Standard conditions for temperature and pressure

Standard temperature and pressure 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.

Not to be confused with Standard sea-level conditions or Standard state.

In chemistry, IUPAC changed the definition of standard temperature and pressure (STP) in 1982:[1][2]

STP should not be confused with the standard state commonly used in thermodynamic evaluations of the Gibbs energy of a reaction.

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). Please note that 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).[3][4]

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.[5]

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 К (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 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.[6]

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).

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.[7][8][9] 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 Relative
humidity
(%)		 Publishing or establishing entity
°C °F kPa mmHg psi inHg
0 32100.000 750.06 14.5038 29.530 IUPAC (STP) since 1982[1]
0 32101.325 760.00 14.6959 29.921 NIST,[10] ISO 10780,[11] formerly IUPAC (STP) until 1982[1]
15 59101.325 760.00 14.6959 29.9210[5][12]ICAO's ISA,[12] ISO 13443,[5] EEA,[13] EGIA (SI Definition)[14]
20 68101.325 760.00 14.6959 29.921 EPA,[15] NIST.[16] This is also called NTP, Normal Temperature and Pressure.[17]
22 72101.325 760.00 14.6959 29.92120–80American Association of Physicists in Medicine[18]
25 77100.000 750.06 14.5038 29.530 IUPAC (SATP)
25 77101.325 760.00 14.6959 29.921 EPA[19]
20 68100.000 750.06 14.5038 29.5300CAGI[20]
15 59100.000 750.06 14.5038 29.530 SPE[21]
20 68101.3 760 14.69 29.950ISO 5011[22]
20 68101.33 760.014.69629.920GOST 2939-63
16 60101.33 760.0 14.69629.92 SPE,[21] U.S. OSHA,[23] SCAQMD[24]
16 60101.6 762 14.7330.0 EGIA (Imperial System Definition)[14]
16 60101.35 760.21 14.729.93 U.S. DOT (SCF)[25]
15 5999.99 750.0 14.50329.5378U.S. Army Standard Metro[26][lower-alpha 1]
15 59101.33 760.0 14.69629.9260ISO 2314,[27] ISO 3977-2[28]
21 70101.3 760 14.70 29.920AMCA,[29][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[30]
15 59101.3 760 14.70 29.92 Federal Aviation Administration (FAA)[31]
20 68101.325 760.00 14.6959 29.9210EN 14511-1:2013[32]
15 59101.325 760.00 14.6959 29.9210ISO 2533:1975[33] ISO 13443:2005,[34] ISO 7504:2015[35]
0 32101.325 760.00 14.6959 29.9210DIN 1343:1990[36]

Note: This table needs careful checking. For example the American Association of Physicists in Medicine paper quotes a temperature of 22 °C. It does not quote a Fahrenheit equivalent. The correct Fahrenheit equivalent is 71.6 °F, not 72 °F as stated in the table. Note: the covert function can display decimal places if used correctly. With no decimal places data, it defaults to "precision comparable to that of the input value "

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 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.[37] 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[38] 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.[39]

