We will proceed to elaborate on the “Methods” and “Procedure”s of environmental standards of MIL-STD-810 in our article series. Instead of writing down the obvious information already given in the standard, we will be discussing more practical information on product design, features regarding “Equipment Under Test” (EUT), and conducting tests.
Test Method 511.5 – Explosive Atmosphere
The explosive atmosphere test is performed to:
a. demonstrate the ability of materiel to operate in fuel-air explosive atmospheres without causing ignition, or
b. demonstrate that an explosive or burning reaction occurring within encased material will be contained, and will not propagate outside the test item.
This method applies to all materiel designed for use in the vicinity of fuel-air explosive atmospheres associated with aircraft, automotive, and marine fuels at or above sea level. Procedure II specifically relates to atmospheres in a space in which flammable fluids or vapors exist or can exist, either continuously or intermittently (e.g., in fuel tanks or within fuel systems).
Procedure I – Explosive Atmosphere. This procedure applies to all types of sealed and unsealed materiel. This test evaluates the ability of the test item to be operated in a fuel vapor environment without igniting the environment.
Procedure II – Explosion Containment. 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 material malfunction.
• Fuel – Unless otherwise specified, use n-hexane as the test fuel. The fuel is used either as reagent grade or 95 percent n-hexane with 5 percent other hexane isomers. This fuel is used when its ignition properties in flammable atmospheres are equal to or more sensitive than the similar properties of aviation gasoline and jet engine fuel.
• Effect of Humidity on Flammable Atmosphere – There is no need to consider the effect of humidity if the ambient air dew point temperature is less than 10 degrees Celsius. The concentration of water vapor increases the n-hexane fuel concentration.
• Altitude Simulation – The MIL 810 test evaluates whether a test item can operate safely in a fuel/air mixture. The purpose is to operate without creating a spark that could ignite the atmosphere. This test is not performed above the altitude of 16 km due to the lack of oxygen in the atmosphere.
• Fuel-Vapor Mixture – This testing method recommends the use of a homogenous fuel-air mixture in the correct ratios. The required information to determine the fuel weight is as follows: Chamber air temperature during the test, fuel temperature, the specific gravity of n-hexane, test altitude, and net volume of the test chamber.
• Temperature – The fuel-air mixture is heated to the highest ambient air temperature at which the material is required to operate. The material should be heated during deployment and provide the greatest chance of ignition. Perform all testing at this maximum air temperature.
• The expected life cycle sequence of events is a risk assessment used by applying the least potentially explosive atmospheres. Vibration, shock, and temperature stresses may distort seals and reduce their effectiveness. This makes ignition of flammable atmospheres more likely.
• Limitations of explosive atmosphere, test method 511, must be noted. First off, this test procedure uses a relatively low flashpoint mixture. This may not be representative of some actual fuel-air aerosol. Next, explosive atmosphere testing is conservative
• There is a low probability of the material igniting prevailing the fuel vapor mixture. This MIL-810 method is not intended to test high surface temperatures. However, it does not preclude this possibility.
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