32 Field Instrumentation GEN Rev) Figure 2.1
pressure scales Differential pressure It is difference between two pressures. The rate of flow through a restriction in a pipe is proportional to the square root of the differential pressure. The loss in pressure increases as the flow rate increases.
Vacuum: Pressure below atmospheric pressure.
Static pressure Force per unit area exerted on a wall by a fluid at rest or flowing parallel to a pipe wall. It is also called
line pressure.
Total pressure: All pressures,
including static, acting in all directions.
Velocity pressure: Pressure exerted by the speed of flow. It is also called
velocity head or impact pressure. Hydrostatic pressure: Pressure below a liquid surface exerted by the liquid above.
33 Field Instrumentation GEN Rev)
Conditions affecting liquid pressure Surface pressure: any pressure acting on the surface is transmitted throughout the liquid and contributes to the pressure at any location beneath the surface.
Depth: pressure is proportional to depth below the surface.
Density: pressure is proportional to the density (or relative density) of the liquid.
Relative density (specific gravity Water is used as reference in comparing the density of liquids or solids, while air is used for gases. The ratio of density of a liquid or a solid to the density of water is called relative density or specific gravity. Relative density is also used to compare pressure in a liquid to the pressure at the same depth in water.
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