Introduction: Sous-vide Cooker / Yoghurt Incubator
This sous-vide cooker was inspired by Burke LaShell's Sous-vide cooker for less than $40, and some of the comments under it. It's sufficiently different that I thought it deserved its own Instructable. And having just stumbled on GeeDeeKay's Greek Yoghurt From Scratch, it occurs to me that this would be ideal for incubating the yoghurt culture overnight too!
Here's what you need (with links for buying them in the UK):
- cooler: £19.99 if you don't already have one
- 4mm acrylic sheet big enough to cover your cooler: ~£14
- immersion heater: £19.95
- digital temperature controller with thermocouple: £10.18
- project box: £3.75 from Maplin
- 4W submersible aquarium pump: £5.99
- Sugru: £12.99. 2 x 5g blue + 1 x 5g white to make colour-matched corner blobs that help the acrylic sheet to sit snugly on top of the cooler; and 1 x 5g white to hold the cup boiler in the hole in the acrylic sheet.
I also bought 4 long self-tapping screws, of the same diameter as those supplied with the project box, but a couple of inches longer, which I used to fix the box to the acrylic sheet.
Total cost: ~£80, including the cooler itself.
Step 1: Physical Assembly
Put the acrylic sheet on top of the cooler, draw round the top using a Sharpie, then use a jigsaw to cut out the shape.
Cut a slot in the side of the project box so you can snugly slot in the temperature controller. I used a hacksaw and a Stanley knife.
Use a 36mm hole saw to make the hole for the Cup Boiler.
UPDATE: after a few months of use, the Cup Boiler burned itself out, so I replaced it with a different immersion heater. This one can drop right through the 36mm hole, so I can now run the cooker with less than a full tank of water when required.
Drill four small holes for the long self-tapping screws which will fix the project box to the acrylic sheet.
Put four big blobs of Sugru near the corners of your acrylic sheet to hold it snugly on top of your cooler. Leave it all for 24 hours for the Sugru to cure.
Step 2: Electrics
The pump should be connected directly to the incoming mains so that it runs continuously, while the Cup Boiler should be controlled by the thermostat. The thermocouple needs connecting to the relevant terminals on the temperature controller too.
Step 3: Setting Up the Temperature Controller
The instructions supplied with the unit I bought were a little cryptic due to their hasty translation from Chinese, but the gist is as follows:
- the unit can control either a heater or a cooling unit; in our case it's connected to a heater, so you need to select 'H', not 'C'.
- you can also set how far the temperature is allowed to wander up or down before the heater is turned on or off; I set both of these to 1ºC
Step 4: Using the Cooker/incubator
In practice, the 300W heater is only sufficient for maintaining the temperature of the water bath, not for heating it up from cold. You'll need to part-fill it will cold water, then boil the kettle and/or heat some pans of water to get it close to the right temperature to start with.
The water level needs to be quite high too, almost to the top of the cooler, to ensure that the water heater is actually immersed in the water.
Obviously the pump should dangle in the water while you're cooking, so that the water circulates and doesn't develop hot spots. The thermocouple should be in the water too, so the controller knows when to turn the heater on and off.
As for the actual sous-vide cooking techniques, start with this basic guide to sous-vide, then google around, ask people, and experiment. And enjoy!
Step 5: Limitations of the Design
- Safety: there's no thermal cut out feature, so it's possible for the cup boiler to overhead if it isn't sufficiently immersed in the water.
- Safety: the immersible pump almost certainly isn't rated to run in water at 60ºC, so it will probably give up at some point, and may trip something on the fuseboard at the same time.
- Size: it's big and bulky, and needs 28 litres of hot water to get started! You could certainly do the same thing in a smaller cooler though.
- Insulation: although it works very well for steaks (~55ºC) and for chicken (60ºC), it just can't maintain the water at 85ºC (for potatoes and other veg).
- Accuracy: commercial sous-vide cookers typically maintain the temperature to ±0.1ºC, but the temperature controller I mention above only does ±1ºC. I'm not sure it makes all that much difference to be honest, but purists may tell you otherwise.
So there you go — a perfectly functional sous-vide cooker (also yoghurt incubator!) for around £80.
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