Durometer
A durometer is a tool used to measure hardness.
There are various Durometer scales in use today. They include: A B C D DO O OO OOO T M
Various scales are used to measure hardness ranging from sponge to steel. For example, the Shore 'A' scale ranges from 0 to 100 units and usually refers to rubber or synthetics. A20 would be an extremely soft silicone rubber while A85 would be a hard rubber skateboard wheel. The Rockwell B scale measures in the range of soft steels. Rockwell C measures harder steels.
Durometers typically work by measuring the resistance, in newtons, of a pin as it penetrates a given material.