[Optional Spanish language version of page].
As soon as divers were able to work at depth (and construction
workers under increased pressure), the issue of diving-related sickness
(also called bends, DCS, caisson disease) arose. The cause was
understood by Paul Bert in 1878 to be from nitrogen gas coming out of
solution and causing bubbles which blocked blood flow. How to manage
this was not really understood until the work of J. S. Haldane. He
experimented with volunteers and animals at different pressures for
different durations. He understood that different tissues have
different time-scales (he used 5: from 5 to 75 minutes) slow tissues
take up nitrogen slowly but release it slowly, etc. One of his
conclusions was that a diver could withstand a pressure decrease to 50%
(so from 10m depth to the surface, for example) without illness. He
introduced staged decompression (stops on the way up). He also
recommended a maximum ascent rate (9m/minute). His work formed the
basis of the Royal Navy dive
tables released in 1908.
Greek sponge divers developed
their own "table" - dive, smoke a cigarette, repeat,.. No wonder some
Greek villages were full of "bent" ex-divers.
I learnt to dive in Switzerland which, at that time (1970s), used the US Navy Tables (also called PADI tables since PADI dive training used them). These tables had a provision for multiple dives - after a dive, you were in a certain "letter" and then, as time passed on the surface, the "letter" changed. When you planned your next dive - this "letter" was part of the input. The Swiss used a metric version of the US Navy tables with the depths rounded down from the "footric" original.
When I came to Britain to dive (from 1975) the BASC tables (pre 1988; basically RN dive tables) were used -
they had a very restricted option for multiple dives - basically treated
as one combined dive unless you waited 6 hours or your second dive was
very shallow.
I remember doing two dives around Anglesey, with the second being
deeper, and worrying that this was potentially going to cause a "bend". When
I checked with my US Navy tables - all was OK, and I immediately felt better.
When the first dive computers became available - the DecoBrain - I saw that it used a fairly simple underlying model - by Bühlmann - of nitrogen uptake and release with 16 time-scales from 4 minutes to 10 hours. This model was published, so I used it to create my own multiple dive tables. The full version of my tables used letters like the US Navy tables. I also created a stripped down version - which listed the maximum time allowed after repeated dives with 1 hour surface interval and no decompression stops. I remember discussing this with John Thornton - who confirmed that it was very similar to what scallop divers were actually using. Some of my colleagues had the confidence in my work to use these tables themselves.
As an example, consider a single dive to 30m depth and return to the surface without decompression stops: the French tables gave a duration of 30 minutes, US Navy gave 25 minutes, BSAC gave 20 minutes, Bühlmann gave 18 minutes and my table gave 17 minutes. My table, of course, catered for repeat dives if an hour or more was spent on the surface.
Eventually, dive computers became affordable, compact and reliable. They had the advantage of dealing with dives to varying depths - for example to 30 metres but with a slow ascent up a reef to the surface. They also facilitate nitrox use.
