Method of, and apparatus for, providing a gas mixture

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;
• a first valve for regulating the flow of the first gas;
• a second valve for regulating the flow of the second gas;
• a mixer located downstream of the first and second valves and arranged, in use, to mix the first and second gases to provide a mixed gas;
• an outlet for said mixed gas at atmospheric pressure;
• a meter arranged to measure the average molecular weight of the mixed gas, comprising a high-frequency planar piezoelectric crystal oscillator in contact with the mixed gas and a sensor operable to determine atmospheric pressure, the meter being operable to determine the density of the mixed gas from said measured resonant frequency and to determine from the density, atmospheric pressure measurement and determined or predetermined temperature of the mixed gas, the average molecular weight of the mixed gas;
• and
• a controller operable, in response to the measured average molecular weight of said mixed gas, to control at least one of said first and second valves in order to control the relative proportion of the first and second gases in said mixed gas.