Standard conditions for temperature and pressure

Standard conditions for temperature and pressure are standard sets of conditions for experimental measurements to be 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 (IUPAC) and the National Institute of Standards and Technology (NIST), although these are not universally accepted standards. Other organizations have established a variety of alternative definitions for their standard reference conditions.

In chemistry, IUPAC changed the definition of standard temperature and pressure (STP) in 1982:[1]

STP 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 (293.15 K, 68 °F) and an absolute pressure of 1 atm (14.696 psi, 101.325 kPa). This standard is also called normal temperature and pressure (abbreviated as NTP).

The International Standard Metric Conditions for natural gas and similar fluids are 288.15 K (15.00 °C; 59.00 °F) and 101.325 kPa.[2]

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 (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).

However, 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, 293 К (20 °C), and 0% humidity.

Definitions

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 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.[3]

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 (1 bar), in contrast to its old standard of 0 °C and 101.325 kPa (1 atm).[4] The new value is the mean atmospheric pressure at an altitude of about 112 metres, which closer to the worldwide median altitude of human habitation (194 m).[citation needed]

Natural gas companies in Europe, Australia, and South America have adopted 15 °C (59 °F) and 101.325 kPa (14.696 psi) as their standard gas volume reference conditions, used as the base values for defining the standard cubic meter.[5][6][7] 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.

Standard reference conditions in current use
Temperature (°C) Temperature (°F) Pressure (kPa) Pressure (mmHg) Pressure (psi) Pressure (inHg) Relative Humidity (%) Publishing or establishing entity
0 32 100.000 750.06 14.5038 29.530   IUPAC (STP) since 1982[1]
0 32 101.325 760.00 14.6959 29.921   NIST,[8] ISO 10780,[9] formerly IUPAC (STP) until 1982[1]
15 59 101.325 760.00 14.6959 29.921 0[2][10] ICAO's ISA,[10] ISO 13443,[2] EEA,[11] EGIA (SI Definition)[12]
20 68 101.325 760.00 14.6959 29.921   EPA,[13] NIST.[14] This is also called NTP, Normal Temperature and Pressure.[15]
22 72 101.325 760.00 14.6959 29.921 20–80 American Association of Physicists in Medicine[16]
25 77 100.000 750.06 14.5038 29.530   IUPAC (SATP)
25 77 101.325 760.00 14.6959 29.921   EPA[17]
20 68 100.000 750.06 14.5038 29.530 0 CAGI[18]
15 59 100.000 750.06 14.5038 29.530   SPE[19]
20 68 101.3 760 14.69 29.9 50 ISO 5011[20]
20 68 101.33 760.0 14.696 29.92 0 GOST 2939-63
16 60 101.33 760.0 14.696 29.92   SPE,[19] U.S. OSHA,[21] SCAQMD[22]
16 60 101.6 762 14.73 30.0   EGIA (Imperial System Definition)[12]
16 60 101 760 14.7 30   U.S. DOT (SCF)[23]
15 59 99.99 750.0 14.503 29.53 78 U.S. Army Standard Metro[24][a]
15 59 101.33 760.0 14.696 29.92 60 ISO 2314,[25] ISO 3977-2[26]
21 70 101.3 760 14.70 29.92 0 AMCA,[27][b] air density = 0.075 lbm/ft3. This AMCA standard applies only to air.; Compressed Gas Association [CGA] applies to industrial gas use in USA[28]
15 59 101.3 760 14.70 29.92   Federal Aviation Administration (FAA)[29]

Notes:

  • EGIA: Electricity and Gas Inspection Act (of Canada)
  • SATP: Standard Ambient Temperature and Pressure
  • SCF: Standard Cubic Foot