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Atmospheric pressure, also known as air pressure or barometric pressure (after the barometer), is the pressure within the atmosphere of Earth.The standard atmosphere (symbol: atm) is a unit of pressure defined as 101,325 Pa (1,013.25 hPa), which is equivalent to 1,013.25 millibars, [1] 760 mm Hg, 29.9212 inches Hg, or 14.696 psi. [2]
is the August equation, after the German physicist Ernst Ferdinand August (1795–1870). The August equation describes a linear relation between the logarithm of the pressure and the reciprocal temp. This assumes a temperature-independent heat of vaporization. The Antoine equation allows an improved, but still inexact description of the change ...
The standard atmosphere was originally defined as the pressure exerted by a 760 mm column of mercury at 0 °C (32 °F) and standard gravity (g n = 9.806 65 m/s 2). [2] It was used as a reference condition for physical and chemical properties, and the definition of the centigrade temperature scale set 100 °C as the boiling point of water at this pressure.
With a temperature lapse rate of −6.5 °C (-11.7 °F) per km (roughly −2 °C (-3.6 °F) per 1,000 ft), the table interpolates to the standard mean sea level values of 15 °C (59 °F) temperature, 101,325 pascals (14.6959 psi) (1 atm) pressure, and a density of 1.2250 kilograms per cubic meter (0.07647 lb/cu ft).
A millimetre of mercury is a manometric unit of pressure, formerly defined as the extra pressure generated by a column of mercury one millimetre high, and currently defined as exactly 133.322 387 415 pascals [1] or approximately 133.322 pascals. [2] It is denoted mmHg[3] or mm Hg. [4][2]
0.019 336 77 psi. The torr (symbol: Torr) is a unit of pressure based on an absolute scale, defined as exactly 1 760 of a standard atmosphere (101325 Pa). Thus one torr is exactly 101325 760 pascals (≈ 133.32 Pa). Historically, one torr was intended to be the same as one "millimeter of mercury", but subsequent redefinitions of ...
Since 1982, STP has been defined as a temperature of 273.15 K (0 °C, 32 °F) and an absolute pressure of exactly 10 5 Pa (100 kPa, 1 bar). NIST uses a temperature of 20 °C (293.15 K, 68 °F) and an absolute pressure of 1 atm (14.696 psi, 101.325 kPa). [ 3 ] This standard is also called normal temperature and pressure (abbreviated as NTP).
With a normal P a O 2 of 60–100 mmHg and an oxygen content of F I O 2 of 0.21 of room air, a normal P a O 2 /F I O 2 ratio ranges between 300 and 500 mmHg. A P a O 2 / F I O 2 ratio less than or equal to 200 mmHg is necessary for the diagnosis of acute respiratory distress syndrome by the AECC criteria . [ 7 ]