Standard temperature and pressure

Not to be confused with Standard sea-level conditions.

For the conditions used in thermodynamic evaluations, see Standard state.

Standard temperature and pressure (STP) are various 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.

In industry and commerce, the 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.

Definitions

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

• 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 has been 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).^[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 (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

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

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				Density	Relative humidity (%)	Publishing or establishing entity
°C	°F	kPa	mmHg	psi	inHg	kg/m³
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	1.225	0^[7]^[16]	ICAO's ISA,^[16] ISO 13443,^[7] EEA,^[17] EGIA (SI Definition)^[18]
20	68	101.325	760.00	14.6959	29.921			EPA,^[19] NIST.^[20] This is also called NTP, Normal Temperature and Pressure.^[21]
22	71.6	101.325	760.00	14.6959	29.921		20–80	American Association of Physicists in Medicine^[22]
25	77	101.325	760.00	14.6959	29.921			SATP,^[23] EPA^[24]
20	68	100.000	750.06	14.5038	29.530		0	CAGI^[25]
15	59	100.000	750.06	14.5038	29.530			SPE^[26]
20	68	101.3	760	14.69	29.9		50	ISO 5011^[27]
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,^[26] U.S. OSHA,^[28] SCAQMD^[29]
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)^[30]
15	59	99.99	750.0	14.503	29.53		78	U.S. Army Standard Metro^[31]^[a]
15	59	101.33	760.0	14.696	29.92		60	ISO 2314,^[32] ISO 3977-2,^[33] ASHRAE Fundamentals Handbook^[34]
21.11	70	101.3	760	14.70	29.92		0	AMCA,^[35]^[b] air density = 0.075 lbm/ft^3. This AMCA standard applies only to air; Compressed Gas Association [CGA] applies to industrial gas use in US^[36]
15	59	101.3	760	14.70	29.92			FAA^[37]
20	68	101.325	760.00	14.6959	29.921		0	EN 14511-1:2013^[38]
15	59	101.325	760.00	14.6959	29.921		0	ISO 2533:1975^[39] ISO 13443:2005,^[40] ISO 7504:2015^[41]
0	32	101.325	760.00	14.6959	29.921		0	DIN 1343:1990^[42]

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 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 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

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.^[43]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

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^[44] 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:

• 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 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.^[45]

See also

• Environmental chamber
• ISO 1 – standard reference temperature for geometric product specifications
• Reference atmospheric model
• Room temperature
• Standard sea-level conditions
• Standard state

Explanatory notes

1. 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.
2. 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).

