Many coffee machines use pressure to enhance brewing parameters. Some of the devices that use pressure are single serve coffee machines, such as Keurig, or Tassimo, and espresso machines. Other devices are the Moka pot, or steam espresso makers. Pressure is measured in bar.
The bar is a non-SI unit of pressure, defined by the IUPAC as exactly equal to 100,000 Pa. It is about equal to the atmospheric pressure on Earth at sea level, and since 1982 the IUPAC has recommended that the standard for atmospheric pressure should be harmonized to 100,000 Pa = 1 bar ≈ 750.0616827 Torr. The same definition is used in the compressor and the pneumatic tool industries (ISO 2787).
Commercial espresso machines brew using the most pressure of any coffee maker. A standard commercial espresso machine will operate using nine bar pressure. Nine bar pressure translates to nine times the amount of pressure at sea level. A standard domestic espresso machine, a single serve coffee maker or a moka pot, on the other hand, will normally only have 1 or 2 bars of brewing pressure although good quality domestic espresso machines can be bought with higher pressure.
In the past, pressure was generated in espresso machines by using pistons. Nowadays, pressure is generated in commercial espresso machines with a motor driven pump. The pressure in cheaper domestic machines is generated by steam, which is a much more affordable but less effective way of producing pressure.