Reference conditions of gas temperature and pressure

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). 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. 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.

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), NIST/CODATA
0	101.325		IUPAC (Former definition), NIST/CODATA, NIST, ISO 10780
15	101.325	0	ISA, ISO 13443, EEA, EGIA
20	101.325		U.S. EPA, NIST
25	101.325		U.S. EPA
25	100.000		SATP
20	100.000	0	CAGI
15	100.000		SPE
°F	psia	% RH
60	14.696		SPE, OSHA, SCAQMD
60	14.73		EGIA, OPEC, EIA
59	14.503	78	Army Standard Metro
59	14.696	60	ISO 2314 and ISO 3977-2
°F	in Hg	% RH
70	29.92	0	AMCA

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:

• IUPAC: International Union of Pure and Applied Chemistry
• NIST: National Institute of Standards and Technology
• CODATA: Committee on Data for Science and Technology
• ISA: ICAO's International Standard Atmosphere
• ISO: International Organization for Standardization
• EEA: European Environment Agency
• EGIA: 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'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. 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:

$P\,V=n\,R\,T$

which can be rearranged as:

${\frac {V}{n}}=V_{\mathrm {m} }={\frac {R\,T}{P}}$

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:

V_m = 8.314472 × 273.15 / 101.325 = 22.414 m³/kmol at 0 °C and 101.325 kPa absolute pressure

V_m = 8.314472 × 273.15 / 100.000 = 22.711 m³/kmol at 0 °C and 100 kPa absolute pressure

• In customary USA units:

V_m = 10.7316 × 519.67 / 14.696 = 379.48 ft³/lb-mol at 60 °F and 14.696 psia

V_m = 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 R_s, which only applies to a specific individual gas. The relationship between the two constants is R_s = R / M, where M is the molecular weight of the gas.

Notes:

• lb-mol is an abbreviation for pound-mol
• °R is degrees Rankine (an absolute temperature scale) and °F is degrees Fahrenheit (a temperature scale).
• °R = °F + 459.67