Downloaded from https://www.everyspec.com MIL-STD-81OD 19 July 1983 1-3.1 mice of test DYOC~ 1-3.1.1 Procedure I - Eg!daD ment oDeratlon n flammable atmow. This procedure is applicable to all types of sealed and unsealed equipment. This test evaluates the ability of the test item to be operated in a fuel-yapor- laden environment without igniting the environment. s: 1-3.1.2 Frocedure II - EXDIOS1On ~ m. This procedure is used to determine the ability of the test item’s case or other enclosures to contain an explosion or flame that is a result of an internal equipment malfunction. .. ~ I-3.2 Wice of test c~. . . The exploiive atmosphere test is a .. conservative test.
If the test item does not ignite the test fuel-air mixture, there is a low probability that the item will ignite fuel vapor mixtures that can”occur in actual use. Conversely, the ignition of the teat fuel-air mixture by,the test item does not mean that the test item will always‘ignite fuel. :. .: vapors that occur in actual deployment. ., I-3.2.1 Fuel for test. The fuel recommended for explosive atmosphere testing shall be the single-component hydrocarbon n-hexane (i.e., normal hexane). This fuel is used because its ignition properties for flammable atmosphere testing are equal to or better than the similar properties of both 100/130 octane aviation’gasoline and .JP-4jet engine fuel.
Optimum mixtures of n-hexane and air will ignite from hot-spot temperatures as low as 222.8oc (433° F) while optimum JP-4 jet engine fuel-air mixtti”resrequire at least a 229”.4° C(445° F) temperature level for autoignition and 100/130 octane aviation gasoline and air requires 44o.60c (8250F) for hot-spot‘ignition. Minimum..spark energy inputs for ignition of optimum fuel vapor‘and air mixtures are essential y “thesame for n-hexane and for 100/130 octane aviation gasoline. Much higher minimum 0 spark energy input is required to ignite JP-4 jet engine fuel and air mixtures. .!, . I-3.2.2 ~. The fuel vapor mixture used.in the ,eXPIOSive ““‘: atmosphere test shall be homogeneous. ...., ,..
I-3.2.3 “’&t temperature. The fuel’’v{por“mixture is heated to the hiihest ambient air temperature at which the test item is required to operate during actual deployment. Heating the ambient air to this temperature gives the fuel vaPor/air mixture its greatest likeliho~d of ,ignition..All testinK should’be”, done at this maximum air temperature. For forced-air-cooled equipment, the test temperature shall be the highest temperature at which equipment per- formance can be evaluated in the absence.of cooling air. * I-3.2.4. ~tv of fw. Unless otherwise,specified,,’ the fuel used shall be~ n-hexane, either reagent grade Or 95 Percent..
The 95 percent n-hex$ne‘fuel actually is ne&.lY 100 percent”hexane, becau9e the i%maihing 5 Percent cOns!sts,, of hexane isomers.; Fuel weight calculated‘to total 3.8 percent by volume of the test atmosphere represents 1.8 atoichiometric equivalents of n-hexane in :. air, giving a mixture needing only minimum en~rgY for ignitiOn. ,. METHOD 511.2 19 +hlly1983 511.2-2