CHAPTER 9 — Air Decompression
Change A 9-37
4.
Pressurize the recompression chamber with air to 20 fsw at a travel rate of 60
fpm.
5.
Upon arrival at 20 fsw in the recompression chamber, the divers are placed on
the Built-in Breathing System (BIBS) mask breathing 100% oxygen.
6.
The 20 foot stop time commences once the divers are breathing oxygen.
Repeat the 20 fsw in-water stop time.
7.
The divers breathe oxygen throughout the 20-foot stop, interrupting oxygen
breathing after each 30 minutes with a 5 minute air break. The air breaks
count as part of the stop time.
8.
Ascend to 10 fsw at 30 fpm. Complete the 10 fsw in-water stop time. The
divers breathe oxygen throughout the 10-foot stop, interrupting oxygen breath-
ing after each 30 minutes with a 5 minute air break. The air breaks count as
part of the stop time.
9.
Ascent to the surface at 30 fpm.
9-11.2
Oxygen System Failure (Chamber Stop). If the oxygen systems fails during a
chamber stop, complete the remaining decompression time on air.
9-12
DIVING AT HIGH ALTITUDES
Because of the reduced atmospheric pressure, dives conducted at altitude require
more decompression than identical dives conducted at sea level. Standard air
decompression tables, therefore, cannot be used as written. Some organizations
calculate specific decompression tables for use at each altitude. An alternative
approach is to correct the altitude dive to obtain an equivalent sea level dive, then
determine the decompression requirement using standard tables. This procedure is
commonly known as the "Cross Correction” technique and always yields a sea
level dive that is deeper than the actual dive at altitude. A deeper sea level equiva-
lent dive provides the extra decompression needed to offset effects of diving at
altitude.
9-12.1
Altitude Correction Procedure. To apply the “Cross Correction” technique, two
corrections must be made for altitude diving. First, the actual dive depth must be
corrected to determine the sea level equivalent depth. Second, the decompression
stops in the sea level equivalent depth table must be corrected for use at altitude.
Strictly speaking, ascent rate should also be corrected, but this third correction can
safely be ignored.
9-12.1.1
Correction of Depth of Dive. Depth of a sea level equivalent dive is determined
by multiplying the depth of the dive at altitude by a ratio of atmospheric pressure
