In chapter 18 you met open circuit (OC): you breathe from a cylinder through a regulator, and every breath you exhale goes into the water as bubbles. That wastes most of the oxygen. The air you breathe out still holds about 16% oxygen, against 21% in the air you breathed in.
Your body uses oxygen at a rate set by how hard you work, not by depth. At depth each breath holds more gas, so more of it is wasted.
A rebreather keeps the gas you breathe out. It removes the carbon dioxide, adds oxygen to replace what your body used, and lets you breathe the same gas again. This chapter shows how the loop works and the main kinds of rebreather.
- Loop gas
The breathing loop
You breathe through a mouthpiece joined to a closed loop of hoses. One-way valves in the mouthpiece send the gas one way round the loop. So you never breathe your last breath straight back.
2 flexible bags, the counterlungs, hold the gas. The exhale counterlung fills as you breathe out. The inhale counterlung empties as you breathe in.
The scrubber removes carbon dioxide
Your body takes oxygen out of each breath and adds carbon dioxide. Even a few percent of carbon dioxide causes hypercapnia (carbon dioxide build-up): breathlessness, headache and confusion.
The gas you breathe out passes through a canister of absorbent, the scrubber. The absorbent reacts with the carbon dioxide and holds it. Modern scrubbers use granules of soda lime. Because the carbon dioxide leaves the loop, the loop slowly shrinks. Think of it like a kitchen extractor fan that cleans the air in a closed room, so the same air can be breathed again.
Oxygen rebreathers
The simplest way to refill the loop is with pure oxygen from a small cylinder. Before the dive you flush the loop with oxygen, so it holds oxygen only. Your body uses the oxygen and the scrubber removes the carbon dioxide, so no gas needs to leave the loop.
Oxygen rebreathers are small and make no bubbles. Their limit is depth. The PO2 (partial pressure of oxygen, also written ppO2) is the pressure of the oxygen alone in the gas you breathe. It is given in ATA (atmospheres absolute, about the same as bar); the surface is 1 ATA and each 10 m of sea water adds 1 more. On pure oxygen the PO2 equals the ambient pressure, the total pressure around you. At 6 m that is 1.6 ATA, which is already the usual upper limit.
Semi-closed rebreathers
A semi-closed rebreather (SCR) adds a steady flow of nitrox (air with extra oxygen) to the loop through a small fixed hole. The flow brings more oxygen than your body uses. The extra gas leaves through a vent on the counterlung.
The oxygen in the loop is lower than in the supply gas, and it changes with how hard you work. Nitrox has less oxygen than pure oxygen, so you can dive deeper than on an oxygen rebreather.
Electronic closed-circuit rebreathers
An electronic closed-circuit rebreather (eCCR, a kind of CCR) measures the oxygen in the loop with 3 oxygen sensors. The setpoint is the PO2 you choose to breathe, for example 1.3. When the PO2 falls below it, a controller opens a valve, the solenoid, to add oxygen.
A second cylinder holds the diluent, usually air or trimix (a mix with helium). The unit adds diluent to keep the loop full as you descend, and it makes up the rest of the mix. Almost no gas leaves the loop, and you breathe the same PO2 at any depth.
The first rebreathers
In 1878 Henry Fleuss patented the first practical self-contained rebreather. It used pure oxygen and a scrubber of rope yarn soaked in caustic potash. In 1880 the diver Alexander Lambert used one in the flooded Severn Tunnel to close a sluice door that divers on air hoses could not reach.
In 1910 Robert Davis designed the Davis Submerged Escape Apparatus, an oxygen rebreather for escaping from a sunken submarine. The Royal Navy adopted it in 1927.
Military oxygen rebreathers
An oxygen rebreather makes no bubbles, so it suited divers who must not be seen. In the Second World War, Italian, British and German navy divers all used them. In the United States, Christian Lambertsen patented one in 1940, later known as the LARU (Lambertsen Amphibious Respiratory Unit).
Wartime diving also showed the danger of oxygen. Divers on pure oxygen had CNS oxygen toxicity: convulsions (fits), sometimes with no warning. Kenneth Donald tested many navy volunteers and found that the time before symptoms varied widely, even in the same diver on different days. Chapter 6 covers it.
Semi-closed and electronic rebreathers
Navies then built semi-closed sets that used nitrox, so divers could go deeper. The first semi-closed rebreathers for sport divers went on sale in 1995.
In 1968 Walter Starck and John Kanwisher designed the Electrolung, the first electronically controlled mixed-gas rebreather to be sold. It had 3 oxygen sensors and used the readings that agreed. It was costly and complex, and it was involved in several fatal accidents.
The first production eCCR for sport divers went on sale in 1997. Modern units still work the same way: 3 sensors, a controller, a solenoid and a diluent. Chapter 20 compares gas use and decompression on a rebreather with open circuit.
Try it
Move the depth. On an oxygen rebreather the PO2 rises by 1 ATA for every 10 m. An eCCR holds its setpoint. Pure oxygen passes the working PO2 of 1.4 and the contingency PO2 of 1.6 within the first few metres. You can check the depth limit of any mix, its MOD (maximum operating depth), with the MOD calculator.
- Pure oxygen
- eCCR, setpoint 1.3
- 1.4 and 1.6 bar
- ppO₂ on pure oxygen
- …bar
- Pure oxygen reaches 1.4 bar at
- …m
- Pure oxygen reaches 1.6 bar at
- …m
Planned with DiveLogicOpen the MOD calculator
- Oxygen rebreather
- Semi-closed
- Electronic closed circuit
What to remember
- A rebreather lets you breathe the same gas again. A scrubber removes carbon dioxide, and oxygen is added to replace what your body uses.
- The counterlungs hold the gas, and one-way valves send it one way round the loop.
- An oxygen rebreather breathes pure oxygen, so its PO2 equals the ambient pressure. So it is for shallow depths only: it reaches 1.6 ATA at 6 m.
- A semi-closed rebreather adds a steady flow of nitrox and vents the extra gas.
- An eCCR uses oxygen sensors and a controller to hold a constant PO2, the setpoint. Diluent makes up the rest of the mix.
Rebreather dives in the planner
DiveLogic plans closed-circuit dives with the setpoints and diluent you choose, including bailout to open circuit. The diluent tool checks a diluent’s PO2 and gas density at the depths you plan.
Sources
- Davis R. H. Deep Diving and Submarine Operations. Siebe Gorman, London (editions from 1935). (Fleuss apparatus and the Davis Submerged Escape Apparatus.)
- Donald K. W. (1947). Oxygen poisoning in man. British Medical Journal 1: 667-672 and 712-717.
- The Electrolung: the first mixed gas rebreather was available to sport divers in 1968. InDEPTH magazine, Global Underwater Explorers.
- US Navy (2016). US Navy Diving Manual, Revision 7. Naval Sea Systems Command. (Closed-circuit oxygen and mixed-gas rebreathers.)