How does an espresso machine flow meter work? This is a brief (and non-technical) description of how an espresso machine flow meter works. We use this device to regulate the volume of water that passes through the group head and it is a very important part of the control system. You usually find a flow meter per group head and checking their integrity forms part of an annual service. When they fail, you will notice irregular shot volumes from your machine. Should anything get stuck in one or the