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Thread: Calculating shock level - conflicting chart / calculator?

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    Join Date
    Aug 2009
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    Central PA
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    Calculating shock level - conflicting chart / calculator?

    Hi all,

    Opened our pool last weekend & found very green walls & floor, but fairly clear 54 degree water. Usually when I close I throw in the floater with chlorine pucks, but last fall I tried Polyquat instead. Maybe a bad move... in the past I've never had much algae on opening.

    Anyway, I got the pump fired up, walls & floor brushed, and began SLAM with 10% Walmart bleach. My CYA was/is about 50, and I figured I shouldn't bring it up to SWG level until the SLAM is done. So the Pool Calculator says with CYA at 50, shock level should be 16. However the Pool School Chlorine/CYA Chart shows a shock of 20 for CYA of 50 (non-SWG; the SWG chart doesn't go down to 50).

    My question is - should I be trusting the Pool Calculator shock of 16 for my circumstances? Or do I need to go higher?

    The pool is looking really good, and I did pass OCLT Thursday morning but ran SLAM one more day for good measure and failed it this morning... So that made me question my process a bit.

    Any thoughts appreciated... Thanks!
    29,000 gal. vinyl in-ground mountain lake (freeform); Hayward DE filter, 2 skimmers, AutoPilot SWG, 2 - 4 X 20 Fafco Solar Bears with Goldline control, Aquabot, LoopLoc winter cover, Pool installed March 2009...

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    Mod Squad tim5055's Avatar
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    Re: Calculating shock level - conflicting chart / calculator?

    Quote Originally Posted by Steverino View Post
    Hi all,

    Opened our pool last weekend & found very green walls & floor, but fairly clear 54 degree water. Usually when I close I throw in the floater with chlorine pucks, but last fall I tried Polyquat instead. Maybe a bad move... in the past I've never had much algae on opening.

    Anyway, I got the pump fired up, walls & floor brushed, and began SLAM with 10% Walmart bleach. My CYA was/is about 50, and I figured I shouldn't bring it up to SWG level until the SLAM is done. So the Pool Calculator says with CYA at 50, shock level should be 16. However the Pool School Chlorine/CYA Chart shows a shock of 20 for CYA of 50 (non-SWG; the SWG chart doesn't go down to 50).

    My question is - should I be trusting the Pool Calculator shock of 16 for my circumstances? Or do I need to go higher?

    The pool is looking really good, and I did pass OCLT Thursday morning but ran SLAM one more day for good measure and failed it this morning... So that made me question my process a bit.

    Any thoughts appreciated... Thanks!
    First, you are correct - do not make any upward CYA adjustments until the SLAM is over.

    Use the FC/CYA Chart for calculations. For now the SWG section is not applicable as you are manually chlorinating the pool.

    When you say Pool Calculator, are you using our PoolMath or the old calculator named Pool Calculator (not controlled by TFP)?
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    Jezza's Avatar
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    Jan 2015
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    Bunbury, WA, Australia
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    Re: Calculating shock level - conflicting chart / calculator?

    That's right. You need to use 20FC for a CYA of 50 to SLAM.
    Not sure if you have done it yet but it is best to turn off the SWG for a SLAM and it absolutely must be off to successfully conduct an OCLT.
    11 000 gallons, IG, Fibreglass, Monarch Sand Filter, Davey Typhoon C100M 1HP pump, Davey Chloromatic Mc16CTO ESR SWG, Davey Pool Wall Climba Robot, Daisy Solar Cover,
    K-2006

  4. Back To Top    #4

    Join Date
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    Location
    Central PA
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    Re: Calculating shock level - conflicting chart / calculator?

    Thanks Tim. Actually I was using the old Pool Calculator - I didn't realize there was a difference between that and PoolMath, as they look very similar. HOWEVER, I just put my CYA, etc. in PoolMath and it says I should be at 20. Aargh, blast and tarnation. I guess I've been somewhat under-shocking for at least some of my SLAM. (Since we both work full time, I've been taking the level a bit higher in the mornings (over shock) as I can't monitor during the day.)

    Guess who's going back to WM for yet more 10%? Me!

    Thank you for setting me straight.

    - - - Updated - - -

    Jezza - I wondered about turning off the SWG, however had figured that with the water barely at 60 it wasn't generating any significant chlorine - especially overnight. But I will make sure it's off tonight. Thanks!
    29,000 gal. vinyl in-ground mountain lake (freeform); Hayward DE filter, 2 skimmers, AutoPilot SWG, 2 - 4 X 20 Fafco Solar Bears with Goldline control, Aquabot, LoopLoc winter cover, Pool installed March 2009...

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