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Standard temperature and pressure
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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 (IUPAC) and the National Institute of Standards and Technology (NIST), although these are not universally accepted. Other organizations have established a variety of other definitions.

In industry and commerce, 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, 273.15 K (0 °C), and 0% humidity.

Definitions

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In chemistry, IUPAC changed its definition of standard temperature and pressure in 1982:[1][2]

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).[3] 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, 100 kPa).[4][5] 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.[6]

The ISO 13443 standard reference conditions for natural gas and similar fluids are 288.15 K (15.00 °C; 59.00 °F) and 101.325 kPa;[7] by contrast, the American Petroleum Institute adopts 60 °F (15.56 °C; 288.71 K).[8]

Past uses

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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.[9]

Current use

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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).[2] 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).[10]

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.[11][12][13] 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.

Standard reference conditions in current use
Temperature Pressure Humidity Publishing or establishing entity
°C °F kPa mmHg psi inHg %
 
0 32 100.000 750.06 14.5038 29.530 IUPAC (STP) since 1982[1]
0 32 101.325 760.00 14.6959 29.921 NIST,[14] ISO 10780,[15] formerly IUPAC (STP) until 1982[1]
15 59 101.325 760.00 14.6959 29.921 0 ICAO's ISA,[16] ISO 13443,[7] EEA,[17] EGIA (SI Definition)[18] Density 1.225 kg/m³
20 68 101.325 760.00 14.6959 29.921 EPA,[19] NIST.[20][21][22]
22 71.6 101.325 760.00 14.6959 29.921 20–80 American Association of Physicists in Medicine[23]
25 77 101.325 760.00 14.6959 29.921 SATP,[24] EPA[25]
20 68 100.000 750.06 14.5038 29.530 0 CAGI[26]
15 59 100.000 750.06 14.5038 29.530 SPE[27]
20 68 101.3 760 14.69 29.9 50 ISO 5011[28]
20 68 101.33 760.0 14.696 29.92 0 GOST 2939-63
15.56 60 101.33 760.0 14.696 29.92 SPE,[27] U.S. OSHA,[29] SCAQMD[30]
15.56 60 101.6 762 14.73 30.0 EGIA (Imperial System Definition)[18]
15.56 60 101.35 760.21 14.7 29.93 U.S. DOT (SCF)[31]
15 59 99.99 750.0 14.503 29.53 78 U.S. Army Standard Metro[32][a]
15 59 101.33 760.0 14.696 29.92 60 ISO 2314,[33] ISO 3977-2,[34] ASHRAE Fundamentals Handbook[35]
21.11 70 101.3 760 14.70 29.92 0 AMCA,[36][b] air density = 0.075 lbm/ft3.[37][38]
15 59 101.3 760 14.70 29.92 FAA[39]
20 68 101.325 760.00 14.6959 29.921 EN 14511-1:2013[40]
15 59 101.325 760.00 14.6959 29.921 0 ISO 2533:1975[41] ISO 13443:2005,[42] ISO 7504:2015[43]
0 32 101.325 760.00 14.6959 29.921 0 DIN 1343:1990[44]

Abbreviations:

  • EGIA: Electricity and Gas Inspection Act (of Canada)
  • SATP: Standard Ambient Temperature and Pressure
  • SCF: Standard Cubic Foot

International Standard Atmosphere

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In aeronautics and fluid dynamics the "International Standard Atmosphere" (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 15 °C (59 °F), pressure of 101,325 pascals (14.6959 psi) (1 atm), and a density of 1.2250 kilograms per cubic meter (0.07647 lb/cu ft). 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).[45][46]

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.[citation needed]

Standard laboratory conditions

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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 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.[47] 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.[citation needed]

Molar volume of a gas

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See also: Standard cubic feet and Oil barrel

It is as important to indicate the applicable reference conditions of temperature and pressure when stating the molar volume of a gas[48] 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 × 288.15 / 101.325 = 23.645 dm3/mol at 15 °C and 101.325 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 in 1976 does recognize that this value is not consistent with the values of the Avogadro constant and the Boltzmann constant.[49]

See also

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Explanatory notes

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References

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[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Standard temperature and pressure

