A method and apparatus detect occlusion in a tube to a passage into a body cavity. An argon gas supply output received by plural selectable flow orifices so one or more of the orifices delivers predetermined metered gas. A duct with a volume of gas selectively provides metered gas to the passage. The tube connects the passage and the duct periodically so a duct pressure transducer can signal. A circuit tests signals of tube equalization with a back pressure. A three way valve controlled by the circuit connects the duct transducer to gas metered to the cavity. The valve alternatively connects the duct transducer to the tube to equalize the tube with metered gas to the back pressure. A monitor of the signals determines the rate of pressure change after the valve connection. The monitor indicates pressure equalization after a timed interval with a timer to establish intervals for checking back pressure. The monitor remembers cavity pressure before connecting the duct transducer to the metered gas and compares remembered pressure with the back pressure after each test interval. Plural orifices provide specific gas flows to the handset and the valve for equalizing them with metered gas as a function of the flow. Connecting plural orifices to meter gas output, delivering the metered gas output with one or more of the orifices, containing the volume of gas in the duct connected to the metered gas output, selectively connecting the duct to the passage, connecting the tube to the passage with the duct and the transducer to periodically signal are steps. Coupling the circuit to one or more orifices, equalizing the tube with back pressure in response to the signals and coupling the transducer to the metered gas output are steps. Connecting the duct transducer to the tube permits periodic back pressure equalization with metered gas output, signaling the monitor with the transducer, determining periodically the rate of pressure change in the tube by the transducer, indicating in the interval with the monitor equalized tube pressure and remembering cavity pressure before connecting the metered gas output and the transducer for intervals and providing specific flows to the handset and the valve to make the back pressure a function of the selectable flow are steps.