MIL-DTL-6396F
4.5.11 Fuel resistance of exterior surfaces. The tank shall be placed in a container large
enough to permit immersion to one-half the depth of the cube using JP-8 or JP-5 aviation fuel.
The tank shall be immersed for 72 hours at the ambient temperature. The tank shall then be
removed and examined. The exterior surface of the tank construction shall show no
unsatisfactory swelling, separation, blistering, dissolution, or other deterioration.
4.5.12 Permeability.
4.5.12.1 Test apparatus. The test apparatus shall consist of:
a. A permeability cup and ring constructed as shown on figure 2.
b. A nylon solution to be used for sealing the test disk to the permeability cup.
4.5.12.2 Preparation of test specimen. The test samples shall be prepared utilizing a
manufacturing method similar to production and shall be given a cure equivalent to that used in
production. Two test samples consisting of the inner liner and fuel barrier (if required) or the
total construction shall be used. The samples shall be conditioned 24 hours at 77 ±5 °F
(25 ±3 °C) and a relative humidity of 50-65 percent prior to test. A nylon solution or other
suitable sealing liquid may be applied to the face of the cup flange and sample edge. The inside
or inner liner surface shall be exposed to the fuel.
4.5.12.3 Method of conducting test. Permeability cups prepared as specified in 4.5.12.2,
shall be placed in a rack at a constant temperature of 77 ±5 °F (25 ±3 °C) and relative humidity
of 50-65 percent. After allowing 1 hour for equilibrium, the cup shall be weighed to the nearest
0.005 gram and placed in the rack with the face of the cup facing upward. The cup shall be kept
at the above constant temperature for 24 hours, then weighed to check for seal integrity. If
necessary, the bolts shall be retorqued at this weighing and at subsequent weighings. The cup
shall be inverted (test disk down) in a rack that permits free access of air to test disk. Cups shall
be weighed at the end of the third, fifth, and eight day after inverting. Defective films or leaks
caused by faulty assembly will usually be found when making the weighing on the third day.
The diffusion rate calculation shall be made on the fifth- to the eighth-day period and expressed
as fluid ounces per square foot per 24 hours. The permeability shall be less than 0.025 fluid
ounce per square foot per 24 hours for each sample tested.
NOTE: Diffusion expressed in fluid ounces per square foot per 24 hours equals the gram loss of
the test specimen per 24 hours multiplied by a factor K which is defined as:
________144_________
K =
(sp.gr.)(29.573)(3.142)R2
Where sp.gr. = Specific gravity of test fluid at 77 °F (25 °C).
R = Inside radius of test cup expressed in inches.
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