Abstract: A closed loop breathing system includes a face mask, a pressurized bottle of oxygen-rich breathing gas, and an exhalation hose and an inhalation hose both coupled to the face mask. Flexible breathing bags smoothen the flow of expired gas through the system, and a container of sorbent material removes CO.sub.2 from the expired gas. A mixing valve mixes expired breathing gas from the face mask with oxygen-rich breathing gas from the pressurized container. A balanced exhalation valve assembly coupled to the exhalation hose includes a main valve which passes the flow of expired gas from the exhalation hose into a chamber and prevents the reverse flow of gas from the chamber into the exhalation hose. A flexible diaphragm forms one wall of the chamber and is coupled to the main valve by a rigid spacer to prevent a pressure increase in the chamber from increasing the force necessary to open the main valve.

Abstract: A closed loop breathing system includes a face mask, a pressurized bottle of oxygen-rich breathing gas, and an exhalation hose and an inhalation hose both coupled to the face mask. Flexible breathing bags smoothen the flow of expired gas through the system, and a container of sorbent material removes CO.sub.2 from the expired gas. A mixing valve mixes expired breathing gas from the face mask with oxygen-rich breathing gas from the pressurized container. A balanced exhalation valve assembly coupled to the exhalation hose includes a main valve which passes the flow of expired gas from the exhalation hose into a chamber and prevents the reverse flow of gas from the chamber into the exhalation hose. A flexible diaphragm forms one wall of the chamber and is coupled to the main valve by a rigid spacer to prevent a pressure increase in the chamber from increasing the force necessary to open the main valve.