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Rebreather Basics

Rebreathers may seem complicated, but the underlying idea is very simple; gas that would normaly be wasted as bubbles is instead re-used. Learn how a rebreather achieves this, what types there are and how the various components involved work.

Recreational and Technical

Recreational rebreathers cater to the diver who appreciates less focus placed on the technical and physiological details and more on the experience. These are usually electronic rebreathers, enabling full automation of safety systems and gas composition which allows the recreational diver to enjoy longer and more comfortable dives than with a traditional scuba setup. Anyone can get certified to use a recreational rebreather thanks to the reduced number of tasks required for the diver to perform.

Technical rebreathers require special training in deep water and are suited for more advanced rebreather divers looking to graduate to the next level. These are usually manual rebreathers, which still monitors the loop partial pressure but the diver needs to add gases manually. As the diver gains more control over the gas content in the loop, possibilities open up for the technical rebreather diver to dive deeper and longer, but also increases risk and requires significantly more skill and attention to maintain safety. The benefits of technical rebreather diving are huge when compared to technical diving on normal scuba diving equipment. The benefits include shorter decompression times, lower gas costs, and a better dive experience.

Unlike most rebreathers that must choose one or the other, Poseidon SE7EN+ has the ease of use and advanced safety systems needed for recreational diving while boasting performance and configurability for expansion into uncompromised technical diving. Upgrades can be made in several stages so that you can choose the pace in which you learn to become a technical diver.

CCR and s-CCR

First off, rebreathers come in either Closed Circuit (CCR) where the air is continuously circulated or semi-Closed Circuit (s-CCR) where air is regularly expelled as bubbles. The SE7EN+ is a CCR as this enables a much higher level of gas-efficiency.

CCRs utilize two gas cylinders: 1 diluent tank (normal air, enriched air, trimix) and 1 oxygen tank. The closed-circuit rebreather will add the exact amount of oxygen needed and will be more efficient and quieter than the semi-closed system.

Semi-Closed Circuit Rebreathers (SCR) only use one gas cylinder with an enriched air content. It usually pushes gas into the loop with a constant flow. The oxymoronically named semi-closed rebreather isn’t as efficient as the closed-circuit rebreather and will need to let gas out from the system every other minute

Closed Circuite Rebreather

Semi-Closed Circuite Rebreather

The Breathing Loop

In CCRs the gas that the diver breaths is circulated in a loop. There are many aspects of the loop to consider. Most importantly, the gas must have the correct composition for the specific depth that the diver is at. Oxygen must be replenished and carbon dioxide must be scrubbed. The loop must also have enough gas for the diver to be able to take a full and comfortable breath. This is achieved by including counter lungs that work as reservoirs for both the inhalation and exhalation air. Water becomes alkaline when mixed with soda-lime, which is used in the carbon dioxide scrubber canister. If the diver suspects a flooded loop, the diver must use the bailout valve, end the dive, and ascend safely.

Gas Management

It comes as no surprise that breathing uses oxygen and produces carbon dioxide. The regulation of these two gases in a rebreather loop is done via two separate processes and is essential in ensuring diver safety.

Oxygen levels in a breathing loop follow a fine balance between two extremes. Obviously, too little oxygen in the loop means the diver will be oxygen deprived and lose consciousness. Therefore, oxygen is added to the loop to maintain a comfortable level. Too much oxygen, on the other hand, leads to oxygen poisoning. The diving depth determines the optimal levels, which in an electronic rebreather is automatically maintained and in a manual rebreather must be controlled by the diver.

The body's waste product from metabolizing oxygen is carbon dioxide (CO2) which must be removed from the breathing loop. This is done with the help of a canister filled with soda lime that "scrubs" the loop clean from the CO2 excess.

The Components of a Rebreather

The heads-up display sits on top of or is integrated into the mouthpiece. The HUD will show the diver that the system is functioning properly or if there are any warnings to be aware of. On the SE7EN+ the HUD is integrated into the mouthpiece.

