Working principle of thermal mass flowmeter-constant temperature difference method

The principle of the constant temperature difference method: the constant temperature difference method is that the temperature of the heating element is higher than the temperature of the gas, and a part of the heat is taken away when the gas flows through, so that the temperature difference between the heating element and the measured gas is kept constant at a certain temperature difference, and the power provided by the heat source is controlled and measured. The power consumption increases with the flow rate, and the gas flow rate is reflected by the power consumption. Add two metal platinum resistors in the measuring pipeline, and one platinum resistor adds a small current (the current is below 4mA, which will not cause the resistor to heat up), which is used to measure the temperature of the measured fluid, which is called a temperature measuring resistor. Another platinum resistance leads to a larger current (the current is generally above 50mA) to measure the speed of the measured fluid, which is called the speed measuring resistance. According to the principle of thermal diffusion, the heat taken away by the heated object by the fluid is related to the temperature difference between the heated object and the fluid, the flow velocity of the fluid, and the properties of the fluid. When working, the temperature-measuring resistor continuously detects the temperature of the medium, and the speed-measuring resistor self-heats to a constant temperature higher than the fluid. When the fluid flows, the resistance value of the speed-measuring resistor changes due to the decrease of the surface temperature of the heat-dissipating speed-measuring resistor, and the Wheatstone bridge is unbalanced. The temperature difference is fed back to the processor through a feedback circuit composed of a Wheatstone bridge to increase the current (or voltage) of the heater to keep the temperature difference constant. The flow rate of the fluid is proportional to the current (voltage) added: the greater the flow rate of the fluid, the greater the current (voltage) added to maintain a constant temperature difference. Therefore, the relationship between heating current (voltage) and mass flow can be calibrated through experiments, and the fluid mass flow can be calculated through current (voltage). There is a problem with the constant temperature difference gas mass flowmeter: with the increase of the medium flow rate, the heat of the speed measuring platinum thermal resistance is quickly taken away, and the constant temperature difference flowmeter requires a rapid increase in the heating energy of the speed measuring platinum thermal resistance to ensure a constant temperature difference . However, because the increase of energy is affected by the power of the circuit itself and the maximum allowable current of the speed measuring platinum thermal resistance, its measurable flow rate is limited.

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