Respiration is designed to provide our tissues with oxygen and to eliminate carbon dioxide produced by metabolism. When we breathe on the surface, we consume about 25 per cent of the oxygen that we inhale with each breath. Thus, if our respiratory minute volume (RMV) were to be 20 lpm, we would breathe in 4 litres of oxygen each minute, of which 1 litre would be consumed and 3 litres would be exhaled back into the surrounding atmosphere. Although this may not seem very efficient, the situation becomes substantially worse when we descend on open-circuit scuba equipment.
As the depth and pressure increase, the amount of gas we inhale with each breath must also increase to compensate. Thus, at 40 metres (5 ATA), we would need to breathe 100 lpm from our cylinder to achieve the same 20 lpm surface RMV. This 100 litres of air would contain about 20 litres of oxygen, of which 19 litres are being exhaled into the ocean unused!
One solution to this inefficiency of gas consumption is to recirculate the gas, removing the carbon dioxide and adding only the oxygen that is consumed by the diver back into the circuit. This is called a ‘rebreather’, and such breathing apparatus can offer substantial reductions in gas consumption over open-circuit systems. The following is a summary of some advantages and disadvantages of rebreather systems, which are expanded upon in the following paragraphs:
- Vastly reduced gas consumption, especially during deep diving.
- Reduction of cold stress and dehydration by the breathing of warm, humidified gas.
- Lack of bubbles good for photography, covert operations, fragile environments such as caves.
- Improved decompression efficiency because of maintenance of ‘optimal PO2’.
- Excellent duration in relatively small unit.
- Significant initial cost.
- Greater complexity and vastly increased need for training, vigilance and maintenance.
- Different hazards to diver, higher overall risk.
Rebreathers fall into one of two main types – closed-circuit rebreathers (CCRs) and semi-closed- circuit rebreathers (SCRs). Although both types recirculate all, or part, of the breathing gas, the main difference lies in the way that the oxygen level is controlled and added into the circuit.
In general, SCRs are less complex but less efficient and have depth limitations dependent on the gas selection. CCRs are the most complex but also the most efficient and most capable with regard to depth and duration.
Because of the similarity between SCR and CCR sets, their common features are discussed first, and features peculiar to each type are then highlighted separately.
The usual gas flow pattern found in a rebreathing set is shown in Figure 4.5. The movement of inhaled and exhaled gas is controlled by one-way valves at the mouthpiece as the gas flows round the circuit. For largely historical reasons, rebreathers of UK or European origin usually have a clockwise gas flow pattern, whereas those of US origin have an anticlockwise pattern. However, this is not universally so.
As the diver descends, gas must be added from a high-pressure cylinder into the breathing loop so that a constant volume is maintained within the system. In most units, the gas is automatically added via a regulator-type valve (automatic diluent valve [ADV]). A manually controlled valve allows the diver to add extra gas if it is required. This addition of gas will affect buoyancy.
The counterlung acts as a gas storage bag that expands and contracts as the diver breathes. It normally incorporates a relief valve (over-pressure valve [OPV]) that releases surplus gas into the water and prevents excess pressure building up. Venting of excess gas is needed in CCR sets when the diver ascends and the gas in the counterlung expands. In SCR sets, excess gas vents regularly through the relief valve.
The carbon dioxide absorbent is usually a mixture of calcium and sodium hydroxides. These chemicals react with carbon dioxide to form carbonates and water, as shown:
Closed-circuit oxygen systems are the simplest CCR sets. The counterlung is filled with oxygen from the cylinder. As oxygen is consumed, the volume of the bag decreases. In some units, a trigger mechanism that operates like a demand valve releases more gas into the bag. In other units, there is a mechanism that releases a continuous flow of oxygen into the circuit. A manually operated method of adding oxygen to the breathing bag is also usually fitted. This will be needed when the diver puts the unit on, when he or she goes deeper and the gas in the breathing bag is compressed, or when the diver needs to increase buoyancy.
