Standard temperature and pressure

Definitions

IUPAC revised its definition of standard temperature and pressure in 1982. Before that year, STP was a temperature of 273.15 K and an absolute pressure of exactly 1 atm. Since 1982, IUPAC has kept the temperature at 273.15 K but changed the pressure to exactly 1 bar. IUPAC also defines SATP (Standard Ambient Temperature and Pressure) as 298.15 K and exactly 1 atm.

NIST uses a temperature of 20 °C and an absolute pressure of 1 atm, a standard also called normal temperature and pressure (NTP). For thermodynamic experiments, NIST often employs 298.15 K and 1 bar, and for refined petroleum products it uses 15 °C, though these values are not exactly consistent. The ISO 13443 standard for natural gas and similar fluids sets reference conditions at 288.15 K and 1 atm, while the American Petroleum Institute adopts 60 °F. Before 1918, metric-system users commonly defined standard reference conditions as 15 °C and 101.325 kPa, and imperial or U.S. customary users employed 60 °F and 14.696 psi, especially in the oil and gas industries.

Many organizations worldwide currently use different definitions; for instance, IUPAC's new pressure value approximates the mean atmospheric pressure at about 112 meters altitude, closer to the global median altitude of human habitation (194 m), and follows the metric system since the pascal is a metric unit. Natural gas companies in Europe, Australia, and South America use 15 °C and 1 atm as their standard gas volume reference conditions. ISO, the U.S. Environmental Protection Agency, and NIST each have multiple definitions across their various standards and regulations.

International Standard Atmosphere

In aeronautics and fluid dynamics, the International Standard Atmosphere (ISA) specifies pressure, temperature, density, and speed of sound at each altitude. At standard mean sea level, it sets a temperature of 15 °C, pressure of 101325 Pa (1 atm), and density of 1.2250 kg/m³, with a temperature lapse rate of −6.5 °C per kilometer. The U.S. Standard Atmosphere matches the ISA at all altitudes up to 65,000 feet above sea level.

Standard laboratory conditions

Standard laboratory conditions deliberately differ from STP because many STP definitions use temperatures far from typical lab environments, such as 0 °C versus roughly 28 °C. What constitutes standard laboratory temperature and pressure depends on geography, as climate, altitude, and workplace heating or cooling vary worldwide. For example, schools in New South Wales, Australia, use 25 °C at 100 kPa. ASTM International publishes Standard ASTM E41 and numerous special conditions for specific materials and test methods.

Molar volume of a gas

When stating the molar volume of a gas, it is essential to indicate the reference temperature and pressure; omitting them causes confusion. Using the ideal gas law, the molar volume of any ideal gas can be calculated at various standard conditions. At 0 °C and 101.325 kPa, the molar volume is 22.414 dm³/mol; at 0 °C and 100 kPa, it is 22.711 dm³/mol; at 15 °C and 101.325 kPa, it is 23.645 dm³/mol; at 25 °C and 101.325 kPa, it is 24.466 dm³/mol; and at 25 °C and 100 kPa, it is 24.790 dm³/mol.

In imperial units, at 60 °F and 14.696 psi, the molar volume is 379.48 ft³/lbmol, and at 60 °F and 14.73 psi, it is 378.61 ft³/lbmol. Technical literature can be confusing because authors often fail to clarify whether they use the ideal gas constant R or the specific gas constant Rs, where Rs = R divided by the molecular mass of the gas. The U.S. Standard Atmosphere (USSA) uses 8.31432 m³·Pa/(mol·K) for R, though the 1976 USSA acknowledges that this value is inconsistent with the Avogadro constant and the Boltzmann constant.