# Standard conditions for temperature and pressure

Standard conditions for temperature and pressure

In chemistry, standard condition for temperature and pressure (informally abbreviated as STP) are standard sets of conditions for experimental measurements, 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. The current version of IUPAC's standard is a temperature of 0 °C (273.15 K, 32 °F) and an absolute pressure of 100 kPa (14.504 psi, 0.986 atm),[1] while NIST's version is a temperature of 20 °C (293.15 K, 68 °F) and an absolute pressure of 101.325 kPa (14.696 psi, 1 atm). International Standard Metric Conditions for natural gas and similar fluids[2] is 288.15 Kelvin 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). However many technical publications (books, journals, advertisements for equipment and machinery) simply state "standard conditions" without specifying them, often leading to confusion and errors. Good practice is to incorporate the reference conditions wherever ambiguity is possible, e.g. V(273.15K, 101.325kPa)m3.

## Definitions

### Past use

In the last five to six decades, professionals and scientists using the metric system of units defined the standard reference conditions of temperature and pressure for expressing gas volumes as being 0 °C (273.15 K; 32.00 °F) and 101.325 kPa (1 atm or 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. However, the above two 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 their old standard of 0 °C and 101.325 kPa (1 atm).[3] Another example is from the oil industry: while a standard of 60 °F and 14.696 psi was used in the past, the current usage (particularly in North America) is predominantly of 60 °F and 14.73 psi.

Natural gas companies in Europe and South America have adopted 15 °C (59 °F) and 101.325 kPa (14.696 psi) as their standard gas volume reference conditions.[4][5][6] 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.

In Russia, State Standard GOST 2939-63 sets the following standard conditions: 20 °C (293.15 K), 760 mmHg (101325 N/m2) and zero humidity.[2]

The SATP used for presenting chemical thermodynamic properties (such as those published by the National Bureau of Standards) is standardized at 100 kPa (1 bar) but the temperature may vary and usually needs to be specified separately if complete information is desired (see standard state). Some standards are specified at certain humidity level.

Standard reference conditions in current use
Temperature Absolute pressure Relative humidity Publishing or establishing entity
°C kPa % RH
0 100.000   IUPAC (present definition)[1]
0 101.325   NIST,[7] ISO 10780,[8] formerly IUPAC[1]
15 101.325 0[9][10] ICAO's ISA,[9] ISO 13443,[10] EEA,[11] EGIA[12]
20 101.325   EPA,[13] NIST[14]
25 101.325   EPA[15]
25 100.000   SATP[16]
20 100.000 0 CAGI[17]
15 100.000   SPE[18]
20 101.3 50 ISO 5011[19]
°F psi  % RH
60 14.696   SPE,[18] U.S. OSHA,[20] SCAQMD[21]
60 14.73   EGIA,[12] OPEC,[22] U.S. EIA[23]
59 14.503 78 U.S. Army Standard Metro[24][25]
59 14.696 60 ISO 2314, ISO 3977-2[26]
°F in Hg  % RH
70 29.92 0 AMCA,[27][28] air density = 0.075 lbm/ft³. This AMCA standard applies only to air.
59 (15c) 29.92 (1013.25 hPa)   FAA, FAA's Pilot Handbook of Aeronautical Knowledge, Chapter 3[29]

Notes:

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

## 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

Due to the fact that 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 culture-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.[30]

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[31] 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 no meaning and it can cause confusion.

The molar gas volumes can be calculated with an accuracy that is usually sufficient by using the universal gas law for ideal gases. The usual expression is:

PV = nRT

…which can be rearranged thus:

$\frac{V}{n} = \frac{RT}{P}$

where (in SI metric units):

P = the absolute pressure of the gas, in Pa (pascal) = amount of substance, in mol = the volume of the gas, in m3 = the absolute temperature of the gas, in K = the universal gas law constant of 8.3145 m3·Pa/(mol·K)

or where (in customary USA units):

P = the absolute pressure of the gas, in psi = number of moles, in lbmol = the volume of the gas, in ft3/lbmol = the absolute temperature of the gas absolute, in °R = the universal gas law constant of 10.7316 ft3·psi/(lbmol·°R)

The molar volume of any ideal gas may be calculated at various standard reference conditions as shown below:

• V/n = 8.3145 × 273.15 / 101.325 = 22.414 m3/kmol at 0 °C and 101.325 kPa
• V/n = 8.3145 × 273.15 / 100.000 = 22.711 m3/kmol at 0 °C and 100 kPa
• V/n = 8.3145 × 298.15 / 101.325 = 24.466 m3/kmol at 25 °C and 101.325 kPa
• V/n = 8.3145 × 298.15 / 100.000 = 24.790 m3/kmol at 25 °C and 100 kPa
• V/n = 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/n = 10.7316 × 519.67 / 14.730 = 378.61 ft3/lbmol at 60 °F and 14.73 psi

The technical literature can be confusing because many authors fail to explain whether they are using the universal gas law constant R, which applies to any ideal gas, or whether they are using the gas law constant Rs, which only applies to a specific individual gas. The relationship between the two constants is Rs = R / M, where M is the molecular weight of the gas.

