What is thermal mass flow meter?

If you have ever lived in a cold environment at any point in your life then you are most probably familiar with a wind chill. When the temperature of your body is cooler than the ambient air temperature then the heat will be transferred to the surrounding air from your body. In the event where there is little or no breeze to help in moving the air past your body, then the air molecules that surround your body will immediately start to heat up as they absorb some heat from you. This will in turn lead to a decrease in the rate of heat loss. On the other hand if your surrounding has even the slightest breeze, then you will come into contact with air molecules which are more cool or unheated leading to a bigger rate of heat loss.  

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.    

To 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 medium. 

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.