What a Rebreather is and The Different Types

At the time of writing we now have 13 people in our Club on Rebreathers. Members within our Club are curious about these machines and want to know more about how they operate, costs, types, advantages & disadvantages. With this thought in mind, I have written this article in order that they can be pointed in the right direction. I have tried to keep it simple.

In order to understand what a rebreather is and how it works, it is necessary to understand how conventional scuba works. All scuba divers use an apparatus presently known as Open Circuit (OC). This system was introduced to the general public by Emile Gagnan and Jacque Cousteau in 1943.

From that time, scuba divers have embraced this system of recreational diving. This system of diving employs the use of a compressed gas supply and a demand regulator from which the diver breathes. The exhaust gas is discarded in the form of bubbles with each breath, hence the term “open-circuit”.

As you may be aware the atmosphere is made up of approximately 21% O2 and 79% Nitrogen. (Air is exactly 78.09% Nitrogen and 20.95% Oxygen. The remaining 1% is made up of Argon 0.93% and Carbon Dioxide 0.03%. Open-circuit scuba is very inefficient system, due to the fact that out of the 21% that we breathe from our cylinder, our metabolism only requires approximately 4%. The other 17% of oxygen is totally wasted in the bubbles that exhale from the regulator. Furthermore, as the Open Circuit Scuba Diver descends the waste increases with depth. In effect if a Scuba diver has 200 bar of air in their tank they will only use approximately 8 bar of the Oxygen, the rest will be wasted.

A rebreather is a very efficient machine that allows you to take less gas on your dive and by the clever use of a scrubbing device, (Scrubber) clean the waste carbon dioxide from your exhaled breath and inject exact amounts of Oxygen (O2) from a cylinder to replenish the 4% of Oxygen used in each breath.

All the different kinds of rebreathers have certain basic components in common. All designs start with a breathing loop equipped with a mouthpiece, through which a diver breathes. This breathing loop is like a collapsible bag and lies tight on your chest. This is like an extension of your lung. When you breathe out, the bag (loop) inflates and when you breathe in, the bag (loop) collapses. The bag is often referred to as a counter lung.

If a diver were to continue breathing in and out from this breathing loop, the carbon dioxide (CO2) exhaled by the diver would soon build up to dangerous levels and cause Hypercapnia. Hypercapnia will cause the diver to go unconscious. Therefore, the breathing loop will include a Carbon Dioxide (CO2) scrubber unit which comes in the form of an absorbent canister containing a lime based chemical substance called Sofnolime.

This is usually accomplished with an upstream check-valve, and a downstream check-valve, located on either side of the mouthpiece. These allow inhaled gas to come from only one direction in the breathing loop, and allow exhaled gas to go only in the opposite direction. Another feature common to the Rebreather designs is some form of shut-off valve in the mouthpiece which can be shut if the mouthpiece is removed underwater. If the diver was to remove the mouthpiece without shutting it off, then the whole system would flood.

In recent years, Rebreathers have commanded a lot of press. Some of it good and some not so good.

Some individuals believe that Rebreathers are dangerous, other including myself believe that we are presently witnessing the death of Open Circuit, with the future of diving being the Rebreather.

Statistically speaking as more and more people head in the direction of “technical diving”, (a term used for off piste diving) then Rebreather is the obvious direction.


The efficiency of gas usage.

Running cost, especially where helium is used.

No long periods waiting on large single and twin set refills.

Up to six hours use from 2: three litre cylinders

Same amount of gas used irrespective of depth.

Less Hydration.

Warmer to use in cold water.

Silent running means getting closer to marine life.



Understanding the complexity of the equipment.

Meticulous Discipline whilst in use.

Time required to master the unit.


In my experience and knowing divers who are interested in “going rebreather”, the main disadvantage is the initial cost, which will be around £6k, by the time you have your training done.

Since the advent of the rebreather into the recreation world of diving, there have been a number of fatalities.

