In his Scuba book series, Simon Pridmore often tells tales of diving close calls or near misses. He tries to identify the key factors and suggest what the divers might have done differently to pre-empt or better deal with what happened. All the stories are true. Some he witnessed, some were recounted by friends, and others just crossed his radar screen at some point to be filed away for future sharing. This is one of the latter.
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Whenever someone gets into difficulty on a dive and takes the time to write down what happened and post the experience on social media or in a magazine, they help make us all safer divers. This is not an easy thing to do as it can expose the diver to criticism or ridicule, but none of us has the right to judge. None of us is immune from human error or the caprices of the ocean.
We can point out lessons to learn though.
Chris’ story
This is Chris’ story. It is not the hairiest “bad dive” story you will ever read but it raises a whole host of valuable talking points, and it shows how easy it is to fall victim to simple body chemistry. I am grateful that he chose to share it.
“It was a typical tropical day: sunny with a chance of showers, although the currents on the reef had been weird all week. So, we chose a site where conditions were usually straightforward. We planned to go down to a maximum depth of 60m (200ft), then come up the reef wall and do a drift. I had a single AL80 of air and an AL40 pony containing nitrox 36. My buddy was diving twin 80s, one with air and one with nitrox 36.
“The deep section of the dive was uneventful; visibility was outstanding. Then we headed up. I switched to my nitrox at 27m (90ft), and we drifted along the reef at 15m (50ft), just sightseeing. Forty minutes into the dive, my computer told me my deco obligation was around 12 minutes at 6m (20ft). By that time, we were hanging out just over the top of the reef at about 10m (35ft), where there was always plenty to see, so we just stayed there, waiting for the deco minutes to count down. My buddy sent up his surface marker buoy (SMB) to let the boat know where we were.
“I had about six minutes of deco left to do when I felt a gentle tug from the ocean. The current was picking up and pulling me away from the reef. I was not overly concerned. I was fit, I was diving with large freediving fins, and I was feeling great: no worries. I started finning horizontally back towards the reef. However, no matter how hard I finned, the reef kept receding into the distance. The current had picked up. I glanced at my buddy and saw him drifting out into blue water too, on the end of his SMB line.
“Every instinct told me, “Get back to the reef!” I looked down and thought I saw a huge swirling vortex. It looked terrifying. The current was still pulling me away, but I was determined to get back to the reef. I had never encountered a current I could not beat, and I was not going to let this one defeat me.
“Then, the current changed. Suddenly, I was being pulled straight down at a terrible pace. I inflated my BCD to try and arrest my descent. It was no help. I was still going down. I started finning as hard as I could to stop myself from going down and, to make things worse, the current started spinning me around violently. Bubbles were everywhere. I had an iron grip on my inflator and the dump valve was screaming as it released the air in my already full BCD. My mask was half full of water and I was panting hard.
“Suddenly, I found myself heading up fast! The ocean had relaxed its grip and released me. Before I knew it, my head broke the surface. My first thought was, “I am in trouble. I am probably going to get bent here.”
“I immediately descended again. I was low on gas in both my cylinders, and I was frightened that the current would pull me down again. I could sense it toying with my feet. I raised my SMB and floated along at 6m (20ft). By the time my computer cleared, my nitrox cylinder was empty, and I had less than 15 bar (300psi) in my AL80. I returned to the surface and the boat picked me up.
“After 90 minutes of rest and no DCS symptoms, we went back in and did a second shallow dive on 36% nitrox. Later, I reviewed my dive log and saw that the current had pulled me down to 18m (60ft). It had felt much, much deeper! The computer clocked my rapid ascent at 45m (150ft) per minute.”
Talking points
There is plenty to unpack in Chris’ story. Let’s start with the idea of doing a 60m (200ft) dive with a single cylinder of air and a pony of nitrox 36 or back-mounted independent twin cylinders, one containing air and the other nitrox 36.
Two cylinders. First, the strategy of carrying two cylinders on your back, one with your deep gas and the other with your deco gas, has been killing divers for decades because it is so easy for a diver with this configuration to breathe the wrong gas at the wrong depth. No training agency teaches it, yet the practice persists.
The configuration issue aside, experienced divers in the tropics do dives like this all the time, without really considering the true risks. If you have an air supply emergency at depth—either a blown cylinder O-ring or a regulator free-flow—you have two options:
- Switch to your nitrox 36, which at 60m (200ft) has a PO2 of 2.52 atmospheres and is likely to give you instant oxygen toxicity convulsions, or
- Ascend immediately to 33m (110ft), where your nitrox 36 will be safe to breathe. This sounds fine until you calculate that this ascent will take you three minutes at a safe speed and that your air cylinder will be empty before you get there, even if you think clearly and act quickly, an impossible feat at 60m (200ft) with the level of narcosis that breathing air at that depth brings on.
So, effectively, you have no viable backup plan for dealing with an air supply emergency at 60m (200ft).
Narcosis. Narcosis is the biggest threat to a diver on an air dive to 60m (200ft). Not only is your breathing gas narcotic, but the density of air at depth causes you to retain more carbon dioxide than usual in your body and this increases your narcosis.
Narcosis diminishes your capacity for rational thought and correct decision-making, and research has shown that narcosis accumulated at depth does not dissipate as you ascend. You are still affected up to 30 minutes after your dive has ended. Narcosis also affects perception and generates confusion, which may explain why Chris’ thinking was flawed from the moment the current took hold of his dive.
As soon as he felt it carrying him away from the reef, he thought he was in danger, and he was afraid of leaving the wall behind. He did not need to fight the current. He could have just popped his SMB up and drifted with the flow, as his buddy did.
Blue orb syndrome. Some divers are affected by a phenomenon known as “blue orb syndrome,” which leads them to become anxious when they can no longer see the reef or seabed. In Chris’ case, this anxiety would have been exacerbated by his frantic finning and contributed to the panic that was about to grip him when he encountered the downcurrent and interpreted his inability to maintain depth control as a risk to his safety. His narcosis-fueled brain perceived the downcurrent to be a terrifying, vicious force dragging him into the abyss. On the contrary, as he found out when he looked at his computer later, it carried him down less than 10m (33ft).
All he had to do when he was caught in the downcurrent was to keep calm and swim laterally until he escaped from its pull. Instead, he chose the worst possible response and inflated his BCD while finning hard upwards. This gave him his first genuine problem, as he discovered when the current released him, and he shot to the surface.
Stay on the surface. Finally, after his rapid ascent, the smartest thing for Chris to do would have been to stay on the surface, instead of returning to depth to clear his computer. He had only blown a few minutes of deco and modern dive computers operate with substantial margins of safety. The greatest risk posed by a rapid ascent is the generation of bubbles in the body. Going back down and starting to breathe gas under pressure again can cause any bubbles generated by the rapid ascent to take on more gas and grow, and big bubbles are more harmful than small bubbles.
This also argues against the wisdom of doing a second dive after an episode like this. Physiologically and psychologically, if you have had a bad dive, it is always best to sit out the rest of the day on the surface, self-monitor and leave any further diving to the next day, assuming you have no residual DCS symptoms or anxiety.
