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Table of Contents

If you have questions about the meaning of a word, be sure to check the glossary.

The Basic questions (and answers)....

  1. What is a rebreather ... ?
  2. What is the difference between semi-closed and fully-closed rebreathers... ?
  3. Why would someone want one and how are they used... ?
  4. How long can you stay underwater?
  5. Do rebreathers make bubbles?
  6. How expensive are rebreathers to purchase and maintain?
  7. What about "consumables"?
  8. Is training required?
  9. How common are rebreathers?
  10. Do the certification agencies have any type of rebreather courses?
  11. How deep can you dive?

The "I'm not completely ignorant" questions (and answers)....

  1. Do Rebreathers keep a constant pO2?
  2. What are "Caustic Cocktails"?
  3. Is there a physiological benefit to diving with a rebreather?

The "pretty darn technical" questions (and answers)....

  1. How does the rebreather know when to add Diluent or O2 to the loop?
  2. Can you use the rebreather to help with decompression obligation?
  3. How do you plan a dive with a constant pO2?

The Basic questions (and answers)....

What is a rebreather ... ?

A rebreather is a self-contained breathing apparatus for use underwater which reuses at least part of each breath. This should be contrasted with "Scuba" (open circuit) where the entire breath is expelled into the surrounding water when the diver exhales.

Rebreathers are able to reuse the oxygen left unused in each exhaled breath while they simultaneously remove C02 with a chemical which "traps" the C02. The net result is greatly extended dive times with relatively small tanks. An added benefit is a relatively quiet dive as there are little or no bubbles produced.

[ Back to Questions - Glossary ]

What is the difference between semi-closed and fully-closed rebreathers... ?

There are two primary classes of rebreathers, semi-closed and fully-closed. Each shares many of the components of the other, but there are some distinct differences as well. In general terms, a semi-closed rebreather expels at least a portion of the breathing gases as a part of its normal operation regardless of depth changes. A fully-closed rebreather completely recirculates the breathing gases. Both types need to expel gases as you acend to relieve pressure on the breathing loop as the gases expand.

Semi-closed rebreathers are generally of simpler design and usually rely on a predetermined nitrox mixture or a constant flow of diluent and Oxygen. Fully closed systems have oxygen sensors which constantly monitor the partial pressure of oxygen as it changes with depth and then based on this information add oxygen to the breathing mix as needed. It would be fair to say that there are benefits and disadvantages to each relative to the requiremens of the type of diving.

[ Back to Questions - Glossary ]

Why would someone want one and how are they used... ?

The initial use and still the predominant use of underwater rebreathers is the military. The long bottom times and stealth ability of rebreathers make them very popular with the military. There are also rebreathers used above the water for such things as mine rescue.

From a non-military perspective, the majority of use is scientific (like underwater archaeology, specimen collection, etc.) and videography (film). Rebreathers are popular in these uses for much the same reasons they are popular in the military, long bottom times and relatively little noise from expired bubbles.

In the very recent past rebreathers have started to make their way into recreational diving, first semi-closed units and recently, fully-closed units. Rebreathers require a substantial commitment in time, money and knowledge to be used effectively and safely, and these are the reasons they have been kept out of the recreational market in the past.

[ Back to Questions - Glossary ]

How long can you stay under water?

The answer to this question is, that it varies widely. The longest times are generally produced by fully-closed rebreathers. Some of the fully closed units like the Cis-lunar or some of the Biomarine units will allow the diver to stay underwater for over 8 hours on very little gas.

[ Back to Questions - Glossary ]

Do rebreathers make bubbles?

Yes, but not many. Semi-closed rebreathers only exhaust a small percentage of each breath and the bubbles that are released are often mechanically diffused. Fully closed rebreathers release no bubbles as the result of breathing. Both types will release some bubbles as the diver ascends and the gases in his breathing loop expand. (It has to go somewhere!)

[ Back to Questions- Glossary ]

How expensive are rebreather to purchase and maintain?

Rebreathers cover a range of prices. From a low of just under $3000.00 USD to over $15,000.00 USD. Maintenance is perhaps a little more intensive than open circuit gear on most units. You are after all connecting an airspace to your lungs which you would probably like to keep clean! There are, of course, other maintenance issues that vary from unit to unit. Generally speaking, the fully-closed units require more maintenance and are more costly to maintain.

[ Back to Questions - Glossary ]

What about "consumables"?

All rebreathers have some consumables which you need to replace on a regular basis. The primary consumable is the scrubber material. As the scrubber material absorbs CO2 it becomes non-reactive and must be replaced. Rebreathers with O2 sensors require replacement of these sensors on a regular basis (fairly expensive).

[ Back to Questions - Glossary ]

Is training required?

YOU BET! Rebreathers are not like diving with open circuit. They have a completely new set of problems that are not covered in open circuit courses. Rebreathers can kill you without you realizing it if you try and use them without extensive training. Some rebreathers have courses available through some of the regular training agencies. The training courses of other units are from the manufacturer or individuals supported by the manufacturers. To give you an idea of the training required, the BMR500 requires 70 hours of training to pick up the unit and you must already have been trained through advanced nitrox. After the 70 hours most people stay shallow and practice for another 30-50 hours. Some semi-closed systems do not require as much training but there are usually more limits on the types of dives for which they are appropriate, which is not to say that there are not uses for which a semi-closed is more appropriate. Rebreathers are not for the casual user.

