Principle of Thermal Mass Flow Meter

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1. Comparison of performance and application of laminar flow principle and thermal principle gas mass flowmeter/controller (II) _ Baidu Knowledge The gas mass flowmeter based on the thermal principle is a device that calculates flow rate by measuring the heat exchange between the probe and the fluid. The basic principle is to use the temperature difference generated when the fluid flows through the metal probe, measure the temperature change of the probe through a temperature sensor, and then calculate the flow rate. The heater inside the probe will create a temperature difference between the probe and the fluid, and the heat carried away by the fluid is proportional to the temperature change of the probe. Through this relationship, the flow rate of the fluid can be calculated. Temperature sensors usually use thermistors, which can accurately measure the temperature changes of the probe. The mathematical model of a thermal mass flowmeter involves multiple parameters of heat transfer, including convective heat transfer, thermal conduction heat dissipation, and radiative heat dissipation. In an ideal situation, convective heat transfer should account for the majority of the total heat transfer. Thermal conductivity and heat dissipation are the main sources of error, as they are related to the installation structure and wires of the probe, and are difficult to accurately calculate under different operating conditions. Radiation heat dissipation can also affect measurement results, but it can usually be ignored in engineering. In heat transfer, dimensionless parameters such as Nusselt number, Prandtl number, and Reynolds number are intermediate variables that describe the heat transfer coefficient. The calculation of these parameters involves physical quantities such as thermal conductivity, specific heat capacity at constant pressure, dynamic viscosity, density, and flow velocity of the fluid. By combining these parameters, the relationship between heating and convective heat transfer can be established. The accuracy of a thermal flowmeter is affected by various factors, including flow rate, flow velocity, incoming temperature, incoming pressure, probe temperature difference, ambient temperature, pipe diameter, probe shape, material, gas working fluid, and probe installation structure. In order to obtain high-precision local jujube degree measurement, a large number of experimental tests are required. This demand has led to the phenomenon of "data trap", which means that in order to meet accuracy requirements, hundreds or even thousands of experiments need to be conducted. This makes the development cycle of thermal flow meters longer, often requiring decades of time. From the perspective of analyzing molecular motion, the core of a thermal flow cylindrical calorimeter is the heat transfer process, which refers to the collision and heat transfer between gas molecules and solid walls. The existence of temperature boundary layer is the key to the heat transfer process of tangerine tree, and its thickness is closely related to temperature distribution and flow boundary layer. The faster the flow velocity, the greater the velocity gradient in the flow boundary layer, the thinner the temperature boundary layer, and the more intense the heat transfer process, which explains why the faster the wind speed, the cooler it feels. The essential obstacle to improving the response speed of the thermal principle gas mass flowmeter lies in the mass an

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