These manometric units are still encountered in many fields. The water-based units still depend on the density of water, a measured, rather than defined, quantity. One millimetre of mercury is approximately equal to one torr. When millimetres of mercury (or inches of mercury) are quoted today, these units are not based on a physical column of mercury rather, they have been given precise definitions that can be expressed in terms of SI units. Fluid density and local gravity can vary from one reading to another depending on local factors, so the height of a fluid column does not define pressure precisely. The pressure exerted by a column of liquid of height h and density ρ is given by the hydrostatic pressure equation p = ρgh, where g is the gravitational acceleration. The most common choices are mercury (Hg) and water water is nontoxic and readily available, while mercury's high density allows a shorter column (and so a smaller manometer) to be used to measure a given pressure. It is approximately equal to typical air pressure at Earth mean sea level and is defined as 101 325 Pa.īecause pressure is commonly measured by its ability to displace a column of liquid in a manometer, pressures are often expressed as a depth of a particular fluid (e.g., centimetres of water, millimetres of mercury or inches of mercury). The standard atmosphere (atm) is an established constant. Oceanographers usually measure underwater pressure in decibars (dbar) because pressure in the ocean increases by approximately one decibar per metre depth. The inch of mercury is still used in the United States. Similar pressures are given in kilopascals (kPa) in most other fields, except aviation where the hecto- prefix is commonly used. Some meteorologists prefer the hectopascal (hPa) for atmospheric air pressure, which is equivalent to the older unit millibar (mbar). The usage of P vs p depends upon the field in which one is working, on the nearby presence of other symbols for quantities such as power and momentum, and on writing style. The IUPAC recommendation for pressure is a lower-case p. Pressure is the amount of force applied perpendicular to the surface of an object per unit area. Manometric units such as the centimetre of water, millimetre of mercury, and inch of mercury are used to express pressures in terms of the height of column of a particular fluid in a manometer. Pressure may also be expressed in terms of standard atmospheric pressure the atmosphere (atm) is equal to this pressure, and the torr is defined as 1⁄ 760 of this. Some of these derive from a unit of force divided by a unit of area the SI unit of pressure, the pascal (Pa), for example, is one newton per square metre (N/m 2) similarly, the pound-force per square inch ( psi, symbol lbf/in 2) is the traditional unit of pressure in the imperial and US customary systems. Various units are used to express pressure. : 445 Gauge pressure (also spelled gage pressure) is the pressure relative to the ambient pressure. Pressure (symbol: p or P) is the force applied perpendicular to the surface of an object per unit area over which that force is distributed.
0 kommentar(er)
