Gas Density
Density of a gas from pressure and temperature — ρ = PM/RT.
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The engineering
The ideal-gas law rearranged for the number a process engineer actually needs — kg/m³ for fan sizing, relief-valve math, and buoyancy checks. Enter absolute pressure, not gauge: that is the most common way this calculation goes wrong. Air at 101.325 kPa and 15 °C gives 1.225 kg/m³, the ISA sea-level standard.
Ideal-gas assumption throughout, good to about 1% for these gases near ambient; CO₂ starts deviating noticeably above ~10 bar. The molar masses are dry-gas values — humid air is *lighter* than dry air (water's 18 g/mol displaces 28.97), a fact that surprises everyone once.
Where this math comes from
Weighing gases precisely was Henri Victor Regnault's specialty in the 1840s — his gas-density measurements for the French government were the metrology backbone of thermodynamics for decades. The method's greatest moment came in 1894, when Lord Rayleigh noticed nitrogen from air was stubbornly 0.5% denser than nitrogen from chemistry, chased the discrepancy with William Ramsay, and discovered argon — a Nobel-winning result found entirely inside a density measurement's error bar.
For engineers the equation is bread and butter: gas density sets stack draft, pipeline capacity, and why a helium balloon pulls up with about 1 kg of lift per cubic metre.
- 1811Amedeo AvogadroMolar volume concept — density becomes PM/RT.
- 1845Henri Victor RegnaultPrecision gas-density measurements (circa).
- 1894Rayleigh & RamsayArgon discovered from a 0.5% density anomaly.
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