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PID control for a high hysteresis

Former Member

Hi

My current pet project is a PH controller for a swimming pool, The sensor and dosing dump are easy and covered.

But PID control is gong to to the kicker. There is plenty of example for PID out there, but can;t find anything for a very high hysteresis system.

Any one got any good reading ?

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  • 0

    Is the pH sensor accurate enough that you could dose an amount based on requirement and then stop? (I would have a set maximum dosage too, just in case something goes a little wonky).

     

    Then I would start a timer to wait however long the lag generally is - I think for my hot tub they say to wait 20 minutes.

     

    So my algorith would be:

     

    repeat {

    1. Test pH

    2. if not in ideal range {

         2.1 calculate dosage of pH-up or pH-down, and do the dose.

         2.2  wait 20 minutes (ie, delay loop, or set a timer/interrupt, or check milliseconds since startup. )

    }

    4. wait 10 minutes

    }

     

    step 4 is time between testing. I don't know how quickly pH changes, so you'd adjust that as you see fit.

     

    Does that help?

     

    ps, I'm more interested in automating Chlorine content in my hot-tub - do you know if any affordable and low maintenance sensors exist for that?

     

    Cheers,

    -Nico

  • 0 in reply to ntewinkel

    Hi Nico,

    that sounds reasonable approach. Maybe I'm being to scientific about this?

     

    Regarding chorine control, that is phase two of my project.

    From my reading so far Oxidation Reduction Potential or Redox measurenes can be used to corilate chlorine levels, however the Ph level needs to be stable..

     

    Atlas Scientic do a ranges of sensors, I'm using the Ph probe & sensor and it works very well.

    The ORP sensor can be found at

    https://www.atlas-scientific.com/product_pages/kits/orp-kit.html

     

     

    regards mike

  • 0 in reply to Former Member

    Hi Mike,

     

    I don't know.. I just like to keep things as simple as possible image  But if there is a huge lag from adding chemicals to seeing the result, I don't think there is any other way except to wait.

     

    Thanks for the link to the sensor kits - they even make it easy to use the results, and the price is not too bad. The sensors look huge though, so it would be tricky to fit those in-line somewhere in my hottub setup. Or maybe the circuit boards are just tiny, heh.

     

    Cheers,

    -Nico

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  • 0 in reply to Former Member

    Hi Mike,

     

    I don't know.. I just like to keep things as simple as possible image  But if there is a huge lag from adding chemicals to seeing the result, I don't think there is any other way except to wait.

     

    Thanks for the link to the sensor kits - they even make it easy to use the results, and the price is not too bad. The sensors look huge though, so it would be tricky to fit those in-line somewhere in my hottub setup. Or maybe the circuit boards are just tiny, heh.

     

    Cheers,

    -Nico

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  • 0 in reply to ntewinkel

    HI Nico,

    the probe is large-ish ( but this is normal for these chemical sensors) In a swimming there are plently of pipes hiden in the filter enclosure

    I modified a comercial solution to insert the probe into a pipe as below. the fitting cost $55 AUD but as the comercial Ph controlller cost welll over $1500 aud. I'm well ahead on the project. (+ I'm having a lot of fun adding web interfaces and monitoring)

    image