Standard conditions for temperature and pressure

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Ambient temperature and air pressure vary from one location to another on the earth, and also vary in the same place over time. These conditions influence many chemical and physical processes. Therefore, it is necessary to define standard conditions for measuring temperature and pressure.

In chemistry, the term standard temperature and pressure (abbreviated STP) denotes an exact reference temperature of 0 °C (273.15 K) and pressure of 1 atm (defined as 101.325 kPa). These values approximate freezing temperature of water and atmospheric pressure at sea level. It is notable that one mole of any gas or combination of gases occupies 22.4 L of volume at STP.

Also in chemistry, the term Standard Ambient Temperature and Pressure (abbreviated SATP) denotes a reference temperature of 25 °C (298.15 K) and pressure of 100 kPa. Although there are many variations of the definition, the most accepted one is the temperature and pressure where the equilibrium constant for the autoionization of water (with [H₂O] omitted in the expression) is 1.0x10^-14. These conditions are also sometimes referred to as Standard Laboratory Conditions (abbreviated SLC). One mole of any gas or combination of gases at SLC occupies 24 L (24 cubic dm).

The International Civil Aviation Organization (ICAO) defines the sea-level International Standard Atmosphere (ISA) as 101.325 kPa or 1013.25 hPa, 15 °C and 0% humidity. These values provide a reference for calculating various aircraft performance figures, such as endurance, range, airspeed, and fuel consumption. When used to calculate performance at any pressure altitude other than sea level, the temperature is adjusted using the prescribed lapse rate which is −6.5 °C/km for the first 11 km.

(Ref: Manual of the ICAO Standard Atmosphere (extended to 80 kilometres (262 500 feet)), Doc 7488 / Third Edition, 1993)

External links

• Online ICAO Atmosphere calculator for aviators http://www.aviation.ch/tools-atmosphere.asp