Many of the skills and procedures we have become accustomed to while using open circuit do not apply while using a rebreather. Simple sensations such as hearing the air move through the hoses to help me control my breathing are absent while using a rebreather. There are no sounds to listen to. The trick of fine-tuning buoyancy by inhaling or exhaling on open circuit does not work with rebreathers.
With a rebreather, inhaling and exhaling simply transfers the gas from one compartment (the lungs) to another (the counter-lungs or breathing-bag). There is no change in volume, therefore, there is no change in buoyancy. I spent many hours bouncing off the sandy bottom before I got accustomed to this difference.
Anybody starting to dive with a rebreather, no matter whether you have logged 3000 dives or 300, make sure that you take it slow and gradually build up your abilities. There have been too many accidents with rebreathers that could have been avoided if the diver would have taken more time to gather experience with the unit first. You will learn that when diving with a rebreather you have to become part of the unit itself. You must monitor your body for different signs and symptoms more frequently than on open circuit as your body becomes part of the gas-loop. One area that is common to both systems is a clear brain. It is one of the best pieces of equipment you need to take with you, always. Remember, take the attitude that you are starting over. Begin SLOWLY.
Rebreathers have been around a lot longer then people would believe. First attempts date as far back as 1680 by a gentleman named Giovanni Borelli. In current use, there are three general types of rebreathers (the simple oxygen rebreather, the semi-closed circuit rebreather and the fully-closed circuit electronically controlled rebreather). Let’s look at the similarities and differences between the three general systems:
Using a rebreather, the gas we breathe is actually recycled from our last exhalation. Since the diver is inhaling and exhaling into, and from, a closed loop, most of the gas volume is retained instead of lost. The diver’s exhalation gas, which has high levels of carbon dioxide, is “scrubbed” out as it passes through a chemical medium. At the same time, oxygen (from some source) is replenished through an oxygen injection system. The mechanism of this oxygen injection system actually helps define the specific type of rebreather.
Henry Fleuss developed the first oxygen rebreather in 1878. Presently, oxygen rebreathers are mostly used by the military, as they are limited to very shallow depths. Oxygen is provided to the loop through a constant bleed or by a mechanical regulator that senses a volume drop in the system (due to the oxygen metabolized by the body). For constant flow-rate units, the set flow is determined by anticipated metabolic consumption needs. Work and effort increase oxygen demands, therefore, flow rates would need to be higher. Generally speaking, flow rates are set anywhere between 0.5 l/min (on a low workload dive) to 3.0 l/min.
Semi-Closed Rebreathers (SCR)
The semi-closed circuit rebreather operates under a similar principal to that of the oxygen rebreather with a constant flow. The difference is that instead of utilizing pure oxygen as the supply source, a pre-mixed nitrox is used. Since the oxygen percentage is diluted, a higher flow rate must be maintained to ensure enough oxygen is always present.
There are four basic components of an SCR.
1. The counter lung
Often called the ‘breathing bag’, it is this that the diver inhales and exhales into via the mouthpiece and hoses. The counter lung is mostly empty after the diver inhales and mostly full after the diver exhales, yet the amount of gas contained within the loop remains fairly stable (the diver’s lungs are part of the loop). SCR’s have an over-pressure valve attached to the counter lung which bleeds off excess gas being added by the constant flow valve.
2. The scrubber caniste
Here is where the CO2 absorption medium is contained. It too must be part of the closed-circuit loop but the exhaled gas mostly passes through this medium and space to remove CO2 from the loop. From the canister, the gas passes into the counter lung.
3. Supply gas cylinder(s)
The oxygen supply to revitalize the loop gas with oxygen is stored here. While some SCR’s utilize two cylinders (one with pure O2 and the other with air), most utilize a single cylinder with a nitrox mixture instead. The actual fO2 that the diver chooses will be determined by what depth they plan on diving too. Even though it is guaranteed that the loop fO2 will not be exactly the fO2 in the supply cylinder, one must assume that it could be. Deeper depths require a lower oxygen percentage and a correspondingly higher constant flow rate.
4. Mass flow control orifice or valve:
It is through this mechanism that a constant gas flow from the supply cylinder is regulated. It must provide a uniform flow rate at all times to ensure appropriate levels of oxygen are available in the loop. The actual flow rate is used in a mathematical calculation to determine equivalent air depths.
Closed Circuit, Electronicallt Controllewd Systems (CCR)
Most of the major components in this system are shared with the SCR with a couple of changes and additions. There is still a counter lung (many CCR’s utilize two), there is still a CO2 canister and there are still supply cylinders (note: at least two). The primary differences are that instead of a constant bleed of gas through a mass flow orifice, pure Oxygen is added (in small amounts) only as you metabolize oxygen from the loop. To achieve this, an oxygen sensor (or sensors) constantly monitor the PO2 in the loop. With this information, the controller will activate a solenoid to manage an injection and maintain the desired level. Since pure oxygen is supplied, a second supply cylinder is required to provide a diluent. For sport diving purposes, the diluent cylinder would contain air. The fully closed, electronically controlled rebreather will give the longest available underwater times because they are the most efficient at utilizing supply gases and they provide the lowest exposure to inert gases (i.e. nitrogen).
