Thermal meters measure gas mass flow directly without the need for additional need pressure, temperature and/or density monitoring. 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.
Thermal mass flowmeters include two temperature sensors and an electric heater between them. The heater can protrude into the fluid stream or may be external to the pipe. By eliminating temperature changes in the process, any heat transfer within the flow stream is directly proportional to its mass flow.