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Mixed-gas closed-circuit rebreathers (CCR) improve upon SCR designs. CCRs are the most advanced rebreather systems and require the most upkeep and maintenance. They are also the most expensive and complex to operate and require the most extensive training. CCRs offer the best gas economy, reduced decompression obligations, and provide extended depth capabilities.

They operate by injecting oxygen, as needed, into the loop to compensate for your metabolic oxygen consumption rate. To do this, they must monitor the PPO2 in the breathing loop. When the PPO2 drops, additional oxygen is injected to keep the PPO2 constant.

Since the PPO2 within the loop must be monitored at all times, CCRs use an electronic control device and oxygen sensors. CCRs were designed to deliver a gas mixture with a constant PPO2 no matter what the depth, which means you’re always breathing the best possible mix for the depth you’re at.

CCRs also use two gas cylinders — one for oxygen and one for a diluent that is used to dilute the oxygen in the breathing loop so that a constant PPO2 is maintained throughout the dive. The most common diluent is air, but trimix and heliox are also used in CCRs for deep diving. Whatever diluent is used, it must be capable of sustaining life if you would have to use it alone.

The electronics in CCRs are responsible for controlling the addition of oxygen through an electrically operated valve. This valve keeps the PPO2 constant no matter what your metabolic consumption rate or changes in depth. The electronics are either analog or computer-controlled and three redundant oxygen sensors are used to maintain a constant PPO2. In addition to electronic valve, manual addition valves allow you to manually add oxygen or diluent to the breathing loop in case of an emergency. Because maintaining an accurate and safe PPO2 is required throughout a dive, most systems have a display of some sort to notify you what the actual PPO2 is within the breathing loop.

Source:

Boxanic, Jeffrey E. Understanding Rebreathers. Flagstaff, AZ: Best Publishing Company, 2002.

Semi-closed circuit rebreathers (SCR) allow for the use of an inert gas to dilute the oxygen, which allows you to dive deeper. The base gas that is typically used is an EANx (nitrox) gas mix. An SCR is very similar to oxygen closed-circuit rebreathers except for the fact that they must have functionality to accommodate the use the inert gas.

One of these changes is the addition of an over pressure valve, which is used to vent gas from the breathing loop so that it stays at ambient pressure. Also, a metering valve is used to inject the breathing gas into the system. The metering valve’s job is to inject enough gas to make up for your metabolic consumption of oxygen.

There are three different classifications of semi-closed circuit rebreathers:

·         Constant mass flow

·         Respiratory minute volume keyed

·         Constant ratio

A constant mass flow SCR is the simplest and most commonly used type of recreational rebreather. They are simple to set up, operate, and maintain. The system meters a constant flow of EANx into the breathing loop. The gas mix set determines the flow rate. The over pressure valve vents gas from the loop corresponding to the flow rate amount of gas. The mass flow controller is an integral part of a constant mass flow system. If must accurately regulate the flow of gas by mass regardless of your depth. Checking the mass flow controller is part of every pre-dive check.

Oxygen closed-circuit rebreathers are oxygen-only breathing rebreathers and are the most simple design and most inexpensive type of rebreather. Because there is no helium or nitrogen to breathe, there are never any decompression requirements.

Oxygen closed-circuit rebreather systems use a valve to add the oxygen into the breathing loop. The valve is designed to keep the oxygen supply constant, meaning that as you metabolize oxygen or descend deeper, the valve adds oxygen to compensate. With this system, there is no way to vent over pressurized gas (no over pressurization valve), which means that an oxygen closed-circuit rebreather is the only true fully closed-circuit rebreather.

Oxygen closed-circuit rebreathers can be operated with zero bubbles as long as you maintain a certain depth or ascend slowly enough to make up for gas expansion. When using this type of rebreather, you must not ascend faster than your metabolic consumption of oxygen if you want to maintain bubble-free diving or risk pulmonary embolism. If you do ascend to quickly, you have to vent gas from your mask to avoid injury.

Breathing oxygen greater than 20 feet (6 m) deep is risky due to an oxygen partial pressure of 1.6 ATM and CNS oxygen toxicity. This means that an oxygen closed-circuit rebreather must be used at depths shallower than 20 feet (6 m). If you want to maintain the recommended 1.4 ATM, you cannot exceed 13 feet (4 m).

The military is the biggest users of oxygen closed-circuit rebreathers. Combat swimmers use this type of rebreather for shallow water operations requiring complete cover. Recreational use of oxygen closed-circuit rebreathers is very limited and thus is not used very often.

Source:

Boxanic, Jeffrey E. Understanding Rebreathers. Flagstaff, AZ: Best Publishing Company, 2002.<>

Rebreathers are diving systems that recycle your breathing gas. This drastically extends the supply of breathing gas by reusing exhaled gas. There are two types of rebreathers on the market today: closed-circuit rebreathers and semi-closed circuit rebreathers. All rebreathers are similar in design and have several of the same components. Every rebreather must incorporate a variable volume container that is responsible for capturing your exhaled breathing gas. This is typically what is known as a breathing bag or counterlung.

Another shared component of all rebreathers is a scrubber. A scrubber is a device that is responsible for removing the expired carbon dioxide from the breathing gas as it passes through the system. Scrubbers today typically use sodalime, which is a disposable chemical carbon dioxide absorbent.

The counterlung and scrubber make up the breathing loop, which is the path taken by the breathing gas. Other pieces of a rebreather make up the entire breathing loop and include the mouthpiece and breather hoses. Your lungs are also considered part of the breathing loop because they contain the gas during a portion of the recycling process.

The two types of rebreathers, closed-circuit and semi-closed circuit, can be broken into three different types of rebreathers: oxygen closed-circuit rebreathers, semi-closed circuit rebreathers, and mixed-gas closed-circuit rebreathers.

Rebreathers are diving systems that recycle your breathing gas. There are two types of rebreathers on the market today: closed-circuit rebreathers (CCR) and semi-closed circuit rebreathers (SCR) and each require specific training.

There are a number of training agencies that can instruct you on diving rebreathers, such as IANTD, GUE, TDI, NAUI, and PADI. All instruct on different rebreathers and different levels of diving when using a rebreather. If you want to dive a mixed-gas closed-circuit rebreather in an overhead environment, you’ll need extensive training. If you only want to dive in open water to recreational levels, such as 130 feet maximum, basic or beginner courses on a specific rebreather is what you need.

When looking for a rebreather course, you first need to first decide how and where you want to dive a rebreather. This can help you decide on the type of rebreather you need. Once you decide on a unit or two, you can then research the training required to operate the unit safely and where you need to go get the proper training. If there is a specific training organization that you want to use, you’ll have to contact them to find out exactly what type of training they offer on what rebreather, where they offer it, how long it takes to complete, costs, etc.

Every rebreather on the market today operates a little differently, even though they all perform the same function. Some rebreathers are mechanically operated only, while others use complex computer and sensor systems. This makes training requirements, costs, training time, etc. very different from rebreather to rebreather. Do your research first.