Compressed Air Work And Diving 1909
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CAISSON AND TUNNEL WORK.
11
such an arrangement is usually preferred by the men
themselves.
Rule.—Decompress to half the absolute pressure
rapidly, and then at the rates given in the ttible.
Table V.—In tunnel work or in caisson sinking, where
the space is available, it is recommencled that a section
of tunnel or large air-lock should be provided where the
pressure should be constantly maintained at a little less
than half the absolute pressure. With such an arrange-
ment the men would be able to pass rapiclly from the
working chamber or section of tunnel into the “purga-
tory ” air-lock or section of tunnel, and after a clelay,
proportionale to the working pressure, would pass rapiclly
out. Whilst in the lock, they could occupy their time
in chanQ-ino- their clothes and washing-, or in havin»' a
meal, and the moving about thus occasioned would con-
siderably hasten the process of desaturation.
This method is a very convenient one for tunnel work,
as the ordinary air-locks are kept free for ingress and
egress of material, except for the few minutes during each
shift actually occupied in passing in and out.
In the table, in the first column, is given the working-
pressure, in the seconcl the absolute working pressure
4- 2.2, which is the pressure at which the air must be
maintained in the purgatory lock, and in the other
columns are given the time in minutes which must be
spent in the lock after different lengths of exposure.
Thus, with a working pressure of 34 Ibs., the pressure
in the purgatory lock or section of tunnel would be
5 = 2 2Ä-lbs. nearly of absolute pressure or 7^ Ibs. by
2.2
gauge. Any one coming out after three hours would
have to spend fifty minutes in the purgatory lock as
shown in the third column from the end.