US Patent No.  4,441,357  April 10, 1984

A closed pressure monitoring system is disclosed in which a pump delivers gas on a supply line through a first accumulator chamber to a load device and returns the gas on a return line through a second accumulator chamber to the pump. After pressures in the first and second chambers stabilize, the pressures are compared, and a leak warning signal is given if the compared pressures changed with respect to one another over time. The system is particularly adapted to use a pressure sensor as the load which is implanted within a human patient. The pressure sensor receives air from a restrictor in the supply line and is operative to maintain the pressure in the supply line substantially equal to the pressure surrounding the sensor.