What is a rebreather? Do I need one as a recreational diver?

In open circuit the bubbles that are exhaled into the water column is basically wasted gas. Let’s try to conserve this wasted gas by creating a more efficient system that recycles the exhaled bubbles. The apparatus that does this is called a “Rebreather”.

How does a rebreather work?

Each breath we inhale contains 21% oxygen. This oxygen is vital to our cellular function and to get energy from nutrients. Thus, we cannot live without it. However, only part of the oxygen is used in each breath, and the rest is breathed out. As a product of metabolism, we exhale carbon dioxide (CO2), as well as water vapour, trace gaseous compounds (e.g., Argon), oxygen (O2), and nitrogen (N2). So, we exhale a mixture that contains 16% oxygen and 4% carbon dioxide. Essentially, nearly 96% of the gas that we breathe is wasted when we exhale using OC SCUBA.

Figure 1 Inhale and Exhale

The Rebreather is a closed-circuit breathing apparatus that recycles exhaled air. The accumulated carbon dioxide exhaled gas is removed by a chemical scrubber, in which a simple chain of chemical reactions takes place, resulting in a moist and warm CO2-free gas. The consumed oxygen is replenished from a separate oxygen cylinder by being injected into the breathing loop. Figure 2: Oxygen Closed Circuit Rebreather – 1. Inhale counterlung: this is a flexible bag that the diver inhales gas from; 2. Dive/Surface valve (DSV): valve that directs the gas flow in the loop and opens/closes for the diver to breathe; 3. Exhale counterlung: this is a flexible bag that the diver exhales into; 4. CO2 scrubber: this absorbs carbon dioxide from the exhaled gas; 5. Oxygen supply cylinder; 6. Oxygen reduction valve; 7. Constant flow nozzle: this feeds oxygen into the loop at a set rate 8. Manual addition valve (MAV): Valve that allows the diver to add oxygen and adjust the loop volume; 9. Oxygen submersible pressure gauge (SPG).

How the oxygen is replenished defines the type of rebreather. There are simple oxygen rebreathers for diving up to 6 m (20 ft), semi-closed rebreathers, manually controlled rebreathers, and finally electronically controlled rebreathers. The latter are the most comprehensive of all the rebreathers and provide the most versatility.

Perhaps one of the most important components of any rebreather are the oxygen sensors. Oxygen sensors measure the amount of oxygen in the breathing loop, and if the oxygen drops below a set value, the computer opens a magnetically controlled valve (solenoid) and injects a precise amount of oxygen that needs to be replenished. The diver monitors the correct functioning of the device by using the display on the handset. Should anything go wrong, the device will notify the diver with an alarm.

Figure 3 Electronically controlled Closed Circuit Rebreather (eCCR) – 1. Inhale counterlung: this is a flexible bag that the diver inhales gas from; 2. Dive/Surface valve (DSV): valve that directs the gas flow in the loop and opens/closes for the diver to breathe; 3. Exhale counterlung: this is a flexible bag that the diver exhales into; 4. CO2 scrubber: this absorbs carbon dioxide from the exhaled gas; 5. Oxygen supply cylinder; 6. Oxygen reduction valve; 7. Constant flow nozzle: this feeds oxygen into the loop at a set rate 8. Oxygen Manual addition valve (MAV): valve that allows the diver to manually add oxygen if needed; 9. Oxygen SPG; 10. Diluent supply cylinder, 11. Diluent reduction valve; 12. Diluent SPG; 13. Automatic Diluent Valve (ADV): valve that maintains the volume of the breathing loop; 14. Diluent Manual Addition Valve (MAV): valve that allows the diver to manually add diluent and also to control the volume of the breathing loop; 15. Oxygen sensors: critical sensors that measure the oxygen level in the loop; 16. Control unit (CU): this will read the oxygen sensors, calculate the amount of oxygen to be added, and thus controls the solenoid valve; 17. Solenoid: this injects oxygen into the loop according to CU data; 18. Handset: the display informs the diver of the loop pO2 and other important data.

