Pressure is defined as force divided by area. Consider trying to cut into a ripe tomato with a butter knife compared to a sharp knife: with the same amount of force, the sharp knife will cut more easily, as the force is concentrated into a smaller area, which means it exerts a higher pressure.
A bar is a unit of pressure, equal to 100 kilopascals, or approximately 14.5 pounds per square inch (psi). One bar is very close to standard atmospheric pressure (1.01325 bar). To get an idea of what this amount of pressure feels like, it’s approximately equivalent to the force of a 1-kilogram weight pressing down on every square centimetre.
This means that inside a steam boiler set at 1 to 1.5 bar, every square centimetre (cm2) is subject to a force equivalent to 1 to 1.5 kilograms. A 10-litre steam boiler in a commercial espresso machine will thus need to be able to withstand a force equivalent to the weight of 3 or 4 tonnes.
Meanwhile, the cross-sectional area of a puck is 26.4 cm2, so 9 bars of pressure pushing down on the puck is equivalent to the force exerted by 245 kg of weight — the weight of three espresso machines.
The pressure gauges in an espresso machine show gauge pressure — how far the pressure is above atmospheric pressure. In other words, a reading of zero on the gauge means that the water inside is at atmospheric pressure. Absolute pressure, on the other hand, measures how high the pressure is compared to a vacuum. So a pump pressure of 9 bars is approximately the same as an absolute pressure of 10 bars.
The unit for gauge pressure is sometimes written as barg (short for bar gauge), but in keeping with general practice in the coffee industry we refer to pressure in bars and assume it’s understood that this means gauge pressure.