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42820Re: [new_distillers] Re: PID Controller

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  • Bob Glicksman
    Nov 22, 2012
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      I'm not an expert at beer brewing, but I doubt that the temperature has to be held absolutely steady e.g. to less than +/-0.1 deg C of the desired temperature.  I'm somewhat surprised that your previous, simple thermostatic controller wasn't acceptable.  I wouldn't think that the temperature wandering around within a few degrees of the desired temperature would make much of a difference to the biochemcial processes at work here, as long as the desired temperature was optimized for the enzymes, pH , etc.  However, if you want to minimize this "wandering around" and have the temperature come up to some point and stay there rock solid, then you want to use a PID controller, as you are doing.  There is no disputing this!  The only issue here is whether actually having the PID controller use an electromechanical contactor to cycle the AC power to your heater reduces the "wandering" down to an acceptable level.  If not, you need to have the PID controller operate a faster device in order to cycle the AC power faster -- and this device would be an SSR.
      -----Original Message-----
      From: Gavin Flett <gavin_flett@...>
      To: new_distillers <new_distillers@yahoogroups.com>
      Sent: Thu, Nov 22, 2012 4:20 pm
      Subject: RE: [new_distillers] Re: PID Controller

       
      Now that's what I have been searching for. a lamens description of what a PID and an SSR does. Thanks

      How do the beer brewers do this then, what kind of W heating element do they use? Is it multiple low wattage elements?


      To: new_distillers@yahoogroups.com
      From: self.adhesive@...
      Date: Thu, 22 Nov 2012 23:48:08 +0000
      Subject: [new_distillers] Re: PID Controller

       

      > Cool, great info. I am actually using it for a barley mash process. So if I understand correctly, an SSR performs the same functions as a PID, but better?

      No, they're completely different things. The PID is a logic "brain" that attempts according to the parameters you program in to shoot for a particular temperature. It determines how long the element is "on" or "off" and makes adjustments according to results.

      SSR stands for Solid State Relay. It's really just an electronic switch - it has no moving parts. It is the equivalent of the contactor but because the contactor has moving parts it is not capable of switching power off and on to the element frequently without destroying itself.

      Your PID itself is able to switch loads of up to 3 amps, but this is well short of 2000 Watts. Remember that Watts = Voltage x Amps. Re-arrange this formula to get W/V = A, and depending on what your voltage is this will tell you how many Amps capability you need.

      So you employ a contactor, or SSR, to do the heavy-duty switching because these are capable of handling many more amps.

      There is one thing to remember with a SSR, and that is if they fail they switch, as far as I understand, to full on and will not turn off. The SSR has its own connection to voltage mains. So, as with all setups like this, you should always supervise operation.

      Paul


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