Air Properties — Density, Viscosity, Heat Capacity, Thermal Conductivity

Overview

Dry air is a mixture of roughly 78 % nitrogen, 21 % oxygen, and 1 % argon by volume. Its thermophysical properties vary with temperature and pressure and are essential inputs for HVAC design, combustion calculations, pneumatic systems, and atmospheric science. This page summarizes key property values at standard conditions, provides compact reference tables, and illustrates common engineering calculations.

Standard-Condition Properties

Bulk Modulus Elasticity

The source air-property reference lists Bulk Modulus Elasticity for air. At standard atmospheric pressure this is approximately 1.01325×10^5 Pa (101.325 kPa), equivalent to 14.7 psi.

Phase-Change and Critical-Point Data

Critical density at the critical point is 302.6 kg/m³.

Interactive Air Phase Diagram Points

The original air phase diagram is preserved below. Its labeled reference points are also represented here as structured data so the phase-change information is not available only inside the bitmap.

Density vs Temperature

Density decreases roughly linearly with temperature over moderate ranges, but the true relationship follows the ideal-gas law ($\rho \propto 1/T$).

Calculator — Air Density (Ideal Gas)

Calculator — Mass of Air

Calculator — Dynamic and Kinematic Viscosity

Source table values are interpolated for dry air at atmospheric pressure.

Calculator — Thermal Conductivity

Calculator — Specific Heat and Sensible Heat

Calculator — Buoyant Lifting Force

Restored Original Source Tables

The following tables are restored from the original source page to preserve the complete reference data.

Source Calculator Signals

The cached source page contains one form and 58 input elements, but these signals come from shared Engineering ToolBox page controls and search/layout widgets rather than 59 independent air-property calculators. The migrated functional equivalents for the substantive engineering calculations are the density, air mass, dynamic and kinematic viscosity, thermal conductivity, sensible heat, and hot-air lifting force calculators above.

Original Source Images

The following original source images are preserved to avoid losing visual reference material. When an image contains chart or tabular data, its extracted values are represented in the page tables, calculators, or interactive charts; remaining images are retained as visual source references.

Engineering Notes

• Moisture matters. Values above are for dry air. Humid air is less dense at the same temperature and pressure because water vapor ($M \approx 18$) is lighter than the N₂/O₂ mixture it displaces. Use humidity-corrected density for accurate HVAC and psychrometric calculations.
• Pressure dependence. At moderate pressures (roughly 0.5–2 bar) air behaves nearly ideally. At high pressures (e.g., compressed-air systems above 10 bar), apply compressibility corrections.
• Standard reference states. Data are commonly reported at 0 °C / 1 bar or at 15 °C / 1 atm (ISA standard). Always confirm the reference state when comparing values across sources.
• Critical point. Above 132.63 K and 3.786 MPa, air cannot be liquefied by pressure alone; this is relevant to cryogenic air-separation plant design.
• Viscosity trend. Unlike liquids, gas viscosity increases with temperature due to greater molecular momentum transfer. Dynamic viscosity rises from approximately 17 μPa·s at 0 °C to approximately 23 μPa·s at 100 °C.
• Specific heat ratio. The ratio $k = C_p/C_v \approx 1.40$ for dry air at ambient conditions is widely used in isentropic-flow and compression calculations.

Key Formulas

Ideal-gas density

$\rho = \frac{p}{R\,T}$

where $p$ is absolute pressure (Pa), $R$ is the specific gas constant for dry air ($287.058\;\text{J kg}^{-1}\text{K}^{-1}$), and $T$ is absolute temperature (K).

Mass from density

$m = V\,\rho$

Buoyant (lifting) force

$F_l = V\left(\rho_{\text{cool}} - \rho_{\text{hot}}\right)g$

Sensible heat transfer

$Q = m\,C_p\,\Delta T$

Variables

References

• Engineering ToolBox — Air Properties
• CRC Handbook of Chemistry and Physics, standard thermophysical tables
• ISO 2533:1975 — Standard Atmosphere