## Standard Temperature and Pressure Conditions

Standard air density is density of air calculated at standard temperature and pressure conditions. Standard temperature and pressure (STP) conditions are used as reference points in thermodynamics of gases. Normally standard and normal temperature pressure conditions are used for specifying gas volume. Values of standard temperature and pressure depend on the organization which defines them. Usually the standard pressure is close to atmospheric pressure and standard temperature is close to room temperature value.

## Standard Air Density Calculation

The standard density value depends heavily on what definition of standard T & P has been used. Standard conditions as per the IUPAC are:

Standard pressure (P) = 100 kPa

Standard temperature (T) = 0 ^{0}C

Let's see how to calculate the air density at these conditions.

Air Molecular weight (MW) = 28.85 gm/mol

Universal gas constant = 8.314 J/K·mol

ρ = P×MW/(R×T) = 100×1000×(28.85/1000)/(8.413×(273.16+0))

**ρ = 1.2554 kg/m ^{3}**

*However, note that we have used an important approximation here. We have use the ideal gas equation to arrive at "ρ = P×MW/(R×T)". If we consider the same equation for real gas, we need to use the compressibility or Z factor to change it to "ρ = P×MW/(Z×R×T). You can check here for the detailed derivation of this equation and a short tutorial for calculation of gas density.*

Alternatively, air density calculator can be used to calculate the air density at these standard temperature and pressure conditions. As per the air density calculator,

ρ = 1.2763 kg/m^{3}

This value matches closely with our calculation for air as an ideal gas at standard temperature and pressure conditions.

You can also use this tutorial as a guideline for calculation for air density

## Standard air density values

But that value corresponds to a specific STP conditions specified by IUPAC. Other organizations have defined slightly different standard T & P conditions.

Air density at these different STP conditions is tabulated below.

Defining Organization |
Std Temperature in ^{0}C |
Std Pressure in kPa |
Standard Air Density in kg/m^{3} |

IUPAC | 0 | 100.0 | 1.2554 |

NIST, ISO 10780 | 0 | 101.325 | 1.2720 |

ISA, ISO 13443 | 15 | 101.325 | 1.2058 |

EPA | 25 | 101.325 | 1.1654 |

SATP | 25 | 100.0 | 1.1501 |

CAG | 20 | 100.0 | 1.1697 |

SPE | 15 | 100.0 | 1.1900 |

ISO 5011 | 20 | 101.3 | 1.1849 |

## Using the standard density

- The standard density for any gas is is an indicator of the composition of the gas. Density of any gas varies widely with temperature and pressure conditions. Hence the standard temperature and pressure conditions are used as the common reference point at which density can be calculated and compared with other gas mixtures also at STP (standard T & P) conditions.
- In case of air, the standard density value is an indicator of humidity in the air. Moisture content present in the air increases its density and makes it heavier.
- Further, you can also use the standard density value for a variety of engineering calculations involving air or vapor streams. Often, it is easier to convert a gas flow to standard conditions for the purpose of engineering calculations.