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  • AutorenbildMichael Mutter

The little decompression theory 5 - diving at altitude

At altitude, the air pressure and hence the ambient pressure underwater are lower than at sea level. This leads to a lesser saturation of the tissues compared to diving in the sea. Nevertheless, the no-decompression limits (NDL) are shorter. Why is that so?

Image template: Karin Aggeler

The readers of the Dekoblog can guess the answer: because of supersaturation tolerance.

At high altitudes, the absolute ambient pressure drops and with it the pressure of the inert gas that is inhaled, as the regulator supplies it at the same level as the ambient pressure. As a result, the inert gas pressure in the tissue increases less. As the absolute ambient pressure for a given diving depth is lower than at sea level, there is less tissue saturation with inert gas (nitrogen or helium). This is actually favorable and an advantage over diving at sea level.

However, as the supersaturation tolerance depends on the ambient pressure, it declines according to

Ptol = Pamb * 1/b + a

as we have seen repeatedly in the last few articles.

For example, the absolute pressure at a depth of 40 m in a mountain lake at 2500 m above sealevel is 4.75 bar: 0.75 bar air pressure at the water surface + 4 bar water column/hydrostatic pressure, compared to 5 bar at sea level. If these values are entered into the saturation equation and the supersaturation tolerances are calculated correctly, the corresponding solutions are obtained for each altitude.

On balance, increasing altitude results in shorter NDL for a given depth compared to a dive at sea level and deeper deco stages, as illustrated in the figure below.

Figure 1: Dive with compressed air to 40 m at sea level (right) and in the Muttsee at 2500 m above sea level (left), shown at compartment 2. As a result of the lower ambient pressure, lower values for the tolerated inert gas pressures result at higher altitudes. Despite the lower saturation (blue line) due to the lower ambient pressure, this results in a shorter no-decompression time at altitude (red horizontal arrow). In addition, at the end of the dive, a decompression stop would have to be taken at 9 m after 20 min, while at sea level it would be possible to ascend to the 6 m decompression stage.

This has been extensively tested and confirmed by Bühlmann and others (A.A. Bühlmann. Dekompression - Dekompressionskrankheit. Springer 1983) in Lake Muttsee among other places. The corresponding tables are well known and popular. Today, computers calculate such dives routinely.

This concluding chapter is intended to illustrate that understanding decompression physiology requires not only the saturation and desaturation behavior of the tissues, but also knowledge of supersaturation tolerance, which is unfortunately hardly ever taught during diving training (because it was not part of the instructors' training and is therefore unfamiliar to them). The principles described here are universally applicable and can be used to understand (almost) any decompression model.

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