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Thread: Does the FC level affect how quickly chlorine is consumed?

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    Does the FC level affect how quickly chlorine is consumed?

    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
    Pool: 16,000 gallons, 16x31, 500SqFt gunite pool, White Pebbletec interior, built by Cody Pools. Pentair Intelliflo VS-3050 pump, Pentair Clean & Clear 420 Cartridge Filter, In-line chlor with Ultra UV, PV3 Infloor cleaning, Pentair Easytouch-P4 control. TF-100
    Spa: 525 Gallons, Coast fiberglass spa of unknown vintage and suspect previous care with a 2-speed main pump and a blower motor, both 4HP. 3-step bromine sanitized.

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    Mod Squad tim5055's Avatar
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    Re: Does the FC level affect how quickly chlorine is consumed?

    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.
    TFP Moderator 39 X 18 23,000(ish) freeform gunite; built 2007ish; Pentair Triton II TR100 600lb Sand filter; 2 HP Pentair pump with 2.2 HP AO Smith single speed motor; 2 skimmers, 1 main drain, 4 returns w/waterfall, Stenner 45MHP2 3GPD running@ 60% - 15 gal Tank; heated by the sun CYA 200+ when I started - 50 now. Dolphin Supreme M5 Pool Cleaner. Hot Springs SX Spa, 285 gallon

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    Re: Does the FC level affect how quickly chlorine is consumed?

    Quote Originally Posted by marcgr View Post
    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.......

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    Divin Dave's Avatar
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    Re: Does the FC level affect how quickly chlorine is consumed?

    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.
    Divin Dave,
    IG Vinyl, 15' x 30', 3 1/2' - 6' deep, Oval, ~15K gal, Intelliclor IC40, Intelliflo VS pump, Clean and Clear 420 Filter, auto-fill-disabled, Retrofit LED Color Light, Dolphin Nautilus Robot, TF100 Test Kit, Taylor K1766 Salt Test Kit, Tftestkit Pressure Gauge.
    www.tftestkits.net Experience- it's what's learned just after you needed it most !!

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    Re: Does the FC level affect how quickly chlorine is consumed?

    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.

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    JoyfulNoise's Avatar
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    Re: Does the FC level affect how quickly chlorine is consumed?

    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.


    Matt
    16k IG PebbleTec pool, 650gal spa, spillway and waterfall, 3HP IntelliFlo VS / 1.5HP WhisperFlo, Pentair QuadDE-100 filter, IC40 SWCG, MasterTemp 400k BTU/hr NG heater, KreepyKrauly suction-side cleaner Dolphin S300i robot, EasyTouch controls, city water, K-1001, K-2006 and K-1766 test kits, Mannitol test for borates

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    Re: Does the FC level affect how quickly chlorine is consumed?

    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.

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    Re: Does the FC level affect how quickly chlorine is consumed?

    Matt - thanks, that link was exactly what I was looking for. This chart in particular: http://troublefreepool.com/~richardf...lorineLoss.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.
    Pool: 16,000 gallons, 16x31, 500SqFt gunite pool, White Pebbletec interior, built by Cody Pools. Pentair Intelliflo VS-3050 pump, Pentair Clean & Clear 420 Cartridge Filter, In-line chlor with Ultra UV, PV3 Infloor cleaning, Pentair Easytouch-P4 control. TF-100
    Spa: 525 Gallons, Coast fiberglass spa of unknown vintage and suspect previous care with a 2-speed main pump and a blower motor, both 4HP. 3-step bromine sanitized.

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    JoyfulNoise's Avatar
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    Re: Does the FC level affect how quickly chlorine is consumed?

    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.


    Matt
    16k IG PebbleTec pool, 650gal spa, spillway and waterfall, 3HP IntelliFlo VS / 1.5HP WhisperFlo, Pentair QuadDE-100 filter, IC40 SWCG, MasterTemp 400k BTU/hr NG heater, KreepyKrauly suction-side cleaner Dolphin S300i robot, EasyTouch controls, city water, K-1001, K-2006 and K-1766 test kits, Mannitol test for borates

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