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Thread: My FC numbers are not matching PoolMath

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    My FC numbers are not matching PoolMath

    Sorry for another round of questions, but I'm in a bit of a quandary. So when I started following this new process I started with 8.25 bleach. I had bought 3 cases at Costco (so 9 jugs). I decided to start going with the Lowe's brand Ket-Tek, because it was 10% and actually a full gallon versus 121oz like the regular bleach. Cost wise and convenience wise this made sense. When I was using the 8.25 my FC numbers were pretty close to what I thought they should be after adding an appropriate amount. But starting this weekend I ran out of 8.25 and started using my Kem-TeK 10% this week.

    My numbers were off Monday and Tuesday with FC shooting up higher then I expected. So yesterday I wanted to be extra careful on all my measurements and be sure to take my time doing my tests. I'm using 19,800 for gallons as this comes from the pool builder and it seems to be right for other chemical additions and previous 8.25% bleach. Here are the numbers I got

    Tuesday night final reading was FC=13 around 5pm
    Weds initial reading FC = 8.5 4:30 PM (barely any color before adding last drop)
    Add 32 oz 10% measured in a Pyrex measuring cup.
    Predicted ~FC 10 (+1.3)
    Actual reading FC=11.5 5:30 PM
    Pump running the entire time.

    It seems like this should be out of the margin of error (double the expected change). I ordered a speed stir to help on the measurement. Should I chalk this up to inexact test measurements? I do tend to add 2 drops at a time and then swirl until I get close to where I think it should be. I didn't think this would matter, but maybe it does. Hoping the speed stir will help with that. I had more out of whack numbers before that but was less exact in my chlorine measurement (Tues 8.5 -> 13 after adding about 36 oz)

    I supposed the gallons could be off but I don't think by enough to make this kind of difference. Seems like it can only be 1) Gallons incorrect 2) Bleach % too low 3) My measuring cup is wrong 4) Operator error 5) PoolMath is wrong - that could be operator error as well

    Usually on this kind of thing I tend to go with 4. I will note, that I user a syringe to get my test sample as exact as possible. This was 3 days in a row the FC went significantly higher then expected. If it was test error you think it would either be random error or I'm doing something wrong and both numbers would be off in the same direction either both high or both low. Also note that repeating the test (with same sample water) gives the same result. I'm getting the water from between a skimmer and return in a different location from where the chlorine was added.

    For reference my other numbers
    pH = 7.5
    TA = 70
    CYA = 60
    CH = 240

    I'm running low on R-0871 because I've done so many test at high FC levels, but I ordered a large refill. I'm expecting that today. So my next plan was to do several test samples from different regions in the pool. Maybe the chlorine is concentrating in an eddy where I take my sample.

    My wife is starting to think I've become obsessed, which might be a little true.
    - Mike

    19.8K Gallon IG, Pebble finish, Hayward SwimClear Cartridge Filter, Hayward Tristar Dual Speed Pump, DeltaUV D-80 UV Sanitizer, K-2006, Finished August 2016

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    pabeader's Avatar
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    Re: My FC numbers are not matching PoolMath

    You remembered to change pool math to 10%, right?
    Bob - INTEX 12x24 owner (not setup yet), 'New to me' Kreepy Krauly Sand Shark, Test kit: TF-100 w/Speed Stir

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    Re: My FC numbers are not matching PoolMath

    Yes, I changed the number to 10%. I get 32oz of 10% Bleach will raise FC 1.3
    - Mike

    19.8K Gallon IG, Pebble finish, Hayward SwimClear Cartridge Filter, Hayward Tristar Dual Speed Pump, DeltaUV D-80 UV Sanitizer, K-2006, Finished August 2016

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    Re: My FC numbers are not matching PoolMath

    You may want to thoroughly clean/rinse the vial and re-test....
    12,000 gal Diamond Brite indoor​ pool. Pentair Triton TR-60 Sand Filter +added DE, Lochinvar Energyrite ERN-200 gas heater, Pentair Multi-valve, Pentair SuperFlo VS pump, TF-100 kit, Pool Math App - iOS

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    Mod Squad pooldv's Avatar
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    Re: My FC numbers are not matching PoolMath

    It is probably a little of all of the above. Chlorine concentrations can vary depending on age and how it is stored.

