Rebreather

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Rebreather

The term rebreather , rebreather or English rebreather called devices for breathing during diving and as respirators for fire brigade , civil protection , military etc. may be used. Another application is emergency medicine: through the efficient use of breathing gas, it is possible to supply an injured person with a comparatively small oxygen supply (e.g. 2-liter gas cylinder) with pure oxygen for well over an hour.

classification

All rebreather devices have in common that the exhaled air is collected in a flexible container (counterlung). The carbon dioxide is chemically in a soda lime bonded (scrubbers), and metabolized oxygen replaced. The great advantage of rebreather diving equipment compared to open diving systems is the much more efficient use of the breathing gas used . In the case of a closed system, ideally only the oxygen consumed and the change in volume during ascent or descent need to be balanced out, while in the semi-closed system gas is released into the environment unused, but this happens to a significantly lesser extent than in open systems. In principle, rebreather diving equipment can be used to dive with the breathing gas mixtures commonly used in technical diving , such as Trimix or Heliox . This is becoming increasingly attractive due to the low consumption of the expensive diluent gas helium.

Other inherent benefits of rebreather can also be found in the following circumstances: warm air you breathe (reduces the chilling of the body); moist air to breathe (reduces the likelihood of accidents due to dehydration / dehydration, no dry mouth); simple ventilation without high breathing effort (delaying diver fatigue); Low noise.

An inherent disadvantage of rebreather diving devices is the fact - in comparison to diving with open systems - that a buoyancy aid via inhalation and exhalation is no longer available. The tare particularly at low depth requires sensitivity and exercise.

A distinction is made between the working methods:

Closed circuit diving devices, oxygen circuit devices

( English closed circuit rebreather - CCR)

Closed loop means that the current consumed in the body oxygen completely with oxygen from a pressurized gas - oxygen cylinder is replaced. The diver therefore breathes pure oxygen in pendulum breathing with a carbon dioxide filter attached. These devices are mainly used in the military sector ( mine divers , combat swimmers ). The exhaled carbon dioxide is bound as a solid by the reactive sodium hydroxide in the form of sodium hydrogen carbonate . In this context, the sodium hydroxide is also known as soda lime and is located in the soda lime cartridge , into which no water may penetrate. In combination with water, sodium hydroxide forms the strongly alkaline reacting sodium hydroxide solution which can cause severe burns. However, due to today's construction of lime containers and exhalations, a small tolerance volume for water entry (e.g. through condensation) is permitted. With some devices, up to 1 liter of water can enter without the lime container being submerged. In principle, the diver does not release any air bubbles into the water, and the volume in the gas circuit (the diver's lungs and breathing bag as a counterlung) remains constant. In practice, volume changes occur at the latest during taring, ascent or descent, which require additional gas or a loss of gas through a pressure relief valve or through exhalation through the mask.

In practice, closed circuit diving devices are operated with at least two cylinders, a compressed air or mixed gas cylinder and an oxygen cylinder. The bottles usually have a volume of 2 or 3 liters and can be filled to 200 or 300 bar.

When breathing pure oxygen, i.e. a very high partial pressure of 1 bar at normal pressure, the diving depth is limited to about 6 m (corresponding to 1.6 bar partial pressure of oxygen), as otherwise oxygen poisoning can occur. With air / oxygen mixtures, depths of up to 40/50 m can be reached. Correspondingly greater depths are possible with mixed gases.

Semi-closed rebreather devices

(English semiclosed rebreather - SCR)

"Half closed" means that the oxygen used in the rebreather is replaced with the help of a (mixed) gas source. Due to the constant or consumption-dependent addition of breathing gas into the circuit, there is a need to release excess breathing gas into the water through a suitable valve or by exhaling through the mask.

