Why can thermal mass flow meters only measure gas flow?

　　Thermal mass flow meters measure gas flow because their operating principle relies on gas heat transfer and flow characteristics. Liquids are not suitable because of their high heat transfer efficiency and complex flow. In addition, the flowmeter is sensitive to the type of gas, and it is difficult to measure special gases accurately. Selection is based on fluid properties and measurement requirements.

　　The reason why thermal mass flow meters can only measure gas flow is mainly based on their operating principle and the physical properties of the fluid. The following is a detailed explanation of this issue:

　　Principle of operation

　　The operating principle of a thermal mass flow meter is based on the transfer of heat in a fluid. When a fluid flows through a pipe heated by an external heat source, there are changes in the temperature field, or a relationship between the energy required to raise the temperature of the fluid by a certain value and the mass of the fluid. By measuring these changes, the mass flow rate of the fluid can be deduced.

　　Physical properties of fluids

　　Thermal conductivity:

　　Gases have good thermal conductivity, when the gas flows through the thermal sensor, it will take away the heat emitted by the sensor, the amount of heat taken away is proportional to the mass flow rate of the gas. This characteristic enables thermal mass flow meters to accurately measure gas flow.

　　Liquids are much more efficient at transferring heat than gases, and they usually have a higher density and specific heat capacity. This can lead to rapid cooling of the sensor, making accurate measurement difficult. Therefore, thermal mass flow meters are not suitable for liquid flow measurement.

　　Flow characteristics:

　　The heat transfer characteristics of gases during flow are directly proportional to the flow rate, which allows thermal mass flow meters to accurately reflect changes in the flow rate of gases.

　　The flow characteristics of liquids are significantly different from those of gases, and their heat transfer and flow patterns are more complex, so thermal mass flow meters may not give accurate results when measuring the flow of liquids.

　　Sensitivity to gas type

　　Thermal mass flow meters are also sensitive to the type of gas. For example, gases such as acetylene and gases with a high moisture content (e.g. steam) may not be accurately measured by a thermal mass flow meter due to differences in thermal conductivity and chemical properties with conventional gases.

　　In summary, thermal mass flow meters can only measure the flow of gases because their operating principle is dependent on the thermal conductivity and flow characteristics of gases. At the same time, some special gases cannot be measured accurately due to the sensitivity of thermal mass flow meters to the type of gas. Therefore, when selecting a flowmeter, it is necessary to choose the right type of flowmeter according to the nature of the measured fluid and the measurement requirements.