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Chapter 6 — 7
Altitude Diving
Diving in waters located above sea level (e.g. a mountain lake or dam) introduces some potential
hazards which are related to the cold temperatures at altitude and buoyancy problems with fresh
water (see below). Other variations with altitude are much more important, but not immediately
obvious.
Consider a dive in a mountain lake where the atmospheric pressure is half that at sea level (this
would be at an unlikely altitude of about 6000 metres or 18,000 ft. elevation, but it makes the
calculations easy). The pressure at the surface of the lake is that of the atmosphere, 0.5 ATA.
Assume it is a salt water lake (fresh water is slightly less dense and so exerts slightly less
pressure at a given depth).
The water in the lake will exert the same pressure at this altitude as it would at any other
altitude.
That is, 10 metres of water will still exert a pressure of 1 ATA.
The pressure at 5 metres depth therefore will be 1 ATA, consisting of 0.5 ATA contributed by
the atmospheric pressure, and 0.5 ATA contributed by the water.
The pressure at 10 metres will thus be 1.5 ATA.
One might think initially that this would give the diver a safety margin since the pressure at a
given depth in a mountain lake is less than that in the ocean. The critical difference, however, is
that the diver in the lake is returning to a lower surface pressure.
This can be illustrated by referring to one of Haldane's hypotheses (see Chapter 13). He
indicated that a diver could spend an unlimited time at 10 metres (2 ATA) and return to the
surface (1 ATA) without developing decompression sickness. In other words, a diver could
return to a pressure of half the original pressure (i.e. a 2 : 1 ratio) without developing nitrogen
bubbles in the tissues.
In the mountain lake, because the surface pressure is only half that at sea level (0.5 ATA), the
diver need dive to only 5 metres (1 ATA) and return to the surface to encounter the same 2 : 1
“safe” ratio. A 10 metre dive exceeds the “safe” decompression ratio. This makes dive tables
designed for sea level unreliable at altitude unless considerable corrections are made.
Decompression at altitude is further complicated by difficulties in estimating depth. Digital
electronic gauges must be calibrated for altitude.
A mechanical depth gauge calibrated for sea level is likely to be unreliable at altitude. The
gauge simply measures pressure and registers this as depth. Since the pressure at the surface of
the lake is 0.5 ATA (half that of sea level), the gauge will be straining its mechanism and
possibly bending the needle, trying to get its pointer past the zero stop to register what it
interprets as negative depth. The gauge may only start to register a depth after it has returned to
1 ATA. This would not happen in the mountain lake until the water pressure and atmospheric
pressure added up to 1 ATA – a depth of about 5 metres.
Even a capillary depth gauge, calibrated at sea level, will not really read accurately. At sea level,
the air-to-water interface in the capillary will move half way along the capillary at 10 metres,
since the pressure there is twice that at the surface. In the mountain lake with a surface pressure
of 0.5 ATA, twice the surface pressure will be encountered at about 5 metres depth. So the
capillary gauge will reach the "10 metre depth mark" (the half volume mark) at 5 metres.