MIL STD 810 G – Test Method 511.5 – Explosive Atmosphere
The explosive atmosphere test is performed to:
demonstrate the ability of materiel to operate in fuel-air explosive atmospheres without causing ignition, or
demonstrate that an explosive or burning reaction occurring within encased materiel 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).
NOTE: Materiel tested to Procedure II is designed such that ignition of an explosive mixture is contained within the materiel without igniting the surrounding explosive atmosphere and, during normal operation, or as a result of any fault, the temperature of any external surface will not rise to a level capable of causing ignition (including hermetically-sealed materiel).
Use other explosive atmosphere safety tests (e.g., electrical or mine safety) if more appropriate.
These procedures use an explosive fuel-air mixture that has a relatively low flash point that may not be representative of some actual fuel-air or aerosol (such as suspended dust) mixtures.
The explosive 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 materiel will ignite prevailing fuel vapor mixtures in service. Conversely, the ignition of the test fuel-air mixture by the test item does not mean the materiel will always ignite fuel vapors that occur in actual use.
These procedures are not appropriate for test altitudes above approximately 16 km where the lack of oxygen inhibits ignition.
This method is not intended to demonstrate ignition due to high surface temperatures. (This method is intended for spark ignition only.)
Procedure I – Operation in an explosive atmosphere
Step 1. With the test item installed, seal the chamber and stabilize the test item and chamber inner walls to the high operating temperature of the test item (±2oC (±4oF)).
Step 2. Adjust the chamber air pressure to simulate the highest operating altitude of the test item (not to exceed 12,200 m (40,000 ft)) plus 2000 meters (6600 ft) to allow for introducing, vaporizing, and mixing the fuel with the air as described in paragraph 2.2.2.
Step 3. Slowly introduce the required volume of n-hexane into the test chamber as the simulated altitude begins to drop.
Step 4. Circulate the test atmosphere and continue to reduce the simulated chamber altitude for at least three minutes to allow for complete vaporization of fuel and the development of a homogeneous mixture, and for the chamber pressure to reach the test altitude.
Step 5. At a pressure equivalent to 1000 m (3300 ft) above the test altitude, verify the potential explosiveness of the fuel-air vapor by attempting to ignite a sample of the mixture taken from the test chamber using a spark-gap device or glow plug ignition source with sufficient energy to ignite a 3.82 percent hexane mixture. If ignition does not occur, purge the chamber of the fuel vapor and repeat Steps 1-4. An alternative method of determining the explosive characteristics of the vapor is by using a calibrated explosive gas meter that verifies the degree of explosiveness and the concentration of the fuel-air mixture.
Step 6. Although above the maximum operational altitude of the test item, attempt to operate the test item and continue operation from this step until completion of Step 8. Note the altitude at which the test item begins proper operation. If the test item fails to operate as intended, follow the guidance in paragraph 4.3.2 for test item failure. Otherwise, make and break electrical contacts as frequently and as reasonably possible.
Step 7. To ensure adequate mixing of the fuel and air, slowly decrease the simulated chamber altitude at a rate no faster than 100 meters per minute by bleeding air into the chamber.
Step 8. Stop decreasing the altitude at 1000 m below the test altitude or ground level, whichever is reached first, and perform one last operational check and switch off power to the test item. If the test item fails to operate as intended, follow the guidance in paragraph 4.3.2 for test item failure.
Step 9. Verify the potential explosiveness of the air-vapor mixture as in Step 5 above. If ignition does not occur, purge the chamber of the fuel vapor, and repeat the test from Step 1.
Step 10. Adjust the simulated chamber altitude to the equivalent of 2000 m above site pressure.
Step 11. Repeat Steps 3-7. At site pressure, perform one last operational check and switch-off power to the test item. See paragraph 5 for analysis of results.
Step 12. Verify the potential explosiveness of the air-vapor mixture as in Step 5, above. If ignition does not occur, purge the chamber of the fuel vapor, and repeat the test from Step 10.
Step 13. Document the test results.
Procedure II – Explosion Containment
Step 1. Place the test item or a model of the test item of the same volume and configuration within the case, and install the case in the explosion chamber.
Step 2. Ensure the air within the test chamber has a water vapor dew point lower than 10°C (50°F) per paragraph 2.2.4.
Step 3. Seal the chamber with the test item inside, and raise the ambient air temperature inside the chamber to the high operating temperature of the test item.
Step 4. When the temperature of the both the test item and the test chamber inner walls come to within 11°C (20°F) of the chamber ambient air temperature, reduce the chamber air pressure to 2000 m of simulated altitude above the site ambient pressure (i.e., ground level).
Step 5. Slowly introduce the required quantity of n-hexane into the test chamber to obtain an optimum fuel-vapor/air mixture, and then introduce it into the interior of the test item.
Step 6. Slowly decrease the simulated chamber altitude (no faster than 100 meters per minute) to return the pressure altitude to site ambient pressure (i.e., ground level).
Step 7. Energize the internal case ignition source and confirm the occurrence of an explosion within the test item using the installed thermocouple. If no explosion occurs, purge the chamber and the test item of all air/fuel vapor and return to Step 3.
Step 8. If an explosion does occur inside the test item’s case and did not propagate to the fuel/air mixture outside the test item, repeat Steps 4-10 four times if the test item’s case is not in excess of 0.02 times the chamber volume. If the test item volume is equal to or greater than 0.02 times the chamber volume, purge the chamber and test item of air/fuel vapor and repeat Steps 3-10 four times.
Step 9. Check the potential explosiveness of the air/fuel vapor mixture by attempting to ignite a sample of the mixture by a spark or glow plug. If the chamber sample does not ignite, purge the chamber of all air/fuel vapor mixture, and repeat the entire test from Step 3.
Step 10. Document the test results.
NOTE: Tailoring is essential. Please, ask to your confidence laboratory for further details about tailoring of test methods.