The standard atmosphere (symbol: atm) is a unit of pressure and is defined as being equal to 101.325 kPa. The
following non-standard units are equivalent: 760 mmHg (torr), 29.92 inHg, 14.696 PSI, 1013.25 millibars. One
standard atmosphere is standard pressure used for pneumatic fluid power (ISO R554), and in the aerospace (ISO
2533) and petroleum (ISO 5024) industries.

In 1999, the International Union of Pure and Applied Chemistry (IUPAC) recommended that for the purposes of
specifying the properties of substances, “the standard pressure” should be defined as precisely 100 kPa (≈750.01
torr) or 29.53 inHg rather than the 101.325 kPa value of “one standard atmosphere”.[1] This value is used as the
standard pressure for the compressor and the pneumatic tool industries (ISO 2787).[2] (See also Standard
temperature and pressure.) In the United States, compressed air flow is often measured in "standard cubic feet"
per unit of time, where the "standard" means the equivalent quantity of moisture at standard temperature and
pressure. However, this standard atmosphere is defined slightly differently: temperature = 20 °C (68 °F), air density
= 1.225 kg/m³ (0.0765 lb/cu ft), altitude = sea level, and relative humidity = 20%. In the air conditioning industry, the
standard is often temperature = 0 °C (32 °F) instead. For natural gas, the petroleum industry uses a standard
temperature of 15.6 °C (60.08 °F), pressure 101.56 kPa (14.73 psi).

Source: WikepediA
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