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Temperature control

Discussion in 'General Electronics Discussion' started by C-41, Jun 25, 2016.

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  1. C-41


    Jun 25, 2016
    Hi !

    I come here because I have no one around me that can help.

    I have a simple project : I want to process my color film at home (C-41 process). With C-41 you need to control with precision the temperature of the chimicals. I need an accuracy of 0,5°C to 1°C, according to the brand of the chimicals used.

    My device will be rather simple : an aquarium pump to keep the water evenly at the same temperature throughout the bath; a water heater (like the ones foud in coffee machines; 110V) and one of those devices: ST-1000 (accuracy: +/- 1°C) / MH1210W (accuracy: +/- 0,3°C) / PID thermostat.

    So I have a few questions :

    1. can I rely on the technical sheet of those instruments? If so, the MH1210W would be my choice (accuracy: +/- 0,3°C). If not, what other (simple) temperature controling device could I use?

    2. "where" is the precision of those instruments? In their calibration? Or in the precision of the sensor? If its in the sensor, which brand of sensor should I use?

    3. what is a "PID thermostat" and should I use it in lieu of the STC?

    4. Now, if there was somewhere a super simple and easy tutorial for making the equivalent with a Raspberry PI I would consider this path since I would learn something (but it has to be super super simple!).

  2. Minder


    Apr 24, 2015
    If you turn a heating element off, it continues to heat as it cools down.
    PID control is to maintain steady control, without overshoot.
    IOW as the temp approaches the set point it anticipate over shoot and starts to turn off before the set point, and then on/off at increasingly smaller intervals.
    This is a simplistic explanation but tells the gist.

  3. C-41


    Jun 25, 2016
    Ok. So the PID is the way to go.

    On eBay, there are many models; are they all alike? Just about any "eBay model" will do the job?

    Some have a heat sink? Does the PID tend to overheat? Is it the solid state relay that will overheat?

  4. Minder


    Apr 24, 2015
    Solid state (SSR's) can be zero crossing switch on and are on for the whole cycle and are usually used in heating applications by burst firing for short intervals, this is opposed to phase angle control such as motor/rpm control and can be noisy.
    A proper rated SSR should not get that hot, they usually come with a heat sink base and can be mounted on a metal surface for extra cooling.
  5. Colin Mitchell

    Colin Mitchell

    Aug 31, 2014
    The temperature differential between the heater and the bath will be at least 3 degrees. You need an external bath of hot water to maintain the temp if you must keep to less than 1 degree.
  6. C-41


    Jun 25, 2016
    Ok, so the SSR should not heat too much. But will it heat a lot if I use a heating element from a kettle of coffee machine (1500W)? I have to admit that I'm not sure for now how to heat the water. Aquarium heaters dont go beyond 34°C when I need 38°C.

    I've seen a PID controller with Auto tune :
    "For PID control mode, if the control with default P, I and d parameters settings are not able to hold
    the target temperature, the built-in auto tune function can find the right PID parameters for the
    system. When on heating mode, it will heat up the system to the target temperature then let the
    system cool down. It will repeat this process for about 3 times, then the controller will calculate the
    parameters automatically".

    Should that be ok just for heating water or should I set manually the hysteresis?

    I'm not sure to understand your comment Colin : should I use 2 baths? Like one with the heating element and a second one with my chimicals?

  7. Amar Dhore

    Amar Dhore

    Dec 2, 2015
    What I would do is: If the Controller has Auto tune, I would use that first and I wold check how my temperature ramp up and ramp down is. Is it too fast or too slow? Also I would check how much is Overshoot and undershoot. if Auto tune is not making my system stable, I would changes one parameter at a time (P, I and D) and analyse the same thing again and again until I feel my system is stable.
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