A gas demand device, comprising a main body, a main body inlet formed in the main body, a device outlet orifice formed in the main body, a vent to atmosphere that is formed in the main body, a timing gas flow path that is formed in the main body and which includes a slave chamber and a secondary slave chamber, a demand gas flow path that is formed in the main body and which includes a primary chamber, a main diaphragm disposed in the secondary slave chamber and which divides the secondary slave chamber into first and second regions, and a slave diaphragm disposed in the slave chamber and which divides the slave chamber into first and second regions, wherein; the main body inlet is adapted and configured to be connected to a compressed gas source; the device outlet orifice is adapted and configured to direct a gas to a user for inhalation thereof via a device outlet; the timing gas flow path extends from the main body inlet through the second region of the slave chamber, through the first region of the secondary slave chamber, and to the vent; the demand gas flow path extends from the main body inlet through the primary chamber and to a device outlet via the device outlet orifice; the second region of the secondary slave chamber is in fluid communication with the device outlet via an outlet passage; open and closed positions of the main diaphragm respectively allows and blocks a flow of gas from the second region of the slave chamber through the timing gas flow path, whose open position allows a flow of gas from the second region of the slave chamber through the timing gas flow path to the vent, the main diaphragm being moved from its closed position to its open position when a vacuum is applied to the device outlet; and open and closed positions of the slave diaphragm respectively allows and blocks a flow of gas from the primary chamber through the demand gas flow path, the slave diaphragm being moved from its closed position to its open position after the second region of the slave chamber has been partially depressurized from a pressure P1 to a pressure P2; and the main diaphragm is moved from its open position to its closed position after partial depressurization of the second region of the secondary slave chamber from pressure P2 to a pressure P3 and the slave diaphragm is moved from its open position to its closed position after repressurization of the second region of the slave chamber to a pressure above P2, wherein the gas demand device is adapted and configured to be operable in a cycle of first, second, third, fourth, and fifth consecutive phases; at an expiration of the first phase and a commencement of the second phase, the slave chamber diaphragm and main diaphragm are in their closed positions to prevent flows of gas through the demand gas and timing gas flow paths, and pressures in the primary chamber and slave chamber second region are equal to a regulated pressure in the main body inlet; in the second phase, a pressure in the slave chamber outlet orifice is sub-atmospheric because of a demand for gas at the device outlet orifice thereby placing the main diaphragm in its open position while the slave diaphragm remains in its closed position and allowing a flow of gas through the timing gas flow path while preventing a flow of gas through the demand gas flow path, a pressure inside the secondary slave chamber first region decreasing during the second phase; the third phase commencing at an expiration of the second phase when a pressure in the slave chamber second region is decreased below a pressure in the slave chamber first region thereby placing the slave diaphragm to its open position while the main diaphragm remains in its open position and allowing a flow of gas through the timing gas and demand gas flow paths, a pressure inside the slave chamber first region decreasing during the third phase; the fourth phase commencing at an expiration of the third phase when a pressure in the secondary slave chamber first region is decreased below a pressure applied by the spring thereby placing the main diaphragm in its closed position while the slave diaphragm remains in its open position to allow a flow of gas through the demand gas flow path and prevent a flow of gas through the timing gas flow path, a pressure inside the slave chamber second region increasing while the pressure inside the slave chamber first region is decreasing during the fourth phase; the fifth phase commencing at an expiration of the fourth phase when a pressure in the slave chamber second region increases beyond that of the slave chamber first region thereby placing the slave diaphragm in its closed position while the main diaphragm remains in its closed position, pressure within the primary chamber increasing during the fifth phase; and the first phase commencing at an expiration of the fifth phase when pressure in the primary chamber builds to a level equal to the regulated pressure pressure in the main body inlet, the slave and main diaphragms remain closed to prevent flows of gas through the demand gas and timing gas flow paths.