What is a flowmeter?
Flowmeters control the flow of water to the group head, only allowing a predetermined amount to pass through. Basically, they have a star/impeller with a magnet inside, and each rotation of the impeller a defined amount of water is passed through, and once the amount required has passed a signal is sent to the board which cuts the water supply to the group head by closing the three way solenoid valve and stopping the pump, and as such it can deliver exactly 30ml (or any other determined amount). Flowmeters are only used in volumetric commercial coffee machines.