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Maintaining Unit Cohesion

If you’re going to go metric or already are, it’s time to get your units straight, explains British tech instructor Rich Walker. Don’t mix your metrics, or your metaphors!



By Rich Walker
Header photo from the GUE archives

For the past few years, I’ve seen a worrying trend when people are discussing gas management topics. It seems to be restricted to regions using the metric system, and in particular some areas of Europe.

Now it’s nothing that is going to cause harm, or be dangerous to divers in any way, but it is simply a factual error that is creeping into teaching, and even into an official scientific diving manual. I think it’s time we fixed the mistakes!

The problem relates to the units that are used when talking about the free liters of gas that are present in a scuba cylinder. For example, the amount of free gas that a 12 Liter  cylinder, pressurised to 200 Bar contains can be calculated as follows:

12 Liters  x 200 Bar = 2400 Liters         Equation (1)

Bar or Bar Liter? Photo courtesy of GUE Archives.

And here is the problem that has been spotted. If you multiply Liters by Bar, the resulting unit should be Bar Liters, right?


To understand why this is so wrong, and the correct unit is still Liters, we need to go back into physics a little. Think about how you calculate the amount of free gas that is present in a cylinder at a depth of 30 m/100 ft. We’re talking about taking the cylinder to a depth of 30 m/100 ft, and then releasing the gas. How much volume does it now occupy, (assuming it doesn’t all float away of course)? Let’s take the same cylinder as the above example. We’d make this calculation:

12 Liters x 200 Bar / 4 Bar = 600 Liters           Equation (2)

Mark your bottles clearly and know your gas management units. Photo courtesy of GUE Archives.

Nobody could argue that the correct unit for the result here was Liters. The mistake that people are making is that, in equation (1), they’re forgetting that they are also dividing by 1 Bar, the ambient pressure at the surface. Now dividing by 1 doesn’t change the result, but the Bar unit of the “1” is important as it keeps the result dimensionally consistent, to use the mathematical terminology. If we rewrite equation (1) properly, you can see it for yourselves:

12 Litres x 200 Bar / 1 Bar = 2400 Liters         Equation (3)

So there is no such thing in diving gas management as the Bar Liter. Let’s stop using it!

You can also watch the video in German.

Rich Walker learned to dive in 1991 in the English Channel and soon developed a love for wreck diving. The UK coastline has tens of thousands of wrecks to explore, from shallow to deep technical dives. He became aware of GUE in the late 1990’s as his diving progressed into the technical realm, and eventually took cave training with GUE in 2003. He began teaching for GUE in 2004. Walker is an active project diver, and is currently involved with the MARS project in Sweden, and cave exploration in Izvor Licanke, Croatia. He is also chairman and founder of Ghost Fishing UK. He is a full time technical instructor and instructor evaluator with GUE, which he delivers via his company, Wreck and Cave Ltd. He sits on the GUE Board of Advisors, and several other industry bodies.

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The Art of Risk: What We can learn from The World’s Leading Risk-Takers




One of the rescuers of the Thai soccer team (now the Netflix series ‘Thai Cave Rescue’) and former Australian of the Year explores why people are attracted to risky pursuits and what we can learn from their expertise. 

In June 2018, with the eyes of the world watching, Dr Richard Harris, or Harry to his mates, dived into a remote cave in northern Thailand in an attempt to rescue a Thai youth soccer team who had become trapped by flash flooding. He used his recreational skills in diving to traverse kilometres of the subterranean cave system, and his professional skills as an anesthetist to sedate the stranded boys so they could be dived and carried to safety. Despite incredible odds, all twelve boys, along with their coach, survived. 

Harry says he was able to succeed in the Thai cave because of decades of experience, comprising thousands of hours of careful planning, risk assessment and management. Often described as the most dangerous sport in the world, Harry never feels like he is doing anything particularly dangerous when he goes cave diving. Despite losing friends to the sport, in his mind the risks can be managed well enough to make the pastime extremely safe. And far from making him anxious or fearful, the planning and execution of potentially high-risk dives have been empowering and fulfilling. In his mind, carefully managed risk-taking gives him the courage to manage the day-to-day stresses of life in the 21st century.

In The Art of Risk, Harry talks to like-minded risk-takers about their adventures and asks them what is it about cheating death that makes them feel so alive. He aims to explore the active pursuit of risk through the lens of risk-takers and adventurers such as soldiers, pilots, mountaineers, rock climbers, deep-sea divers, sailors, big-wave surfers, firefighters, rally-car drivers – both professionals and amateurs. His conversations give us insights into what motivates these people and why a life without risk is no life at all. He believes that by doing ‘the hard things’ in life you can push yourself a little harder and become stronger, more courageous and resilient.

Selling Points

  • FASCINATING SUBJECT: why do deep-sea divers, free climbers and big-wave surfers take the risks that they do? How do soldiers and firefighters manage risk? What can we learn from how they prepare – and what they experience – that we can take into our own lives? Harris shows that in doing ‘the hard thing’, we become more resilient and courageous. Angela Duckworth’s Grit meets Alex Honnold film ‘Free Solo’
  • EXPERT AUTHOR: Dr Harry Harris was at the heart of the Thai Caves rescue, anesthetizing all the boys in order to get them out. A genuine hero and former Australian of the year, Harry Harris explores flooded caves deep underground for fun. For most people, this is the definition of a nightmare. Because Harry understands and can prepare for the risks, for him it’s a pleasurable – even meditative – experience. And, as he says, he feels ‘carefully managed risk-taking gives him the courage to manage the day to day stresses of life in the 21st century’.
  • FAMOUS INTERVIEWEES: Harry talks with people like climber Alex Honnold, sailor Jessica Watson, mountaineer James Scott, film director and deep-sea diver James Cameron and polar explorer Tim Jarvis, amongst many others. 
  • MAN BEHIND THE NETFLIX SERIES: ‘Thai Cave Rescue’ is fresh onto Netflix, further pushing awareness of the story.

The Book

Publication date: July 2023
288 Pages plus color inserts

The Author

Dr Richard ‘Harry’ Harris, SC, OAM and joint 2019 Australian of the Year, is an anaesthetist and cave diver who played a crucial role in the Tham Luang cave rescue in northern Thailand. He has more than thirty years’ experience as a cave diver and also works for the South Australian Ambulance Service’s medical retrieval service. He is the co-author, along with Craig Challen, of Against all Odds, the inside account of the Thai cave rescue and the courageous Australians at the heart of it. He lives in Adelaide, Australia.

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