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Encyclopedia Britannica - Main :: BUN-CAL |
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CAISSON DISEASE . In order to exclude the water, the air pressure within a caisson used for subaqueous works must be kept in excess of the pressure due to the superincumbent water; that is, it must be increased by one atmosphere, or 15 lb per sq. in. for every 331 ft. that the caisson is submerged below the surface. Hence at a depth of too ft. a worker in a caisson, or a diver in a diving-dress, must be subjected to a pressure of four atmospheres or 6o lb per sq. in. Exposure to such pressures is apt to be followed by disagreeable and even dangerous physiological effects, which are commonly referred to as caisson disease or compressed air illness. The symptoms are of a very varied character, including pains in the muscles and joints (the " bends "), deafness, embarrassed breathing, vomiting, paralysis (" divers' palsy "), fainting and sometimes even sudden death. At the St Louis bridge
body
body
long as it is continued, but only after it has been removed; and the view now generally accepted is that they are due to the rapid effervescence of the gases which are absorbed in the body-fluids during exposure to pressure. Experiment has proved that in animals exposed to compressed air nitrogen is dissolved in the fluids in accordance with Dalton
But he also found that dogs exposed, for moderate periods, to similar pressures suffered no ill effects provided that the pressure was relieved gradually, in x-11 hours; and his results have been confirmed by subsequent investigators. To prevent caisson disease, therefore, the decompression should be slow; Leonard Hill suggests it should be at a rate of not less than 20 minutes for each atmosphere of pressure. Good ventilation of the caisson is also of great
long shifts should be avoided, because by fatigue the circulatory and respiratory organs are rendered less able to eliminate the absorbed gas. Another reason against long shifts, especially at high pressures, is that a high partial pressure of oxygen acts as a general protoplasmic poison. This circumstance also sets a limit to the pressures that can possibly be used in caissons and therefore to the depths at which they can be worked, though there is reason to think that the maximum pressure (44 atmospheres) so far used in caisson work
See Paul Bert, La Pression barometrique (1878) ; and Leonard Hill, Recent
Physiology and Biochemistry (1906), (both these works contain bibliographies); also a lecture by Leonard Hill delivered at the Royal Institution of Great
recent
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