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Chapter 13 — 5
DIVE PROFILES
The type of dive has a significant bearing on where and when bubble formation takes place.
Short deep
dives (i.e. deeper than 30 metres)
tend to cause bubbles in the fast tissues
(blood, brain and spinal cord) while
long shallow dives tend to produce bubbles in the slow
tissues
(like the joints). Long deep dives cause bubbles everywhere.
This distribution occurs because:
• in short dives, only the fast tissues take up enough N
2
to form bubbles on ascent and
• after shallow dives, fast tissues eliminate their relatively modest N
2
excess before a
critical pressure differential develops.
It can thus be seen why it is important to ascend slowly from all deep dives. The slower the
ascent rate, the longer the time for fast tissues to eliminate N
2
through the lungs, before a
critical N
2
pressure-differential develops. Slow tissues are more affected by the total exposure
(duration + pressure effects) and are more influenced by this and the duration of staging.
Diving folklore contains a myth that a diver using a single 2000 litres (72 cu. ft) tank cannot
develop DCS. The air supply available was said to limit the diver to safe dive profiles. This is
not true. For example, A single dive to 20 metres, may last 30-40 minutes, within the no-
decompression time given by most tables, but it is not entirely safe. Remember, as mentioned
previously, that even a single dive in excess of 10 metres can produce DCS.
The myth may become more apparent for deeper dives. For example, a single 2000 litre tank
will give around 10 minutes duration for a 50 metre dive. According to most decompression
tables, a 10 minute dive to 50 metres will require 10 minutes of decompression — but there
may be no air remaining to complete these stops. Even if there was sufficient air, dives to this
depth have a significant risk of DCS despite the tables being followed correctly.
FACTORS INFLUENCING
DCS
DCS is unpredictable. In general, anything that increases blood flow to an organ will increase
the rate of N
2
loading. Anything that interferes with blood flow from an organ will reduce the
capacity to off-load N
2
. These alterations may explain some of the possible predisposing
factors that increase the likelihood of DCS.
!
Depth/duration.
Any dive deeper than 10 metres can produce DCS although in general, the deeper the dive,
the more gas absorbed, the greater the risk. The longer the dive at any one depth, the more
gas absorbed (until saturation), the more the DCS risk.