O2ptima Scrubber Duration Test Results

Testing and qualification of the O2ptima CO2 scrubber duration has been done using the ExtendAir CO2 absorbent cartridge model 101C and scrubber designed by Micropore. The recommended O2ptima rebreather scrubber duration for sport diving depths is 240 liters of CO2, if the diver is monitoring their oxygen consumption. If the diver does not wish to monitor oxygen consumption, they may use default durations of three hours in warm water and two hours in cold water.

Micropore Rebreather Hyperbaric Testing Laboratory

Micropore Rebreather Hyperbaric Testing Laboratory

These recommendations are based on testing performed by Micropore using similar procedures and equipment as the U.S. Navy. At depths beyond recreational limits or near freezing temperatures, scrubber duration is significantly shorter.

The test results are provided solely to document the basis for recommendations regarding O2ptima CO2 scrubber duration. The test results are provided for informational and educational purposes only, see the disclaimer near the bottom of this page.

Lab Results for Unmanned O2ptima CO2 Scrubber Duration Tests

Note these are unmanned tests, using machines in a lab to reproduce the test conditions. The test environment is created by immersing the O2ptima breathing loop in water inside a hyperbaric chamber, then using a sinusoidal breathing machine to simulate a human on the breathing loop under a set of carefully controlled conditions. Keep in mind that laboratory simulations using machines do not model all the possible variables in conditions, nor do they address variations in human physiology.

The breathing rate, called the respiratory minute volume (RMV), and the amount of CO2 injected into the breathing loop are measured in standard liters per minute (lpm). These values have a wide range depending upon the physiology and diet of the individual, with extreme athletes able to produce the highest numbers. The table below offers some breathing rate guidelines adapted from open-circuit regulator testing.

22.5 lpm RMV
0.77 lpm CO2
Most relaxed divers, doing little or no swimming,
can sustain an RMV of 22.5 lpm almost indefinitely.
37.5 lpm RMV
1.28 lpm CO2
A physically fit diver, taking slow deep breaths while swimming hard,
can sustain an RMV of 37.5 lpm for a few minutes.
75 lpm RMV
2.55 lpm CO2
A diver athlete in superb physical condition, doing severe work,
can sustain an RMV of 75 lpm for one or two minutes.

CO2 = 85% of VO2 and VO2 = 4% of RMV

 

After several years of research, the Navy Experimental Diving Unit (NEDU) selected 40 lpm RMV and 1.35 lpm CO2 for use in their rebreather tests. Keep in mind these values are NOT representative of recreational divers, who exhibit a much wider range of fitness levels. Thus, among the recreational rebreather diving community there is always debate regarding selection of testing parameters. However, the Navy values have become the convention for evaluating scrubber duration in the U.S. and they are reflected in test conditions for the O2ptima.

Another important test condition variable to consider is temperature. CO2 absorption is a chemical process that is most efficient when warm and moist, so as the temperature drops the effectiveness of the scrubber decreases as well. As the temperature approaches the freezing point of water, the chemical process and CO2 absorption are so slow that for practical purposes they have ceased. The U.S. Navy tests over a temperature range that goes as low as 40 °F, but considering that we are discussing a recreational activity, test results for the O2ptima have been selected at 50 °F for cold water and 75 °F for warm water.

Pressure, expressed in Atmospheres Absolute (ATA), also affects scrubber duration. As the pressure increases the gas molecule density increases, i.e. there are more molecules in the breathing loop under 2 ATA of pressure than are present at 1 ATA of pressure. However, the number CO2 molecules in the loop is a function of metabolizing oxygen, not pressure. As pressure increases the total number of molecules, the relative concentration of CO2 molecules in the loop is reduced, slowing the chemical absorption process. Thus as depth increases, scrubber efficiency will decrease, although not proportionately. Again, the U.S. Navy tests over a range of values, but for sport diving 5 ATA (130 fsw) is a reasonable value. Also included are results for an extreme dive to 10 ATA (300 fsw).

Scrubber duration is determined by defining an end point when the gas passing through the absorbent still contains a significant amount of CO2. The 0.5% CO2 "breakthrough" point is widely accepted as the first indication the absorbent is beginning to lose effectiveness. At 3% CO2, respiration rates will increase but there are no central nervous system effects. At 4% to 6% CO2 sustained over a few minutes most individuals will begin to have minor symptoms of CO2 poisoning, such as perceptive changes or distracting discomfort. As CO2 concentrations reach the 10% level, hypercapnia symptoms such as headache or muscle weakness are usually (but not always) noticeable and some individuals can fall into a stupor or even become unconscious. At CO2 concentrations exceeding 10% most individuals are very rapidly incapacitated.

