DIY closed breathing system. In a device with a closed breathing cycle. Semi-closed cycle rebreather with active feed with mixture preparation during feeding

This is a device that purifies gas used for breathing. The oxygen necessary for breathing continuously flows (forced) into the gas mixture circuit. The exhaust gas remains in the circuit: it passes through a unidirectional channel and is purified of CO2. After purification, the gas is reintroduced into the inhalation bag, then the cycle is repeated.

Rebreather: new technology?

Did you know that the first diving device was a rebreather? It was created in 1878 by the engineer Fleuss and consisted of a rubber mask connected to a breathing bag, which was filled with oxygen supplied from a copper cylinder; carbon dioxide was absorbed by a “filter”: interwoven fibers impregnated with caustic potash (potassium carbonate). In 1915, Fleuss’s idea was borrowed by Sir Robert Davis when creating an apparatus for emergency ascent from submarines, which then began to be produced all over the world. Hans Hass is the first underwater photographer to dive on a rebreather.

ARO - (oxygen rebreather closed loop) originally from Italy, was created in the period between the I and II World Wars. In 1933-34, the Italian military divers Teseo Tesei and Elios Toschi appreciated the indispensability of this device in military operations, some changes were made to the device, and it began to play first fiddle in the operations of the Gamma and Maiali detachments.

After the war, the ARO was used by the Navy to train divers.

The ARO is still used today for training and for diving to very great depths.

Meanwhile, in 1969, the Dra "ger company develops very current semi-closed cycle nitrox devices and produces FGT (this device is still used by many military divers).

Later came the FGT III, a semi-closed cycle heliox for diving to depths of up to 200 meters.

In subsequent years, Dra'ger perfected the system to ensure continuous flow and took a leading position in the production of these components.

In 1995, the first semi-closed cycle rebreathers for sports began to be produced.

Today there are three main types of rebreathers - oxygen, semi-closed and closed devices.

Oxygen rebreathers

This type of device uses pure oxygen and is completely closed. The history of their creation and use dates back to the 19th century. These devices were actively used by Hans Haas and his wife Lota Haas, the most famous underwater explorers and photographers. During the war, these devices were actively used by underwater saboteurs from all countries participating in the war. Currently, oxygen rebreathers have undergone minor changes and are used mainly by naval forces. Devices of this type are the most compact, simple in design and reliable. Typically they contain one breathing bag, one oxygen canister and a canister of chemical absorbent. Pure oxygen is supplied into the breathing bag through a special nozzle hole at a certain speed, or periodically. Then you inhale the oxygen and exhale into a canister of soda - where the resulting carbon dioxide is absorbed and everything goes in a circle again. No electronics, only a pressure gauge. The most famous products of this class are LAR-V from the German company draeger, Oxyng from the French company spirotechnique, Italian products from OMG and of course a large number of Soviet devices - IPSA, IDA-64, IDA-76, IDA-71, etc. .d. The main disadvantage of these devices was and is the depth limitation - 6 meters.

Semi-closed rebreathers

These devices are divided into two types: aSCR - devices with active feed gas and pSCR - with passive supply, respectively.

aSCR- these devices were developed in the fifties and were used, as is always the case by the military, mainly by divers - sappers. The operating principle is extremely simple. The cylinders are filled with nitrox (mostly), the gas flows in a constant flow through a special nozzle (draeger Dolphin, Ray) or through an adjustable needle valve (Azimuth, Ubs-40) into the inhalation bag, then you exhale accordingly into the exhalation bag, then the gas enters the a canister with a chemical absorbent and again into the inhalation bag. During these procedures, as a rule, excess gas occurs, which is removed into the water through a special valve.

aSCR– the most popular recirculation devices on the amateur market today. They are simple, reliable and easy to learn. Their main advantage is gas savings, the use of nitrox mixtures and low noise. The devices, in the basic configuration, do not have any electronics and recommended temperature conditions operation from -1 to +35 degrees, which is also an advantage. Disadvantages are limited depth, lack of advantages in decompression modes and a large difference between the gas in the cylinders and the gas in the breathing circuit, which should be taken into account when planning. The greater the physical activity, the greater the difference and can vary from 5 to 20%.

