CHAPTER 22 — Recompression Chamber Operation
Change A 22-17
valve a given number of turns (or fraction of a turn), which will provide a certain
number of cubic feet of ventilation per minute at a specific chamber depth, and to
use the supply valve to maintain a constant chamber depth during the ventilation
period. Determination of valve settings required for different amounts of ventila-
tion at different depths is accomplished as follows.
WARNING
This procedure is to be performed with an unmanned chamber to avoid
exposing occupants to unnecessary risks.
1.
Mark the valve handle position so that it is possible to determine accurately
the number of turns and fractions of turns.
2.
Check the basic ventilation rules above against probable situations to
determine the rates of ventilation at various depths (chamber pressure) that
may be needed. If the air supply is ample, determination of ventilation rates
for a few depths (30, 60, 100, and 165 feet) may be sufficient. It will be
convenient to know the valve settings for rates such as 6, 12.5, 25, or 37.5
cubic feet per minute (acfm).
3.
Determine the necessary valve settings for the selected flows and depths by
using a stopwatch and the chamber as a measuring vessel.
a.
Calculate how long it will take to change the chamber pressure by 10
feet if the exhaust valve lets air escape at the desired rate close to the
depth in question. Use the following formula.
Where:
T = time in seconds for chamber pressure to change 10 feet
V = internal volume of chamber (or of lock being used for test) in
cubic feet (cf)
R = rate of ventilation desired, in cubic feet per minute as measured at
chamber pressure (acfm)
P = Change in chamber pressure in fsw
D = depth in fsw (gauge)
Example: Determine how long it will take the pressure to drop from 170
to 160 feet in a 425-cubic-foot chamber if the exhaust valve is releasing 6
cubic feet of air per minute (measured at chamber pressure of 165 feet).
1.
List values from example:
T =
unknown
V = 425 cf
R = 6 acfm
P = 10 fsw
T
V
60
D
P
R
D
33
+
(
)
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=
