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Thread: ORP values have never really correlated with the Cl values

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    ORP values have never really correlated with the Cl values

    Mike, Have you tested your sensor using fresh city tap water?
    Split from this topic. JasonLion



    I would be very interested in the results you get with a new ORP sensor.

    I have had two HI 98121 pH/ORP/Temp meters from Hanna for over a year (one has an intermittent value on the pH reading) and I really like them for the temp and pH values, but the ORP values have never really correlated with the Cl values I have in the pool.

    I also recently purchased one of the epool monitors from Leslie's and that correctly shows cl ppm over or under 2.

    I think it would be great to be able to use ORP as a chlorine indicator but the values I'm getting don't make sense.

    I just ran a bunch of comparison tests and the results are below. Key takeaway from these tests is that for tap water I am getting the "perfect" > 600 mv whereas with the pool I'm getting much much lower values. Ive seen the 150 mV I measured today even go lower than 100.

    I have seen the ORP values affected by static (plastic measuring cup on a dry plastic pool accessories storage box) so I wanted to rule out any container effect, and I checked the cl with different test kits and it's ok (pool is also very clear and clean looking).

    Leslie's says I have high phosphates, could that be causing a problem?
    I have some minor metal (copper probably) staining on the stairs, could that cause it?
    Does the salt from the SWG have any effect?
    Any other ideas?





    SWG: 3300 ppm salt

    TF-100
    fc 3 ppm
    cc 0.5 ppm
    ta 80 ppm
    ca 30 ppm

    Thermometer temp 32.2 °C

    Fresh DPD kit
    cl 3 ppm
    pH 7.5 ppm

    pool orp values (meter held directly in pool)
    ORP 150 mV
    pH 7.47 pH
    temp 32.5 °C

    Standing pool water in plastic measuring cup
    214 mV
    28.3 °C
    7.55 pH

    Cold tap water in plastic measuring cup
    7.16 pH
    637 mV
    22.7 °C

    Cold tap water in glass
    7.25 pH
    665 mV
    24.7 °C

    cold tap water in metal saucepan
    7.24 pH
    688 mV
    25.4 °C

    Fridge filtered cold water in plastic measuring cup (has only trace cl)
    7.3 pH
    648 mV
    15.7 °C

    Gas heated tap water in plastic measuring cup
    7.21 pH
    645 mV
    45.8 °C
    7600 gal, IG rectangular, SWG wood-fired pool heater, solar heating (pool not too warm, hot tub too big)

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    JasonLion's Avatar
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    Re: ORP values have never really correlated with the Cl valu

    Phosphates, as usual, have nothing to do with it. Dissolved metals will, if anything, raise the ORP valve.

    One of the factors that affect ORP readings is dissolved hydrogen gas, which lowers ORP readings dramatically. Hydrogen gas is produced by a SWG while it is working. Often the hydrogen bubbles out of the water, but some of it dissolves and lowers the ORP level.

    Another one is CYA. Nearly any CYA level will lower ORP dramatically, and higher CYA levels will lower ORP even more.
    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: ORP values have never really correlated with the Cl valu

    What is your CyA?
    IG 24k plaster with overflow spa. Goldline PS-8 SWG. Tristar 0.75 HP filter pump, Polaris 280, large cartridge filter, 400k BTU NG Max-E-Therm HD, SR Smith Turbo Twister, Life Saver pool fence, ORP managed.

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    Re: ORP values have never really correlated with the Cl valu

    CYA is a bit low - just barely makes the dot invisible at 20-30 on the tube scale.

    I'm thinking the hydrogen is the issue. To keep the pool clean and temperature up I usually keep the solar cover on it when it's not being used, and we're usually in it only a few hours per day max.

    It seemed like ORP went even into negative numbers after I had the cover on for a while.

    I also have an over-dimensioned cell for the amount of water in the pool so I'm guessing that when it is generating, it is really blasting. At water temps in the 70s I'll get a chlorine buildup when the swg runs 5 hrs per day at 5% (lowest setting). I've had to increase the generation recently because the water is now in the mid 90s, thanks to a new solar heater, and the ORP seems to have gone down (more negative) since then too.


    Good stuff to know.

    Is there any way to reduce the hydrogen or remove it from the water? Does it hurt anything? Does it even matter?
    7600 gal, IG rectangular, SWG wood-fired pool heater, solar heating (pool not too warm, hot tub too big)

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    JasonLion's Avatar
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    Re: ORP values have never really correlated with the Cl valu

    Dissolved hydrogen gas doesn't affect anything except ORP sensors. Hydrogen gas will come out of the water over many hours to a couple of days, but that only helps if you stop adding more, ie stop using a SWG.
    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: ORP values have never really correlated with the Cl valu

    Quote Originally Posted by pool-newb
    Is there any way to reduce the hydrogen or remove it from the water? Does it hurt anything? Does it even matter?
    that would leave a pool of oxygen, which will hurt when you dive in

    Maybe you are members of http://www.dhmo.org/research.html

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    Re: ORP values have never really correlated with the Cl valu

    Quote Originally Posted by JasonLion
    Dissolved hydrogen gas doesn't affect anything except ORP sensors. Hydrogen gas will come out of the water over many hours to a couple of days, but that only helps if you stop adding more, ie stop using a SWG.

    Hydrogen gas doesn't act as a reducing agent and dechlorinate to some extent?
    21' Leslies Beachland Ag Pool, 10,000 gallons, professionally installed (best money I ever spent) Hayward 16" sand filter w/Pentair two speed pump Fafco 4x20 solar heater,Aqua Trol RJ. Borates added. Hard plumbed.

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    JasonLion's Avatar
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    Re: ORP values have never really correlated with the Cl valu

    There are a number of reactions that can happen with hydrogen gas, and all of them are going on to one extent or another, but as far as I can tell they are all at rates that are slow enough to be essentially irrelevant.
    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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