It is absolutely necessary to define the standard reference conditions of temperature and pressure when expressing a gas volume or a volumetric flow rate because the volume of a gas varies with the temperature and pressure of the gas.

Why necessityEdit

The data in this article show quite clearly that there is no universally accepted single definition of the standard conditions of temperature and pressure. For that reason, simply stating that a gas flow rate is 10,000 m³/h (i.e. cubic meters per hour) at "standard conditions" has no meaning unless the actual conditions are clearly stated.

Definitions used in the past Edit

For 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, the Imperial or the customary USA system of units. Some of the many different definitions currently in use are presented in the following section.

It was also common in the past, when using the metric system of units, to refer to a Normal Cubic Meter (Nm³) and to define it as being at 0°C (273.15 K) and 101.325 kPa (i.e. 1 atmosphere of absolute pressure). As shown in the following section, that notation is no longer appropriate unless the specific reference conditions are explicitly stated, since there are currently many different metric system definitions of what constitutes standard reference conditions.

In the same manner, it is also no longer appropriate to refer to a Standard Cubic Foot (scf) unless the specific reference conditions are explicitly stated, again because there are currently many different definitions of the standard reference condition in both the Imperial and the customary U.S. systems of units. In particular, 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 for expressing natural gas volumes and flow rates (rather than the 60°F and 14.696 psia commonly used previously).

Definitions in current use Edit

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

As shown in the table, the IUPAC (International Union of Pure and Applied Chemistry) currently defines standard reference conditions as being 0°C and 1 bar (i.e. 100 kPa) of absolute pressure rather than the 1 atmosphere (i.e. 101.325 kPa) of absolute pressure used in the past. In fact, the IUPAC's current definition has been in existence since 1997.

As further shown in the table, the oil and gas industries have to a large extent changed from their past usage of 60°F and 14.696 psia to their current usage of 60°F and 14.73 psia. This is especially true of the natural gas industry in North America as well as elsewhere.

It should also be noted that the International Organization for Standardization (ISO), the U.S. 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.

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 flowrate. It is equally important to state whether the gas volume is expressed on a dry basis or a wet basis. As noted in the table, 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
°CkPa% RH
0100.000 IUPAC (post-1997)
0101.325 IUPAC (pre-1997) , NIST , ISO
15101.3250 [4], [5]ISA , ISO
20101.325 EPA
25101.325 EPA
25100.000 SATP
15100.000 SPE
°Fpsia % RH
6014.696 SPE , OSHA , SCAQM
5914.50378Army Standard Metro


  • 101.325 kPa = 1 atmosphere = 1.01325 bar ≈ 14.696 psi
  • 100.000 kPa = 1 bar ≈ 14.504 psi
  • 14.503 psi = 750 mm Hg = 99.992 kPa ≈ 1 bar
  • All pressures are absolute pressures (not gauge pressures)
  • 59°F = 15°C
  • 60°F ≈ 15.6°C
  • dry = 0 percent relative humidity = 0 % RH

The full names of the entities listed in Table 1:

  • IUPAC: International Union of Pure and Applied Chemistry[[1]]
  • NIST: National Institute of Standards and Technology[[2]]
  • ISA: ICAO's pp[[3]]
  • ISO: International Organization for Standardization[[4]]
  • EEA: European Environment Agency[[5]]
  • EGIA: Electricity and Gas Inspection Act (of Canada)
  • EPA: U.S. Environmental Protection Agency[[6]]
  • SATP: Standard Ambient Pressure and Temperature
  • CAGI: CAGI|Compressed Air and Gas Institute[[7]]
  • SPE: Society of Petroleum Engineers[[8]]
  • OSHA: U.S. Occupational Safety and Health Administration[[9]]
  • SCAQMD: California's South Coast Air Quality Control District
  • OPEC: Organization of Petroleum Exporting Countries[[10]]
  • EIA: U.S. Energy Information Administration[[11]]
  • Std. Metro: U.S. Army's Standard Metro (used in ballistics)

Molar volume of a gas Edit

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 much confusion.

