A gas mixer arrangement (700; 800; 900; 1000) 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 electronic valves (714, 716) for regulating the respective flow of the first and second gases from the first and second gas sources; a mixer; and an outlet, the mixer being located downstream of the first and second electronic valves and arranged, in use, to mix the first and second gases to provide a mixed gas to the outlet, wherein the gas mixer arrangement further comprises a meter (750; 850; 950; 1050), the 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 first or second gas downstream of the respective one of the first or second electronic valves and upstream of the mixer; 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 electronic valves 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. 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 electronic valves (714, 716) for regulating the respective flow of the first and second gases from the first and second gas sources; a mixer; and an outlet, the mixer being located downstream of the first and second electronic valves and arranged, in use, to mix the first and second gases to provide a mixed gas to the outlet, wherein the gas mixer arrangement further comprises a meter (750; 850; 950; 1050), the 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 first or second gas downstream of the respective one of the first or second electronic valves and upstream of the mixer; 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 electronic valves 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. 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 first or second gas downstream of the respective one of the first or second electronic valves and upstream of the mixer; 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 electronic valves 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.