The Durometer is the international standard for the hardness measurement of rubber, plastic and other non-metallic materials. Durometers are described in the American Society for Testing and Material specification ASTM D2240, which is the recognized specification for the instrument and test procedures.
"A" type is suitable for softer non ferrous materials, such as rubber o-rings, car tires, rubber bands etc.
"D" type is suitable for harder non ferrous materials. such as a hard plastic (HDPE), truck tire, lexan etc.