The molar gas volumes can be calculated with an accuracy that is usually sufficient by using the Universal Gas Law for ideal gases:

P • V = n • R • T
… is the usual expression of the Universal Gas Law and it can be rearranged thus:
V ÷ n = R • T ÷ P
P = the gas absolute pressure (in Pa or psia)
n = number of mols (kgmol or lbmol)
V ÷ n = the gas molar volume (in units of volume per kgmol or lbmol)
T = the gas absolute temperature (in K or °R)
R = the Universal Gas Law constant 8.3145 m³•kPa/(kgmol•K) or 10.7316 ft³•psia/(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/kgmol at 0°C and 101.325 kPa absolute pressure
  • V ÷ n = 8.3145 × 273.15 ÷ 100.000 = 22.711 m³/kgmol at 0°C and 100 kPa absolute pressure
  • V ÷ n = 10.7316 × 519.67 ÷ 14.696 = 379.48 ft³/lbmol at 60°F and 14.696 psia absolute pressure
  • V ÷ n = 10.7316 × 519.67 ÷ 14.730 = 378.61 ft³/lbmol at 60°F and 14.73 psia absolute pressure

The technical literature can be very 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.

References Edit

  1. note label|IUPAC|1|a}}{{note label|IUPAC|1|b"Compendium of Terminology", 2nd Edition, 1997, IUPAC Secretariat, Research Triangle Park, P.O. Box 13757, NC, USA (pre-1997 and post-1997 definitions)  IUPAC Compendium
  2. note label|NIST|2|^"NIST Standard Reference Data Base 7 Users Guide", December 1969, NIST, Gaithersburg, MD, USA  NIST Data Base 7
  3. note label|ISO|3|^"Stationary Emissions-Measurement of Velocity and Volume Flow Rate of Gas in Ducts", ISO 10780, International Standards Organization, Geneva, Switzerland  ISO
  4. note label|Handbook|4|^"Handbook of Physics and Chemistry", 56th Edition, pp.F201-F206, CRC Press, Boca Raton, FL, USA
  5. note label|2ISO|5|^"Natural Gas-Standard Reference Conditions", ISO 13443, International Standards Organization, Geneva, Switzerland  ISO
  6. note label|EEA|6|^"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
  7. note label|EGIA|7|anote label|EGIA|7|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
  8. note label|EPA|8|^"Standards of Performance for New Sources", 40 CFR--Protection of the Environment, Chapter I, Part 60, Section 60.2, 1990  New Source Performance Standards
  9. note label|2NIST|9|^"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
  10. note label|2EPA|10|^"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
  11. note label|SATP|11|^"Table of Chemical Thermodynamic Properties", National Bureau of Standards (NBS), Journal of Physics and Chemical Reference Data, 1982, Vol. 11, Supplement 2.
  12. note label|CAGI|12|^"Glossary", 2002, Compressed Air and Gas Institute, Cleveland, OH, USA  Glossary
  13. note label|SPE|13|anote label|SPE|13|b"The SI Metric System of Units and SPE Metric Standard (Notes for Table 2.3 on page 25)", June 1982, Richardson, TX, USA (defines standard cubic foot and standard cubic meter)  SPE
  14. note label|OSHA|16|^"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
  15. note label|SCAQMD|17|^"Rule 102, Definition of Terms (Standard Conditions)", Amended December 2004, South Coast Air Quality Management District, Los Angeles, California, USA  SCAQMD Rule 102
  16. note label|OPEC|19|^"Annual Statistical Bulletin", 2004, Editor-in-chief: Dr. Omar Ibrahim, Organization of the Petroleum Exporting Countries, Vienna, Austria  OPEC Statistical Bulletin
  17. note label|EIA|20|^"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
  18. note label|Army|21|^"Effects of Altitude and Atmospheric Conditions", Exterior Ballistics Section, Sierra's "Rifle and Handgun Reloading Manual, 5th Edition", Sedalia, MO, USA  Exterior Ballistics
  19. note label|3ISO|22|^"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 Standards Organization, Geneva, Switzerland ISO

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