Compressed Air Work And Diving 1909
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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