Standard conditions for temperature and pressure are standard sets of conditions for experimental
measurements established 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, although these are not universally accepted
standards. Other organizations have established a variety of alternative definitions for their
standard reference conditions. In chemistry, IUPAC established standard temperature
and pressure as a temperature of 273.15 K and an absolute pressure of 100 kPa. An unofficial,
but commonly used standard is standard ambient temperature and pressure as a temperature
of 298.15 K and an absolute pressure of 100 kPa. The STP and the SATP should not be confused
with the standard state commonly used in thermodynamic evaluations of the Gibbs energy of a reaction.
NIST uses a temperature of 20 °C and an absolute pressure of 101.325 kPa. The International
Standard Metric Conditions for natural gas and similar fluids are 288.15 K and 101.325
kPa. 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. However, many technical publications simply state "standard conditions" without
specifying them, often leading to confusion and errors. Good practice is to always incorporate
the reference conditions of temperature and pressure.
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 15 °C and 101.325 kPa. During those same years, the most commonly
used standard reference conditions for people using the imperial or U.S. customary systems
was 60 °F and 14.696 psi because it was almost universally used by the oil and gas
industries worldwide. The above definitions are no longer the most commonly used in either
system of units. Current use
Many different definitions of standard reference 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. For example, IUPAC has, since 1982,
defined standard reference conditions as being 0 °C and 100 kPa, in contrast to its old
standard of 0 °C and 101.325 kPa. Natural gas companies in Europe and South
America have adopted 15 °C and 101.325 kPa as their standard gas volume reference conditions.
Also, the International Organization for Standardization, the United States Environmental Protection
Agency and National Institute of Standards and Technology each have more than one definition
of standard reference conditions in their various standards and regulations.
In Russia, State Standard GOST 2939-63 sets the following standard conditions: 20 °C,
760 mmHg and zero humidity.
Notes: EGIA: Electricity and Gas Inspection Act
SATP: Standard Ambient Temperature and Pressure International Standard Atmosphere
In aeronautics and fluid dynamics the "International Standard Atmosphere" is a specification of
pressure, temperature, density, and speed of sound at each altitude. The International
Standard Atmosphere is representative of atmospheric conditions at mid latitudes. In the USA this
information is specified the U.S. Standard Atmosphere which is identical to the "International
Standard Atmosphere" at all altitudes up to 65,000 feet above sea level.
Standard laboratory conditions Due to the fact that many definitions of standard
temperature and pressure differ in temperature significantly from standard laboratory temperatures,
reference is often made to "standard laboratory conditions". 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.
For example, schools in New South Wales, Australia use 25 °C at 100 kPa for standard laboratory
conditions. ASTM International has published Standard
ASTM E41- Terminology Relating to Conditioning and hundreds of special conditions for particular
materials and test methods. Other standards organizations also have specialized standard
test conditions. 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 very little meaning and can cause confusion.
The molar volume of gases around STP can be calculated with an accuracy that is usually
sufficient by using the ideal gas law. The molar volume of any ideal gas may be calculated
at various standard reference conditions as shown below:
Vm = 8.3145 × 273.15 / 101.325 = 22.414 m3/kmol at 0 °C and 101.325 kPa
Vm = 8.3145 × 273.15 / 100.000 = 22.711 m3/kmol at 0 °C and 100 kPa
Vm = 8.3145 × 298.15 / 101.325 = 24.466 m3/kmol at 25 °C and 101.325 kPa
Vm = 8.3145 × 298.15 / 100.000 = 24.790 m3/kmol at 25 °C and 100 kPa
Vm = 10.7316 × 519.67 / 14.696 = 379.48 ft3/lbmol at 60 °F and 14.696 psi
Vm = 10.7316 × 519.67 / 14.730 = 378.61 ft3/lbmol at 60 °F and 14.73 psi
Technical literature can be confusing because many authors fail to explain whether they
are using the ideal gas constant R, or the specific gas constant Rs. 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. However, the USSA,1976
does recognize that this value is not consistent with the values of the Avogadro constant and
the Boltzmann constant. See also
Atmospheric models Environmental chamber
ISO 1 – standard reference temperature for geometric product specifications
Standard Dry Air Standard state
standard sea level Notes
References
External links "Standard conditions for gases" from the IUPAC
Gold Book. "Standard pressure" from the IUPAC Gold Book.
"STP" from the IUPAC Gold Book. "Standard state" from the IUPAC Gold Book.