lar v draeger training course manual
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lar v draeger training course manualRebreather diving is used by recreational, military and scientific divers in applications where it has advantages over open circuit scuba, and surface supply of breathing gas is impracticable. The main advantages of rebreather diving are extended gas endurance, and lack of bubbles.Gas reclaim systems used for deep heliox diving use similar technology to rebreathers, as do saturation diving life support systems, but in these applications the gas recycling equipment is not carried by the diver. Atmospheric diving suits also use rebreather technology to recycle breathing gas, but this article covers the technology, hazards and procedures of ambient pressure rebreathers carried by the diver.As the diver goes deeper, much the same mass of oxygen is used, which represents an increasingly smaller fraction of the inhaled gas. Since only a small part of the oxygen, and virtually none of the inert gas is consumed, every exhaled breath from an open-circuit scuba set represents at least 95 wasted potentially useful gas volume, which has to be replaced from the breathing gas supply. The saving is proportional to the ambient pressure, so is greater for deeper dives, and is particularly significant when expensive mixtures containing helium are used as the inert gas diluent. The rebreather also adds gas to compensate for compression when depth increases, and vents gas to prevent overexpansion when depth decreases.With open circuit scuba, the entire breath is expelled into the surrounding water when the diver exhales.The diver's metabolic rate is independent of ambient pressure (i.e. depth), and thus the oxygen consumption rate does not change with depth. The production of carbon dioxide does not change either since it also depends on the metabolic rate. This is a marked difference from open circuit where the amount of gas consumed increases as depth increases since the density of the inhaled gas increases with pressure, and the volume of a breath remains almost unchanged.http://www.7pub.pl/7b/userfiles/99-s10-repair-manual.xml
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The economy of gas consumption is also useful when the gas mix being breathed contains expensive gases, such as helium. In normal use at constant depth, only oxygen is consumed: small volumes of inert gases are lost during any one dive, due mainly to venting of the gas on ascent. For example, a closed circuit rebreather diver effectively does not use up any diluent gas after reaching the full depth of the dive. On ascent, no diluent is added, however most of the gas in the loop is lost. A very small amount of trimix could therefore last for many dives. It is not uncommon for a 3 litre (19 cubic foot nominal capacity ) diluent cylinder to last for eight 40 m (130 ft) dives.This can offer a dramatic advantage at the end of deeper dives, where a diver can raise the partial pressure of oxygen during decompression, permitting shorter decompression times. Care must be taken that the partial pressure of oxygen is not set to a level where it can become toxic.A malfunctioning rebreather can supply a gas mixture which contains too little oxygen to sustain life, too much oxygen which may cause convulsions, or it may allow carbon dioxide to build up to dangerous levels.This makes them more expensive to produce, more complex to maintain and test, and sensitive to getting their circuitry wet. With open circuit, this type of failure can only occur if the diver selects an unsuitable gas, and the most common type of open circuit failure, the lack of gas supply, is immediately obvious, and corrective steps like changing to an alternative supply would be taken immediately.When an open-circuit scuba diver inhales, a quantity of highly compressed gas from their cylinder is reduced in pressure by a regulator, and enters the lungs at a much higher volume than it occupied in the cylinder. This means that the diver has a tendency to rise slightly with each inhalation, and sink slightly with each exhalation.http://primary-insulation.com/userfiles/99-r1-manual.xml This does not happen to a rebreather diver, because the diver is circulating a roughly constant volume of gas between his lungs and the breathing bag. This is not specifically an advantage or disadvantage, but it requires some practice to adjust to the difference.In some dry open environments, such as a recompression chamber or a hospital, it may be possible to put fresh absorbent in the canister when break through occurs.If not enough new oxygen is being added, the proportion of oxygen in the loop may be too low to support life. In humans, the urge to breathe is normally caused by a build-up of carbon dioxide in the blood, rather than lack of oxygen. The resulting serious hypoxia causes sudden blackout with little or no warning. This makes hypoxia a deadly problem for rebreather divers.The loop often has a pressure relief valve to prevent over-pressure injuries caused by over-pressure of the loop. In some modern oxygen rebreathers, the pressure in the breathing bag controls the oxygen flow like the demand valve in open-circuit scuba; for example, trying to breathe in from an empty bag makes the cylinder release more gas. The volume in the loop is usually controlled by a pressure controlled automatic diluent valve, which works on the same principle as a demand valve, to add diluent when inhalation lowers the pressure in the loop during descent or if the diver removes gas from the loop by exhaling through the nose.You can help by adding to it. ( October 2019 ) The feedback of actual oxygen partial pressure measured by the oxygen sensors is compared with the set-points, and if it deviates outside of the limits of upper and lower set-points, the