Compressed Air Work And Diving 1909
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CH APTER II.
Stage Decompression (conlinued).
Table IV.—'This table is for use in caisson or tunnel
work where long shifts may be employed by reason of
the comparatively low pressure usually necessary for this
class of work.
With such long shifts and low pressures the calculated
theoretical rate of decompression is nearly uniform atter
the first rapid drop in pressure, and the rules for clecom-
pression are greatly simplified by adopting uniform de-
compression. The table gives the number of minutes
for each pouncl of decompression after a first rapid drop
in the proportion of two to one in absolute pressure.
Suppose, for instance, that men working at a pressure
of 30 Ibs. came out for a meal after three hours’ exposure,
they would first decompress rapidly to J° +1 = 22^- Ibs.
absolute pressure, or 7^ Ibs. by gauge. They would then
lower the pressure 1 Ib. every six minutes, taking 6x7!
= 45 minutes to pass through the air-lock. After a seconcl
or thircl exposure the time required would be 7 x 7^-
= 52^- minutes. After six hours or more continuous
exposure 8x7^ = 60 minutes would be required. It is
clear that in order to economise time spent in the air-lock
it will be better for pressures over 2 5 Ibs. to keep the
men continuously under pressure during each shift, and