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Thread: ORP readings vs. Free Chlorine readings

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    ORP readings vs. Free Chlorine readings

    Hello, everyone,
    Reading about ORP on another thread brought up some of my own questions. First, what is the ideal ORP range for swimming pools? The WHO says that 650mV is suitable for drinking water, than why have I seen pools with ORP setpoints at around 525mV? ANd how would you decide what the ORP setpoint should be?
    THe higher the pH, the lower the ORP because ORP meauseres the quality, not quantity of sanitizer. So if the ORP was, for example, 750mV, and the Free Chlorine reading was 5.0, why does it take so long for the ORP probe to realize that the Free Chlorine level has been met?
    If ORP is not trustworthy, than why are there no chemical controllers that can measure in ppm rather than ORP?

    Thank you, for helping me to better understand ORP.
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    JasonLion's Avatar
    Join Date
    May 2007
    Silver Spring, MD

    Re: ORP readings vs. Free Chlorine readings

    There are sensors that can measure HOCl concentrations or FC ppm directly, they are just far more expensive.

    You need to calibrate your ORP target based on measuring appropriate levels in your pool with a test kit, and when your levels are where you want them, your current ORP level becomes your ORP target. This calibration needs to be repeated at regular intervals.

    There is no ideal ORP level for swimming pools mostly because there is no standard way to calibrate ORP sensors. Different sensors read different levels on the exact same water and there is no agreed upon way to calibrate, so the numbers are meaningless unless comparing two readings from the same sensor. It also impossible to set absolute targets because there are various things which can change the ORP reading which have nothing at all to do with sanitizing ability.

    ORP sensors respond slowly, just the way it is. Even if the ORP reading changed very quickly, it would still only reflect the water currently at the sensor. That water may or may not be representative of the pool as a whole.

    ORP readings are nearly useless unless you are also regulating the PH so that PH does not change.

    ORP is not measuring the quality of the sanitizer, though it is correlated with that. ORP takes into account a large number of different things, one of which is the HOCl level. You hope that the other things that affect the ORP reading don't change so that when the ORP reading does change it will presumably reflect changes in the HOCl level, but that isn't always true. That is why you need to recalibrate the target level regularly.
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    Patrick_B's Avatar
    Join Date
    Jun 2011
    Midland TX

    Re: ORP readings vs. Free Chlorine readings

    Jason, I can attest to everything you say. I have twelve plus years dealing with ORP/ FC contollers as part of my job. I have worked with the best the industry has to offer, and some of the cheapest. What is inconsistency in all of them, no matter the cost. I've had some moderate success, but preciuos little.
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