Temperature probe arrived

Finally, got hold of a temperature probe (PT100 from eBay).

Most conductive materials have properties that change with temperature, materials whose resistance changes reliably with temperature are used as thermistors. PT100 devices uses platinum as that material, and has a very linear response with temperature compared with other devices.

More importantly though, I bought this PT100 probe simply because it’s housed in a nice stainless steel probe!

PT100 will be connected like this:

With the equation:

Vout = Vin R2 / (R1RTD + R2)

The resistance of a PT 100 is given in this PT100 chart, the temperature-to-resistance profile isn’t exactly linear, so the code in the microcontroller must make a conversion

Although as a demonstration, this chart shows the error when trying to linearise the PT100 temperature response curve

Now my PT100 is terminated with three wires, this is to reduce the errors introduced by the resistance of the connecting wires. For example, if it only had one wire, the equivalent circuit would look like this:

The effective resistance of the RTD (resistive temperature device) is therefore the resistance of the RTD plus the resistance of the two connecting wires, skewing the results a bit.

But with three wires connected like this:

The effective circuit looks like:

Note now that the resistance introduced by the wire is symmetrical now, and so does not skew the results as much.

Also, four-wire probes are available:

Equivalent:

WordPress won’t let me upload zip files, so I’ll put the code up on GitHub at some point


Got myself a mini fridge

Purchased a cheap mini fridge for converting into a sous-vide cooker. I hear these things are pretty rubbish at cooling your beer with, but from experience, peltier devices are much better at heating duty.

Not much space inside, but that’s a good thing – less thermal mass to worry about, and as long as it fits a steak or two, all good.

The thing has a three way switch to enable cooling or heating mode, that’s good, just what we need for sous vide

As an engineer, the urge to take this thing apart to see how it works is overwhelming.

So the insides look pretty standard, but they are EXACTLY what is needed.

At the top there is a heatsink, under which must be the peltier device.

On the right is the yellow mains AC to 12V DC board, with its rectification and smoothing and regulation circuits. Two wires come off this board carrying 12V DC and GND to the green switch board on the left.

This green switch board on the left does two things – switch between mains power and 12V DC power from the external socket, and switch between heat and cool mode. It does this by simply reversing the polarity of the peltier device – feeding in 12V one way causes the inside of the fridge to cool down (and the back to heat up), and feeding in 12V in the other direction causes the inside of the fridge to heat up (and the back to cool down).

So it looks like all I need to do is to add a transistor in there to control the power to the green board, and add some temperature sensors.


Sous Vide

I’ve been looking at doing some sous vide cooking, but all the sous-vide equipment is so expensive out there.  I’ve looked at some DIY projects out there, but it seems to me that there aren’t really any decent ones out there that require minimal electronic components.

I’m going to go out and buy a cheap mini-fridge lying, and I have a few old prototype micro-controller development boards that I thought I’d use up (these guys: http://www.universalair.co.uk/forebrain).  The conclusion is obvious.

Microcontroller + mini-fridge = Sous Vide cooke

This blog shall chronicle these exploits