Buoyancy of gas-filled bladders at great depth

At high hydrostatic pressures exceeding 20MPa or 200bar, equivalent to depths exceeding ca.2000m, the behaviour of gases deviates significantly from the predictions of standard equations such as Boyle's Law, the Ideal Gas Law and Van der Waals equation. The predictions of these equations are compared with experimental data for nitrogen, oxygen and air at 0°C and 15°C, at pressures up to 1100bar (110MPa) equivalent to full ocean depth of ca. 11000m. Owing to reduced compressibility of gases at high pressures, gas-filled bladders at full ocean depth have a density of 847kgm−3 for Oxygen, 622kgm−3 for Nitrogen and 660kgm−3 for air providing potentially useful buoyancy comparable with that available from man-made materials. This helps explain why some of the deepest-living fishes at ca. 7000m depth (700bar or 70MPa) have gas-filled swim bladders. A table is provided of the density and buoyancy of oxygen, nitrogen and air at 0°C and 15°C from 100 to 1100bar.