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Until the early 1980s almost all the recreational diving was taught by
the standards of U.S. Navy tables. Because the Navy tables are not based
on multi-level diving profiles, scientists developed algorithms that take
into account changes in nitrogen uptake with continuous changes in depth.
These algorithms were mainly theoretical models until the microchip revolution
made them accessible and workable in a hand-held computer. When the algorithm
is programmed into a computer that also senses depth (a simple depth gauge)
and measures time, you have a "dive computer."
Automated decompression monitors were discussed for years before they
became reality. The first serious attempt at making a decompression meter
was in the mid-fifties by the US Navy. The mechanical device tested was
not accurate enough to replace the tables and was abandoned. The first
mechanical decompression meter available to the public came out of Italy
in 1959. Through the sixties and seventies many companies and military
departments worked on various designs with little success. The technology
they needed to make their design work would not become widely available
until the eighties. In the early eighties the microchip technology became
more affordable for many applications. At last the number crunching power
necessary for computing decompression in real time was available.
The first commercially available dive computer was the Orca Edge developed
by Karl Huggins and Craig Barshinger in 1983. This computer was a multi
level computer that used the Haldanian model based on the silent bubble
work by Merill Spencer. Since then dive computers have become smaller
and more versatile. They are now manufactured by many companies, and incorporate
one of several algorithms for calculating nitrogen uptake and elimination.
Haldane's decompression theory
The first scientific approach to adequate decompression was taken by
J.S Haldane in the early 1900. Haldane and his colleagues. Working for
the British Admiralty, published their theory in 1908. Haldane's work
cantered on the use of half-time theory to describe how tissues accumulate
and released nitrogen during diving. Haldane put forward the concept that
tissues could be described by time factors, these factors ranging from
"fast tissues" to "slow tissues", which controlled the rate at which different
tissues took on gases and released them. Tissue half-lives is a method
used to describe the time necessary to allow the amount of gas in a particular
tissue to change by 50%. Haldane chose 5, 10, 20, 40 and 75 minutes as
tissue times based on what was considerate at the time to be the outer
limits of diving capabilities. In Haldane's time, bottom time and maximum
depths were severely limited by not only decompression, but by the limits
of technology. Haldane's theory was based on the following postulates:
- Tissues absorb and secrete nitrogen at an exponential rate, based
on the pressure difference between ambient pressure and the pressure
of nitrogen in the tissue
- Tissues in the human body form a continuous spectrum with various
rates of absorption and secretion
- This continuous spectrum can be modeled by selecting a finite number
of tissues
Haldane's method had an immediate impact on diving and tunnel workers,
as the rate of injury and death was reduced be half. Even so, the limits
of Haldane's theory were soon apparent. Haldane himself recognized the
need to continue improve his theory as the concept of half-life to describe
the amount of nitrogen tolerated by a tissue during decompression was
increasingly inaccurate at deeper depths and longer bottom times. Over
the years many researchers, both British and American, have worked on
expanding and improving Haldane's work. His theory, much reworked by various
researchers since 1908, is known as the Neo-Haldanian and is still the
basis for modern recreational diving decompression schedules.
Buhlmann's decompression
theory
Haldane considered the tissues as "parallel" tissues so there was no
influence in between the tissues. Only the gas dissolved in the blood
was affecting those tissues but they were not affecting each other. Others
developed Haldane's ideas over the years. In the mid-1960's US Navy Medical
Corps Captain Robert Workman refined the idea of allowable overpressure
in tissues, discounting oxygen and considering only inert gasses in the
breathing mix, such as nitrogen and helium. Workman's maximum allowable
overpressure values (what he called "M-values") were more complex than
Haldane's, varying with depth and with tissue type. At around the same
time Professor Albert Buhlmann was working on similar research at the
University Hospital in Zurich. Buhlmann's research spanned over 30 years
and was published as a book, Dekompression - Dekompressionskrankheit in
1983. This book, published in English in 1984, made fairly comprehensive
instructions on how to calculate decompression available to a wide audience
for the first time and therefore Buhlmann's work became the basis for
many dive tables, computers and desktop decompression programs. Three
other editions were published, the last in 1995, on which this document
is based. See "An explanation Professor A A Buhlmann's ZH-L16 Algorithm
by Paul Chapman" in the annexes.
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