Volumetric
Volumetric espresso machines are also called automatic espresso machines. The volume of water passing the hyraulic system for the espresso shot is measured by volume. After reaching the pre-defined volume, like one ounce or two ounces or whatsoever is defined in the settings, the machine stops the extraction by itself. Technically there is a litte device with a propeller in the inbound water flow, giving electronic information to the brain of the machine about the flow.