A flowmeter has an inlet body including a flow detector and an outlet body including a flow adjuster. The inlet body is fixed to the outlet body and forms a single piece assembly comprising an inlet tube and a measurement tube. The flow adjuster includes a disk turned by a knob. The disk has holes so at least one hole always faces the outlet tube, regardless of the position of the disk, to allow flow distribution without the risk of interrupting it even while flow is adjusted, and enable gradual flow adjustment.