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Thread: Does anyone have experience with automated chemical dosing?

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    Join Date
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    Does anyone have experience with automated chemical dosing?

    I run a school pool built in the 1960s.

    The pool is hand dosed. We use a combo of trichlor tablets in a feeder to give us some continuous addition and 14% sodium hypochlorite for topping it up. I know all about CYA thanks to TFP and am able to manage this combination of chemicals pretty well. I am very proud of my sparkly water!

    However, the requirement for two-hourly testing in a hand-dosed pool is killing me! The alternative is an automatic dosing machine, but I have a feeling that this will bring its own set of problems.

    For instance, the turnover of the pool is really long compared to what it would be in a modern pool. We've never had to worry about chlorine levels being too high - unless a pixie adds stuff to the water behind my back then that's simply not going to happen - but with an automated machine then there could be a delay between the chlorine being added and the sensors in the system registering the new levels, and the pool could end up overdosed. Or is this something that the people who make this kit have already thought of? Is there some way to limit the amount of chlorine that can be added in a certain period of time?

    The same concern goes for acid. We have hard water and couldn't use NaHOCl without also adding acid.

    Is there somewhere better to ask these kinds of questions? The people who sell this equipment seem to have too much vested in selling the equipment to want to talk about potential problems using it. Automated dosing would be a huge step for us, so I want to make sure it's the right one.

    Thanks in advance for any advice!
    90k litres, outdoor in ground pool, concrete, painted (I hate paint)
    Sta-Rite 5P2R 1.5 Hp pump, undersized sand filter (needs upgrading).

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    Patrick_B's Avatar
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    Re: Does anyone have experience with automated chemical dosing?

    Interesting questions loop pea,

    I understand your concerns and deal with them daily. Not for pools, but systems where "over-shoot" of acid or chlorine is a concern. Systems where either of these can be extremely critical and cost hundreds of thousands of dollars overnight with a serious mistake. That "overshoot" is what you mention as a fear, and could be quite possible, but it can be overcome. Not knowing what you would be able to purchase in the UK and what capabilities the automation has, makes it more difficult to answer you. The shortest answer without more information is putting a limit timer on feeds when you figure out what that time is.

    I assume this control for FC would be ORP/Redox? If so, Just remember that ORP is affected by many factors and can sometimes be tricky at best. What gives you good FC level at 500mV, may not do the same at a pool next door with the same fill water. Why this happens can be a mystery from system to sytem, sometimes very difficult to figure out as well. As for Acid, your basic water chemistry will determine whether you need acid more than whether or not you use Bleach as a FC source. Hard water is generally Alkaline as well, so that is the determining factor more than NaHOCl. Bleach raises pH, yes, but it is temporary. I'm not arguing you should use it, just thinking aloud. I'm curious what they are selling, and what they are telling you. Obviously you are wise to have the concerns you have raised. If you are truly interested in the automation, it might be worth digging into, but stay the course, and don't let them fool you.
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    JasonLion's Avatar
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    Re: Does anyone have experience with automated chemical dosing?

    Automated dosing machines still need to be calibrated against manual testing more or less continuously, several times a day at least. And most areas have required manual testing at least every four hours.

    Overshoot on the sensors is rarely a problem as long as you keep the injection point in the same plumbing loop as the sensor (but after the sensor), and keep the feed rate relatively low. You are then in effect treating the water as it passes through the system, rather than treating the whole pool on the basis of the sensor. When you work that way, it is essentially impossible to overshoot.
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    Patrick_B's Avatar
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    Re: Does anyone have experience with automated chemical dosing?

    I agree that low dosing rates will help a lot, but finding the sweet spot to meet demand and not overshoot can be more easily said than done sometimes.

    I also agree that pH and ORP need frequent calibration, but if you have to calibrate them more than once or especially twice a day, they aren't hardly worth having in my opinion. Depending whether or not how well they do on single point calibrations, this could become a real burden.

    I honestly would not have something that needed more than once per day calibration for these parameters.
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