I have adapted a refrigerator to create a temperature controlled environment for brewing beer. I have mounted an electric heater coil inside the fridge and configured a basic controller to maintain a constant temperature inside the fridge. When the temperature at the temperature probe, $T_p$, is below the target temperature, $T$, the heater is turned on. When $T_p$ is above $T$, the fridge compressor is turned on. The target temperature, $T$, is in fact a small range to stabilise the system.

$$ T_p \lt T-c \ \ \rightarrow \ \ Heat $$ $$ T_p \gt T+c \ \ \rightarrow \ \ Cool $$

My question is this: I am trying to decide, more for interest than anything else, whether it is more effective to probe the temperature of the air inside the fridge, or the liquid (beer) inside the plastic vessel inside the fridge?

My instinct says that I should probe the air temperature, because the temperature of the beer will follow the temperature of the air, subject to a time delay, and by probing the temperature of the beer I could get unstable feedback due to this delay? Would the answer change if I were to put the fridge outside, where the air temperature is subject to much more variation? I suspect in this case it might be better to probe the beer so that the controller can over-compensate with air temperature and limit temperature variation of the beer.

For the purposes of this application, the control system is fixed as I'm using a fixed controller ie. heating and cooling are binary on/off functions. However I'd be interested to learn of any control techniques that could be implemented with a raspberry pi, if they were suggested!

Thanks in advance.

  • $\begingroup$ The thermal capacity of the air will small compared with the beer (unless you are brewing a very small amount of beer in a huge fridge) but the thermal capacity of the heater itself might be bigger, especially if it operates at a high temperature. You might also want to think about how much heat is being radiated direct from your heater to the beer container rather than being transferred by convection through the air. $\endgroup$
    – alephzero
    Nov 29 '18 at 12:23
  • 2
    $\begingroup$ You have a strongly nonlinear exothermic process subject to damaging runaway depending on the size of the carboy. You definitely want to measure the wort. The air temps needed to regulate the fermentation will very by quite a lot, like 15 degrees F. But you also need to limit the air temp excursion to prevent the carboy walls from reacting differently and producing off flavors. The air temps need to stay within the healthy range for your yeast pitch. I'd put high and low level cutouts in the airspace and monitor the wort. $\endgroup$
    – Phil Sweet
    Dec 29 '18 at 15:54
  • $\begingroup$ Depending on the temperature range, heating/cooling power, and relative sizes of the air and liquid, monitoring the liquid could create an unstable situation.. Say the liquid is too cool, so the system heats the air until the liquid becomes warm enough, but by then the air is very hot and even though the heater is off the liquid continues to heat until the cooling is triggered. Then the whole thing happens over again in the other direction. The liquid will continually cycle between too warm and too cool. To avoid this, monitor the air as well, with a larger, but not too large, range. $\endgroup$ Apr 28 '19 at 13:56

I would probe the liquid and control it with a PID + SSR relay, it'll take an initial time to stabilize, but once it figures out its equation, your good to go. You probe the liquid simply because that's what you really care about. The liquid's container will have some effect on heat transfer from the environment to the liquid, so unless the liquid is to be in the refrigerator for a very long time, you can guarantee the two temperatures will be the same. Symmetry is important here, unless everything is geometrically/ volumetrically center, temperatures will vary inside until time takes over ( may not even be possible to create complete uniform temperature in the environment, insulation,sealing,etc) So it may be possible to have temperature fluctuations throughout the liquid.

May I suggest... Use the fridge as is, always on, and control the heaters, submerged in the liquid, inside its container, inside the fridge, with a PID/SSR,probe in the liquid. Stirring the liquid would ensure proper temp throughout, or multiple heater/PID/SSR/probe would help without stirring. And depending on the set point temp, you might not even need the fridge. Cold is the absence of heat, if your set temp is always above ambient, all you need to control is heat input.

  • $\begingroup$ Wouldn‘t using a ssr relay effectively be a 2point control system controlled via pid with beyond excessive switching? (Since one is effectively continuous and the other discrete?) $\endgroup$
    – morbo
    Sep 18 at 12:34

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