View source for Reference conditions of gas temperature and pressure

{{subpages}}
In [[chemistry]], [[physics]] and [[engineering]] as well as industry and commerce, the '''reference conditions of gas temperature and pressure''' define the [[Density (chemistry)|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 ''Nm<sup>3</sup> ([[normal cubic metre]]s)'' 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 [[Celsius (unit)|°C]] (273.15 [[Kelvin (unit)|K]]) and 101.325 k[[Pascal (unit)|Pa]] (i.e., 1 [[Atmosphere (unit)|atmosphere]] of absolute pressure).  During those same years, the most commonly used standard reference conditions for people using the [[Imperial units|Imperial]] or [[U.S. customary  units|customary USA system of units]] was 60 [[Fahrenheit (unit)|°F]] (520 [[Rankine (unit)|°R]]) and 14.696 [[U.S. customary units|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).<ref name=IUPAC>[http://goldbook.iupac.org/S05921.html Standard pressure] IUPAC Goldbook</ref><ref>[http://goldbook.iupac.org/S05910.html Standard  conditions of gases] IUPAC Goldbook</ref><ref>[http://goldbook.iupac.org/S06036.html STP] IUPAC Goldbook</ref> 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.<ref name=EGIA>"Electricity and Gas Inspection Act", SOR/86-131 (defines a set of standard conditions for Imperial units and a different set for metric units) &nbsp;[http://laws.justice.gc.ca/en/ShowFullDoc/cr/SOR-86-131///en Copy of the Canadian "Electricity and Gas Inspection Act"]</ref><ref name=OPEC>"Annual Statistical Bulletin", 2004, Editor-in-chief: Dr. Omar Ibrahim, Organization of the Petroleum Exporting Countries, [[Vienna]], [[Austria]] &nbsp;[http://www.opec.org/library/Annual%20Statistical%20Bulletin/pdf/ASB2004.pdf OPEC Statistical Bulletin]</ref><ref name=EIA>"Natural Gas Annual 2004", DOE/EIA-0131(04), December 2005, [[U.S. Department of Energy]], Energy Information Administration, [[Washington (U.S. state)]], [[District of Columbia|D.C.]], USA &nbsp;[http://tonto.eia.doe.gov/FTPROOT/natgas/013104.pdf Natural Gas Annual 2004]</ref> 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.<ref>[http://www.gassco.no/sw3138.asp Gassco] (a [[Norway|Norwegian]] pipeline supplying gas to [[Europe]])</ref><ref>[http://www.nord-stream.com/uploads/media/Nord_Stream_Route_Status_ENGLISH.pdf Nord Gas AG] (a consortium pipeline suppling gas from [[Russia]] to [[Germany]])</ref><ref>[http://www.secinfo.com/dsD7y.1a.7.htm Metrogas] (a natural gas company in [[Argentina]])</ref>

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 (unit)|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).