The most famous models Mix-55 , Mixegers 78(France) Aromix OMG(Italy), Dräger FGT I(Germany) AKA – 60(Russia).The most famous models for the amateur market are Dräger Dolphin(Germany) Draeger Ray(Germany) – discontinued. Fieno(Japan) – discontinued. Azimuth Pro(Italy) UBS-40(Italy) - still in production.

pSCR- differ from aSCR the fact that the gas is supplied not through a nozzle, but through a standard regulator in accordance with the minute consumption of the diver’s mixture. As a result of the direct forced addition of gas, the composition of the actual respiratory mixture in the passive system circuit is more constant than that of devices with active gas supply and does not change significantly with changes in physical activity.

Since the device passive type linked to the RMV value, dive planning is made easier.

The main disadvantage of these devices is the increased resistance to inhalation and exhalation, since the breathing bag is located in the lumbar region. (meaning Halcyon devices and its clones - Ron, SF-1, etc.). An interesting development in this direction is the K2-advantage device (it has a breathing bag on the chest).

Devices of this type are not widespread and are not certified in Europe.

Closed rebreathers

Divided into eCCR and mCCR.

eCCR– this type of device is the most complex, advanced and, accordingly, expensive.

The price of products ranges from 9 to 14 thousand dollars. These are the quietest devices, but their most important advantage is the ability to maintain constant partial pressure oxygen, due to this, effective and rapid decompression occurs, and also increases the no-decompression limits. As a rule, the device uses two cylinders - one with oxygen, the second with diluent (air, trimix, heliox). The rebreather uses electronics to monitor the partial pressure of oxygen and to supply oxygen to the circuit as needed through a solenoid valve. In principle, this is all; the devices differ in nuances - the number of oxygen sensors, the location of the breathing bags, the presence of built-in decompression meters, etc. The most famous and popular devices of this type are Inspiration Vision (England), Megalodon (USA). Currently, quite a lot of electronic devices have appeared on the market. closed type– Optima (USA), Sentinel (England), Voyager (Italy), etc. But the leaders remained the same.

The most important thing is that eCCRs require respect, increased attention and very good training. Descents on closed devices require more discipline and responsibility, therefore their users should be people who dive regularly and are well versed in the specifics of rebreathers. When working with CCR, there is an increased risk of experiencing hypoxia or hyperoxia.

mCCR- they differ from electronic devices in that oxygen is not supplied to the circuit through a solenoid at the command of a computer, but constantly flows through a nozzle (almost like in an SCR or in a simple oxygen device), but it is supplied in a smaller quantity than is necessary for the human body, i.e. .e. somewhere 0.6-0.7 l/min. Electronics are present to monitor po2 values. Lack of oxygen is supplied manually. As is usually the case in our country, what we don’t keep, we lose through tears. Foreigners took our IDA-71s and made mCCRs out of them. Today, the most popular devices of this type are KISS (Canada), rEVO (Belgium), Submatix (Germany), Pelagian (Thailand).

Prices range from 5 to 8 thousand dollars.

The device meets the requirements of GOST R 53256-2009. Autonomous Breathe-helping machine closed cycle, operating on compressed oxygen with excess submask pressure, designed to protect the respiratory system and human vision during long-term use in a smoky or toxic gas environment. It is used during rescue operations in mines, fires, in confined spaces, during rescue operations in tunnels and when working with hazardous substances.

All modifications of the AP "Alpha" are made in the form of a backpack, the load from which, when worn, is distributed on the shoulders and hips. The device is equipped with a pressure gauge that shows the remaining amount of oxygen and produces two visual alarms and one sound signal, showing the state of the system.

The closed-loop system recycles exhaled air, eliminates carbon dioxide, replaces consumed oxygen, absorbs condensation and cools inhaled and exhaled air.

Overpressure ensures the internal pressure under the mask is slightly higher than the external pressure atmospheric pressure. This provides 100% protection of the respiratory organs and vision from the external atmosphere entering under the mask.

