Abstract: A respiratory air or gas flow volume indicating instrument comprises a tubular stator within a casing which forms an annular gallery around the stator. Oblique slots are formed through the stator so that air or gas flow through them forms a spirally swirling flow which drives a rotor. A barrier constrains air or gas flow in one direction round the gallery. The length of each slot is increased progressively in the direction of flow around the stator (20) from a radial port to the barrier. The slots open at one end of the stator and have sloping side and end faces so that the aperture at the radially inner end of each slot is smaller than the aperture at the radially outer end of that slot.