**Standard conditions for temperature and pressure** (**STP**) are standard sets of conditions for experimental measurements to be established to allow comparisons to be made between different sets of data. STP should not be confused with the standard state of a material (pure substance, mixture, or solution) which is a reference point used to calculate its properties under different conditions.

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.

**Standard Temperature and Pressure defined by NIST**

- Standard temperature is 0 ℃ or 273.15 K or 32 ℉.
- Standard pressure is 1 atm or 101.325 kPa or 760 mmHg (or torr) or 14.6959 psi.

**Standard Temperature and Pressure defined by IUPAC**

- Standard temperature is 0 ℃ or 273.15 K or 32 ℉.
- Standard pressure is 1 bar or 100.000 kPa or 750.6 mmHg (or torr) or 14.5038 psi.

## Normal Temperature and Pressure (NTP)

**Normal Temperature and Pressure** (**NTP**) is introduced by the National Institute of Standards and Technology (NIST); it is commonly used as a standard condition for testing and documentation of fan capacities. Normal temperature as per NTP is 20 °C (293.15 K, 68 °F; this temperature is a more realistic and convenient condition in the world compared to 0 ℃ which is the standard temperature of STP) and 1 atm (101.325 kN/m^{2}, 101.325 kPa, 14.7 psia, 0 psig, 29.92 in Hg, 407 in H_{2}O, 760 torr). Density 1.204 kg/m^{3} (0.075 pounds per cubic foot). At these conditions, the volume of 1 mol of a gas is 24.0548 liters.

## Standard Ambient Temperature and Pressure (SATP)

**Standard Ambient Temperature and Pressure** (**SATP**) is a reference brought by the International Union of Pure and Applied Chemistry (IUPAC) with temperature of 25 °C (298.15 K, 77 ℉) and pressure of 101.325 kPa. At these conditions, the volume of 1 mol of a gas is 24.4651 liters. It is used in Chemistry as a reference standard condition.

## International Standard Atmosphere (ISA)

**International Standard Atmosphere** (**ISA**) is used as a reference to aircraft performance: it is defined to 101.325 kPa, 15 °C and 0% humidity. The International Standard Atmosphere is a static atmospheric model of how the pressure, temperature, density, and viscosity of the Earth’s atmosphere change over a wide range of altitudes or elevations. It has been established to provide a common reference for temperature and pressure and consists of tables of values at various altitudes, plus some formulas by which those values were derived. The International Organization for Standardization (ISO) publishes the ISA as an international standard, ISO 2533:1975. Other standards organizations, such as the International Civil Aviation Organization (ICAO) and the United States Government, publish extensions or subsets of the same atmospheric model under their own standards-making authority.

## ICAO Standard Atmosphere

Standard model of the atmosphere adopted by the **International Civil Aviation Organization** (**ICAO**) follow an ideal model of the average real atmosphere, considered at a latitude of 45°:

- atmospheric pressure: 760 mmHg = 14.7 lbs-force/sq inch
- temperature: 15 °C (288.15 K, 59 °F)
- dry air (relative humidity: 0%) and free of impurities
- air density at average sea level: 1.225 kg/m
^{3} - vertical baric gradient: −1 hPa every 8.2296 m (27 ft) of altitude. In reality, this value can only be considered valid up to 3-4000 feet, since the variation does not follow a linear law
- vertical thermal gradient: -6.5 °C every 1000 m of altitude up to 11000 m; null from 11000 to 20000 m of altitude; irregular above 20000 m altitude.

The use of this ideal environment is useful in applied sciences to calculate and compare aircraft efficiency and performance, to calibrate navigation and measuring instruments, to test equipment, in standardized conditions.

## References

- International Standard Atmosphere. Wikipedia. https://en.wikipedia.org/wiki/International_Standard_Atmosphere