Screw-type flow meters belong to the group of positive displacement flow meters. Two measuring spindles (pos. 2) with a screw-shaped profile engage with each other. They are low-friction mounted using roller contact bearings (pos. 3) and enclosed by a housing (pos. 1). The liquid flow causes the spindles to rotate and passes through the device in an axial direction. In so doing, self-contained part volumes are formed which are continuously filled and emptied. This principle of measurement causes no pressure and/or flow pulsations. A sensing wheel attached to the measuring spindle (pos. 4) is scanned by two non-contact sensors. Using two sensors, a bidirectional flow with detection of flow-direction is possible. The inflow and outflow occurs almost without deflection. Consequently, the device exhibits a relatively low drop in pressure. Measuring this way does not require the flow to be calmed at the inlet and outlet. All moving parts are lubricated by the measurement medium.