[ Back to Questions - Glossary ]


How common are rebreathers?

Rebreathers are most common in the military. After the military the use of rebreathers has been primarily in the commercial, scientific and filmaking fields. With the introduction of the Draeger Atlantis, the recreational market has started to grow and you should expect to see a number of newly available units in 1997. 1997 may very well be the year that rebreathers really start to make inroads into the recreational market. As for actual numbers, as of January of 1997, Dreager claimed over 1000 of the Atlantis units had been sold. Fully closed rebreathers are expected into the recreational market for the first time this year.

In other words, as of this date, there are not very many rebreathers out of military hands.

[ Back to Questions - Glossary ]


Do the certification agencies have any type of rebreather courses?

The technical diving agencies are getting into training on rebreathers.   IANTD is training on most of the available rebreathers, and some companies such as Steam Machines, and Halcyon are training on individual units.  Rebreathers are even becoming available on some live aboard dive boats (usually the semi-closed units).   If you look carefully it's possible to rent units.

[ Back to Questions - Glossary ]

How deep can you dive?

This is a difficult question to answer because it varies based on the unit and circumstances.   For instance, an O2 rebreather should be limited to 20 fsw, while there have been chamber lock out dives on closed circuit rebreathers over 1000fsw!   In general, the limits for diving fully closed circuit units revolve around bail-out requirements more than the rebreathers themselves.  It's difficult or impossible to carry enough bailout gas for a total system failure on the rebreather when you are making long deep dives.  Having said this there are people in the world who regularly dive rebreathers over 400fsw.  More typically rebreather divers stay under 200fsw just like regular divers but with extended bottom times.


The "I'm not completely ignorant" questions (and answers)....

Do Rebreathers keep a constant pO2?

Mostly. First you must always remember that there are many different ways to make a rebreather. Semi-closed units do not keep constant pO2's. Fully closed units make every attempt to keep a constant pO2 by monitoring the oxygen partial pressures with galvonic sensors and then adding only enough O2 to maintain a set point. Military specs are a pO2 of .7, most other rebreather divers usually use a set point from 1.2 to 1.6.

[ Back to Questions - Glossary ]


What are "Caustic Cocktails"?

"Caustic Cocktails" were named for the corrosive mixture common to older rebreathers when water came in contact with the scrubber medium. This is less of a problem now as the soda lime which is usually used as scrubber medium is less susceptable to this chemical reaction. However, it is still a very good idea to keep water out of your breathing loop as it reduces the efficiency of the soda lime, and still makes a less than savory mix, which, if less likely to kill you, is still to be avoided.

[ Back to Questions - Glossary ]


Is there a physiological benefit to diving with a rebreather?

Yes, there is a physiological benefit at least with a constant pO2 rebreather. If you maintain a constant higher pO2 than you would have during an open curcuit dive, you will absorb less Nitrogen into the body. This can lessen the chance of DCS relative to open circuit on the same dive profile. Of course you need to be more aware of CNS toxicity when diving a closed-curcuit rebreather due to the higher exposures to O2 . Semi-closed rebreathers do not generally have the same physiological benefits, because the do not maintain a constant pO2.

[ Back to Questions - Glossary ]


The "pretty darn technical" questions (and answers)....

How does the rebreather know when to add Diluent or O2 to the loop?

A fully closed curcuit rebreather has O2 sensors and electronics which monitor the sensors to control the addition of Oxygen. As the pO2 drops below the set point a solinoid fires which injects O2 into the breathing loop. There is typically a set of three sensors which are polled with various schemes to be sure of maintaining the correct Oxygen partial pressure.

Diluent is added as a function of loop volume, as you go deeper more is added to maintain the loop volume, on many units, if the counter-lung bottoms out, a mechanical valve is pressed to release more diluent into the counter lung loop.

[ Back to Questions - Glossary ]


Can you use the rebreather to help with decompression obligation?

Yes. It is quite common on fully closed rebreathers to finish a dive by purging the counter lung at the 20ft. stop and manually injecting Oxygen to get the benefits of pure O2 for decompression. Also, there is generally a lower decompression obligation to begin with because of the high pO2's during the dive. This is not usually the case with semi-closed rebreathers which are similar to open circuit in terms of decompression.

[ Back to Questions - Glossary ]


How do you plan a dive with a constant pO2?

Currently, with rare exception divers use constant pO2 tables which are similar to recreational tables in use but which take into account the constant pO2's. There is also starting to be a small number of software packages which will calculate deco tables on a constant pO2. In the near future, constant pO2 computers are due to come on the market.

As a side not, the most commonly used algorithm to calculate tables and used within computers is some variation of the Buhlman algorithm. This algorithm already allows for a constant pO2 but this feature of the algorithm has been rarely implemented because of the relatively small need. This should change with the growing popularity of rebreathers.

[ Back to Questions - Glossary ]

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Revised: September 07, 2005.