Air - Density and Specific Volume vs. Altitude
The density of air varies with pressure (the Ideal Gas Law) and the altitude above sea level. Mean absolute pressure at sea level is approximately 760 mmHg (101.325 kPa, kN/m2) with a variation is about +/- 5%.
Altitude and Air Pressure
The air barometer pressure varies with altitude as shown in the table below:
| Altitude (m) | Pressure | |
|---|---|---|
| (mmHg) | (kPa) | |
| 0 | 760 | 101.3 |
| 75 | 753 | 100.4 |
| 150 | 746 | 99.5 |
| 250 | 739 | 98.5 |
| 300 | 733 | 97.7 |
| 450 | 719 | 95.9 |
| 600 | 706 | 94.1 |
| 750 | 693 | 92.4 |
| 900 | 681 | 90.8 |
| 1000 | 668 | 89.1 |
| 1200 | 656 | 87.5 |
| 1350 | 644 | 85.9 |
| 1500 | 632 | 84.3 |
| 1800 | 609 | 81.2 |
| 2100 | 586 | 78.1 |
| 2400 | 564 | 75.2 |
Altitude and Air Density
Air density and specific volume as functions of altitude above sea level can be indicated as in the diagrams below:
STP - Standard Temperature and Pressure
- STP - Standard Temperature and Pressure is 273.15 K (0 °C, 32 °F) and absolute pressure exactly 105 Pa (100 kPa, 1 bar) - according IUPAC - International Union of Pure and Applied Chemistry - since 1982
NTP - Normal Temperature and Pressure
- NTP - Normal Temperature and Pressure (commonly used for testing and documentation of fan capacities) is 293.15 K (20 °C, 68 °F) and absolute pressure 1 atm (101.325 Pa, 14.696 psi) according NIST - National Institiute of Standards and Technology
Note! - other definitions of STP and NTP are widely used.
Altitude and Specific Volume Correction
The specific volume of standard air at a certain altitude can be calculated by multiplying with the volume correction factor below
| Altitude | Volume Correction Factor | |
|---|---|---|
| (m) | (ft) | |
| 0 | 0 | 1.00 |
| 480 | 1600 | 1.05 |
| 990 | 3300 | 1.11 |
| 1500 | 5000 | 1.17 |
| 1980 | 6600 | 1.24 |
| 2460 | 8200 | 1.31 |
| 2970 | 9900 | 1.39 |
Altitude and Air Pressure
| Altitude | Pressure | ||||
|---|---|---|---|---|---|
| feet | metre | inHg | torr | psia | kPa |
| 0 | 0 | 29.92 | 759.97 | 14.7 | 101.3 |
| 500 | 152 | 29.39 | 746.51 | 14.43 | 99.4 |
| 1000 | 305 | 28.86 | 733.04 | 14.17 | 97.6 |
| 1500 | 457 | 28.34 | 719.84 | 13.91 | 95.8 |
| 2000 | 610 | 27.82 | 706.63 | 13.66 | 94.1 |
| 2500 | 762 | 27.32 | 693.93 | 13.41 | 92.4 |
| 3000 | 914 | 26.82 | 681.23 | 13.17 | 90.7 |
| 3500 | 1067 | 26.33 | 668.78 | 12.93 | 89.1 |
| 4000 | 1219 | 25.84 | 656.34 | 12.69 | 87.4 |
| 4500 | 1372 | 25.37 | 644.4 | 12.46 | 85.8 |
| 5000 | 1524 | 24.9 | 632.46 | 12.23 | 84.3 |
| 5500 | 1676 | 24.44 | 620.78 | 12 | 82.7 |
| 6000 | 1829 | 23.98 | 609.09 | 11.77 | 81.1 |
| 6500 | 1981 | 23.53 | 597.66 | 11.55 | 79.6 |
| 7000 | 2134 | 23.09 | 586.49 | 11.34 | 78.1 |
| 7500 | 2286 | 22.66 | 575.56 | 11.13 | 76.7 |
| 8000 | 2438 | 22.23 | 564.64 | 10.91 | 75.2 |
