Reference conditions of gas temperature and pressure

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In chemistry, physics and engineering as well as industry and commerce, the reference conditions of gas temperature and pressure define the density of a gas and document a stated gas volume. The reference conditions must be specified when expressing a gas volume or a volumetric flow rate because gas volumes vary with the temperature and pressure of the gas.

The available data on the various definitions of so-called standard conditions clearly indicates that there is no universally accepted definition of the standard reference conditions of temperature and pressure (see Table 1 below). For that reason, simply stating gas volumes or gas volumetric flow rates are at standard conditions or at STP (Standard Temperature and Pressure) has no meaning unless the specific reference conditions are clearly stated. For the same reason, the notations Nm3 (normal cubic metres) and scf (standard cubic feet) are also meaningless unless the specific reference conditions are clearly stated.

Past and present definitions

For a great many years, most engineers, chemists, physicists and other 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) and 101.325 kPa (i.e., 1 atmosphere of absolute pressure). During those same years, the most commonly used standard reference conditions for people using the Imperial or customary USA system of units was 60 °F (520 °R) and 14.696 psia (i.e., 1 atmosphere of absolute pressure) because it was almost universally used by the oil and gas industries worldwide.

The above two definitions are no longer the most commonly used definitions in either the metric, Imperial or the customary USA system of units. Some of the many different definitions currently in use are presented in the next section (see Table 1).

When stating that a gas volume or flow is in Normal Cubic Meters (Nm³) or Standard Cubic Feet (scf) or any other notation (nm, Scf, STP, etc.), the specific reference conditions of temperature and pressure should be explicitly stated. Not to do so can lead to confusion since there is no universally accepted set of reference conditions. For example, as noted above, the International Union of Pure and Applied Chemistry (IUPAC) now defines the standard reference conditions as 0 °C and 100 kPa (rather than 0 °C and 101.325 kPa).[1][2][3] As another example, the Organization of the Petroleum Exporting Countries (OPEC) and a majority of the natural gas industry in North America have adopted 60 °F and 14.73 psia as their standard reference conditions (instead of 60 °F and 14.696 psia) for expressing natural gas volumes and flow rates.[4][5][6] Also, natural gas companies in some other countries have adopted 15 °C (59 °F) and 101.325 kPa (14.696 psia) as their standard gas volume reference conditions.[7][8][9]

Many technical publications (books, journals, equipment vendor advertisements and Internet online articles) still state gas volumes or flows in Nm³ or scf without stating the reference temperature and pressure. That practice assumes that the reader will understand what reference conditions are applicable. Such assumptions can and will lead to confusion and to errors.

Definitions in current use

There are a great many different definitions of the standard reference conditions currently being used. Table 1 presents thirteen such variations of standard condition definitions - and there are quite a few others as well.

For the "SATP" (Standard Ambient Temperature and Pressure) used in presenting chemical thermodynamic properties such as those published by the National Bureau of Standards (see Table 1), the pressure is standardized as 1 bar (100 kPa) and the temperature is usually (but not always) specified at 25 °C.

It should also be noted that 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 publications. Also note that the NIST/CODATA listing of fundamental physical constants includes values at both 100 kPa and 101.325 kPa standard pressure for the molar volume of an ideal gas.

The table makes it quite obvious that it is absolutely necessary to clearly state the temperature and pressure reference conditions whenever expressing a gas volume or gas volumetric flow rate. It is equally important to state whether the gas volume is expressed on a dry basis or a wet basis. As noted in Table 1, some of the current definitions of the reference conditions include a specification of the percent relative humidity (% RH).

Table 1: 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] NIST/CODATA[10]
0 101.325   IUPAC (Former definition),[1] NIST/CODATA,[10] NIST,[11] ISO 10780 [12]
15 101.325 0[13][14] ISA,[13] ISO 13443,[14] EEA,[15] EGIA[4]
20 101.325   U.S. EPA,[16] NIST[17]
25 101.325   U.S. EPA[18]
25 100.000   SATP[19]
20 100.000 0 CAGI[20]
15 100.000   SPE[21]
°F psia  % RH
60 14.696   SPE,[21] OSHA,[22] SCAQMD[23]
60 14.73   EGIA,[4] OPEC,[5] EIA[6]
59 14.503 78 Army Standard Metro[24]
59 14.696 60 ISO 2314 and ISO 3977-2[25]
°F in Hg  % RH
70 29.92 0 AMCA[26]

Notes:

  • 101.325 kPa = 1 atm = 1.01325 bar = 760 mm Hg = 29.92 in Hg ≈ 14.696 psi
  • 100.000 kPa = 1 bar ≈ 14.504 psi ≈ 750 torr (or mm Hg)
  • 14.73 psi ≈ 30 inHg ≈ 1.0156 bar ≈ 101.560 kPa
  • All pressures are absolute pressures (not gauge pressures)
  • 59 °F = 15 °C,   60 °F ≈ 15.6 °C,   70 °F ≈ 21.1 °C
  • dry = 0 percent relative humidity = 0 % RH

The full names of the entities listed in Table 1:

Molar volume of a gas

The molar volume of a gas is a fundamental physical constant.[10] 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 no meaning and it can cause confusion.

