How can I control my beer temperature?
May 18, 2008 9:31 PM   Subscribe

I'm looking for a serial-port temperature controller thingie for a bizarre cooling system I'm building, for a beer fermenter.

The Problem: Beer needs to be kept around 65-70 degF in order to ferment properly; too cold and everything slows down, too hot and the yeasts die.

The Tools: I have a mini-fridge, a very small computer with only a serial port and 3 network ports, and not a lot of money.

The Solution so far: I'm going to have the beer fermenting in a glass carboy, and the mini-fridge running right beside it. Inside the fridge will be a tank of water, and inside the tank will be an aquarium pump.

The aquarium pump is connected to some plastic hose which attaches to a copper coil (refrigeration grade) that circles around the carboy, then dumps the water back inside the fridge container. Basically, when the aquarium pump turns on, it cycles a new length of cold water into the copper tubing and cools the carboy. The whole carboy will be inside of a styrofoam box or something to insulate it.

To control the pump, I'm going to have a thermistor or some other temperature sensor floating in the beer, or even better a couple of them at different positions inside the carboy. This is where my question comes in:

I can either try and build a circuit to switch on the aquarium pump when the temperature hits a certain trigger, using a prototype board and solder. Or, since I have this tiny computer sitting un-used, I can just find a temperature sensor that will plug into the serial port, then write a program to read the temperature and...I guess somehow use it to turn on the aquarium pump.

Basically I want to know if anyone can think of a better setup than this (keeping in mind that i'm poor,) or suggest another way to do it?
posted by geodave to Food & Drink (11 answers total) 3 users marked this as a favorite
Best answer: Are you handy with programming/soldering? Do you already have the prototype board lying around? If not, the cheapest (but hardest) solution might be to create a small circuit board that utilizes the parallel port to both measure temperature and turn the pump on.

You'd want a relay for the pump, and a cheap serial thermometer circuit. Modify the software so that one of the unused pins connects to the relay switch. Temperature output from sensor goes into computer, which then controls the output to the relay, which then controls the pump.

All in all, this shouldn't cost you more than $15 in parts, less if you have diodes, relays, etc. lying around.
posted by suedehead at 10:18 PM on May 18, 2008

Hm. Buy some DS1Wire or I2C temperature sensors from Digi-Key or Mouser; write a program to bit-bang them with some of the serial port's control lines (or use one of the open source ones). Use another one of the serial lines to control an optotriac which triggers a relay which controls the aquarium pump (leave off the relay if the pump's small enough to be controlled by the optotriac directly— remember to have a snubber for the inductive kick). WAG parts cost around $15, maybe?

Or, y'know, use a thermistor and a comparator instead of a computer. Dunno if that would actually be cheaper fin this situation.
posted by hattifattener at 10:23 PM on May 18, 2008

Heh, simul-$15. Aberto Ricci Bitti's page exemplifies the I2C suggestion; martybugs', the DS1Wire suggestion.
posted by hattifattener at 10:34 PM on May 18, 2008

Can you put the copper in the fermenter, instead of outside it? Your coil will be a different shape, but you'll get a much better heat exchanger. If you're not wedded to the glass carboy, go for a plastic bucket with a snap-on lid instead (about $5 from home depot if you're in the states) -- just don't clean it with anything scratchy. Honestly, I really dislike glass carboys. They're impossible to clean well, they smash if you drop them, and they weigh a ton.

I'd tend to go the thermistor/parallel port route if for no other reason than that you then have a built-in data logger too. You'll be able to see the temperature trends over a day, better map the response of the system to fluctuations and so on.

