The hot-wire anemometer which is used to measure the speed of air is one of the simplest form of thermal mass flow meter. The thermal flow meter includes a metal wire which is used to pass electric current to heat it up, and then an electric circuit controls the resistance of the metal wire. If there is an increase in the speed of air past the wire, then there will be more heat being drawn away from the metal wire leading to a decrease in its temperature. The electric circuit will sense the change in temperature and will compensate it by adding more current through the wire so that the temperature gets back to its set point.
maintain the hot wire at an elevated constant temperature, a certain amount of
electrical power is needed. The power is a direct function of the rate of mass
air flow which is past the wire. The design of industrial thermal mass flow
meters usually includes a flow tube which has 2 temperature sensors (thermal
resistance) in it. One of the RTD sensors should be heated while the other
should not. The heated one RTD will act as a mass gas flow sensor which cools
down as the rate of the gas flow increases while the other sensor which is
unheated acts as a compensation for the ambient temperature of the measured
The thermal dispersion gas flow meter can now be manufactured in small sizes, this is because of their simple design which gives them the freedom. Another significant factor in the calibration of the thermal flow meter is the specific heat of the fluid. The specific heat can be defined as a measure of the amount of heat energy which is required to alter the temperature of a standard amount of substances by a specific quantity. Substances have different specific heat values with some being higher than the others this means that that the substances with higher values can easily release or absorb a lot of heat energy without having to face a big change in temperature.
You can also find some of the designs of the thermal flow meters to be having walls of heated tubes, this acts as the heated element which will be cooled by the fluid. The difference between the turbulent flow stream and the rate of heat transferred by a laminar flow stream will be big. Therefore if the flow changes from laminar to turbulent and vice versa there will be a shift in the calibration for the design of the thermal flow meter.