Adding a PID controller to an espresso machine

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A PID Controller - what is it?

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A PID controller is a device that allows accurate and stable temperature control of a system. It does this by using a mathematical algorithm, which attempts to predict the response of the system based on parameters that you program into it. It does this using 3 different calculations: Proportional, Integral & Differential. We won't go into detailed descriptions here of how each of these work (there is plenty of very technical information available on the internet), but the most commonly used analogy is that of trying to stop a car by driving up to a line, but only hitting the brakes when you reach that line. You will overshoot. A sensible driver will predict when he needs to start braking to stop at the line, and then he will regulate his braking along the way to ensure he stops at the line smoothly. The sensible driver is basically how a PID works.

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Why Use One?

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Espresso machines (leaving aside HX machines which work differently) normally use a thermostat fitted to the boiler to control the heating element and thus the temperature of the water. These devices have built in hysteresis to stop them from constantly switching i.e. it will switch off at, say, 110°C and turn back on at say 100°C. As a result of this hysteresis the temperature of the boiler (and therefore the brew temperature) can vary by quite a lot. The common method of getting over this is by "temperature surfing", which is running water through the group until the thermostat turns on and then waiting for a set amount to roughly achieve the desired brew temp. This method is wasteful of water, time consuming, and not that consistent.

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The PID controller, as described above, takes this hassle out of the equation by ensuring that the boiler is held at a particular temperature whenever you wish to pull a shot (leaving recovery time aside for the moment). The second advantage of this control method is that you are able to easily change the boiler temperature and, consequently, the brew temperature so you can fine tune your brew temperature, and change it for different bean blend requirements if required. The third advantage is that it looks cool!