Passive semi-closed rebreather devices

(English passive semiclosed rebreather - pSCR)

"Passive half-closed" means that the oxygen used in the rebreather is replaced with the help of a (mixed) gas source. With each breath, a constant proportion (usually 1:10, i.e. one tenth) of the circulatory volume is removed from the device and released to the outside. The reduced volume is then automatically replenished (via valves) with mixed gas. A constant breathing gas is thus established within a short time. This form of rebreather has found its way into technical cave diving and is used worldwide by cave divers for deep and long, usually several hours long, cave exploration (see European Karst Plain Project and Woodville Karst Plain Project ).

dosage

Manual dosing

Original form of the rebreather devices: used breathing gas is replaced by manual control with a gas valve. Practical only with oxygen cycle devices.

passive

Continuous addition of an oxygen-containing breathing gas mixture to the breathing circuit with the aim of keeping the gas mixture breathable despite consumption. Often used with depth control (depth compensation).

Depth compensation

This is understood to mean a (mechanical) control system that is additionally introduced with constant gas addition or, more rarely, with volume control, which takes into account gas consumption at a greater depth or a desired, different mixture and is regulated via automatic depth control or manual switching.

A change of the gas mixture as well as the scrubber or the circuit itself is provided as part of the depth switching in some devices.

Mechanical / active (volume controlled)

Another early form was the volume controlled oxygen rebreather. If the gas volume in the counterlung decreases (due to consumption or pressure increase), a valve is actuated during volume control and oxygen or breathing gas is introduced. The control often required complex mechanics and was common with fire-fighting or mining equipment, but not with diving.

Electronic regulation

The oxygen content (partial pressure) of the breathing gas is monitored with sensors. Oxygen, inert gas or a mixture is introduced via one or more electronically controlled solenoid valve (s) so that the previously set partial pressure of the oxygen is kept constant. Thus, the percentage of oxygen in the breathing gas changes depending on the current diving depth.

Historically, the deaths with the first type of device of this type, the American ElectroLung, should be mentioned. The cause, however, was missing or used soda lime. With adequate training and maintenance, these technically complex devices can now also be described as largely mature.

Manually controlled by divers

Another method of oxygen enrichment widely used in closed circuit diving equipment follows the KISS principle (Keep it simple and stupid). The first devices were developed and built by Gordon Smith, who named his company and the devices according to this general development principle.

With KISS devices, pure oxygen is usually constantly fed into the circulatory system at a “metabolic” rate (0.8 liters / min) through a nozzle. If there is an increased need, for example during exertion, the diver can add more oxygen manually (by hand). The oxygen concentration is determined by oxygen sensors and displayed to the diver.

Chemically

The scrubber contains a substance that releases oxygen to the same extent when it binds carbon dioxide. Since these chemical reactions are dependent on the operating temperature and therefore an adequate oxygen supply is not guaranteed, these devices, which are possible in principle, have not caught on.

Cryogenic

By cooling down and liquefying the gas, CO 2 components of the breathing air are separated and (liquid) removed. Oxygen is added from a source of liquid oxygen. The function itself was proven with a test device, but it turned out to be consumption and temperature dependent. So far there has been no practical use.

Breathing gases and rebreather diving equipment

Recirculating diving devices with electronically regulated oxygen dosing or manually diver-controlled devices have the unique ability to continuously adapt the composition of the breathing gas during the dive to the requirements of the respective depth compared to all other diving devices. With these devices, the oxygen partial pressure of the gas mixture is continuously measured by means of redundant sensors and kept constant at an adjustable value by adding pure oxygen - the percentage composition of the breathing gas changes constantly. (The alternative to a rebreather is to carry different gas mixtures, each with a complete regulator, from which you can breathe depending on the phase of the dive.)

Example: The partial pressure for oxygen is set to 1 bar. The diluent gas is normal compressed air .

  • At a depth of 20 m, the diver needs breathing gas with an ambient pressure of 3 bar. This corresponds to a composition of approx. 33% oxygen and 67% nitrogen ( Nitrox ).
  • At a depth of 40 m, the diver breathes at 5 bar ambient pressure. At an oxygen partial pressure of 1, this corresponds to a composition of approx. 20% oxygen and 80% nitrogen and thus almost the mixing ratio of normal compressed air (21% oxygen and 78% nitrogen plus small proportions of other gases).

This type of control therefore allows diving with all the advantages of Nitrox , whereby the risk of " oxygen poisoning " due to diving too deep with a non-variable, pre-filled higher level of oxygen saturation is practically excluded. A fixed Nitrox mixture of 40% oxygen means a restriction to a max. Depth of 25 m. An electronically controlled rebreather with this adaptation enables free diving with an optimally high oxygen content, without having to accept a fixed limit.

