Thermal mass flowmeters measure the mass flow rate of gases and liquids directly. They are used in monitoring or controlling mass-related processes such as chemical reactions that depend on the relative masses of unreacted ingredients. Thermal flowmeters provide high rangeability (10:1 to 100:1) if they are operated in constant-temperature-difference mode. This meter is available in high pressure and high temperature designs, and in special materials including glass, Monel, and PFA. It is most often useful for the regulation of low gas flows. They operate either by introducing a known amount of heat into the flowing stream and measuring an associated temperature change, or by maintaining a probe at a constant temperature and measuring the energy required to do so. The components of a basic thermal mass flowmeter include two temperature sensors and an electric heater between them. Some common gas-flow applications for thermal mass flowmeters include combustion air measurement in large boilers, semiconductor process gas measurement, air sampling in nuclear power plants, process gas measurements in the chemical and petrochemical industries, research and development applications, gas chromatography, and filter and leak testing. Thermal mass flowmeters are well suited for high rangeability measurements of very low flows, but also can be used in measuring large flows such as combustion air, natural gas, or the distribution of compressed air.