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 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 industrial-grade thermal dispersion mass flowmeters finally emerged.
This standard covers the thermal dispersion type of thermal mass flowmeter. The second type is the capillary tube type of thermal mass flowmeter, typically provided in its mass flow controller configuration. Both types measure fluid mass flow rate by means of the heat convected from a heated surface to the flowing fluid. In the case of the thermal dispersion, or immersible, type of flowmeter, the heat is transferred to the boundary layer of the fluid flowing over the heated surface. In the case of the capillary tube type of flowmeter, the heat is transferred to the bulk of the fluid flowing through a small heated capillary tube. The principles of operation of the two types are both thermal in nature, but are so substantially different that two separate standards are required. Additionally, their applications are much different. Thermal dispersion flowmeters are commonly used for general industrial gas flow applications in pipes and ducts, whereas capillary tube flowmeters are primarily used for smaller flows of clean gases in tubes.