2.1 Selecting the Explosive Atmosphere Method
After examining requirements documents and applying the tailoring process in Part One of this Standard to determine where explosive atmospheres are foreseen in the life cycle of the test item, use the following to confirm the need for this Method and to place it in sequence with other methods.
MIL-STD-810H
METHOD 511.7
2.1.1 Procedure I - Explosive Atmosphere
This procedure is applicable 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.
2.1.2 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 materiel malfunction.
2.1.3 Effects of Explosive Atmosphere Environments
Low levels of electrical energy discharge or electrical arcing by devices can ignite mixtures of fuel vapor and air. Fuel vapors in confined spaces can be ignited by a low energy discharge such as a spark from a short-circuited flashlight cell, switch contacts, electrostatic discharge, etc. High surface temperatures in excess of the auto-ignition temperature of flammable/explosive vapors may result in ignition of the vapors.
2.1.4 Sequence Among Other Methods
- a. General. Use the anticipated life cycle sequence of events as a general sequence guide (see Part One, paragraph 5.5).
- b. Unique to this Method. Considering the approach to conserve test item life by applying what are perceived to be the least damaging environments first, generally apply the explosive atmosphere test late in the test sequence. Vibration, shock, and temperature stresses may distort seals and reduce their effectiveness, thus making ignition of flammable atmospheres more likely. Recommend the test item(s) first undergo the above tests (on the same item(s)) to better approximate the actual operational environment.
2.2 Selecting Procedure Variations
Before conducting this test, complete the tailoring process by selecting specific procedure variations (special test conditions/techniques for this procedure) based on requirements documents, Life Cycle Environmental Profile (LCEP), and information provided with these procedures. Consider the following:
2.2.1 Fuel
Unless otherwise specified, use n-hexane as the test fuel, either reagent grade or 95 percent n-hexane with 5 percent other hexane isomers. This fuel is used because its ignition properties in flammable atmospheres are equal to or more sensitive than the similar properties of 100/130-octane aviation gasoline, JP-4 and JP-8 jet engine fuel. Optimum mixtures of n-hexane and air will ignite from temperatures as low as 223 °C, while optimum JP-4 fuel-air mixtures require a minimum temperature of 230 °C for auto-ignition, and 100/130 octane aviation gasoline and air requires 441 °C for hot-spot ignition (see paragraph 1.3d). Minimum spark energy inputs for ignition of optimum fuel vapor and air mixtures are essentially the same for n-hexane and for 100/130-octane aviation gasoline. The minimum ignition energy for n-Hexane in air at standard atmospheric pressure is 0.29mJ. A much higher spark energy input is required to ignite JP-4 or JP-8 fuel-air mixtures. Use of fuels other than n-hexane is not recommended.
WARNING: N-hexane is the flammable liquid used to test products in an explosive atmosphere. This solvent is listed as a hazardous material under Section 313 of the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA) (paragraph 6.1, reference a). It is classified by the Clean Air Act as a hazardous air pollutant and a hazardous air contaminant, is a Class 3 hazardous material, and has been identified by the Occupational Safety and Health Administration (OSHA) as requiring a maximum permissible exposure limit. The current OSHA permissible exposure limit (PEL) for n-hexane is 500 parts per million (PPM) (in air at 25 °C (77 °F), 760 Torr) for an 8-hour workday, time weighted average (TWA). OSHA directs an individual shall not exceed this average level per an 8-hour period (workday) based on a 40-hour workweek. N-hexane does not have a specified ceiling limit (as established by OSHA). OSHA has not established a specific PEL for the other fuels listed above. These fuels, AvGas 100/130 octane, JP-4, and JP-8 are blends of various simple and complex organic compounds. In many cases, the fuel formulas can include chemical compounds identified in 29 CFR 1910-1000, Table Z-1 (paragraph 6.1, reference b). If a specific producer/product is consistently employed, the formula for
this fuel blend can be analyzed and a specific warning prepared for the individual product. However, it should not be necessary, as the recommended chemical for the test is n-hexane.