Breathing 100% oxygen beyond a depth of 6 m (20 ft) is fatally toxic to the human body. Therefore, for a diver to dive to deeper depths, it will be necessary to dilute the oxygen with a safe breathing gas. So, we use a second cylinder, which contains a diluent gas on the rebreather. The diluent gas is added to the loop to 1) reduce the partial pressure of the oxygen, and 2) maintain the volume of the breathing loop. As the diver descends, the increased pressure compresses the gas necessitating an increased volume of gas in the loop to ensure that the diver can take a full breath. The diluent gas that is used is either compressed air, or TRIMIX or heliox, that is used to dive to deeper depths.

Rebreather Gives Us Liberty

So, what are the advantages of a rebreather? The answer is many. To begin with, the actual gas consumed on a dive is minimal compared to OC SCUBA. Using classic SCUBA apparatus the gas that is exhaled is lost to the water column, whereas with a rebreather the gas that is consumed is approximately 1-1.5 litres of oxygen per minute. The point is that whatever the depth, the tissues in the human body still need the same amount of oxygen no matter what the ambient pressure is.

The oxygen requirement remains the same whether the diver is at 10 m (33 ft) or 100 m (329 ft). The diluent is practically consumed on the descent. An experienced diver can efficiently manage gas consumption to an absolute minimum. For a dive to a depth of 40 m (131 ft), about 60 - 100 litres of oxygen will be needed, and similar volume of diuent

Consumption (SAC 22 l/min)

• 10 m (33 ft) - OC SCUBA: 44 l/min, Rebreather: 1.5 l/min
• 20 m (65 ft) - OC SCUBA: 66 l/min, Rebreather: 1.5 l/min
• 30 m (98 ft) - OC SCUBA: 88 l/min, Rebreather: 1.5 l/min
• 40 m (131 ft) - OC SCUBA: 110 l/min, Rebreather: 1.5 l/min
• 50 m (164 ft) - OC SCUBA: 132 l/min, Rebreather: 1.5 l/min

SAC - Surface Air Consumption. Also known as Respiratory Minute Volume (RMV)

Thanks to the oxygen and dilution systems, the diver always has the most suitable mixture for a given depth. Rebreathers, simply put, are a blending station on your back. Whether spending time on the bottom, ascending or decompressing, the diver always has the optimal gas - made-to-measure, if you will - which can significantly increase no-decompression times or reduce any decompression incurred on a dive.

So, how long can you stay underwater with a Rebreather? The most limiting factor is the CO2 scrubber which can last for 4-8 hours independent on the depth. As I described before the gas is not a limit in this case.

How effective is a rebreather?

Let's take a specific example. Three divers, A, B, & C, are diving on a magnificent wreck that lies at a depth of 30 m (98 ft) in a tropical sea. Diver A is using recreational SCUBA equipment (air: 12 litre cylinder), Diver B is using OC technical diving equipment (air: twinset of 2 x 12 litre cylinders), and Diver C has an electronically controlled rebreather with air diluent. How long can each diver explore the wreck if they have to return to their boat within one hour?

Diver A. This diver can spend as little as 20 minutes on the wreck. After that they must begin their ascent to ensure that they have enough gas in a 12-litre cylinder (consumption rate of 22 l/min and a reserve of 30 bar). Diver A spends a total of 18 minutes on decompression. That's almost the same time as on the wreck. This diver will return to the surface support vessel after 38 minutes .

Diver B. This diver is better off. This is because they are wearing a heavy twinset. Therefore, their gas reserve is sufficient for a full hour of diving with 70 bar left by the time they get to the surface. But to make it back on board in time, they can only enjoy 26 minutes on the wreck, because a tiring 34 minutes of ascent and decompression lay ahead. That's assuming that Diver B is using compressed air. If they are a little smarter, then they can extend their bottom time by up to 9 minutes by using NITROX.

Diver C This diver doesn't have to worry very much about their gas supply. In fact, Diver C will have enough oxygen in reserve for up to six dives, and possibly enough diluent depending on excursion during the dive. They will typically enjoy the wreck for 45 minutes, and the ascent with decompression will take only 15 minutes .

