Standard temperature and pressure

Definitions

Main definitions

In chemistry, IUPAC changed its definition of standard temperature and pressure in 1982:

• Until 1982, STP was defined as a temperature of 273.15 K and an absolute pressure of exactly 1 atm.
• Since 1982, STP has been defined as a temperature of 273.15 K and an absolute pressure of exactly 1 bar. IUPAC also defines SATP (Standard Ambient Temperature and Pressure) as a temperature of 298.15 K and an absolute pressure of exactly 1 atm.

NIST uses a temperature of 20 C and an absolute pressure of 1 atm. This standard is also called normal temperature and pressure (abbreviated as NTP). However, a common temperature and pressure in use by NIST for thermodynamic experiments is 298.15 K and 1 bar. NIST also uses 15 C for the temperature compensation of refined petroleum products, despite noting that these two values are not exactly consistent with each other.

The ISO 13443 standard reference conditions for natural gas and similar fluids are 288.15 K and 1 atm; by contrast, the American Petroleum Institute adopts 60 F.

Past uses

Before 1918, many 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's new value is the mean atmospheric pressure at an altitude of about 112 metres, which is closer to the worldwide median altitude of human habitation (194 m), and follows the metric system (Pascal is a metric unit, while atmosphere isn't).

Natural gas companies in Europe, Australia, and South America have adopted 15 C and 1 atm as their standard gas volume reference conditions, used as the base values for defining the standard cubic meter. 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.

Comparison table

International Standard Atmosphere

In aeronautics and fluid dynamics the "International Standard Atmosphere" (ISA) is a specification of pressure, temperature, density, and speed of sound at each altitude. At standard mean sea level it specifies a temperature of 15 C, pressure of 101325 Pa (1 atm), and a density of 1.2250 kg/m3. It also specifies a temperature lapse rate of −6.5 °C (−11.7 °F) per km (approximately −2 °C (−3.6 °F) per 1,000 ft).

The International Standard Atmosphere is representative of atmospheric conditions at mid latitudes. In the US 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

Because many definitions of standard temperature and pressure differ in temperature significantly from standard laboratory temperatures (e.g. 0 °C vs. ~28 °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 geography-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 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 and at atmospheric pressure 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 dm3/mol at 0 °C and 101.325 kPa
• Vm = 8.3145 × 273.15 / 100.000 = 22.711 dm3/mol at 0 °C and 100 kPa
• Vm = 8.3145 × 288.15 / 101.325 = 23.645 dm3/mol at 15 °C and 101.325 kPa
• Vm = 8.3145 × 298.15 / 101.325 = 24.466 dm3/mol at 25 °C and 101.325 kPa
• Vm = 8.3145 × 298.15 / 100.000 = 24.790 dm3/mol at 25 °C and 100 kPa
• Vm = 10.7316 × 519.67 / 14.696 = 379.48 ft3/lbmol at 60 °F and 14.696 psi (or about 0.8366 ft3/gram mole)
• 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 mass of the gas.

The US Standard Atmosphere (USSA) uses 8.31432 m3·Pa/(mol·K) as the value of R. However, the USSA in 1976 does recognize that this value is not consistent with the values of the Avogadro constant and the Boltzmann constant.

Explanatory notes

References

References

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