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Find steady state of constant heat application

  1. Mar 8, 2012 #1
    Hey all. I sure hope somebody can assist with this!

    I am a hobby beer brewer and am writing my own heat control algorithms. Part of the process involves heating water to a desired temperature setpoint. I'm using an electric water heater element (5500W) inside the water vessel (~5-10 gal). I can pulse the heater element for a desired duty cycle (0 to 100%). The vessel is not well insulated as to allow quicker recovery from a temperature overshoot. Precise temperature control within about 1F is my goal.

    Let's say that I apply 20% duty cycle. Over a VERY LONG time (hours) , the temperature stabilizes at say 127F. The curve is similar to a cooling curve but it rises toward an "asymptote" which is the steady state. I believe it is logarithmic.

    My question is this- how can I predict the steady state with a few minutes of data instead of waiting hours upon hours?

    Thanks in advance !!!
    -Micah
     
  2. jcsd
  3. Mar 8, 2012 #2

    LCKurtz

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    A logarithmic curve will not approach an asymptote. It just grows so slowly it might give that appearance. Perhaps you could model it with a logistic curve with appropriate scaling, the right side of which seems to fit your description. You might take a look at:
    http://en.wikipedia.org/wiki/Logistic_function
     
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