8-22 Change A
U.S. Navy Diving ManualVolume 2
intercoolers, are designed to perform with maximum efficiency at the rated pres-
sure of the compressor. Operating at any pressure below this rating reduces the
efficiency of the unit. Additionally, compression reduces water vapor from the air.
Reducing the amount of compression increases the amount of water vapor in the
air supplied to the diver.
The air supplied from the compressor expands across the pressure regulator and
enters the air banks or volume tank. As the pressure builds up in the air banks or
volume tank, it eventually reaches the relief pressure of the compressor, at which
time the excess air is simply discharged to the atmosphere. Some electrically-
driven compressors are controlled by pressure switches installed in the volume
tank or HP flask. When the pressure reaches the upper limit, the electric motor is
shut off. When sufficient air has been drawn from the volume tank or HP flask to
lower its pressure to some lower limit, the electric motor is restarted.
All piping in the system must be designed to minimize pressure drops. Intake
ducting, especially, must be of sufficient diameter so that the rated capacity of the
compressor can be fully utilized. All joints and fittings must be checked for leaks
using soapy water. Leaks must be repaired. All filters, strainers, and separators
must be kept clean. Lubricant, fuel, and coolant levels must be periodically
Any diving air compressor, if not permanently installed, must be firmly secured in
place. Most portable compressors are provided with lashing rings for this purpose.
High-Pressure Air Cylinders and Flasks. HP air cylinders and flasks are vessels
designed to hold air at pressures over 600 psi. Convenient and satisfactory diving
air supply systems can be provided by using a number of these HP air cylinders or
flasks. Any HP vessel to be used as a diving air supply unit must bear appropriate
Department of Transportation (DOT) or military symbols certifying that the cylin-
ders or flasks meet high-pressure requirements.
A complete air supply system includes the necessary piping and manifolds, HP
filter, pressure reducing valve, and a volume tank. An HP gauge must be located
ahead of the reducing valve and an LP gauge must be connected to the volume
In using this type of system, one section must be kept in reserve. The divers take
air from the volume tank in which the pressure is regulated to conform to the air
supply requirements of the dive. The duration of the dive is limited to the length of
time the banks can provide air before being depleted to 200 psi over minimum
manifold pressure. This minimum pressure of 200 psi must remain in each flask or
As in scuba operations, the quantity of air that can be supplied by a system using
cylinders or flasks is determined by the initial capacity of the cylinders or flasks
and the depth of the dive. The duration of the air supply must be calculated in
advance and must include a provision for decompression.