5 basic principles of rebreather and one extra

In a previous article we explained the basic function of rebreathers. But there is not just one rebreather, there are several fundamentally different types of Closed-Circuit Rebreather (CCR) and Semi-Closed Circuit Rebreather (SCR). So, let's review the basic principles of rebreathers that are used in recreational and professional diving.

There are 5 of them plus one extra:

• O2 CCR
• SCR
• PSCR
• mCCR
• eCCR

I consider all types to be proof of human ingenuity, but each of them has its own strengths, but of course also weaknesses. Let's focus on a basic understanding of the different principles, from the simplest to the most complex, and derive the advantages and disadvantages from there.

Finally, we will then show one solution that tries to use the most advantages with the least disadvantages.

O2 CCR

The oxygen closed circuit is an ingeniously simple principle at first glance and is actually the first ever independent breathing apparatus.

All we need to construct it is an oxygen source, which we inject into the breathing circuit via a demand valve sometimes supplemented by a constant flow nozzle, and a carbon dioxide scrubber to remove CO2 from the exhaled gas. For this, one or two counterlungs are sufficient to give the diver something to breathe from and a mouthpiece with directional valves to direct the flow of the breathing gas. In this way the device can operate without any electronics, virtually without the need to know the oxygen content of the loop, because if the device is well flushed with oxygen at the surface, the oxygen content of the breathing loop will always be very high during use.

Because the instrument uses only oxygen, it is the only unit that does not have to vent a single bubble, even at the ascent, where all other units must purge the excess gas volume. Oxygen can be metabolized, so if you surface slowly enough, your body can process all the excess oxygen.

The complete absence of bubbles, simplicity and compactness make it ideal for military divers - combat swimmers who need to infiltrate the enemy's rear undetected.

The maximum compactness and very lightweight construction make it easy to transport on dry land, so it can also be used in areas other than diving. Such devices are used, for example, by firefighters, mine rescuers, pilots, etc.

Recreational diving with an oxygen CCR is unfortunately very limited to a maximum dive depth of 6 metres due to oxygen toxicity. Even so, it can still be great for photography in shallow water, exploring mountain lakes and otherwise inaccessible places where we have to carry the device on our backs.

Oxygen Closed Circuit Rebreather – 1 Inhale counterlung; 2 Dive/Surface valve (DSV); 3 Exhale Counterlung; 4 CO2 scrubber; 5 Oxygen supply; 6 Oxygen reduction valve; 7 constant flow nozzle; 8 Manual addition valve (MAV); 9 Oxygen SPG

SCR

To go deeper than 6 metres, we need to dilute the oxygen. So, we will use a very similar principle to the oxygen CCR, but instead of oxygen we will add NITROX. This will be injected into the circuit through a constant flow nozzle so that the oxygen consumed is always replaced by the oxygen from the nitrox. Since we metabolize the oxygen, but cannot metabolize the nitrogen in the nitrox, we must get rid of the ever-increasing nitrogen by venting some of the gas out of the circuit. Hence the semi-closed circuit. The simplicity of the design has been preserved, but the opportunity to go deeper has been traded off for a not very high gas consumption efficiency.

Because we consume oxygen, the fraction of oxygen in the breathing circuit will always be less than that in the cylinder with the supplied gas. How much smaller it will be depending on how much oxygen we consume. If we are at rest we use about 0.6 litres, but on exertion we may use 2.5 to 3 litres of oxygen per minute. In the case of a lower value, a flow rate of 8 litres of Nitrox 36 per minute would be sufficient to maintain breathable gas, but we have to allow for the worst-case scenario of never dropping below breathable levels. In that case, we need up to 18 litres per minute.

This is already like an open circuit, however, unlike an open circuit, this flow rate can remain constant for any achievable depth. In this particular case, the maximum is 35 metres. This means that the SCR is still up to tens of litres per minute better than the open circuit.

The unstable oxygen consumption also leads to an unstable gas composition in the breathing loop. More consumption means less oxygen and thus the need for longer decompression. PO2 naturally varies with depth, so the best mixture is out of the question. This is another disadvantage. Without an oxygen sensor connected, the diver cannot be sure of what he is breathing and how to calculate decompression, so he has to start from worst-case scenarios for both maximum depth and decompression calculations. We can also consider the fact that we cannot fill the cylinders with available air as a disadvantage. Or we could, but then we couldn't dive directly from the surface because the partial pressure of oxygen would be too low.

