Thermal dispersion mass flowmeters comprise a family of instruments for the measurement of the total mass flow rate of a fluid, primarily gases, flowing through closed conduits. The operation of the thermal dispersion mass flowmeters is attributed to L.V. King who, in 1914 (Ref. 1), published his famous King's Law revealing how a heated wire immersed in a fluid flow measures the mass velocity at a point in the flow. King called his instrument a "hot-wire anemometer". However, it was not until the 1960's and 1970's that the industrial-grade dispersion mass flowmeters finally emerged. This standard covers the thermal dispersion type of thermal mas flowmeter.