This is just an overview of the various types of rebreathers.

I will assume that you have little or no previous experience in this field, so I will explain everything simply, but not in great detail.

This guide is intended to help you sort out the various types, and decide which might be best for you.

What is a rebreather?

Simply stated, a rebreather is a breathing system that allows you to re-breath the same breath over and over again. It does this by adding oxygen as you use it, and removing carbon dioxide as you produce it.

In its simplest form, it is a breathing bag that you breath into and out of, a scrubber, which removes carbod dioxide, and commonly resembles a cannister of cat litter, and an oxygen tank to replenish your oxygen.

ADVANTAGES of using rebreathers are

Lighter

Smaller tanks

Few or no bubbles

Quiet operation

 Longer duration

Greater depth.

 Note that not all rebreathers offer all these features.

DISADVANTAGES of rebreathers.  Note that some of the disadvantages seem to negate the advantages above. This is because not all rigs offer the same advantages.  Hey, if it was that simple, you wouldn't need a guide, right?

Heavier

Shorter duration

Restricted depth

High cost of unit

Additional cost: oxygen

Additional cost: nitrox

Additional cost: sodasorb.

More difficult to use

More training required

Increased WOB

More maintenance.

Additional hazards.

Again, each of these factors may or may not apply to an individual unit.  Some, such as  your adsorbent, will be pretty constant among all the units currently available.

PURE O2

These rebreathers are most often used by the military. They are a form of CCR (closed circuit rebreather) but they form a class all by themselves.  They among the safest, simplest, and most reliable. They usually contain no electronics, and have a single cylinder, filled with pure oxygen. Though the cylinder is quite small, often no larger than 6 cubic feet, they can last for hours.  These are among the smallest and lightest rebreathers available.  They are also the quietest. A good operator with a true oxygen rebreather never has to make any bubbles at all. Perhaps the most familiar example of this kind of rebreather would be the LAR V.  The limitations of this kind of rig are imposed by oxygen toxicity levels, they are generally restricted to about 20 feet of depth or less.

SCR (semi closed circuit rebreathers)

Semi closed rebreathers were a half-step in rebreather design.  They bridged the capability gap between pure O2 and CCR (closed circuit rebreathers)  They were larger and more complex than pure oxygen rebreathers, but offered depths similar to regular SCUBA divers (open circuit)  These units required special oxygen mixes (Nitrox) to operate, and also required special orifices to meter the flow of those gasses. This put the diver in the position of picking both the mix and the orifice before a dive.  This limits the diver to a specific range of depth, the plan cannot be changed any time after the cylinder is filled. Semi closed rebreathers also require NITROX, which is more expensive than air. This kind of rebreather will also make bubbles periodically, every couple of breaths or so, depending on the orifice used. The bubbles do make noise, but will be less than normal scuba divers make. Bubble diffusers can also be added to reduce this problem. The orifice selection will also determine the noise level.  Some orifices are louder than others.  This may or may not be a distraction to the diver, but marine life definitely notices it.  Because the breathing bag is constantly being filled, and must be burped every few breaths, a constant buoyancy fluctuation will be experienced.   SCRs may have oxygen monitoring, but they are designed to operate without it.

In terms of efficiency, SCRs fell somewhere between closed circuit and SCUBA. The tank size required falls about half way in between, ranging in size from about 27- 50 cubic feet. The deeper you want to go, the larger tank you need.  Like normal SCUBA, SCRs use more gas at depth.

Now that oxygen monitoring is affordable and reliable, SCRs are giving way to CCR diving.  Nevertheless, there will be many  of these around for some time to come.  A popular example of this kind of rig is the Drager Dolphin. The major plusses of this kind of rig are relative simplicity, more depth range than pure O2.  The disadvantages are reduced gas efficienty, (compared to  CCR) bubbles, noise, and added expense of Nitrox.

CCR or closed circuit rebreathers are the fastest growing segment of the rebreather community.

These rebreathers have extended depth capability, the capability to change your gas mix on the fly, and generally, extended time as well, due to their gas efficiency.

The down side of this kind of rig is complexity and expense.  It takes more training, and more maintenance. These rigs are generally larger and heavier than other rebreathers.  They requier two tanks, one with pure oxygen, the other with a diluent. there are several kinds of CCR:

CCCR: chemically closed circuit rebreather, most favored by the CCCP. (soviets) The scrubber material gave off oxygen as it absorbed carbon dioxide.  Clever, but dangerous. Scrubber material was highly reactive. Although these are still available in large numbers, the adsorbent for these is not.  Best example: IDA-71.

MCCR: Manually closed circuit design, means you add oxygen or diluent by hand. Some CCRs are designed this way, most CCRs have this as their bacup  mode. Thebest example of a CCR that uses this as its primary mode is the KISS.  An innovative design, combines a SCR style bleed, but of pure oxygen, at slightly less than normal biological consumption.  Excellent plan, well executed, has the slight drawback of exaggerating oxygen spikes on descent.  Advantages are, of course, you don't need anything but O2 monitoring to use it.

ECCR: Electronically closed circuit Rebreather.  Probably this is the one most people think of when they think about rebreathers.  This kind of rebreather will monitor your oxygen levels, and add more automatically. To avoid the limitations of pure oxygen rebreathers, they also have a cylinder of air or other diluent to dilute the Oxygen. To many, this kind represents the best system available today.  In many ways, this is correct, but these are also the most tricky and dangerous to use. (based on fatalities/thousand) They will get better with time, of course, but the tendency is for divers to take their brain out of the loop and let the computer handle it. The training manuals for these insist that you check your PPO2 every thirty seconds anyway.  Well, that's a fine idea, and the prudent diver will do this anyway, but if you have to do that, why not just get an MCCR in the first place?   The drawbacks to the ECCRs are that they have the most stringent maintenance requirements, are hardest to use, generally the largest and heaviest, and of course, the most expensive.

Best example of this class used to be the MK 15, but lately, I would have to say it is better represented by the Inspiration (or YBOD, which means yellow box of death.)

No matter which kind of rebreather you use, some things are common to all rebreathers.

WOB: Work of breathing. The best of rebreathers have more WOB than even a bad SCUBA reg. In general, you will find the WOB varies as follows:

Chest mounted: WOB is hard on exhale.

Back mounted: WOB is high on inhale.

Over the shoulder bags: pretty good in general, still harder than open circuit.

Buoyancy:  Open circuit divers experience two major buoyancy factors associated with their apparatus: the weight of the air itself,  which decreases with use, and of course, the variations produced by breathing.  This latter effect is probably more pronounced than you realize. With a rebreather, whether you inhale or exhale, it is unlikely to produce much variation in your buoyancy.  If you are used to using this effect in your normal scuba life, you will miss it badly at first.

Poorly designed systems, with overly large breathing bags will also require extra weight, which may negate  the benefit of any lightness of the system itself.

Scrubber: the scrubber is going to be a pretty universal feature on rebreathers that are available to the public for some time to come. Adsorbent is going to be an added expense on all dives. Reusable scrubber material is in existance, and may be available soon, so this may be less of a concern in the future.

Nitrox. Even if you go with a pure O2 rebreather, it is important to understand Nitrox.  Oxygen under pressure is toxic, which is what limits the depth of pure O2 rigs. The rules of Nitrox must be well understood for SCR and CCR, and Trimix for extended depth. There are dive computers that can measure and calculate your oxygen dosage, but as with regular dive computers, you should still understand what it is doing.

If you found this guide helpful, please click "Yes" below before you leave.  If you would like to know more, Email me and I will revise this article to include your info.

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