Despite some drawbacks, however, the Constant Flow SCR is very simple to operate, which is why this principle was used in the first recreational rebreathers. It has also been dusted off in the recent past and in a somewhat hybrid form combined with electronics is used in one of the recreational rebreathers of the current day.

Semi-closed Circuit Rebreather – 1 Inhale counterlung; 2 Dive/Surface valve (DSV); 3 Exhale Counterlung; 4 CO2 scrubber; 5 Nitrox supply; 6 Nitrox reduction valve; 7 constant flow nozzle; 8 Manual addition valve (MAV); 9 Nitrox SPG; 10 Overpressure valve

PSCR

As a solution for those who needed to save as much gas as possible without the need for any electronics, a solution was found in the 1980s in the form of a passive semi-closed circuit. The word passive means that the mixture does not flow into the breathing circuit in a constant flow of gas but is injected by an ingenious mechanism every xth breath. Usually, every eighth or ninth breath.

The so-called breath keying is solved by the diver exhaling through a CO2 absorber partly into a large counterlung in the form of a bellows, and partly into a small counterlung, which is placed inside the large one. Importantly, the small counterlung is separated from the large counterlung by a one-way valve, so that exhalation can be directed into it, but inhalation cannot be taken from it. Instead, when the diver inhales, it collapses along with the large counterlung as the diver sucks the gas out of the large counterlung, and with its other end with a second non-return valve, exhausts some of the gas into the surrounding environment.

By venting some of the gas to the surrounding environment, both counterlungs gradually shrink until they collapse completely. At that point, the diver can no longer breathe from the closed circuit, so the demand valve (essentially the second stage of the regulator) opens to supply the diver with a completely fresh mixture. With the exhalation, the counterlung fills and the whole cycle starts again. The ratio of breaths from the closed circuit to the addition of fresh gas determines the ratio of the volumes of the two counterlungs. As I mentioned above, the ratio is most often 8:1 or 9:1.

This system is actually very clever and retains a completely mechanical solution without the need for any electronics. It was and is therefore used for long and difficult cave explorations even at great depths. However, the mixture ratio in the loop and thus the partial pressure of oxygen is very unstable, and again always slightly lower than the oxygen content of the supply bottle.

The big disadvantage is the inefficiency of decompression, because you always have worse gas than in an open circuit, and the PO2 changes with depth, and breathing. Of course, the biggest disadvantage of all is the poor work of breathing, which comes from the counterlung placement usually very far from the diver's lungs. So any change in the diver's position will immediately have a negative effect on inspiratory or expiratory resistance.

The keying of the inspiration also means that the consumption, although lower than in an open circuit, increases proportionally with depth, as in an open circuit.

Passive Addition Semi-closed Circuit Rebreather – 1 Inhale/Exhale counterlung bellows; 2 Discharge bellows; 3 Dive/Surface valve (DSV); 4 CO2 scrubber; 5 and 6 Directional valves; 7 Demand valve; 8 Manual addition valve (MAV); 9 reduction valve; 10 SPG

eCCR

To counteract the disadvantages of semi-closed circuits, where the gas consumption and decompression process is not very efficient, we have to use a fully closed circuit, similar to the oxygen circuit at the beginning. However, in order to dive deep, we need to dilute the oxygen. This time, however, we do it by leaving the oxygen alone and diluting it with another gas containing an inert gas (nitrogen, helium). This gas could be air, trimix or even heliox.

The oxygen is injected into the circuit by means of a solenoid (electro-magnetically controlled valve). The computer evaluates how much oxygen needs to be injected based on information from the oxygen sensors. The oxygen sensors react to the partial pressure by changing the electric current. The gas is diluted during descent by the previously mentioned diluent gas, which is injected by an automatic (mechanical) valve. The computer adjusts the consumed oxygen to a pre-set oxygen partial pressure level - setpoint. Therefore it is also referred to as Constant PO2 CCR.

From the above, it follows that the diver receives only the best mixture at any depth. This achieves maximum gas consumption efficiency while minimizing decompression. See previous article (link) for details.

Despite all the advantages and convenience of electronics that can also calculate decompression on-line, the vulnerable point of such devices tends to be the faulty electronic components. While today's electronics are of course at a different level than 40 years ago, the issue of oxygen sensors remains the same. Each eCCR has at least 3 sensors for comparing their readings and possibly identifying a faulty sensor.

However, the other elements of the system are often not backed up, and in the event of their failure, the diver is forced to use emergency procedures.

