Standard conditions for temperature and pressure

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In chemistry and other sciences, STP or standard temperature and pressure is a standard set of conditions for experimental measurements, to enable comparisons to be made between sets of data. Internationally, the current STP defined by the IUPAC (International Union of Pure and Applied Chemistry) is an absolute pressure of 100 kPa (1 bar) and a temperature of 273.15 K (0 °C).[1] Other organizations have established a variety of alternative definitions for the standard reference conditions of temperature and pressure.

In industry and commerce, it is 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. The available data on the various definitions of standard reference conditions clearly indicates that there is no universally accepted 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" or at "STP" has no meaning unless the reference conditions that were applied are clearly stated.

Definitions used in the past

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.

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

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 1982.[2]

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.

For the SATP used in presenting chemical thermodynamic properties (such as those published by the National Bureau of Standards as included in Table 1) that the pressure is standardized at 1 bar (100 kPa) but the temperature may vary and needs to be specified separately.

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 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
°C kPa % RH
0 100.000   IUPAC (present definition)[1]
0 101.325   IUPAC (former definition),[1] NIST,[3] ISO 10780[4]
15 101.325 0 [4], [5] ISA,[5] ISO 13443,[6] EEA,[7] EGIA[8]
20 101.325   EPA,[9] NIST[10]
25 101.325   EPA[11]
25 100.000   SATP[12]
20 100.000 0 CAGI[13]
15 100.000   SPE[14]
°F psia  % RH
60 14.696   SPE,[14] OSHA,[15] SCAQMD[16]
60 14.73   EGIA,[8] OPEC,[17] EIA[18]
59 14.503 78 Army Standard Metro[19]
59 14.696 60 ISO 2314, ISO 3977-2[20]
°F in Hg  % RH
70 29.92 0 AMCA,[21] air density = 0.075 lbm/ft³

Notes:

  • 101.325 kPa = 1 atmosphere = 1.01325 bar ≈ 14.696 psi
  • 100.000 kPa = 1 bar ≈ 14.504 psi
  • 14.503 psi ≈ 750 mmHg ≈ 100.0 kPa ≈ 1 bar
  • 14.696 psi ≈ 1 atm = 101.325 kPa
  • 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
  • dry = 0 percent relative humidity = 0 % RH

The full names of the entities listed in Table 1:

International Standard Atmosphere

In aeronautics and fluid dynamics the term "International Standard Atmosphere" is often used to denote the variation of the principal thermodynamic variables (pressure, temperature, density, etc.) of the atmosphere with altitude at mid latitudes.

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. This concept of "standard laboratory conditions" is taught as part of the New South Wales high school chemistry syllabus.[22]

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[23] 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:

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

…which can be rearranged thus:

<math>\frac{V}{n} = \frac{R \cdot T}{P}</math>

where (in SI metric units):

P = the gas absolute pressure, in Pa
n = number of moles, in mol
V / n = the gas molar volume, in m³/mol
T = the gas absolute temperature, in K
R = the universal gas law constant of 8.3145 m³·Pa/(mol·K)

or where (in customary USA units):

P = the gas absolute pressure, in psia
n = number of moles, in lbmol
V / n = the gas molar volume, in ft³/lbmol
T = the gas absolute temperature, in °R
R = the universal gas law constant of 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/kmol at 0 °C and 101.325 kPa absolute pressure
  • V / n = 8.3145 × 273.15 / 100.000 = 22.711 m³/kmol 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 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. 1.0 1.1 1.2 "Compendium of Terminology", 2nd Edition, 1997, IUPAC Secretariat, Research Triangle Park, P.O. Box 13757, NC, USA (former and present definitions)  IUPAC Compendium
  2. IUPAC recommended standard pressure of 1 bar in 1982 IUPAC Compendium
  3. "NIST Standard Reference Data Base 7 Users Guide", December 1969, NIST, Gaithersburg, MD, USA  NIST Data Base 7
  4. "Stationary source emissions – Measurement of velocity and volume flow rate of gas streams in ducts", ISO 10780, International Organization for Standardization, Geneva, Switzerland  ISO
  5. "Handbook of Physics and Chemistry", 56th Edition, pp.F201-F206, CRC Press, Boca Raton, FL, USA
  6. "Natural gas – Standard reference conditions", ISO 13443, International Organization for Standardization, Geneva, Switzerland  ISO
  7. "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
  8. 8.0 8.1 "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
  9. "Standards of Performance for New Sources", 40 CFR--Protection of the Environment, Chapter I, Part 60, Section 60.2, 1990  New Source Performance Standards
  10. "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
  11. "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
  12. "Table of Chemical Thermodynamic Properties", National Bureau of Standards (NBS), Journal of Physics and Chemical Reference Data, 1982, Vol. 11, Supplement 2.
  13. "Glossary", 2002, Compressed Air and Gas Institute, Cleveland, OH, USA  Glossary
  14. 14.0 14.1 "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
  15. "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
  16. "Rule 102, Definition of Terms (Standard Conditions)", Amended December 2004, South Coast Air Quality Management District, Los Angeles, California, USA  SCAQMD Rule 102
  17. "Annual Statistical Bulletin", 2004, Editor-in-chief: Dr. Omar Ibrahim, Organization of the Petroleum Exporting Countries, Vienna, Austria  OPEC Statistical Bulletin
  18. "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
  19. "Effects of Altitude and Atmospheric Conditions", Exterior Ballistics Section, Sierra's "Rifle and Handgun Reloading Manual, 5th Edition", Sedalia, MO, USA  Exterior Ballistics
  20. "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
  21. 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
  22. Peter Gribbon (2001). Excel HSC Chemistry Pocket Book Years 11-12. Pascal Press. ISBN 1-74020-303-8. 
  23. Fundamental Physical Properties: Molar Volumes (CODATA values for ideal gases as listed on a NIST website page)


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