Comparison of working principles of two thermal mass flowmeters

Flowmeter is one of the important instruments in industrial measurement. With the development of industrial production, the requirements for the accuracy and range of flow measurement are getting higher and higher, and the flow measurement technology is changing with each passing day. In order to adapt to various purposes, various types of flowmeters have come out one after another. At present, there are more than 100 kinds of flowmeters that have been put into use, and the thermal gas mass flowmeter is one of the most widely used ones. Thermal mass flowmeters can be divided into: constant temperature difference method (power consumption measurement method) flowmeter and constant power method (temperature measurement method) flowmeter. From the development history of the thermal gas mass flowmeter, the constant temperature difference flowmeter was first applied to the actual industrial medium measurement. This is because the constant temperature difference type is easier to implement than the constant power type, so from now on the production of gas mass flow From the perspective of meter manufacturers, there are more manufacturers using the principle of constant temperature difference, such as Kurx, Sirren, Brooks, etc. However, with the continuous improvement of production requirements, the constant temperature difference flowmeter has been difficult to meet the needs of some special production, which makes the constant power flowmeter become the key research direction. 1 Principle of constant temperature difference method: constant temperature difference method (power consumption measurement 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 control And measuring the power provided by the heat source, the power consumption increases with the increase of 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 maximum measurable flow is limited. 2. Constant power method The constant power method (temperature measurement method) is to provide heat to the platinum thermal resistance with constant power to heat it to a temperature higher than that of the gas. The fluid flow takes away part of the heat on the surface of the platinum thermal resistance. The greater the flow rate, the lower the temperature. The larger the measurement is, the temperature that changes with the fluid flow can reflect the gas flow. There are the following two implementation methods: (1) Only one platinum resistance is heated, and the temperature difference is measured by the principle of thermal diffusion. Principle: Similar to the structure of the constant temperature differential flowmeter, two metal platinum resistors are also added to the measuring pipeline, one is a temperature measuring resistor used to measure the temperature of the measured fluid, and the other is used to measure the velocity of the measured fluid. Speed ​​measuring resistor. Add a constant power to the heater to heat the speed-measuring platinum resistance. When the fluid is stationary, the surface temperature difference between the speed-measuring platinum resistance and the temperature-measuring platinum resistance is ΔT21=TS2-TS1. With the flow of the medium, the surface temperature of the two platinum resistance The difference decreases. The greater the flow rate of the fluid, the smaller the temperature difference between the two platinum resistors.
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