    Your wife will appreciate the sparkly pool all summer long, it only takes one summer and she'll never question your sanity again.
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    2012 build and pics, 20k gal gunite, black onyx pebblesheen, OK flagstone, IntellifoVS, cart filter w/Pleatco, IC40 SWG, Solartouch, 5 12'x4' solar panels, HP50HA heat pump, 8mil solar cover, borates, TF-100 test kit, SONOS, Doheny's Discovery Robot, hot tub on bleach

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    Re: My FC numbers are not matching PoolMath

    I keep telling her she will have the best/safest swimming pool in the neighborhood. Need it to warm up a bit so she can start enjoying it. I went from skeptical to full throttle in about 2 weeks.
    - Mike

    19.8K Gallon IG, Pebble finish, Hayward SwimClear Cartridge Filter, Hayward Tristar Dual Speed Pump, DeltaUV D-80 UV Sanitizer, K-2006, Finished August 2016

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    Mod Squad pooldv's Avatar
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    Re: My FC numbers are not matching PoolMath

    Ditto! And then I went from lurker to moderator in a few years!
    TFP Moderator
    If TFP helped you or saved you money - Become a TFP Supporter! <--Click here
    2012 build and pics, 20k gal gunite, black onyx pebblesheen, OK flagstone, IntellifoVS, cart filter w/Pleatco, IC40 SWG, Solartouch, 5 12'x4' solar panels, HP50HA heat pump, 8mil solar cover, borates, TF-100 test kit, SONOS, Doheny's Discovery Robot, hot tub on bleach

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    Re: My FC numbers are not matching PoolMath

    And then I went from lurker to moderator in a few years!
    I can't decide if that's a good thing or sort of a downhill slide !!!
    Dave S.
    42k vinyl and concrete pool, 1.5hp pump, 140gpm filter
    TFTestkits , PoolMath , Pool School

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    Mod Squad pooldv's Avatar
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    Re: My FC numbers are not matching PoolMath

    TFP Moderator
    If TFP helped you or saved you money - Become a TFP Supporter! <--Click here
    2012 build and pics, 20k gal gunite, black onyx pebblesheen, OK flagstone, IntellifoVS, cart filter w/Pleatco, IC40 SWG, Solartouch, 5 12'x4' solar panels, HP50HA heat pump, 8mil solar cover, borates, TF-100 test kit, SONOS, Doheny's Discovery Robot, hot tub on bleach

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    Re: My FC numbers are not matching PoolMath

    Quote Originally Posted by mww View Post
    I keep telling her she will have the best/safest swimming pool in the neighborhood. Need it to warm up a bit so she can start enjoying it. I went from skeptical to full throttle in about 2 weeks.
    Mine is rapidly turning into a pool snob. She used to make fun of me and my chemistry set, but after a first season of being able to see the screw threads in the drain covers, she now looks at public and friends' pools, cringes, and tells me how much better ours looks.
    8,500gal plaster in-ground with attached spa, two floor drains, one skimmer. Chlorine, City Water.
    Pentair WhisperFlo 1hp Pump, MagnaTek Midcentury EPlus motor, Pentair CleanNClear 200SF Cartridge filter, Purex Triton MiniMax 250 Gas Heater, Pentair SunLite, AutoFill.
    Kreepy Krauly Cruiser, TF-100 with SpeedStir. :D

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    Mark R's Avatar
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    Re: My FC numbers are not matching PoolMath

    ^^^ that's funny cause I have a wife like that too ^^^
    12k gal Gunite/Hayward filter C5520BH with quad 33 inch filters. 2.5 hp motor. Pool is on south side, so full sun light. Spill over spa.
    Laar's heater LT250-Low NOx Gas Fired for pool/spa 250,000 BTU. Roof mount 6 panel solar system installed east/south. TF-100 test kit. May-September 100+ degrees out side.

