Thermal mass flow meter working principle
The working principle of thermal mass
flowmeters (TMF) is mainly based on the heat exchange relationship between the fluid and the heat source to measure the mass flow of the fluid. This type of flowmeter is also often called a calorimetric flowmeter in China. Its working principle can be summarized into two main types:
1. Measurement based on temperature field changes
Measurement principle: The mass flow of the fluid is measured by using the temperature field changes generated when the fluid flows through the pipe heated by the external heat source. When the fluid flows through the heating element, it will take away some heat, causing the temperature of the heating element to drop. By measuring the degree of this temperature drop, the mass flow of the fluid can be indirectly calculated.
Implementation method: Usually, a thermal mass flowmeter contains one or more heaters and one or more temperature sensors. The heater is used to generate heat, while the temperature sensor is used to monitor the temperature of the fluid or the temperature change of the heating element. When the fluid flows through, it takes away the heat from the heating element, causing the temperature detected by the temperature sensor to drop. This temperature drop is proportional to the mass flow of the fluid, so the mass flow of the fluid can be calculated by measuring the degree of temperature drop.
2. Measurement based on the relationship between energy and fluid mass
Measurement principle: Another type of thermal mass flow meter works by measuring the mass flow of a fluid by using the relationship between the energy required to increase the temperature of the fluid when the fluid is heated and the mass of the fluid. Specifically, it measures the energy required to increase the temperature of the fluid by a certain value, and then calculates the mass flow of the fluid based on the relationship between this energy and the mass of the fluid.
Implementation: In this type of flow meter, heaters and temperature sensors are also used. But the difference is that the main focus here is on the energy required to heat the fluid and the degree of temperature rise of the fluid. By accurately controlling the power of the heater and monitoring the change in the temperature of the fluid, the energy required to increase the temperature of the fluid by a certain value can be calculated, and the mass flow of the fluid can be further obtained.
3. Comprehensive application
In practical applications, thermal mass flow meters are usually used to measure the mass flow of gases. It has the advantages of low pressure loss, large flow range, high accuracy, high repeatability and high reliability. At the same time, since it has no moving parts, it is less likely to be damaged or malfunction during use. In addition, thermal mass flow meters can also be used for monitoring and control of extremely low gas flows.
However, it should be noted that thermal mass flow meters have certain requirements for the cleanliness of the fluid. If the measured gas contains more impurities or moisture, it may affect the accuracy of the measurement results. Therefore, it is necessary to pre-treat the measured gas or select a suitable flow meter model before use to meet the measurement requirements.
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