If they are to properly fulfill all their functions, insulaation systems must be designed with considerable care. Such care includes discriminating engineering judgment in evaluating tests on materials that are to be used in the system. To illustrate: an engineer might know that the ultimate strength of a particular steel, by test, is 98,000 lb./sq. in. In the design of structural members he would probably use a value of 20,000 to 22,000 lb./sq. in. to allow a safety factor for the unknowns—corrosion, fatigue, etc.