Disadvantages of using open circuit
Poor Gas efficiency – when the diver exhales, all gas is released into the water. Your body only utilizes approx 4-6% of the total volume lost. The deeper you go the greater the volume loss with each exhalation.
Poor Weight to Duration Ratio – A large waste in volume requires that more gas is carried. This means more and larger tanks must be strapped to the diver causing more physical exercise.
Limited Mix Optimization – Since the fO2 is fixed; it can only be optimized at a single depth. This usually means only at the deepest depth planned. While traveling ‘to or from’ or, in circumstances where you elect to stay shallower than planned, you are stuck with the mix that is in your cylinder.
Maximum Noise & Environmental Disruption – Noise from the release of bubbles as you exhale and from high-pressure gas moving through the hoses scare away marine life. Some environmental destruction also occurs in caves and shipwrecks as this bubble volume collects on the roof literally exposing marine life to air instead of water.
Dehydration – Gas from compressed cylinders is dried as much as possible before filling. This means that the diver is always breathing a gas that is super-dry. Physiologically, this is a negative.
Heat Loss – Not only is the supply gas at equilibrium with the ambient temperature of the water, it becomes super-cooled as it passes through the first stage regulator due to gas physics. The cooled gas robs the diver of heat energy and it is physiologically undesirable.
Benefits of a Rebreather
Increased Gas efficiency – Since gas used is re-circulated, a vast improvement in efficiency is realized. Additionally, this efficiency is totally depth independent. CCR’s have the best gas efficiency.
Good Weight to Duration Ratio – Since the gas we use with a rebreather is reused, the volume we are required to carry becomes reduced. This translates into smaller, fewer and much lighter weight tanks for support. CCR’s have the best ratio (i.e. a 3-liter cylinder, filled with Oxygen can sustain the average diver-workload for 400+ minutes).
Little Noise – No more gas whistling through regulators. No more (or very few) bubbles to annoy the fish. Simply put, it is now quiet.
Little Heat Loss – The gas that the diver is breathing is actually warmer than the ambient surroundings due to the CO2 removal reaction. Heat is created in this process.
Reduced Dehydration – The same chemical reaction that makes the gas warmer also adds moisture. This coupled with the fact that the normally moist exhalation of a human being is also trapped within the loop. Together, this provides a comfortably moist inhalation gas.
What Are The Training Options & Where Can you Get it?
American Nitrox Divers International (ANDI) has a full line of rebreather courses.
LEVEL 1 – Introduction to Closed Circuit Rebreathers
This is for people who want to try rebreather diving. It gives them a chance to use a unit with the help of a qualified instructor. You only need to be Open water certified to get a chance to do a shallow dive with a rebreather unit. This is a very safe course and it is done in a controlled environment. This can be accomplished in one short day. It is a good way to see how a rebreather works. If you do like what you have done, you can progress to the next level.
Level 2 – Semi-Closed rebreather
The length of this course is approx. 4-days. There is one day of theory, one day in a swimming pool and two days on the boat. This course gives most people plenty of enough time to get comfortable with the SCR unit. You must remember, the buoyancy characteristics are not the same as with open circuit. You will be shown how to assemble and disassemble your unit to develop the confidence to achieve it on your own. The two days on the boat will provide no less than 150 minutes of bottom time, again to provide ample experience. Don’t miss out on the opportunity to dive silent and more comfortable. The prerequisites for this course are; you must have level-II enriched air training (ANDI, CSU certification or equivalent) and a minimum of 50 logged open water dives.
Level 3 – Closed Circuit Rebreather
This course is to the level 2, SCR program except that you will be using a fully closed circuit unit. Minimum open water time is increased to 180 minutes. The prerequisites are the same as in level 2, SCR.
Level 4 – Technical Rebreather Diver
For the more experienced, certified rebreather divers who would like to do dives beyond the recreational limits. During this course, we will take you to a maximum depth of 50 metres. We will be doing deco no deeper than 9 metres with a maximum deco of 30-minutes. During this course, there are four to five dives with a minimum bottom time of 180 minutes and a minimum run time of 210 minutes. Plenty of time will be devoted to practicing decompression procedures. This course is obviously more extensive. The prerequisites for this are; the student needs to be TSD qualified and also have a CCR certification with a minimum of 100-open water dives.
Level 5 – CCR/ERE
This is the highest level you can reach. The maximum depth for this course is 100 metres which should satisfy all needs. The minimum time using the unit during the course is 240 minutes bottom time with a total run time of 300 minutes. To be able to dive these deeper depths you have to definitely be familiar with your equipment and your specific unit. You will have the confidence and ability to plan and execute these dives when you finish this course. The prerequisites are, you must be ERD & TRD certified and have a minimum of 200 open water dives.
Rebreathers are the way of the future. They have made quite a leap in technology in the last few years.