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Closed circuit rebreathers

Closed Circuit Oxygen Rebreather - O2-CCR

This is the ancestor of rebreathers in general. The first such apparatus was created and used by the British inventor Henry Fluss in the mid-19th century while working in a flooded mine. A closed-cycle oxygen rebreather has all the main parts typical for any type of rebreather: a breathing bag, a canister with a chemical absorbent, breathing hoses with a valve box, a bypass valve (manual or automatic), a bleed valve and a cylinder with a reducer high pressure. The principle of operation is as follows: oxygen from the breathing bag enters through a non-return valve into the diver’s lungs, from there, through another non-return valve, oxygen and carbon dioxide formed during breathing enter the chemical absorbent canister, where carbon dioxide is bound by caustic soda, and the remaining oxygen is returned to the breathing bag. Oxygen, replacing that consumed by the diver, is supplied to the breathing bag through a calibrated nozzle at a rate of approximately 1 - 1.5 liters per minute or is added by the diver using a manual valve. During a dive, compression of the breathing bag is compensated either by the operation of an automatic bypass valve or by a manual valve controlled by the diver himself. It should be noted that, despite the name “closed”, any closed-circuit rebreather releases breathing gas bubbles through an exhalation valve during ascent. To get rid of bubbles, caps made of fine mesh or foam rubber are installed on the etching valves. This simple device is very effective and reduces the diameter of the bubbles to 0.5 mm. Such bubbles completely dissolve in water after just half a meter and do not unmask the diver on the surface.

The limitations inherent in closed-cycle oxygen rebreathers are primarily due to the fact that these devices use pure oxygen, the partial pressure of which is the limiting factor in the depth of immersion. Thus, in sports (recreational and technical) training systems, this limit is 1.6 ata, which limits the diving depth to 6 meters in warm water with minimal physical activity. In the German Navy, this limit is 8 meters, and in the USSR Navy - 22 meters.

Closed circuit rebreather with manual oxygen supply - mCCR or KISS

This system is also called K.I.S.S. (Keep It Simple Stupid) and was invented by Canadian Gordon Smith. This is a closed-cycle rebreather with on-the-fly mixture preparation (selfmixer), but in the simplest possible design. The operating principle of the device is that 2 gases are used. The first, called diluent, is automatically or manually supplied to the breathing bag of the apparatus through a lung demand valve or bypass valve, respectively, to compensate for the compression of the breathing bag during immersion. The second gas (oxygen) is supplied to the breathing bag through a calibrated nozzle with constant speed, however, less than the rate of oxygen consumption by the diver (approximately 0.8-1.0 liters per minute). When diving, the diver must himself monitor the partial pressure of oxygen in the breathing bag according to the readings of electrolytic oxygen partial pressure sensors and add the missing oxygen using a manual supply valve. In practice, it looks like this: before diving, the diver adds a certain amount of oxygen to the breathing bag, setting the required partial pressure of oxygen using sensors (within 0.4-0.7 ata). During the dive, to compensate for depth, a diluent gas is automatically or manually added to the breathing bag, reducing the oxygen concentration in the bag, but the partial pressure of oxygen still remains relatively stable due to the increase in water column pressure. Having reached the planned depth, the diver uses a manual valve to set any partial pressure of oxygen (usually 1.3) and works on the ground, monitoring the readings of the oxygen partial pressure sensors every 10-15 minutes and adding oxygen, if necessary, to maintain the required partial pressure. Typically, within 10-15 minutes, the partial pressure of oxygen decreases by 0.2-0.5 ata, depending on physical activity.

Not only air, but also trimix or heliox can be used as a diluent gas, which allows diving with such a device to very decent depths, however, the relative variability of the partial pressure of oxygen in the breathing circuit makes it difficult to accurately calculate decompression. Usually, with devices that only have an indication of the partial pressure of oxygen in the circuit, they dive no deeper than 40 meters. If a computer is connected to the circuit, capable of monitoring the partial pressure of oxygen in the circuit and calculating decompression on the fly, then the depth of the dive can be increased. The deepest dive with a device of this type can be considered the dive of Matthias Pfizer, who dived to 160 (one hundred and sixty) meters in Hurghada. In addition to oxygen partial pressure sensors, Mathias also used a VR-3 computer with an oxygen sensor, which monitored the partial pressure of oxygen in the mixture and calculated decompression taking into account all changes in the breathing gas.

There are a large number of conversions of commercial, military and sports rebreathers to the K.I.S.S. system, but all this, of course, is unofficial and under the personal responsibility of the diver converting and using them.

