used them, CYA doesn't get measured very well in the test strips.  The melamine turbidity test is better, though even that is somewhat subjective (though probably within +/- 10 ppm).  It's in Taylor kits as shown [url=http://www.taylortechnologies.com/ChemistryTopicsCM.ASP?ContentID=44]here[/url].  You can get a great test kit including just a CYA test from tftestkits.net [url=http://www.tftestkits.net/index.php?act=viewProd&productId=24]here[/url] or the full TF100 [url=http://www.tftestkits.net/index.php?act=viewProd&productId=2]here[/url] but with shipping it won't be cheap.  I am not familiar with what you've got available in France.