A capacitance-to-digital modulator converts capacitance of a differential pressure sensor to a pulse code modulation output signal. The system uses a novel exterior surface mixing tip device wherein oxygen is supplied through a predetermined portion of one supply line, and a predetermined gas is supplied through a predetermined portion of another supply line enveloping the first predetermined portion. The ridge cap vent may be formed from a corrugated plastic material and includes a multiplicity of air passages extending generally transversely to a longitudinal axis of the ridge cap vent. A wheel monitoring unit provides the status of the wheels of the trailer, specifically whether there is rotation of the wheels or not. The resulting oscillation of the membrane serves to maintain the flow rate originally set by the clinician. According to the invention, the dimension of the light-sensitive surfaces of the receptor arranged perpendicular to the orientation of the rows is greater than that oriented along the row direction.