Electronically controlled closed circuit rebreather - eCCR

Actually, a real closed-cycle rebreather (electronically controlled selfmixer). The first such device in history was invented by Walter Starck and was called Electrolung. The principle of operation is that a diluent gas (air or trimix or heliox) is supplied by a manual or automatic bypass valve to compensate for the compression of the breathing bag during diving, and oxygen is supplied using a microprocessor-controlled solenoid valve. The microprocessor interrogates 3 oxygen sensors, compares their readings and, averaging the two closest ones, issues a signal to the solenoid valve. The readings of the third sensor, which differ most from the other two, are ignored. Typically the solenoid valve is activated once every 3-6 seconds depending on the diver's oxygen consumption.

The dive looks something like this: the diver enters two oxygen partial pressure values ​​into the microprocessor, which the electronics will maintain for different stages dives. Typically this is 0.7 ata to exit from the surface to the working depth and 1.3 ata to stay at depth, undergo decompression and ascent to 3 meters. Switching is carried out using a toggle switch on the rebreather console. During the dive, the diver must monitor the operation of the microprocessor to identify possible problems with electronics and sensors.

Structurally, electronically controlled closed-cycle rebreathers have virtually no restrictions on depth, and the actual depth at which they can be used is mainly determined by the error of the oxygen sensors and the strength of the microprocessor housing. Usually the maximum depth is 150-200 meters. Electronic closed-cycle rebreathers have no other restrictions. The main disadvantage of these rebreathers, which significantly limits their distribution, is the high price of the device itself and consumables. It is important to remember that conventional computers and decompression tables are not suitable for diving with electronic rebreathers, since the partial pressure of oxygen remains constant throughout almost the entire dive. With rebreathers of this type, either special computers must be used (VR-3, VRX, Shearwater Predator, DiveRite NitekX, HS Explorer) or the dive must be pre-calculated using programs such as Z-Plan or V-Planer based on the minimum possible partial pressure of oxygen (in this case, it is necessary to very strictly ensure that the value of the partial pressure does not decrease below the calculated value, otherwise the risk of getting DCS increases many times over). Both programs are recommended for use by manufacturers and creators of all electronic rebreathers.

Semi-closed cycle rebreathers

Semi-closed cycle rebreather with active feed - aSCR

This is the most common type of rebreather in sport diving. The principle of its operation is that the EANx Nitrox breathing mixture is supplied into the breathing bag at a constant speed through a calibrated nozzle. The feed rate depends only on the oxygen concentration in the mixture, but does not depend on the depth of immersion and physical activity. Thus, the oxygen concentration in the breathing circuit remains constant during constant physical activity. Obviously, with this method of supplying breathing gas, excess gas appears, which is removed into the water through the etching valve. As a result, a semi-closed cycle rebreather releases several bubbles of the breathing mixture not only upon ascent, but also with each exhalation of the diver. Approximately 1/5 of the exhaled gas is released. To increase secrecy, deflector caps, similar to those used in closed-cycle oxygen rebreathers, can be installed on the bleed valves.

Depending on the oxygen concentration in the EANx (Nitrox) breathing mixture, the flow rate can vary from 7 to 17 liters per minute, thus, the time spent at depth when using a semi-closed cycle rebreather depends on the volume of the breathing gas cylinder. The immersion depth is limited by the partial pressure of oxygen in the breathing bag (should not exceed 1.6 ata) and the set pressure of the reducer. The fact is that the flow of gas through a calibrated nozzle has a supersonic speed, which makes it possible to keep the flow constant as long as the set pressure of the reducer exceeds the pressure environment two or more times.

Passive feed semi-closed cycle rebreather - pSCR

The operating principle of the device is that part of the exhaled gas is forcibly released into the water (usually 1/7 to 1/5 of the inhaled volume), and the volume of the breathing bag is obviously less than the volume of the diver’s lungs. Due to this, for each breath, a fresh portion of breathing gas is supplied through the lung demand valve into the breathing circuit. This principle allows you to use any gases other than air as a breathing mixture and very accurately maintain the partial pressure of oxygen in the breathing circuit, regardless of physical activity and depth. Since the supply of breathing gas is carried out only on inspiration, and not constantly, as is the case with rebreathers with an active supply, a semi-closed cycle rebreather with a passive supply is limited in depth only by the partial pressure of oxygen in the breathing circuit. A significant negative point in the design of semi-closed cycle rebreathers with passive supply is that the automation is activated by the respiratory movements of the diver, which means that the severity of breathing is obviously greater than on other types of devices. Devices using a similar operating principle are preferred by underwater speleologists and followers of the DIR teachings in diving.

