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Thread: ORP Still Low Following Shock - Large Indoor Pool

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    ORP Still Low Following Shock - Large Indoor Pool

    Hello. First post, but have read through a lot of valuable information here, and hope to get some good feedback!

    Here's my problem: I manage two pools at an indoor aquatic facility; a 350,000gal 25yd pool and a 420,000gal 50m pool. The 350,000gal pool has received complaints from the swim teams we host that Cl levels have been to high, and the swimmers are experiencing respiratory problems. Cl was a little high for an indoor facility, at about 4-5 ppm but this was to maintain an ORP value around 680mV.

    So, it was time for a shock. Pre-shock my measured values were:

    ORP: ~680
    FC: ~4.0
    TC: ~5.0
    pH: 7.2
    TA: 80ppm
    CH: 300ppm
    CYA: 0ppm
    Temp: 82 f

    A little more info on the pool's filtration system: We run a large sand filter, with UV-sterilization, and feed Cl using Cal-hypo pellets. I calculated that adding an additional 65lbs of granulated Cal-hypo would raise Cl values 15ppm to a goal of 20ppm. I thought this would be significant enough to shock the entire system.

    I let the super-chlorinated water sit over night, with all our exhaust fans turned on and several portable fans placed around the pool to help vent chloramines. I returned the next morning to lower the Cl values with sodium thiosulfate, which was brought our Cl level to approx 0ppm very quickly.

    The next day, once our pulsar chlorinator had stabilized the system, we were reading slightly higher, but still low (IMO) ORP values.

    Here's where we sit today, approx 5 days after the shock.

    ORP: 666mV
    FC: 3.3ppm
    TC: 3.3ppm
    pH: 7.4
    TA: 80ppm
    CH: 300ppm
    Temp: 82 f

    I was expecting our ORP value to climb to around 800mV. Our 50m pool sits at around 3ppm, which corresponds to an ORP value of 815mV. Any thoughts as to why the shock did not significantly raise our ORP values? Thanks for the response!

  2. Back To Top    #2

    Join Date
    Feb 2008
    Flower Mound, TX

    Re: ORP Still Low Following Shock - Large Indoor Pool

    Welcome to TFP! I cannot really help you answer the question about your ORP issue but we have many folks here who do. They should be along soon.
    10K gallon IG gunite with waterfall; Pentair CC320P filter; WhisperFlo 2 HP pump
    TF Test Kits - Pool Math - Pool School
    "It depends."- JohnT

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    JasonLion's Avatar
    Join Date
    May 2007
    Silver Spring, MD

    Re: ORP Still Low Following Shock - Large Indoor Pool

    Welcome to TFP!

    Have you cleaned your ORP sensor recently? That seems like the most likely possible problem.
    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

  4. Back To Top    #4

    Re: ORP Still Low Following Shock - Large Indoor Pool

    I have not, but I'm planning a second shock this weekend, and will do that as part of the process. I'll report back my results.

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