Standard temperature and pressure (STP) or standard conditions for temperature and pressure are various standard sets of conditions for experimental measurements used 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 Outline of p ...

(NIST), although these are not universally accepted. Other organizations have established a variety of other definitions. In industry and

commerce Commerce is the organized Complex system, system of activities, functions, procedures and institutions that directly or indirectly contribute to the smooth, unhindered large-scale exchange (distribution through Financial transaction, transactiona ...

, the standard conditions for temperature and pressure are often necessary for expressing 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 (Sm³/s), and normal cubic meters per second (Nm³/s). 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, (), and 0% humidity.

Definitions

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 quantitatively expresses the attribute of hotness or coldness. Temperature is measurement, measured with a thermometer. It reflects the average kinetic energy of the vibrating and colliding atoms making ...

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 unit of measurement, units of force per unit of surface area. Many techniques have been developed for the me ...

of exactly 1 atm (101.325

kPa The pascal (symbol: Pa) is the unit of pressure in the International System of Units (SI). It is also used to quantify internal pressure, stress, Young's modulus, and ultimate tensile strength. The unit, named after Blaise Pascal, is an SI ...

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

of 273.15 K (0 °C, 32 °F) and an

of exactly 1 bar (100 kPa, 10⁵ Pa).

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 or ψ), the twenty-third letter of the Greek alphabet * Psi (Cyrillic), letter of the early Cyrillic alphabet, adopted from Greek Arts and entertainment * "Psi" as an abbreviat ...

, 100

). NIST also uses 15 °C (288.15 K, 59 °F) for the temperature compensation of refined petroleum products, despite noting that these two values are not exactly consistent with each other. The ISO 13443 standard reference conditions for natural gas and similar fluids are and 101.325 kPa; by contrast, the

American Petroleum Institute The American Petroleum Institute (API) is the largest U.S. trade association for the oil and natural gas industry. It claims to represent nearly 600 corporations involved in extraction of petroleum, production, oil refinery, refinement, pipeline ...

adopts .

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. Also, the

International Organization for Standardization The International Organization for Standardization (ISO ; ; ) is an independent, non-governmental, international standard development organization composed of representatives from the national standards organizations of member countries. M ...

(ISO), the

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

(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

aeronautics Aeronautics is the science or art involved with the study, design process, design, and manufacturing of air flight-capable machines, and the techniques of operating aircraft and rockets within the atmosphere. While the term originally referred ...

and

fluid dynamics In physics, physical chemistry and engineering, fluid dynamics is a subdiscipline of fluid mechanics that describes the flow of fluids – liquids and gases. It has several subdisciplines, including (the study of air and other gases in motion ...

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 ...

" (ISA) is a specification of pressure, temperature, density, and speed of sound at each altitude. At standard mean sea level it specifies a temperature of , pressure of (1 atm), and a density of . It also specifies a temperature lapse rate of −6.5 °C (−11.7 °F) per km (approximately −2 °C (−3.6 °F) per 1,000 ft).Batchelor, G. K., ''An Introduction to Fluid Dynamics'', Cambridge Univ. Press, 1967. The International Standard Atmosphere is representative of atmospheric conditions at mid latitudes. In the US 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.

Standard laboratory conditions

Because many definitions of standard temperature and pressure differ in temperature significantly from standard laboratory temperatures (e.g. 0 °C vs. ~28 °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. Laboratories are found in a variety of settings such as schools ...

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 New South Wales (commonly abbreviated as NSW) is a States and territories of Australia, state on the Eastern states of Australia, east coast of :Australia. It borders Queensland to the north, Victoria (state), Victoria to the south, and South ...

Australia Australia, officially the Commonwealth of Australia, is a country comprising mainland Australia, the mainland of the Australia (continent), Australian continent, the island of Tasmania and list of islands of Australia, numerous smaller isl ...

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

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

has published Standard 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 specified procedure that produces a test result. To ensure accurate and relevant results, a test method should b ...

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 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 (''V'') occupied by a substance to the amount of substance (''n''), usually at a given temperature and pressure. It is also eq ...

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 ''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 () is the mass of a given molecule, often expressed in units of daltons (Da). Different molecules of the same compound may have different molecular masses because they contain different isotopes of an element. The derived quan ...

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

Avogadro constant The Avogadro constant, commonly denoted or , is an SI defining constant with an exact value of when expressed in reciprocal moles. It defines the ratio of the number of constituent particles to the amount of substance in a sample, where th ...

and the

Boltzmann constant The Boltzmann constant ( or ) is the proportionality factor that relates the average relative thermal energy of particles in a ideal gas, gas with the thermodynamic temperature of the gas. It occurs in the definitions of the kelvin (K) and the ...

.''U.S. Standard Atmosphere, 1976''
U.S. Government Printing Office, Washington, D.C., 1976.

Explanatory notes

References

{{Reflist

External links

"Standard conditions for gases"
from the

IUPAC 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 ...

''Gold Book''.
"Standard pressure"
from the

''Gold Book''.
"STP"
from the

''Gold Book''.
"Standard state"
from the

''Gold Book''. Atmospheric thermodynamics Aerodynamics Engineering thermodynamics Gases Measurement Physical chemistry Standards Thermodynamics