Mechanical self mixer - mSCR

A very rare design of a semi-closed cycle rebreather. The first such device was created and tested by Drägerwerk in 1914. The principle of operation is as follows: there are 2 gases (oxygen and diluent), which are supplied through calibrated nozzles into the breathing bag, as in a semi-closed cycle rebreather with an active feed. Moreover, oxygen is supplied at a constant volumetric velocity, as in a closed rebreather with manual supply, and the diluent enters through the nozzle at a subsonic flow rate, and the amount of supplied diluent increases with increasing depth. Compensation for compression of the breathing bag is carried out by supplying diluent through an automatic bypass valve, and excess breathing mixture is released into the water in the same way as in the case of a semi-closed cycle rebreather with an active supply. Thus, only due to changes in water pressure during the dive, the parameters of the breathing mixture change, and in the direction of decreasing oxygen concentration with increasing depth. Mechanical self-mixers tend to change the oxygen concentration in the breathing bag when physical activity changes, and this is a direct consequence of the fact that their principle of operation is very similar to the principle on which semi-closed rebreathers with active feed are built.

The depth restrictions for a mechanical self-mixer are the same as for a semi-closed cycle rebreather with an active feed, with the exception that only the set pressure of the oxygen reducer must exceed the ambient pressure by 2 or more times. In terms of time, the selfmixer is mainly limited by the volume of diluent gas, the supply rate of which increases with depth. Air, Trimix and HeliOx can be used as diluent gases.

Semi-closed cycle rebreather with active feed with mixture preparation during feeding

A very rare semi-closed cycle rebreather design. This type of rebreather, in its operating principle, is completely similar to a semi-closed cycle rebreather with an active feed, except that the breathing mixture is not prepared in advance, but during the operation of the rebreather. The principle of operation is as follows: there are 2 gases (oxygen and diluent), which are supplied through calibrated nozzles into the breathing bag, just like in a semi-closed cycle rebreather with an active feed. The supply of both oxygen and diluent occurs at a constant rate, regardless of depth, while the gases are mixed in the breathing bag. Depending on the supply rate of oxygen and diluent, we obtain the gas we need. This type of rebreather has all the disadvantages of a semi-closed type rebreather with an active feed; in addition, it is more complex in design and requires at least two gas cylinders (while for normal operation aSCR only requires one gas cylinder). The advantage of rebreathers of this type is that there is no need to prepare the breathing mixture in advance and it is possible to set the desired gas in the circuit (by adjusting the flow rate of O2 and diluent) without changing the source gases, but only their proportion. The following diluent gases can be used: air, Trimix and HeliOx.

Regenerative rebreathers

Regenerative rebreathers can operate using both closed and semi-closed breathing patterns. Their main difference is that in addition to (instead of) the usual carbon dioxide absorber, a regenerative substance is used: O3 (o-tri), ERW or OKCh-3 created on the basis of sodium peroxide. The regenerative substance is capable of not only absorbing carbon dioxide, but also releasing oxygen. The principle of operation of a regenerative rebreather is that the diver’s oxygen consumption is compensated not only by the supply of fresh breathing mixture from a cylinder, but also by the release of oxygen by the regenerative substance.

Classic representatives of regenerative rebreathers include the devices IDA-59, IDA-71, IDA-72, IDA-75, IDA-85.

Separately, as the most successful design, one can note devices of the IDA-71 type, which are still used in units of combat swimmers and reconnaissance divers. The design of the device and the principle of its operation are simple and accessible. When used correctly, it is very reliable. Despite its “venerable” age (in principle, the device is considered morally obsolete), it is considered the most successful design of devices of this type and is still produced today (the “Respirator” plant). The IDA-75 and IDA-85 devices were produced in a pilot series, but due to the collapse of the USSR they never went into production. After the collapse of the USSR design bureaus until they invented a device superior in its characteristics to the IDA-71.

When descending in closed-circuit devices using pure oxygen, decompression modes are not used. According to the Navy Diving Service Rules, descents using pure oxygen are allowed to depths of up to 20 meters. When using AKS and AAKS type mixtures, non-decompression descents are allowed to depths of up to 40 meters - in the IDA-71 apparatus, and up to 60 meters in the IDA-75 and IDA-85 apparatus. The maximum permissible non-decompression time at these depths is 30 minutes. If the specified residence time is exceeded, exit is carried out in compliance with the decompression regime.