Electronically controlled Closed Circuit Rebreather (eCCR) – 1 Inhale counterlung; 2 Dive/Surface valve (DSV); 3 Exhale Counterlung; 4 CO2 scrubber; 5 Oxygen supply; 6 Oxygen reduction valve; 8 Oxygen Manual addition valve (MAV); 9 Oxygen SPG; 10 Diluent supply, 11 Diluent reduction valve; 12 Diluent SPG; 13 Automatic Diluent Valve; 14 Diluent Manual Addition Valve (MAV); 15 Oxygen sensors; 16 Control unit (CU); 17 Solenoid; 18 Handset display.

mCCR

The trend to avoid electronics at all costs, but still enjoy the benefits of a fully closed circuit, such as maximum gas savings and minimum decompression, has led to the design of the mechanical CCR (mCCR).

This has been achieved by retaining the eCCR design but instead of a solenoid, oxygen is injected through a constant mass flow. The oxygen flow is achieved by a nozzle connected to a constant intermediate pressure. This means that the first stage of the oxygen regulator has a blanked section which registers ambient pressure. The ambient pressure therefore does not affect the intermediate pressure and so the oxygen flow through the nozzle decreases with increasing ambient pressure and therefore gas density. However, the amount of oxygen molecules that pass through the nozzle is independent of depth, so the oxygen flow still affects PO2 in the same way.

Sounds fantastic, doesn't it? Except that to keep oxygen from accumulating too much in the breathing loop, we have to set the oxygen flow rate lower than our metabolic consumption in order to maintain breathable gas. This creates the need to inject oxygen manually. Thus, our oxygen is measured by the same sensors as in the case of eCCR, but everything is monitored and controlled by the fallible human brain, which is easily distracted. Not adding oxygen, especially during ascent to the surface, would of course have fatal consequences.

A blinded first stage on oxygen is a very clever solution, but it comes with one drawback. The fact that the mean pressure does not vary with depth implies that when the mean pressure equalizes with the ambient pressure, oxygen stops flowing altogether. Further, it is not even possible to use separate manual injectors. mCCR is therefore limited by depth.

Manual Closed Circuit Rebreather (mCCR) – 1 Inhale counterlung; 2 Dive/Surface valve (DSV); 3 Exhale Counterlung; 4 CO2 scrubber; 5 Oxygen supply; 6 Oxygen reduction valve (blinded); 8 Oxygen Manual addition valve (MAV); 9 Oxygen SPG; 10 Diluent supply, 11 Diluent reduction valve; 12 Diluent SPG; 13 Automatic Diluent Valve; 14 Diluent Manual Addition Valve (MAV); 15 Oxygen sensors; 17 Constant mass flow; 18 Handset display.

Fault tolerant eCCR

If we think about all the previous principles, we would like to have only advantages and get rid of disadvantages. This is clear. But what to do about it? We have thought about this too, and the solution we bring is that we can't do without electronics in this case. We also have to give up maximum simplicity. But what if by making the system more complex, or rather, by backing it up and securing it, we achieve greater security than with a simple system?

We know this from advanced technologies on which human lives depend, such as aerospace. All systems are minimally duplicated or triplicated and ingeniously put together to complement or replace each other. We have attempted the same in the case of the Liberty CCR which we dare to call the Fault Tolerant Rebreather.

If we build two systems side by side, each with its own power source, its own set of sensors and its own solenoid, then everything is duplicated. In addition, if we interconnect the two systems so that they can communicate with each other continuously, sharing information from all sensors and evaluating it independently, then we achieve that the failure of one or more components will not affect the operation of the entire system.

The instrument can continue to operate without the need for any emergency diver intervention. The diver, on the other hand, has maximum control of the situation. He knows the readings from all the sensors and can evaluate or directly verify the accuracy of their readings.

The smart software also makes sure that the handling of this already quite complex device is user-friendly and the diver has as much or much less work to do with controlling the device than with conventional eCCRs.

The only disadvantage is the relative overall design complexity of the device, which is for the better and makes the diver's life rather easier.

Fault tolerant electronically controlled Closed Circuit Rebreather – 1 Inhale counterlung; 2 Dive/Surface valve (DSV); 3 Exhale Counterlung; 4 CO2 scrubber; 5 Oxygen supply; 6 Oxygen reduction valve; 8 Oxygen Manual addition valve (MAV); 9 Oxygen SPG; 10 Diluent supply, 11 Diluent reduction valve; 12 Diluent SPG; 13 Automatic Diluent Valve; 14 Diluent Manual Addition Valve (MAV); 15 Oxygen sensors; 16 Control units (CU); 17 Solenoids; 18 Handset display.