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Estimating Your Scrubber Duration

Global Underwater Explorers (GUE) rebreather divers use the Absorbent Canister Endurance (ACE) method for estimating scrubber duration.

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Reprinted from GUE’s CCR Standard Operating Procedures. 

Header photo: JJ-CCR in GUE configuration with twin seven-liter diluent cylinders, Photo by Kees Beemster Leverenz.

Global Underwater Explorers (GUE) rebreather divers use the Absorbent Canister Endurance (ACE) method for estimating scrubber duration. The volume of CO2 produced by a diver is variable and incremental in relationship to the level of exercise, and consequently the diver’s Respiratory Minute Volume (RMV). If you know your RMV, you can estimate your O2  consumption (VO2) and CO2 produced (VCO2) and finally, your Absorbent Canister Endurance (ACE). The calculation shown below is for the JJ-CCR [Ed. note: GUE has standardized on the JJ-CCR with a specific configuration] but can be applied to other units including those with radial scrubbers.

Divers peer into the hold of the steamship Numidia in the Red Sea. Photo by Kees Beemster Leverenz.

Follow these steps: 

1. Theoretical Absorbent Capacity (TAC) Molecular Products Sofnolime 797 (1.0-2.5 mm granular size), which is recommended for use with the JJ-CCR, has a Typical Usage Capacity (TUC) of 150 liters of CO2/kg of sofnolime. The standard axial absorbent canister in the JJ-CCR contains 2.5 kg to 2.7 kg of sofnolime when it is properly packed. Based on these numbers the Theoretical Absorbent Capacity (TAC) is calculated using the following formula: 

Formula 

TAC = TUC x kg 

Example: 2.7 liters Sofnolime 797 

TAC = 150 x 2.7 

TAC = 405 liters of CO2 can, in theory, be absorbed. 

2. Volume of Oxygen Consumed per Minute (VO2The ratio of VO2 consumption to Respiratory Minute Volume (RMV) is about 4.0% and are indicative of what would be expected of divers performing graded exercise underwater.1

VO2 = RMV x 0.04 

3. Volume of Carbon Dioxide Produced per Minute (VCO2For practical purposes, the VCO2 production, and the VO2 consumption, are assumed to be equal (respiratory exchange rate 1:1). 

The ratio of VCO2 consumption to Respiratory Minute Volume (RMV) is about 4.0% and are indicative of what would be expected of divers performing graded exercise underwater.2  

Formula 

VCO2 = RMV x 0.04 

Example 1: RMV 40 lpm                   Example 2: RMV 20 lpm 

VCO2 = 40 x 0.04                              VCO2 = 20 x 0.04 

VCO2 = 1.6 lpm                                 VCO2 = 0.8 lpm 

4. Real Absorbent Canister Capacity (RAC) 

The Real Absorbent Canister Capacity (RAC) varies depending on your actual RMV. A lower RMV equates to a longer dwell time and consequently better CO2 absorption. The estimated percentages below are based on tests conducted by GUE.

20 lpm RMV = 95% of the Theoretical Absorbent Capacity (TAC) 

30 lpm RMV = 85% of the Theoretical Absorbent Capacity (TAC) 

40 lpm RMV = 80% of the Theoretical Absorbent Capacity (TAC) 

Example 1: RMV 40 lpm and VCO2 1.6 lpm 

RAC = TAC x 0.8 

RAC = 324 liter CO2 (The amount of liters of CO2 the absorbent canister can absorb under real world conditions). This compares to theoretical absorption capacity, TAC=405 liters CO2.

Example 2: RMV 20 lpm and VCO2 0.8 lpm 

RAC = TAC x 0.95 

RAC = 384.75 liter CO2 (The amount of liters of CO2 the absorbent canister can absorb under real world conditions), compared to the theoretical limit. 

5. Absorbent Canister Endurance (ACE) The estimated absorbent canister endurance (ACE) can now be calculated,  based on the diver’s average VCO2

Formula 

ACE = RAC ÷ VCO2 

Example 1: RMV 40 lpm, VCO2 1.6 lpm and 324 liter RAC 

ACE = 324 ÷ 1.6 

ACE = 202.5 min 

Example 2: RMV 20 lpm, VCO2 0.8 lpm and 384 liter RAC 

ACE = 384 ÷ 0.8 

ACE = 480.9 min 

This should provide an estimate of the expected duration of your scrubber.

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Footnotes

1: U.S. Navy Experimental Diving Unit Report 3-81 Standardized NEDU Unmanned UBA Test Procedures and Performance Goals. James R. Middleton. Edward D. Thalmann. July 1981 

2. U.S. Navy Experimental Diving Unit Report 3-81 Standardized NEDU Unmanned UBA Test procedures and Performance Goals. James R. Middleton. Edward D. Thalmann. July 1981

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

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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.

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