Advanced Espresso

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Æ 6.01 – What Does 9 Bars Mean?

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.