Downloaded from https://www.everyspec.com Appendix A RELATIONSHIP BETWEEN AR 70-38 AND MIL-STD-210B* *Available from Naval Publications and Forms Center, 5801 Tabor Avenue, Philadelphia, PA 19120 A–1. Background. To provide background for the proper use of this regulation, its relationship to MIL-STD-210B is outlined. There are two main differences between the two publications. a. MIL-STD-210B applies only to materiel developed for worldwide use. AR 70-38 gives design values for most Army materiel, whether intended for worldwide or for more limited use. b. The treatment of withstanding values.
Both publications reflect a philosophy that accepts a small risk of failure during periods of extreme weather. They also require a complete return to operation after exposure to extreme conditions has ended. MIL-STD-210B, however, considers only the likelihood of natural extremes occurring, whereas AR 70-38 considers the induced conditions many items are exposed to during transit and storage. This difference means that Army materiel must be able to withstand much higher air temperatures than those in MIL-STD-210B, although the high temperatures for operation are the same. A–2. Risk policy.
In the Ground Environment section, MIL-STD-210B contains single worldwide values for each climatic element to be considered in the design of materiel for operations. For most climatic elements, the design value selected was the value exceeded not more than one percent of the hours in the most extreme month in an average year at the most severe location for that element. (For low temperature the level selected was 20 percent of the hours and for rainfall the level selected was 0.5 percent of the hours.) These values have become known as one percent design values. When they are applied collectively they are often referred to as a one percent risk policy.
Although this is a convenient short designation, it can be misleading to those who are not aware of this specific definition of a one percent risk policy. In fact, there is no way to quantify, with any degree of accuracy, the probability that materiel will ever encounter a given extreme of an environmental element. It can be stated with assurance that the designated one percent risk levels as used in MIL-STD-210B are very conservative. For example, on a year-round basis, the risk of encountering the design level of a selected element approaches 1/12 of one percent (there is some likelihood of occurrence in other than the most extreme month).
Also, for many of the climatic elements, the design value applies only to the most severe location in the world. Therefore, the risk of materiel encountering this extreme may be very small, particularly if the value at the most severe location is representative of only a small area or the location is in a remote part of the world. The above considerations led to the adoption of the system now used in AR 70-38. It provides alternate design values for items not intended for worldwide use. Consequently, the world was divided, on the basis of temperature, into four types. The design temperatures in this four-type division are somewhat arbitrary.
However, the geographic areas encompassed by the basic design type contain most of the world’s population and landmass. In general, the lines delimiting the areas included in a design type have the same basis as MIL-STD-210B; that is, one percent of the hours in the most severe month on average exceeded the design temperature. Note, however, that it is only along the demarcation line that this criterion applies exactly. For example, if more than one percent of the hours in the coldest month at a given location are below −50 ° (−46 ° C), the area represented by that location is considered part of the cold climatic design type.
Yet, at that location, there may be almost no chance of occurrence of −60 ° F (−51 ° C), which is the lower design value for that type. On the other hand, there are stations in the areas included in the severe cold design type that have temperatures below −60 ° F (−51 ° C) for as much as 20 percent of the hours in the coldest month. This kind of variation within the regions could be eliminated only by creating a large number of small regions, a procedure that would make this regulation unduly complex. A–3. Additional guidance. A general regulation such as this one cannot possibly address in detail the environmental considerations for all materiel.
Thus, users are encouraged to seek additional or more specific guidance from the proponent agency. 25 AR 70–38 • 15 September 1979