By Tim Curran
Unknown date
Years ago there was a song about Duluth with the phrase "Where else in this entire nation, can you find cheaper refrigeration". When it comes to natural refrigeration, Minnesota has plenty. If you want to make a true lager style beer, then a means of cold storage (lagering) is needed. To take advantage of our 6 months of winter, I built a lagering chamber that sits in the garage and uses a heater to keep it from freezing. The chamber is just a insulated container large enough for a couple of carboys. An old refrigerator would work equally well provided a heat source is included. A working refrigerator would have the advantage of year-round lagering.
An electric heat source can be provided to the chamber in a few different ways. A light bulb will do it and can even be pretty small (40watts or less) if the chamber insulation is good. Heating elements or coils have an advantage in that there is no light emitted that might affect the beer. The heat source should be limited or controlled in some manner to provide a steady temperature. There are a number of controllers on the market available thru homebrew supply shops and industrial suppliers.
Most controllers operate like your furnace thermostat. When the desired temperature is too low, the heater kicks on until the temperature rises above a turn off threshold. There are usually a few degrees Fahrenheit difference between the turn-on and turn-off points to prevent chattering or oscillation. This is important for your home furnace so the fan motor isnt going on and off every second or two, leading to rapid wear out. If you are just controlling a heating element, then this is generally not a problem as there is no wear-out mechanism due to rapid on-off, although this may not be true in the case of a light bulb if the bulb cools completely between cycles. It is possible to run a heating element, including light bulbs, by switching the heater on and off many times per second.
Light dimmers work in this fashion by clipping the 60 hertz power cycle by a percentage, such that the light is turned on/off 60 times per second, with a varying duty cycle (percentage) of on-time. This is fast enough that the bulb does not cool completely each cycle and no flicker is visible to the human eye.
The difference between the on and off temperatures of ordinary controllers is called the dead-band or hysterysis. As you would guess, smaller is better for more constant temperature, but it probably doesnt make too much difference since the liquid in a carboy wont vary as quickly as the air temperature (which is what the controller measures). For people like myself that like to do things the hard way just to make a marginal improvement, a more sophisticated controller can be used that runs the heater with a variable duty cycle, just the right amount to keep the temperature very constant.
The best controllers are known as PID controllers, which stands for Proportional-Integral-Differential. A discussion of PID is beyond the scope of this article, but I may post a separate article on our club web site. Commercial PID example is the Watlow 935 series. I believe this one costs about $100. It is a very small but advanced controller that has a numeric display of temperature and set point. It uses thermocouples as the temperature sensor. It is a controller only you supply the heater.
I have built my own PID controller, mostly as an exercise to learn about control theory. The electrical schematic is posted below and is based on a circuit from Bob Pease of National Semiconductor (Whats All This P-I-D Stuff Anyhow?, Electronic Design, June 26, 1995). The circuitry is not all that complex, just a few low cost integrated circuits and a bunch of knobs. The harder part I found was the mechanics of putting it in a box with a display, and making a separate heater module that incorporated a heating element, solid-state switch (to take the low voltage control signal and switch the 120 volts to the heater on/off), muffin fan for air circulation and a temperature sensor. Even if you buy the controller, the heater itself is still needed. The muffin fan is not a necessity, but I think it keeps the air temperature uniform within the chamber and makes the sensor more responsive. I won't bore you further with technical details, but if anyone wants to see the controller or has an interest in building one of these, let me know and I'll provide my usual sage and somewhat marginal advice.
PID Schematic
