9-40 Change A
U.S. Navy Diving ManualVolume 2
deeper than 145 fsw (actual depth). At altitudes above 1000 ft., correction is
required for all dives.
Depth Measurement at Altitude. The preferred method for measuring depth at
altitude is a mechanical or electronic gauge that can be re-zeroed at the dive site.
Once re-zeroed, no further correction of the reading is required.
When using a recompression chamber for decompression, zero the chamber depth
gauges before conducting surface decompression.
Most mechanical depth gauges carried by divers have a sealed one atmosphere
reference and cannot be adjusted for altitude, thus they will read low throughout a
dive at altitude. A correction factor of 1 fsw for every 1000 ft of altitude should be
added to the reading of a sealed reference gauge before entering Table 9-3.
Pneumofathometers can be used at altitude. Add the pneumofathometer correction
factor (Table 9-1) to the depth reading before entering Table 9-3. The pneumofath-
ometer correction factors are unchanged at altitude.
A sounding line or fathometer may be used to measure the depth if a suitable depth
gauge is not available. These devices measure the linear distance below the
surface of the water, not the water pressure. Though fresh water is less dense than
sea water, all dives will be assumed to be conducted in sea water, thus no correc-
tions will be made based on water salinity. Enter Table 9-3 directly with the depth
indicated on the line or fathometer.
Equilibration at Altitude. Upon ascent to altitude, two things happen. The body
off-gases excess nitrogen to come into equilibrium with the lower partial pressure
of nitrogen in the atmosphere. It also begins a series of complicated adjustments to
the lower partial pressure of oxygen. The first process is called equilibration; the
second is called acclimatization. Twelve hours at altitude is required for equilibra-
tion. A longer period is required for full acclimatization.
If a diver begins a dive at altitude within 12 hours of arrival, the residual nitrogen
left over from sea level must be taken into account. In effect, the initial dive at alti-
tude can be considered a repetitive dive, with the first dive being the ascent from
sea level to altitude. Table 9-4 gives the repetitive group associated with an initial
ascent to altitude. Using this group and the time at altitude before diving, enter the
Residual Nitrogen Timetable for Repetitive Air Dives (Table 9-7) to determine a
new repetitive group designator associated with that period of equilibration. Deter-
mine sea level equivalent depth for your planned dive using Table 9-3. From your
new repetitive group and sea level equivalent depth, determine the residual
nitrogen time associated with the dive. Add this time to the actual bottom time of
Example: A diver ascends rapidly to 6000 feet in a helicopter and begins a dive
to 100 fsw 90 minutes later. How much residual nitrogen time should be added to