The use of a dive computer, which ensures the setting of a fixed P O 2 (oxygen partial pressure), can de facto rule out the development of deep intoxication, whereby all the advantages of the closed circuit diving device are fully retained. This applies in particular with regard to gas mixtures, especially with Trimix, the exact mixture of which must be determined depending on the depth during deep dives.

The composition of the breathing mixture, which changes with the depth, has considerable positive effects on decompression and the decompression times to be observed . Due to the higher oxygen content at shallow depths and thus significantly lower nitrogen content in the air you breathe, you get a certain gain in safety with regard to the decompression obligation when diving with a conventional dive computer that assumes nitrogen saturation by air. If you want to exchange this safety gain for an extension of the no-stop time or a shortening of the decompression stop, you can also use a dive computer specially designed for rebreather diving (with or without a connection to the current oxygen values ​​of the rebreather). It should be noted that the calculations of all decompression computers can only be based on general models. The personal circumstances can differ. There is therefore always a risk of accident.

Circular diving devices require thorough training on the respective device and ongoing practice. They allow very deep (up to 200 m and deeper) and / or very long dives (up to well over 3 hours). In the event of poor maintenance or incorrect operation, they may lead to device failure. If emergency bottles are not or not sufficiently carried and used due to the gas pressures in the body, especially during dives with high gas saturation, fatal accidents can occur with a probability bordering on certainty.

Description of the CO 2 absorption in soda lime

The binding of CO 2 in soda lime is carried out in three phases. In the first phase, the CO 2 is bound by water H 2 O to form carbonic acid H 2 CO 3 . It is therefore very important that the soda lime has a certain amount of moisture. (Basic humidity and breathing air)

The carbonic acid now reacts with a component in the soda lime ( caustic soda , NaOH, sodium hydroxide) to form sodium carbonate (Na 2 CO 3 ) and water:

The sodium carbonate now reacts with calcium hydroxide (Ca (OH) 2 ) to form (carbonate) lime ( calcium carbonate , CaCO 3 ) and sodium hydroxide (which is used again in the reaction above):

The reaction substances water and sodium hydroxide are renewed during the process, only the calcium hydroxide is consumed and converted to chemically inactive lime .

Theoretically, 100 g calcium hydroxide (Ca (OH) 2 ) can bind approx. 30 standard liters of CO 2 .

Average soda lime consists of: 5% NaOH, 1% KOH, 0.2% silicon / kieselguhr, 14–19% water and approx. 75% Ca (OH) 2 .

Thus, a theoretical CO results for 1 kg of soda lime at 20 ° C 2 binding ability of 225 standard liters CO 2 . The manufacturers indicate a binding capacity of 120 standard liters for 1 kg of soda lime.

However, the effectiveness decreases with the temperature of the soda lime: 100% at 21 ° C, 80% at 15.5 ° C, 65% at 10 ° C and <50% at 1.5 ° C.

history

British naval frogman with rebreather, 1945

In the 16th century, the first helmet diving suits were used in England and France at depths of up to 20 meters. Air was supplied from the surface as breathing gas by hand pump. Soon the helmets were made of metal, and greater depths were visited. These helmet diving devices were already a kind of rebreather, although their breathing gas supply was purely surface-based and CO 2 was not yet chemically bound, but only flushed out.

Giovanni Borelli wanted to build a closed breathing apparatus in 1680. According to his idea, the breathing air should circulate through a seawater-cooled pipe. He hoped that all impurities would condense on the inside wall of the pipe and could be separated, but it cannot work that simply. However, if you replace the seawater with liquid nitrogen under (constant absolute) pressure and have a tank with liquid oxygen in it, the gas phase of which is in connection with the breathing circuit, you have a cryogenic rebreather that actually works. The CO 2 is frozen out on the wall of the nitrogen tank, which also keeps the temperature of the oxygen tank constant, and the oxygen partial pressure is kept constant by the gas phase above the liquid oxygen. The constant nitrogen pressure above the liquid nitrogen means a constant boiling temperature, so the temperature in the oxygen tank and thus the oxygen partial pressure are also kept constant.

