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The testing of dive computers using human subjects
has been very limited; that means most of the support for
computers’ use has resulted from their operational success.
But operational safety does not translate to decompression-
algorithm safety since most real-world dives do not push the
algorithms to their limits. The simplest way to understand the
operational benefits a particular dive computer truly offers is to
simulate dives using the computer’s software and then analyze
the generated profiles using validated dive tables. If the results
are very similar, the risk of DCS should be roughly equivalent.
There is a lack of information on how different models
compute decompression, and this is sometimes perceived
as a lack of verifiable safety. The problem is an absence of
standards specific to dive computers that allow assessment
of their functional safety. Applied to a dive computer as a
safety-critical system, such standards would mean the device
performs according to requirements and that in case of failure,
no harm would occur. There is a need for a consolidated dive-
computer safety standard that uses the essential safety and
health requirements of CE Marking Directives (a set of widely
accepted European product conformity standards).
Applicability of Dive-Computer Algorithms
Dive-computer models employ more conservative versions of
dive tables; they achieve this primarily by reducing the tolerated
levels of supersaturation. While it’s obvious that using a
decompression model outside of its validated range carries risk,
even using one within its validated range does not guarantee
safety. A multilevel dive computed as an extension of the
multicompartment theory (which was validated using square
dives) cannot be assumed to follow the same rules.
There is little agreement among various computers
with regard to repetitive dives with short surface intervals
(one hour or less). While a relatively standard Haldanean
implementation is at the core of most dive computers,
different mathematical manipulations are employed to
account for residual nitrogen. This indicates that the true
impact of residual nitrogen is not fully understood.
To manage the risk of DCS, dives are conducted according
to decompression schedules that have parameters that
account for depth, time and breathing gas. These schedules are
derived from algorithms that aim to limit bubble formation
by slowing decompression, typically by interrupting ascent
with decompression stops to allow time for washout of inert
gas from tissues. Many decompression models use DCS as a
measurable endpoint, but it’s not generally practical to commit
time and money to the large number of dives necessary for this
type of validation, nor is it particularly ethical to provoke DCS.
Venous gas emboli (VGE) nearly always accompany DCS,
although their presence does not have a direct relationship with
clinical symptoms. However, VGE are an accepted indicator of
the level of decompression stress that a diver has been subjected
to and can thus be used as a tool to help in the validation process.
Conclusions
Twenty-nine years of
operational experience
with dive computers
has demonstrated that
their advantages over
tables outweigh their
disadvantages. The
primary issue with
computers remains their
mechanism of accounting
for repetitive dives. The
significant variability among dive computers means selection
criteria should be established to meet the specific needs
of particular dive communities. An important element
of this approach is the creation of a community-specific
universe of “safe” dive profiles for which the computer is
effective. This can be accomplished through the use of
a dive-computer monitoring program. Traditionally, the
limits of decompression models were established using
trials with human subjects, but this is not likely to occur
in dive-computer validation because of the time and
expense involved as well as the infinite combination of dive
computers and settings.
At present, DCS is the measurable negative outcome.
There is a need to specify an acceptable level of DCS risk
and a method for measuring that risk. A defined window
of applicability for each computer is also needed. A dive
computer should have the support of a dive planner, and
the computer’s functionality and safety must be verified
and documented. To understand “what’s in the box,”
documentation of designers’ logic and equations is needed.
There is no evidence that multilevel dives with dive
computers are more risky than square dives when they
follow the same algorithm. The risk of DCS in no-
decompression recreational and scientific diving is no
greater now than when tables were prevalent. This is largely
because dive computers are not pushed to the limits of their
decompression models or algorithms.
Dive-computer validation should include specification of
training standards for divers who use the computer, thorough
assessment to ensure the computer meets all requirements
for validation and monitoring of the computer’s operational
performance. AD
For more information on dive-computer validation, see:
Blogg, S.L., M.A. Lang, and A. Møllerløkken, eds. Proceedings of the
Validation of Dive Computers Workshop, 24 August 2011, Gdansk,
Poland, pp. 93-97. Trondheim: European Underwater and Baromedical
Society, 2012.
Further Reading
Clarity of information and
comfort are important aspects
of dive-computer validation.
STEPHEN FRINK