Does the FC level affect how quickly chlorine is consumed?

[marcgr]

Is the rate of chlorine extinction dependent on the FC concentration?

Imagine 2 outdoor pools, with optimal TFP Chemical levels, side by side, identical in every way, except that one runs FC of 3 and the other runs FC of 6. To make it easier, let's say that they're not being used by bathers and don't have significant leaf litter or other contaminants getting into them.

How will they differ in their consumption of chlorine? Is it related to the FC concentration, or related to other mechanisms that are less dependent on the chlorine concentration?

Just curious...

Marc

 

[tim5055]

Chlorine demand, not considering organics is a % of FC. Each pool will be a little different, so I'll just use 25% as an example. If you start with 20 FC day one drops to 15, day two drops to 12.5, day 3 drops 9.2, day four drops to 7 and so on.

Regular accurate testing will reveal your consumption and you will get a feel for what you need to add daily.

 

[ewkearns]

Is the rate of chlorine extinction dependent on the FC concentration?

Imagine 2 outdoor pools, with optimal TFP Chemical levels, side by side, identical in every way, except that one runs FC of 3 and the other runs FC of 6.

How will they differ in their consumption of chlorine?

That is a curious question! Do we lose a percentage of the total chlorine per day or do we use X ppm as a constant. Then there is the effect of CYA, whether it be 40 ppm or 90 ppm. Perhaps the question should be rephrased such that: "For a given ppm of CYA, one could expect a loss of [something].... Intuitively, for a given CYA ppm, I would guess that the 6 ppm would lose more than the 3 ppm pool, but I, now, have a headache....

This is fodder for ChemGeek and the Deep End.......

 

[Divin Dave]

Primarily, there are 2 things which consume chlorine - organincs in the water and UV from the sun.

with 2 pools side by side and peaked up to TFP recommendations, including the correct CYA for each FC level, then theoretically, while omitting organic contaminents and considering exposure to the sun to be equal, then FC would deplete equally over the coarse of a day.

As Tim mentioned, this would be a percentage and not a finite X ppm.

 

[JamesW]

Loss to the sun is primarily based on the hypochlorite ion concentration and the intensity and duration of the UV. The hypochlorite ion concentration is determined by the fc, cya and ph. Assuming that the fc/cya ratio and ph are equal, the hypochlorite ion concentration should be roughly equal and the loss of chlorine in ppm should be roughly the same.

There is possibly some shielding effect at higher cya levels, which might protect chlorine at lower depths, resulting in lower chlorine loss in higher cya pools.

 

[JoyfulNoise]

Marc,

A lot of information about the advanced chemistry of pool water can be found in this thread written by Richard Falk (aka, chem geek) - Pool Water Chemistry

It quite clearly shows the relationship between CYA/FC/pH.

The thing to remember about chlorine is that FC is a catch-all measured value. It consists of chlorine bound to CYA (no disinfecting/oxidizing power), hypochlorous acid (HOCl, powerful disinfectant & oxidizer) and hypochlorite anion (OCl-, lesser disinfectant). Those three forms of chlorine exist in a chemical equilibrium determine by pH and temperature. FC is the sum of all three components and so it, all by itself, tells you nothing. The most important form of chlorine in water for pools is the active chlorine level (hypochlorous acid) and so you need to know CYA and pH to figure that out.


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[JamesW]

Maintaining an fc/cya of about 7.5% is going to give you your best efficiency point. Anything lower risks algae, which will increase fc loss. Anything higher, is usually unnecessary and wastes chlorine.

For pools where mustard algae has been a problem, maintaining an fc/cya of about 15% for about a week after completing a proper Slam can provide extra insurance.

 

[marcgr]

Matt - thanks, that link was exactly what I was looking for. This chart in particular: http://troublefreepool.com/~richardfalk/pool/ChlorineLoss.gif shows that the chlorine loss at a fixed CYA level is proportional to the concentration of FC.

In other words, chlorinating beyond the optimal FC/CYA ratio is just a waste of chlorine - unless you're overchlorinating for some other reason, for example, to keep it from dropping below a safe minimum level before the next chlorination - which is basically what James said.

 

[JoyfulNoise]

Yes. That is because the concentration of hypochlorite anion (OCl-) is proportional to FC. At a pH of 7.5, the ratio of HOCl:OCl- is roughly 1:1 (equal amounts). However, it is the OCl- that is reduced to chloride (Cl-) by UV. When that happens the equilibrium chemical reactions shift and the concentrations of HOCl and OCl- are restored to their equilibrium values by the reserve of chlorine that is bound to the CYA.

This is the (not really) complicated chemical theory that the pool industry does not recognize either through willful ignorance or outright ignorance or both. If the pool industry truly understood these concepts, then the use of stabilized chlorine would be much more carefully managed and the guidelines of 1-3ppm FC absolute would change to incorporate the role of CYA concentration. Apparently the pool industry is unable or unwilling to do simple, 1st year college chemistry to under this.

Many people, including Richard, have tried to get these concepts recognized formally within the industry and regulatory agencies but simply have been ignored or rebuffed in their efforts. So TFP exists to educate those willing to learn that pool water management is really a lot easier than most are led to believe.


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