See also

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 presure". IUPAC. Compendium of Chemical Terminology (2nd ed.). Oxford: Blackwell Scientific Publications. doi:10.1351/goldbook.S05921. ISBN 978-0-9678550-9-7.
  3. Helrich, Carl S. (2008-11-14). Modern Thermodynamics with Statistical Mechanics. Springer Science & Business Media. ISBN 978-3-540-85418-0.
  4. "A Guide to the NIST Chemistry WebBook". webbook.nist.gov. Retrieved 2020-10-06.
  5. Natural gas – Standard reference conditions (ISO 13443). Geneva, Switzerland: International Organization for Standardization. 1996.
  6. 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.
  7. Gassco. "Concepts – Standard cubic meter (scm)". 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.
  8. Nord Stream (October 2007). "Status of the Nord Stream pipeline route in the Baltic Sea" (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.)
  9. 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.
  10. NIST (1989). "NIST Standard Reference Database 124 – Stopping-Power and Range Tables for Electrons, Protons, and Helium Ions". 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).
  11. ISO (1994). "ISO 10780:1994 : Stationary source emissions – Measurement of velocity and volume flowrate of gas streams in ducts".
  12. Robert C. Weast (Editor) (1975). Handbook of Physics and Chemistry (56th ed.). CRC Press. pp. F201–F206. ISBN 978-0-87819-455-1.CS1 maint: extra text: authors list (link)
  13. Extraction, First Treatment and Loading of Liquid & Gaseous Fossil Fuels (Emission Inventory Guidebook B521, Activities 050201 – 050303) (PDF). Copenhagen, Denmark: European Environmental Agency. September 1999.
  14. "Electricity and Gas Inspection Act", SOR/86-131 (defines a set of standard conditions for Imperial units and a different set for metric units)  Canadian Laws.
  15. "Standards of Performance for New Sources", 40 CFR—Protection of the Environment, Chapter I, Part 60, Section 60.2, 1990  New Source Performance Standards.
  16. Wright, J. D.; Johnson, A. N.; Moldover, M. R. (2003). "Design and Uncertainty for a PVTt Gas Flow Standard" (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.
  17. "What is the difference between STP and NTP?". Socratic. Archived from the original on 2015-11-27. Retrieved 2018-08-28.
  18. Almond, Peter R.; Biggs, Peter J.; Coursey, B. M.; Hanson, W. F.; Huq, M. Saiful; Nath, Ravinder; Rogers, D. W. O. (1999). "AAPM's TG-51 protocol for clinical reference dosimetry of high-energy photon and electron beams". Medical Physics. 26 (9): 1847–1870. Bibcode:1999MedPh..26.1847A. doi:10.1118/1.598691. PMID 10505874. S2CID 12687636.
  19. "National Primary and Secondary Ambient Air Quality Standards", 40 CFR—Protection of the Environment, Chapter I, Part 50, Section 50.3, 1998  National Ambient Air Standards.
  20. "Glossary". Cleveland, OH, USA: Compressed Air and Gas Institute. 2002. Archived from the original on 2007-09-02.
  21. "The SI Metric System of Units and SPE Metric Standard (1982)" (PDF). Society of Petroleum Engineers. Standard Temperature (Page 24), and Notes for Table 2.3, (on PDF page 25 of 42 PDF pages), define two different sets of reference conditions, one for the standard cubic foot and one for the standard cubic meter.
  22. Air Intake Filters (ISO 5011:2002). Geneva, Switzerland: International Organization for Standardization. 2002.
  23. "Storage and Handling of Liquefied Petroleum Gases" and "Storage and Handling of Anhydrous Ammonia", 29 CFR—Labor, Chapter XVII—Occupational Safety and Health Administration, Part 1910, Sect. 1910.110 and 1910.111, 1993  Storage/Handling of LPG.
  24. "Rule 102, Definition of Terms (Standard Conditions)", Amended December 2004, South Coast Air Quality Management District, Los Angeles, California, USA  SCAQMD Rule 102
  25. "49 C.F.R. § 171". Retrieved 22 May 2018.
  26. Sierra Bullets. "Chapter 3 – Effects of Altitude and Atmospheric Conditions (Exterior Ballistics Section)". Rifle and Handgun Reloading Manual (5 ed.). Sedalia, MO, USA. Archived from the original on 2006-03-09. Retrieved 2006-02-03.
  27. Gas turbines – Acceptance tests (ISO 2314:1989) (2 ed.). Geneva, Switzerland: International Organization for Standardization. 1989.
  28. Gas turbines – Procurement – Part 2: Standard reference conditions and ratings (ISO 3977-2:1997). Geneva, Switzerland: International Organization for Standardization. 1997.
  29. 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.
  30. Association, Compressed Gas (2012-12-06). Compressed Gas Handbook. ISBN 9781461306733. Retrieved 22 Nov 2017.
  31. "Chapter 3, Principles of Flight" (PDF). Pilot's Handbook of Aeronautical Knowledge. Federal Aviation Administration.
  32. Air Conditioners, liquid chilling packages and heat pumps with electrically driven compressors for space heating and cooling. UK: BSI EN. 2013.
  33. Standard Atmosphere. Geneva, Switzerland: International Organization for Standardization. 1975.
  34. Natural gas - Standard reference conditions. Geneva, Switzerland: International Organization for Standardization. 1996.
  35. Gas analysis - Vocabulary. Geneva, Switzerland: International Organization for Standardization. 2015.
  36. Referenzzustand, Normzustand, Normvolumen; Begriffe und Werte. Germany: Deutsches Institut für Normung. 1990.
  37. Peter Gribbon (2001). Excel HSC Chemistry Pocket Book Years 11–12. Pascal Press. ISBN 978-1-74020-303-6.
  38. "Fundamental Physical Properties: Molar Volumes (CODATA values for ideal gases)". NIST.
  39. U.S. Standard Atmosphere, 1976, U.S. Government Printing Office, Washington, D.C., 1976.
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