The Complete Guide To Rebreather Systems
At first, rebreather systems can seem a little confusing. The system has two big cylinders with a container in the middle, fixed in a cage! This confusion on how they work is because they use a more complicated system than the traditional SCUBA system.
Today, this guide will introduce you to rebreather systems and how they work!
So let’s get into this system in more detail!
What is a Rebreather System?
A rebreather system recirculates your exhaled air back into the system instead of releasing it as bubbles. The system removes the Carbon Dioxide (CO2) from the exhaled gas and inserts oxygen. This creates the correct air mixture to be rebreathed.
Whether you are an experienced Scuba diver or not, it is commonly known that a traditional SCUBA system releases your exhalation and produces bubbles after each breath. This is known as an open circuit because the air exits the circuit after each breath.
The deeper you dive, the more compressed the air is and the more extreme the loss of air per breath.
The Rebreather system restricts this loss of air by recycling, replenishing then reusing! Instead of releasing the exhaled air, it is kept within the system and recycled!
As mentioned above, a rebreather system is a lot more complicated than the typical SCUBA open circuit! Due to this higher level of complexity, I suggest becoming experienced at regular SCUBA before attempting rebreather training.
Diving with a rebreather demands a higher level of knowledge, skill and awareness of the situation. In order to achieve this, divers are required to fully understand the system and practice all the operational skills required in the event of a fault that needs to be recovered from.
This is extremely impressive training, ensuring the rebreather diver can recognise, analyse and recover from every single possible issue that could occur when underwater with a rebreather system.
There are two different types of rebreathing systems, semi-closed and closed-circuit rebreathers.
What is a Semi-Closed Rebreather?
Semi-closed rebreathers expel some gas from the system after a few breaths. Therefore it is not a fully closed system, even though most of the air remains in the system and is recycled.
Where the gas is vented depends on the rebreather itself. It could be from the back of the system, from a vent on the top or from the mouthpiece itself. These systems are slightly cheaper than closed-circuit rebreathers, but can still increase your no decompression time at depth.
Semi-closed systems have a mixed gas supply that is breathed from. One benefit of this system is that the divers can go to greater depths without the risk of suffering oxygen (O2) toxicity.
What is a Closed Circuit Rebreather?
Closed circuits keep all the gas within the system, therefore is completely dependent on the scrubber. No gas is expelled here and it is all recycled. Closed-circuit rebreathers have both pure Oxygen (O2) and mixed gases to maintain the oxygen concentration.
How does a Rebreather System Work?
During a dive, the air that is exhaled travels back through the system, through a scrubber. CO2 is removed by the scrubber and O2 is replaced by being inserted from an O2 tank. The gas reaches the desired mix and is then rebreathed.
This hugely differs from conventional SCUBA systems, known as an open system. Once taking a breath from your tank, you exhale and the gas is released as bubbles. A rebreather recycles this exhalation and no bubbles are produced.
The scrubber is required to remove the CO2 from the exhaled breath. Removing CO2 is extremely important as CO2 in high volumes is poisonous to our bodies.
If the rebreather system failed to remove the CO2 in the exhaled breath, the concentration of CO2 will build up and the O2 concentration will go down. The increased CO2 inhaled will cause a stress response in the brain, leading to a higher breathing rate and a state of panic.
The scrubber is therefore extremely important for removing the CO2 from the exhaled breath.
In the case of a scrubber failure, if you notice it in time, you can use the bailout system. This is where you remove the rebreather system and breath with a regulator from a cylinder of a gas mix. This allows you to breathe while you ascend to the surface.
So we know how important it is for the scrubber to remove CO2 from the rebreather system, but how do they do this?
One of the main types of scrubbing processes uses a canister of sodium hydroxide (NAOH2). The CO2 and NAOH2 react to produce solid Calcium Carbonate (CaCO3). This removes the CO2 from the system.
Another method to remove CO2 uses crushed limestone. Rebreather divers compact this limestone into the scrubber container, then place it into the system.
During the dive, the CO2 gets absorbed by the limestone, and the O2 continues to move through the system. The limestone and absorbed CO2 begin to move up the scrubber container. Once it reaches the top, the scrubber needs to be replaced.
Using the crushed limestone method, scrubbers can easily be changed by emptying the used limestone and replacing it with fresh limestone. Rebreather divers need to ensure they compact the limestone well and keep it dry! (This is very important!)
Depending on the rebreather system, a small cylinder of either pure O2 or a mixture is injected into the system to replace the O2 used.
The concentration of O2 needs to be monitored by sensors, to ensure the partial pressure of oxygen remains constant. These sensors control the amount of O2 inserted, to ensure the correct partial pressure of O2 is always maintained.
What are the Benefits of the Rebreather System?
Benefits of rebreather systems include increased gas efficiency (resulting in longer dive times), it is lighter, there is a reduced risk of the bends and no bubbles are produced! The air you breathe from a rebreather is more moist than air from a regular tank, eliminating the post-dive dry mouth some people experience.
