Fluid mixing

Claim 1

A gas mixing apparatus including: a. at least two gas inlet passages for the gases to be mixed; b. a gas pressure regulator in each inlet passage; c. a plurality of gas mixture distribution passages; d. a plurality of flow rate control passages, each flow rate control passage being connected between an inlet passage and a gas mixture distribution passage, and each gas mixture distribution passage being connected by flow rate control passages to both of said at least two gas inlet passages; e. a calibrated flow impedance in each flow rate control passage, for passing gas at a chosen flow rate to the gas mixture distribution passage downstream thereof, whereby gas mixtures of different chosen compositions are able to be formed in the gas mixture distribution passages; f. a common outlet to which all the gas mixture distribution passages are connected, and from which a gas mixture can be supplied; and g. valve means operable to close selected gas mixture distribution passages, whereby the composition of the gas mixture supplied from the common outlet depends on which of the gas distribution passages is left open to the passage of gas therethrough.