A rebreather is a recirculating breathing apparatus, that is, a device in which, unlike scuba gear (SCUBA), when exhaling, the respiratory mixture is not completely removed into the water or is not completely removed. Instead, the spent mixture is processed so that it can be re-breathed (re-breathe). For this you need remove carbon dioxide from the mixture(carbon dioxide) and add oxygen to the mixture.
The first task is solved in the same way in all rebreathers - they always contain a container (absorption canister) included in the breathing circuit, which is filled with a chemical substance that actively absorbs carbon dioxide.
The second task - adding oxygen to the mixture - is solved in various types rebreathers in different ways. Let's take a closer look at this...

What types of rebreathers are there?

All rebreathers according to their operating principle can be divided into two large groups: semi-closed And completely closed.
IN closed In rebreathers (CCR - Closed Circuit Rebreathers), the exhaled mixture is completely processed and, after removing carbon dioxide, pure oxygen is added to it. This is not to say that the mixture in these types of rebreathers is not etched into the water at all; rather, it is not etched when swimming at a constant depth. When ascending, that is, when the external pressure decreases, the breathing mixture expands and its excess is removed into the water through the exhaust valve.
Semi-closed rebreathers (SCR - Semi Closed Rebreathers) differ from closed ones in that the mixture is removed from the breathing circuit even when swimming at a constant depth, but the amount of the mixture removed is much less than that of a conventional scuba tank. Removing part of the mixture is necessary because to maintain the required level of oxygen in the breathing mixture, it is not pure oxygen that is used here, but artificial breathing mixtures such as Nitrox, Trimix and Heliox. Therefore, it is necessary to remove excess neutral gases: nitrogen and helium.
In turn, both closed and semi-closed rebreathers can be of several types according to the principle that supports optimal composition breathing mixture.
Closed:
1) Oxygen rebreathers(CCOR - Closed Circuit Oxygen Rebreather) operate on pure oxygen, i.e. the diver breathes pure oxygen without any admixture of neutral gases. This principle simplifies the design and reduces the size, but also introduces its own limitations. You and I know that oxygen becomes toxic when the partial pressure increases above 0.5 bar. In this case, toxicity manifests itself in two forms: pulmonary (calculated in OTU - Oxygen Tolerance Units) and convulsive (calculated by the effect on the central nervous system CNS - Central Nervous System). The maximum safe partial pressure of oxygen for divers is considered to be 1.6 bar (usually 1.4 for long exposures) and only in emergency cases is it allowed to briefly increase it to 2.0 bar (3.0 in the French and Russian Navy). Considering that there is still some neutral gas remaining in the breathing circuit of the device, maximum depth diving in such devices is limited to 7 meters (10 meters in emergency cases).
Another negative factor of the action of pure oxygen is that it “feeds” any manifestations of caries or other diseases of the oral cavity. Therefore, when using such devices, do not forget to visit the dentist regularly (which, by the way, is recommended for all divers) and you will not have problems with your teeth.
Thanks to small sizes, great autonomy and, most importantly, the absence of exhaled bubbles, such devices are very popular among military and underwater biologists.
Most famous representatives of this type: Draeger LAR VI and OMG Castoro C-96.
2) Oxygen rebreathers with chemical regeneration of the breathing mixture(CCCR - Closed Circuit Chemical Rebreather). They are similar in design to rebreathers of the previous type, but differ in the principle of restoring the oxygen content in the mixture. The fact is that, unlike an absorption substance, which simply absorbs carbon dioxide, the canisters of such devices are charged with a regenerating substance, which, when absorbing 1 liter of carbon dioxide, releases approximately 1 liter of oxygen.
Despite their small size, such devices have fantastic autonomy. For example, when using typical representative This group of the Soviet apparatus IDA-71 managed to swim underwater for 6!!! hours.
Unfortunately, the regenerative substance is very capricious to use. When water enters the absorption canister, a foam-like alkali is released, resulting in the same “caustic cocktail” that is used to scare divers when talking about rebreathers (this is one of the most common myths). This “cocktail” can greatly damage the diver’s mouth, larynx, trachea, and even the lungs. An ordinary absorbent substance behaves much calmer. Yes, alkali is released when wet, but without a violent reaction, you can determine the flow of water without tasting the mixture, but simply by difficulty breathing.
