Both thermal and Coriolis flowmeters measure mass flow. However, thermal meters measure mass flow quite differently from Coriolis meters. Instead of using fluid momentum, as do Coriolis meters, thermal flowmeters make use of the thermal or heat-conducting properties of fluids to determine mass flow. While most thermal flowmeters are used to measure gas flow, a small percentage also measure liquid flow.

In the early 1990s, new environmental regulations began requiring companies to detect and reduce the emission of sulfur dioxide (SO2) and nitrous oxide (NOx) into the air. SO2 and NOx are two principal causes of acid rain. The Environmental Protection Agency (EPA, initiated a program to reduce pollution in the atmosphere. It is possible to determine how much of these substances are released into the atmosphere by combining a measurement of the flowrate with a measurement of the concentration of SO2 and NOx.

In response to CEM requirements, thermal flowmeter companies developed multipoint thermal flowmeters. In many cases, continuous emissions monitoring occurs in large stacks that emit pollution from industrial sources. Single-point thermal flowmeters measure flow at a point, making it difficult to accurately compute flow in a large pipe or smokestack. Multipoint thermal flowmeters measure gas flow at multiple points, and use these values to compute flow for the entire pipe, duct, or stack. Some multipoint flowmeters have as many as 16 measuring points.

While the need for CEM is ongoing, the 21st century has brought new environmental awareness and requirements. This is providing new opportunities for thermal flowmeters to measure emissions in applications such as recovery of landfill gas, monitoring of flue gas, measuring boiler emissions, ethanol refining, and biomass gasification. Expect these types of environmental applications to expand and give new life to thermal flowmeters.