Rebreathers: The Basics

First, a Few Questions:

If you answered "yes" to any of the above questions, you might be ready to try diving with a rebreather. In addition to avoiding these issues, you would get out of the water feeling great, and also be warmer and hydrated. You probably already enjoy the benefits of Nitrox diving. Rebreather diving is even better. The dive itself becomes almost silent. Couple that with minimized buoyancy changes from breathing to achieve a gliding effect. Diving a rebreather feels more like flying. You merge with the dive site and your environment, rather than existing as a noisy distant observer. You're really swimming with the fish, rather than just watching them swim away. And all while breathing comfortably and normally.

More and more in the dive magazines, there is a growing buzz about rebreathers. These devices are not new. They have, in fact, been around much longer than the now traditional open circuit SCUBA. The earliest rebreathers were made from pigskin bladders. Before anyone gets any ideas and takes that old football and garden hose into the garage, realize that the technology has advanced a lot since 900 BC.

It is modern technology which is now bringing rebreathers into the reach of more than the just the military and the rich. Technology is improving and lowering costs in manufacturing. Digital controls and microprocessors have made electronics smaller and more survivable in a hostile marine environment. In short, technology has finally caught up with the concept. Technology is making rebreathers more affordable, safe, and practical.

What Is A Rebreather?

There are several categories of rebreathers, but let's start with the basics. A rebreather is a device which re-circulates the diver's breathing gas. A typical person consumes less than 10% of the Oxygen (O2) in the breathing gas in every breath. This means that the other 90+% is wasted if it is just exhaled, and not "rebreathed". There are three primary categories of rebreathers. They vary from Oxygen rebreathers to Semi-Closed Circuit (SCR) to state of the art Fully-Closed Circuit Rebreathers (CCR). These systems vary in the manner in which gas is added or replenished. All rebreathers must scrub out the CO2 from the breathing gas; otherwise the diver will become hypercapnic (which can cause blackout and other problems)

The Basic Rebreather Consists of:

  1. Breathing loop:
    1. Mouthpiece. (Dive Surface Valve or DSV).
    2. Counter-Lung(s). To contain the diver's exhaled gas for processing and rebreathing.
    3. Scrubber. To remove carbon dioxide (CO2) from the breathing gas.
    4. Check Valves. To keep gas flow directional.
    5. Breathing Hoses. To connect the DSV, Counter-Lung(s), and Scrubber.
  2. Gas Addition/Replenishment System.
    1. Compressed gas cylinder(s).
    2. Regulators. To reduce the high pressure gas in the cylinders to ambient pressure.
    3. Oxygen measuring device.
    4. Gas addition controls.

Oxygen Rebreathers:

In this design, only pure O2 is injected into the breathing loop. This is done via a demand valve and manual override. These are Fully Closed Circuit systems which are extremely depth limited because the diver is breathing pure O2. The typical use is military. These are the only class of rebreathers which are totally "bubbleless". There is never a decompression obligation with these units, as there is no inert gas in the system to be absorbed by the diver's tissues.

Semi-Closed Rebreathers (SCR):

In this design, Nitrox (O2 enriched air) is injected into the breathing loop via a mass flow orifice valve, or a system which keys the injection to the divers breathing rate. Because gas is continually injected into the loop, it must also be expelled from the loop (hence the term semi-closed). These systems offer gas use economy over open circuit SCUBA, but there is no decompression advantage over nitrox open circuit diving.

Fully Closed Circuit Rebreathers (CCR):

In this design, there are two gases; pure O2 and a diluent gas (air, tri-mix or heliox, etc). Diluent is added either automatically or manually to the loop to make up the bulk of the loop volume during descent. In a CCR system, only O2 is consumed or burned by the diver. That consumption rate is based on the diver's metabolic rate. It is not affected by the depth of the dive, but by the diver's workload. This means that the diver will consume the same amount of O2 at 300 ft as at 30 ft or 3ft, provided he is performing the same task.

CCR's require the gas in the breathing loop to be monitored. This is done via oxygen sensors (typically 2 or 3). O2 is injected into the loop to maintain a set partial pressure of O2. There are two injection strategies within this topic. Electronically Controlled (automatic) and Manually Controlled. Some CCR's are designed to allow both strategies.

CCR's offer the maximum decompression advantage over open circuit SCUBA. CCR's are truly the best choice if your diving ranges from advanced recreational to heavy technical.

Q & A

Q: What Are Some Basic Differences Between Rebreathers and Open Circuit SCUBA?

A: Open circuit SCUBA has seen a great deal of development over the years, and now even low-end regulators breathe better than most of the high-end regulators of a decade ago. Of course, further improvements can be made.

