Estimating Your Scrubber Duration

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.

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 (VO₂) The 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 (VCO₂) For 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