Definitions

Standard conditions for temperature and pressure

In physical sciences, standard conditions for temperature and pressure are standard sets of conditions for experimental measurements, to allow comparisons to be made between different sets of data. The most used standards are those of the International Union of Pure and Applied Chemistry and the National Institute of Standards and Technology but are far from being universal standards. Other organizations have established a variety of alternative definitions for their standard reference conditions. The current version of IUPAC's standard is a temperature of 0 °C (273.15 K, 32 °F) and an absolute pressure of 100 kPa (14.504 psi), while NIST's version is a temperature of 20 °C (293.15 K, 68 °F) and an absolute pressure of 101.325 kPa (14.696 psi).

In industry and commerce, standard conditions for temperature and pressure are often necessary to define the standard reference conditions to express the volumes of gases and liquids and related quantities such as the rate of volumetric flow (the volumes of gases and liquids vary significantly with temperature and pressure). However many technical publications (books, journals, advertisements for equipment and machinery) simply state "standard conditions" without specifying them, often leading to confusion and errors.

Definitions

Past use

In the last five to six decades, professionals and 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 (1 atm). During those same years, the most commonly used standard reference conditions for people using the Imperial units or U.S. customary units was 60 °F (520 °R) and 14.696 psi (1 atm) because it was almost universally used by the oil and gas industries worldwide. However, the above two definitions are no longer the most commonly used in either system of units.

Current use

Many different definitions of standard references conditions are currently being used by organizations all over the world. The table below lists a few of them, but there are more. Some of these organizations used other standards in the past, such as IUPAC which currently defines standard reference conditions as being 0 °C and 100 kPa (1 bar) of pressure rather since 1982, in contrast to their old standard of 0 °C and 101.325 kPa (1 atm). Another example is from the oil industry. While a standard of 60 °F and 14.696 psi was used in the past, the current usage (particularly in North America) is predominantly of 60 °F and 14.73 psi.

Natural gas companies in Europe and South America have adopted 15 °C (59 °F) and 101.325 kPa (14.696 psi) as their standard gas volume reference conditions. Also, 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 SATP used for presenting chemical thermodynamic properties (such as those published by the National Bureau of Standards) is standardized at 100 kPa (1 bar) but the temperature may vary and usually needs to be specified separately if complete information is desired (see standard state). Some standards are specified at certain humidity level.

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)
0 101.325   IUPAC (former definition), NIST, ISO 10780
15 101.325 0 ICAO's ISA, ISO 13443, EEA, EGIA
20 101.325   EPA, NIST
25 101.325   EPA
25 100.000   SATP
20 100.000 0 CAGI
15 100.000   SPE
°F psi % RH
60 14.696   SPE, U.S. OSHA, SCAQMD
60 14.73   EGIA, OPEC, U.S. EIA
59 14.503 78 U.S. Army Standard Metro
59 14.696 60 ISO 2314, ISO 3977-2
°F in Hg % RH
70 29.92 0 AMCA, air density = 0.075 lbm/ft³. This AMCA standard applies only to air.

Notes:

• EGIA: Electricity and Gas Inspection Act (of Canada)
• SATP: Standard Ambient Pressure and Temperature
• SCAQMD: California's South Coast Air Quality Management District

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. The concept of "standard laboratory conditions" taught as part of the New South Wales high school chemistry syllabus is 25 °C at 100 kPa.

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 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:

$P V = nRT$

…which can be rearranged thus:

$frac\left\{V\right\}\left\{n\right\} = frac\left\{RT\right\}\left\{P\right\}$

where (in SI metric units):

P = the absolute pressure of the gas, in Pa
n = amount of substance, in mol
V = the volume of the gas, in m3
T = the absolute temperature of the gas, in K
R = the universal gas law constant of 8.3145 m3·Pa/(mol·K)
or where (in customary USA units):
P = the absolute pressure of the gas, in psi
n = number of moles, in lbmol
V = the volume of the gas, in ft3/lbmol
T = the absolute temperature of the gas absolute, in °R
R = the universal gas law constant of 10.7316 ft3·psi/(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
• V/n = 8.3145 × 273.15 / 100.000 = 22.711 m3/kmol at 0 °C and 100 kPa
• V/n = 10.7316 × 519.67 / 14.696 = 379.48 ft3/lbmol at 60 °F and 14.696 psi
• V/n = 10.7316 × 519.67 / 14.730 = 378.61 ft3/lbmol at 60 °F and 14.73 psi

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)