When using open circuit SCUBA, regardless of model or manufacture, after each breath you exhale into the surrounding water. That exhaled gas is gone. The deeper the diver goes, the greater the volume of gas that is expelled on every breath. The bottom time with open circuit SCUBA is typically limited by how deep the diver is going and how much gas can be carried. By comparison, in a CCR where only O2 is consumed, bottom time is limited by scrubber duration (typically 3 to 5 hours), decompression obligation (optimized), and oxygen toxicity (CNS and OTU's).

SCUBA cylinders are cooled due to adiabatic expansion as the gas leaves the tank. With open circuit SCUBA, the breathing gas coming from the tank will almost always be at a lower temperature than the surrounding water by the time the diver inhales it. The diver's body warms the incoming gas only to have it expelled on the next breath. This removes heat directly from the diver's core. The open circuit diver is also breathing very dry gas. This is hard on the body because the lungs require moisture for efficient gas exchange. The body must hydrate the incoming gas, primarily in the lungs. This moisture is then expelled into the surrounding water. These two factors act to dehydrate the diver. Dehydration of the body and lung tissue and heat loss are primary Decompression Sickness (DCS) Factors.

In rebreathers, the scrubbing of CO2 from the breathing gas is an exothermic chemical reaction (it produces heat). The reaction by-product is water vapor. The result of this scrubbing is warm, moist breathing gas. With rebreathers, the exhaled breath is re-circulated, which means that the moisture level is maintained. The loop gas is typically at 100% humidity and is much warmer than the surrounding water. This means that the diver stays warmer and hydrated which reduces the primary DCS factors.

Q: Rebreathers Look Technical, Are They More Difficult to Dive?

A: The short answer is no. There is a learning curve. There is more to keep track of than with open circuit, but after a little while it's like driving a car, the needed skills become automatic. The actual breathing is generally much easier than with open circuit SCUBA.

It is important to understand, though, that rebreathers are not for lazy or complacent individuals.

The breathing gas in a rebreather is dynamic. It can vary from being life supporting to hypoxic (too little O2) or to hyperoxic (too much O2). There are also concerns with hypercapnia (too much CO2). It is not our intent to scare people away from rebreathers, only to emphasize the importance of training, practice and respect for the rebreather.

Q: Do Rebreathers Require a Lot of Maintenance?

A: Any SCUBA equipment should undergo a pre-dive check before each use. Unlike open circuit, a rebreather requires a little more than a couple of puffs on the mouthpiece to make sure everything is working properly. Rebreathers require some preparation, pre-dive checks and calibration, but these can typically be done in 20 to 30 minutes. Post-dive cleaning can routinely be done in about 15 minutes. An advantage is that the pre-dive work can be done days in advance of the dive. Another plus is that once the rig is set, you can dive the entire weekend without touching it again (except for draining condensation, and provided the scrubber has not been consumed).

Q: Are Rebreathers Expensive?

A: Rebreathers can cost from $2,000 for a SCR to $70,000 for some military CCR's. An electronically controlled CCR can be purchased for approximately $8,000, and manual-addition CCR's such as the StingRay for approximately $6000, but reserve $1000 - $1500 for the training. Obviously, if you only make 3 dives annually while on family vacation, it is difficult to justify such an expense. If, however, diving is a serious hobby or a business, a rebreather could prove quite cost-effective. Dives using air as a diluent are about the same cost per dive hour as open circuit. If you are using helium-based gases, the cost advantages of a CCR add up very quickly.

Q: Why use the KISS approach to CCR's?

A: First, the KISS approach eliminates many of the failure-prone electronics of a CCR (after all, electronics and water don't mix very well), while also simplifying the overall operation of the unit.

Second, the motto of the CCR diver is "Complacency Kills".

Dr. Richard L. Pyle said,

"I return again to "Richard's Reliable Rebreather Paradox": that a more reliable rebreather can be more dangerous, because reliability encourages complacency, and as we all know, complacency kills rebreather divers more than anything else.

By this metric, manual rebreathers are the safest, because the diver has no choice but to always be cognizant of the PO2. A KISS diver is, almost by definition, an excellent rebreather diver, and one who is almost immune to the complacency killer."

By requiring that the diver monitor his PO2, the KISS-type Manual CCR helps the diver avoid the complacency which can be engendered by the automatic controls of an electronic CCR. In addition, the KISS approach reduces task loading by lengthening the interval between O2 additions from once every few seconds (for a completely manual CCR) to once every five to ten minutes (for a properly adjusted KISS-type CCR).

In Conclusion:

Open circuit SCUBA has reached a pinnacle of design and is now at or near the point of diminishing returns. Rebreathers are here to stay, and as the technology continues to advance there will be more and more of these systems showing up on dive boats and at dive destinations. If you have a chance to try one, do it, but be warned that you will never look at your open circuit SCUBA gear the same way.