Gas efficiency is improved as the gas exhaled is recycled instead of released as bubbles! Therefore less air is required for a longer dive!
Another advantage is that rebreather systems can provide the same dive duration as twin cylinders, but without the weight of the heavy cylinders. This is great, as you can have an amazing long dive, without the weight or discomfort of twins on your back.
Diving with a rebreather also increases comfort, as the air breathed in is warmer and moister, and breathing does not affect buoyancy. This avoids that dry mouth feeling you often experience with recreational open-circuit SCUBA.
As the amount of nitrogen in the rebreather system is very small, the risk of “the bends” is reduced. This, therefore, means divers can remain at depth for longer, having a longer no-decompression time!
Who wouldn’t want to dive deeper for longer!?
Staying down for longer with less chance of going into deco is awesome, allowing you to see more marine life and explore the underwater world more!
Depending on the gas mix, your dive time at 20m could almost quadruple in time.
Another amazing thing about closed-circuit rebreathers is that no bubbles are produced! This is awesome as it does not scare marine life. You no longer feel like an intruder in their world, creating noise pollution and scary bubbles.
Marine animals sometimes come really close as they are intrigued as to what the rebreather diver is! As you do not produce bubbles, it’s as if you belong in their world! Sharks have been known to come really close, trying to figure out what this unknown creature is!
Rebreathers were originally designed for the military, to reach great depths and stay down for longer durations. However they are now widely used within recreational and commercial diving, but very intense training is required.
How Much Does a Rebreather System Cost?
The minimum cost of a Rebreather is $5,000 but prices reach much higher depending on the rebreather. They are very expensive due to their complexity and are often built specifically for the individual diver.
Although these systems are very expensive, packages are available which come with an extensive list of equipment. This includes the scrubber, a wing BCD, pressure gauges and tanks with their valves.
Rebreathers also come with their own built-in computer, which is already around £1,000 for a high-quality technical dive computer. Packages come fully complete, which is why they are so expensive. You will be required to assemble the package, which when doing so, ensure you follow the instructions meticulously.
Other expenses that are not included in the package are things such as batteries for the oxygen sensors. These sensors continuously measure the set partial pressure of the gas, inserting oxygen when needed.
Rebreather systems are very expensive, but the price does not stop there!
A massive cost that needs to be implemented is the training. Rebreather training is very pricey, as the system is so complicated the training needs to be intense.
As mentioned before, there are more aspects of the system which could go wrong, so a lot of training needs to be completed to get yourself out of any bad situation.
It is therefore crucial that you only begin your Rebreather journey when you are an experienced diver and fully prepared to invest a lot of time and money into equipment and training.
How Long Should a Rebreather System Last?
Rebreather systems are built to last, therefore they should last for quite a few years. However different components of the rebreather system will need to be replaced at different times. The scrubber will last around 2-5 hours, depending on the dive and the conditions.
The scrubber only lasts a few hours, and this is dependent on the following factors: breathing rate, water temp, depth and care of the scrubber.
For example, if the diving conditions or depth cause your breathing rate to be high, you will release more carbon dioxide and the scrubber will have a shorter life span.
When you return from the dive, it is important to dry out the canister and make sure the scrubber has not been soaked through. If your scrubber gets wet before or during the dive, the diver will be breathing in toxins that are produced by the scrubber reacting with water.
These toxins are corrosive and create acid, which can destroy your lungs.
To prevent this, ensure you store the scrubber in a cool dry place. This is why Technical diving rooms, especially in the tropics, are always inside with the air-conditioning on to prevent heat and humidity.
When the scrubber is filled in preparation for the next dive, it is recommended to always follow the manufactures instructors perfectly. This ensures the scrubber is filled correctly.
Scrubbers should be used within a couple of days of your last dive. You need to change your scrubber if it has been left a few days, or if it has been used for its maximum use. To empty the limestone scrubber, dump all the limestone into a bin, then dry the inside.
You need to make sure you take extremely good care of your rebreather system. Not only are they very expensive, they are complicated, increasing the opportunities for something to be faulty.
Therefore you need to ensure you take the correct measures to keep your rebreather system in perfect condition, to minimise the risks of things breaking underwater.
How to take care of your Rebreather system.
- After a dive, externally rinse the system.
- Break the system down, making sure there is no water inside.
- Keep all the components of the system as dry as possible at all times when outside of water.
- Always make sure the electronics are dry. These are very expensive and extremely crucial to the workings of the system.
Don’t Forget Your Dive Insurance!
Before you go out on any dive trip or holiday, it is essential to make sure you have insurance that covers you if something goes wrong. Check out our dive insurance article for more information.
Or go straight to these dive insurance company websites:
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Conclusion
Rebreather systems recycle divers exhaled air, removes the carbon dioxide and inserts oxygen to the pre-set partial pressure of oxygen. These systems are very complicated, therefore are very expensive!
Their complexity requires intense training, however once mastered, deeper and longer dives on rebreathers are incredible!
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