An apparatus (10) which measures and monitors the pressure within a human body includes a pressure sensor (11) having a housing (30) closed by a pressure sensitive diaphragm (34) to define a plenum (35), an exhaust tube (36) mounted within the plenum (35) which terminates adjacent the inner surface of the diaphragm (34), and tubes (13, 15) which transmit a substantially constant flow of gas into the plenum and which withdraw gas from the exhaust tube (36) to a location remote from the body. Changes in pressure within the body cause the diaphragm (34) to alternately close and open the end of the exhaust tube (36), thereby resulting in an increase or decrease in the pressure within the plenum until an equilibrium pressure is reached. A source (12) of gas pressure is provided, as well as a means for providing a substantially uniform flow rate (R) and means for measuring the pressure (14) within the tube (13) that transmits the gas flow to the sensor (11). The flow rates to and from the sensor (11) are compared, and the flow of gas to the sensor (11) is cut off if the flow rates are unequal, a condition indicative of a leak in the sensor (11) or in the tubing (13, 15, 17) that transmits the gas to and from the sensor.