The purpose of this article is to educate you regarding the different type of temperature controls found on different espresso machines. There are three types of regulators found on pump driven espresso machines.
First and least expensive (roughly $9.95 to $14.95 in US dollars), thermostats measure the surface temperature of the boiler. These thermostats typically are found in espresso machines that have a single boiler (some inexpensive higher end espresso machines also have thermostats) or on home model coffee makers. The way thermostats work is allowing electrical current to pass through to the heating element until the desired temperature is reached. Although the electrical current is severed when the maximum temperatre is reached at the surface of the boiler, the heating element remains to heat the water as full current was delivered to the heating element right up to the point of cutoff. Thermostats can not be adjusted to change the temperature of the water in the boiler. In addition, most machines will usually have 2-3 thermostats - one for coffee temperature regulation, one for steam temperature regulation, and/or one for a safety cut-off.
Examples of brands with traditional home machines that have thermostats include: Rancilio, FrancisFrancis!, Saeco, La Pavoni, Gaggia, Capresso, Isomac, La Pavoni, Saeco, and Solis to name a few.... In the picture, the thermostat has a small, blue plastic cover housed over an aluminum base. There are two electrical contacts opposite of each other.
The second type of temperature regulator is a pressurstat. The presurestat doesn't actually regulate the temperature of the boiler. It regulates the steam pressure inside the boiler. The pressure is directly related to the temperature. As the pressure of the boiler increases, the temperature increases. Vice versa, as well. Typically, the pressure regulator can be adjusted to increase or decrease the pressure. This is performed by changing the adjustment screw usually located on top center of the pressurstat.
There are two versions of pressurstats. There is a smaller, semi-commercial version and the full-size commercial version. The beauty of pressurstats is that they can be adjusted so that the steam pressure and water temperature "both" can be increased or decreased.
The full-size commercial version usually has three sets of electrical contacts. If one contact does not work, the wiring could be moved over to the next set of contacts. Carbon build-up, which occurs on the contacts of pressurstats, can be cleaned with a metal wire brush on the full-size commercial versions. However, with the smaller, semi-commercial pressurstats, the carbon build-up usually can not be removed. Therefore, the smaller pressurstats need to be replaced at an approximate cost of $45-$50. The full-size pressurstat replacements range in price between $70 and $100.
The semi-commercial version can be found on semi-commercial machines, such as the Pasquini Livia 90, the Isomac semi-commercial line, Elektra a leva and semiautomatica lines, La Pavoni lever models, Nuova Simonelli Oscar Professional, and ECM Giotto.
In the picture, there are two types of semi-commercial pressurstats - the newer version is blue and yellow with three contacts on top and the older version is black and gold (or tan and gold) with three contacts on the side.
The commercial version can be had on the La Valentina, Rancilio S24, and the Grimac La Uno. In the picture, the full-size commercial pressurstat has a black plastic housing that can be removed from the base. The adjustment screw is lcoated on top under the cover.
The third type of temperature regulator conforms to a new category where there is a) a probe that directly measures the temperature of the water in the boiler or b) a sensor (thermocouple) that monitors the temperature of the boiler which indirectly measures the temperature of the water. This type of regulation maintains a high degree of accuracy as the controller regulates the electrical current that is dispersed to the heating element. In other words, as the temperature of the water gets closer to the optimal temperature set by the operator, the controller will reduce the amount of electrical current sent to the heating element. The end result is a water temperature that "slides" into the optimal temperature.
Home models that include an electronic sensor are the Solis SL90 and the superautomatic espresso machines manufactured by Solis, Saeco, Spidem, Capresso, and Jura.
With the above understanding, home espresso machines enthusiasts have devised methodologies and hardware changes to acquire improved performance for delivering the perfect espresso extraction - no matter which regulator above is found inside the machine. Methodologies have included "Cheating Miss Silvia" or "Temperature Surfing". Hardware changes have included PID'ing espresso machines so that the exact degree can be achieved in the quest for the perfect espresso!
Now, the real question is why do care about temperature/pressure regulation when extracting espresso. The main reason is that temperature stability is paramount when relentlessly pursuing the perfect espresso shot. And, temperature regulation is one of the several (not the only)important factors that provide that stability.