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Is There A Maximum Safe Level Of Fc


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I have converted to liquid chlorine but I know a good number of people who still use chlorine pucks and I have a quick question.

Given that many of their pools will have CYA above 400ppm is it safe to swim in them if the FC is kept at the recommended 30 – 45ppm. I recognise that with CYA at this level the chlorine is effectively only 5ppm but is this still true in terms of its detrimental effect on skin, swimming costumes or if taken orally.

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You are incorrect about the effective or equivalent active level of chlorine. With an FC/CYA ratio of 10%, the active chlorine level is roughly equivalent to only 0.1 ppm FC with no CYA (technically at 77ºF, 40 ppm FC with 400 ppm CYA has the same hypochlorous acid concentration as 0.09 ppm FC with no CYA; at 88ºF it's the same as 0.20 ppm FC with no CYA).

There is a small oxidation effect from chlorine bound to CYA that is no more than 1/150th the rate of that of hypochlorous acid. So perhaps at such higher FC levels the equivalent with no CYA might have as much as 0.5 ppm added to it. This is still lower than commercial/public pools with no CYA in them such as indoor pools. The effects on swimsuits, skin, and hair will be no worse.

Drinking pool water is different since there it is the total capacity of chlorine that would have an effect so the 40 ppm FC would be relevant, but remember that the EPA allows up to 4 ppm FC in drinking water and that assumes drinking around 2 liters or quarts per day for a lifetime and you shouldn't be ingesting very much chlorine.

However, a pool with such a high CYA level is risky in that if algae starts to develop trying to get rid of the algae using chlorine alone will be very difficult since it would take extraordinary FC levels. So it's better to lower the CYA level via water dilution. Usually, those pools won't have 400 ppm CYA because they'll often get algae at their industry-normal FC level of 1-3 ppm unless they are very lucky to have very low algae nutrient (phosphate and nitrate) levels or are using algaecides.

Also, don't forget that CYA contributes to Total Alkalinity (TA) so maintaining water balance to protect plaster pools means having a higher TA level so you have enough carbonate alkalinity.

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Once again many thanks for your reply, but oh dear you have just blown my limited understanding of the CYA/FC relationship and with it the level of FC I need in my own pool! I have always used the poolcalculator.com calculator which tells me that with CYA at 400ppm 40ppm of FC are needed, I had assumed (by the sounds of it wrongly!) that this would give me an equivalent (CYA=0) FC or 1 – 3 ppm. For my own pool the CYA is 50ppm and I try to keep the FC above 5ppm but from what you say this is only giving me equivalent FC of 0.1ppm. Is this correct or am I totally muddled?

However, you have answered one point that has puzzled me for years and this is why with such high CYA levels people do not suffer from permanently green pools, whereas in practice most people who only use their pools themselves, just two people who take a dip once or twice a day, rarely have an algae problem but those who let their houses out, families who are in the pool for eight or more hours a day, get a green pool once a season. I think it is because we are very lucky in that our water is from mountain springs and there is no significant agriculture for a hundred miles or more, when I tested some years ago for phosphates and nitrates I could not get a reading at all. So thanks for clearing that one up for me.

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What you are missing is that the equivalent of 0.1 ppm FC with no CYA is more than you need to both kill pathogens and prevent algae growth. The "1-3 ppm FC" you quote is an irrelevant number by itself without saying what the CYA level is. If you have 1-3 ppm FC with no CYA, then that is a HIGH active chlorine level, though is one found in commercial/public pools that don't use CYA such as most indoor pools. This is why such pools oxidize swimsuits, skin and hair faster (my wife has personal experience with this).

You are wrong that most poole including those with high CYA don't have problems. Most do, but not all because some have low algae nutrient (phosphate, nitrate) levels. Between 1/4th and 1/3rd of all pool owners have problems in their pools in the current season while in the past 3 years it's closer to 70-80%. The problems are most commonly cloudy water or visible algae growth or unusual chlorine demand, all of which can occur from algae growth including that which is nascent and not yet visible (cloudy water has other causes, but early algae growth is the most common). On another pool forum website there are over 400,000 visitors PER MONTH during peak summer months because people have problems with their pools. So don't think that people "rarely have an algae problem". It is not rare.

Of course, you may be describing your local experience and as you point out your water may not have phosphates in it. Just note that some municipal water supplies add orthophosphate to the water to prevent metal corrosion. Also, anyone using HEDP-based metal sequestrants will have increasing phosphate levels.

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Thanks. You are quite right when I wrote "However, you have answered one point that has puzzled me for years and this is why with such high CYA levels people do not suffer from permanently green pools, whereas in practice most people who only use their pools themselves, just two people who take a dip once or twice a day, rarely have an algae problem but those who let their houses out, families who are in the pool for eight or more hours a day, get a green pool once a season. I think it is because we are very lucky in that our water is from mountain springs and there is no significant agriculture for a hundred miles or more, when I tested some years ago for phosphates and nitrates I could not get a reading at all. So thanks for clearing that one up for me." I was referring to local experience.

However, I am sorry but I'm still a bit confused as to what you are saying about CYA/FC. I normally use www.poolcalculator.com which shows that with CYA at 0 the recommended FC is 1 -3 ppm and 10 ppm shock. With CYA=50ppm FC=4-8 and Shock 16ppm. This is also where I originally got figures for CYA=400 FC=30-43ppm. Reading your comments on another forum, your chart http://troublefreepool.com/~richardfalk/pool/HOCl.htmand http://www.troublefreepool.com/threads/2177-Chlorine-CYA-Chart it would appear that an easy guide to FC is to aim for FC=CYA*11.5% with a minium FC=CYA*7.5% and FC=CYA*40% for shock, unless I'm demonstrating that I still dont understand the CYA FC relationship!!!!!!!

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You understand this correctly. A minimum 7.5% FC/CYA ratio will prevent green and black algae growth regardless of algae nutrient (phosphate, nitrate) level. The Pool Calculator (which is now replaced with PoolMath) does say 1-3 ppm FC with 0 ppm CYA, but that is not because that is the level required to prevent algae growth. It is impractical to be able to maintain only 0.06 ppm FC with no CYA even though that is all that would be needed. With no CYA in the water, chlorine breaks down quickly in sunlight. Also, such low levels of chlorine are not easily measured nor maintained.

I now understand why you were confused. While the calculator used an FC/CYA ratio when CYA is present, it used a practical range when CYA was zero. Sorry for that confusion.

As for the 400 ppm CYA, yes the calculator is just computing a ratio, but it's not very practical to have CYA over 100 ppm since if anything goes wrong and you need to shock the pool with chlorine to get rid of any algae growth, it takes extraordinarily high FC levels to do so. Even the maintenance FC level is impractical. Just because the calculator lets you put in a high CYA level doesn't mean it's a proper thing to do.

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