Durometers are instruments used for measuring the indentation hardness of materials. Durometers are pushed against the sample to be tested and the hardness information is displayed. This is considered a form of nondestructive testing, as the sample is not permanently deformed.

Durometers use a specific scale, the durometer hardness scale, for measuring the hardness of rubber, sponge rubber, plastic, and other nonmetallic materials.  The durometer hardness scale is an internationally used standard.  Additionally, other organizations, such as ASTM, DIN, JIS and ISO all have their own standards for durometers.

The ASTM D2240 standard covers type A, B, C, D, DO, and OO durometers. The most popular durometers are the types A and D. Very soft materials are rated on the Shore OO scale. Rubber and polyurethane are rated on the Shore A scale and hard materials are rated on the Shore D scale. There is some overlap between scales, so a material may be rated on more than one scale.  For the qualities of specific durometers, refer to the list below:

Type A - Soft Rubber, Elastomers, Waxes, Printing Rollers.
Type B - Rubber, Elastomers.
Type C - Medium-hard Rubber, Plastics.
Type D - Hard Rubber, Plastics.
Type DO - Dense Textile Winding, Medium Printing Rollers.
Type O - Soft Printing Rollers, Textile Windings.
Type OO - Sponge Rubber, Very soft Rubber.
Type OOO - Very Soft Materials, Open Cell Foam.

Durometers are available in three main styles: pencil, round or block, and quadrant.  Pencil style durometers are small and lightweight. These are typically handheld devices, with an extended point that can be used in confined or hard to reach areas other durometers cannot test.  Round or block style durometers are of the shape implied, and are typically mounted on a test stand. Quadrant durometers are small and lightweight.  The durometers housing is the shape of a quarter circle.

Durometers function by pressing an indenter into the test sample.  The indenter used depends upon the relative hardness and configuration of the material being tested. A calibrated spring provides the resistance to the movement of the indenter.