TM 5-1940-321-13&P
( 4 ) If keel pressure exceeds 240 mb (3.5 psi), release a small amount of air. To
release air from the keel, unscrew the metal valve insert approximately 1/2
turn. Monitor the pressure gauge reading. Continue to release air in small
amounts while observing the pressure gauge until the pressure is 220-240 mb
(3.2 to 3.5 psi).
( 5 ) If keel pressure is below 220 mb (3.2 psi), remove the gauge, insert the foot
pump hose, and operate the pump to add air to the keel, remove the hose,
reinsert the pressure gauge, and observe the pressure. Add air until the
pressure is 220-240 mb (3.2 to 3.5 psi).
( 6 ) Tighten the keel valve metal insert firmly by hand and replace the metal valve
cap. Tighten the metal valve cap firmly by hand.
NOTE:
Some air loss from the craft is normal. Excessive air loss indicates a need for
corrective maintenance. A craft is considered to be within specifications if air
pressure loss does not exceed 30 mb (.44 psi) over a 24-hour period. Before
determining that the craft has an air leak and that corrective maintenance is
required, it is necessary to consider normal causes of air pressure drop.
Operators must be aware of the following normal causes of air pressure drop.
The temperature of the craft has an effect on inflation pressure. A change of only
1.8F results in a 4 mb (.06 psi) pressure change. Temperature increases cause
air pressure increases. Temperature decreases cause air pressure decreases. A
craft inflated on shore at an ambient temperature of 80F and then placed in
water with a temperature of 60F will lose 44 mb (.64 psi) of pressure. This
pressure loss can incorrectly attribute to air chamber leaks. A craft inflated in the
morning when it is cool will have increased pressure as he temperature
increases later in the day.
Operators must be aware of these temperature induced pressure changes and
check inflation pressure regularly.
After initial inflation of a deflated or partially deflated craft, the pressure must be
allowed to stabilize for one hour and then must be checked again with the
pressure gauge. The stabilization time is necessary because the fabric that forms
the buoyancy tube stretches. The fabric stretch results in slightly greater air
chamber volume and corresponding decrease in pressure. In addition, operating
the craft in rough seas can cause the overpressure valves to release small
amounts of air. This is normal. The air lost due to any of the above causes must
be replaced using the foot pump as part of the daily pressure check procedure
described in this section. Paragraph 5-19 describes the pressure retention test to
be performed every 6 months or when a leak is suspected. This test determines
if corrective maintenance is required.
5-7