This type of device was used only by the military and then only in two countries - the USSR and France. Now, due to the complexity of handling regenerative substances, this type of device is becoming a thing of the past.
3) Rebreathers using breathing mixtures with electronic control(CCMGR - Closed Circuit Mixed Gas Rebreather). As the name suggests, this type of rebreather has electronic system control, which includes an oxygen partial pressure sensor, an electronic circuit that analyzes the oxygen content in the mixture and signals the electric valve to add pure oxygen to the breathing circuit to the optimal level. The advantages of such a scheme are clear: the ability to work with gas mixtures (and not pure oxygen) and, as a result, dive to almost any depth, always optimal partial pressure of oxygen at any depth, absence of bubbles when swimming, maximum possible saving of breathing gas and greater autonomy. On the other hand, it is a complex design with the possibility of electronic failure, difficult and expensive to maintain. Sensors operating on the electrochemical principle have a limited service life when high price and usually require replacement at least once a year.
The most famous representatives of the type: Buddy Inspiration, CIS Lunar.
4) Rebreathers using breathing mixtures with semi-automatic control(KISS rebreather). They differ from the previous type in that the sensors and electronic circuit They only monitor the partial pressure of oxygen, and the diver himself adds oxygen to the breathing circuit if necessary.
The most competent design of this type of device provides for an automatic constant supply of oxygen through the nozzle in quantities less than what the diver needs, and the diver adds oxygen only to maintain the optimal level of partial pressure. In this case, the number of manual manipulations with the device is greatly reduced on the one hand, and on the other hand, one of the points of failure - the solenoid valve - is absent.
Semi-closed:
1) With active supply of breathing mixture(CMF SCR - Constant Mass Flow Semi Closed Rebreathers). In these devices, when the valve of the cylinder containing the breathing mixture is opened, it begins to flow continuously through a calibrated nozzle into the breathing circuit. The partial pressure of oxygen is maintained by removing exactly the same (!!!) amount of the waste mixture into the water. The fresh mixture supply rate (liters per minute) depends on bandwidth nozzle and is selected depending on the depth of immersion and the composition of the breathing mixture.
The attractive features of using this type of rebreather are simplicity of design, ease of calculations, and ease of maintenance. The duration of the dive (in terms of the reserves of the breathing mixture) practically does not depend on the depth, because at all depths the consumption of the mixture from the cylinder changes very little, on the other hand, the partial pressure of oxygen in the breathing circuit is very strong (even more than that of a conventional scuba gear!!!) depends on two factors: the depth of the dive and the physical activity of the diver (that is, oxygen consumption).
The most famous representatives of the type: Draeger Dolphin and Ray, OMG Azimuth.
2) With passive supply of breathing mixture(PA SCR - Passive Addition Semi Closed Rebreather). In this type of rebreather, the partial pressure of oxygen is also maintained by etching part of the spent mixture into water, but (!!!) the amount of the mixture clearly established by the design is removed from the breathing circuit with each exhalation (usually from 8 to 25% of the exhalation volume). Instead of the one removed from the cylinder, an equal amount of fresh respiratory mixture is supplied. It is known that the breathing rate is directly related to the oxygen consumption of the diver, therefore the partial pressure in the breathing circuit of such devices practically does not depend on oxygen consumption and depends only on the depth of the dive (the same as in conventional scuba gear). To put it simply, we can say that when swimming with this type of rebreather, the diver uses all the calculations associated with the use of gas mixtures in conventional scuba gear, but carries with him a supply of gas that is 4-10 times (depending on the bleeding coefficient) greater than the actual volume of the cylinder.
The most famous representatives of the type: Halcyon RB-80, K-2 Advantage, DC-55.

How do rebreathers work?

