5. How Do Rebreathers Work?

Let's say from the start that this article will be a bit long. Hopefully, it will be useful and enjoyable for those who are curious. Now, let's get serious and try to understand the mechanics behind rebreathers.

This article will focus on three main topics:

1. Basic Gas Flow Circuit

2. Filtering Carbon Dioxide Gas from the System

3. Oxygen Level Monitoring and Sensors

Basic Gas Flow Circuit

First and foremost, let's understand that rebreather is playing with fire. Sending a gas containing carbon dioxide back into a system where you'll breathe it again, and having your life dependent on a chemical and electronic system underwater, doesn't sound very appealing from a high vantage point. Therefore, understanding how rebreathers work in general, as well as being familiar with the specifics of the unit you're using, is extremely critical.

Basic Components of a Rebreather;

1. Breathing hose

2. Diver Surface Valve - dsv

3. Counterlung

4. Scrubber canister 

5. Oxygen Cylinder

6. Diluent Cylinder

Now let's examine how gas flow occurs in a fully enclosed electronic rebreather and how the rebreather works. Above, we see an example rebreather diagram. This diagram includes two separate counterlungs for inhalation and exhalation. While some models have only one counterlung, the use of dual counterlungs has become common in recent years to reduce work of breathing (WOB). Let's examine the flow step by step:

1. Gas is pumped from the diluent cylinder into the breathing bag (counterlung).

2. The diver breathes in through a mouthpiece and a breathing hose, so that clean gas passes from the counterlungs into the diver's lungs.

3. When the diver exhales, carbon dioxide-rich gas reaches the exhale counterlung.

4. From the exhale counterlung, the gas passes through a scrubber containing a carbon dioxide-capturing chemical (soda-lime), where the carbon dioxide is trapped by this chemical.

5. The decarbonized gas passes through a fully enclosed circuit, in an area called the HEAD, which contains oxygen sensors, electronics, and a solenoid valve that enables automatic oxygen addition.

6. Based on the oxygen level measured by the sensors, the missing amount of oxygen is automatically added to the system by the solenoid valve and reaches the breathing bag again. Here, the solenoid valve is connected to the pure oxygen cylinder via a regulator hose. As the solenoid opens, pure oxygen enters the system from the pure oxygen cylinder.

The cycle continues in this way, essentially the same gas, carbon dioxide, is captured, oxygen is added, and the gas is re-breathed to the diver. The first point I want to emphasize here is that there are one-way valves on either side of the mouthpiece in the breathing loop; "mushroom valves". Thanks to mushroom valves, all gas flow in the system occurs in one direction. Maintaining the correct flow direction of the gas, or in other words, the proper functioning of these one-way (check valve) valves, is critically important for the health of the system. If the check valves do not function properly, carbon dioxide-rich gas may return to the inhale counterlung without passing through the chemical absorber. This situation can lead to accidents that may result in death due to carbon dioxide inhalation during diving. For this reason, we perform a series of tests every dive day to ensure that the check valves are working correctly (we will discuss the details in later sections).

Let's try to understand the function of the diluent gas entering the inhaler and the oxygen gas entering the exhaler in the diagram above. First, we might consider why we would need to refill the system with diluent gas, since we theoretically breathe it continuously throughout the dive. The primary reason is that as we descend deeper, our inhaler will shrink due to pressure, and we need to add new gas to the system to be able to breathe gas at ambient pressure. This can be done manually or by adding diluent gas using an ADV (automatic diluent valve). Essentially, this is the main reason we need to add diluent gas to the system. An ADV is a type of second-stage regulator connected to the inhaler in many units. If there isn't enough gas in the inhaler, and the diver continues to inhale, this part acts like a demand regulator, adding diluent gas to the inhaler. Gas leaks, such as those caused by mask equalization, also cause the inhaler to lose gas/volume, necessitating the addition of diluent gas from the cylinder. Another reason is that, in case of various sensor, solenoid, scrubber, or other system malfunctions (which we will examine in detail later), we can "refresh" the gas in the breathing circuit by releasing the gas and adding new diluent. The oxygen gas entering the exhalation bag can be used to manually add oxygen when the system cannot automatically pump oxygen due to a solenoid or other malfunction.