Head-up display (HUD)

To make sure that the gas in the loop travels in the right direction, a rebreather utilizes one-way valves or mushroom valves. This will prevent the air from simply going back and forth in the loop, and it will instead be channeled around the diver; in through the scrubber and past the oxygen sensors.

One-way valves

If water for some reason gets into the loop, it is important to divert it away from the scrubber. If water reaches the scrubber, an alkaline solution will be created. This solution is commonly known as a caustic cocktail amongst rebreather divers. If this solution travels back to the diver’s mouthpiece, there is a risk that the diver will “burn” his/her lungs from the chemical solution. If the diver suspects a flooded loop, the diver must use the bailout valve, end the dive, and ascend safely.

Water diversion manifold

Human lung volume is generally about 5-8 liters, and when you dive a rebreather this volume of gas needs to be collected somewhere. Therefore, a rebreather has counterlungs that collect the gas that you exhale. Normally a rebreather has two sets of counterlungs: an inhalation side and an exhalation side.


If you need to drain the loop of gas or water, you will be able to do so from the exhalation lungs OPV. The OPV can be adjusted so that you always have the right amount of gas in the loop. You will learn more about this during your Poseidon rebreather diver course.

Over pressure valve (OPV)

The canister housing sits on the diver’s back in most cases and protects the scrubber. On top of the housing is typically where thee-module will reside.

Canister housing

The scrubber is a soda-lime based product that is designed to deduct carbon dioxide from the gas that passes through it. When a scrubber is being used in a rebreather, it is integrated into the loop. It will then absorb the CO2 in the gas that is produced by the diver. This process is called “scrubbing” amongst rebreather divers.


The e-module contains the controlling computers, oxygen sensors, and solenoids. This is where the oxygen content of the loop gets analyzed. If needed, oxygen is added into the loop.


Solenoids or pneumatic solenoids, sit at the end of a low-pressure hose from either the diluent or oxygen cylinder. When the rebreather or the diver detects that the machine needs either more oxygen or diluent in the loop the solenoid is activated. When the solenoid is activated it will inject the respective gas into the loop in small bursts. Whether the diver manually or the rebreather automatically triggers the solenoids depends on if the rebreather is a recreational or technical machine.


The oxygen and diluent cylinders are placed on each side of the canister housing. The oxygen cylinder is primarily used for the diver's metabolism rate and contains 100% oxygen. The diluent cylinder is used for compensating the decreased loop volume upon descent and filling up your buoyancy devices. The Diluent contains normal air in most cases and if you are a technical diver the diluent contains different types of gas mixtures of Trimix.

Oxygen & Diluent Cylinder

The first stage regulators make sure to decrease the pressure of the cylinder to an intermediate pressure that will go to the solenoids for an even distribution into the loop. The diluent gas will also be directed to the mouthpiece if the loop doesn’t contain enough air for a single breath. Then the diluent gas will be added into the loop automatically through the Automatic Diluent Valve.

Oxygen & Diluent first stage regulators

The diluent cylinder on a rebreather is usually between 2-3 liters (14-20 cu ft). This means that if you would need to bailout from the loop and go over to open-circuit mode, you will have very limited time to get to the surface before the diluent supply runs out. The reason for this is that the gas is not being reused anymore, instead, you are now exhaling all gas right into the water. This is why a lot of rebreather divers carry a bailout cylinder. The bailout cylinder has a greater volume than the diluent cylinder and can also be used to fill your dry suit when coldwater diving. The bailout cylinder gives you extended time to make a safe ascent to the surface in a bailout situation.

As soon as you are planning a dive below 18 meters /60 feet most dive agencies say that you should carry a bailout cylinder. A dive down to 30 meters / 100 feet it is sufficient with a 5 liter / 40 cu ft bailout cylinder if the diver is not going to do a decompression stop.

Technical divers need to consider a lot of different scenarios when it comes to reaching the surface in a safe manner if they need to bailout from the loop. They would need several bailout cylinders with different gas mixtures to safely make it to the surface.

