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Abstract: Fluid measurement and control is a complex and cutting-edge technology. Although mass flow meters available today are highly versatile, to better address issues in processes and experiments, it is still necessary to understand the principles behind different types of mass flow meters. First, let’s talk about the thermal mass flow meter. Most thermal mass flow meters available on the market are capillary-type. This thermal flow meter is mainly composed of a proportional divider, two temperature sensors, a capillary, and two heating wires. Those who have used thermal mass flow meters know that they need to be preheated before use, and the time required varies depending on the brand and product model. What is the purpose of this preheating time? In fact, two heating wires are needed to heat the capillary to a constant temperature, and this process takes a relatively long time. After the capillary is heated to a fixed temperature, there must be a difference between the two power meters; the greater this power difference, the higher the gas flow rate. However, there is no linear relationship between the two; the relationship between this power difference and volumetric flow rate is calibrated (calibration involves determining the flow rate through numerous experiments, with more frequent calibrations resulting in higher accuracy, while values outside the range of calibration must be estimated using mathematical models and experience). Therefore, another core component of a thermal mass flow meter is its calibration database. The diagram below is the schematic of the thermal mass flow controller I drew earlier, based on the design principles of Bronkhorst’s thermal mass flow controllers. Next, let’s take a look at the Coriolis flow meter. The principle behind mass measurement in a Coriolis mass flow meter is Newton’s second law, F=Ma. When fluid flows through a vibrating tube, a Coriolis force is generated that is proportional to the mass flow rate. When no fluid is flowing through, the vibrating tube does not twist, and the signals detected by the electromagnetic signal detectors on both sides of the vibrating tube are in the same phase ； When a fluid passes by, the vibrating tube twists under the action of torque, resulting in a phase difference between the two detectors. The transmitter measures the lag time between the left and right detection signals; by multiplying this time difference by the flow calibration coefficient, the mass flow rate can be determined. Finally, let’s take a look at the laminar flow differential pressure mass flow meter. It is a type of differential pressure flow meter, but there are differences in its principle; moreover, it is a flow meter that offers advantages over traditional differential pressure flow meters. The laminar pressure drop mass flow meter is based on Hagen-Poiseuille’s law, which states that, under constant conditions such as temperature and pipe diameter, the volumetric flow rate of an incompressible fluid flowing in a circular pipe in a laminar state is linearly related to the pressure drop. By reading the pressure difference signal at both ends of the laminar flow element, the volumetric flow rate is calculated; this volumetric flow rate is then corrected for pressure and temperature to obtain the standard volumetric flow rate and mass flow rate. It can be seen from this that the prerequisite for Hagen-Poiseuille’s law is that the fluid must flow in a laminar manner; therefore, the key aspect of a laminar pressure differential mass flow meter is \"laminar flow\". After understanding the principles of laminar flow differential pressure, thermal, and Coriolis mass flow meters, let’s take a look at their applications. Coriolis mass flow meters are primarily used for low-pressure liquids, high-pressure liquids, and high-pressure gases ； Thermal mass flow meters are primarily used for low-pressure gases ； Laminar pressure difference mass flow meters are mainly used for low-pressure liquids and low-pressure gases. It can be seen from this that Coriolis mass flow meters are generally used in high-pressure markets, while thermal and laminar flow differential pressure mass flow meters are mainly used in low-pressure markets. Another significant difference lies in the requirements regarding the cleanliness of the fluid. Coriolis mass flow meters have no particular requirements as far as fluid cleanliness is concerned; they can be used with both solid-liquid mixtures and pure fluids. However, thermal and laminar pressure difference mass flow meters can only be used with pure gases or liquids. From the above introduction, it can be seen that thermal and laminar flow differential mass flow meters are primarily used in the low-pressure, clean fluid market, while Coriolis and other types of mass flow meters are mainly used in the high-pressure fluid market. Finally, it should be noted that fluid measurement and control is a complex and cutting-edge technology. Although mass flow meters available today are quite versatile, to better address issues related to processes and experiments, it is still necessary to understand the principles behind different types of mass flow meters. After all, fluid measurement and control depends not only on the performance of the measuring equipment but also on a well-designed gas delivery system.