The durometer is a measuring instrument used to measure the hardness of materials. There are different types of durometers, each intended for a specific application and has its own scale of measurement.
To measure the hardness of a material, the durometer makes on it a penetration, leaving an imprint on the tested point (called witness).
The test using the hardness tester can be defined as a non-destructive testing, because the object in question could maintain their integrity and functionality. The durometer leaves however an imprint, so it must be used, or in areas where the witness does not bother you, or on special specimens made of the same material to be tested.
The test with the durometer can not be repeated in the same location (or even in the vicinity of other witnesses): in fact the surface in the vicinity of a witness is geometrically and structurally modified and the repetition of the test would lead to large measurement errors.