Thermal meters are used to calibrate the mass flow of gas without needing any other pressure, temperature or density monitoring. Thermal meters work by introducing a certain amount of heat to the flow of the gas and measuring the temperature change produced or by creating a specific temperature environment and measuring the energy required to maintain the temperature with the assistance of a probe. Thermal mass flowmeters use an electric heater that is placed between two temperature sensors. The heater can either be used externally on the pipe or in the fluid stream. Using this technique eliminates temperature changes during the process. Any heat transfer in the flow stream is proportional to the mass flow.
There are 3 varieties of thermal mass flowmeters used in the industry. Immersible mass flowmeters, also known as a thermal dispersion or industrial thermal mass flowmeters include a number of instruments that measure the mass flow rate of fluids (primarily gases) that flow through closed conduits. Another type of thermal mass flowmeter is a capillary tube thermal mass flowmeter. Mass flow controllers, abbreviated MFC, combine electronics, a valve and a mass flowmeter. A thermal mass flowmeter is built measures the temperature differential across silicon-based MEMS chips.
Both of these types of thermal mass flowmeters regulate the rate of fluid mass flow by measuring the heat that is produced from a heated surface that the fluid flows across. In immersible mass flowmeters, the heat is transferred to a barrier zone of the fluid that is flowing over the heated area. In a capillary tube mass flowmeter, the heat is transferred to the fluid flowing through a heated capillary tube. The two types of thermal mass flowmeters are similar in principle; however, the standards used are substantially different as well as the applications they are used for. Thermal dispersion flow meters are typically used for gas flow applications in ducts or pipes while capillary tube mass flow meters are used for liquids or clean gases in tubes. Both of these types of thermal mass flow meters are used in the industry.
The main reason that thermal mass flowmeters are used in industrial applications is because of the way they are built. The thermal mass flowmeter does not contain any moving parts and has an unobstructed flow path. Additionally, these thermal mass flowmeters do not require corrections to the pressure or temperature and they retain their accuracy over a number of flow rates. The run in a straight pipe can be reduced using a dual plate flow conditioning element. Installation is simple and has minimal pipe intrusions.
The thermal properties of a fluid depends on the composition of the fluid in many applications. The thermal flow measurement can be affected by varying the composition of the fluid during the operation. Therefore, it is essential that the supplier of the thermal flow meter know the exact composition of the fluid to properly calibrate the meter so that it determines the flow rate correctly. Suppliers can give the user with the information needed to properly calibrate the thermal mass flowmeter for other mixtures of gas. however, the accuracy depends on the gas mixture being the same as that used for calibration purposes. If the actual gas that is flowing has a different composition than that which was calibrated, the mixture may be incorrect and degrade. This is why it is essential that the gas used in calibration is the same as that used during the manufacturing process.