The unit can be operated as a closed system because, unless something goes wrong, the gas in the breathing bag will contain a high concentration of oxygen, diluted with nitrogen that was in the lungs and body of the diver when he or she put the unit on. It is standard practice to flush the counterlung with oxygen at set intervals to ‘denitrogenate’ before starting the dive to prevent a build-up of diluting gases.
Possible problems with these units include carbon dioxide toxicity if the absorbent fails, dilution hypoxia if the oxygen is impure or the diver neglects to flush nitrogen from the lungs and the counterlung and oxygen toxicity if the diver descends too deep.
To reduce the risk of oxygen toxicity, a depth limit of about 6 to 8 metres is often imposed on the use of these units to limit the PO2 to 1.6 to 1.8 ATA, a range generally deemed acceptable for military operations, although too high for recreational technical diving, where a lower risk is appropriate and consequently a PO2 significantly lower than 1.6 ATA is usually maintained.
Closed oxygen rebreathing apparatus has the particular advantage that a small unit may give a long endurance. A unit weighing less than 15 kg can allow dives of more than 2 hours. The lack of bubbles and quietness of this unit are also important in some specialized roles such as clandestine operations.
Rebreathing units are quieter and have a greater endurance than scuba units. The extra hazards and costs involved restrict their use and demand significant extra training, maintenance and vigilance.
In closed-circuit mixed gas systems, oxygen and a diluting gas are fed into the breathing loop at rates required to keep the PO2 within safe limits and to provide an adequate volume of the mixture.
Figure 4.6 shows the fundamental features of this system.
As with the closed-circuit oxygen unit, the diver inhales breathing gas from the counterlung and exhales through the carbon dioxide absorber back into the counterlung. As the diver consumes oxygen, the PO2 in the counterlung falls, and this fall is detected by oxygen sensors. At a certain level, a valve injects more oxygen into the circuit. Both mechanically and electronically controlled units are commonly seen in recreational diving, and there has been much controversy as to which arrangement is safer.
Although all rebreather divers should know their PO2 at all times, the above argument hinges on the requirement for the diver in the manual system to be forced to know his or her PO2 at all times (although in most systems a basal flow of oxygen is continually bled into the unit). However, the requirement to manage the PO2 in this system can create problems during times of high task loading. In contrast, there is less obvious compulsion for the diver using an electronic rebreather to know the PO2 at all times, and should the controlling computer fail, the diver would be at risk, although the chances that the computer will fail in an electronic rebreather are very low. The reality is that, to date, neither system has been shown to offer a survival advantage and all rebreather divers should make a habit of knowing their PO2 at all times.
Most modern mixed gas CCRs use a series of three redundant oxygen sensors (galvanic fuel cells) to track the PO2 in the loop. This allows for the comparison of the outputs of the sensors because they are relatively fragile and prone to failure, as well as having a limited life (usually ~18 months).
If the volume of gas in the bag falls, this triggers a second valve that adds diluting gas (diluent) from a separate cylinder. Air, trimix (helium, nitrogen and oxygen) or heliox (helium and oxygen) may be used as the diluent depending on the planned depth and profile of the dive. The selection of the gas is determined largely by oxygen toxicity and work of breathing issues. For the former, the diluent gas should not have a PO2 greater than a predetermined set-point at depth (preferably a little lower so that an ‘oxygen spike’ does not happen during descent when diluent is added to the loop). To manage the latter, the diver should calculate the density of the gas at the proposed maximum depth, such that it does not exceed the manufacturer’s recommendation and maintains the work of breathing within the specifications of the unit.
Manual controls and displays indicating the oxygen concentration are often fitted to allow the diver to override the controls if the automatic control fails. In many cases, divers also either raise the ‘set-point’ or flush the unit with oxygen during the final decompression stop at 6 metres to shorten decompression time.
This system would appear to be the most efficient breathing system. It is more economical in terms of gas usage than any other gear apart from the oxygen-breathing apparatus. It enables a diver to go deeper for longer and with fewer encumbrances than other equipment.
