What is a rebreather?
The concept of a rebreather has been around since time began – in fact everyone (without exception) has experienced a rebreather of sorts. How? The planet has a giant rebreather system that we give no thought to at all. Why? Easy! As we inhale we breathe in air, a mixture of oxygen (21%), nitrogen (79%) and a few other gases. We metabolise some of the oxygen (about 4%) and exhale the remainder; that’s why CPR works so well at sustaining the brain in a cardiac incident. Whilst exhaling the by-product of metabolism, carbon dioxide, is released. The exhaled carbon dioxide is absorbed by plants etc through the process of photosynthesis, the result of which is oxygen. Lucky for us! We then re-breathe some of the exhaled oxygen we didn’t use and also the oxygen produced by the plants. Perfect!
So what about rebreathers for diving?
Giovanni Borelli (1860) illustrated a giant leather bag utilizing chemical components, which he suggested should allow the air to be breathed again by a submerged diver. Apparently the device was never made, but his design was the first to conceptualise a diver as we think of them today. Since then there has been much development in this field to arrive at the choice of rebreathers we have today.
A rebreather is a very simple contraption in its basic form, nothing more than the bag that Borelli depicted all those years ago. With a bag to breathe in and out of (counterlungs), this raises the question of oxygen starvation (hypoxia) as we metabolise 4% of the oxygen each time we take a breath. So the introduction of oxygen replenishment is required, this can be by a direct injection of pure oxygen in a passive or active method as in the case of CCR (Closed Circuit Rebreather) or by the replenishment of larger amounts of gas (usually nitrox) as in the case of SCR (Semi-Closed Rebreather). With the metabolism of oxygen, comes the production of carbon dioxide and elevated levels of carbon dioxide in a sealed breathing environment like the counterlungs of a rebreather can lead to hypercapnia a dangerous condition that will be discussed in a later article. Carbon dioxide is removed using a container filled with a chemical absorbent referred to as the “scrubber”.
Rebreathers are divided into three categories which are broadly separated by the way that gas is added to the loop (counter lungs, scrubber and mouthpiece) and the control of the gas mixture the diver breathes.
An oxygen rebreather is the most basic of the three types; a cylinder of pure oxygen feeds gas into the loop as the oxygen is metabolized by the diver. The amount of oxygen required by the diver varies depending on activity. During periods of inactivity the body requires much less oxygen than during periods of heavy work. This is why we breathe heavily during exercise; to facilitate greater amounts of oxygen transfer through the lungs. From a design standpoint, oxygen rebreathers are very simple because they do not require a complex control system. However, they are also extremely limited in function because the potential for CNS Toxicity (too much oxygen) dives to depths in excess of about 20 feet/6 meters. Oxygen rebreathers are not commonly used by recreational divers, and are no longer used in military operations from which they originate for this reason. In order to use rebreathers deeper than this, the gas mixture in the breathing loop must contain a gas or gases other than pure oxygen (e.g. nitrogen or helium). Mixed-gas rebreathers usually come in one of two forms: semi-closed rebreathers and closed-circuit rebreathers.
Semi-Closed Rebreathers (SCR)
This is the first type of mixed gas rebreather of which there are two different types; active and passive addition systems. The most common are active-addition systems, which are commercially available both to hire and buy in many locations across the world. It is mechanical in operation; with gas constantly flowing into the breathing loop. This constantly replenishes the gas within the loop to counteract the lowering oxygen levels brought on by metabolism. The flow rate of the gas is adjusted according to the fraction of oxygen in the supply gas. This ensures that the rate of oxygen in the breathing loop meets or exceeds the rate at which the diver metabolises oxygen; this is set to maintain a constant fraction of oxygen. The other gases in the loop; generally nitrogen or helium, are not metabolised, and as the volume of these constituents increases. For this reason, the semi-closed rebreather vents gas periodically to reduce the volume of gas in the breathing loop.
Closed-Circuit Rebreathers (CCR)
This type of rebreather is becoming more popular and commercially available. A closed circuit rebreather uses at least two gas supplies, one of which is oxygen. The other is most commonly air, trimix, heliar or heliox (referred to as diluent). As with the other types it has a breathing loop consisting of a scrubber, breathing hoses and counterlungs. Unlike SCR, which is a constant fraction of oxygen, CCR maintains a constant partial pressure of oxygen. This can be done in a number of ways, computer controlled, constant flow and manual addition.
Computer controlled – the computer software takes the measurements from the sensors and if the level is too low a solenoid opens to allow more oxygen to the breathing loop.
Constant flow – the oxygen flows into the loop at a rate which closes matches the metabolism rate of the diver. If the rate is too fast, manual intervention is required to stop the flow of gas into the loop (turn of the tank valve). If the rate is too slow manual intervention is required in the form of manual addition (usually a low pressure inflator similar to a BCD inflator).
Manual addition – just like it says! The diver must manually add oxygen to the breathing loop throughout the dive, as above with something similar to the BCD inflator.
So why dive a rebreather?
Ask 10 different rebreather divers and get 10 different answers! Each type of rebreather has advantages and disadvantages to diving over open circuit scuba and each other.
Advantages and Disadvantages of Rebreathers versus SCUBA
Gas efficiency; economical for expensive gases
Reduced Decompression obligation
Silence; diver can get closer to marine life and in over head environments there is a lesser chance is dislodging silt.
Cost; generally more expensive to purchase.
Discipline; rebreathers are more complex to operate and as such require additional training. With some manufacturers refusing to sell to anyone who has not attended a training course. There are also many more failure modes than open circuit, which is generally working or not!