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Standard conditions for temperature and pressure are standard sets of conditions for experimental measurements established to allow comparisons to be made between different sets of data. The most used standards are those of the International Union of Pure and Applied Chemistry (IUPAC) and the National Institute of Standards and Technology (NIST), although these are not universally accepted standards. Other organizations have established a variety of alternative definitions for their standard reference conditions.
In chemistry, IUPAC established standard temperature and pressure (informally abbreviated as STP) as a temperature of 273.15 K (0 °C, 32 °F) and an absolute pressure of 100 kPa (14.504 psi, 0.987 atm, 1 bar). An unofficial, but commonly used standard is standard ambient temperature and pressure (SATP) as a temperature of 298.15 K (25 °C, 77 °F) and an absolute pressure of 100 kPa (14.504 psi, 0.987 atm). The STP and the SATP should not be confused with the standard state commonly used in thermodynamic evaluations of the Gibbs energy of a reaction.
NIST uses a temperature of 20 °C (293.15 K, 68 °F) and an absolute pressure of 101.325 kPa (14.696 psi, 1 atm). The International Standard Metric Conditions for natural gas and similar fluids are 288.15 K (59.00 °F; 15.00 °C) and 101.325 kPa.
In industry and commerce, standard conditions for temperature and pressure are often necessary to define the standard reference conditions to express the volumes of gases and liquids and related quantities such as the rate of volumetric flow (the volumes of gases vary significantly with temperature and pressure). However, many technical publications (books, journals, advertisements for equipment and machinery) simply state "standard conditions" without specifying them, often leading to confusion and errors. Good practice is to always incorporate the reference conditions of temperature and pressure.
In the last five to six decades, professionals and scientists using the metric system of units defined the standard reference conditions of temperature and pressure for expressing gas volumes as being 15 °C (288.15 K; 59.00 °F) and 101.325 kPa (1 atm or 760 Torr). During those same years, the most commonly used standard reference conditions for people using the imperial or U.S. customary systems was 60 °F (15.56 °C; 288.71 K) and 14.696 psi (1 atm) because it was almost universally used by the oil and gas industries worldwide. The above definitions are no longer the most commonly used in either system of units.
Many different definitions of standard reference conditions are currently being used by organizations all over the world. The table below lists a few of them, but there are more. Some of these organizations used other standards in the past. For example, IUPAC has, since 1982, defined standard reference conditions as being 0 °C and 100 kPa (1 bar), in contrast to its old standard of 0 °C and 101.325 kPa (1 atm).
Natural gas companies in Europe and South America have adopted 15 °C (59 °F) and 101.325 kPa (14.696 psi) as their standard gas volume reference conditions. Also, the International Organization for Standardization (ISO), the United States Environmental Protection Agency (EPA) and National Institute of Standards and Technology (NIST) each have more than one definition of standard reference conditions in their various standards and regulations.
In Russia, State Standard GOST 2939-63 sets the following standard conditions: 20 °C (293.15 K), 760 mmHg (101325 N/m2) and zero humidity.
|Temperature||Absolute pressure||Relative humidity||Publishing or establishing entity|
|0||101.325||NIST, ISO 10780, formerly IUPAC|
|15||101.325||0||ICAO's ISA, ISO 13443, EEA, EGIA|
|60||14.696||SPE, U.S. OSHA, SCAQMD|
|60||14.73||EGIA, OPEC, U.S. EIA|
|59||14.503||78||U.S. Army Standard Metro[a]|
|59||14.696||60||ISO 2314, ISO 3977-2|
|°F||in Hg||% RH|
|70||29.92||0||AMCA,[b] air density = 0.075 lbm/ft³. This AMCA standard applies only to air.|
|59||29.92||Federal Aviation Administration (FAA)|
In aeronautics and fluid dynamics the "International Standard Atmosphere" (ISA) is a specification of pressure, temperature, density, and speed of sound at each altitude. The International Standard Atmosphere is representative of atmospheric conditions at mid latitudes. In the USA this information is specified the U.S. Standard Atmosphere which is identical to the "International Standard Atmosphere" at all altitudes up to 65,000 feet above sea level.
Due to the fact that many definitions of standard temperature and pressure differ in temperature significantly from standard laboratory temperatures (e.g., 0 °C vs. ~25 °C), reference is often made to "standard laboratory conditions" (a term deliberately chosen to be different from the term "standard conditions for temperature and pressure", despite its semantic near identity when interpreted literally). However, what is a "standard" laboratory temperature and pressure is inevitably culture-bound, given that different parts of the world differ in climate, altitude and the degree of use of heat/cooling in the workplace. For example, schools in New South Wales, Australia use 25 °C at 100 kPa for standard laboratory conditions.
ASTM International has published Standard ASTM E41- Terminology Relating to Conditioning and hundreds of special conditions for particular materials and test methods. Other standards organizations also have specialized standard test conditions.
It is equally as important to indicate the applicable reference conditions of temperature and pressure when stating the molar volume of a gas as it is when expressing a gas volume or volumetric flow rate. Stating the molar volume of a gas without indicating the reference conditions of temperature and pressure has no meaning and it can cause confusion.
The molar gas volumes can be calculated with an accuracy that is usually sufficient by using the universal gas law for ideal gases. The usual expression is:
...which can be rearranged thus:
where (in SI metric units):
|P||= the absolute pressure of the gas, in Pa (pascal)|
|n||= amount of substance, in mol|
|V||= the volume of the gas, in m3|
|T||= the absolute temperature of the gas, in K|
|R||= the universal gas law constant of 8.3145 m3·Pa/(mol·K)|
The molar volume of any ideal gas may be calculated at various standard reference conditions as shown below:
The technical literature can be confusing because many authors fail to explain whether they are using the universal gas law constant R, which applies to any ideal gas, or whether they are using the gas law constant Rs, which only applies to a specific individual gas. The relationship between the two constants is Rs = R / M, where M is the molecular weight of the gas.
The US Standard Atmosphere (USSA) uses 8.31432 m3·Pa/(mol·K) as the value of R (see Gas constant) for all calculations. However, the USSA,1976 does recognize that this value is not consistent with the values of the Avogadro constant and the Boltzmann constant.