Standard temperature and pressure

In industry and commerce, the standard conditions for temperature and pressure are often necessary for expressing the volumes of gases and liquids and related quantities such as the rate of volumetric flow (the volumes of gases vary significantly with temperature and pressure): standard cubic meters per second (Sm³/s), and normal cubic meters per second (Nm³/s).

Many technical publications (books, journals, advertisements for equipment and machinery) simply state "standard conditions" without specifying them, often substituting the term with older "normal conditions", or "NC". In special cases this can lead to confusion and errors. Good practice always incorporates the reference conditions of temperature and pressure. If not stated, some room environment conditions are supposed, close to 1 atm pressure, 25 °C (77.00 °F), and 0% humidity.

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 (0.00 °C; 32.00 °F) and an absolute pressure of exactly 1 standard atmosphere (101,325.000 Pa).
• Since 1982, STP has still been defined as a temperature of 273.15 K (0.00 °C; 32.00 °F) but defined with an absolute pressure of exactly 1 bar (100,000.000 Pa).

IUPAC also defines SATP (Standard Ambient Temperature and Pressure) as a temperature of 298.15 K (25.00 °C; 77.00 °F) and an absolute pressure of exactly 1 atm (101.325 kPa).

NIST uses a temperature of 20 °C (293.15 K; 68.00 °F) and an absolute pressure of 1 atm (101.325 kPa). 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 (25.00 °C; 77.00 °F) and 1 bar (100.000 kPa). NIST also uses 15 °C (288.15 K; 59.00 °F) 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 (15.00 °C; 59.00 °F) and 1 atm (101.325 kPa); by contrast, the American Petroleum Institute adopts 60 °F (15.56 °C; 288.71 K).

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 (288.15 K; 59.00 °F) and 101.325 kPa (1.00 atm; 760 Torr). During those same years, the most commonly used standard reference conditions for people using the imperial or U.S. customary systems was 60 °F (15.56 °C; 288.71 K) and 14.696 psi (1.00 atm) 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 is not).

Natural gas companies in Europe, Australia, and South America have adopted 15 °C (288.15 K; 59.00 °F) and 1 atm (101.325 kPa) 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