Same lesson, with the figures in motion.
Build a Fermentation Chamber
Why a fermentation chamber?
1. The yeast makes its own heat
Fermentation generates its own heat. During the first day, when the yeast works hardest, the beer becomes several degrees warmer than the room — and the room itself swings between day and night. That is exactly when the flavour is set: too warm gives fusel alcohols and more fruit than you asked for.
In a fermentation chamber the yeast works just as hard, but the heat is carried away and the beer stays where you set it. In the graph, the orange line is the fermenter in the room and the blue one is the fermenter in the chamber.
2. Choose the box
A second-hand fridge or chest freezer is plenty — it doesn't need to look good, just be intact and seal properly. Measure the inside height against your fermenter with the airlock fitted, and watch out for the compressor step at the bottom and the door shelves that eat into the depth.
A fridge is the easiest to load. A chest freezer often holds two fermenters and loses less cold when you open it, but you lift 25 kilos over the edge — and one day it can become a keezer for kegged beer.
3. Give the box a brain
A fridge is built to keep milk cold, not to hold 19 °C. Its thermostat rarely goes above about 8 °C, and it can't heat at all — a chest freezer only wants to head for −18 °C. Plug the box in as it is and it will cool your beer far below the target.
That is why you need an external controller. It measures the beer and switches the power to the box on and off through a switched socket: cooling when the beer is too warm, resting when it is right. In the picture it is a YeastMaster landing on top, and its screen shows what it has decided. A simpler thermostat with one probe does the basic job.
4. Add some heat
As well as cooling, the beer sometimes needs warming: in a cold garage, in winter, and when you raise the temperature towards the end for a diacetyl rest. So the controller can also switch on a heater, letting the cooling rest in the meantime. It doesn't take much — 20–60 watts is enough. Two common choices are a tube heater, the kind that keeps frost out of greenhouses and cellars, and a heat mat of the type that warms reptiles in terrariums. Both are sold in DIY and pet shops. Warm the air rather than the fermenter; if the mat sits right against the fermenter, put the beer probe on the opposite side.
Cooling and heating each get their own switched socket. The controller only switches the power on and off, so you never have to open up the fridge's electronics.
5. Place the probes
It is the beer you control, not the air. The best way to measure it is with a thermowell: a thin stainless steel tube that goes down through the lid and stands in the beer, with the probe inside. Without a thermowell you can tape the probe to the fermenter at beer level with a piece of insulation over it — foam or folded bubble wrap — otherwise it measures the air brushing past.
The air probe hangs freely in the chamber, not against the wall and not close to the heat mat. The air swings quickly and the beer slowly, so a controller that sees both can ease off before the beer has time to follow.
6. Turn the fridge to max
The fridge keeps its own thermostat, and it still decides when the compressor may run. With the dial in the middle it cuts out at around 5 °C — so a cold crash never gets down to 2 °C, however much the controller asks for cooling. Turn the dial to the coldest setting and let the controller decide.
If the cooling takes a few minutes to kick in, nothing is wrong: the compressor needs to rest between cycles, and YeastMaster gives it at least five. Wipe out the chamber between batches and leave the door ajar, or mould will thrive in the damp.
Want to feel what it's like to be the controller – in the game Fermentation Chamber?
Play →