A method of safety monitoring a respirator which is connected to a patient through an endotracheal tube which has a respiratory gas jet nozzle delivering respiratory gas thereto and a measuring tube opening into the tube, comprising supplying at least one jet nozzle with a respiratory gas discharging into the tube, directing a fluid through the measuring tube at a substantially constant flow rate so that the variations in the pressure in the endotracheal tube will vary the pressure in the measuring tube, sensing the pressure in the measuring tube and controlling the flow through the jet nozzle in accordance with the pressure in the measuring tube, and wherein the flow of fluid in the measuring tube is set sufficiently low that in the event of a negligible pressure drop in the tube the pressure measured in the connection of the measuring tube at least approximately corresponds to the ambient pressure in the opening of the measuring tube.