2.2.2 Fuel-Vapor Mixture
Use a homogeneous fuel-air mixture in the correct fuel-air ratios for the explosive atmosphere test. Fuel weight calculated to total 3.8 percent by volume of the test atmosphere represents 1.8 stoichiometric equivalents of n-hexane in air, giving a mixture needing only minimum energy for ignition. This yields an air/vapor ratio (AVR) of 8.33 by weight (paragraph 6.1, reference c).
- a. Required information to determine fuel weight:
- (1) Chamber air temperature during the test.
- (2) Fuel temperature.
- (3) Specific gravity of n-hexane (see Figure 511.7-1).
- (4) Test altitude: ambient ground or as otherwise identified.
- (5) Net volume of the test chamber: free volume less test item displacement.
- b. Calculation of the volume of liquid n-hexane fuel for each test altitude:
- (1) In metric units:
Volume of 95 percent n-hexane (ml) =
- (2) In English units:
Volume of 95 percent n-hexane (ml) =
(150.41)
Figure description:
Specific Gravity vs. Temperature
| Temperature (°C) | Specific Gravity |
|---|---|
| 0 | 0.6775 |
| 5 | 0.6730 |
| 10 | 0.6685 |
| 15 | 0.6640 |
| 20 | 0.6595 |
| 25 | 0.6550 |
Notes: The chart shows a linear relationship between Specific Gravity and Temperature, with the data line terminating at 25°C even though the axis extends to 35°C.
Figure 511.7-1. Specific gravity of n-hexane.
2.2.3 Temperature
Heat the fuel-air mixture to the highest ambient air temperature at which the materiel is required to operate during deployment and provide the greatest probability of ignition. Perform all testing at this maximum air temperature. For forced-air-cooled materiel, use a test temperature that is the highest temperature at which the materiel can be operated and performance evaluated in the absence of cooling air. Chamber air temperature is typically controlled by indirect heating such as heated chamber walls.
2.2.4 Effect of Humidity on Flammable Atmosphere
The effect of humidity upon the fuel-air composition need not be considered in the test if the ambient air dewpoint temperature is 10 °C (50 °F) or less because this concentration of water vapor only increases the n-hexane fuel concentration from 3.82 percent to 3.85 percent of the test atmosphere. For example, if the atmospheric pressure is cycled from an equivalent of 1524 m (5000 ft) above the test level to 1524 m (5000 ft) below, (a 34 percent change in pressure), the volume of n-hexane will decrease from 4.61 percent to 3.08 percent. This decrease will compensate for the fuel enrichment effect that results from water vapor dilution of the test air supply. Tailoring may be required to simulate specific geographic areas.
2.2.5 Altitude Simulation
This test evaluates whether a test item can operate safely in a fuel/air mixture without creating a spark that could ignite the atmosphere. Since the components used in certain test items may make them more susceptible to creating sparks at high altitudes, all items must be tested up to their maximum operating altitude. The energy required to ignite a fuel-air mixture increases as pressure decreases. Ignition energy does not drop significantly for test altitudes below sea level. Therefore, unless otherwise specified, perform all tests with at least two explosive atmosphere steps, one at the highest anticipated operating altitude of the materiel (not to exceed 12,200 m (40,000 ft) where the possibility of an explosion begins to dissipate), and one between 7,000 ft and -1,500 ft (11.3 and 15.5 psi) that is representative of most ground ambient pressures. As noted in paragraph 1.3, because of the lack of oxygen at approximately 16 km (9.94 miles), do not perform this test at or above this altitude.
2.3 Definitions
For the purpose of this Method, the following definitions apply:
- a. Simulated altitude. Any height that is produced in the test chamber by reducing air pressure.
- b. Test altitude. The nominal simulated height(s) (generally, above sea level) at which the test item will be tested, i.e., the maximum altitude identified in paragraph 2.2.5.
MIL-STD-810H
METHOD 511.7