Thermal Flow Meters and the Various Principles
The thermal-mass flow meters feature 2 temperature sensors as well as an electric-heater that is positioned between them. The heaters used can either reach into the stream of fluid or it can be outside of this pipe. Elimination of temperature changes that are associated with these processes means that any type of heat transfer that occurs inside the flow-stream is said to be proportional directly to the mass flow.
Today in the industries there are 2 types. These include the industrial thermal flow meter that also goes by the name of immersible or "thermal dispersion" mass-flow meters. These meters form a part of a group of instruments that measure the totaled “mass flow” rates of fluids, in most cases gases that flow through a closed conduit. The 2nd type is known capillary-tube thermal flow meters. Various MFC (mass flow controllers) that combine a valve, electronics and mass-flow meters will be based on these design types. Additionally, these types of thermal-mass flow meters have been designed to measure temperature differences along MEMs silicon-based chips.
Both of the above mentioned types measure the fluid “mass flow rate” using heat convected that comes from the heated surface that goes into the flowing fluids. In regards to the thermal immersible or dispersion flow meters, heat will be transferred onto the fluids’ boundary layer that flows over heated surfaces. In regards to the capillary-tube types, heat will be transferred to the majority of the flowing fluids from the heated and small capillary tube.
The main principles on both these types are thermal, but are still significantly different which necessitates the need for different standards. In addition, the applications are very different. The thermal dispersion type flow meters will typically be used for industrial-based applications for gas flow that occurs in ducts and pipes, while the capillary versions are used more for a smaller flow of clean liquids or gases in tubes.
The reason as to why the thermal flow meters are widely used in various industrial applications have been associated with the way in which they are built and designed. This involves the very important aspect regarding the absence of moving parts a very close to unobstructed flow path, they also do not need to retain accuracy, pressure or temperature corrections over various flow rates. The straight-pipe runs have the ability to be reduced when using conditioning elements that are dual-plate flow. Overall installation is extremely simplified with very few pipe intrusions.
However, when it comes to various applications, thermal properties based on fluid types will depend on the composition of the fluid. In these types of applications, the variation in compositions of the fluids that occur during operation can have an effect on thermal flow measurements. This is why it is of importance for the supplier of the thermal-flow meters to know about the fluids composition in order for the correct calibration factor to be used in order to determine flow rates in an accurate manner.