control system will activate a solenoid valve to add oxygen or diluent gas to the loop to correct the oxygen content until it is within the set-point limits. Usually the user can override the gas addition by manual activation of injection valves.http://gbb.global/blog/bose-ipod-speaker-manual Some control systems allow depth activated switching of set-points, so that one pair of set-points can be selected for the main part of the dive, and another, usually richer, for accelerated decompression above the limiting depth. The changeover is automatic during ascent.The calculation depends on the mose of gas addition.Dump rate is equal to feed rate minus oxygen consumption for this case.In the interests of safety, the range can be determined by calculating oxygen fraction for maximum and minimum oxygen consumption as well as the expected rate.The volume may be low because the internal bellows has discharged a part of the previous breath to the environment, or because an increase in depth has caused the contents to be compressed, or a combination of these causes. The oxygen used by the diver also slowly decreases the volume of gas in the loop.After a diluent flush the gas must be breathable, and this limits MOD, but it is possible to use more than one option for diluent, and switch the gas to a hypoxic mix for the deeper sector of a dive, and a normoxic mix for the shallower sectors.MOD calculations can also be done for loop gas as calculated, but this is subject to variations which are not always accurately predictable. Loop gas calculated values for passive addition systems could possibly be used for working MOD calculation, and supply gas for emergency MOD given the relatively stable loop fraction in the passive addition systems, however the loop gas concentration may be closer to full strength if the diver works hard and ventilation increases beyond the linear extraction ratio.In this case the diver needs an alternative breathing source: the bailout gas. Often the planned dive is limited by the capacity of the bailout and not the capacity of the rebreather.While this option has the advantages of being permanently mounted on the rebreather and not heavy, the quantity of gas held by the rebreather is small so the protection offered is low.http://hhwebshop.com/images/candy-csf-4590-e-manual.pdf The extra cylinders are heavy and cumbersome but larger cylinders let the diver carry more gas providing protection for the ascent from deeper and long dives. The breathing gas mix must be carefully chosen to be safe at all depths of the ascent, or more than one set will be necessary. This can save time in an emergency, as the bailout demand valve is in place for immediate use. This can be important in a situation of severe acute hypercapnia, when the diver physically cannot hold their breath long enough to change mouthpieces.A major safety issue is that many divers become complacent as they become more familiar with the equipment, and begin to neglect predive checklists while assembling and preparing the equipment for use - procedures which are officially part of all rebreather training programmes. There can also be a tendency to neglect post-dive maintenance, and some divers will dive knowing that there are functional problems with the unit, because they know that there is generally redundancy designed into the system. This redundancy is intended to allow a safe termination of the dive if it occurs underwater, by eliminating a critical failure point. Diving with a unit that already has a malfunction, means that there is a single critical point of failure in that unit, which could cause a life-threatening emergency if another item in the critical path were to fail.This can be caused by the rise in ambient pressure caused by the descent phase of the dive, which raises the partial pressure of oxygen to hyperoxic levels.The scrubber must be configured so that no exhaled gas can bypass it; it must be packed and sealed correctly, and it has a limited capacity for absorption of carbon dioxide. Another problem is the diver producing carbon dioxide faster than the absorbent can handle; for example, during hard work, fast swimming, or high work of breathing caused by excessive depth for the loop configuration and gas mixture combination. The solution to this is to reduce effort and let the absorbent catch up. Divers need to lose any air conservation habits that may have been developed while diving with open-circuit scuba. In closed circuit rebreathers, this also has the advantage of mixing the gases preventing oxygen-rich and oxygen-lean spaces developing within the loop, which may give inaccurate readings to the oxygen control system. The diver is normally alerted to this by a chalky taste in the mouth.The gas mixture is known and reliable providing the loop is adequately flushed at the start of a dive and the correct gas is used. There is little that can go wrong with the function other than flooding, leaking and running out of gas, both of which are obvious to the user, and there is no risk of decompression sickness, so emergency free ascent to the surface is always an option in open water. The critical limitation of the oxygen rebreather is the very shallow depth limit, due to oxygen toxicity considerations.Therefore, if the gas addition system fails, the volume of gas in the loop will generally remain sufficient to provide no warning to the diver that the oxygen is depleting, and the risk of hypoxia is relatively high.If the addition to make up for depth increases is disregarded, the endurance of the unit is basically fixed for a given orifice and supply gas combination. However, the oxygen partial pressure will vary depending on metabolic requirements, and this is generally predictable only within limits. The uncertain composition of the gas means that worst case