Compressed Air Work And Diving 1909

2 COMPRESSED AIR WORK. most convenient way of timing the decompression in practical working. Stage decompression has several great advantages over uniform decompression. One is that the nitrogen in the blood is kept at a maximum safe pressure in excess of the air breathecl, and therefore the rate of desaturation is also at a maximum. In this way the greatest economy possible in the time spent in decompression is obtained. Thus to decompress from 2 13 ft., or a pressure of 95 Ibs., woulcl take 309 minutes, or about five hours, whereas to obtain the same standard of safety by uniform clecom- pression woulcl take ten hours, showing an advantage of 50 per cent, in favour of the stage methocl. Then, when decompressing from great depths or high pressures atter short exposures, the saturation, which woulcl otherwise be going on during decompression, is obviated by the first rapid drop in pressure, so that after such short exposures the rate of decompression can be kept within reasonable limits. Again, the rate of decompression is slowest when most neecled, whereas uniform decompression is either unnecessarily slow at the beginning, or dangerously quick at the end. To those accustomed to uniform decompression, a sudden drop from 45 to 15 Ibs., or to half the absolute pressure, might appear dangerous. 1 his, however, is not the case ; it is ås safe to decompress quickly from 45 to 15 Ibs. as it is from 15 Ibs. to atmosphere, since in each instance the volunie of gas (not the mass) tending to be liberated will be the same. In other worcls it is the relative, and not the absolute, drop in pressure which has to be considerecl. Lastly, this method has been tested by a large number of experimental dives carried out under the