The molar gas volumes can be calculated with an accuracy that is usually sufficient by using the ideal gas law:

<math>P\,V = n\,R\,T</math>

which can be rearranged as:

<math>\frac{V}{n} = V_\mathrm{m} = \frac{R\,T}{P}</math>

where (in SI metric units):

P = the gas absolute pressure, in Pa
n = number of moles, in mol
Vm = the gas molar volume, in m3/mol
T = the gas absolute temperature, in K
R = the universal gas law constant of 8.314472 m3·Pa·mol-1·K-1

or where (in customary USA units):

P = the gas absolute pressure, in psia
n = number of moles, in lb-mol
Vm = the gas molar volume, in ft3/lb-mol
T = the gas absolute temperature, in degrees Rankine (°R)
R = the universal gas law constant of 10.7316 ft3·psia·lb-mol-l·°R-1

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

  • In SI metric units:
Vm = 8.314472 × 273.15 / 101.325 = 22.414 m3/kmol at 0 °C and 101.325 kPa absolute pressure
Vm = 8.314472 × 273.15 / 100.000 = 22.711 m3/kmol at 0 °C and 100 kPa absolute pressure
  • In customary USA units:
Vm = 10.7316 × 519.67 / 14.696 = 379.48 ft3/lb-mol at 60 °F and 14.696 psia
Vm = 10.7316 × 519.67 / 14.730 = 378.61 ft3/lb-mol at 60 °F and 14.73 psia

The technical literature can be confusing because some 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 specific 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.

Notes:

References

  1. 1.0 1.1 1.2 Standard pressure IUPAC Goldbook
  2. Standard conditions of gases IUPAC Goldbook
  3. STP IUPAC Goldbook
  4. 4.0 4.1 4.2 "Electricity and Gas Inspection Act", SOR/86-131 (defines a set of standard conditions for Imperial units and a different set for metric units)  Copy of the Canadian "Electricity and Gas Inspection Act"
  5. 5.0 5.1 "Annual Statistical Bulletin", 2004, Editor-in-chief: Dr. Omar Ibrahim, Organization of the Petroleum Exporting Countries, Vienna, Austria  OPEC Statistical Bulletin
  6. 6.0 6.1 "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
  7. Gassco (a Norwegian pipeline supplying gas to Europe)
  8. Nord Gas AG (a consortium pipeline suppling gas from Russia to Germany)
  9. Metrogas (a natural gas company in Argentina)
  10. 10.0 10.1 10.2 Fundamental Physical Constants, Physico-chemical Constants (NIST's listing of CODATA's data)
  11. NIST (1989). NIST Standard Reference Database 7. (NIST Electron and Positron Stopping Powers of Materials Database)
  12. International Organization for Standardization (1994), ISO 10780:1994 : Stationary source emissions - Measurement of velocity and volume flow rate of gas streams in ducts  ISO Standards Catalogue
  13. 13.0 13.1 Robert C. Weast (Editor) (1975). Handbook of Physics and Chemistry, 56th Edition. CRC Press, pp. F201-F206. ISBN 0-87819-455-X. 
  14. 14.0 14.1 "Natural gas – Standard reference conditions", ISO 13443, International Organization for Standardization, Geneva, Switzerland  ISO Standards Catalogue
  15. "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
  16. "Standards of Performance for New Sources", 40 CFR--Protection of the Environment, Chapter I, Part 60, Section 60.2, 1990  New Source Performance Standards
  17. "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
  18. "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
  19. "Table of Chemical Thermodynamic Properties", National Bureau of Standards (NBS), Journal of Physics and Chemical Reference Data, 1982, Vol. 11, Supplement 2. The NBS Tables of Chemical Thermodynamic Properties
  20. "Glossary", 2002, Compressed Air and Gas Institute, Cleveland, Ohio, USA  Glossary
  21. 21.0 21.1 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)
  22. "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
  23. "Rule 102, Definition of Terms (Standard Conditions)", Amended December 2004, South Coast Air Quality Management District, Los Angeles, California, USA  SCAQMD Rule 102
  24. Robert Hayden, Ted Almgren, Kevin Thomas and William McDonald. Rifle and Handgun Reloading Manual (Section 3), 5th Edition. Sierra Bullets.  Exterior Ballistics
  25. "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 Standards Catalogue
  26. ANSI/AMCA Standard 210, "Laboratory Methods Of Testing Fans for Aerodynamic Performance Rating", as implied in the Greenheck website, accessed on October 17, 2007