estimates are usually made for both maximum operating depth and decompression considerations. Unless the gas is monitored in real time by a decompression computer with an oxygen sensor, these rebreathers have a smaller safe depth range than open circuit on the same gas, and are a disadvantage for decompression.This can result in hypoxia and unconsciousness without warning. This can be mitigated by monitoring the partial pressure in real time using an oxygen sensor, but this adds to the complexity and cost of the equipment.The fresh gas addition is made by controlling the pressure in a dosage chamber proportional to the counterlung bellows volume. The dosage chamber is filled with fresh gas to a pressure proportional to bellows volume, with the highest pressure when the bellows is in the empty position. When the bellows fills during exhalation, the gas is released from the dosage chamber into the breathing circuit, proportional to the volume in the bellows during exhalation, and is fully released when the bellows is full. Excess gas is dumped to the environment through the overpressure valve after the bellows is full.Dosage ratio is constant once the gas has been selected, and the variations remaining on oxygen fraction are due to variations in the extraction ratio. This system provides a fairly stable oxygen fraction which is a reasonable approximation of open circuit for decompression and maximum operating depth purposes.To prevent this, a system is needed that warns the diver that there is a feed gas supply failure so the diver must take appropriate action. This can be done by purely mechanical methods.This will provide warning to the diver if the addition system stops working for any reason, as the discharge system will continue to empty the loop and the diver will have a decreasing volume of gas to breathe from. This will generally provide adequate warning before hypoxia is likely.A large bellows ratio adds a larger proportion of the breath volume as fresh gas, and this keeps the gas mix closer to supply composition at shallow depth, but uses the gas up faster.It is more likely to leak than block, which would use gas faster, but not compromise the safety of the gas mixture. Oxygen fraction of the loop gas is considerably less than of the supply gas in shallow water, and only slightly less at deeper depths, so the safe depth range for a given supply gas is smaller than for open circuit, and the variation in oxygen concentration is also disadvantageous for decompression. Gas switching may compensate for this limitation at the expense of complexity of construction and operation. The ability to switch to open circuit in shallow depths is an option which can compensate for the reduction in oxygen content at those depth, at the expense of operational complexity and greatly increased gas use while on open circuit. This may be considered a relatively minor problem if the requirement for bailout gas is considered. The diver will be carrying the gas anyway, and using it for decompression at the end of a dive does not increase the volume requirement for dive planning.If this is chosen incorrectly the oxygen fraction may differ significantly from the calculated value. Very little information on variation of extraction ratio is available in easily accessible references.The volume of gas dumped by the system is, for a given depth, a fixed fraction of the volume breathed by the diver, as in the case of the non-depth-compensated system. The effect is for an amount of gas of reasonably constant mass proportion to oxygen usage to be discharged, and the same amount, on average, is supplied by the addition valve, to make up the loop volume at steady state. This is very similar to the demand controlled SCR in effect on the oxygen fraction of the loop gas, which remains nearly constant at all depths where the compensation is linear, and for aerobic levels of exercise. The limitations on this system appear to be mainly in the mechanical complexity, bulk and mass of the equipment. The linearity of depth compensation is limited by structural considerations, and below a certain depth the compensation will be less effective, and finally dissipate. However, this does not have a great effect on oxygen fraction, as the changes at those depths are already small. The slightly higher concentrations in this case are a bit nearer to the supply gas value than if the compensation was still effective. The depth compensated PASCR can provide almost identical breathing gas to open circuit over a large depth range, with a small and nearly constant oxygen fraction in the breathing gas, eliminating a major limitation of the non-compensated system at the expense of complexity.Many of the failure modes are not easily identified by the diver without the use of sensors and alarms, and several failure modes can reduce the gas mixture to one unsuitable for supporting life. This problem can be managed by monitoring the state of the system and taking appropriate action when it diverges from the intended state. The composition of the loop gas is inherently unstable, so a control system with feedback is required. Oxygen partial pressure, which is the characteristic to be controlled, must be measured and the value provided to the control system for corrective action. The control system may be the diver or an electronic circuit. The measuring sensors are susceptible to failure for various reasons, so more than one is required, so that if one fails without warning, the diver can use the other(s) to make a controlled termination of the dive.It relies on electrochemical sensors and electronic monitoring instruments to provide the diver with the information required to make the necessary decisions and take the correct actions to control the gas mixture. The diver is required to be aware of the status of the