Flow Sensor Technology

1. Flow sensor Since the development of gas mass flow controllers, the sensors used mainly include thermal sensors, volumetric sensors and differential pressure sensors. Among them, thermal type and differential pressure type are mostly used. The thermal sensor is a sensor that calculates the fluid mass flow rate by measuring the heat transfer and heat transfer when the fluid is heated. The differential pressure mass sensor is a sensor that calculates the fluid flow by measuring the pressure difference generated by the gas flowing through the resistor. The greater the resistance, the greater the pressure difference, and the more sensitive it is to the flow. Thermal flow sensors are widely used in the development of flow controllers because they are sensitive to gas flow and gas composition, and are applicable to a wide range of gas types and flow rates. 2. Application of thermal flow sensor in gas mass flow controller The gas mass flow controller is composed of sensors, flow divider channels, flow regulating valves and amplification control circuits and other components. The principle is that the flow signal measured by the sensor heating bridge is sent to the amplifier for amplification, and the amplified flow detection voltage is compared with the set voltage, and then the difference signal is amplified to control and adjust the valve, thus realizing closed-loop flow control. flow through the channel. Among them, the sensor is the core component of the gas mass flow controller. It mainly uses the principle of capillary heat transfer thermoelectric calorimetry to measure the mass flow of gas, and has the characteristics of automatic temperature and pressure compensation. Usually, the thermal flow sensor is designed with two coils, as shown in Figure 1, the temperature of the coil will increase after the current is applied. Figure 1 Schematic diagram of the sensor When there is airflow passing through, the temperature of the intake air is generally around 20°C, while the temperature of the coil is generally around 100°C. Cold air will reduce the temperature of the coil, and the temperature difference between the upstream coil and the intake air is large, and the effect of being cooled Also better. After being heated by the upstream coil, the temperature of the gas gradually approaches the temperature of the coil, and the cooling effect of the downstream coil by the gas is much smaller. Therefore, the average temperature of the downstream coil is generally higher than the average temperature of the upstream coil. Therefore, there is a temperature difference between the upstream and downstream gases. The greater the gas flow, the greater the cooling of the upstream coil, and the greater the difference in resistance between the two coils. If we take the voltage difference between the upstream and downstream coils as the output signal of the sensor, the gas flow can be controlled after being amplified by the amplifier circuit. The figure below shows the temperature change and temperature difference between the upstream and downstream coils after the gas is passed into the sensor. Figure 2 The average temperature of the sensor coil. It can be seen from the above figure that when the flow rate of the gas reaches a certain level, the difference between the upstream and downstream temperatures will tend to decrease, and the output of the sensor will also decrease, and then it will become stable. That is, the full-scale state. 3. Advantages of thermal flow sensors At present, the semiconductor industry is developing in the direction of technology, and its requirements for the accuracy of flow controllers are getting higher and higher. Because of its sensitivity to gas flow, thermal flow sensors can not only measure and control micro-flow gases, but also measure different gases or mixed gases, while ensuring high control accuracy and stability. Therefore, thermal flow sensors have always occupied a dominant position in the field of gas mass flow controllers (flow meters).

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