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Theory
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The idea of using a bag to bring air to breath , dates back to the time of "Ancient Romans," A flexible bag is subjected to external pressure, also case for the gas content in it, and then the bag in question provides gas pressure environment.
If the next exhalation gas back in the bag breathing is called "closed loop".
In this case it is necessary to control the composition of gas inhaled, in fact, for every respiratory act, we have an increase of carbon dioxide and a decrease of oxygen, which makes the gas unbreathable even after a few breaths.
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To remove carbon dioxide is sufficient to achieve a filter containing an absorbent chemical.
The search for chemical elements capable of absorbing the carbon dioxide goes back at the end of'800.
Today we have produced widely tested and safe.
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ll next problem is to restore the oxygen consumed by metabolism, which resolved ...... we made a Rebreather!
As we shall see, this system consists considerable advantages if compared with open circuit: breathe into a bag properly constructed can recover the gas, humidity and heat.
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To better understand the following topics is necessary to make certain preconditions of physiology.
The metabolic oxygen consumption has averaged about one liter per minute, a low consumption if compared to the gas consumption of the open circuit , you must also show that the metabolic consumption does not increase with depth: with the open circuit is wasting tens of liters per minute of precious gas, to take about one liter per minute oxygen.
A side of a table comparing gas consumption shows the enormous waste of open circuit, compared with the Closed-Circuit systems (thin green stripe, bottom), both with Semi-Closed systems (Chart yellow)
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The gas we breathe in open circuit is cold because of expansion due to the sharp reduction in pressure that occurs in providing the heat loss due to a cold gas with the lung surface, which is very extensive, is critical. The gas recovered from a Rebreather keeps the heat of the expired gas, which greatly reduces the problem of cold draft. Another important advantage relates to the moisture gas inhaled the gas in cylinders is dried by the charging process, the inhalation of dry gas involves a dehydration level lung and blood, which constitutes an obstacle to eliminate inert gas in decompression. This problem does not exist if we are breathing into a Rebreather, because the moisture emitted during exhalation is fully recovered. Contrary to the circuit open, the rebreathers provide warm and humid gas also because of the contribution of the chemical on the filter carbon dioxide. The chemical reaction produces also heat and humidity. The next table shows the temperature of gas inhaled as function of the environment's temperature.
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The essential structure of a Rebreather consists of a flexible container in which to breathe, called "counterlung", hoses that convey the respiratory gases and f a scrubber to absorbs CO2: this set is tcalled "loop" and is the "machine" that allows the pulmonary ventilation, delivery of the equivalent circuit open.
That circuit is common to all types of rebreathers and may consist of one or two counterlungs. In the second case there ia a bag for exhalation and the other for inspiration.
The next figure shows an hypothetical Loop composed of two bags, corrugated thoses with the mouthpiece and down the filter of carbon dioxide.
The mouthpiece is fitted with a "DSV" (Dive Surface Valve), a tap that must remain closed when not breathe in the circuit to prevent flooding of the circuit itself. On the sides of the mouthpiece there are two check valves that are used to convey the respiratory flow in the right direction.
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The first rebreathers did use simply pure oxygen, In these devices monitoring of oxygen is related to the decrease in volume of counterlung.
The loop is filled with pure oxygen that is consumed by metabolism, while the carbon dioxide produced is eliminate by the scrubber, this leads to a reduction in volume of counterllungs that is restored from O2 supplied by the cylinder, manually or automatically by a regulator a demand.
The pure oxygen is failing to limit the depth of immersion at very low depth (about 6 mt/20 feet), so if you want to enhance the operational depth, is necessary to dilute the oxygen with inert gas to breathe and a suitable mixture.
These devices are called "Oxygen Rebreather".
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When there is a mixture into the loop, the fraction of oxygen decreases due to metabolic consumption.
In other words, the volume of gas decreases slightly, but the fraction of oxygen decreases very up to levels insufficient to support life: If you do not enter new oxygen to replace what is metabolized an hipoxya may occur.
The metabolic oxygen consumption in volume (lt / min) is indicated by the abbreviation "VO2"
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The metabolic oxygen consumption does not vary with depth but varies greatly depending on the diver's work effort.
The date indication so far of about a liter per minute is an average value and very approximate.
There are many scientific studies that have helped to find the relationship between the diver's work effort and metabolic consumption , tables obtained and the method are very different and showed variations ranging from 0.3 l / min in absolute rest until and beyond 3lt/min particularly in subjects trained.
The next figure shows a table obtained experimentally
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Considering the variability of metabolic oxygen consumption is not possible to think of balancing consumption with a fixed flow of pure oxygen: small differences cause a sudden decrease or increase the fraction of inspired oxygen (fO2)
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From a theoretical point of view, we can expect to charge the cylinder of an oxygen rebreather, with a gas mixture instead of pure oxygen and create a system to regularly renew the mixture in the bag (which is impoverished oxygen) with that from the cylinder .
This is the idea that is the basis of these SCR, Semiclosed Circuit Rebreather
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Active Addiction-Continous mass flow systems- SCR-CMF (Semiclosed Circuit Rebreather Constant Mass Flow)
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There are two main systems for achieving the above: one is to achieve a continuous flow of the mixture can balance the VO2 max predicted, the system is said "active addiction" or constant flow CMF (constant mass Flow).
These systems are called SCR-CMF (Semiclosed Circuit Rebreathers-Constant Mass Flow).
The continuous flow of these systems is obtained by inserting a special nozzle supplied by IP originating from the pressure reducer (1st stage).
