What are standard temperature and pressure conditions?

The Standard Temperature and Pressure Conditions (STPCs), also called normal conditions, are a set of parameters for atmospheric pressure and temperature that are considered to be representative and consistent in different applications.

They are used to enable the comparison of different data sets that have been measured and evaluate the physical, chemical, and thermodynamic properties of substances. The STPCs come from the field of science (physics, chemistry, and engineering), but they are widely used in the industrial sector. Standard temperature and pressure conditions are also important for navigation and aeronautics.

While there is consensus on the concept, there is no unified set of values considered to be normal or standard. Each discipline and institution determines its own values.

What are the main standard temperature and pressure conditions?

In the past, the values of the International Union of Pure and Applied Chemistry (IUPAC) were used; these set the normal conditions as:

• Temperature: 0 degrees Celsius (equivalent to 273.15 degrees Kelvin and 32 degrees Fahrenheit).
• Pressure: 1 atmosphere (equivalent to 101,325 kilopascals or 760 Torrs).

However, these standards are obsolete. Today, each leading body has different values. The most commonly used are:

• The IUPAC, based in Switzerland, establishes the STPCs at 273.15 degrees Kelvin (0°C) and 10⁵ pascals (1 bar or 0.98692327 atm). For gases, this is set at 273.15 K (0 °C) and a pressure of 10⁵ pascals (1 bar or 0.98692327 atm).
• The National Institute of Standards and Technology (NIST) in the United States sets a temperature of 20°C and an absolute pressure of 1 atm.
• The International Standard Atmosphere (ISA) is a model of the Earth’s atmosphere that makes it possible to obtain the values of temperature, pressure, density, and viscosity of the air, which provides a frame of reference according to altitude.
• The  International Organization for Standardization (ISO) is made up of several national standardization organizations.

Given the wide range of references currently available, the application of the STPCs absolutely must indicate which reference values are being used.