In all of the cases the weak link has been proven in a number of court cases to be “user error”. This “diver error” occurred due to one of the following:

A: Training: Or rather either the lack of or misinformed training.

B: Lack of ability.

C: Lack of experience.

D: Diver attitude.

E: Diver ability.

These “Diver errors” can also be found in Open Circuit diver incidents, however with the Rebreather, the unit is less forgiving. However to the aware, educated, trained and “properly switched on” Rebreather Diver, there is no problem that can not be recognized and remedied.

At all times the Rebreather diver carries an Open Circuit bail out cylinder. “If in doubt, bail out”

What to look out for when buying a Rebreather.
I first looked at buying a Rebreather in 2006; it took me just short of three years to decide on what I needed.

From my experience, this is what I looked for.

There is no particular order for the criteria except for the first rule.

1: Training

There have been a number of fatalities on Rebreathers. In most cases it has been proven at the Coroners Inquest that it was Diver error, caused by either no training or individuals being badly taught. I searched for a dedicated, skilled Instructor and found Michael Whitley (02866341638). Michael has dived the Inspiration for 10 years and has a thorough knowledge of the unit. With a combination of a very thorough teaching process and the patience of a saint, he is one of the best Instructors, where I have received tuition. This is a MUST for the Novice Rebreather Diver. I had a choice of several Instructors, however having felt their patience wearing thin on prior courses, I knew that I would require a patient type of individual.

If the instructor asks a question off the student and they are either afraid of looking stupid or afraid of upsetting the Instructor, then this is a recipe for disaster. If there is anything that you do not understand, ensure that you fully understand what is being taught before moving on. The whole learning process must be thorough.

Likewise the Instructor must be totally committed to the fact that the novice rebreather diver is fully conversant with all that is being taught. Nothing must be taken for granted. More importantly it is of vital importance that a methodical procedure is put in place for setting up the unit to work and that this strict methodical procedure is strictly adhered to. Never become complacent with the unit.

It is also excellent if you are able to do your course with another club member or diving buddy / friend. During your training you will learn from one another and be able to focus on a common goal. The companionship will also make the course more enjoyable. I was fortunate to have Jimmy Lyons, one of our club’s founder members, do the BSAC Rebreather course with me in January. The memories of 10 hours diving, required in the 4 degree fresh water thermoclines of Donegal Bay will stay with us forever.

2: What type of unit will I buy?

There are 4 basic types of rebreather.

1. The Fully Closed Rebreather, such as the Inspiration, Cis Lunar, Megalodon, Prism, CCR1000, and MK15 series. (no bubbles electronically controlled)

2. The Kiss Rebreather (This is a fully manual unit where the oxygen and diluent must be manually injected to maintain the correct partial pressure in the breathing loop.

3. Semi Closed Passive rebreather. Halcyon, Halcyon 80, Odyssey, K3 and BMD, (Occasional venting of bubbles, gas addition keyed to respiratory need)

4.The Semi Closed Active rebreather: Dolphin/Atlantis, Azimuth, Drager Ray (mass flow bubbles every 5 breaths or so.)

I looked at most of the above units and in the end purchased the Ambient Pressure Inspiration with Vision electronics. The main reason was as follows;

A: Other members of our club were using the units and subsequently in the event of a problem with the unit I could easily get information B: The AP Valves Inspiration is the most popular unit on the market and parts can be delivered next day from Cornwall. There was also a good chance that one of the other users in our club would have a spare part, should I require it in a hurry. C; When diving the unit, the other users in our club could keep an eye on what I was doing during the set up stage. This would also be applicable on the dive.

3: What is the pedigree of the unit?

A large number of Rebreather Manufactures have gone into liquidation over the past number of years, taking customers deposits as they went down. Cis lunar has stopped making units, although it services them. When you decide to buy a unit, it is vitally important that the Company that you are buying the unit from will be around for years to come in order to provide backup, service and upgrades where necessary.

