CO₂ Scrubber Use in the O₂ptima CM CCR

Disclaimer: This article is a refresher for certified CCR divers. It does not replace manufacturer recommendations, formal training, or time with an instructor. If you are unclear on any points here, contact your instructor and schedule a refresher. This information is specific to the O₂ptima CM unit—other rebreathers have different scrubber designs, test data, and recommendations.

It’s easy to get comfortable stretching limits:

  • Maybe you know someone who did “X hours” and it worked out fine.
  • Maybe you’ve done an extra dive with no issue.

The problem is:

  • There are small, invisible differences in scrubber performance from fill to fill.

  • A low-workload scooter dive is not the same as a high-effort working dive.

  • The risks of pushing beyond proven limits far outweigh any benefit.

Rebreather CO₂ scrubbers don’t “filter” gas the way a house air filter catches dust — they chemically remove CO₂ from the breathing loop. Inside the scrubber, soda lime (or an ExtendAir® cartridge) contains alkaline compounds, primarily calcium hydroxide. When exhaled gas passes through, CO₂ dissolves in the thin layer of water on the surface of each granule, forming carbonic acid. This acid reacts with the alkaline material, producing calcium carbonate and releasing heat. Because this is a chemical reaction, the scrubber’s effectiveness depends on precise conditions — moisture content, temperature, and contact time all affect how completely CO₂ is removed. Thinking of the scrubber as a tiny “test tube experiment” instead of a dust filter makes it clear why changes in storage, handling, or packing matter so much.

How Long Does the Scrubber Last?

This is one of the most common questions among CCR divers.
The simple answer for the O₂ptima CM is 4–5 hours.
The real answer: It depends on many variables that can shorten or extend that duration.

What Affects Scrubber Duration?

  1. Depth

    • Greater depth means denser gas. Denser gas cools the scrubber bed faster, reducing chemical reaction efficiency.

  2. Ventilation Rate (RMV)

    • The faster you breathe, the less time the CO₂ has to react with the absorbent before leaving the scrubber bed.

  3. CO₂ Production

    • Hard work can increase your CO₂ production. More CO₂ to scrub = faster absorbent use.

  4. Temperature

    • The CO₂ absorption reaction is temperature-dependent. Cold reduces efficiency, which is why the test duration drops from 320 minutes at 75°F to 240 minutes at 50°F.

  5. Storage Between Dives

    • Scrubber absorbent performance changes after it has been used, cooled, and then rewarmed on later dives. Moisture patterns in the bed can shift, and efficiency drops over repeated dives.

Manufacturer test data is based on:

  • CO₂ addition: 1.35 L/min

  • Ventilation rate (RMV): 40 L/min

  • Pressure: 5 ATA

  • Temperatures tested: 75°F and 50°F

  • Duration: 320 minutes at 75°F, 240 minutes at 50°F

These numbers represent baseline performance under controlled testing—not a guarantee of real-world duration.

Why the Test Numbers Are a Baseline, Not a Target

The test values use high CO₂ addition and high RMV. These are separate but related stressors on the scrubber.

For real dives:

  • One 4-hour dive is not the same as four 1-hour dives.

  • Multiple dives reduce scrubber efficiency because of repeated drying/wetting cycles.

  • Always leave a buffer—never plan to dive until the exact tested limit.

If you need to stay longer due to an emergency or problem, having that buffer is critical.

Post-Dive Scrubber Care

After a rebreather dive, you need to think about how you store your scrubber material if you plan to reuse it. A scrubber’s ability to remove CO₂ depends heavily on its moisture content — too dry and the chemical reaction won’t start efficiently, too wet and the reaction is slowed or flooded. Either extreme can reduce scrubber performance and shorten its usable duration.

If you have a partially used canister and you plan to dive again the next day, simply storing it in your rebreather or a sealed container is usually fine. In that short time frame, the storage method isn’t likely to make a noticeable difference. However, if you’re going to wait more than a day before your next dive, it’s best to limit the scrubber’s exposure to ambient air. One effective method is to seal the partially used canister in a plastic bag. This reduces moisture exchange with the environment, which helps preserve the scrubber’s remaining capacity. Never dump the granular material into another container and repack it. This can cause channeling.

For divers using ExtendAir® cartridges, storage is easier. These cartridges should be kept in t heir original storage tube, with both the plastic wrapper and the aluminum end cap in place. This dual barrier helps prevent moisture from entering or leaving the cartridge. Even small changes in humidity can cause the cartridge to dry out or absorb excess moisture, both of which can reduce its ability to scrub CO₂ effectively.

In short, CO₂ scrubbing efficiency is tied directly to controlling moisture exchange. Good post-dive storage practices help ensure your scrubber material performs as expected on the next dive and can extend its usable life between changes.

Why This Matters – The Role of Moisture in CO₂ Scrubbing
Soda lime removes CO₂ through a chemical reaction that requires water. Moisture helps dissolve CO₂ into a weak acid (carbonic acid), which then reacts with the alkaline compounds in the scrubber material to form calcium carbonate. If the scrubber dries out, this reaction slows dramatically, allowing CO₂ to pass through unreacted. On the other hand, too much water can fill the air spaces between granules or in the cartridge, restricting airflow and reducing contact between gas and absorbent. Keeping the moisture level stable is key to maintaining predictable scrubber performance from one dive to the next.

Key Concepts for the O₂ptima CM Scrubber

  • Never empty, mix, and repack used granular absorbent.

  • Remove excess moisture, but do not try to “dry out” the scrubber—moisture is essential for proper function.

  • Single long dives and multiple short dives affect scrubber life differently.

  • Manufacturer test data is a baseline—not a dive plan target.

  • Scrubber failure means bailout. Expect high breathing rates and high stress—things will escalate quickly.

Bottom line: Treat scrubber duration conservatively. Understand the factors that reduce efficiency. Respect the test numbers as a baseline—not a challenge—and you’ll give yourself the margin you need when something unexpected happens underwater.