Standard temperature and pressure (STP) are

standard Standard may refer to: Symbols * Colours, standards and guidons, kinds of military signs * Standard (emblem), a type of a large symbol or emblem used for identification Norms, conventions or requirements * Standard (metrology), an object th ...

sets of conditions for experimental measurements to be 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 The International Union of Pure and Applied Chemistry (IUPAC ) is an international federation of National Adhering Organizations working for the advancement of the chemical sciences, especially by developing nomenclature and terminology. It is ...

(IUPAC) and the

National Institute of Standards and Technology The National Institute of Standards and Technology (NIST) is an agency of the United States Department of Commerce whose mission is to promote American innovation and industrial competitiveness. NIST's activities are organized into physical sc ...

(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 changed its definition of standard temperature and pressure in 1982: * Until 1982, STP was defined as a

temperature Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer. Thermometers are calibrated in various temperature scales that historically have relied o ...

of 273.15 K (0 °C, 32 °F) and an

absolute pressure Pressure measurement is the measurement of an applied force by a fluid (liquid or gas) on a surface. Pressure is typically measured in units of force per unit of surface area. Many techniques have been developed for the measurement of pressur ...

of exactly 1 atm (101.325

kPa KPA may refer to: * Keele Postgraduate Association, Keele University, UK, formerly Keele Research Association (KRA) * Kensington (Olympia) station, London, England, National Rail station code * Kenya Ports Authority * ''Kiln phosphoric acid'', a ...

). * Since 1982, STP has been defined as a

of 273.15 K (0 °C, 32 °F) and an

of exactly 10⁵ Pa (100 kPa, 1 bar). STP should not be confused with the

standard state In chemistry, the standard state of a material (pure substance, mixture or solution) is a reference point used to calculate its properties under different conditions. A superscript circle ° (degree symbol) or a Plimsoll (⦵) character is used ...

commonly used in thermodynamic evaluations of the Gibbs energy of a reaction.

NIST The National Institute of Standards and Technology (NIST) is an agency of the United States Department of Commerce whose mission is to promote American innovation and industrial competitiveness. NIST's activities are organized into physical s ...

uses a temperature of 20 °C (293.15 K, 68 °F) and an absolute pressure of 1 atm (14.696 psi, 101.325 kPa). This standard is also called normal temperature and pressure (abbreviated as NTP). However, a common temperature and pressure in use by NIST for thermodynamic experiments is 298.15 K (25° C, 77° F) and 1 bar (14.5038

psi Psi, PSI or Ψ may refer to: Alphabetic letters * Psi (Greek) (Ψ, ψ), the 23rd letter of the Greek alphabet * Psi (Cyrillic) (Ѱ, ѱ), letter of the early Cyrillic alphabet, adopted from Greek Arts and entertainment * "Psi" as an abbreviatio ...

, 100 kPa). NIST also uses "15 °C (60 °F)" for the temperature compensation of refined petroleum products, despite noting that these two values are not exactly consistent with each other. The International Standard Metric Conditions for natural gas and similar fluids are and 101.325 kPa. In industry and

commerce Commerce is the large-scale organized system of activities, functions, procedures and institutions directly and indirectly related to the exchange (buying and selling) of goods and services among two or more parties within local, regional, nation ...

, 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):

standard cubic meters per second Flow measurement is the quantification of bulk fluid movement. Flow can be measured in a variety of ways. The common types of flowmeters with industrial applications are listed below: * a) Obstruction type (differential pressure or variable area ...

(Sm³/s), and normal cubic meters per second (Nm³/s). However, many technical publications (books, journals, advertisements for equipment and machinery) simply state "standard conditions" without specifying them; often substituting the term with older "normal conditions", or "NC". In special cases this can lead to confusion and errors. Good practice always incorporates the reference conditions of temperature and pressure. If not stated, some room environment conditions are supposed, close to 1 atm pressure, 293 K (20 °C), and 0% humidity.

Definitions

Past uses

Before 1918, many professionals and scientists using the metric system of units defined the standard reference conditions of temperature and pressure for expressing gas volumes as being and . During those same years, the most commonly used standard reference conditions for people using the imperial or U.S. customary systems was and 14.696

(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.

Current use

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). The new value is the mean atmospheric pressure at an altitude of about 112 metres, which is closer to the worldwide median altitude of human habitation (194 m). Natural gas companies in Europe, Australia, and South America have adopted 15 °C (59 °F) and 101.325 kPa (14.696 psi) as their standard gas volume reference conditions, used as the base values for defining the

standard cubic meter A standard cubic foot (scf) is a unit used both in the natural gas industry to represent an amount of natural gas and in other industries where other gases are used. It is the unit commonly used when following the customary system, a collection ...

. Also, the

International Organization for Standardization The International Organization for Standardization (ISO ) is an international standard development organization composed of representatives from the national standards organizations of member countries. Membership requirements are given in Ar ...

(ISO), the

United States Environmental Protection Agency The Environmental Protection Agency (EPA) is an independent executive agency of the United States federal government tasked with environmental protection matters. President Richard Nixon proposed the establishment of EPA on July 9, 1970; it ...

