Controlling a heating element

Discussion in 'The Projects Forum' started by jmajor817, Apr 8, 2013.

  1. jmajor817

    Thread Starter New Member

    Apr 7, 2013
    3
    0
    Hello I am currently working on a project using an Arduino Uno, 1500 W immersion heating element from mcmastercarr, and PT100 RTD temperature sensor. I am wanting to read in the temperature from the PT100 to the Arduino, and based on the reading control the heating element. I am looking to control the temperature of around three gallons of water for upwards of an hour with a +/- .5 F precision. I currently have a mechanical relay that can control the on/off of the heating element. However, I am not sure whether or not I be able to control the element enough with a simple on/off tactic. I am considering a couple of options:

    1. First, I realize that the mechanical relay will need to be replaced with a Solid State Relay because of the amount of switches that will occur per use. But if I had a SSR, I am thinking of trying to tinker with an equation to program into the micro-controller that will be able to control the on/off to desired precision. Something like as the temp nears the desired temp, i.e. within 10 F of 150 F, cycle on 10 seconds and off one second, and them within 9 F a different cycle. The SSR shouldn't have any problem handling this amount of switches, or so I have read. Or does anyone have any experience using the Arduino PID library?

    2. I am also considering using a TRIAC. Kind of like the way a light dimmer works, but with a longer period because the heating is a lot slower process. It is my understanding that a TRIAC can be used like a relay, only the output can be controlled based on the input into the TRIAC that can be controlled by the micro-controller. So maybe something like lower the output as it gets within X degrees of the desired. Then simply use the on/off without cycling but just at a lower percent.

    Any comments, concerns, or improvements on these two approaches would be greatly appreciated. Also if anyone has a different/better idea as to do this I am definitely open. Thanks in advance to any responses.
     
  2. #12

    Expert

    Nov 30, 2010
    16,248
    6,745
    That size heater can move the entire 3 gallons by 1/2F in less than 8.8 seconds. Stratification is part of the process, as is its enemy, convection. Have you considered movement of the water?
     
  3. jmajor817

    Thread Starter New Member

    Apr 7, 2013
    3
    0
    Sorry I forgot to say that the micro-controller also controls a pump that will be circulating the water in order to keep the water evenly heated. The process will need to heat to one temp, around 150, hold for an hour +/- .5 F, then after this bring it to a boil for another hour.
    Both the pump and heating element will be plugged into a GFCI outlet that will be connected to a 120V 15A main. The relays, controlled on/off by the micro-controller based on the read in from the RTD, will control the pump and heating.
     
  4. #12

    Expert

    Nov 30, 2010
    16,248
    6,745
    Both of your approaches will work, but you are going to be using rather small duty cycles.
     
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