Mechanical thermostat or PID

Water temperature is one of the three parameters that decide an espresso, along with grind and pressure. It is also the only one the user cannot see: you read a number on a display, or you read nothing at all, and you assume the water is at that number. It need not be. How the machine holds temperature matters as much as the value it claims.

What a mechanical thermostat does

A thermostat is a switch that opens and closes according to heat. Below one threshold it turns the element on, above another it turns it off. In between: nothing. No modulation, no anticipation.

The result is an oscillation — temperature overshoots the cut-off point because the element keeps giving off heat after it is switched off, then undershoots the switch-on point for the same reason in reverse. The width of that swing depends on the water mass and on the distance between the two thresholds, and it is why on a thermostat machine the moment you press the button changes the coffee: pressing just after the element has fired is not the same temperature as pressing just before.

On a large boiler, with plenty of water and plenty of inertia, the oscillation is slow and wide but predictable: a café operator learns to read it. On a small boiler it is quicker and harder to anticipate.

What a PID does

PID stands for proportional-integral-derivative, the three ways the controller looks at the error between measured and target temperature: how big it is now (proportional), how much has accumulated over time (integral), how fast it is changing (derivative). From those three readings it decides not whether to switch the element on, but for what fraction of the time to hold it on.

In practice the element no longer works in jolts but in doses: it eases off before reaching the target instead of overshooting and then chasing. Temperature does not oscillate around a value: it settles on it.

The second effect, less often told and just as important, is that the value becomes settable. On a thermostat machine the temperature is whatever the manufacturer calibrated; with a PID you choose it, and you can change it for a lightly roasted single origin as against a dark blend.

Where the difference is felt, and where it is not

It is felt when the variable you are chasing is small: single origins, light roasts that ask for higher temperatures, the search for repeatability from one morning to the next. With those coffees two degrees change the cup perceptibly.

It is felt less if you drink a dark blend, in a small cup, with sugar. Not because temperature does not matter, but because the margin of error that coffee tolerates is wider than the margin a PID recovers.

And it is not felt at all if the machine is unstable for other reasons: a PID does not compensate for a group that never came up to temperature, nor for water flowing too fast.

What our catalogue says

As of 7 September 2026 the portafilter machines that declare the data number 54, and 51 of them have PID control.

That number does not say PID is now everywhere. It says PID is everywhere in the segment this catalogue covers, which is the prosumer and professional one: mass-market machines, where the mechanical thermostat is still the norm, barely appear here. Read as a market statistic it would be false. Read as "if you are looking at this range, PID is the expectation and its absence is the thing that needs explaining", it is exact.

On the product pages the data appears as a filterable attribute: you can narrow the catalogue to the machines that declare it, and on each one you find the manufacturer page the value was taken from.