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High-precision mass flowmeters are used for flow measurement; since all types of flowmeters are affected by one or several parameters related to the properties of the fluid, these fluid properties play a significant role in determining the choice of flowmeter. Therefore, the selected measurement method and flowmeter must not only suit the properties of the fluid being measured, but also take into account the influence of changes in one parameter of the fluid’s physical properties during the measurement process on another parameter. For example, the effect of temperature changes on liquid viscosity. Common fluid properties include density, viscosity, vapor pressure, and other parameters. 1. Temperature and pressure: Carefully analyze the operating pressure and temperature of the fluid inside the flow meter; especially when measuring gases, significant changes in temperature and pressure can lead to substantial changes in density, which may require changing the measurement method used. For example, when temperature and pressure affect performance aspects such as the accuracy of flow measurement, temperature or pressure corrections must be made. Furthermore, the structural strength design and material of the flow meter housing also depend on the temperature and pressure of the fluid. Therefore, it is necessary to know exactly the maximum and minimum values of temperature and pressure. Careful selection is even more necessary when temperature and pressure vary greatly. It should also be noted that when measuring gas, whether the volumetric flow rate is based on the temperature and pressure under the operating conditions or those under standard conditions. 2. Density: For liquids, their density remains relatively constant in most applications; unless there are significant temperature changes that cause substantial variations, no density correction is generally necessary. In gas applications, the range and linearity of the flow meter depend on the gas density. There are also methods for converting the values in a flowing state to certain established reference values, and this approach is widely used in oil storage and transportation. Low-density gases affect certain measurement methods, particularly those instruments that use gas momentum to drive the detection sensors. 3. Viscosity: Viscosity varies little among different gases, and its value is low. And it does not change significantly due to changes in temperature and pressure. Because the viscosity of liquids is higher than that of gases. At 20°C and 100 kPa, the dynamic viscosity of water is Pa·s, while that of air is Pa·s; therefore, the effect of viscosity must be taken into account for liquids. Viscosity has varying degrees of impact on different types of flowmeters. For example, in the case of electromagnetic flowmeters, ultrasonic flowmeters, and Coriolis mass flowmeters, the range of viscosity values over which flow measurements can be taken is wide, so the effect of liquid viscosity is minimal. The error characteristics of positive displacement flowmeters are related to viscosity, and they are thus affected to some extent. As for float flowmeters, turbine flowmeters, and vortex flowmeters, viscosity levels above a certain threshold may render them unusable. The characteristics of some flow meters are described using the pipe Reynolds number as a parameter, and the pipe Reynolds number is a function of the fluid viscosity, density, and pipe flow velocity. Therefore, viscosity has a certain impact on the characteristics of the instrument.