This is Part 2 of a series delving into the intricacies’ of Mil-Std-901G.
Mil-Std-810G, Method 500.5 details testing equipment in a low pressure environment to simulate effects of altitude. This section comprises 7 pages starting at page 63 of the Specification.
The detrimental effects of high altitude operation for computers and LCD displays includes:
- Poor cooling due to less air to carry away the heat
- Coronal arcing (air is an insulator)
- Structural deformities or failure of the device such as delamination in an LCD screen
Method 500.5 includes four procedures:
- Procedure I – Storage/Air Transport – Procedure I is appropriate if the materiel is to be transported or stored at high ground elevations or transported by air in its shipping/storage configuration.
- Procedure II – Operation/Air Carriage – Use Procedure II to determine the performance of the materiel under low pressure conditions. If there are no low pressure storage, rapid, or explosive decompression requirements, this procedure can stand alone.
Procedure III – Rapid Decompression – Use Procedure III to determine if a rapid decrease in pressure of the surrounding environment will cause a materiel reaction that would endanger nearby personnel or the platform (ground vehicle or aircraft) in which it is being transported.
Procedure IV – Explosive Decompression – Procedure IV is similar to Procedure III except that it involves an “instantaneous” decrease in the pressure of the surrounding environment.
An interesting note in the Specification says that Procedure IV is not intended for material to be transported in a cargo bay. Analysis shows that for a C-5 aircraft, for example, to reduce the cabin altitude from 40,000 feet to 8,000 feet in 1 second would require a 350 square foot hole (19 x 19-feet) which would most likely be catastrophic for the aircraft leading to aircraft loss. Procedure IV is more appropriate for small pressurized spaces such as cockpits where loss of the canopy would lead to instantaneous depressurization.
For ground operation, the highest altitude anticipated for ground military operations is currently 15,000 feet. For transport in an aircraft, for decompression, the initial cabin altitude is specified at 8,000 feet with a final altitude of 40,000 feet after decompression. For rapid decompression, it is specified to reduce the altitude over a period not to exceed 15 seconds. Explosive decompression is to be accomplished in 0.1 second or less, clearly not possible in a cargo aircraft short of catastrophic airframe failure.
Low temperature testing at higher altitudes may be appropriate depending on the intended use. Remember that 810G does not necessarily specify test procedures but instead stipulates a test procedure be crafted to mimic the environment. As such, at higher altitudes, the ambient temperature will most likely be much lower than in a test lab. For a computer operation, this lower temperature may be advantageous in cooling when less air is available for heat transfer.
Test duration of at least one hour has historically been considered adequate for most material.
The test is performed as you would expect by placing the equipment into a pressure chamber in which the temperature can be controlled, then the pressure is lowered to the equivalent of the desired test altitude and the test run for a minimum of one hour.