{| class="wikitable"
|+ '''Table 1: Standard reference conditions in current use'''<br><br>
! Temperature !! Absolute pressure !! Relative humidity
!rowspan="2"| Publishing or establishing entity
|-
!°C!!kPa!!% RH 
|-
|align="center"|0||align="center"|100.000||&nbsp;||IUPAC (Present definition),<ref name=IUPAC/> NIST/CODATA<ref name=NISTCODATA>[http://physics.nist.gov/cgi-bin/cuu/Value?mvolstd|search_for=physchem_in! Fundamental Physical Constants, Physico-chemical Constants] (NIST's listing of CODATA's data)</ref>        
|-
|align="center"|0||align="center"|101.325||&nbsp;||IUPAC (Former   definition),<ref name=IUPAC/> NIST/CODATA,<ref name=NISTCODATA/> [[National Institute of Standards and Technology|NIST]],<ref name=NISTDataBase7>{{cite web |url=http://www.nist.gov/srd/WebGuide/nist7/07_2.htm |title=NIST Standard Reference Database 7|author=[[NIST]]|year=1989}} (NIST Electron and Positron Stopping Powers of Materials Database)</ref>  ISO 10780 <ref name=ISO10780>{{Citation|author=International Organization for Standardization|title=ISO 10780:1994 : Stationary source emissions - Measurement of velocity and volume flow rate of gas streams in ducts |year=1994}} &nbsp;[http://www.iso.org/iso/iso_catalogue/catalogue_tc/catalogue_detail.htm?csnumber=18855 ISO Standards Catalogue]</ref>
|-
|align="center"|15||align="center"|101.325||align="center"|0<ref name=Handbook>{{cite book|author=Robert C. Weast (Editor)|title=Handbook of Physics and Chemistry|edition=56th Edition|publisher=CRC Press|pages=pp. F201-F206|year=1975|id=ISBN 0-87819-455-X}}</ref><ref name=ISO13443>"Natural gas – Standard reference conditions", ISO 13443, International Organization for Standardization, Geneva, Switzerland &nbsp;[http://www.iso.org/iso/iso_catalogue.htm ISO Standards Catalogue]</ref>||ISA,<ref name=Handbook/> ISO 13443,<ref name=ISO13443/> [[European Environment Agency|EEA]],<ref name=EEA>"Extraction, First Treatment and Loading of Liquid & Gaseous Fossil Fuels", Emission Inventory Guidebook B521, Activities 050201 - 050303,  September 1999, European Environmental Agency, [[Copenhagen]], [[Denmark]] &nbsp;[http://reports.eea.eu.int/EMEPCORINAIR3/en/B521vs3.1.pdf Emission Inventory Guidebook]
</ref> EGIA<ref name=EGIA/>
|-
|align="center"|20||align="center"|101.325||&nbsp;||[[U.S. EPA]],<ref name=NSPS>"Standards of Performance for New Sources", 40 CFR--Protection of the Environment, Chapter I, Part 60, Section 60.2, 1990 &nbsp;[http://a257.g.akamaitech.net/7/257/2422/08aug20051500/edocket.access.gpo.gov/cfr_2005/julqtr/pdf/40cfr60.2.pdf New Source Performance Standards]</ref> NIST<ref name=NISTJournal>"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 &nbsp;[http://www.cstl.nist.gov/div836/836.01/PDFs/2003/j80wri.pdf NIST Journal]</ref> 
|-
|align="center"|25||align="center"|101.325||&nbsp;||U.S. EPA<ref name=NAAQS>"National Primary and Secondary Ambient Air Quality Standards", 40 CFR--Protection of the Environment, Chapter I, Part 50, Section 50.3, 1998 &nbsp;[http://a257.g.akamaitech.net/7/257/2422/08aug20051500/edocket.access.gpo.gov/cfr_2005/julqtr/pdf/40cfr50.3.pdf National Ambient Air Standards]</ref> 
|-
|align="center"|25||align="center"|100.000||&nbsp;||SATP<ref name=NBS>"Table of Chemical Thermodynamic Properties", National Bureau of Standards (NBS), Journal of Physics and Chemical Reference Data, 1982, Vol. 11, Supplement 2. [http://stinet.dtic.mil/oai/oai?verb=getRecord&metadataPrefix=html&identifier=ADD095481 The NBS Tables of Chemical Thermodynamic Properties]</ref> 
|-
|align="center"|20||align="center"|100.000||align="center"|0||CAGI<ref name=CAGI>"Glossary", 2002, Compressed Air and Gas Institute, Cleveland, [[Ohio]], USA &nbsp;[http://www.cagi.org/toolbox/glossary.htm Glossary]</ref> 
|-
|align="center"|15||align="center"|100.000||&nbsp;||SPE<ref name=SPE>[http://www.spe.org/spe-site/spe/spe/papers/authors/Metric_Standard.pdf 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)</ref> 
|-
!°F!!psia!! % RH 
|-
|align="center"|60||align="center"|14.696||&nbsp;||SPE,<ref name=SPE/> OSHA,<ref name=OSHA>"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 &nbsp;[http://ecfr.gpoaccess.gov/cgi/t/text/text-idx?c=ecfr&sid=f169acd0f57a17565c9984fa0f57d285&rgn=div8&view=text&node=29:5.1.1.1.8.8.33.10&idno=29 Storage/Handling of LPG]</ref> [[South Coast Air Quality Management District|SCAQMD]]<ref name=SCAQMD>"Rule 102, Definition of Terms (Standard Conditions)", Amended December 2004, South Coast Air Quality Management District, [[Los Angeles, California|Los Angeles]], California, USA &nbsp;[http://www.aqmd.gov/rules/reg/reg01/r102.pdf SCAQMD Rule 102]</ref> 
|-
|align="center"|60||align="center"|14.73||&nbsp;||EGIA,<ref name=EGIA/> OPEC,<ref name=OPEC/> EIA<ref name=EIA/>   
|-
|align="center"|59||align="center"|14.503||align="center"|78||Army Standard Metro<ref name=ArmyStdMetro>{{cite book|author=Robert Hayden, Ted Almgren, Kevin Thomas and William McDonald|title=Rifle and Handgun Reloading Manual (Section 3)|edition=5th Edition |publisher=Sierra Bullets|year= |id= }}  [http://www.exteriorballistics.com/ebexplained/5th/31.cfm Exterior Ballistics]</ref>  
|-
|align="center"|59||align="center"|14.696||align="center"|60||ISO 2314 and ISO 3977-2<ref name=ISO3977-2>"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 [http://www.iso.org/iso/iso_catalogue.htm ISO Standards Catalogue]</ref>
|-
!°F!!in Hg!! % RH 
|-
|align="center"|70||align="center"|29.92||align="center"|0||AMCA<ref name=AMCA>[[ANSI]]/[[AMCA]] Standard 210, "Laboratory Methods Of Testing Fans for Aerodynamic Performance Rating", as implied in the [http://www.greenheck.com/pdf/centrifugal/Plug.pdf Greenheck website], accessed on October 17, 2007</ref> 
|}