It will also add to the residual value of the unit if the manufacturer is still in existence. Ambient Pressure Diving – an entity of AP Valves – (Really its AP Valves with the name changed for legal purposes) the manufacturer of the Inspiration is far and away the largest manufacturer of Fully Closed Circuit Rebreathers, with shipped units numbering around 5,000 worldwide. AP Valves has been around for 30 years and it’s a fair bet they will be around for as long again. This was a major deciding factor on my purchasing my unit from Ambient Pressure Diving.

4: Residual Second Hand Value

At sometime in the future you may wish to sell your unit. You may wish to retire from diving or you may wish to upgrade to another unit. At this stage it is important that you have a unit that is attractive to a buyer With a market leader unit, people are familiar with the name and comfortable purchasing a well known brand name second hand. This was yet another deciding reason for my purchase of the Inspiration unit.

5: How many units have been sold?

Find out how many units have been sold and the history of those units in service. Reliability of electronics, reliability of parts, work of breathing, comfort, surface attitude, its buoyancy characteristics, and manufacturing failure points are all considerations.

The best test for how these things stand up is for the units to be out there and dived by the Public or the Military.

If a unit has been dived by a large number of people you will get a better idea of its performance. If you are diving one of the first 20 units produced by a company then in all likelihood you are a guinea pig.

6: When to Buy

In order to secure the deal, the manufacturer / sales agent will promise you the unit within a week. That’s almost impossible. If you purchase at the beginning of the summer, be prepared to wait. Most people buy at Easter at the beginning of the diving season. I would not do this. Firstly everyone is doing the same thing, so you may end up having to wait longer than normal. Secondly there is nearly always a Dive Show around October. Even without visiting the Dive Show, you can avail of the Dive Show Discounts. I was able to purchase my unit, approximately £1000 cheaper than the retail advertised price, due to a show price special.

Due to the fact that I ordered the machine in October, the end of the dive season, I had the Inspiration unit delivered to my Instructor in 5 weeks from the date of order. I had my training completed by the end of Jan 2010.

7: The cost of the Unit and approximate numbers that have been manufactured.

The figures below are in some cases best guess. For exact details call the manufacturer and ask them. Contact info at the bottom of article.

Fully Closed rebreathers.

Ambient Pressure Diving

Inspiration with Vision Electronics + Trimix Software £5000.00. Second Hand Classic Units @ £2500.00. Units produced: 5000 plus.

Megalodon £5000.00 Second hand unlikely. Units produced approximately 200.

8: Running Costs and Maintenance.

The running costs of a Rebreather are roughly around the same as a diver on Open Circuit, however for Divers diving on Trimix, it can be up to 90% cheaper than the divers on Open Circuit. A 20kg tub of Sofnolime is approximately £70 delivered. This will give you around 8 fills. With a scrubber use of 3 hours, you are looking at 24 hours diving per tub. A set of batteries via the internet, will cost £5.00. On average every 3 to 4 months. Be prepared to part with at least £500.00 for your training. (2 days class room and 8 hrs under water)
This is is a list of useful websites.

1. The original rebreather web site.

2. One of the most informed rebreather divers out there is Richard Pyle. His information and advice is some of the very best.

3. IANTD is the largest and oldest technical agency and teaches nearly all the rebreathers out there. 5. Inspiration Rebreather site

4. Kevin Jurgensen. Site information on MK15 Cis Lunar maintenance and Inspiration additions such as the Hammerhead.

5. CCR2000 website . Steam Machines Prism rebreather.

6. Tracey Robinette.

7. Kiss rebreather web site. Halcyon rebreather web site.

9. Megalodon rebreather web site.

10. Azimuth rebreather web site.

11. An excellent site on all things Inspiration.

12. Web site of Bob Howell. Bob is a very experienced Rebreather diver who makes all sorts of high quality additions for Rebreathers.


These views are the personal thoughts of the author (written at 12.30am) and must / should not be a substitute to proper training”

Peter McCamley