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Scrubers. Soda lime is a mixture of chemicals, used in granular form in closed breathing environments, such as general anaesthesia, submarines, rebreathers and recompression chambers, to remove carbon dioxide from breathing gases to prevent CO2 retention and carbon dioxide poisoning. The main components of soda lime are : Calcium hydroxide - Ca(OH)2 (about 75%) Sodium hydroxide - NaOH (about 3%) Potassium hydroxide - KOH (about 1%) Sofnolime is an example of a commercial soda lime product that is used in diving and medicine. Exhaled gas must be passed through a "carbon dioxide scrubber" where the carbon dioxide is adsorbed before the gas is made available to the breather to be re-inhaled. In rebreathers the scrubber is a part of the breathing loop. In larger environments, such as recompression chambers, a fan is used to pass gas through the canister. When carbon dioxide in the gas passing through the scrubber comes into contact with the active ingredients of the soda lime, an exothermic chemical reactionelements to form a solid and the active ingredients of the adsorbent are consumed. The reaction occurs along a "front", which is a cross section the canister, of the unreacted soda lime that is exposed to carbon dioxide-laden gas. This front moves through the canister, from the gas input end to the gas output end, as the reaction consumes the active ingredients. The term "break through" describes the failure of the "scrubber" to continue removing carbon dioxide from the exhaled gas mix. In rebreather diving, the typical effective duration of the scrubber will be one to five hours of breathing, depending on the granularity and composition of the soda lime, the ambient temperature and the design of the rebreather. In some dry, open environments, such as a recompression chamber or a hospital, it may be possible to load the canister with fresh adsorbent when breakthrough occurs. There are several approaches to avoiding or dectecting breakthrough: An indicating dye is incorporated in the soda lime, which changes the colour of the soda lime after the active ingredient is consumed.
With a transparent canister, this may be able to show the position of the reaction "front".
This is useful in dry, open environments but is not useful on diving equipment, which is mounted behind the diver. As the reaction between carbon dioxide and soda lime is exothermic, temperature sensors, such as thermocouples along the length of the scrubber can be used to measure the position of the front and therefore the life of the scrubber. Divers are trained to monitor and plan the exposure time of the soda lime in the scrubber and replace it within the recommended time limit. In addition to the normal reason for breakthrough, complete consumption of the active ingredient,
there are at least two other mechanisms that cause the scrubber to fail or become less efficient: In a rebreather, the soda lime must be packed tightly so that all exhaled gas comes into close contact with the granules of soda lime and the loop is designed so that any spaces or gaps between the soda lime and the loop walls that would allow gas to avoid contact with the adsorbent are minimised. If any of the seals, such as o rings, or spacers that prevent the bypassing of the scruber are not cleaned, lubricated or fitted properly, the scrubber will be less efficient. Break through occurs sooner on deep dives than on shallow dives because the scrubber is less efficient at depth. The reduction in efficiency is caused by a lower proportion of carbon dioxide molecules passing over the soda lime granules at depth than the proportion of molecules of other component gases. This means the carbon dioxide molecules find it difficult to come in contact with the soda lime granules because the other gases cover the granules. There are two factor that reduce of the proportion of carbon dioxide in the mix at depth: The metabolism, for a given rate of exertion, produces the constant partial pressure of carbon dioxide irrespective of depthThe partial pressures of the other gases in the breathing gas increases with depth. Soda lime is caustic and can cause burns to the eyes and skin. 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. PADI 5 Star National Geographic Instructor Development Center. 49 Thaweewong Road, Patong Beach, Phuket, Thailand. Phone: (+66) 076292052 Fax: (+66) 076293034
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