The diver is required to perform a purge procedure prior to or during any dive in
which closed-circuit oxygen UBA is to be used. The purge procedure is designed
to eliminate the nitrogen from the UBA and the diver’s lungs as soon as he begins
breathing from the rig. This procedure prevents the possibility of hypoxia as a
result of excessive nitrogen in the breathing loop. The gas volume from which this
excess nitrogen must be eliminated is comprised of more than just the UBA
breathing bag. The carbon dioxide-absorbent canister, inhalation/exhalation hoses,
and diver’s lungs must also be purged of nitrogen.
Immediately prior to entering the water, the divers shall carry
out the appropriate purge procedure. It is both difficult and unnecessary to eliminate
nitrogen completely from the breathing loop. The purge procedure need only
raise the fraction of oxygen in the breathing loop to a level high enough to prevent
the diver from becoming hypoxic, as discussed in paragraph 18-2.2. For the MK 25 UBA, this value has been determined to be 45 percent. For further information
on purge procedures, see paragraph 18-7.4.
If the dive is part of a tactical scenario that requires a turtleback phase, the purge
must be done in the water after the surface swim, prior to submerging. If the
tactical scenario requires an underwater purge procedure, this will be completed
while submerged after an initial subsurface transit on open-circuit scuba or other
UBA. When the purge is done in either manner, the diver must be thoroughly
familiar with the purge procedure and execute it carefully with attention
This procedure is approved for
turtleback emergency descents:
1. Open the oxygen supply.
2. Exhale completely, clearing the mouthpiece with the dive/surface valve in the
surface position.
3. Put the dive/surface valve in the DIVE position and make the emergency
descent.
4. Immediately upon reaching depth, perform purging under pressure
(pressurized phase) (IAW the appropriate MK 25 Technical Manual).
The following errors may result in a dangerously
low percentage of oxygen in the UBA and should be avoided:
-
Exhaling back into the bag with the last breath rather than to the atmosphere
while emptying the breathing bag.
-
Underinflating the bag during the fill segment of the fill/empty cycle.
-
Adjusting the waist strap of the UBA or adjustment straps of the life jacket too
tightly. Lack of room for bag expansion may result in underinflation of the bag
and inadequate purging.
-
Breathing gas volume deficiency caused by failure to turn on the oxygen-supply
valve prior to underwater purge procedures.
The following errors may result in a dangerously
low percentage of oxygen in the UBA and should be avoided:
-
Purging Procedures for the Draeger LAR V Underwater Breathing Apparatus;
NEDU Report 5-84
-
Underwater Purging Procedures for the Draeger LAR V UBA; NEDU Report
6-86
-
MK 25 UBA (LAR V) Operation and Maintenance Manual; NAVSEA SS600-
AJ-MMO-010, Change 1, January 1, 1985
