Fluid Pressure Author: Su Yanxun Li Jinhai We define the force that acts vertically and uniformly on a unit area as the fluid pressure. The formula is Pu003d where P fluid acts pressure; F acts force; A acts area. In the International System of Units, the unit of force F is Newton (N), the unit of area A is meter 2 (m2), and the unit of pressure P is Newton/m2, that is, the pressure of the fluid in Pascal (Pa). The force uniformly acting on a unit area is defined as the pressure of the fluid. The formula is Pu003dIn the formula, P——fluid pressure; F——acting force; A——acting area. In the International System of Units, the unit of force F is Newton (N), the unit of area A is meter 2 (m2), and the unit of pressure P is Newton/m 2 or Pascal (Pa). There are different concepts of pressure in real life and production. (1) Atmospheric pressure: Atmospheric pressure is the pressure produced by the weight of a column of air on the surface of the earth. Expressed by the symbol PB, the atmospheric pressure value varies with weather conditions, altitude and geographic latitude, etc. (2) Gauge pressure: The pressure indicated by the pressure measuring instrument is called gauge pressure, which is the pressure from which atmospheric pressure is zero. It is represented by the symbol PG. Gauge pressure is the practical pressure in general engineering. (3) Absolute pressure: refers to the total pressure calculated without any conditions. That is, the full pressure of the space where liquid, gas, and steam are located. It is equal to the sum of atmospheric pressure and gauge pressure. Expressed by the symbol PA: PA u003d PG ten PB (4) Vent pressure: When the absolute pressure is less than the atmospheric pressure, the difference between the atmospheric pressure and the absolute pressure is called the vacuum pressure, also called vacuum pressure, negative pressure. Expressed by the symbol PH: PHu003dPB-PA(5) Differential pressure (pressure): the difference between two related pressures, usually expressed by the symbol ΔP. The relationship between absolute pressure, gauge pressure, atmospheric pressure, vacuum pressure and differential pressure can be represented by the following diagram. In addition to Chen's above-mentioned pressure concept, the concepts of static pressure and dynamic pressure are often used in flow measurement: (6) Static pressure: Static pressure refers to the gauge pressure value measured in a fluid that is not affected by the flow rate. For example: For pipeline flow, the pressure measured by the pipe wall is the static pressure value. In the past, the static pressure expressed by the height of the liquid column was called the pressure head, and the static pressure expressed by the height of the water column was called the hydrostatic head. Relationship between absolute pressure, gauge pressure, atmospheric pressure and real space Flow velocity distribution diagram in the tube (7) Dynamic pressure: Dynamic pressure refers to the kinetic energy of the fluid unit volume. Usually calculated with 1/2ρν2. In the formula, ρ——fluid density; v——fluid movement speed. Dynamic pressure is also called dynamic pressure head. The pressure of the fluid is measured by various pressure measuring instruments. Commonly used pressure measuring instruments include spring-type pressure gauges and liquid pressure gauges. The pressure of the fluid is an extremely important parameter in flow measurement. The differential pressure flowmeter uses the pressure difference between the two ends of the throttle to achieve flow measurement. In addition, the working pressure of the flowmeter can be known through the pressure measurement, and necessary correction calculations are made to ensure the accuracy of the flowmeter measurement.
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