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In industrial production, due to material balance, heat balance, as well as requirements related to storage and economic accounting, it is mass rather than volume that is needed. Therefore, in measurement tasks, the measured volumetric flow rate often has to be multiplied by density in order to convert it into a mass flow rate. However, since density changes with temperature and pressure, when measuring the volumetric flow rate of a fluid, it is necessary to measure both the pressure and density of the fluid in order to determine the mass flow rate. It is difficult to achieve the purpose of measurement in situations where temperature and pressure change frequently. At this point, a mass flow meter is used to measure the mass flow rate, eliminating the need for manual conversion as mentioned above. http://yunrun.com.cn/upload/201901/07/201901071634458858.png Classification of mass flow meters: ① Direct-type mass flow meters – These are those that directly measure a quantity proportional to the mass flow rate; the sensing element directly reflects the mass flow rate. ②A derived mass flow meter is a device that uses a combination of a volume flow meter and a density meter to measure mass flow; it simultaneously detects the volume flow rate and the fluid density, and then generates an output signal related to the mass flow rate through calculations. ③A compensating mass flow meter simultaneously measures the volumetric flow rate of the fluid, as well as its temperature and pressure values. Based on the relationship between fluid density and temperature and pressure, the calculation unit determines the density of the fluid under those conditions, and from this, the mass flow rate of the fluid is calculated. There are many types of direct-type mass flow meters, and the Coriolis mass flow meter is the one that is widely used at present. A Coriolis mass flow meter (abbreviated as CMF) is a direct-type mass flow measuring instrument that utilizes the Coriolis force – a force proportional to the mass flow rate – generated when fluid flows through a vibrating tube. Mass flow meters that utilize the Coriolis force come in various forms such as straight-tube, bent-tube, single-tube, and double-tube types. Among them, the double-bend tube type is the most common; it consists of two metal U-shaped tubes whose ends are connected to each other and to the pipeline under test, as shown in Figure 1. In this way, the fluid can flow in both U-tubes simultaneously, with a set of piezoelectric transducers installed at points A, B, and C in the middle of the two tubes. http://yunrun.com.cn/upload/201901/07/201901071627489300.png Figure 1: Coriolis mass flow meter (CMF). Transducer A generates an alternating force under the action of an applied alternating voltage, causing the two U-shaped tubes to vibrate back and forth; this is equivalent to the two hoses swinging in opposite directions. Transducers B and C are used to detect the vibration of these two tubes. Since B is on the inlet side and C is on the outlet side, according to the principle that the vibration phase on the outlet side leads that on the inlet side, the phase of the alternating signal output by C will lead the phase of B by a certain amount. The magnitude of this phase difference is proportional to the mass flow rate. If the phase difference between these two AC signals is further converted into standard signals such as DC 4-20mA using circuitry, it becomes a mass flow transmitter. Mass flow meters have high measurement accuracy and are primarily used for measuring the flow rate of single-phase and multiphase fluids with relatively high viscosity and density. Due to structural reasons and others, this mass flow meter is suitable for measuring flow rates in pipes of small to medium size.