The medium measured by the flowmeter can be either a liquid or a gas. Although the gas and the liquid are both fluids, there are great differences in physical properties. Therefore, fluid density, viscosity, and changes in gas composition all have different effects on fluids. Be sure to pay attention to fluid changes during use and selection. (1) The effect on density First of all, we all know that the density of gas and liquid is very different. In the standard state (at a temperature of 20° C. and an absolute pressure of 101.325 Pa), the density of air is 1.2 kg/m 3 and the density of pure water is 999.8 kg/m 3 . This feature of low gas density determines that the gas flow standard device of the mass method must be designed with high pressure in order to increase the gas density. Otherwise, due to the problem of tare weight, the phenomenon of using large scales and small objects can be reduced and the accuracy can be reduced. (2) The effect on temperature is again the temperature. The influence of temperature on the gas is also much greater than that of the liquid. For example, under the pressure of 100kPa, the air temperature rises from 20°C to 21°C, and the density decreases by 0.4%. In the same situation, the density of pure water only decreased by 0.03%. Therefore, in the flow measurement of water, the influence of temperature is often not considered, and even if considered, the accuracy of the temperature measurement is not required to be high. However, the volume of gas flow meter is not the same, and it is necessary to adjust and control the temperature as well as the pressure, accurately measure the temperature, and correctly correct the temperature. (3) The effect of pressure on the gas is that the density is very small and it can be easily compressed. Therefore, the effect on the pressure is much greater than that of the liquid. For example, at 20°C, the air pressure rises from 100kPa to 200kPa, and its density increases by 100%. In the same situation, the density of water increases only by 0.01%. Therefore, in water flow measurement, water can be regarded as an incompressible fluid. In addition to requiring highly accurate measurement, pressure correction is generally not considered. In the case of gas flow meters, the pressure must be well regulated and controlled, the pressure accurately measured, and the correct pressure corrected. (4) Viscosity effects Viscosity of gas is much lower than that of liquid. For example, at 20°C and 100kPa, the dynamic viscosity of air is 1.8×10-5Pa·s, and the dynamic viscosity of water is 1×10-3Pa. ·s. Therefore, the influence of viscosity must be considered in the flow measurement of liquids, especially the volumetric method of liquid flow standard devices. Due to the large viscosity of the liquid, the container will adhere to the wall of the container when the container is empty, resulting in an error. For gases, the effect of viscosity is generally not considered.
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