A device to accumulate supplemental oxygen in a respiratory treatment apparatus, the device comprising: a gas conduit having an input aperture and an output aperture, the gas conduit configured as a channel (4106) for movement of a gas between the input aperture and the output aperture, the input aperture configured to receive gas output from a blower of the respiratory treatment apparatus, the output aperture configured to convey gas to a patient interface from the blower, the gas conduit including a supplemental oxygen input (4200) proximate to the output aperture and a barrier (4110) in the channel between the input aperture and the output aperture of the gas conduit, the barrier operable to selectively prevent vapor passing from the output aperture to the input aperture. The barrier is further configured to permit accumulation of supplemental oxygen in the gas conduit by reducing back flow of the supplemental oxygen during user expiration.
