I was diving off the Torran Rocks at the south-west corner of Mull. It was 1980 and we were on a club summer trip, two inflatables, an old GPO van dangerously overloaded with diving gear and 12 soggy divers trying to survive in tents.I was diving off the Torran Rocks at the south-west corner of Mull. It was 1980 and we were on a club summer trip, two inflatables, an old GPO van dangerously overloaded with diving gear and 12 soggy divers trying to survive in tents.
The weather had turned unusually good and we drove our boats as far south and west along the Torran Rocks as possible. We picked a flat topped rock that would barely show at high tide, dropped anchor on the steeper looking east side of it and snoozed off in the sunshine. Well, I did, because everyone else was diving except my buddy and she was keeping watch.
Half-hour later it is our turn. We descend a steep rocky slope to 25 meters and a coarse sandy seabed. Visibility is excellent, marine life is plentiful and we are having a fantastic dive.
We follow the rocks away from the boat. Nearing the end of our no-stop time the sand ends and a wall drops below us. We check air, point and gesticulate at deco tables, and decide to stretch our dive over the wall for a few minutes.
It seemed like a good idea at the time; the view was fantastic. Five minutes later, rising above the wall we find ourselves heading into a rapidly building current.
On the old BSAC/RNPL tables we had already accumulated 10 minutes of stops and could not afford to accumulate any more. We make a blue water ascent and deco stops while swimming against the current on a compass bearing.
We eventually surface a couple of hundred metres off the opposite side of the island. The sea is flat calm, but there is no way the boats can see us. We shout and blow whistles whilst swimming toward the island. No response.
Beginning to feel nervous we started swimming harder. We move into top gear and begin to make headway. It didn't cross our minds to ditch any kit. I used to play a lot of octopush and was quite fit at the time. My buddy had recently been told to slow down by the rest of the women's octopush team because they couldn't keep up with her (you wouldn't get this from the current crop of tough female players). It was not easy, but we made it to the island and scrambled ashore. Fins in hands and with the rest of our kit still in place, we walked back to our boats across the island.
“Wondered where you had got to. We were giving it another 5 minutes before we came looking” was the comment when we rejoined the rest of the group.
Now, all sorts of dive planning and execution points that could be made from this near-incident, but the real point is that with one item of equipment now taken for granted we wouldn't even have noticed anything abnormal about this dive. I'm talking about the delayed surface marker buoy.
Ok, we could have taken an SMB at the start and towed it around for the dive, but we had planned to follow the rocks down and back. The only time an SMB would have had a role to play was during the unplanned method of our ascent. A delayed SMB would have allowed us the freedom to make the dive while giving the cover boats something to follow while we were decompressing in the current.
The principle behind the DSMB has been with us for a long time. Wreck divers with a spare lifting bag would attach 5 metres of light rope and hang on the bag for their shallowest decompression stops. It saved crowding on the shot line and removed the absolute necessity of returning to the shot at the end of a dive.
The hassle of laying a bottom line was no longer an absolute requirement for divers making long deco stops. The worst case was that you missed the shot and had to ascend in blue water until shallow enough to hang on the lifting bag.
Pretty soon, however, the weaknesses of this system were exposed. Lifting bags required a lot of air to stick up well above the surface. Inflating a bag at shallow stop depth placed divers at risk of loosing control of their buoyancy.
Specialised decompression bags, what we now know as DSMBs, were developed; long and thin with less lift and more sticking out of the water. Safe use was now much easier, less positive buoyancy was involved in sending a DSMB up. With a reel attached it could be released from a greater depth, giving more time to bring things under control before breaking a decompression ceiling.
Another advantage of the narrow tube pattern became evident in heavy seas. Whereas a lifting bag would ride at a constant height in the water, jostling the divers hanging below up and down with the waves, a dedicated DSMB could ride the waves more easily, smoothing the worst of the movement.
Divers using reels to a DSMB made the next step. Rather than playing with buoyancy, they wanted to hang on to a rock or piece of wreckage when inflating it, but this would often involve more than one pair of hands. The answer was to tie the reel to a wreck temporarily with a short piece of line. An added advantage was that this allowed time to sort things out should the reel jam.
It was at this stage of using DSMBs that I had another dive that could be described as a "near incident". My buddy and I were diving the Aeolian Skye, a large freighter in 35 metres off the Dorset coast. We had built up 15 minutes or so of planned decompression and it was time to ascend.
I tied my reel in, inflated my DSMB fully and let it go. It shot up and a few seconds later the line went slack, indicating that it was at the surface. I untied and started to ascend, reeling in the line as I went. Half way up we met the bag coming down.
I re-inflated the bag fully and let it go again. As it ascended I could see it tumble in the water, tipping air but still buoyant enough to reach the surface. On our 3-metre stop I could just see it above us. It was almost empty with only the tip showing above the water. We were all right decompressing, but in a strong current and a choppy sea could the boat see us?
Fortunately I could hear the boat engine and knew it was not far away. Our diligent skipper had seen the tip of the DSMB and staying nearby. When we got back to shore I dropped in to the local dive shop and bought a DSMB with a closed end.
These closed-bag variants are not strictly closed; a set of flaps allows air to enter the bag whilst preventing the air escaping, and overpressure is handled by a separate relief valve. The important thing is that such bags stay fully inflated no matter how they tip on the way up.
The current state of the art is a self-inflating DSMB. Essentially a closed end DSMB with a small air cylinder attached. Rather than having to fill it from a demand valve or air gun, you just crack the cylinder.
DSMBs are now an almost essential piece of equipment for decompression dives, to the extent that many boat skippers now require divers to "ascend on a bag" rather than on the shot line. The last thing a skipper wants is half a boat load hanging on to a shot line while the other half drift off on DSMBs. Circumstances can dictate exceptions to this, but what used to be an item of precautionary equipment has now become the preferred method of ascent.
DSMBs have given us the option to conduct such dives with freedom and safety. Which brings me to a question I would like regular users of such equipment to ponder: are you a safer diver by virtue of any degree of caution, or is it just that modern diving equipment lets you get away with it?