3. Rebreather Working Principles

Rebreathers can work on different principles to basically recirculate the gas you breathe. Before choosing a brand - unit in rebreather, understanding how rebreathers work and which rebreather model is right for you can make your job easier.

Basically, we can talk about two types of rebreathers;

1. Semi Closed Circuit Rebreather (SCCR)

2. Closed Circuit Rebreather (CCR)

Semi-closed rebreathers are a closed circuit system that is not fully closed. We can say that it is an intermediate form between fully closed rebreathers and open circuit Scuba. Semi-closed systems generally operate in a way that provides a constant oxygen ratio, as in open circuits. Semi-closed rebreathers continuously and at a constant flow rate flow oxygen into the breathing circuit. The diver can adjust the flow rate of the constant flow here by knowing his own metabolic oxygen consumption, and thus the required, safe and minimum flow is preferred. In a circuit with continuous flow, there will of course be bubble exits at certain intervals. This creates a disadvantage for semi-closed rebreathers as loosing gas from the loop. The advantage is that thanks to a continuous oxygen flow, we can say that in case of any problem in the carbon dioxide trap, it remains in a safer position.

There are also two different types of semi-closed circuit rebreathers; active and passive semi-closed circuits. In active semi-closed circuits, oxygen or an oxygen-rich mixture is continuously injected into the system at a constant flow. The flow rate of this flow can be adjusted by the diver. Inert gas (such as nitrogen or helium) can be included in the gas mixture injected into the system, or it is possible to manually add gas to the system from a second cylinder. Since the oxygen flow in these rebreathers is adjusted to be more than what the diver will use in his metabolic activities, the volume of the breathing bags will increase during the dive. This causes gas to be released out of the system periodically. In practice, bubbles are released out of the system approximately every 4-5 breaths. On the other hand, fresh gas is constantly injected into the system in this way. It is still very efficient in terms of gas consumption compared to open circuit.

In passive semi-closed circuits, instead of a constant flow gas, an oxygen-rich mixture is injected depending on the diver's respiratory minute volume (RMV) or work rate. In passive units, as the breathing bag shrinks, the system automatically injects fresh gas with the pressure difference in the circuit. Gas efficiency can be better than in constant flow semi-closed circuits. Since the gas flow depends on the person's respiratory rate, it can be difficult in high-effort dives. On the other hand, calculating the oxygen ratio in the mixture is more complicated.

In semi-closed circuits, calculating the oxygen partial pressure according to the diver's effort brings different side effects and difficulties. When estimating the oxygen rate that the diver will breathe, hypoxia or oxygen poisoning risks will arise and it will need to be calculated in a very safe area. When calculating to stay in the safe area, it will not be possible to be sure of the oxygen rate inhaled. When making a deco calculation considering that it is within a certain range, it will be necessary to make a deco calculation according to the lowest oxygen and highest nitrogen rate that can occur during the dive. This situation will also become a disadvantage that prolongs deco times. In the event that the flow rate of the valve providing flow to the system cannot be calculated efficiently, there will be excess gas output from the system and this will negatively affect efficiency in terms of gas consumption.

Closed circuit rebreathers are mostly encountered today as electronic rebreathers. In closed circuits, the oxygen in the circuit is constantly measured by sensors. In order to complete the oxygen that decreases by turning into carbon dioxide in the circuit, new oxygen is added to the circuit by a solenoid valve. The driver circuit that controls this entire system and the controller that manages this driver (which is similar to a dive computer and also performs dive computer functions) consist entirely of electronic systems. In this article series, we will mainly talk about fully closed, electronic circuits (rebreathers). Today, they are the most widely used and most common rebreather systems in the sector. Semi-closed circuits do not have many areas of use other than dives for certain purposes (e.g. scientific, archaeological). In these dives, semi-closed circuits are preferred if you want to stay underwater for a long time and if there is a problem with the system and you want to stay in a safe zone even if carbon dioxide is not retained, but you will not go too deep anyway.

Closed circuits differ from semi-closed circuits in three main points:

1. There is no periodic gas release. Instead, there is only gas release as the breathing bag volume expands as the lungs ascend.

2. The proportion of oxygen in the inspired gas is continuously variable, the partial pressure of oxygen is constant.

3. They usually have two different gas sources; pure oxygen and diluent cylinders.

In closed circuits, electronically controlled rebreathers are frequently encountered today. These are also called ECCR - Electronic Closed Circuit Rebreather. In JJ CCR, Revo, AP Diving and many other popular closed circuit rebreather systems, even if there are electronic controls, oxygen can be managed by adding manually. The biggest question mark regarding electronically controlled units is what will happen if the electronics in the system fail. Although the electronics here are quite simple systems, the probability of failure (bug) is low, of course there is always such a possibility exists. Against this, if your unit has a manual control feature, how to do it is explained in the trainings.

Although not as common as ECCR systems, there are MCCR - Manual Closed Circuit Rebreather systems as an alternative. MCCR units provide partial pressure control of oxygen with “passive” methods such as constant flow, variable flow or manual addition. If the flow control is not calculated correctly in these, you will suddenly be in a semi-closed circuit, meaning you may lose efficiency in gas consumption. Nevertheless, since it is not connected to an electronic system, it is one of the most preferred closed circuit diving systems after ECCR.

SCR (semi-closed circuit) vs CCR (fully closed - electronic circuits) Comparison Chart

Bonus: oxygen rebreathers. We talked about it a little in the previous section, but let's also briefly talk about pure oxygen closed circuit systems, which we will not focus on much in this article series and which are not used much among sports or technical divers. Pure oxygen rebreathers or simply oxygen rebreathers have been used for military diving for many years. While military divers can travel long distances without making bubbles and leaving a trace, they can also get out of the water after diving for hours without the risk of decompression sickness since they do not breathe any nitrogen, helium or any other inert gas thanks to the pure oxygen rebreather. Of course, as a disadvantage of this, they cannot go deeper than 6-9 meters due to the risk of oxygen poisoning. On the other hand, there are those who use pure oxygen rebreathers in surface operations, such as firefighters, in addition to military divers. There is no electronic sensor mechanism in these rebreathers, the volume of the breathing bag decreases as the inhaled oxygen is converted into carbon dioxide and held, and after a certain level, an automatic mechanical valve opens to send oxygen to the bag.