Here is some great information from Rex Durometers.
What is a Durometer
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.
Rex Gauge Durometers
Rex Durometers are known World-Wide for their quality, dependability and accuracy. Except where noted, all gauges comply with ASTM D2240. All are in stock and ready for immediate delivery. Custom designed gauges and operating stands, along with many accessories are available.
To make a simple operation check, take a reading on smooth glass or steel (WARNING: THIS IS NOT RECOMMENDED FOR TYPE D DUROMETERS – THE GAUGE MAY BE SEVERELY DAMAGED). The gauge should read no more than 100 and no less than 99. In a Rex Durometer, the unique built-in linearity assures the gauge is in proper operation throughout the entire scale. Should a Rex Durometer not give the above reading on a smooth, hard surface, it may be assumed that the Durometer has been damaged, and it should be sent to Rex for inspection and/or repair.
Durometer Test Blocks give the user the ability to develop the proper “feel”. Test Blocks are not to be used for calibration purposes, however it does enable the user to perform a quick check for proper durometer operation.
Durometers measure hardness by relating the penetration of an indentor into a specimen, and because the indentor travel may reach .100″, it follows that a specimen must be of sufficient thickness to ensure a proper, sensitive test. Generally, samples to be tested should not be less than 1/4″ (6mm) thick. Exceptions may be made for harder materials because the indentor is at less than half stroke. What must be avoided is a specimen so thin that the indentor may sense the hardness of the underlying surface. This will give a false reading due to the “anvil effect”. The Rex Type M durometer can be used when checking materials thinner than 1/4″ (as thin as .050″).
When testing flat specimens too thin to give accurate readings, the specimens may be stacked to provide the required thickness.
In addition to sufficient thickness for testing, sufficient material around the sides of the indentor must be present. With soft materials, a minimum 1/4″ (6mm) is recommended.
|A||Rubber, Elastomers||Flat Cone Point 35°||822 g|
|B||Harder Elastomers, Plastics, Paper, Fibrous Materials
Use above 93 A scale.
|Sharp Cone Point 30°||822 g|
|C||Medium Hard Elastomers, Plastics.
Useful to avoid surface marks.
|Flat Cone Point 35°||10 lb|
|D||Hard Rubber, Plastics, Thermo Plastics||Sharp Cone Point 30°||10 lb|
|DO||Dense Granular Materials, Textile Windings||3/32″ Spherical||10 lb|
|O||Very Soft Elastomers, Textile Windings, Soft Granular Materials Use below 20 A scale.||3/32″ Spherical||822 g|
|OO||Foams, Sponge Rubber, Gels, Animal Tissue.||3/32″ Spherical||113 g|
|OOO||Open Cell Foams, Ultra-soft Gels, Sponge Rubber.||1/2″ Spherical||113 g|
Materials as thin as .050″
|Sharp Cone Point 30°||78 g|
Note: This chart is for comparison purposes only.
Durometer Selection Guide
See our durometer selection guide for more information.
Source: Rex Durometers