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Chapter 14 — 1
Chapter 14
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DECOMPRESSION
TABLES & METERS
Since the work of J.S.
Haldane
(a British physiologist) early last century, decompression
tables have been based on mathematical models of gas uptake and elimination in the body. He
believed (wrongly) that the exponential rates of uptake and elimination of gases would be
equal, and that body tissues could tolerate a supersaturation pressure gradient of air equal to
2:1, without causing bubble formation. He experimented on goats – the closest animal model
that he could get to the diver – to determine this gradient. Thus a goat could tolerate 2 ATA
of air pressure in its tissues, even when walking around at 1 ATA! Or, it could ascend from 6
to 3 ATA, 4 to 2 ATA and from 2 to 1 ATA, without bubble formation.
Haldane devised mathematical equations (later referred to as models or algorithms) that
would represent a diver, and applied this to dive exposures. He postulated 5 hypothetical
tissues (with half-times of 5, 10, 20, 40, 75 min) and decompressed the diver so that the
supersaturation gradient in each of these 5 tissues would never exceed 2:1. Once any tissue
reached that ratio, the diver would stop his ascent and “stage” there until the tissue “off-
gassed” to allow him to ascend to the next stop. Thus bubble formation would be avoided by
this decompression.
Although used by the Royal Navy for 50+ years, it was evident that some of Haldane’s dive
stops, or stagings, were too conservative and others too radical. Also, the assumptions on
which the procedure was based were wrong.
The acclaimed
1957
U.S. Navy Tables
are based on Haldane's theories. These include equal
exponential uptake and elimination of inert gases and supersaturation gradients – as described
in Chapters 1 and 13 – but have been modified by experimental trials and practical diving
experience. In an attempt to overcome the flaws in Haldane’s tables and to make
decompression safer, they increased the number of hypothetical tissues to 6 (5, 10, 20, 40, 80
and 120 minutes), and calculated different maximum safe supersaturation ratios (now called
M values) for the different tissues at different depths.
Following experiments and reviewing of the established decompression tables, between 1960
and 2000 a whole series of innovations and modifications were introduced. Dr Bruce Bassett
(a USAF physiologist) concluded that the US Navy Tables resulted in an excessive incidence
of about 6% DCS, when pushed to the no-decompression limits. Merrill Spencer in Seattle
verified this observation and supported it with extensive Doppler monitoring, showing that
bubbles developed in many routine dives – implying inadequate decompression. Many others
observed similar inadequacies and in an attempt to cope with this new information, tables