Abstract: A pump piston assembly has a pump cylinder, at least one hollow piston slidably mounted for reciprocating motion in the pump cylinder, the piston having an inner surface of a first diameter and an outer surface of a second diameter, and a liner sleeve of a different material from the piston mounted on the inner or outer surface of the piston with a close or friction fit to the diameter of the piston surface on which it is mounted. The liner sleeve is of an acoustic dampening material which has a lower acoustic frequency than the piston material and is configured to absorb harmonic acoustic vibrations resulting from reciprocation of the piston in the cylinder.

Abstract: 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.