Method of and apparatus for measuring the mass flow rate of a gas

A meter for measuring the mass flow rate of a gas through a conduit, comprising:

• a flow restriction orifice dividing the conduit into an upstream portion upstream of said orifice and a downstream portion downstream of said orifice, the orifice having a cross-sectional area, wherein in use choked flow of the gas occurs through the orifice;
• a sensor assembly including a piezoelectric crystal oscillator comprising at least two planar tines in said upstream portion such that said piezoelectric crystal oscillator is in contact with said gas when the meter is in use, said sensor assembly being arranged:
• to drive the piezoelectric crystal oscillator such that the piezoelectric crystal oscillator resonates at a resonant frequency;
• to measure said resonant frequency of said piezoelectric crystal oscillator and to determine the density of the gas upstream of the orifice based on the measured resonant frequency;
• and
• to determine the mass flow rate through the conduit based on the determined density of the gas upstream of the orifice, the cross-sectional area of the orifice, and the speed of sound in the gas.