As I’ve noted before (here), I am intrigued with sous vide cooking at the moment. But, to do it properly you need a precision water bath, and they are pretty expensive (like over £1000) … more than I can justify on playing by about an order of magnitude. However, it is also possible to use something like a slow cooker, but with a much more precise temperature controller … and you can buy units to do exactly this – for example, from Auber instruments you can get a precision controller for just over $200 shipped (here). The right answer for me would simply be to buy one of these, but I was interested in just how simple this could be, since you can buy precision temperature control modules for about £25 (look for PID temperature controller on ebay), and the solid state relay to control the power is only about £12.
As projects go, this is close to the level of lego bricks. The only bit that took any time was cutting holes in the case. In the photo the temperature sensor is a thermocouple, but I’ve replaced this with a platinum sensor that offers more precision (tenths of a degree, not just degrees … and I didn’t need a range going up over 2000 centigrade!). I used a 40 amp solid state relay which is massively over-rated, but I thought I might one day want to control an oven, and this would allow it. It needed no calibration, with a check in a ice water bath reading between -0.1 and 0.1 degrees Centigrade.
As a first long time period cooking trial, I thought I’d go for a rack of lamb. First step is to seal the bags :-
As a water bath I am using a slow cooker. I first ran it without any food in to auto-tune the temperature controller. This allows it to measure the response of the controlled device, and so hold the temperature much more closely than it would otherwise do. Then I put the lamb in, with a heavy metal clip to ensure that if any gas did form then it didn’t leave the bags above water (reminder – this is long cooking at low temperatures, so you need to take special care to avoid issues with bacteria) :-
Then, lid on and a 24 hour wait whilst the controller holds the water at 56 centigrade. It is holding in a band between 55.8 and 56.2 from periodic checks, though photo shows it at 55.6 as it heated the lamb up :-
At lunchtime the next day, I removed the lamb, and browned it. It did plan to use a gas torch, but as a good example that I need to clean the garage, couldn’t actually find it … so, I did it in a pan instead, which is a little tricky for something as awkwardly shaped as a rack of lamb. The end result was superb, albeit probably a little rarer than we might go for next time. It tasted absolutely excellent, even without any sauce. It was also very clear how little shrinkage there was – as the cooking temperature was much lower, the muscle fibres hadn’t contracted nearly as much.
All in all a very pleasing test, and soooo much easier than the first trial that held the temperature by hand adjustment. I also realise that I can use this for a few other temperature control applications like precision deep fat frying and plant incubation.