Espresso! My Espresso!
How A Pressurestat Works
by Randy Glass - Copyright 2009 - All rights reserved
E-mail me at firstname.lastname@example.org
One function of an espresso machine is to deliver water at a predetermined temperature to the coffee, or at least to deliver it within an acceptable range. The trick is for the manufacturer to find a way to do that. Some do it quite well, and others not so much. With lesser machines a simple button thermostat is screwed to the top of the boiler and it measures the temperature of the metal surface. At the other end of the spectrum are complicated, proprietary digital controls, custom designed for that specific machine. Most commercial machines, and most having heat exchangers have a device called a pressurestat. The word itself describes their design- a pressure-operated thermostat... Well, technically, they do not measure "thermo" (temperature) at all, but do a good job of controlling it. On its simplest level, it is merely a switch that turns the heating element on and off. Let's take a look at the how and why....
Here's a diagramatic look at a boiler in an espresso machine:
The espresso machine boiler, while it is at idle, just sitting there and on, is a sealed container. Simplified here, you see the major elements that we are going to discuss. The metal container (the boiler) with some water in it, the heating element submerged in teh water, steam in the upper portion of the boiler, and the pressurestat on top of the boiler.
If the heating element is energized it will heat the water and some of the water will turn to steam. As more water turns to steam the pressure inside of the boiler will increase. It the heating element is left on the pressure will continue to rise until something happens. If left unchecked the boiler will explode with deadly force (NOTE: No need to worry. All boilers are equipped with a pressure relief device that in the case of too high pressure they open before the machine is damaged.). If the heating element is left off then the boiler will eventually return to room temperature. Obviously, we want something between room temperature and explosion— that's the job of the pressurestat.
Here is a simplified, diagramatic view of a pressurestat. There is some sort of diaphragm that can move (the green line) stretched inside the pressurestat. There are also electrical contacts as are found inside most switches.
As pressure builds inside the boiler (indicated by the red arrow), the diaphragm moves and an electrical circuit is closed as the metal switch contacts are connected. This signal goes to the control box (or directly to the heating element) telling the machine to de-energize the heating element. (NOTE: This is a very simplified view to illustrate the concept of how a pressurestat works. For example, there can be other designs, some with two sets of contacts, one below and one above with one set turning on and one set turning off the heating element depending on the movement of the diaphragm.)
Whatever the design of the pressurestat, it operates based on the principle that in a sealed chamber, when a fixed volume of gas (in this case, steam) is heated to a specific pressure the temperature will rise a predictable amount. That explanation is sufficient for an art major (like me) but the science majors out there will see the minor flaws. Still, the important point to remember is that the pressurestat reacts to changes in pressure and these pressure levels are used to indirectly control temperature.
Adjusting a pressurestat will effectively set the contacts so that a lower or higher pressure will close the contacts. This normally accomplished by a screw located on top of the pressurestat as seen above.
All pressurestats have a deadband. That is the number of degrees between when the pressurestat turns on and when it turns off. If too large there will be a wide range of temperatures reached in the boiler and brew quality will be compromised. If too narrow, the heating element will go on and off too often and the electrical contacts inside the pressurestat will not last very long.
The positives of using a pressurestat is that they are relatively dependable, quite accurate, and easy to diagnose and change if there is a problem. They are also quite simple and inexpensive when compared to more complicated electronic devices or systems. The negatives are that they use electrical contacts that will wear out eventually and the moving diaphragm is subject to mineral deposits from the water that can eventually cause it to operate erratically or inaccurately.
But what about that deadband— that swing of temperature range that takes place between the on and off cycle? That is not important because the espresso machines that I know of that use a pressurestat also have a Heat Exchanger, discussed HERE.