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 diving is significant in recreational diving because the depths and durations used for dives at altitude are different from those used for the same dive profile at sea level.
Measurement of depth at altitude
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.
Decompression when diving at altitude
At altitude, atmospheric pressure is lower than at sea level, so surfacing at the end of an altitude dive leads to a greater reduction in pressure and an increased risk of decompression sickness. 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 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 most common of the modifications to decompression tables at altitude are the "Cross Corrections" which use a ratio of atmospheric pressure and sea level to that of the altitude to provide a conservative equivalent sea level depth. The Cross Corrections were later looked at by Bassett and by Bell and Borgwardt.
Paulev and Zubieta have created a new conversion factor in order to make any sea level dive table usable during high altitude diving in 2007.
Repetitive dives should be conducted in the same manner as other dives including "Cross Corrections" for altitude. The US Navy does not allow repetitive diving for surface-supplied helium-oxygen diving and a 12 hour surface interval is required. An 18-hour surface interval is required if the dive requires decompression.
Pre- and post-dive ascents
In addition to making depth adjustments using the Cross Conversions, dives at altitude often require pre- and post-dive altitude ascents which must be taken into consideration. Several methods for performing post-dive ascents are used. One is to adjust the dive times needed for an altitude ascent. Another is to use surface intervals to allow for an ascent.
Extreme altitude diving
Although no official records are recognized, until 2007 the highest recorded altitude at which a scuba dive had been conducted was 19,300 feet (5,900 m), by a team led by Charles Brush and Johan Reinhard in 1982 in Lago Licancabur. This record was equalled by a team led by Nathalie Cabrol (SETI Institute/NASA Ames) in 2006. That year, Cabrol set the highest recorded altitude scuba diving for women. She also free dived at Lake Licancabur in 2003 and 2004.
The deepest known staged decompression altitude dive was conducted by Nuno Gomes at Boesmansgat (Bushman's hole) in South Africa. Conducted at an altitude of approximately 5,000 feet (1,500 m), Gomes dived to a depth of 927 feet (283 m). Gomes's decompression schedule was calculated as being equivalent to a dive to 1,112 feet (339 m) if it had been conducted at sea level.
- Prior to this the highest scuba dive in the continental US was a 1997 dive by Peter Hemming and David Moore at California's Tulainyo Lake, altitude 12,818 feet (3,907 m).
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- Altitude Diving: Understanding the Tables – Todd Stedl, PhD and PADI Instructor
- Altitude Exposure after Diving – Edmond Kay, MD
- Diving At Altitude – John Ware, PhD
- At-Altitude Arithmetic – Larry "Harris" Taylor, PhD
- Altitude Diving Calculator – Online calculator to determine the theoretical ocean depth while scuba diving at altitude.