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    Re: My FC numbers are not matching PoolMath

    Quote Originally Posted by triptyx View Post
    Mine is rapidly turning into a pool snob. She used to make fun of me and my chemistry set, but after a first season of being able to see the screw threads in the drain covers, she now looks at public and friends' pools, cringes, and tells me how much better ours looks.
    funnier that I am the wife and my husband brags to any of his friends who have pools... he keeps telling them "you should listen to her" lol
    Christina
    ~14,000gal 14x30 IG vinyl, Hayward DE4820 filter (everything else is too old to read lol), TF100 Test Kit, TFPC method
    POOL SCHOOL, ABC'S of WATER CHEMISTRY, POOL MATH, SUPPORT TFP!!, TEST KIT!

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    Re: My FC numbers are not matching PoolMath

    Well, howdy neighbor! I started right in with TFP, as I started lurking while doing research before even deciding to build a pool. Still a noob with zero summers under my belt. But, anxious for the first to start.

    Any chance that the bleach you had been using was "old" and not up to snuff, and that it validated a contractor-overstated pool size? I think it is rare that they estimate smaller than actual, though mine did. Is it a geometrically- or orgainically-shaped pool? Mine is geometric, so it makes measuring easier. But, I broke mine up into small areas and added all the volumes together. Builder was off quite a bit. Of course, I nerded out and guessed by fill time to within a few minutes, winning a bet against the PB, and verifying my calculated volume based on fill rate.

    I find really fresh liquid chlorine is usually higher concentration than stated.

    My wife makes fun of my water obsession less and less. I'm sure she's starting to realize the benefits.
    Chris & Lea Stoddard
    20k gal, IG shotcrete/pebble pool/spa combo
    Filled 11/19/2016
    All Jandy: DEV 60, PS Stealth 2 hp 2 spd, LXi (gas) heater, PDA-PS8
    Stenner 45MPHP22 for chlorination

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    Re: My FC numbers are not matching PoolMath

    Mine is geometric also and would fairly trivial to calculate it, but I have not taken the measurements. A rough calculation though shows the contractor may have over estimated by a little bit. I need to do a real calculation though. I will say that numbers are closer since I started using the speed stir. Bleach is very fresh so it may be higher concentration as well. Probably some combination of all of the above, but I'm getting a good feel for how much I need now. Hope we get some sunny days soon to start enjoying the pool more. Too many cloudy days for April.
    - Mike

    19.8K Gallon IG, Pebble finish, Hayward SwimClear Cartridge Filter, Hayward Tristar Dual Speed Pump, DeltaUV D-80 UV Sanitizer, K-2006, Finished August 2016

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    clambert1273's Avatar
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    Re: My FC numbers are not matching PoolMath

    Quote Originally Posted by mww View Post
    Mine is geometric also and would fairly trivial to calculate it, but I have not taken the measurements. A rough calculation though shows the contractor may have over estimated by a little bit. I need to do a real calculation though. I will say that numbers are closer since I started using the speed stir. Bleach is very fresh so it may be higher concentration as well. Probably some combination of all of the above, but I'm getting a good feel for how much I need now. Hope we get some sunny days soon to start enjoying the pool more. Too many cloudy days for April.
    To get a good calc on mine (even though it is a rectangle, it is very deep with large slopes - I do not have a shallow end), I used Pool Math and went to the bottom and in the section that will allow you to put how many ounces of bleach should raise FC by ___. I tested that theory and added 2 gals of 10%. I knew what my level should rise to so went back to top and input figures in the FC and gallons until it matched the figure.

    That took my size to 12,200 gallons which has been spot on for chems ever since... hopefully that was clearer than mud


    Christina
    ~14,000gal 14x30 IG vinyl, Hayward DE4820 filter (everything else is too old to read lol), TF100 Test Kit, TFPC method
    POOL SCHOOL, ABC'S of WATER CHEMISTRY, POOL MATH, SUPPORT TFP!!, TEST KIT!

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