By Nuno Gomes

The higher the altitude above sea level, the deeper is the relative exposure
in terms of the sea level equivalent depth. When diving at altitude the
actual depth, the ascent rate and the depth of the decompression stops has
to be corrected because of the reduced atmospheric pressure at altitude
(see table 1.0 below). The reduced atmospheric pressure at altitude is not
able to keep the gases in solution, in the diver’s body, following
a dive if the standard decompression procedures are followed.
TABLE 1.0 – ATMOSPHERIC PRESSURE AT ALTITUDES ABOVE SEA LEVEL

ALTITUDE
ABOVE SEA LEVEL (Meters) |
ALTITUDE
ABOVE SEA LEVEL (Feet) |
ATMOSPHERIC
PRESSURE (fsw) |
ATMOSPHERIC
PRESSURE (Kpa) |

0 |
0 |
33.0 |
100 |

305 |
1000 |
31.9 |
96.4 |

610 |
2000 |
30.8 |
93.0 |

915 |
3000 |
29.7 |
89.6 |

1219 |
4000 |
28.5 |
86.4 |

1524 |
5000 |
27.5 |
83.2 |

1929 |
6000 |
26.5 |
80.1 |

2134 |
7000 |
25.4 |
77.2 |

2438 |
8000 |
24.5 |
74.3 |

2743 |
9000 |
23.6 |
71.5 |

3048 |
10000 |
22.7 |
68.8 |

Much research has been carried out over the years by various researchers
such as Buhlmann, Cross, Wienke and others on the topic of high altitude
diving.

In Switzerland and other mountainous or plateau countries, located at
ambient atmospheric pressures lower than the atmospheric pressure at sea
level, it becomes imperative to correct the depth, ascent rate and decompression
stops depths for altitude (see table 2.0 below), in order to avoid serious
decompression sickness.

Dives at high altitude, because of the relative increased depths and thus
longer total decompression times, carry a far greater risk than same depth
dives at sea level.

TABLE 2.0 – ALTITUDE CORRECTION FACTORS

ALTITUDE
ABOVE SEA LEVEL (Meters) |
ALTITUDE
ABOVE SEA LEVEL
(Feet) |
CORRECTION
FACTOR |

0 |
0 |
1.00 |

305 |
1000 |
1.04 |

610 |
2000 |
1.07 |

915 |
3000 |
1.11 |

1219 |
4000 |
1.16 |

1524 |
5000 |
1.20 |

1929 |
6000 |
1.24 |

2134 |
7000 |
1.29 |

2438 |
8000 |
1.34 |

2743 |
9000 |
1.40 |

3048 |
10000 |
1.45 |

For example a dive to 100 meters (328.10 feet) at an altitude of 2700
meters (9000 feet) above sea level:

Corrected depth is: 100 meters x 1.40 = 140 meters (for the calculation
of deco times)

Corrected depth is: 328.10 feet x 1.40 = 459.34 feet (for the calculation
of deco times)

Corrected ascent rate (Buhlmann): 10 meters/minute / 1.40 = 7 meters/minute

Corrected ascent rate (US Navy): 60 feet/minute/ 1.40 = 40 feet/minute

Corrected decompression stop depth: 3 meters / 1.40 = 2 meters (applies
to all deco stops)

Corrected decompression stop depth: 10 feet / 1.40 = 7 feet (applies to
all deco stops)

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