Abstract: The monitor employs a sensing piston whose position is balanced by offsetting the pressure to be monitored on one side of the piston by a reference pressure of approximately equal magnitude on the other side of the piston. A balance valve on the sensing piston controls the rate of flow of the reference gas from a first chamber bounded on one side by the sensing piston, into a second chamber bounded on one side by a signal piston having a striker whose surface is positioned proximate to the outlet of a gas jet tube. The gas jet tube is connected to a sensing gas pressure source and pressure detector and will produce a pneumatic signal when the striker surface of the signal piston blocks the outlet of the jet tube.