Why Divers Run Low on Gas

In 2006, I started a PhD program in diving physiology at the University of Western Australia, aiming to make a small but measurable improvement to diving safety. From the scientific literature, it was clear that running low on, or out of, gas was a common risk factor for diving injuries and deaths. It was a common theme in the annual reports that Divers Alert Network once published about North American dive incidents and fatality reports, and in the U.K. incident reports that the British Sub-Aqua Club still does publish. The Western Australian coroner's records confirmed that running out of gas was common among diving fatalities in Western Australia, too. But why do divers really run low on gas?

I invited 27 diving experts to help me out, using a Delphi survey to gather the data. I found nine diving doctors who had published research and treated countless injured divers, plus nine expert divemasters who had taken thousands of divers out on dive boats, and between them, had probably seen it go wrong in all possible combinations. Then I found nine expert divers who were well-published diving authors and who had made thousands of dives worldwide. I wrote to these experts and asked them, "Why do divers run out of gas?" I arranged all the suggestions into alphabetical order, then sent that list back out and asked each expert to rank the top five reasons they thought people might run out of gas. Once everyone had picked their five, then I gave each first-choice selection five points, each second-choice selection four points, and so on. Then I added up the points. I sent out this new list, with the reasons ranked by order of popularity, and asked the group to reconsider their top five reasons, bearing in mind the weight of the opinion of the expert panel. Some people reconsidered their reasons, and the group as a whole moved closer to consensus.

Ultimately, here were the most popular reasons that my expert panel thought led to divers running out of gas (below right). Now I knew what to look for.

Next, I bought 15 Sensus Ultra data-loggers, which estimate a depth reading every 10 seconds. I loaded them into my 1985 Nissan, which I drove all over Western Australia. Wherever I found divers, I would clip a logger onto each volunteer and fill out a fourpage survey, recording their tank volumes, start and finish air pressures, the dive site, who was diving, the weather, and, using a Secchi disk, the underwater visibility (if you're asking what the heck a Secchi desk is, read up at http://en.wikipedia.org/wiki/Secchi_disk ). I recorded more than 1,000 recreational dives over two summers.

My plan was to compare anyone who returned from a dive with less than 50-bar remaining (similar to the 500 psi you Americans use as a benchmark) to the other divers who were diving in the same place at the same time. My question was, what is the difference between someone who ran low on gas and the other divers who did not? For this analysis, I used a "conditional logistic regression," and a statistics program from SAS in North Carolina, pressed the big green button and watched the numbers roll down the screen. If it is a random answer, then no one will publish your results; if it's controversial, then you'll have to defend your methods. Years of work, and it all comes down to just a couple of seconds. And then, there it was: <0.0001. The likelihood of my data being a random finding was less than a hundredth of a percent.

The reasons adult recreational divers making organized group dives in Western Australia ran low on gas are shown at right:

What this chart means is that, taking into account all the possibilities, people who were surprised by how little gas they had left after the dive were 21.7 times more likely to have less than 500 psi than their dive buddies who were not surprised. Remember, failing to monitor the gauge was the experts' number-one reason why people run out of gas. Men were 13.3 times more likely to come back with less than 500 psi than women. For each five meters deeper someone went, they were 3.5 times more likely than their buddies to come back with less than 500 psi. Young divers were twice as likely to get low on air for every 10 years younger they were than their buddies, and so on.

Another interesting finding was that divers on liveaboards tended to get deeper each day, and come back with less and less gas.

How these results are interpreted depends largely on who is interpreting them. For example, not everyone dives in organized groups, so these results might not apply to all recreational divers. However, if I was asked for a less scientifically rigorous opinion, like if a dive operator asked me what he might want to keep an eye out for, then I would interpret our results like this:

Issue a check-your-gauge reminder. Most divers don't intend to run low on gas. Some are surprised at the end of the dive when they check their gauge. If a dive site has potential for divers to go deeper than planned, then it might be worth reminding everyone to check their pressure gauges more often.

Keep an eye on the men, especially the younger ones. Men and women reached similar average and maximum depths, but women's gas lasted longer (55 minutes versus 49 minutes). Personally, I think this might be because we're bigger, on average, so we have to do more work to move through the water, meaning we breathe more gas per minute. If a couple of young men are buddied up, then it might be worth reinforcing their need to regularly check their gauges.

Save shallower sites toward trip's end. On liveaboard trips, some people tend to dive deeper each day, perhaps as they gain confidence. It may be worth monitoring this and selecting shallower sites toward the end of a trip, especially if people need to fly home.

Get your weights right. Lastly, although the surface air consumption per kilogram and per minute only slightly increased anyone's risk of running low on gas, bear in mind that our workload during any dive is greatly reduced if we level off horizontally, which means getting our weighting and weight distribution spot on. Anything less than horizontal and your drag increases, as does your workload.

Dive safe -- and check your gauge.

Peter Buzzacott holds a PhD in diving physiology from the University of Western Australia, and is currently is a research associate at the University of Western Australia's School of Sports Science, specializing in scuba diving research. The citation for the published results is P. Buzzacott, M. Rosenberg, J. Heytworth and T. Pikora, "Risk factors for running low on gas in recreational divers in Western Australia," Diving and Hyperbaric Medicine, June 2011, pgs. 85-9).