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Thread: FC Levels - Pool Calculator vs Chlorine / CYA chart

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    Firefly2000's Avatar
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    FC Levels - Pool Calculator vs Chlorine / CYA chart

    I just noticed that the pool calculator and chlorine/CYA charts give different target and shock FC levels for various CYA levels. Which once should I go with?
    Daniel

    47000L (12500 gal)
    IG Quartzon Interior
    Rolachem Acid & Chlorine dosing
    Paramount In-floor cleaner
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    Solar heating under the great Aussie sun!

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    JasonLion's Avatar
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    Re: FC Levels - Pool Calculator vs Chlorine / CYA chart

    Either one, they both work.

    There are some subtile differences in algae kill rates, numerical rounding, and safety margins between the various sources. We haven't gotten around to working through all the details and developing a consensus of which set of levels is more "ideal" yet.
    19K gal, vinyl, 1/2 HP WhisperFlo pump, 200 sqft cartridge filter, AutoPilot Digital SWG, Dolphin Dynamic cleaning robot
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    Re: FC Levels - Pool Calculator vs Chlorine / CYA chart

    Quote Originally Posted by Firefly2000
    I just noticed that the pool calculator and chlorine/CYA charts give different target and shock FC levels for various CYA levels. Which once should I go with?
    Hey Firefly -
    I have found CYA of 50 works well for me. I've had a lot of rain and now the summertime evap rates are going up (as much as 3/4" a day), so I've had to keep a close watch on CYA levels since the addition of water to the pool will lower the CYA.
    20x40 30,000 gal IG Liner, Pentair II Sand

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    Re: FC Levels - Pool Calculator vs Chlorine / CYA chart

    CYA doesn't evaporate, does it? If you lose water to evaporation, then add it back, shouldn't your CYA ppm be the same?

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    frustratedpoolmom's Avatar
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    Re: FC Levels - Pool Calculator vs Chlorine / CYA chart

    Correct, CYA doesn't change from evaporation.
    Helpful links: Pool School; CYA/Chlorine Chart
    24' round AG pool, 52" high, Raypak heater; Waterway 2 spd Pump;
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    Re: FC Levels - Pool Calculator vs Chlorine / CYA chart

    Quote Originally Posted by frustratedpoolmom
    Correct, CYA doesn't change from evaporation.
    Right, but when you add water to the pool due to evaporation, it will change. That's what I meant.
    20x40 30,000 gal IG Liner, Pentair II Sand

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    JasonLion's Avatar
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    Re: FC Levels - Pool Calculator vs Chlorine / CYA chart

    Quote Originally Posted by Rangerman
    Right, but when you add water to the pool due to evaporation, it will change. That's what I meant.
    As water evaporates the total amount of water goes down, while the amount of CYA remains the same. Then, when you add water to top off the pool, the amount of water goes back to what it was before and the amount of CYA again remains the same. In the middle the CYA level is slightly higher and then you end up with the same CYA level you had to start.
    19K gal, vinyl, 1/2 HP WhisperFlo pump, 200 sqft cartridge filter, AutoPilot Digital SWG, Dolphin Dynamic cleaning robot
    Creator of PoolMath and Pool Calculator. Other handy links: Support this site, TF Test Kits, Pool School

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    Re: FC Levels - Pool Calculator vs Chlorine / CYA chart

    Take a simple, but extreme, example. Say that the CYA level is 50 ppm and that half of the pool water evaporates. The CYA does not evaporate so you have the same absolute quantity of CYA in half the amount of water so its concentration is doubled so is now 100 ppm (i.e. the "parts" is the same absolute quantity of CYA but the "per million" is half as much water).

    You now fill the pool with water that has no CYA in it. You end up with the same absolute quantity of CYA that you started with and now have the same total amount of water that you started with so the CYA concentration is back to 50 ppm.

    An even more extreme example would be to evaporate ALL of the water from the pool. You would then have a pile of solid CYA (or equivalent, such as sodium cyanurate) sitting at the bottom of the pool. Then add water to where you started and you end up with exactly the same CYA concentration that you started with.

