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This article mainly introduces how to select laminar flow mass flow meters. It should be noted that the mass flow meters referred to here are MFM types, which have only a measurement function and do not come equipped with control valves. When selecting a laminar flow mass flow meter, there are several key parameters to take into consideration. That is, pressure, temperature, flow rate, communication, and medium. I. Pressure: Regarding pressure, two factors need to be considered: one is the maximum pressure of the air source, and the other is the minimum pressure. The parameter corresponding to the maximum pressure of the gas source depends on the maximum operating pressure of the flow meter; for example, if the operating pressure specified in the flow meter’s parameters is 0–1 MPa, it means that the maximum pressure of the gas source required during use must not exceed 1 MPa. It should be noted here that a parameter of the flow meter is its pressure tolerance – that is, the maximum air pressure the flow meter can withstand. This pressure tolerance refers to the instantaneous pressure; short-term overloads will not affect the normal operation of the flow meter. For the minimum pressure of the gas supply, what we need to consider is the pressure drop across the flow meter; it is well known that there is a pressure loss as gas flows. Similarly, there is also pressure loss as the gas passes through the flow meter; this requires that the minimum pressure of the gas supply be higher than the pressure loss caused by the flow meter, in order to avoid disrupting the normal flow of the gas and resulting in inaccurate measurement values. The typical pressure loss of laminar flow mass flow meters is 4 KPa. When the gas supply pressure is relatively low, such as only 6–7 KPa, it is necessary to use flow meters with lower pressure loss. This is because pressure loss occurs not only in the flow meter itself but also in various components within the piping system, such as pipes and valves; therefore, flow meters with even lower pressure loss are required to ensure the reliability of the readings. If the gas source pressure is even lower, at only 1–2 KPa, we need to use a flow meter with extremely low pressure loss. II. Temperature: The temperature referred to here is that of the medium being measured. The normal operating temperature range for laminar flow mass flow meters is between -20°C and 60°C. If the temperature of the medium exceeds 60°C, a high-temperature flow meter is required; if it is below -20°C, a low-temperature flow meter is necessary. III. Flow Rate: One of the most important parameters in selecting a laminar flow mass flow meter is the flow rate range of the gas to be measured. The operating range of conventional laminar flow mass flow meters is 1%–100% of the Full Scale value. Take a flow meter with a capacity of 100 SCCM as an example; its normal measurement range is 1 ml/min to 100 ml/min. All flowmeters have an operating range, which is by no means from 0 to 100% F.S.; therefore, special attention should be paid to this when selecting a flowmeter. Additionally, the flow rate model of the flow meter should be selected in consideration of its guaranteed accuracy range. Taking a flow meter with a capacity of 100 SCCM as another example, if the manufacturer specifies an accuracy of 1% RD (reading value) within a range of 20% to 100% of the scale ； 0.2%F.S. (within the measurement range of 1%–20%), which indicates that this flow meter has an accuracy of 1%RD within a flow rate range of 20 ml/min to 100 ml/min ； Within the flow rate range of 1 ml/min to 20 ml/min, the accuracy is 0.2% F.S., which corresponds to an error of ±0.2 ml/min. IV. Communication methods: The choice of communication method depends mainly on the control scheme used in the associated equipment; common options include analog signals at 4-20 mA or digital signals via RS485, with the selection depending on the control module used. Analog signals suffer from signal loss and are susceptible to strong currents and electromagnetic interference; their communication accuracy is inferior to that of digital signals ； Digital signals will experience a slight delay in communication speed compared to other digital signals. Of course, digital signals have a clear advantage over analog signals, as they offer high accuracy and low error rates. V. Medium: When selecting a flow meter, it is also important to consider whether the medium being measured is corrosive. If it is a highly corrosive gas, a corrosion-resistant flow meter is necessary; for ordinary gases, a regular flow meter will suffice. The differences between corrosion-resistant and conventional flowmeters mainly lie in the materials and packaging methods. VI. All flow meters have measurement errors; what we refer to as accuracy is actually the degree of precision. Common precision levels include 1% RD and 1% F.S., and there is a significant difference between these two forms of accuracy. 1% RD is what we call the reading error; the magnitude of this error is the flow meter’s reading value multiplied by 0.01. 1% F.S. is what we call the full-scale error; the magnitude of this error is the flow meter’s full scale multiplied by 0.01. Taking a flow meter with a capacity of 100 SCCM as another example, when measuring a flow rate of 50 ml/min, if it is a flow meter with an accuracy of 1% RD, the measurement error will be ±0.5 ml/min ； For a flow meter with an accuracy of 1% F.S., the measurement error is ±1 ml/min, and this error remains at ±1 ml/min across the entire range. Therefore, understanding the concept of accuracy is also important for selecting flow meters. In addition to the parameters mentioned above, there is one more thing to keep in mind: the accurate accuracy indicated on flow meters comes with a guaranteed range, which includes but is not limited to requirements regarding the flow rate range, temperature range, and pressure range – these are actually also requirements related to the operating conditions. Finally, it should be noted that when measuring gas flow under negative pressure, a specialized negative pressure flow meter is required. The negative pressure referred to here is absolute pressure in the range of 0~101 KPa. Negative pressure flow meters are designed differently from positive pressure flow meters; therefore, regular flow meters must not be used to measure gas flow under negative pressure conditions. Even if it can be measured, the readings it displays are inaccurate.