Soda lime
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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.
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Scrubber
The canister holding the soda lime is called the "carbon dioxide scrubber". Exhaled gas is passed through the scrubber before being re-inhaled. In a rebreather the canister is a part of the loop and the diver's exhaled gas is passed through it. In larger environments, such as recompression chambers, a fan is used to pass gas through the canister.
Chemical components
The main components of soda lime are :
- Calcium hydroxide - Ca(OH)2 (about 75%)
- Water - H2O (about 20%)
- Sodium hydroxide - NaOH (about 3%)
- Potassium hydroxide - KOH (about 1%)
Caustic cocktail
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 breather to switch to an alternate source of breathing gas and rinse his or her mouth out with water.
Break through
The term "break through" describes the failure of the "scrubber" to continue removing carbon dioxide from the exhaled gas mix. In rebreather diving, it is the diver's responsibility to monitor the exposure time of the soda lime in the "scrubber" and replace it within the recommended time limit. Typically, the duration will be one to five hours of breathing depending on the granularity and composition of the soda lime and the design of the rebreather.
In rebreather diving, "break through" occurs sooner on deep dives than on shallow dives because the scrubber is less efficient at depth. The reason for this is the proportion of carbon dioxide molecules passing over the soda lime granules is much lower 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. So, the carbon dioxide is not "scrubbed" but can be re-inhaled. There are two reasons for the proportion of carbon dioxide falling at depth:
- The metabolism, for a given rate of exertion, produces the same partial pressure of carbon dioxide irrespective of depth
- The partial pressures of the other gases in the breathing gas increases with depthru:Натронная известь