| 8500 | 2591 | 21.81 | 553.97 | 10.71 | 73.8 |
| 9000 | 2743 | 21.39 | 543.31 | 10.5 | 72.3 |
| 9500 | 2896 | 20.98 | 532.89 | 10.3 | 71.0 |
| 10000 | 3048 | 20.58 | 522.73 | 10.1 | 69.6 |
| 10500 | 3200 | 20.19 | 512.83 | 9.91 | 68.3 |
| 11000 | 3353 | 19.8 | 502.92 | 9.72 | 67.0 |
| 11500 | 3505 | 19.41 | 493.01 | 9.53 | 65.7 |
| 12000 | 3658 | 19.03 | 483.36 | 9.34 | 64.4 |
| 12500 | 3810 | 18.66 | 473.96 | 9.16 | 63.1 |
| 13000 | 3962 | 18.3 | 464.82 | 8.99 | 61.9 |
| 13500 | 4115 | 17.94 | 455.68 | 8.81 | 60.7 |
| 14000 | 4267 | 17.58 | 446.53 | 8.63 | 59.5 |
| 14500 | 4420 | 17.24 | 437.9 | 8.46 | 58.3 |
| 15000 | 4572 | 16.89 | 429.01 | 8.29 | 57.1 |
| 15500 | 4724 | 16.56 | 420.62 | 8.13 | 56.0 |
| 16000 | 4877 | 16.22 | 411.99 | 7.96 | 54.8 |
| 16500 | 5029 | 15.9 | 403.86 | 7.81 | 53.8 |
| 17000 | 5182 | 15.58 | 395.73 | 7.65 | 52.7 |
| 17500 | 5334 | 15.26 | 387.6 | 7.47 | 51.5 |
| 18000 | 5486 | 14.95 | 379.73 | 7.34 | 50.6 |
| 18500 | 5639 | 14.65 | 372.11 | 7.19 | 49.5 |
| 19000 | 5791 | 14.35 | 364.49 | 7.05 | 48.6 |
| 19500 | 5944 | 14.05 | 356.87 | 6.9 | 47.5 |
| 20000 | 6096 | 13.76 | 349.5 | 6.76 | 46.6 |
| 20500 | 6248 | 13.48 | 342.39 | 6.62 | 45.6 |
| 21000 | 6401 | 13.2 | 335.28 | 6.48 | 44.6 |
| 21500 | 6553 | 12.92 | 328.17 | 6.34 | 43.7 |
| 22000 | 6706 | 12.65 | 321.31 | 6.21 | 42.8 |
| 22500 | 6858 | 12.38 | 314.45 | 6.08 | 41.9 |
| 23000 | 7010 | 12.12 | 307.85 | 5.95 | 41.0 |
| 23500 | 7163 | 11.86 | 301.24 | 5.82 | 40.1 |
| 24000 | 7315 | 11.61 | 294.89 | 5.7 | 39.3 |
| 24500 | 7468 | 11.36 | 288.54 | 5.58 | 38.4 |
| 25000 | 7620 | 11.12 | 282.45 | 5.46 | 37.6 |
| 25500 | 7772 | 10.88 | 276.35 | 5.34 | 36.8 |
| 26000 | 7925 | 10.64 | 270.26 | 5.22 | 36.0 |
| 26500 | 8077 | 10.41 | 264.41 | 5.11 | 35.2 |
| 27000 | 8230 | 10.18 | 258.57 | 5 | 34.5 |
| 27500 | 8382 | 9.96 | 252.98 | 4.89 | 33.7 |
| 28000 | 8534 | 9.74 | 247.4 | 4.78 | 32.9 |
| 28500 | 8687 | 9.53 | 242.06 | 4.68 | 32.2 |
| 29000 | 8839 | 9.31 | 236.47 | 4.57 | 31.5 |
| 29500 | 8992 | 9.11 | 231.39 | 4.47 | 30.8 |
| 30000 | 9144 | 8.9 | 226.06 | 4.37 | 30.1 |
Temperature vs. Elevation Density Correction
The chart above can be used to estimate "temperature vs. elevation density correction factor". The density at a given temperature and elevation can then be calculated as
ρ = ρref / f (1)
where
ρ = density at the given temperature and elevation (lbm/ft3)
ρref = density reference at elevation 0 ft and temperature 70 oF - (0.075 lbm/ft3)
f = correction factor from the chart above
Example - Density at Elevation 4000 ft and Temperature 300 oF
From the chart above - at elevation 4000 ft and temperature 300 oF the correction factor is approximate 1.7. The density can be calculated as
ρ = (0.075 lbm/ft3) / 1.7
= 0.043 lbm/ft3
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