This flow serves only to control the fraction of oxygen in the bag, Lung ventilation is ensured by the loop and is not influenced by the continuous flow.
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Passive addiction systems - PASCR (Passive Addiction Semiclosed Circuit Rebreather)
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In the other system, the renewal of the mixture is relized by venting outside a fraction of the breathing volume that is replaced by the mixture supplied by the tank.
To achieve this system use two bellows counterlungs a great one and a little one that is put internally the great one: The expired gas fills both the counterlungs; later during inspiration, gas in the large bag is rebreathed, in this way the great counterlung, compresses the smaller forcing it to vent its contents outside. The relationship between the volume of small counterlung and the total volume of the two counterlungs is calculated to renew, for every respiratory act a certain fraction of gas ventilated. This ratio depends by the fraction of mixture of the tank and is fixed by the manufacturer's design. The volume of gas expelled from the small bag is restored, for every respiratory act, by a demand valve.
The picture shows the functioning of being exhalation. At this stage the expired gas fills both the bellows counterlungs.
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This image shows what happens durino the inspiration. The gas content in the internal bellows is discharged into the environment, while the remainder is rebreathed.
The volume of vemnted gas is restored with new gas from the cylinder. The renewal of this mixture into the respiratory system provides oxygen consumed by metabolism.
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Elettrochemical Automatic Closed Circuit Rebreathers - ECCR (Electronic Closed Circuit Rebreather)
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Until now we have seen how works systems based on the estimation of metabolic oxygen consumption, the ideal thing, of course, is to be able to measure the oxygen in the respiratory circuit, to do so we need of oxygen sensors.
With the reading of oxygen sensors we can add oxygen exactly as required: this is the working principle of CCR (Closed Circuit Rebreather).
The SCR systems, and then added oxygen "estimates", while closed systems "on balance".
The first sensor for oxygen for underwater use was developed by John Kanwisher Rebreather for the first "electronic rebrethaer", the Electrolung in 1968.
The operating principle of a CCR is very simple: there are two cylinders, one of oxygen and one of diluent which is never pure inert, but a mixture of gases whose FO2 is breathing at the maximum depth provided, usually it is air up to 40 meters and in the case of diving to depths greater consists of an appropriate mixture of Trimix or Heliox. During the descent diluent fills the loop, automatically through a demand valve (ADV) or manually, with a by-pass operated by the diver. Then underwater electronic controls a solenoid which provides oxygen to the achievement of PO2 chosen (the setpoint). Thus the fraction of O2 varies with the depth. At constant metabolism, consuming oxygen down the pO2 and still is restored by electronics that receives information from sensors.
While surfacing, the decline in PA involves an equal decrease in pO2: even in this case electronic wlii supply O2 thus providing the optimum amounts of oxygen to the depths reached. Even oxygen can be added manually via a bypass, in case of need or solenoid's failure.
The heart of the system is the oxygen sensor (generally are three sensors for redundancy), which is a small electrochemical current generator, this is practically a "fuel cell" that provides a voltage proportional to pO2, so reading the voltage sensor can be traced back directly to pO2 in the loop.
In these devices the control of the mixture is entrusted with electrochemical sensors, therefore, should be called "electrochemical" rather than "electronic", especially given that electronics are not interposes between the diver and control of the mix, but rather acts in parallel, as an autopilot.
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Elettrochemical Manual Closed Circuit Rebreathers MCCR (Manual Closed Circuit Rebreather)
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As said, in a Closed Circuit system, controlled by oxygen sensors, the responsibility of the composition of the mixture inspired is not entrusted blindly to electronics, but remains dependent on the diver who must periodically check the "displays" and if necessary must operate manually the oxygen by-pass valve. The manual addition of oxygen does not involve a heavy commitment and is just the first skill of a rebreather's training course.
These considerations led to the creation of a closed system manually controlled, where the diver supply oxygen through a lever or button acting on a system of opening the By-Pass. These devices are called MCCR (Closed Circuit Rebreather Manual).
When oxygen is supplied manually, the ranges of action on by-pass is scanned by minutes, not seconds. Despite this, generally these rebreathers are equipped with a nozzle that supplies a continuous flow of oxygen. This flow must be less than the metabolic consumption at resti to avert the danger of an increase in the proportion of oxygen in the circuit (eg 0.7-0.8 l / min against an average consumption of 1 l / min). In the presence of that flow ranges intervention on By-Pass are dilated to about 5 times
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Chemically Cotrolled Systems - CCR (Chemical Closed Circuit Rebreather)
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Developed in Russia, these devices, constructively very simple, uses a superoxide which is a chemical compound that releases oxygen in proportion to absorbing CO2, from the physiological point of view there is a ratio between CO2 production and O2 consumption, the control of O2 percentage is in principle satisfied. Their design is very simple, is enough to put a filter containing peroxide in "parallel" with that of soda lime (a cylinder of oxygen or mixture is present to ensure the volume of gas into the loop), the rest of design is the same of an oxygen rebreather, even that this principle is mainly employed for Russian oxygen rebreathers.
This system that may seem that "the egg of Columbus" for the control of the mixture has its greatest deterrent in that superoxide is a highly incendiary: If goes in contact with fuel burns even under water, if wet initiates an exothermic reaction that can also merge its canister aluminum made, producing furthermore highly noxious vapors, if all goes well this chemical product is stabilized with asbestos that is not the best thing to breathe. In practice, if you do not die before then probably you will die anyway.
For information this superoxide is the chemical that caused the fire on the space station MIR.
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