So, Diver C be on the wreck 27 minutes longer than Diver A, and 19 minutes longer than Diver B. Note that the decompression is still 3 minutes shorter than Diver A, and 19 minutes shorter than Diver B. Diver C will return to the ship with Diver B.

However, while Diver B now has to fill or change cylinders for the next dive, the rebreather diver now calmly leaves the apparatus as it is and goes off for a snack. The same will be true between the second and third, or third and fourth dives on the same day. In the evening, Diver C simply unplugs the rebreather head to let the oxygen sensors dry out, recharges any batteries, tops up the onboard cylinders and changes the scrubber as appropriate, and tomorrow will off to a new start.

If something unexpected happened to our three divers A, B, or C on the wreck, the first would be very limited in time to resolve the situation, and the second would be only slightly better off. The third diver with a rebreather would have literally hours to resolve the situation, even if they started breathing very fast because of the stress. In such a situation, ventilation can jump to 50-70 litres per minute. Try calculating how fast you lose air at 30 meters in such a situation! In this situation, using a rebreather, would have comparatively little effect on your gas supply.

How deep can you dive with a rebreather?

The truth is that rebreathers, especially the eCCRs, were made for deep diving. Until you get enough practice, you will only dive within recreational limits with air to a maximum depth of 40 metres (131 ft). The use of helium mixtures will greatly expand your options and it will all depend on your experience, physical and physiological principles. For moderately experienced divers, a dive to 100 m (328 ft) is not difficult. Dives below 120 m are only for very experienced divers and exceptionally the CCR can be used to depths of over 300 m (984 ft). As depth increases, the number of complex factors affecting the human body increases. The greatest depth limitation is the density of the gas being breathed. At great depths, the density is such that one cannot ventilate sufficiently and CO2 accumulates dangerously in the body.

I won't be so cold with a rebreather

So now we know that even recreational divers’ benefit from a rebreather, and that we don't even have to be technical divers! The savings on gas are incredible. Nonetheless, the benefits don't end there. When the CO2 is removed from the exhaled gas, a chemical reaction creates heat as well as moisture, therefore, we breathe significantly warmer gas than with an open circuit.

You won't be able to tell at first, but if you did the same test as I did, you would see a major difference in the retention of your own heat during the dive. I conducted two identical dives in a cold lake in one day. The water was 4°C, the dives had the same depth of 20 m (60 ft), time 45 minutes and path, I wore the same dry suit and undersuit on both dives. The difference was that the first dive was done with a classic SCUBA open circuit and the second with a rebreather.

During the first dive, I started to feel the cold after 30 minutes of the dive and by the end I was already thoroughly chilled. After about an hour break, I put on the rebreather and did an identical dive. Even though it was the second divethe cold feeling did not come until the very end of the dive, so I walked out of the water with a smile on my face.

Closer to nature

Another advantage that nature lovers, photographers, and even scientists particularly appreciate is the absence of bubbles. We don't exhale the gas into the water, but back into the loop, so we don't make any disturbing noise underwater. Thanks to this, you'll find that aquatic animals will take you in their stride, and let you come within an unexpectedly short distance and may even come to investigate you!

Photo by Kristof Goovaerts - IG, diver PJ Prinsloo

Comparison of rebreather (closed circuit) with standard SCUBA (open circuit)

Rebreather - closed circuit
Exhaled gas is returned to the system
No bubbles disturbing the underwater life around you
Minimal gas consumption
Oxygen consumption is independent of depth
Minimal decompression
Enough time to deal with emergency
Chemical reaction of the absorber saves body heat
Minimal maintenance between dives for repetitive dives
More demanding preparation and testing
High acquisition cost

SCUBA - open circuit
Exhaled gas is lost to the surrounding water
Bubbles chase away underwater life
High gas consumption
Gas consumption is multiplied by pressure
Long decompression (unless more gases are used for accelerated decompression)
Very limited time to deal with emergency
Inhaled cold expanded gas contributes to loss of body heat
Need to change or refill cylinders on repetitive dives
Easy preparation and checking
Relatively low acquisition cost

• Jakub Šimánek, Divesoft Factory IT