Altitude diving

Altitude diving is underwater diving using scuba or surface supplied diving equipment where the surface is 300 meters (1,000 feet) or more above sea level (for example, a mountain lake). The U.S. Navy tables recommend that no alteration be made for dives at altitudes lower than 91 meters (300 feet) and dives between 91 meters and 300 meters correction is required for dives over 44 meters sea water (145 feet sea water). Altitude is significant in diving because the depths and decompression used for dives at altitude are different from those used for the same dive profile at sea level.

Special consideration must be given to measurement of depth given the effect of pressure on gauges. The use of bourdon tube, diaphragm, and digital depth gauges may require adjustment for use at altitude. Capillary gauges have been shown to be a conservative method for measurement of compensated depth at altitude. Modern dive computers detect changes in altitude and automatically adjust their calculation of a safe decompression regime for a dive at that altitude. If an altitude-aware computer is not used, altitude decompression tables must be used.

At altitude, atmospheric pressure is lower than at sea level, so surfacing at the end of an altitude dive leads to a greater relative reduction in pressure and an increased risk of decompression sickness compared to the same dive profile at sea level. The dives are also typically carried out in freshwater at altitude so it has a lower density than seawater used for calculation of decompression tables. The amount of time the diver has spent acclimatising at altitude is also of concern as divers with gas loadings near those of sea level may also be at an increased risk. The US Navy recommends waiting 12 hours following arrival at altitude before performing the first dive. The tissue supersaturation following an ascent to altitude can also be accounted for by considering it to be residual nitrogen and allocating a residual nitrogen group when using tables with this facility.

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