Type A durometer

Two types of durometers are most commonly used— Type A and Type D. The basic difference between the two types is the shape and dimension of the indentor. The hardness numbers derived from either scale are just numbers without any units. The Type A durometer is used with relatively soft material and Type D is used with slightly harder material. A commercially available durometer hardness measuring instrument is shown in Figure 2-62. 

Hardness - Hardness, as applied to rubber products, is the relative resistance of the surface to indentation under specified conditions. Hardness of rubber is usually measured with a small spring-loaded hardness gauge known as a durometer (ASTM D-2240). The durometer may be handheld or mounted on a stand. The measurement is made by pressing the indentor against the sample and reading the scale, which is calibrated in arbitrary units ranging from 0 (soft) to 100 (hard). A Type A durometer is used for most soft rubber products there is also a Type D durometer for hard rubber and plastic-like materials. On the A scale, a gum rubber band would measure around 40, a tire tread 60, and a shoe sole 80. 

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