Education
Density Discords: Understanding and Applying Gas Density Research
Do you know the density of your breathing gas at your planned working depth? New research conducted by Gavin Anthony and Simon J. Mitchell suggests that you better! A gas density of 6 grams/liter (g/l)—the equivalent of diving nitrox 32 at 110 ft/34 m, or trimix 18/35 at 200 ft/61 m—significantly increased the risk of dangerous CO2 retention, resulting in test subjects experiencing problems at three times the rate of divers using gas even 1 g/l less dense. Divers Alert Network risk mitigation leader Reilly Fogarty explains.
By Reilly Fogarty
Header photo by Derek Remmers
As divers we’ve learned to adapt to the effects of depth without fully understanding the mechanisms behind them. Gas density is one of those misunderstood dynamics lurking in the background of many of our dives. We minimize our work at depth, improve our fitness, and add helium to reduce narcosis and work of breathing (WOB) at depth, but these are reactionary responses. While we typically can’t discern the difference during a dive, gas density and the accompanying increase in WOB causes decreased respiratory capacity, increased CO2 production, and decreased ability to eliminate CO2 in the blood. Some agencies (e.g., GUE) have been proactive in modifying standard gases and tailoring training to adapt to these concerns, but the forefront of hyperbarics research is constantly pushing us toward increasingly conservative gas choices.
Just to clear the air, we’re working with two terms here — WOB and gas density. WOB is an integral of pressure as a function of volume that’s used to measure the effort required to breathe. A high WOB means it takes more effort (measured in energy, typically joules) to draw breath (a measure of volume, typically in liters). High WOB results in increased CO2 production, and that CO2 increase can result in hypercapnia, narcosis, and loss of consciousness among other symptoms.
Gas density is a measure of mass per unit volume, measured in grams per liter (g/l). A high gas density means a given volume of gas weighs more and takes more effort to move, resulting in increased WOB. Increased gas density also skews the pressure gradient between inspired and arterial CO2, resulting in further decreased CO2 off gassing efficiency and a recurring system that results in further complications.
In the past, these factors were taken for granted, but recent research by Gavin Anthony and Simon J. Mitchell from the University of Auckland Department of Anaesthesiology (see link below) has cast gas density in a new light. Working with both open-circuit and rebreather divers, Anthony and Mitchell found that gas density near the 6 g/l mark significantly increased the risk of dangerous CO2 retention during dives, resulting in their test subjects failing more than half their attempted dives and experiencing issues at more than three times the rate of divers using gas even 1 g/l less dense. Their takeaway from this research was an ideal maximum gas density of 5.2 g/l (equivalent to air at 102 fsw/31 msw), and a hard maximum of 6.2 g/l (equivalent to air at 128 fsw/39 msw).
The implications of these results are both complex and far-reaching. Recreational and technical divers alike face issues with gas density under these new guidelines. The use of EANX 32 (32% oxygen, 68% nitrogen) at 110 fsw/34 msw exceeds recommendations with a gas density of 5.66 g/l and more than 6.54 g/l at 132 fsw/40 msw. Technical divers using trimix 18/35 (18% oxygen, 32% helium, balance nitrogen) will overshoot recommendations, reaching 6.93 g/l at 200 fsw/61 msw and a PO2 of just 1.26, and trimix 10/70 (10% oxygen, 70% helium, balance nitrogen) reaches an impressive 6.73 g/l at 396 fsw/121 msw and 10.29 g/l at 495 fsw/151 msw.
The reality is that gas density is another in a series of dynamic risk factors that divers of all levels must contend with. Treating gas density like DCS by acknowledging and mitigating the hazards via personal fitness, decreased work at depth, and appropriate dive planning has worked in the past and will continue to work. What this research shows us is why we face the issue we do at depth, a possible understanding of corollary hazards like DCS and IPE, and how we might be able to use that data to keep ourselves safer.
View the Respiratory Physiology of Rebreather Diving research in full. For questions about gas density or comments about this and future articles, reach out to the author at RFogarty@DAN.org.
Gas Density Calculator
Here is a simple gas density calculator for you to download, created by Brendon Allen aka RainPilot, at ScubaBoard.com, that enables one to calculate the gas density of their bottom mix at their planned depth (in ATA). It also includes equivalent narcotic depth (END), and partial pressure of oxygen (PPO2) at the planned max depth.
Reilly Fogarty is a team leader for its risk mitigation initiatives at Divers Alert Network (DAN). When not working on safety programs for DAN, he can be found running technical charters and teaching rebreather diving in Gloucester, MA. Reilly is a USCG licensed captain whose professional background also includes surgical and wilderness emergency medicine as well as dive shop management.
Community
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
$34.99
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.