As an example of the efficiency of this type of equipment, it has been estimated that in a helium saturation dive program involving a prolonged series of dives to 180 metres, the cost of helium for a CCR apparatus was about 2.5 per cent of the cost of an SCR diving apparatus. These advantages must be balanced against the greater initial cost and complexity of the system. This complexity can lead to fatal malfunctions.
SCRs offer some of the saving in gas obtained in the closed systems while avoiding the depth limitations of the oxygen sets and the greater complexity of the closed mixed gas sets. The basic system is shown in Figure 4.7.
SCRs typically use oxygen-enriched air (nitrox) as the breathing mix instead of oxygen. Two major types of SCR are in common use. The most common is the constant mass flow (CMF) type, but the keyed respiratory minute volume (RMV keyed) type has some advocates, especially with US cave divers.
In a typical CMF SCR system, the gas flow and composition are chosen for maximum efficiency for the proposed dive. First, the composition of the gas is chosen, with as high an oxygen concentration as possible without creating an unacceptable risk of oxygen toxicity at the planned maximum depth. This level may be changed depending on the duration of exposure. The flow is then chosen so that the diver will receive sufficient oxygen while working on the surface.
The oxygen concentration in the diver’s inspired gas is determined by the flow into the system, the diver’s consumption and loss through the relief valve. It ranges from close to that in the supply bottle when the diver is resting down to about 20 per cent when the diver is working at the maximum expected rate.
In the RMV keyed sets, a fixed volume of gas is dumped from the circuit with each breath, with ‘new’ gas added via a demand valve.
As a safety precaution with an SCR, almost invariably an excess of gas is added to the loop and is vented through the relief valve, thus making these devices less efficient on gas than CCRs. However, an SCR system with a flow of 12 lpm gives an eightfold saving of gas compared with a demand system when the diver is consuming 1 lpm of oxygen. This saving would increase if the scuba diver was working harder and consuming more air.
The high oxygen concentrations in both SCR and CCR systems mean that the diver may not absorb as much nitrogen as he or she would if breathing air. This can give a decrease in the decompression needed, but unless the PO2 in the loop is actually measured, the diver must ‘guesstimate’ the PO2 for decompression purposes. On occasion, this has resulted in problems.
Military divers have traditionally been the main users of SCR sets, although they have become increasingly popular in recreational diving. The reduced gas flow with these sets means that they can be designed to make little noise. If they are constructed from non-magnetic materials, they can be used for dives near mines, although CCRs are now more often used for mine countermeasures.
Problems with rebreathers
Both CCR and SCR systems introduce a variety of potential hazards (see Chapter 62):
Carbon dioxide accumulation can occur if the scrubber fails. This can occur if the scrubber material is used for too long, if the scrubber is incorrectly attached or if the scrubber’s instantaneous capacity to remove carbon dioxide is exceeded because of a diver’s high demand, such as with exertion.
Oxygen toxicity can occur if the diver exceeds his or her depth limit, descends too quickly when diving at a particular PO2 (set-point) or uses a mixture with too much oxygen in it. This can occur if an excessively oxygen-rich mix was added to the diluent cylinder or if the solenoid or manual oxygen injection valves jam open.
Hypoxia may result if the gas flow decreases. This can be caused by omitting to turn on the gas supply before descending, exhausting the oxygen supply, solenoid or electronics failure, and ascending too rapidly to enable the solenoid to add sufficient oxygen to the loop. Hypoxia can also occur if the diver works harder than expected or if a mix with too little oxygen is used.
A review of the 181 reported recreational CCR-related deaths that occurred between 1998 and 2010 estimated that the fatality rate for CCR users was about 10 times that of recreational open-circuit divers. The author also suggested that CCRs have a 25-fold increased risk of component failure compared with manifolded twin-cylinder open-circuit systems. It was suggested that this risk could be partly offset by carrying a redundant bail-out system.