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To fill in more detailÂ… The divers were: Leigh Bishop (organiser) Kevin Emans Nick Hope Bob Hughs Chris Hutchinson Oliver McClory Jamie Powell Richard Tully David Wilkins Support Diver - Mick Cousins All the divers used a Trimix 9/57 bottom mix, in twin 15l cylinders. Two basic arrangements of nitrox cylinders were used, with most divers using a 7l cylinder mounted between/ on the back or the main cylinders plus two 2l side mounts. Gases for this arrangement were typically air (in the 7l cylinder), nitrox 40 and nitrox 80. To be different, I used four side mounts, with a 7l and a 4l on each side (air, nitrox 50, nitrox 80 and oxygen). All the divers carried all their gases throughout the dive. Decompression tables were cut mostly using Proplanner, but I used Abyss - the results were not greatly different. Typically air was used on the descent to 30-40m. On the ascent, the changeover from bottom mix to air was at 60-50m according to taste. One diver had a narcosis 'event' (dizziness) on changing onto air at 60m, but this was short-lived. There was debate within the team over the relative merits of nitrox 80 and oxygen for the last stop(s). With the advantage of four deco cylinders, I used oxygen on the 3m stop, which gave a noticeable reduction in deco time over nitrox 80 at that depth. This benefit of oxygen appears to become significant when high helium partial pressures are used on the bottom - ie very deep dives on strong trimix. Bottom times were generally 15 minutes (run time, ie including descent time), giving in-water times of about 2.5 hours. The in-water time was constrained by the water temperature, and two divers used electrical suit heaters. The "minor DCS problem" referred to by John was a type 2 bend in the left elbow and shoulder, requiring recompression treatment at Aberdeen hospital. The hospital and emergency services, who were advised of the expedition in advance, acted with great efficiency and have the wholehearted thanks of the team. The diver in question has made a full recovery, and wishes others to learn from his misfortune. A number of other divers had very minor 'niggles' on surfacing, all joint/ muscle aches (ie possible type 2 DCS), which resolved very quickly with surface oxygen. The general opinion of the team is that currently available decompression programs/ tables/ algorithms are highly experimental (ie don't work very well) over 100m, and the problems were not specific to the particular programs used. I would note the Jean Pierre Imbert, at the UK IANTD conference at Coventry earlier this year, gave the opinion that compartment/ tissue based algorithms break down below 120m. I would offer the suggestion that using oxygen on the 3m stop (typically about 40 minutes) is beneficial for this type of diving in preventing DCS arising from nitrogen in very slow compartments - there is some evidence that this may have been the source of the problems. Equipment problems were remarkably few. A nite-rider (sic) battery pack couldn't take the pressure and flooded (perhaps it should have had a tank parked on top of it?). The new type Uwatec timers refused to read over 99.9m, but the older (4 memory) ones were fine. An amazing number of contents gauge (spg to the Yanks) hoses failed, all when cylinders were turned on during kitting up (this was also noticed on the 1994 Lusitania trip) - why is their failure rate so much higher after very deep dives? Several cases of stratification of gases in trimix cylinders were observed. In all these, helium was being haskelled on top of a weaker trimix. For example, before the first dive my cylinders contained a part fill of trimix 14/33 and had helium pumped on top to get the desired mix. An hour after pumping, the cylinders analysed at 6% oxygen, but analysed as expected after a further hour upside down. The wreck turned out to be upside-down, and sunk into the bottom so that the 6" tertiary armament is about 1-2 feet above the seabed. The Admiralty hydrographic printout says a ROV report indicated the wreck was on its side. Because of the tidal conditions, most of the decompression time was done on a decompression station, assembled by the support diver. This was detached from the shot line when the last diver moved onto it at about 30m on the ascent, to drift with the tide. The deco station comprised three large buoys, each with a 30-40m drop rope under it, and attached to each other with rigid poles. Bungee cord under the buoys took out most of the wave action. The poles held the station in a triangular formation after detachment from the shot line. The dive boat stayed mobile all the time, as is normal UK practice. Points arising: 1) does anyone make a depth gauge designed to read (accurately) below 100m? and 2) constructive discussion on decompression below 100m welcome. Dave (dwilkins@fi*.ci*.co*.uk*) -- Send mail for the `techdiver' mailing list to `techdiver@aquanaut.com'. Send list subscription requests to `techdiver-request@aquanaut.com'.
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