Carbon Dioxide Filtration

Now let's talk about the component that enables this wonderful device, which we call a rebreather, to rebreathe: the "scrubber." A scrubber, which translates to "cleaner" in Turkish, is a chemical compound that filters carbon dioxide from the exhaled gas. This chemical, known by various names such as soda-lime, "absorbent," and "sorb," is a hard, white chemical product with a granular structure, composed of a mixture of calcium oxide (CaO) (75%), water/moisture (20%), and sodium hydroxide (NaOH) (5%). Its main application is in the medical sector, in anesthesia devices. Another application is in submarines. By trapping carbon dioxide in the air breathed by submarine personnel, it allows them to remain underwater for extended periods without surfacing.

I don't know how much production of such a product would be worthwhile for a relatively small market like diving, if it weren't used in such large sectors. Nevertheless, soda lime used for diving is produced specifically for this purpose, unlike other sectors. It is produced in optimum sizes, called 8-12 mesh, to keep breathing resistance low and prevent voids within the canister. If the particle size is too large, air pockets will form within the canister, leading to a "channeling" effect. Channeling is when the gas in the system passes through a low-resistance point as it passes through the scrubber, preventing other parts of the chemical from being used and causing carbon dioxide to be captured much faster than expected. If the particle size is too small, like sand, it will require more effort for the gas to pass through the chemical and reach the diver, increasing the breathing effort (WOB). This is something we certainly don't want, as it can cause many different problems, including hypercapnia, during diving. Soda lime from brands like Sofnolime and Intersorb, produced for diving, are manufactured in optimum sizes and quality, taking these sensitivities into account. Also, unlike other soda limes, no extra chemicals are added to change the color or add flavor so you can tell when it's finished.

The removal of carbon dioxide through soda lime is achieved through a chemical reaction. Carbon dioxide reacts with soda lime to form water and calcium carbonate. This chemical reaction is an exothermic reaction, meaning it releases heat. Those who remember their high school chemistry lessons will easily understand what we mean from the diagram below;

CO2 + Ca(OH)2 (soda lime) -> CaCO3 + H20 + heat

The carbon dioxide in the exhaled gas is trapped through this chemical reaction, while the release of heat and moisture provides additional benefits during diving that we wouldn't want to ignore. Because the gas we breathe is warmer than in an open circuit due to the heat generated, it reduces the feeling of cold during diving. At the same time, the moisture released as a result of the reaction allows us to breathe a moist gas instead of dry air (gas), offering a more comfortable diving experience.

Of course, it's not natural for everything to be so beneficial, so let's also consider some of the problems this chemical brings. First of all, "underwater" and "chemical" don't sound like a pleasant combination. The contact of soda lime with water causes the formation of an alkaline soda that has a bitter cocktail taste called a "caustic cocktail." The resulting product has a pH level between 12-14, meaning it is quite acidic. If inhaled, caustic cocktail can cause painful discomforts such as coughing, shortness of breath, and difficulty swallowing. Therefore, the airtightness of the compartment (canister) where the soda lime chemical is stored is critically important. The O-rings on the canister lids should be carefully checked and maintained. Its airtightness should be ensured through pre-dive checks (which we will detail later). On the other hand, if this compartment starts to fill with water, it will both reduce its ability to hold carbon dioxide, causing carbon dioxide to appear in the gas you breathe and leading to hypercapnia, and it will increase breathing resistance, causing you to exert more effort and produce more carbon dioxide.

