A thermal flowmeter is an instrument that measures fluid flow based on the principle of heat exchange. It mainly has the following three working principles:
Principle of heat conduction
Basic principle: Place a heating element and one or more temperature sensors in the measuring pipeline of a thermal flowmeter. When the fluid flows through the measuring pipeline, heat exchange occurs between the fluid and the heating element, transferring the heat from the heating element to the fluid. By measuring the temperature difference or heat transfer rate between the heating element and the fluid, the flow rate of the fluid can be calculated.
Specific process: In the absence of fluid flow, the heating element reaches thermal equilibrium with the surrounding environment, and the temperature measured by the temperature sensor remains stable. When the fluid starts to flow, the flowing fluid takes away the heat from the heating element, causing a temperature drop, and the larger the flow rate, the more heat is taken away, resulting in a more significant temperature drop. By measuring temperature changes and using pre calibrated curves or formulas, the fluid flow rate can be determined.
Thermal diffusion principle
Basic principle: Utilizing the phenomenon of thermal diffusion, that is, when there is a temperature gradient in the fluid, heat will diffuse from high temperature areas to low temperature areas. In a thermal flowmeter, two sensors with different temperatures are installed in the pipeline to measure the temperature difference caused by fluid flow and calculate the flow rate.
Specific process: One sensor is heated as a heat source, while the other sensor serves as a temperature measuring element. When there is no fluid flow, the temperature difference between two sensors is mainly caused by thermal conduction. When the fluid flows, the flow of the fluid accelerates the diffusion of heat, causing a change in the temperature difference between the two sensors, and the larger the flow rate, the greater the change in temperature difference. Obtain the fluid flow rate based on the correspondence between temperature difference and flow rate.
Calorimetry principle
Basic principle: Based on the relationship between fluid absorption or release of heat and flow rate. Input a certain amount of known heat into the fluid, measure the increase or decrease in fluid temperature, and calculate the fluid flow rate based on its specific heat capacity and other characteristics.
Specific process: By installing a heating device in the pipeline, a certain amount of heat is input into the fluid. Meanwhile, temperature sensors are installed upstream and downstream of the heating point to measure the temperature changes of the fluid before and after heating. Calculate the fluid flow rate using formulas based on parameters such as heat input, temperature changes, and specific heat capacity of the fluid.