View on Grokipedia
from Grokipedia
Standard temperature and pressure (often abbreviated as STP) is a reference set of conditions used in physical sciences, particularly chemistry and , to standardize measurements of gas properties and volumes for comparability across experiments and datasets. Defined by the International Union of Pure and Applied Chemistry (IUPAC), STP specifies a of exactly 0 °C (273.15 K) and an absolute pressure of 100 kPa (equivalent to 1 bar or 105 Pa). This convention facilitates calculations involving es, such as determining the molar volume—the volume occupied by one mole of an under these conditions, which is approximately 22.711 L/mol according to the IUPAC definition. Prior to 1982, STP was defined using a of 0 °C and a of 1 (101.325 kPa), yielding a molar volume of 22.414 L/mol; the update aligned the standard with the (SI) by adopting the more precise bar as the unit. Although the pre-1982 definition persists in some older literature and educational contexts, the IUPAC standard is now the internationally recognized benchmark for reporting gas volumes and thermodynamic data. Variations exist for other reference conditions, such as normal temperature and pressure (NTP) at 20 °C (293.15 K) and 101.325 kPa, or standard ambient temperature and pressure (SATP) at 25 °C (298.15 K) and 100 kPa, which are used in specific applications like ambient environmental testing or biological systems. The U.S. National Institute of Standards and Technology (NIST) often employs 20 °C and 101.325 kPa for practical standards in thermophysical property measurements, reflecting real-world calibration needs. These standards ensure consistency in fields ranging from gas law derivations to industrial , underscoring STP's as a foundational concept in scientific .

Definitions

Historical Uses

Before 1918, reference conditions for temperature in metric systems were commonly defined as 15 °C (288.15 K) in European and for calibrating and volume standards to minimize errors in measurements. Standard pressure was often 101.325 kPa ( or 760 mmHg). In the Imperial system prior to 1918, the corresponding standard was 60 °F (15.56 °C) and , widely adopted in the United States for documenting gas volumes, such as in the natural gas industry where volumes were reported as standard cubic feet under these conditions. In chemistry, standard temperature and pressure came to be defined as 0 °C (273.15 K) and 1 (101.325 kPa) to align with the reproducible freezing point of , facilitating precise thermometry in gas law experiments and determinations. This emphasized conceptual consistency in thermodynamic calculations over ambient convenience. The pressure unit of 1 , equivalent to 760 (mmHg), remained prevalent in science and gas measurements through the mid-20th century due to the historical reliance on mercury barometers for . These evolutions culminated in the IUPAC update to 1 bar, marking a transition from atmosphere-based to SI-aligned standards.

Current IUPAC STP

The current International Union of Pure and Applied Chemistry (IUPAC) definition of standard temperature and pressure (STP), adopted in 1982, establishes a temperature of 273.15 (0 °C) and a pressure of 100 kPa, equivalent to 1 bar or exactly 10510^5 Pa. This definition reflects a deliberate shift from the pre-1982 standard, which used a pressure of 101.325 kPa (1 atm), to better align with the (SI) and facilitate precise calculations in . STP under this definition serves as a reference condition primarily in chemistry for standardizing the reporting of gas volumes and concentrations, particularly in applications involving the , expressed as PV=nRTPV = nRT, where the VmV_m of an is given by Vm=RT/PV_m = RT/P. The adoption of 100 kPa simplifies thermodynamic computations by using a round SI-derived value, avoiding the fractional aspects of the former standard. This STP specification is detailed in the IUPAC Green Book (Quantities, Units and Symbols in , 3rd edition, 2007), which reaffirms the 1982 parameters for reference states in gaseous systems, and remains unchanged in subsequent IUPAC publications as of 2025.