Notes:
* 101.325 kPa = 1 [[Atmosphere (unit)|atm]] = 1.01325 [[Bar (unit)|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, &nbsp; 60 °F ≈ 15.6 °C, &nbsp; 70 °F ≈ 21.1 °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
*NIST: [[National Institute of Standards and Technology]]
*CODATA: [[Committee on Data for Science and Technology]]
*ISA: [[International Civil Aviation Organization|ICAO]]'s [[International Standard Atmosphere]]
*ISO: International Organization for Standardization
*EEA: [[European Environment Agency]]
*EGIA: [[Canada|Canadian]] Electricity and Gas Inspection Act
*U.S. EPA: [[United States Environmental Protection Agency]]
*SATP: Standard Ambient Pressure and Temperature
*CAGI: [[Compressed Air and Gas Institute]]
*SPE:  [[Society of Petroleum Engineers]]
*OSHA: [[U.S. Occupational Safety and Health Administration]]
*SCAQMD: [[California (U.S. state)|California]]'s [[South Coast Air Quality Management District]]
*OPEC: [[Organization of Petroleum Exporting Countries]]
*EIA: [[U.S. Energy Information Administration]] of the [[U.S. Department of Energy]]
*Std. Metro: [[U.S. Army]]'s Standard Metro (used in ballistics)
*AMCA: Air Movement and Control Association (the AMCA standard applies only to air)

== Molar volume of a gas ==

The [[molar volume]] of a gas is a fundamental physical constant.<ref name=NISTCODATA/> 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|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):
{| border="0" cellpadding="2"
|-
!align=right| ''P''
|align=left|= the gas absolute pressure, in [[pascal (unit)|Pa]]
|-
!align=right|''n''
|align=left|= number of moles, in [[mole (unit)|mol]]
|-
!align=right| ''V''<sub>m</sub>  
|align=left|= the gas molar volume, in m<sup>3</sup>/mol
|-
!align=right| ''T''
|align=left|= the gas absolute temperature, in [[kelvin|K]]
|-
!align=right| ''R''
|align=left|= the [[molar gas constant|universal gas law constant]] of  8.314472 m<sup>3</sup>·Pa·mol<sup>-1</sup>·K<sup>-1</sup>
|}
or where (in customary USA units):
{| border="0" cellpadding="2"
|-
!align=right| ''P''
|align=left|= the gas absolute pressure, in [[pound-force per square inch|psia]]
|-
!align=right|''n''
|align=left|= number of moles, in [[mole (unit)|lb-mol]]
|-
!align=right| ''V''<sub>m</sub>
|align=left|= the gas molar volume, in ft<sup>3</sup>/lb-mol
|-
!align=right| ''T''
|align=left|= the gas absolute temperature, in [[Rankine (unit)|degrees Rankine]] (°R)
|-
!align=right| ''R''
|align=left|= the universal gas law constant of 10.7316 ft<sup>3</sup>·psia·lb-mol<sup>-l</sup>·°R<sup>-1</sup>
|}

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

* In SI metric units:
:'''''V'''''<sub>'''m'''</sub> = 8.314472 × 273.15 / 101.325 = 22.414 m<sup>3</sup>/kmol at 0 °C and 101.325 kPa absolute pressure
:'''''V'''''<sub>'''m'''</sub>  = 8.314472 × 273.15 / 100.000 = 22.711 m<sup>3</sup>/kmol at 0 °C and 100 kPa absolute pressure

* In customary USA units:
:'''''V'''''<sub>'''m'''</sub> = 10.7316 × 519.67 / 14.696 = 379.48  ft<sup>3</sup>/lb-mol at 60 °F and 14.696 psia 
:'''''V'''''<sub>'''m'''</sub>  = 10.7316 × 519.67 / 14.730 = 378.61 ft<sup>3</sup>/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 '''''R<sub>s</sub>''''', which only applies to a specific individual gas.  The relationship between the two constants is '''''R<sub>s</sub>''''' = '''''R / M''''',  where '''''M''''' is the [[Molecular mass|molecular weight]] of the gas.

Notes:

* lb-mol is an abbreviation for [[Mole (unit)|pound-mol]]
* °R is [[Rankine (unit)|degrees Rankine]] (an absolute temperature scale) and °F is [[Fahrenheit (unit)|degrees Fahrenheit]] (a temperature scale).
* °R = °F + 459.67

== References ==
{{reflist}}