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Gas mass flow is directly measured with thermal meters without the need for extra pressure, density and/or temperature monitoring. They can operate either by maintaining a probe and a consistent temperature as well as measuring the energy needed to do so, or by introducing a certain amount of heat into the stream that is flowing.
 
Two temperature meters as well as an electric heater between them is what thermal mass flow meters include. The heater can be external to the piper or may protrude into the fluid system. By having temperature changes eliminated in the process, any heat transfer found in the flow stream will be directly proportionate to its mass flow. 
 
In the industry, you can find three varieties of meters for thermal mass flow. There is the thermal mass flow meters that are industrial, also called immersible mass flow or thermal dispersion meters, are made up of a group of instruments used to measure a fluid's total mass flow, mainly gas which flow through conduits that are closed.
 
The second kind is the thermal mass flow meter of capillary-tube kind. MFC (many mass flow controllers) which are comprised of a mass flow meter a valve and electronics are based off of this design. On top of that, you can build a mass flow meter by measuring temperature differences across a MEMS chip that is silicone based.
 
Fluid mass flow rate is measured by both types with the use of the heat convected from a surface that is heated to the flowing fluid. When it comes to the immersible or thermal dispersion type of flow meter, the heat is sent to perimeter area of the fluid which is flowing over the surface which is heated.
 
When it comes to the capillary-tube kind, the heat is sent to the larger mass of flowing fluid via a small capillary tube that is heated. Both types share the principles of operation which is thermal in nature, they are however so significantly different that there is a requirement for two separate standards.
 
On top of that, their applications are very different. Commonly a thermal dispersion flow meter will be used for general industrial gas flow, such as in ducts and pipes; capillary tubes on the other hand, are mainly used for smaller flows of gases or liquids that are clean in tunes. In industry, this kind is most commonly used for thermal mass flow meters. However, the subject of discussion here is not the capillary type.
 
The main reasons for the popularity of thermal mass flow meters in industrial applications are due to their design and construction. They contain no moving parts, require no corrections of temperature of pressure, they are basically unobstructed directly through the flow path and over a wide range of flow rates, they retain accuracy. By using conditioning elements of dual plate flow, straight pipe runs can be reduced and it is very simple to install, there are minimal pipe intrusions.
 
That being said, in many applications, the fluid's thermal properties can depend on the composition of the fluid. In these applications, the thermal flow measurement can be affected by the fluid´s varying composition during actual operation. This is why it is crucial for a thermal flow meter supplier to be aware of the fluid's composition is that the correct calibration factor is used to accurately determine the flow rate.
 
Although suppliers are able to provide information on proper calibration for other gas mixtures, how accurate the thermal flow meter depends on the actual gas mixture and the gas mixture that is used for calibration purposes, being the same. To put it simply, of the actual gas that is flowing has a different composition, the accuracy of a thermal flow meter which has been cdesigned for a specific gas mixture, will become degraded.