The US Standard Atmosphere uses 8.31432 m3·Pa/(mol·K) as the value of R for all calculations. (See Gas constant)

## References

1. ^ a b c A. D. McNaught, A. Wilkinson (1997). Compendium of Chemical Terminology, The Gold Book (2nd ed.). Blackwell Science. ISBN 0865426848. "Standard conditions for gases: Temperature, 273.15 K [...] and pressure of 105 pascals. IUPAC recommends that the former use of the pressure of 1 atm as standard pressure (equivalent to 1.01325 × 105 Pa) should be discontinued."
2. ^ ISO 13443
3. ^ A. D. McNaught, A. Wilkinson (1997). Compendium of Chemical Terminology, The Gold Book (2nd ed.). Blackwell Science. ISBN 0865426848. "Standard pressure: Chosen value of pressure denoted by po or p°. In 1982 IUPAC recommended the value 105 Pa, but prior to 1982 the value 101 325 Pa (= 1 atm) was usually used."
4. ^ Gassco. "Concepts – Standard cubic meter (scm)". 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."
5. ^ Nord Stream (October 2007). "Status of the Nord Stream pipeline route in the Baltic Sea". 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.)"
6. ^ 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."
7. ^ NIST (1989). "NIST Standard Reference Database 7 – NIST Electron and Positron Stopping Powers of Materials Database". Retrieved 08-07-25. "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)."
8. ^
9. ^ a b Robert C. Weast (Editor) (1975). Handbook of Physics and Chemistry (56th ed.). CRC Press. pp. F201–F206. ISBN 0-87819-455-X.
10. ^ a b "Natural gas – Standard reference conditions", ISO 13443, International Organization for Standardization, Geneva, Switzerland  ISO Standards Catalogue
11. ^ "Extraction, First Treatment and Loading of Liquid & Gaseous Fossil Fuels", Emission Inventory Guidebook B521, Activities 050201 - 050303, September 1999, European Environmental Agency, Copenhagen, Denmark  Emission Inventory Guidebook
12. ^ a b "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
13. ^ "Standards of Performance for New Sources", 40 CFR--Protection of the Environment, Chapter I, Part 60, Section 60.2, 1990  New Source Performance Standards
14. ^ "Design and Uncertainty for a PVTt Gas Flow Standard", Journal of Research of the National Institute of Standards and Technology, Vol.108, Number 1, 2003  NIST Journal
15. ^ "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
16. ^ "Table of Chemical Thermodynamic Properties", National Bureau of Standards (NBS), Journal of Physics and Chemical Reference Data, 1982, Vol. 11, Supplement 2.
17. ^ "Glossary", 2002, Compressed Air and Gas Institute, Cleveland, OH, USA  Glossary
18. ^ a b The SI Metric System of Units and SPE Metric Standard (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)
19. ^ "Air Intake Filters", ISO 5011:2002, International Organization for Standardization, Geneva, Switzerland ISO
20. ^ "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
21. ^ "Rule 102, Definition of Terms (Standard Conditions)", Amended December 2004, South Coast Air Quality Management District, Los Angeles, California, USA  SCAQMD Rule 102
22. ^ "Annual Statistical Bulletin", 2004, Editor-in-chief: Dr. Omar Ibrahim, Organization of the Petroleum Exporting Countries, Vienna, Austria  OPEC Statistical Bulletin
23. ^ "Natural Gas Annual 2004", DOE/EIA-0131(04), December 2005, U.S. Department of Energy, Energy Information Administration, Washington, D.C., USA  Natural Gas Annual 2004
24. ^ Sierra Bullets L.P.. "Chapter 3 – Effects of Altitude and Atmospheric Conditions". Rifle and Handgun Reloading Manual, 5th Edition. "Effects of Altitude and Atmospheric Conditions", Exterior Ballistics Section, Sierra's "Rifle and Handgun Reloading Manual, 5th Edition", Sedalia, MO, USA  Exterior Ballistics
25. ^ The pressure is specified as 750 mmHg. However, the mmHg is temperature dependant, as mercury expands as temperature goes up. Here the values for the 0-20°C range are given.
26. ^ "Gas turbines – Procurement – Part 2: Standard reference conditions and ratings", ISO 3977-2:1997 and "Gas turbines - Acceptance tests", ISO 2314:1989, Edition 2, International Organization for Standardization, Geneva, Switzerland ISO
27. ^ ANSI/AMCA Standard 210, "Laboratory Methods Of Testing Fans for Aerodynamic Performance Rating", as implied here: http://www.greenheck.com/pdf/centrifugal/Plug.pdf when accessed on October 17, 2007
28. ^ 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)
29. ^ [1]
30. ^ Peter Gribbon (2001). Excel HSC Chemistry Pocket Book Years 11-12. Pascal Press. ISBN 1-74020-303-8.
31. ^ Fundamental Physical Properties: Molar Volumes (CODATA values for ideal gases as listed on a NIST website page)

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