    If your fill water contains any substance, then that gets added to the pool water. So the above analysis works for Calcium Hardness (CH), but whatever CH is in the fill water will get added to that in the pool water which is why the CH goes up in hot climates with lots of evaporation when the fill water has CH in it, even if the CH is at a lower concentration then the pool water itself. Same goes for Total Alkalinity (TA) as well.

    Richard
    16,000 gallon outdoor in-ground 16'x32' plaster pool; Pentair Intelliflo VF pump; Pentair IntelliTouch i9+3s control system; Jandy CL-340 square foot cartridge filter
    12 Fafco solar panels; Purex Triton PowerMax 250 natural gas heater (200,000 BTU/hr output); automatic electric pool safety cover; 4-wheel pressure-side "The Pool Cleaner"

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    Re: FC Levels - Pool Calculator vs Chlorine / CYA chart

    I apologize. Evaporation has nothing to do with dumping water. I guess I'll go back to the best form of forum participation - ie, read more, post less.
    20x40 30,000 gal IG Liner, Pentair II Sand

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    Firefly2000's Avatar
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    Re: FC Levels - Pool Calculator vs Chlorine / CYA chart

    I think some of you may have read my original post wrongly.. Sorry about the ambiguity.

    I was actually after some direction on the correct FC levels to use.

    Say I have a CYA level of 50. if I use the pool calculator, the target FC is 8, and the shock FC is 16. However, if I use the Chlorine / CYA chart, target FC is 6, and the shock FC is 20.

    For higher CYA, the discrepancy in FC levels (especially for shock) is much greater...
    Daniel

    47000L (12500 gal)
    IG Quartzon Interior
    Rolachem Acid & Chlorine dosing
    Paramount In-floor cleaner
    1.5HP pump + Sand filter
    Solar heating under the great Aussie sun!

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    Re: FC Levels - Pool Calculator vs Chlorine / CYA chart

    The Pool Calculator at a CYA level of 50 actually says "Normal: 4 to 8; Shock 16; Mustard Algae Shock 29".
    The Chlorine / CYA chart says "Minimum 4; Target 6; Shock 20".

    The minimums are the same. The mid-point in the Normal in The Pool Calculator is equivalent to the Target in the Chlorine/CYA chart so is consistent. The high number in the Normal range is equivalent to a number that used to be called "Max", but there really isn't a hard maximum. So basically, you will get similar numbers between these two for Normal if you understand that the Normal range means "Min to Max" with the mid-point being a "Target".

    As for the shock level, the actual number isn't as critical since higher numbers will kill the algae faster. It's not like the Minimum level (or low number in the Normal rangae) where going below this number significantly increases the chances for algae to grow. The Chlorine/CYA chart shock levels are for the same technical amount of "active" chlorine, but these numbers get quite high at higher CYA levels so from a pragmatic point of view The Pool Calculator uses lower numbers. Also note that the Chlorine/CYA chart is a rounded number and simplified version of this chart that has actual calculated amounts of chlorine with consistent disinfection/oxidizing power in each column, but has all sorts of caveats described in footnotes.

    Richard
    16,000 gallon outdoor in-ground 16'x32' plaster pool; Pentair Intelliflo VF pump; Pentair IntelliTouch i9+3s control system; Jandy CL-340 square foot cartridge filter
    12 Fafco solar panels; Purex Triton PowerMax 250 natural gas heater (200,000 BTU/hr output); automatic electric pool safety cover; 4-wheel pressure-side "The Pool Cleaner"

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    Firefly2000's Avatar
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    Re: FC Levels - Pool Calculator vs Chlorine / CYA chart

    Thankyou for the clarification. Thanks also for all your valuable contributions and effort on this web site!
    Daniel

    47000L (12500 gal)
    IG Quartzon Interior
    Rolachem Acid & Chlorine dosing
    Paramount In-floor cleaner
    1.5HP pump + Sand filter
    Solar heating under the great Aussie sun!

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