Other Standards

International Standard Atmosphere

The (ISA) defines the atmospheric conditions at mean as a of 15 °C (288.15 K) and a of 101.325 kPa (1013.25 hPa or 1 ). This model provides a hypothetical, static representation of the Earth's atmosphere, assuming dry air and no variability due to or location. The ISA was established by the International Civil Aviation Organization (ICAO) in 1954 through Document 7488, building on the 1924 standards set by the International Commission for Aerial Navigation (ICAN). These foundational agreements aimed to unify atmospheric references amid post-World War I discrepancies in national models. Subsequent extensions, such as the 1964 ICAO update to 32 km altitude, refined the model while preserving its core sea-level parameters. The primary purpose of the ISA is to standardize aircraft performance calculations, altimetry, and atmospheric modeling in and , enabling consistent predictions under varying real-world conditions. It assumes —where atmospheric pressure balances gravitational forces—and ideal gas behavior for air, facilitating reliable engineering designs and . Unlike the IUPAC standard temperature and pressure, which uses 0 °C for chemical applications, the ISA's 15 °C reflects average global sea-level conditions more relevant to aeronautical use. In the troposphere, the ISA specifies a vertical temperature profile with a constant lapse rate of -6.5 °C/km from sea level up to 11 km altitude, after which temperature stabilizes at -56.5 °C until the tropopause. This linear decrease models the typical cooling with elevation in the lower atmosphere. Pressure variation with altitude follows the hydrostatic equation integrated over the temperature profile: P=P0(TT0)−gλRP = P_0 \left( \frac{T}{T_0} \right)^{-\frac{g}{\lambda R}}, where P0P_0 and T0T_0 are sea-level values, gg is gravitational acceleration, λ\lambda is the lapse rate, and RR is the gas constant for air; this form arises from combining the ideal gas law with hydrostatic balance.

Standard Ambient Temperature and Pressure

Standard Ambient Temperature and Pressure (SATP) refers to the conditions of 298.15 K (25 °C) and 100 kPa (1 bar), as recommended by the International Union of Pure and Applied Chemistry (IUPAC) for reporting thermodynamic properties in . This standard was introduced in the late , specifically through IUPAC recommendations dating to 1982, to ensure uniformity in data presentation under conditions approximating typical experiments. It aligns with the SI-unit framework adopted for STP, promoting consistency across scientific measurements while reflecting practical ambient environments. SATP finds primary application in the tabulation of key thermodynamic quantities, including standard enthalpies of formation, Gibbs free energies of reaction, and equilibrium constants, as detailed in chemistry textbooks and compilations. These conditions facilitate the of experimental results by standardizing the state for substances in gaseous, , or solution phases, emphasizing conceptual reliability over variations in local laboratory setups. Earlier definitions of SATP in some pre-2000s employed a pressure of 101.325 kPa (1 ), but the IUPAC Green Book (3rd edition, 2007) explicitly standardized it to 100 kPa to harmonize with modern units and eliminate discrepancies from the legacy atmospheric standard. Unlike STP, which also uses 100 kPa but specifies 273.15 K (0 °C) for normalizing gas volumes, SATP's elevated better suits thermodynamic evaluations at ambient conditions.

Applications

Laboratory Conditions

In laboratory settings, standard conditions for and typically range from 20 °C to 25 °C and 100 kPa to 101.325 kPa, varying by and application to ensure consistency in experimental reproducibility. The National Institute of Standards and Technology (NIST) often employs 20 °C and 1 (101.325 kPa) for measurements involving gas properties and calibrations, aligning with practical room temperatures for precision work. These conditions facilitate controlled environments without strictly adhering to theoretical ideals like those in STP definitions. Regional variations reflect local standards and SI unit preferences. In the and under (ISO) guidelines, laboratories commonly use 23 °C and 101.325 kPa for conditioning and testing materials, as specified in standards for environmental control. In , particularly in educational and industrial contexts, 25 °C and 100 kPa are adopted as standard laboratory conditions to simplify calculations with metric units. These laboratory conditions serve practical purposes such as instrument , fluid determinations, and simulated environmental testing, where deviations from assumptions are acceptable to match real-world scenarios. For instance, the American Society of Heating, Refrigerating and Air-Conditioning Engineers () specifies 20 °C and 101.325 kPa for HVAC system evaluations in its 2023 guidelines, ensuring reliable performance assessments under typical indoor air densities. Similarly, the U.S. Environmental Protection Agency (EPA) uses 25 °C and 101.3 kPa for ambient air quality monitoring, standardizing pollutant concentration reporting to account for volume corrections at these reference points. This approach prioritizes operational accuracy over universal thermodynamic constants, briefly aligning with broader ambient standards like SATP for certain thermodynamic validations.