system at all times, which increases task loading, but along with the experience, the diver develops and retains the skills of keeping the mixture within planned limits, and is well equipped to manage minor failures. The diver remains aware of the need to constantly check the status of the equipment, as this is necessary to stay alive.It is generally very effective at this function until something goes wrong. When something does go wrong the system should notify the diver of the fault so that appropriate action can be taken. Two critical malfunctions may occur which may not be noticed by the diver.If the diver or the control system respond to this by adding oxygen, a hyperoxic gas can be caused which may result in convulsions. To avoid this, multiple sensors are fitted to ECCCRs, so that a single cell failure does not have fatal consequences. Three or four cells are used for systems which use voting logic.If extensive testing of failure modes is not done, the user can not know what might happen if the circuit fails, and some failures may produce unexpected consequences. A failure which does not alert the user to the correct problem may have fatal consequences.This occurs when the reaction front reaches the far end of the absorbent. This will occur in any scrubber if used for too long. Some rebreathers may be assembled without all the components essential for ensuring that the breathing gas passes through the scrubber, or without the absorbent, and with no way of visually checking after assembly. In deeper diving, the scrubber needs to be bigger than is needed for a shallow-water or industrial oxygen rebreather, because of this effect. A caustic cocktail is a mixture of water and soda lime that occurs when the scrubber floods. It gives rise to a chalky taste, which should prompt the diver to switch to an alternative source of breathing gas and rinse his or her mouth out with water.This may occur gradually, over several minutes, with enough warning to the diver to bail out, or may happen in seconds, often associated with a sudden increase in depth which proportionately increases the partial pressure of the carbon dioxide, and when this happens the onset of symptoms may be so sudden and extreme that the diver is unable to control their breathing sufficiently to close and remove the DSV and swap it for a bailout regulator.It changes the colour of the soda lime after the active ingredient is consumed. This is useful in dry open environments, but is not useful on diving equipment, where:These limits will be conservative for most divers based on reasonably predictable levels of exertion. At present, there is no effective technology for detecting the end of the life of the scrubber or a dangerous increase in the concentration of carbon dioxide causing carbon dioxide poisoning. The diver must monitor the exposure of the scrubber and replace it when necessary. This test relies on the sensitivity of the diver to detect a raised concentration of carbon dioxide. Such systems should be used as an essential safety device to warn divers to bail off the loop immediately. If the diver does not bail out to a breathing gas with low carbon dioxide fairly quickly, the urge to breathe may prevent removal of the mouthpiece even for the short time required to make the switch.They are also subject to gradual failure due to using up the reactive materials, and may lose sensitivity in cold conditions. Any of the failure modes may lead to inaccurate readings, without any obvious warning. Cells should be tested at the highest available oxygen partial pressure, and should be replaced after a use period and shelf life recommended by the manufacturer.An electronically controlled CCR generally uses a minimum of three oxygen monitors to ensure that if one fails, it will be able to identify the failed cell with reasonable reliability.Bailout is the only safe option.Appropriate action will depend on circumstances, but this is not an immediately life-threatening event.If one assumes that only one cell will fail, then comparing three or more outputs which have been calibrated at two points is likely to pick up the cell which has failed by assuming that any two cells that produce the same output are correct and the one which produces a different output is defective. If the third cell output deviates sufficiently from the other two, an alarm indicates probable cell failure. If this occurs before the dive, the rebreather is deemed unsafe and should not be used. If it occurs during a dive, it indicates an unreliable control system, and the dive should be aborted. Continuing a dive using a rebreather with a failed cell alarm significantly increases the risk of a fatal loop control failure. This system is not totally reliable.A majority of cells must not fail for safe function of the unit. In order to decide whether a cell is functioning correctly, it must be compared with an expected output. This is done by comparing it against the outputs of other cells. In the case of two cells, if the outputs differ, then one at least must be wrong, but it is not known which one. In such a case the diver should assume the unit is unsafe and bail out to open circuit. With three cells, if they all differ within an accepted tolerance, they may all be deemed functional. If two differ within tolerance, and the third does not, the two within tolerance may be deemed functional, and the third faulty.Improvements are only in the order of one to two orders of magnitude.This test does not only validate the cell. If the sensor does not display the expected value, it is possible that the oxygen sensor, the pressure sensor (depth), or the gas mixture F O 2, or any combination of these may be faulty.Unless action is taken, the breathing gas will become hypoxic with potentially fatal consequences. An alternative mode of