References

1. A. D. McNaught and A. Wilkinson (1997). Nič, Miloslav; Jirát, Jiří; Košata, Bedřich; Jenkins, Aubrey; McNaught, Alan (eds.). IUPAC. Compendium of Chemical Terminology (PDF) (2nd ed.). Oxford: Blackwell Scientific Publications. p. 54. doi:10.1351/goldbook. ISBN 0-632-03583-8. Standard conditions for gases: ... and pressure of 10⁵ pascals. The previous standard absolute pressure of 1 atm (equivalent to 101.325 kPa) was changed to 100 kPa in 1982. IUPAC recommends that the former pressure should be discontinued.
2. A. D. McNaught and A. Wilkinson (1997). "standard pressure". IUPAC. Compendium of Chemical Terminology (2nd ed.). Oxford: Blackwell Scientific Publications. doi:10.1351/goldbook.S05921. ISBN 978-0-9678550-9-7.
3. Doiron, Theodore D. (January 2007). "20 Degrees Celsius--A Short History of the Standard Reference Temperature for Industrial Dimensional Measurements". Nist. 112 (1): 1–23. doi:10.6028/jres.112.001. PMC 4654601. PMID 27110451.
4. Helrich, Carl S. (2008-11-14). Modern Thermodynamics with Statistical Mechanics. Springer Science & Business Media. ISBN 978-3-540-85418-0.
5. "A Guide to the NIST Chemistry WebBook". webbook.nist.gov. Retrieved 2020-10-06.
6. "Specifications, Tolerances, and Other Technical Requirements for Weighing and Measuring Devices" (PDF). p. 3-1.
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8. API Petroleum Measurement
9. Doiron, Ted (Jan–Feb 2007). "20 °C – A Short History of the Standard Reference Temperature for Industrial Dimensional Measurements". Journal of Research of the National Institute of Standards and Technology. 112 (1): 1–23. doi:10.6028/jres.112.001. PMC 4654601. PMID 27110451.
10. Cohen, Joel E.; Small, Christopher (November 24, 1998). "Hypsographic demography: The distribution of human population by altitude". Proceedings of the National Academy of Sciences. 95 (24): 14009–14014. Bibcode:1998PNAS...9514009C. doi:10.1073/pnas.95.24.14009. PMC 24316. PMID 9826643.
11. Gassco. . Archived from the original on October 18, 2007. Retrieved 2008-07-25. Scm: The usual abbreviation for standard cubic metre – a cubic metre of gas under a standard condition, defined as an atmospheric pressure of 1.01325 bar and a temperature of 15°C. This unit provides a measure for gas volume.
12. Nord Stream (October 2007). (PDF). Archived from the original (PDF) on 2008-02-16. Retrieved 2008-07-25. bcm: Billion Cubic Meter (standard cubic metre – a cubic metre of gas under a standard condition, defined as an atmospheric pressure of 1 atm and a temperature of 15 °C.)
13. Metrogas (June 2004). "Natural gas purchase and sale agreement". Retrieved 2008-07-25. Natural gas at standard condition shall mean the quantity of natural gas, which at a temperature of fifteen (15) Celsius degrees and a pressure of 101.325 kilopascals occupies the volume of one (1) cubic meter.
14. NIST (1989). . Archived from the original on October 6, 2010. Retrieved 2008-07-25. If you want the program to treat the material as an ideal gas, the density will be assumed given by M/V, where M is the gram molecular weight of the gas and V is the mol volume of 22414 cm³ at standard conditions (0 deg C and 1 atm).
15. ISO (1994). "ISO 10780:1994 : Stationary source emissions – Measurement of velocity and volume flowrate of gas streams in ducts".
16. Robert C. Weast, ed. (1975). Handbook of Physics and Chemistry (56th ed.). CRC Press. pp. F201–F206. ISBN 978-0-87819-455-1.
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19. "Standards of Performance for New Sources", 40 CFR—Protection of the Environment, Chapter I, Part 60, Section 60.2, 1990.
20. Wright, J. D.; Johnson, A. N.; Moldover, M. R. (2003). (PDF). Journal of Research of the National Institute of Standards and Technology. 108 (1): 21–47. doi:10.6028/jres.108.004. PMC 4844527. PMID 27413592. Archived from the original (PDF) on 2004-07-21.
21. . Socratic. Archived from the original on 2015-11-27. Retrieved 2018-08-28.
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30. "49 C.F.R. § 171". Retrieved 22 May 2018.
31. Sierra Bullets. . Rifle and Handgun Reloading Manual (5 ed.). Sedalia, MO, US. Archived from the original on 2006-03-09. Retrieved 2006-02-03.
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35. ANSI/AMCA Standard 210, "Laboratory Methods Of Testing Fans for Aerodynamic Performance Rating", as implied by http://www.greenheck.com/pdf/centrifugal/Plug.pdf when accessed on October 17, 2007.
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45. U.S. Standard Atmosphere, 1976, U.S. Government Printing Office, Washington, D.C., 1976.

External links

• "Standard conditions for gases" from the IUPAC Gold Book.
• "Standard pressure" from the IUPAC Gold Book.
• "STP" from the IUPAC Gold Book.
• "Standard state" from the IUPAC Gold Book.