All rebreathers, without exception, are more complex than scuba tanks. This is understandable, since their operating principle is more complicated. However, they all have similarities design features, which make their work possible.
Firstly, unlike scuba gear, where one hose running from the cylinder to the mouthpiece has long become the norm, the rebreather uses two hoses- one for supplying the mixture to the mouthpiece, the other for returning the mixture to the breathing circuit.
Since the respiratory mixture is not exhaled into the water, but is returned, you need a container into which it can be returned. In addition, the breathing mixture in this container must have the same pressure as the surrounding water. Therefore, each rebreather has one or two breathing bag(breathing bag) from which the diver inhales and exhales a gas mixture under pressure equal to the ambient pressure. The bags can be soft or semi-rigid (on semi-closed rebreathers with passive feed).
To clean the mixture from carbon dioxide, all rebreathers have canister, into which it is poured chemical absorbent.
As mentioned above, the absorbent substance really does not like water getting into the canister. Therefore, most rebreathers have in their design water traps or hydrophobic membranes. The purpose of such devices is to intercept water entering through the mouthpiece and prevent it from entering the absorber. Typically, a second breathing bag (exhalation bag) is used as a trap, which also helps reduce the exhalation resistance of the rebreather.

Advantages of rebreathers.

Speaking about the advantages, we need to start with another myth: that rebreathers are cheaper to use than scuba tanks because they consume less breathing mixture... This is true, but provided that mixtures based on helium (!!!) are used, which is expensive. When using relatively cheap Nitrox, savings on mixture consumption may even be offset by the cost of the absorber. In addition, for complex types of rebreathers, such as closed circuit devices with electronic control, one must take into account the need to replace sensors, which are also expensive, and provide a superficial support team in case of unforeseen circumstances!!!
Another myth is that rebreathers allow you to swim for so long and so deep that this is unattainable with conventional scuba gear. This is also true, but not all types of rebreathers fit this rule, but only closed-cycle rebreathers operating on mixtures! All other types of rebreathers do not fall under this definition...
Now about the real benefits:
1) Less noise and fewer bubbles, which usually scare away all wary marine life;
2) Almost constant buoyancy during the inhalation-exhalation cycle. Since the total volume of the respiratory mixture in the lung-rebreather system remains almost unchanged, the diver is not pulled up when inhaling, and does not pull down when exhaling. A very valuable feature for underwater photographers and videographers, isn't it?;
3) When carbon dioxide is absorbed, a certain amount of water vapor and heat is released, so the diver breathes heated and humidified air. This increases comfort and reduces the risk of decompression sickness, especially when swimming in cold water. For the same reason, rebreathers do not go into free flow.
4) When organizing serious expeditions that require the use of gas mixtures, it is necessary to deliver significantly fewer gas cylinders to the dive site. Although, as written above, you are unlikely to gain in cost, rebreathers consume significantly less gas mixtures than scuba tanks, so an expedition with rebreathers will actually require less gases.

Disadvantages of rebreathers.

Let's start with myths again. We have already talked about the caustic cocktail above, as well as about ways to combat this phenomenon. It only remains to note that it is very difficult to obtain such a cocktail in modern rebreathers, even if you specifically try. Even when releasing the mouthpiece from the mouth, it floats up due to the positive buoyancy of the hoses and begins to bleed the mixture from the inhalation bag, so the amount of water that gets into the exhalation bag is insignificant.
Difficulty of learning. Partially true, at least regarding closed-mix rebreathers. Training for all other types of rebreathers certainly requires basic knowledge of the student, but is no more difficult than any of the scuba diving courses.
Difficult to maintain. Yes, servicing any rebreather takes more effort and time than scuba gear, but the procedures are standard and do not cause difficulties. It just takes habit, just like when servicing SCUBA.
The most important myth is that buying a rebreather will cost much more than scuba gear. It is true that most rebreathers are more expensive than the average SCUBA kit, but some models, especially semi-closed active feed rebreathers, are quite comparable in price to a good SCUBA kit.
Now let's move on to the real disadvantages:
1) A rebreather is not an individualistic device; it requires training and teamwork much more than scuba gear. Should this be considered a disadvantage though?
2) The difficulty of using one device by two divers in an emergency. Although some divers now practice this exercise, they mostly use the emergency diver’s open-circuit breathing from a separate emergency cylinder or a cylinder with breathing mixture rebreather
3) The greater weight and dimensions of the device itself (not including cylinders) makes it difficult to travel.
4) The need to provide consumables (gas mixtures and absorber) at the dive site. Although the gas mixtures used are mostly standard ones, the absorber will appear when rebreathers become common on our reservoirs.