According to the divers I know and respect, decompression theory is more fiction than fact, less science than art. They seem to agree that there for every constant we have to account for when we dive, there are about ten times more variables in play, and so the management of decompression stress – a nice euphemism for the best ways to avoid getting bent – is mostly a blend of luck and weighted dice.
Over the course of several thousand dives, a large percentage of them requiring staged decompression, I have been extraordinary lucky and have never ended a dive with a trip to a re-compression chamber.
There may be several factors at play in my case and any one of them may have helped me avoid getting bent… or more correctly, chamber bent. Genetics, running conservative dive schedules, making generous use of heliox, nitrox or oxygen during my ascents (sometimes all three), and having a medical condition that requires me to be pretty well hydrated all the time… at least better hydrated than the average North American male. However, one thing that I feel has helped is that I have avoided short-duration dives (bounce dives) after a deep staged dive like the plague. And on the few occasions a scheduled second dive ended up much shorter than planned – mostly because of a bailout situation following something hitting the proverbial spinning fan – I’ve followed the tactic of pulling a long ascent schedule followed by a long surface interval (four hours or more) before getting back in the water to conduct another dive.
During a conversation about decompression strategies with John Crea – an anesthesiologist and an early member of the technical diving community – he mentioned that the outcome of bounce dives – specifically the behavior of absorbed gas in a diver’s body – is notoriously unpredictable. Crea and Bill Hamilton once told the audience at a technical diving conference that decompression is a crap shoot. During that conversation, he said that bounce dives elevate the crap shoot up to a game of Russian roulette. I have no reason to question his logic.
Here’s part of the issue. For most people, the bulk of off-gassing is likely to take place when a diver is back on the surface and at one bar or one atmosphere. Doppler ultrasound scans tell us that after a few minutes out of the water, it’s party-time for inert gas bubbles with levels of free-phase gas at their highest relative to what happened in the water during ascent. These bubbles continue to grow in number and size as time passes, with the peak – especially following a deep dive – not being reached until one or two hours into the SIT.
This is nothing new to those who regularly conduct staged deco dives, but the key here is to understand what happens when most divers follow up a decompression dive with a second “shorter” dive… say to retrieve a hook or a piece of kit left on the bottom. Essentially that second dive is comparable to opening up a shunt between the venous side of one’s bloodstream to the arterial. Diving MAY compress bubbles sufficiently to bypass the lungs and re-circulate them. A bounce dive profile can easily allow these bubbles to then expand during ascent and cause all sorts of complications… including bringing on the bends.
Naturally, bounce dives are easy to avoid… in most cases. For example, I would hope most divers understand that dropping a stage bottle in 6-10 metres (20-30 feet) of water after a deco dive and jumping in to fish it out, is EXACTLY the thing to avoid. However, here’s another scenario that I was reminded of last week while diving a trimix-depth wreck in Lake Ontario.
The wreck has no permanent buoy and so our dive boat made a pass and dropped a small mushroom anchor with a length of line with a small tuna ball tied to the end. The first team dropped in but the line had been fouled and did not deploy correctly. When they arrived at 45 metres (about 150 feet), there was no sign of the two-masted schooner we’d come to dive – only mud. They did a short search and opted to run a bailout profile that called for a few minutes of hang-time. According to all that’s outlined above, they decided their day was shot and they did not bother to try another dive with a short surface interval between, which is what would have been called for given the day’s schedule. Their actions were conservative, but in my opinion, justified and correct given the circumstances.
By definition, a second dive would have been shorter than the first since their available gas volume would have been limited. Admittedly, the second dive would not have been a shallow bounce – probably it would have been almost as deep as their aborted dive – and it would have required a short staged ascent – perhaps similar to their bailout profile – but they figured the odds of bubble trouble would be greatly increased by getting back in the water to try again.
Were their actions correct? Could we really have classified their second dive a bounce? Good questions, but think about this possible scenario.
Given all we think we know about off-gassing and the state of the bubbling following their first dive, what might have happened if they had opted to try that second dive, and when they got to 9 metres or 30 feet, they saw that the down-line was fouled again? Might they have aborted the dive and surfaced? That would have been a bounce now wouldn’t it?
In effect, there are several possible situations that would have the potential to make their second dive a bounce dive, including having to bailout again because of the anchor missing its mark or one of their three-person team thumbing the dive during its early stages. I would suggest that the specific situation they found themselves in after their first dive is one that many technical wreck divers may be faced with when diving sites that are not part of the regular “tourist” fare. I think their choice to hang up the fins for the day, was the right one. But then, I think we already established that when it comes to decompression theory, I am conservative. That’s fine by me. How about you?