(EPA) and

(NIST) each have more than one definition of standard reference conditions in their various standards and regulations. Abbreviations: * EGIA: Electricity and Gas Inspection Act (of Canada) * SATP: Standard Ambient Temperature and Pressure * SCF: Standard Cubic Foot

International Standard Atmosphere

In aeronautics and

fluid dynamics In physics and engineering, fluid dynamics is a subdiscipline of fluid mechanics that describes the flow of fluids— liquids and gases. It has several subdisciplines, including ''aerodynamics'' (the study of air and other gases in motion) an ...

the "

International Standard Atmosphere The International Standard Atmosphere (ISA) is a static atmospheric model of how the pressure, temperature, density, and viscosity of the Earth's atmosphere change over a wide range of altitudes or elevations. It has been established to provide a ...

" (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 The U.S. Standard Atmosphere is a static atmospheric model of how the pressure, temperature, density, and viscosity of the Earth's atmosphere change over a wide range of altitudes or elevations. The model, based on an existing international ...

which is identical to the "International Standard Atmosphere" at all altitudes up to 65,000 feet above sea level.

Standard laboratory conditions

Because 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 A laboratory (; ; colloquially lab) is a facility that provides controlled conditions in which scientific or technological research, experiments, and measurement may be performed. Laboratory services are provided in a variety of settings: physicia ...

temperature and pressure is inevitably geography-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 ) , nickname = , image_map = New South Wales in Australia.svg , map_caption = Location of New South Wales in AustraliaCoordinates: , subdivision_type = Country , subdivision_name = Australia , established_title = Before federation , es ...

, Australia use 25 °C at 100 kPa for standard laboratory conditions.

ASTM International ASTM International, formerly known as American Society for Testing and Materials, is an international standards organization that develops and publishes voluntary consensus technical standards for a wide range of materials, products, systems, an ...

has published

Standard Standard may refer to: Symbols * Colours, standards and guidons, kinds of military signs * Standard (emblem), a type of a large symbol or emblem used for identification Norms, conventions or requirements * Standard (metrology), an object th ...

ASTM E41- Terminology Relating to Conditioning and hundreds of special conditions for particular materials and

test method A test method is a method for a test in science or engineering, such as a physical test, chemical test, or statistical test. It is a definitive procedure that produces a test result. In order to ensure accurate and relevant test results, a test m ...

s. Other

standards organization A standards organization, standards body, standards developing organization (SDO), or standards setting organization (SSO) is an organization whose primary function is developing, coordinating, promulgating, revising, amending, reissuing, interpr ...

s also have specialized standard test conditions.

Molar volume of a gas

It is equally as important to indicate the applicable reference conditions of temperature and pressure when stating the

molar volume In chemistry and related fields, the molar volume, symbol ''V''m, or \tilde V of a substance is the ratio of the volume occupied by a substance to the amount of substance, usually given at a given temperature and pressure. It is equal to the molar ...

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 very little meaning and can cause confusion. The molar volume of gases around STP and at atmospheric pressure can be calculated with an accuracy that is usually sufficient by using the

ideal gas law The ideal gas law, also called the general gas equation, is the equation of state of a hypothetical ideal gas. It is a good approximation of the behavior of many gases under many conditions, although it has several limitations. It was first stat ...

. The molar volume of any ideal gas may be calculated at various standard reference conditions as shown below: * ''V''_m = 8.3145 × 273.15 / 101.325 = 22.414 dm³/mol at 0 °C and 101.325 kPa * ''V''_m = 8.3145 × 273.15 / 100.000 = 22.711 dm³/mol at 0 °C and 100 kPa * ''V''_m = 8.3145 × 288.15 / 101.325 = 23.645 dm³/mol at 15 °C and 101.325 kPa * ''V''_m = 8.3145 × 298.15 / 101.325 = 24.466 dm³/mol at 25 °C and 101.325 kPa * ''V''_m = 8.3145 × 298.15 / 100.000 = 24.790 dm³/mol at 25 °C and 100 kPa * ''V''_m = 10.7316 × 519.67 / 14.696 = 379.48 ft³/lbmol at 60 °F and 14.696 psi (or about 0.8366 ft³/gram mole) * ''V''_m = 10.7316 × 519.67 / 14.730 = 378.61 ft³/lbmol at 60 °F and 14.73 psi Technical literature can be confusing because many authors fail to explain whether they are using the

ideal gas constant The molar gas constant (also known as the gas constant, universal gas constant, or ideal gas constant) is denoted by the symbol or . It is the molar equivalent to the Boltzmann constant, expressed in units of energy per temperature increment per ...

''R'', or the specific gas constant ''R''_s. The relationship between the two constants is ''R''_s = ''R'' / ''m'', where ''m'' is the

molecular mass The molecular mass (''m'') is the mass of a given molecule: it is measured in daltons (Da or u). Different molecules of the same compound may have different molecular masses because they contain different isotopes of an element. The related quant ...

of the gas. The US Standard Atmosphere (USSA) uses 8.31432 m³·Pa/(mol·K) as the value of ''R''. However, the USSA,1976 does recognize that this value is not consistent with the values of the

Avogadro constant The Avogadro constant, commonly denoted or , is the proportionality factor that relates the number of constituent particles (usually molecules, atoms or ions) in a sample with the amount of substance in that sample. It is an SI defining c ...

and the Boltzmann constant.''U.S. Standard Atmosphere, 1976''
U.S. Government Printing Office, Washington, D.C., 1976.

Explanatory notes

References

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