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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 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.

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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Project Divers Are We

Diving projects aka expeditions—think Bill Stone’s Wakulla Springs 1987 project, or the original explorations of the Woodville Karst Plain’s Project (WKPP)—helped give birth to technical diving….

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Header image: Divers positioning a decompression habitat during a recent GUE Project Diver core module. Photo by SJ Alice Bennett, courtesy of GUE.

Diving projects, or expeditions—think Bill Stone’s Wakulla Springs 1987 project, or the original explorations of the Woodville Karst Plain’s Project (WKPP)—helped give birth to technical diving, and today continue as an important focal point and organizing principle for communities like Global Underwater Explorers (GUE). The organization this year unveiled a new Project Diver program, intended to elevate “community-led project dives to an entirely new level of sophistication.” Here, authors Guy Shockey and Francesco Cameli discuss the power of projects and take us behind the scenes of the new program.


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