An enrichment device for mixing ambient air with a gas such as oxygen has a rigid outer housing defining a reservoir. The housing has an outlet port for attachment to a cyclic low pressure source, an ambient air inlet, and a second inlet for connection to a supply of pressurized gas. The reservoir has a plurality of internal walls defining a passageway having a plurality of turns defining a tortuous path for gas flow between the ambient air inlet and the outlet port, and the second inlet communicates with the passageway at a location at or close to the outlet port.