Soda lime shelf life; in fully closed-loop rebreather (CCR) systems, the first factor limiting dive time is often the shelf life of the soda lime chemical. Most manufacturers allow a maximum usage time of 180 minutes, depending on the chemical brand they support/test. As we mentioned in the previous section, it's quite difficult to give a precise shelf life for the chemical. The values ​​provided by manufacturers are, as they themselves state, tests conducted at a specific depth, for a specific mixture, at a specific temperature, and under scenarios with high carbon dioxide production. In practice, these tests depend on multiple variables such as depth, temperature, and the diver's metabolic oxygen consumption, making it difficult to give a precise value. Therefore, sticking to the 180-minute (3-hour) value is as safe as possible. The chemical may lose its properties within 180 minutes. The production date, or more precisely the expiration date, and storage conditions will also affect this time. Expired soda lime should not be used. Soda lime should be stored in its original container in an airtight container, preferably in a dry, well-ventilated, and, importantly, dark place. The chemical must also be replaced after 180 minutes* of use. Some units have electronic systems that can show the remaining lifespan of the scrubber. Others use sensors that monitor the carbon dioxide level in the system. Unfortunately, neither system is 100% accurate and has its own error points. In this case, you need to decide on the remaining chemical lifespan yourself, based on your training, and replace it without taking any risks.

*Note: The 180-minute value might also be given because it somewhat resembles the 1.3 atm oxygen partial pressure limit for 100% CNS (oxygen poisoning).

Storing used soda lime; now, this chemical we call soda lime certainly comes with a cost. Actually, the average cost of filling a canister is around 10 euros, so we're not talking about an exorbitant price. However, let's say you fill your canister, do a one-hour dive, and your next dive is in three days. What do you do? The best thing to do is actually pour it out and then refill it. But still, sometimes you just can't bring yourself to do that :) Let me tell you what I, and many CCR divers worldwide, do in such a situation, and you can decide. Without removing the soda lime from the canister, I leave the canister to dry at room temperature for a few hours or overnight. You'll see that the outside of the canister is wet at the end of the dive due to the condensation of water from the chemical reaction described above and the water vapor from our breath. I dry the outside with a towel or paper napkin and leave it in a suitable environment to dry the inside as well. On the other hand, to prevent it from drying out too much and losing its natural moisture, I don't put it under an air conditioner or anything like that. Afterwards, I put the canister inside a plastic garbage bag and tie the opening tightly so that no air gets in. I even repeat this by stacking 4-5 bags on top of each other. I choose a black garbage bag for the last bag if possible and store it in a place where it doesn't get light. Using it this way, I have been able to easily do my dives in the following weeks as well. Still, is this risk worth it? The decision is yours.

Oxygen Level Monitoring and Sensors

When discussing basic gas flow, we mentioned the HEAD component, which houses the electronics. Now let's examine this component in more detail.

First of all, there are usually 3 or 4 oxygen sensors here. Oxygen sensors are electrochemical in structure and are a type of battery that produces different current values ​​according to the oxygen pressure they receive. Like all batteries, they have a limited lifespan. The oxygen tanks we use in closed-loop systems typically lose their function within 1-2 years. Therefore, regular replacements are necessary, paying attention to the manufacturer's instructions.

What is an Oxygen Sensor?

An oxygen sensor consists of two parts:

• Galvanic Cell: This is the central part where oxygen undergoes a chemical reaction to generate current.

• Circuit Board: Ensures that sensor values ​​do not change according to different ambient temperatures, and ensures the reliability of sensor values ​​under different conditions.

The sensor measures the partial pressure of oxygen by converting the chemical reaction into a low electrical current. This current is then converted into a voltage that we can read.

Critical Point;

• Sensors are a source of current, not voltage.

• The output is proportional to oxygen levels, but is affected by temperature. Therefore, by using electronic components, the sensor output values ​​are balanced according to temperature, aiming for accurate readings.

Sensor Calibration: Each dive day, the sensors are calibrated to ensure they provide the correct oxygen level during the dive. Ultimately, these sensors can only tell you the voltage generated based on the current they produce. At the beginning of each dive, we need to tell the dive computer (or controller) what voltage value (e.g., 51 mV) corresponds to 100% oxygen. This value will change over time as the sensor ages. While different procedures are used in each unit to determine this value, fundamentally, the system is filled with 100% oxygen and the voltage value from the sensors is read. This process is called calibration. After calibration, the dive computer (controller) can calculate the partial pressure of oxygen from the voltage values ​​from the sensor and works to maintain it at the level we have set.