Molar Volume of Gases

The molar volume VmV_m of an ideal gas, defined as the volume occupied by one mole of gas, is given by the ideal gas law as Vm=RTPV_m = \frac{RT}{P}, where R=8.314 462 618R = 8.314\,462\,618 J/mol·K is the molar gas constant, TT is the absolute temperature in kelvin, and PP is the pressure in pascals. Under current IUPAC STP conditions of 0 °C (273.15 K) and 100 kPa, the is 22.711 dm³/mol (or 22.711 /mol). At the historical STP of 0 °C and 101.325 kPa, it is 22.414 dm³/mol (22.414 /mol). For SATP at 25 °C (298.15 K) and 100 kPa, the is 24.789 dm³/mol. Under ISA conditions of 15 °C (288.15 K) and 101.325 kPa, it is approximately 23.644 dm³/mol. The difference between the historical and current IUPAC STP molar volumes arises solely from the pressure variation, yielding a conversion factor of approximately 1.01325 (i.e., volumes at 100 kPa are 1.01325 times those at 101.325 kPa for the same ). These values assume behavior and are essential for converting measured gas volumes to standard conditions in chemical analyses and experiments, enabling consistent comparisons across datasets despite varying laboratory environments.
StandardTemperature (°C)Pressure (kPa)Molar Volume (dm³/mol)
IUPAC STP010022.711
Historical STP0101.32522.414
SATP2510024.789
ISA15101.32523.644

References

  1. https://goldbook.iupac.org/terms/view/S06036
  2. https://cen.acs.org/articles/94/i47/Teaching-IUPAC-recommendations.html
  3. https://www.engineeringtoolbox.com/stp-standard-ntp-normal-air-d_772.html
  4. https://nvlpubs.nist.gov/nistpubs/TechnicalNotes/NIST.TN.1887.pdf
  5. https://chem-textbook.ucalgary.ca/chapter-9-main/standard-conditions-of-temperature-and-pressure/
  6. https://emtoolbox.nist.gov/publications/nistjresjan-feb2007-112-1.pdf
  7. https://law.justia.com/cases/federal/appellate-courts/F2/716/1/278283/
  8. https://old.iupac.org/publications/books/gbook/green_book_2ed.pdf
  9. https://www.nist.gov/pml/owm/metric-si/unit-conversion/pressure-and-gas-flow-unit-conversions
  10. https://iupac.org/wp-content/uploads/2019/05/IUPAC-GB3-2012-2ndPrinting-PDFsearchable.pdf
  11. https://skybrary.aero/articles/international-standard-atmosphere-isa
  12. https://www.ngdc.noaa.gov/stp/space-weather/online-publications/miscellaneous/us-standard-atmosphere-1976/us-standard-atmosphere_st76-1562_noaa.pdf
  13. https://eaglepubs.erau.edu/introductiontoaerospaceflightvehicles/chapter/international-standard-atmosphere-isa/
  14. https://www.nature.com/articles/179299a0
  15. https://ntrs.nasa.gov/api/citations/20020092087/downloads/20020092087.pdf
  16. https://www.faa.gov/sites/faa.gov/files/06_phak_ch4_0.pdf
  17. https://www.grc.nasa.gov/www/k-12/airplane/atmosmet.html
  18. https://www.e-education.psu.edu/meteo300/node/594
  19. http://fisicaatmo.at.fcen.uba.ar/practicas/ISAweb.pdf
  20. https://www.aerodynamics4students.com/properties-of-the-atmosphere/variation-with-altitude.php
  21. https://www.ausetute.com.au/moledefs.html
  22. https://www.ashrae.org/technical-resources/standards-and-guidelines/read-only-versions-of-ashrae-standards
  23. https://www.epa.gov/sites/default/files/2018-07/documents/33-15-02_ambient_air_quality_standards_final.pdf
  24. https://physics.nist.gov/cgi-bin/cuu/Value?r
  25. https://chem.libretexts.org/Bookshelves/General_Chemistry/Map%3A_Chemistry_-_The_Central_Science_(Brown_et_al.)/10%3A_Gases/10.04%3A_The_Ideal_Gas_Equation
  26. https://physics.nist.gov/cgi-bin/cuu/Value?mvolstd
  27. https://faculty.fiu.edu/~mebela/chm3410_chapter1.pdf
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