Please keep in mind these tests are for the O2ptima breathing loop, using the Micropore canister and reactive plastic cartridge CO2 absorbent known as the ExtendAir EP cartridge. The O2ptima scrubber is more compact and light weight than granular adsorbent scrubbers, because ExtendAir cartridges are more efficient with more consistent performance. Because the horizontally mounted O2ptima scrubber canister and ExtendAir cartridge are very unique in their properties, the test results can not be extrapolated to other canister designs, orientations or to granular absorbents.

When looking at the data graphs below, you may find it easier to
print them using a color printer rather than view them on screen.

Click Here
to view a graph
of lab results (69Kb PDF)

1 ATA (Surface), 40 lpm RMV, 1.35 lpm CO2, and 50°F

Under high breathing-rate and cold water test conditions, the scrubber duration to reach 0.5% CO2 concentration was very consistently near four hours and at the surface. Once the cartridge absorbs 320 liters of CO2, the concentration increases very rapidly.

This test is the typical "benchmark" used when comparing scrubber durations between different rebreather designs. For the O2ptima, it demonstrates the exceptional pound-for-pound efficiency and consistency of the reactive plastic cartridge CO2 absorbent.

Click Here
to view a graph
of lab results (39Kb PDF)

5 ATA (130 fsw), 40 lpm RMV, 1.35 lpm CO2, and 50°F

Doing a simulated dive under similar high breathing-rate and cold water test conditions at a pressure equal to a depth of 130 fsw, the scrubber duration to reach a surface equivalent of 0.5% CO2 concentration was very consistently just over two hours. The time to reach 1% CO2 concentration was almost twice as long, about four hours.

Keep in mind, the 120 minute scrubber duration would be over ten times the no-stop-required time limit for constant PO2 sport diving at that depth.

Click Here
to view a graph
of lab results (25Kb PDF)

5 ATA (130 fsw), 40 lpm RMV, 1.35 lpm CO2, and 75°F

Under high breathing-rate and warm water test conditions at a pressure equal to 130 feet, the scrubber duration to reach a surface equivalent of 0.5% CO2 concentration was about three and one-half hours. However, the time to reach a surface equivalent of 1% CO2 concentration was just 30 minutes more.

No surprise, warm water almost doubled the scrubber duration for this dive, but the scrubber rapidly depletes following breakthrough. Interestingly, warm or cold, shallow or deep, the time to reach 1% CO2 is around four hours.

Click Here
to view a graph
of lab results (67Kb PDF)

10 ATA (300 fsw), 30 lpm RMV, 1.1 to 0.8 lpm CO2 and 45°F

This is a simulated 300 fsw technical dive using a 60% Helium Trimix Dil and a 35 minute bottom time in very cold water. With a constant PO2 of 1.3 ATA and stops beginning at 230 fsw, the total runtime for the dive would have to be well over three hours. After clearing required deco the diver could hang at 15 fsw for another one and half hours before reaching surface equivalent of 0.5% CO2 concentration. That's a total scrubber duration time of almost five hours. The RMV and CO2 values were selected by starting with the biometrics published for a well known endurance athlete, and scaling them for a 250 pound body.

You may notice the graph indicates a momentary increase in the CO2 level about 35 minutes into the dive just as the initial ascent began; this is an insignificant artifact of testing. The operator injected a known quantity of CO2 after the absorbent in order to verify the CO2 detection equipment.

References

  • NEDU Technical Manual No. 01-94, U.S. Navy - Chapter 3, UBA Test Procedures and Performance Goals
  • Tek Closed Circuit Rebreather, IANTD - Chapter "CO2 & The Diver" by David Sawatzky, MD
  • Mastering Rebreathers by Jeffrey E. Bozanic - Chapter 5, Physiology
DISCLAIMER: All information is provided "as is" without any warranties of any kind. Dive Gear Express LLC makes no representations and disclaim all express and implied warranties and conditions of any kind, including, without limiting the generality of the foregoing, representations, warranties or conditions regarding accuracy, timeliness, completeness, or fitness for any particular purpose.

Test Data ©2005 Copyright Micropore, Inc: The test results may be used and reproduced solely for non-commercial, personal and/or educational purposes, provided that it is not modified and provided that you maintain all copyright and other notices contained in the contents. Such information may not be otherwise used, reproduced, broadcast, published or re-disseminated without the prior written consent of Micropore.

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