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Thread: ORP and chlorine

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    travisr100's Avatar
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    ORP and chlorine

    I just kind of found this graph interesting with respect to ORP and chlorine. I don't use a SWG, I use bleach. You can see where my bleach pump cuts on and it seems to have no affect on ORP. Time of day sure does though.

    IG 13,000 gal, WetEdge Prism Matrix Indigo Blue, Hayward C4030 cartridge filter, Hayward Ecostar VS main pump, Hayward Tristar VS feature pump, Hayward OmniLogic, Hayward Sense & Dispense (pH only), Stenner 45MPHP22 bleach, Hayward H400FDN Heater, Hayward Color UCL lights, Pentair LED Bubblers. TF-100 XL test kit. Test Results. Build Thread.

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    Re: ORP and chlorine

    Sunlight depresses ORP which is ironic because 1) the instantaneous hypochlorous acid concentration actually isn't lower because it's replenished from the chlorine bound to CYA much faster than it is depleted by sunlight and 2) when chlorine does break down it produces hydroxyl radicals that are very powerful oxidizers so that in fact ORP in the bulk pool water is higher but because these radicals are so short-lived they don't make it to the ORP sensor.

    So what is going on? Who knows, but it's yet another reason why ORP is pretty pointless and can only be used for very rough process control. Supposedly, this effect from sunlight on ORP is exacerbated by higher CYA levels (what's your CYA level?) and has something to do with poisoning of the ORP sensor membranes at higher CYA level. I don't know what this has to do with sunlight, however. Maybe the breakdown of chlorine bound to CYA produces a CYA radical and that persists long enough to find its way to the ORP sensor and interfere with the membrane, but that's a wild guess on my part (see the formation of •HCY- near the bottom of this post).
    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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    travisr100's Avatar
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    Re: ORP and chlorine

    I didn't really expect an explanation but thanks chemgeek. I don't use ORP for chlorination control but I have the probe so I capture the data. You can see my bleach pump dosing in the graph. Each on cycle of the bleach pump represents 17.3 oz of 10% bleach being added. Has no affect at all on ORP. My cya is 50. Full test results in my sig.
    IG 13,000 gal, WetEdge Prism Matrix Indigo Blue, Hayward C4030 cartridge filter, Hayward Ecostar VS main pump, Hayward Tristar VS feature pump, Hayward OmniLogic, Hayward Sense & Dispense (pH only), Stenner 45MPHP22 bleach, Hayward H400FDN Heater, Hayward Color UCL lights, Pentair LED Bubblers. TF-100 XL test kit. Test Results. Build Thread.

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