Introduction to the principle of flow sensor

1. Traffic definition

The two concepts of volume flow and mass flow are involved in the research process of flow sensors.

The flow rate of a certain cross-sectional area is defined as the volume or mass of the flow passing through the cross-sectional area of ​​the flow channel per unit time, where the volumetric flow rate $q_v$The definition formula of is:
$$q_v=dV/dt=Sv$$
where$V$ represents the volume of the fluid,$t$ stands for time,$S$ represents the cross-sectional area of ​​the flow channel,$v$ represents the average flow velocity of the fluid.

mass flow $The\; formula\; for\; defining\; q$ is:
$$q=dm/dt=Sv\rho$$
where$m$ represents the mass of the fluid,$\rho$ represents the density of the fluid.

The gas volume flow sensor simply measures the volume flow through the effective cross-sectional area per unit time . Since the change of the external environment always affects the size of the gas volume, the gas volume has the problem of unstable size. In the actual measurement process It is necessary to consider the changes of various physical quantities. The gas mass flow sensor measures the mass flow through the effective cross-sectional area per unit time , and is divided into indirect measurement and direct measurement according to different measurement methods.

2. Classification of volume flow sensors

Volumetric flow sensors await investigation, but are expected to be less stable.

3. Classification of mass flow sensors

According to the different working methods of key components, gas mass flow sensors can be roughly divided into Coriolis flow sensors, mass flow sensors based on thermal principles, and differential pressure mass flow sensors.

3.1 Coriolis sensors

The Coriolis mass flow sensor is a specific application of Coriolis force. When a gas flows through a rotating pipeline, a Coriolis force related to the mass flow will be formed in the pipeline. By measuring the The Coriolis force generated by the gas flow can directly obtain the mass flow rate of the gas. Due to the particularity of its working principle, the Coriolis sensor has the advantages of high measurement accuracy, high reproducibility, large measurement range, and can also measure liquid flow of various properties. It is widely used in petroleum, chemical, pharmaceutical and other industrial fields. , but the Coriolis sensor has the disadvantages of large weight and volume, and is sensitive to external vibration interference, etc., and its application field has certain limitations.

3.2 Thermal Sensors

The working principle of the thermal sensor is to heat the temperature-sensing resistance of the sensor through an external heating source to make its temperature higher than the ambient temperature. When there is airflow passing through, the movement of the airflow will take away the heat on the temperature-sensing resistance, reducing the temperature of the temperature-sensing resistance. , by measuring the temperature change of the temperature-sensing resistance to calculate the required gas mass flow rate, that is, the change of air flow is converted into the change of temperature through the sensor.

3.3 Differential pressure sensor

The differential pressure flow sensor is generally a sensor composed of a pair of throttles placed on both sides of a reduced caliber. The volume flow of the measured flow is obtained by measuring the pressure difference on both sides, and then passed between the volume flow and the mass flow. The transformation relationship finally obtains the mass flow rate of the flow rate. The differential pressure flow sensor is a specific application of the law of conservation of mass and law of energy conservation. From the traditional orifice flowmeter to the current tower flowmeter, the differential pressure flowmeter has a development history of hundreds of years and is widely used in various fields such as industry, energy, transportation, and environment.

4. References

[1] Xiao Shijin. Development of MEMS thermal mass flow sensor [D]. Shandong University, 2021. DOI: 10.27272/d.cnki.gshdu.2021.004000.