Exposure—How Long, How Deep, How Cozy?
Staying Alive: Application of Risk Management in Scuba Diving.
Tags & Taxonomy
The Royal Mail Ship, Empress of Ireland, was an ocean-going luxury liner on her way to Liverpool from Quebec City when she sank in the Saint Lawrence River, 14 minutes after colliding with a Norwegian collier in the early morning fog of 29 May 1914. She had 1,477 people on board—passengers and crew—and the accident claimed the lives of 1,012, more than 800 of them passengers.
I’ve had the privilege to dive on the wreck several times; the first was in the aftermath of Hurricane Hortense, which blew its way up the eastern seaboard of North America, and although it did not hit Rimouski directly, turned that late Quebec summer into a mini-maelstrom. The weather was awful—windy, wet and bleak. It had kept us out of the water and holed up in a small hotel for days, playing euchre and praying for a break in the weather. When a narrow window of opportunity finally opened up early in the morning on our last scheduled day in French Canada, we suited up on at the dock, threw our gear onto our charter boat, and hoped for the best.
The dive was fantastic—truly historic, but my most vivid memory is staring at my dive computer towards to end of it and seeing that I had earned 45-minutes of decompression. The water was between 3°C and 5°C. I had on inadequate thermal underwear, the current changed direction every few minutes and carried a force that varied from the relative comfort of flag-waving three-quarters of a knot to an extremely unfriendly “hold your hat on Maude, we’re going for a ride.” The only up-side was a seal that seemed to delight in smart-bombing us relentlessly and at regular intervals throughout the various stops from about nine metres to the surface. It took a liking to my fins.
I learned two lessons about “exposure” that day: never rely solely on a personal dive computer to track your decompression obligation (especially a second-generation dinosaur) because there might be a better way; and the speed at which time passes follows a curve proportional to falling water temperature.
Exposure in the context of diving and more especially risk management in diving, relates to surfacing safely without suffering decompression stress, hypothermia, heat-stroke, or wounding from passing critters, and without drifting off into the cosmos far from your lift back to harbor.
The focus in most texts is primarily on the part of a dive that begins around the time we leave the bottom and ends when we are back on the surface (or more correctly, when our surface interval is over and we know we are safe from DCS). This is the usual focus since DCS is a real risk on all dives even those on which the broader issues such as staying warm and comfortable, surviving other environmental conditions such as current, boat traffic, wildlife; and even being able to pee when the need arises are less compelling!
So, to conform to convention, let’s start with that pesky decompression thing.
Following that first dive on the Empress, I understood viscerally that to follow a dive computer blindly and without question was not the best possible option. It can get one in over one’s head, figuratively and literally. The PDC I was using – a demo from a European manufacturer – suggested I stay in freezing water and horrible conditions for far longer than necessary. Thankfully, my buddy and I carried lots and lots of “spare” decompression gas: most of which was consumed by the time our computers cleared us to surface, since both our respiration rates were easily double our norm.
Back in those days
Bear in mind, this episode was back in the dawn of personal dive computers (PDCs). They were reasonably new and those that did not lock-up when their user exceeded the no decompression limit (NDL), had strangely and well-padded degrees of conservatism built-in.
What made the issue worse was that the user had zero jurisdiction over which level of conservatism was used. It seemed that each manufacturer had its programmers conditioned to think like litigation lawyers. If the Buhlmann algorithm (and they all seemed to use Buhlmann back then… it was free after all) called for four minutes at six metres followed by 12 minutes at three, it would add automatically something along the lines of a three-minute stop at nine metres and increase the duration of “real stops” by 80 percent or more. So effectively, on a dive that would merit a stodgy 16-minutes of deco time on tables, would have an ascent time twice as long using a PDC.
Modern PDCs are much more user-friendly even allowing divers to adjust levels of conservatism to suit their particular needs and proclivities. I wear one – occasionally two – especially for cave diving and when using a closed-circuit rebreather (CCR); however, I never dive without consulting custom dive tables created by using proprietary decompression software. This ensures me and my dive buddies are perfectly clear what the penalty we’ll have to pay for hanging about as long and as deep as called for in the dive plan. It’s simply part of our understanding of Exposure and its control.
I want to explain something called ascent behavior. It’s a technique that came about because of the way I felt after that Empress dive, but before we go down that pathway, it’s worth spending a few minutes explaining what works for me when it comes time to plan my personal decompression dives.
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X-Ray Mag #58
Cuba's Gardens of the Queen; Indonesia's Bunaken; Solomon Islands' New Georgia; Norway's Frankenwald wreck; Fluorescent Night Diving; Doug Allan profile; Dive Fitness on Back & Biceps; Scuba Confidential on O2; Sharks Close to Extinction; Tech Talk on Exposure; Blu Rivard portfolio; Plus news and discoveries, equipment and training news, books and media, underwater photo and video equipment, turtle news, shark tales, whale tales and much more...