Standard conditions for temperature and pressure
Standard temperature and pressure (STP) are standard 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 (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.
- Until 1982, STP was defined as a temperature of 273.15 K (0 °C, 32 °F) and an absolute pressure of exactly 1 atm (101.325 kPa).
- Since 1982, STP is defined as a temperature of 273.15 K (0 °C, 32 °F) and an absolute pressure of exactly 105 Pa (100 kPa, 1 bar).
NIST 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). These stated values of STP used by NIST have not been verified and require a source. However, values cited in Modern Thermodynamics with Statistical Mechanics by Carl S. Helrich and A Guide to the NIST Chemistry WebBook by Peter J. Linstrom suggest a common STP in use by NIST for thermodynamic experiments is 298.15 K (25°C, 77°F) and 1 bar (14.5038 psi, 100 kPa).
The International Standard Metric Conditions for natural gas and similar fluids are 288.15 K (15.00 °C; 59.00 °F) 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): standard cubic meters per second (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.
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 15 °C (288.15 K; 59.00 °F) and 101.325 kPa (1.00 atm; 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). 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 (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.
|Publishing or establishing entity|
|0||32||100.000||750.06||14.5038||29.530||IUPAC (STP) since 1982|
|15||59||101.325||760.00||14.6959||29.921||0||ICAO's ISA, ISO 13443, EEA, EGIA (SI Definition)|
|20||68||101.325||760.00||14.6959||29.921||EPA, NIST. This is also called NTP, Normal Temperature and Pressure.|
|22||71.6||101.325||760.00||14.6959||29.921||20–80||American Association of Physicists in Medicine|
|15.56||60||101.33||760.0||14.696||29.92||SPE, U.S. OSHA, SCAQMD|
|15.56||60||101.6||762||14.73||30.0||EGIA (Imperial System Definition)|
|15.56||60||101.35||760.21||14.7||29.93||U.S. DOT (SCF)|
|15||59||99.99||750.0||14.503||29.53||78||U.S. Army Standard Metro[lower-alpha 1]|
|15||59||101.33||760.0||14.696||29.92||60||ISO 2314, ISO 3977-2|
|21.11||70||101.3||760||14.70||29.92||0||AMCA,[lower-alpha 2] air density = 0.075 lbm/ft3. This AMCA standard applies only to air;|
Compressed Gas Association [CGA] applies to industrial gas use in USA
|15||59||101.3||760||14.70||29.92||Federal Aviation Administration (FAA)|
|15||59||101.325||760.00||14.6959||29.921||0||ISO 2533:1975 ISO 13443:2005, ISO 7504:2015|
- 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 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.
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 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, 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.
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 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 molar volume of any ideal gas may be calculated at various standard reference conditions as shown below:
- Vm = 8.3145 × 273.15 / 101.325 = 22.414 dm3/mol at 0 °C and 101.325 kPa
- Vm = 8.3145 × 273.15 / 100.000 = 22.711 dm3/mol at 0 °C and 100 kPa
- Vm = 8.3145 × 298.15 / 101.325 = 24.466 dm3/mol at 25 °C and 101.325 kPa
- Vm = 8.3145 × 298.15 / 100.000 = 24.790 dm3/mol at 25 °C and 100 kPa
- Vm = 10.7316 × 519.67 / 14.696 = 379.48 ft3/lbmol at 60 °F and 14.696 psi (or about 0.8366 ft3/gram mole)
- Vm = 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 Rs. The relationship between the two constants is Rs = R / m, where m is the molecular mass 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 and the Boltzmann constant.
- Environmental chamber
- ISO 1 – standard reference temperature for geometric product specifications
- Reference atmospheric model
- Room temperature
- Standard sea level
- Standard state
- The pressure is specified as 750 mmHg. However, the mmHg is temperature-dependent, since mercury expands as temperature goes up. Here the values for the 0–20 °C range are given.
- The standard is given as 29.92 inHg at an unspecified temperature. This most likely corresponds to a standard pressure of 101.325 kPa, converted into ~29.921 inHg at 32 °F (0 °C).
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