![]() ![]() The Standard Atmospheric Pressure approximates to the average pressure at sea-level at the latitude 45° N. The atmospheric pressure varies with temperature and altitude above sea level. Atmospheric pressure is the pressure in the surrounding air at – or “close” to – the earth’s surface. ![]() Gauge pressures are positive if they are above atmospheric pressure and negative if they are below atmospheric pressure. The latter pressure scale was developed because almost all pressure gauges register zero when open to the atmosphere. When pressure is measured relative to atmospheric pressure (14.7 psi), gauge pressure (psig), the term gauge pressure is applied when the pressure in the system is greater than the local atmospheric pressure, p atm. Since atmospheric pressure at sea level is around 101.3 kPa (14.7 psi), this will be added to any pressure reading made in air at sea level. Pounds per square inch absolute (psia) is used to clarify that the pressure is relative to a vacuum rather than the ambient atmospheric pressure. When pressure is measured relative to a perfect vacuum, it is called absolute pressure (psia). Although absolute pressures must be used in thermodynamic relations, pressure-measuring devices often indicate the difference between the absolute pressure in a system and the absolute pressure of the atmosphere existing outside the measuring device. But in engineering, we often deal with pressures that are measured by some devices. In this article, the term pressure refers to absolute pressure unless explicitly stated otherwise. Often it will be important to distinguish between absolute pressure and gauge pressure. Who was the first person to use a barometer to measure differences in altitude.Pressure, as discussed above, is called absolute pressure. The unit is named after Blaise Pascal (1623 - 1662), French philosopher and mathematician, Air pressure is also measured in hectopascals (hPa), Thus pressure is moreĬommonly measured in kilopascals (kPa), with 1 kPa = 0.145 lbf/in 2. Terms a pascal is only 0.000 145 pounds per square inch (0.020 885 lbf/ft 2 or 0.007 50 mmHg). Sounds impressive, but in traditional English Per square meter or one "kilogram per meter per second per second." Values in "standard atmosphere" Elevation Temperature Air PressureĪir pressure depends on temperature and indirectly on altitude Pascal (Pa) is the metric SI unit of pressure and the standard pressure unit in the MKS metric system, equal to one newton Pressure, temperature and standard height (altitude or elevation) ![]() Pressures of several gigapascals can convert graphite to diamond or make hydrogen a metallic conductor. Such high pressures are rarely encounterd. The prefix "kilo" means "1,000", so one kilopascal = 1,000 Pa.
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