Standard temperature and pressure

Standard temperature and pressure (STP), or standard conditions for temperature and pressure, are various standard sets of conditions for experimental measurements used 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 (IUPAC) and the National Institute of Standards and Technology (NIST), although these are not universally accepted. Other organizations have established a variety of other definitions.

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).