DIN 53 505 (draft February 1985) gives this definition: "Shore hardness is understood to be the resistance to penetration by a body of a specific shape under a defined spring force. The hardness scale covers a range from 0 to 100 hardness units, 0 being the minimum and 100 the maximum hardness rating." It is customary to state Shore hardness (A or D, depending on the testing equipment) for elastic materials. O-rings made of elastomers for mechanical seals have Shore hardness ratings of between 30 and 95 according to the case of application, the operating conditions or material.