Method of, and apparatus for, providing gas mixture

There is provided a gas mixer arrangement comprising a first gas source for supplying a first gas; a second gas source for supplying a second gas different from said first gas; first and second flow regulation devices for regulating the respective flow of the first gas and second gases from the first and second gas sources; a mixer; and an outlet. The mixer is located downstream of the first and second flow regulation devices and arranged, in use, to mix the first and second gases to provide a mixed gas to the outlet. The gas mixer arrangement further comprises a meter comprising a first sensor assembly operable to determine the average molecular weight of the mixed gas and including a high-frequency planar piezoelectric crystal oscillator in contact with the mixed gas, a second sensor assembly operable to determine the pressure of the gas downstream of one of the first or second flow regulation devices, and a controller operable, in response to the average molecular weight of the mixed gas and said gas pressure, to control automatically said first and second flow regulation devices to control the relative proportion of the first and second gases in said mixed gas and the pressure or mass flow rate of the mixed gas from the outlet.