Altitude diving

Altitude diving is scuba diving where the surface is 300 meters (1,000 feet) or more above sea level (for example, a mountain lake).^[1][2] 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) (actual depth).^[3] 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.^[4]

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.^[2] Capillary gauges have been shown to be a conservative method for measurement of compensated depth at altitude.^[5] 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.^[6] 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.^[6] 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.^[6] The US Navy recommends waiting 12 hours following arrival at altitude before performing the first dive.^[3]

Decompression Tables

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.^[7][8] The Cross Corrections were later looked at by Bassett and by Bell and Borgwardt.^{[9][10][11][12]}

Hennessy formulated that is was possible to convert standard air decompression tables for no-stop diving at altitude or from a habitat based on phase equilibration theory.^[13]

Albert A. Bühlmann recognized the problem^[14][15][16] and proposed a method which calculated maximum nitrogen loading in the tissues at a particular ambient pressure.^[17][18]

Wienke proposed guidelines for decompression diving at altitude in 1993.^[19]

Egi and Brubakk reviewed various models for preparing tables for diving at altitude.^[20]

Repetitive Diving

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.^[3]

References

External links

• 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

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