Stephen Hales used the first CO 2 absorber in 1726 : a towel soaked with tartar and seawater inside a helmet for mine rescue equipment.

In 1774, JF Zöllner suggested using pure oxygen for diving.

The Swedish chemist Carl Wilhelm Scheele discovered in 1777 that bees could stay alive in a closed container if you put in a bowl of lime water, which filtered out the CO 2 .

In 1825, William H. James had the idea of ​​attaching a pressure vessel in the shape of a belt to the diver's belly and allowing him to breathe autonomously (without connection to the surface). But he had no idea how to reduce the filling pressure to ambient pressure.

Regnault and Reise made the discovery in 1847 that dogs can stay alive in a sealed chamber when oxygen is added and CO 2 is removed.

The real history of the rebreather began in 1876. Although all previous devices can be called rebreather (in this case pendulum breathing ), none of these devices did not breathe the breathing gas through a circuit.

Henry Fleuss , a German officer in the English merchant navy , began developing an oxygen rebreather . He did not use a mouthpiece in the modern sense, but a rubber mask. The oxygen tank was made of copper. As a scrubber, he used a ball of rope soaked in a quick lime solution. In 1879 he tested his new device for an hour in a water tank, and later he dived it in a bay up to 5.5 meters deep. Henry Fleuss thus became the first " scuba diver". His device was used in the construction of the Severn Railway Tunnel in 1880 and was later used several times to rescue miners from flooded tunnels. The further development led to a device that was used in 1905 as a rescue device for abandoning submarines.

Achilles de Khotinsky and Simon Lake patented the use of barium hydroxide as a scrubber for rebreather in 1881 .

Siebe Gorman patented Oxylite in 1904 . This is a mixture of KO 2 and Na 2 O 2 that gives off oxygen when it reacts with water or CO 2 .

The Drägerwerk manufactured its first submarine diving rescue in 1907. Since its foundation in 1889, the Dräger company has developed into one of the market-leading manufacturers of rebreather diving equipment.

In 1911 Dräger made its first attempts with a circulatory helmet diving device.

In 1912, Bernhard and Heinrich Dräger presented the freely portable, tubeless Dräger diving apparatus. At first glance, it was hardly distinguishable from the usual helmet diving equipment, but the air hose was missing. The back weight was also missing, in its place two oxygen bottles and the absorber were attached.

In 1913, Dräger made deep diving experiments with circuit diving equipment. On July 17, 1913, a 40-minute dive in the diving tower led successfully to 9 bar (80 meters).

In 1914, Dräger designed a self-mixing (oxygen-air) rebreather for diving depths of up to 40 meters.

Jacques-Yves Cousteau was one of the first to use oxygen rebreather for diving. After two dives in 1938, both of which resulted in oxygen poisoning , he lost interest in these diving equipment.

Christian Lambertsen invented the Lambertsen Amphibious Respiratory Unit (LARU) in 1939 .

In 1942, Hans Hass and the chief engineer Hermann Stelzner at Dräger developed the swim diving device from the Gegenlunge. Siebe-Gorman presented the Salvus ANS oxygen circulation device, the breathing bag of which was carried on the back in the form of a rucksack.

The well-known IDA-71 appeared in 1957: a partly chemically metering, switching mixed gas rebreather (CCCR) from the Soviet Union .

The Trimix circulation device IDA-59M was developed in 1959 as a submarine rescuer for depths of up to 300 meters.

In 1968 the first electronically controlled rebreather with automatic breathing gas mixture appeared. The ElectroLung was used in the following Conself project. Some deaths from used soda lime overshadowed the development.

In 1984 Dräger presented the CCBS deep diving system for depths of up to 600 meters.

The British company Ambient Pressure Diving brought the Inspiration (formerly Buddy Inspiration Rebreather ) onto the market in 1998 , one of the currently most widely used rebreathers.

See also

literature

Web links

Commons : rebreather  - collection of images, videos and audio files

Individual evidence

  1. ↑ The strong man, the rebreather and the tunnel divernet.com, (accessed February 23, 2010)