a respiratory tract differential - pressure flow sensor , including a tube having a distal end that is insertable into a respiratory tract , an interfering body disposed within the tube proximal to the distal end and having a first edge facing one end of the tube and a second edge facing away from the end , the interfering body extending across the diameter of the tube , a first pressure sensing port operative to sense an air pressure , the first port being disposed in the first edge not abutting the wall of the tube , and a second pressure sensing port operative to sense an air pressure , the second port being disposed in the second edge not abutting the wall of the tube , where either of the edges is inclined inwardly towards the axis of the interfering body extending across the diameter of the tube .