Or, could you fit the fermenter into the mini-fridge (taking out the shelves, natch) and just use the native, passive, unpowered insulative properties therein to maintain a reasonable temperature?
posted by 5MeoCMP at 5:33 AM on May 19, 2008

would it not be easier to put the carboy in the fridge? im not sure if $52.99 is too pricey but plenty of homebrew shops sell thermostat overrides. if it is too expensive it sounds like it is essentially the same device you would need to build in order to determine when to circulate water.
posted by phil at 6:20 AM on May 19, 2008

if cost is the determining factor a swamp cooler may by the way to go.
posted by phil at 6:37 AM on May 19, 2008

I would have to recommend against 5MeoCMP's plan simply because it's really hard to be sure that you've cleaned the coil enough to put it in the unfermented wort, which as you know is essentially a huge fungal/bacterial growth medium. You want as little stuff as possible to touch the wort.

I'd essentially go with phil's suggestion and get a thermostat override. I suspect you can't fit your carboy in your minifridge; only a few models accommodate them. But even if you keep this weird heat exchanger in place, you can slide the thermostat probe in between the coil and the glass, and it'll be close. Put a plastic thermometer strip directly on the glass and you can calibrate the thermostat reading with what temperature you actually achieve.

I suspect this arrangement will be quite inefficient, however, and you'll need to leave the fridge on almost all the time and the precise thermostat control will be redundant. Anyway, try it out.
posted by rkent at 7:06 AM on May 19, 2008

Best answer: ...or suggest another way to do it?

This all sounds way too complicated for your needs -- maybe I'm nuts, but here's a simple 1-sensor controller that would cost maybe a couple of dollars in parts (beyond the cost of a thermistor, which a quick Digikey check says is $3-5).

(Wow, this got long. Summary: make a couple of voltage dividers, one with a thermistor and another with a pot for reference; feed it into a comparator; use that to control the relay.)

You can set this up with whatever power supply you want, say 5-12V. A 9V battery would work easily, or just grab and wire-strip a wall wart. For this, let's say you need a 5k negative-coefficient thermistor (so its resistance decreases as temp increases), two 5k resistors, a 5k potentiometer, and a standard-issue 741 op-amp.

The first chain you need to make is the thermistor and 5k resistor in series from power to ground -- that's (V+)---(therm.)---(5k)---(GND). Call the voltage where the thermistor and the 5k meet Vtherm. As the temp increases, the thermistor lowers its resistance, and Vtherm gets closer to V+. Temp goes down, thermistor increases resistance, Vtherm goes closer to GND.

The next chain is almost the same: (V+)---(5k ^ pot)---(5k resistor)---(GND). That's the left leg of the pot to V+, right leg to the 5k resistor, middle leg is your output, which will move around like Vtherm, but this time in response to you turning the knob of the pot.

So -- connect Vtherm to the (+) input of the op-amp; connect the middle leg of the pot to the (-) input. With no feedback, you've got a comparator. When Vtherm is higher than your reference voltage (from the pot), the output of the 741 will be V+ (roughly); when it's lower, the ouput is 0. Connect that output to the relay that's controlling your pump, calibrate the thing with a kitchen thermometer, and you're done. High temp = pump on, low temp = pump off.

One caveat -- you'll almost definitely need some hysteresis so that the relay doesn't drive you nuts clacking back and forth when you're near the threshold. The first example given by Maxim might be an easy place to start. Any computer control you write is going to have to do this too. If you want multiple sensors, you could use another couple of op amps to average their inputs and use this same technique to control the pump.
posted by range at 7:57 AM on May 19, 2008

Try it out. You can do it either way – or both – and it depends on which you'd like to try first: putting together a little circuit, or programming the serial port. Have fun! The journey is the reward.
posted by exphysicist345 at 9:43 AM on May 19, 2008

Come on man, just buy a temp controller. They're not that expensive, and you won't have a complicated setup to debug if things go wrong.

Johnson Controls and Ranco are the two brands that I know of. This page sells Johnson.
posted by jclovebrew at 9:43 AM on May 19, 2008

Response by poster: Thanks guys. Ain't no way a 6 gallon carboy is fitting in that tiny fridge.

Plus, it just wouldn't be me unless it involved some elaborate overkill mad-scientist crap.
posted by geodave at 12:43 PM on May 19, 2008

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