For many years, espresso machines operated on the same electro/mechanical principals. The boiler temperature is controlled by a mechanical pressurestat. When the pressure rises too high, electrical current is shut off to the heating element. When the pressure falls far enough, electrical contact is mechanically reestablished. This is old-school thinking. It works about as well as it did 50 years ago; Lots of temperature fluctuation.
21st century electronic devices are ideally temperature controlled by computer. A PID Controller (proportional–integral–derivative controller) takes temperature readings constantly & commands exacting adjustments in order to maintain precise and constant temperature control.
My Bunn ES-1A Espresso machine is a small professional model. It is a heat exchanger machine which was built in 1993. This past week, I converted it from “old school” to “new school” by disengaging the analog pressurestat and adding a PID controller in its place. By virtue of rock-solid temperature control, my espresso making should become more consistent, and hopefully better. Additionally, the pressurestat is a noisy device, clicking & clacking throughout the day as it makes & breaks electrical contact. The PID is virtually silent. A nice benefit for a home setting.