Bailout cylinder

The battery or batteries will provide the power for the computer or computers. Different manufacturers use different types of batteries. Some use normal AA batteries and others, like the SE7EN+, use smart batteries, which will store dive logs and contain buddy alarms. Poseidon Smart Batteries come in two different versions that let you get access to different depths by enabling deco-mode and Trimix diving.


When a diver descends, the gas in the loop will be compressed as pressure increases from the surrounding water. After a couple of meters/feet, the gas will be so compressed that the diver won’t have enough gas in the loop to take one full breath. This is when the ADV is activated and makes sure that the diver gets enough additional gas from the diluent cylinder, which is injected directly into the loop. The ADV can be placed on different locations: on a t-valve, counterlung or on/in the mouthpiece. On the SE7EN+ it is placed inside of the mouthpiece.

Automatic Diluent Valve (ADV)

The bailout valve is either attached or integrated into the mouthpiece. When the diver needs to go off or on the loop he/she activates the BOV. The reason for bailing out from the loop is mainly that you have an “un-breathable loop”. Once bailed out the diver will exhale straight out into the water like a normal open-circuit diver. On the SE7EN+ the BOV is integrated into the mouthpiece.

Bailout valve (BOV)

All rebreathers use a display or computer of some sort. Some use an external computer, which the diver will need to purchase themselves and add to the rebreather configuration. Other rebreathers have a display and computer built into the system from the start, like the SE7EN+. The display or computer will show the diver all of the important data they will need to know during the dive and on land.


Keeping the level of carbon dioxide low in the loop is one of the biggest concerns of a rebreather diver. If the carbon dioxide gets too high, a diver can get in a stage called hypercapnia, which means that the level of carbon dioxide in the body is too high and will prevent oxygen from reaching all parts of the body. That is why some rebreathers are equipped with carbon dioxide monitoring systems. If the system is triggered, the diver needs to use the bailout valve, end the dive, and ascend to the surface.

Carbon dioxide monitoring systems that currently exist are temp-sticks and CO2 sensor technologies. The systems are there to sense if you get an elevated level of CO2 in the loop or if the scrubber is starting to lose the capability to scrub.

When the scrubber is actively scrubbing CO2 from the loop, heat is created from the reaction. Having a “temp-stick” running through the canister will allow the diver to register the temperature shift in the scrubber during the dive. When the temperature falls to a lower level, the diver can react appropriately.

When using a CO2 sensor the rebreather will try to find elevated levels of CO2 in the loop. This would indicate that the scrubber is depleted or that the gas in the loop is surpassing the scrubber somehow.

The scrubber will actively be scrubbing CO2 from the loop until the scrubber is depleted. This means that there isn’t a fading effect of the scrubber capability; it will either be active or depleted. The CO2 sensor is designed to warn you when you get a higher level of CO2 inside your system. This means that the sensor can only warn you once the scrubber is completely depleted and then gives you a very short time frame to bail out onto open-circuit before the elevated carbon dioxide levels force the diver into hypercapnia. Because of this, the sensor could give you a false sense of security. A rebreather diver must always know the theoretical amount of time left on the scrubber and plan the dive accordingly.

CO2 monitoring systems

The most important thing when it comes to rebreather diving, except for breathing continuously, is to always know what gas you are breathing. For this reason, you will need oxygen sensors in the loop to analyze the gas that passes through. Depending on what type of rebreather you are using, you will add oxygen manually (technical rebreather) or the machine will add oxygen automatically (technical and recreational rebreather) to make sure that the fraction of oxygen is correct in the loop.

Oxygen sensors

A rebreather mouthpiece connects the diver to the rebreather, so he/she can breathe from the loop.


The loop hoses connect the different parts of the rebreather so the gas can flow through the system.

Loop hoses

The O-rings in the rebreather are very important for the functionality of the rebreather. They prevent water from entering the loop and gas from escaping the loop.