Abstract: A pneumatic oxygen conserver used with a single-tube cannula. The conserver includes a body having first and second cavities. A main diaphragm divides the first cavity into first and second chambers. An inlet passage delivers oxygen from a supply to the first chamber, and an outlet passage delivers oxygen from the first chamber to the cannula. The main diaphragm is movable between a closed position preventing oxygen flow through the outlet passage and an open position permitting flow. A first flow control passage connects the inlet passage and the second chamber, and a first flow control orifice in the passage restricts flow. A sensing diaphragm divides the second cavity into third and fourth chambers. A second flow control passage connects the second and third chambers, and a second flow control orifice in the passage restricts flow. The sensing diaphragm is movable between a closed position preventing flow through the second passage and an open position permitting such flow.