failure is one in which the injection valves are kept open, resulting in an increasingly hyperoxic gas mix in the loop, which may pose the danger of oxygen toxicity.Either a redundant independent control system may be used, or the risk of the single system failing may be accepted, and the diver takes the responsibility for manual gas mixture control in the event of failure.If the electronic injection fails, the user can take manual control of the gas mixture provided that the oxygen monitoring is still reliably functioning. Alarms are usually provided to warn the diver of failure.The negative pressure test is most important for this purpose. This test requires that the breathing loop maintains a pressure slightly below ambient for a few minutes to indicate that the seals will prevent leakage into the loop. This valve should always be closed when the mouthpiece is out of the mouth underwater.A caustic cocktail is usually a sign of a fairly extensive flood and is only likely if there are a lot of small particles in the scrubber material, or a relatively soluble absorbent material is used.The positive pressure test checks that the assembled unit can maintain a slight internal positive pressure for a short period, which is an indication that gas does not leak out of the loop. Inspection and replacement of soft components should detect damage before component failure.Manufacturer's operating manuals generally require the user to identify the cause of any leak and rectify it before using the equipment. Leaks which develop during a dive will be assessed by the dive team for cause and risk, but there is not often much that can be done about them in the water.The forced addition of gas will bring up the oxygen content, but the dive should be terminated as this problem can not be rectified during the dive.This is a fatal accident rate of over 100 times that of open circuit scuba. The statistics indicate that equipment choice has a dramatic effect on dive safety. Review shows that the risk of death while diving on a rebreather is in the region of 5.33 deaths per 100,000 dives, roughly 10 times the risk of open circuit scuba or horseriding, five times the risk of skydiving or hang gliding, but one eighth the risk of base jumping. No significant difference was found when comparing mCCRs with eCCRs or between brands of rebreather since 2005, but accurate information on numbers of active rebreather divers and number of units sold by each manufacturer are not available. The survey also concluded that much of the increased mortality associated with CCR use may be related to use at greater than average depth for recreational diving, and to high-risk behaviour by the users, and that the greater complexity of CCRs makes them more prone to equipment failure than OC equipment. EN 14143 also requires compliance with EN 61508.This does not compensate for poor maintenance and inadequate pre-dive checks, high risk behavior, or for incorrect response to failures.You can help by adding to it. ( September 2020 ) Two leak tests are usually conducted. These are generally known as the positive and negative pressure tests, and test that the breathing loop is airtight for internal pressure lower and higher than the outside. The positive pressure test ensures that the unit will not lose gas while in use, and the negative pressure test ensures that water will not leak into the breathing loop where it can degrade the scrubber medium or the oxygen sensors.The technique involves simultaneously venting the loop and injecting diluent. This flushes out the old mix and replaces it with a known proportion of oxygen.Several methods may be possible:This is easy to do and works well even when the diver is hypercapnic, as there is no need to hold the breath at all. This also requires no removal of the mouthpiece. It requires a suitable model full-face nask. This is simple, but requires the diver to hold their breath while switching the moutpiece, which may not be possible in cases of hypercapnia. This is not really bailing out to open circuit, but has logistical advantages in dives where the bulk of sufficient open circuit gas to reach the surface may be excessive, and a second rebreather is less bulky. There may be an intermediate stage where the diver bails out to open circuit on diluent gas while preparing the bailout rebreather. In all cases when bailing out the rebreather loop should be isolated from the water to avoid flooding and loss of gas which could adversely affect buoyancy. It may be necessary to close the gas supply valves to prevent a malfunctioning control system from continuing to add gas to the loop, which would also adversely affect buoyancy, possibly making it impossible for the diver to remain at the correct depth for decompression.The alarms are electronically controlled and therefore rely on input from a sensor.There is a high risk that it will soon be unsuitable to support consciousness. A good general response is to add diluent gas to the loop as this is known to be breathable. This will also reduce carbon dioxide concentration if that is high.The system used a copper diving helmet and standard heavy diving suit. The breathing gas was circulated by using an injector system in the loop.These were successfully used during the rescue of the crew and salvage of the USS Squalus in 1939. The US Navy Mark V Mod 1 heliox mixed gas helmet is based on the standard Mark V Helmet, with a scrubber canister mounted on the back of the helmet and a inlet gas injection system which recirculates the breathing gas through the scrubber to remove carbon dioxide and thereby conserve helium.They both used an injector system to recirculate the breathing gas and did not increase work of breathing.Innovations include:Diving for Science 2005. American Academy of Underwater Sciences. Retrieved 2011-01-09.