CHAPTER 9 Air Decompression
Change A 9-45
Equivalent Depth for the second dive is 110 fsw. The residual nitrogen time for
Group H at 110 fsw is 27 min. The equivalent single dive time therefore is 57 min.
The appropriate decompression schedule from the Surface Decompression Table
Using Oxygen is 110 fsw for 60 min. A 26 min stop at 40 fsw in the chamber is
required by the schedule. This stop is taken at a chamber depth of 40 fsw.
Figure 9-25 shows the filled-out Repetitive Dive at Altitude Worksheet for these
two dives. Figure 9-26 and Figure 9-28 shows the filled out Diving Charts for the
first and second dives.
ASCENT TO ALTITUDE AFTER DIVING/FLYING AFTER DIVING.
Leaving the dive site may require temporary ascent to a higher altitude. For
example, divers may drive over a mountain pass at higher altitude or leave the dive
site by air. Ascent to altitude after diving increases the risk of decompression sick-
ness because of the additional reduction in atmospheric pressure. The higher the
altitude, the greater the risk. (Pressurized commercial airline flights are addressed
in Note 3 of Table 9-5.)
Table 9-5 gives the surface interval (hours:minutes) required before making a
further ascent to altitude. The surface interval depends on the planned increase in
altitude and the highest repetitive group designator obtained in the previous 24-
hour period. Enter the table with the highest repetitive group designator obtained
in the previous 24-hour period. Read the required surface interval from the
column for the planned change in altitude.
Example: A diver surfaces from a 60 fsw for 60 minutes no-decompression dive
at sea level in Repetitive Group J. After a surface interval of 6 hours 10 minutes,
the diver makes a second dive to 30 fsw for 20 minutes placing him in Repetitive
Group C. He plans to fly home in a commercial aircraft in which the cabin pres-
sure is controlled at 8000 feet. What is the required surface interval before flying?
The planned increase in altitude is 8000 feet. Because the diver has made two
dives in the previous 24-hour period, you must use the highest Repetitive Group
Designator of the two dives. Enter Table 9-5 at 8000 feet and read down to Repet-
itive Group J. The diver must wait 17 hours and 35 minutes after completion of the
second dive before flying.
Example: Upon completion of a dive at an altitude of 4000 feet, the diver plans to
ascend to 7500 feet in order to cross a mountain pass. The diver's repetitive group
upon surfacing is Group G. What is the required surface interval before crossing
The planned increase in altitude is 3500 feet. Enter Table 9-5 at 4000 feet and read
down to Repetitive Group G. The diver must delay 1 hour and 23 minutes before
crossing the pass.
Example: Upon completion of a dive at 2000 feet, the diver plans to fly home in
an unpressurized aircraft at 5000 feet. The diver's repetitive group designator upon
surfacing is Group K. What is the required surface interval before flying?