TUTORIAL
Controlling temperature in the kitchen
emperature control is hugely
important in working with food.
Sometimes we want to heat it to a high
temperature, and for this we have ovens
and microwaves. Sometimes we want to
chill it, and for this we have fridges and
freezers. However, sometimes we want to warm it a
little, and for this there are fewer options. For example,
you might want to warm bread dough to help it rise, or
keep milk at the right temperature for yoghurt to form.
To help with this, we’re going to build an insulated,
heat-controllable box.
The mechanics of a heat-controllable box are pretty
simple. We need something to heat it, something to
sense the temperature, and something to turn the
heater on or off, depending on what it reads from the
sensor. There are lots of options for all of these and, in
many ways, there’s not a ‘best’ option.T
Ben Everard
@ben_everardBen loves cutting stuff,
any stuff. There’s no
longer a shelf to store
these tools on (it’s now
two shelves), and the
door’s in danger.Reuse waste plastic to make a fermenting box
Controlling temperature
in the kitchen
Above
Insulating our box with shredded plastic
packaging – just make sure it’s all cleanThe first real decision is what voltage to run your
system at. Many off-the-shelf heating components are
designed to run at mains voltages. For example, some
people use filament light bulbs. However, we wanted
to run ours at a lower voltage, so we didn’t have to
worry too much about the electrical safety. However,
if you go too low in voltages, you need a high current
to get enough power to provide enough heat for your
box, and that introduces its own problems. We went
with a 12 V system.
For heating, we went with a 12 W silicon heating
pad. These are available cheaply from a variety of
sources (ours cost £2.78, including delivery).
For temperature and control, we could have used
more or less any microcontroller and programmed our
own interface, but we wanted to make this project
super-simple, so opted for an off-the-shelf temperature
control unit. This has a temperature probe, a display,
and a relay. You can set a temperature you want it to
hold, and it will turn the relay on and off to help you
achieve this. You just have to wire the heating pad into
the relay.THERMAL CUT-OFF
Our temperature controller claims to have a built-in
thermal cut-off, but it’s still relying on a single unit
to ensure that it never overheats, and we don’t have
a good idea of how reliable this unit is yet. The risk
is that the unit somehow jams on and continues to
push power into the heat pad, even when it’s too
hot. Whether or not this is a problem depends on
how hot your unit can get with the heating element
permanently on. It’s a good idea to test this scenario
out by putting your heat pad on permanently while you
monitor it for temperature to see what the risk is.
If you’re concerned about it getting to a dangerous
temperature, you may want to consider putting a
separate heat monitoring unit in there that can cut
off the supply to the first unit. This could be as simple
as another identical temperature controller that’s set
to the same temperature with both relays rigged in
series. That way, as long as one of the two units is
working properly, it will cut off the power once it gets
above the predefined temperature.