Higher numbers on the scale indicate a greater resistance to indentation and thus harder materials. Lower numbers indicate less resistance and softer materials.
The term is also used to describe a material's rating on the scale, as in an object having a “‘Shore durometer’ of 90.”
The scale was defined by Albert Ferdinand Shore, who developed a suitable device to measure hardness in the 1920s. It was neither the first hardness tester nor the first to be called a durometer (ISV duro- and -meter; attested since the 19th century), but today that name usually refers to Shore hardness; other devices use other measures, which return corresponding results, such as for Rockwell hardness.
Method of measurement
Durometer, like many other hardness tests, measures the depth of an indentation in the material created by a given force on a standardized presser foot. This depth is dependent on the hardness of the material, its viscoelastic properties, the shape of the presser foot, and the duration of the test. ASTM D2240 durometers allows for a measurement of the initial hardness, or the indentation hardness after a given period of time. The basic test requires applying the force in a consistent manner, without shock, and measuring the hardness (depth of the indentation). If a timed hardness is desired, force is applied for the required time and then read. The material under test should be a minimum of 6 mm (0.25 inches) thick.
|Durometer||Indenting foot||Applied mass (kg)||Resulting force (N)|
|Type A||Hardened steel rod 1.1 mm – 1.4 mm diameter, with a truncated 35° cone, 0.79 mm diameter||0.822||8.064|
|Type D||Hardened steel rod 1.1 mm – 1.4 mm diameter, with a 30° conical point, 0.1 mm radius tip||4.550||44.64|
The ASTM D2240 standard recognizes twelve different durometer scales using combinations of specific spring forces and indentor configurations. These scales are properly referred to as durometer types; i.e., a durometer type is specifically designed to determine a specific scale, and the scale does not exist separately from the durometer. The table below provides details for each of these types, with the exception of Type R.
|Durometer type||Configuration||Diameter||Extension||Spring force|
|A||35° truncated cone (frustum)||1.40 mm (0.055 in)||2.54 mm (0.100 in)||8.05 N (821 gf)|
|C||35° truncated cone (frustum)||1.40 mm (0.055 in)||2.54 mm (0.100 in)||44.45 N (4,533 gf)|
|D||30° cone||1.40 mm (0.055 in)||2.54 mm (0.100 in)||44.45 N (4,533 gf)|
|B||30° cone||1.40 mm (0.055 in)||2.54 mm (0.100 in)||8.05 N (821 gf)|
|M||30° cone||0.79 mm (0.031 in)||1.25 mm (0.049 in)||0.765 N (78.0 gf)|
|E||2.5 mm (0.098 in) spherical radius||4.50 mm (0.177 in)||2.54 mm (0.100 in)||8.05 N (821 gf)|
|O||1.20 mm (0.047 in) spherical radius||2.40 mm (0.094 in)||2.54 mm (0.100 in)||8.05 N (821 gf)|
|OO||1.20 mm (0.047 in) spherical radius||2.40 mm (0.094 in)||2.54 mm (0.100 in)||1.111 N (113.3 gf)|
|DO||1.20 mm (0.047 in) spherical radius||2.40 mm (0.094 in)||2.54 mm (0.100 in)||44.45 N (4,533 gf)|
|OOO||6.35 mm (0.250 in) spherical radius||10.7–11.6 mm (0.42–0.46 in)||2.54 mm (0.100 in)||1.111 N (113.3 gf)|
|OOO-S||10.7 mm (0.42 in) radius disk||11.9 mm (0.47 in)||5.0 mm (0.20 in)||1.932 N (197.0 gf)|