Gas flow measurement using the thermal principle

The same applies to temperature: if it changes, the speed of the particles also changes and the pressure increases while the volume remains the same. Like this, volume, pressure and temperature always depend on each other. The particles present in a volume, and therefore the mass, on the other hand, always remain the same. Therefore, if the gas flow is determined by its volume, the pressure and temperature have to be be determined in order to calculate the mass flow of the gas. If, on the other hand, the gas flow is determined by its mass, this takes place independently of pressure and temperature. This eliminates the need for conversation and makes it is easier to measure the mass flow directly. Next to mass flow measurement  based on the Coriolis principle, the use of mass flow sensors based on the thermal principle are the most popular.

The thermal principle of mass flow measurement is based on the fact that a heated body in a gas flow heats up the surrounding gas. This heat difference is measured. A thermal mass flow sensor consists of two temperature sensors and a heating body in the middle. The first sensor measures the reference temperature and the second sensor a possible temperature difference. This also allows to determine the flow direction of the gas. If there is no gas flow, the temperature at both sensors is identical. If there is a gas flow, the temperature at the second sensor will be higher (if the flow direction is positive) because heated gas moves in the flow direction. The difference between the two temperature sensors therefore becomes larger and is a direct indicator of the size of the gas mass flow.