No matter what method the espresso machine uses to control the temperature, it’s important to be able to measure the exit temperature accurately in order to be able to calibrate it. Multi-boiler machines will usually be accurately calibrated by the manufacturers, but if you want to check or adjust the calibration yourself, you’ll need special equipment to measure the temperature correctly.
The reason for this is that in most machines the exit temperature will be affected by the flow rate. Machines are designed to maintain a steady temperature during brewing, when the water flows relatively slowly. If cold water is being pumped into the boiler more quickly than this, the exit temperature may drop. Alternatively, higher flow could result in more superheated water from the heat exchanger reaching the group, and the exit temperature may go up. The hot water will also lose heat to the atmosphere very quickly if it is not contained in some way.
The ideal setup for measuring the temperature is a modified portafilter that simulates brewing a shot. It should leave approximately the same amount of headspace that a coffee disco de hockey would, to simulate the initial flow from the group, but then control the flow rate, with some sort of flow restrictor, to approximate the flow rate in brewing. A thermocouple (an accurate and fast digital thermometer) can then measure the temperature of the water passing through the portafilter. The best-known example is the ‘Scace’, invented by and named after Gregory Scace. Here are Scace’s instructions on how to safely use the thermfilter.
When measuring the exit temperature, it’s also important to replicate actual brewing conditions as far as possible — for example, by controlling the length and timing of flushes between shots. World Coffee events have established a protocol for measuring temperature that takes this into account.
In this video Andy Schecter — the originator of the use of PID technology in espresso machines —