Downloaded from https://www.everyspec.com f. Sand and dust. Sand and dust are usually differentiated on the basis of particle size, although there are no generally accepted specific size limits for the two kinds of particles. For most military applications, it is important to distinguish between the smaller particles (dust) and the larger particles (sand) because of their different effects on equipment. Dust can penetrate small openings, cause undue wear to moving parts, and interfere with electrical contacts. Blowing sand, which may be too large to penetrate the smaller openings, can erode and abrade the outside of equipment.
Sand and dust present in the air affect the performance of electro-optical systems because of their atenuation and degradation of electromagnetic radiation in the atmosphere. Particles vary in diameter from 0.1 to 1,000 micrometers (3.94 Ч 10 inches to 3.94 Ч 10 inches), but most airborne particles are less than 74 micrometers (2.91 Ч 10 inches). (1) Three operational levels are given; selection of the appropriate one depends on intended use of the materiel under consideration.
Items likely to be used in close proximity to aircraft operating over unpaved surfaces should be designed for particle concentrations of about 1.32 Ч 10 lb/ft 3 (2.19 Ч 10 kg/m 3 ) in multidirectional strong winds (downwash from helicopter rotors). The extinction coefficient for this particle concentration is estimated to be 100 km for visible wavelengths through the middle infrared (12 micrometers). Such particles range in size up to 500 micrometers (1.97 Ч 10 inches) in diameter.
Items never used or never exposed in close proximity to operating aircraft but which may be found near operating surface vehicles, should be designed for particle concentrations of 6.61 Ч 10 lb/ft 3 (106 Ч 10 kg/m 3 with windspeeds up to 59 fps (18 mps) at a height of 10 feet (3 in). Particle sizes will range from less than 74 micrometers (2.91 Ч 10 inches) in diameter to 1,000 micrometers (3.94 Ч 10 inches), with the bulk of the particles ranging in size from 74 to 350 micrometers (13.8 Ч 10 inches). (2) The above two categories are likely to include most military items.
However, items that are ensured of being subjected only to natural conditions should be designed for particle concentrations of 1.10 Ч 10 lb/ft 3 (0.177 Ч 10 kg/m 3 ) with windspeeds of 59 fps (18 mps) at a height of 10 feet (3 in). Under these conditions, the bulk of the particle sizes are likely to be less than 150 micrometers (5.90 Ч 10 inches) except that some large particles (up to 1,000 micrometers) may be in motion within several feet of the ground. In all categories, temperatures are typically above 70 ° F (21 ° C) and relative humilities are less than 30 percent.
For testing purposes, particle sizes up to 150 micrometers should be used if the primary concern is with the penetration of fine particles. If the abrasion effect of blowing sand is the primary concern, particle sizes up to 1,000 micrometers should be used, but the bulk of the particles should be between 150 and 500 micrometers. Many items, such as rifles, vehicles, and helicopters, may be exposed to particles up to 1,000 micrometers that can penetrate the space between moving parts. g. Ozone concentration. For operations, a value of 1.37 Ч 10 lb/ft 3 (220 Ч 10 kg/m 3 ) is recommended. h. Atmospheric pressure.
Atmospheric pressure usually is riot considered in the design and testing of military equipment. Ambient pressure, however, may be important for a few types of equipment, for example, items which require oxygen for combustion and sealed units, which might explode or collapse under abnormally low or high pressure. (1) High pressure. The operational extreme high pressure is 1,080 mb (31.89 inches). (2) Low pressure. The operational extreme low pressure is estimated to be 508 mb (15.00 inches) at 15,000 feet (4, 572 in), the highest. elevation at which Army equipment is likely to be used. At sea level the operational extreme is 877 mb (25.90 inches). 2–9. Combined environmental effects.
The climatic design types in this regulation are based primarily on temperature extremes and secondly on humidity extremes. The climatic elements discussed in paragraph 2-8, however, may interact concurrently with temperature and humidity and with each other to produce effects on materiel either different or more severe than the sum of the effects caused by the separate elements acting independently. These are known as combined or synergistic environmental effects.
The fact that these synergistic effects exist is one of the prime arguments for conducting field tests, because it is extremely difficult or impossible to reproduce the interacting environmental factors concurrently in a test chamber. Section III DISTRIBUTION OF CLIMATIC DESIGN TYPES 2–10. Map of climatic design types. Figure 2-1 shows land areas where the four climatic design types apply. Discussion of the delimitation of the climatic conditions (para 2-11) is included to permit proper interpretation and use of the map. 2–11. Delimitation of climatic design types.
The primary basis for delimiting the climatic conditions in this regulation is temperature; secondary consideration is given to humidity conditions. a. Hot climatic design type. The areas where hot conditions apply include most of the lowlatitude deserts of the 21 AR 70–38 • 15 September 1979