Number of Sensors and Voting Algorithm: In fully enclosed circuits, we generally see 3 or 4 oxygen sensors. This is to provide a backup sensor in case of a sensor reading error. Of course, the idea that 2 sensors would suffice comes to mind. If there are 2 sensors, it would be quite difficult to determine which sensor is incorrect if one gives an erroneous reading. Therefore, by adding a third sensor to the system, we can quickly conclude that two sensors showing the same value are correct and the sensor showing a different value is incorrect, using what we call the voting algorithm logic. Although there is a possibility that both sensors will give incorrect readings while one sensor gives a correct reading, this probability is low, so the controller accepts the two sensors giving approximately the same value as correct and continues to operate the system. If we want to further reduce this probability, a fourth sensor can be added to the system. Some brands of units use 4 sensors instead of 3 for this reason. Of course, in that case, there is still a possibility that 3 sensors will give incorrect readings and the remaining sensor will give a correct reading. Therefore, after initially noticing inaccurate readings on the sensors, a rebreather diver should consider the required oxygen partial pressure at their current depth. They should then observe changes in the sensors as they change depth or manually add diluent/oxygen to the system before making a definitive judgment. It's even worse than two sensors showing the same value while one shows a different value; three sensors showing different values ​​can occur.

While the sensors mentioned above are called galvanic oxygen sensors, recently, sensors using near-field infrared (NFI) technology, also known as solid-state oxygen sensors, have begun to find their place in various products. These sensors can be used for many years without needing calibration before each dive. Of course, their initial cost is somewhat high; we can say between 1200-1400 USD. Solid-state sensors, used by Poseidon since 2017-2018, have not yet been adopted by other major brands due to their high cost and doubts about their efficiency in humid environments. We can see this used as a differentiating factor by slightly newer brands in the market.

The solenoid is an electromechanical component that enables the automatic oxygen addition process, a hallmark of electronic rebreathers. Essentially found in many different electromechanical systems, the solenoid acts like an oxygen valve in rebreathers, controlling the on/off operation. The controller, based on readings from the sensors, opens the solenoid valve when oxygen addition is needed. Solenoid valves generally have their own separate batteries within the system. A depleted battery will cause the solenoid to malfunction. Therefore, the solenoid battery is checked via the controller before the dive. Even if the battery runs out and the solenoid malfunctions during the dive, the manual oxygen addition mechanism allows the dive to be easily and safely terminated. A more challenging situation is when the solenoid valve remains open. We will examine in more detail in later articles how to act in such oxygen level anomalies.

Sensor storage: Rebreather sensors are manufactured specifically for the rebreather brand. The manufacturing date is also critically important and is printed on them when shipped to the user. It is generally accepted that an unused sensor has a lifespan of 2 years from its date of manufacture, and 1 year after it starts being used. For example, you can use a sensor manufactured in January 2024 from January 2025 to January 2026, but even if you start using it in June 2025, it is still recommended to stop using it by January 2026 at the latest. In practice, sensors can last one and a half or even two years. Nevertheless, it is best to replace them to avoid unnecessary risks. Considering that the average cost of sensors is 60-80 euros per unit, a sensor malfunction during a dive or missing a dive due to a faulty reading beforehand would result in a much greater cost in value. Personally, I always carry a set of spare sensors with me. Especially during long and consecutive diving days, sensors become damp due to condensation of water vapor inside the unit, and if they don't have enough time to dry, they start giving inaccurate readings. When replacing sensors, it is recommended to replace one sensor every 6 months instead of replacing all sensors at once. Therefore, if there is a manufacturing defect, it will affect all sensors purchased at the same time, meaning you will be using products from different batches. Finally, if you buy spare sensors like I did, or if you want to maximize the use of your existing sensors, you might consider storing them in an airtight and moisture-proof plastic bag. This way, they will wear out more slowly.