Using a Vickers indenter and ultrasonic technology, the hardness tester is first calibrated using a sample of known hardness. The working principle, in short, is the use of an oscillating rod inside your hardness probe. When you apply force to the indenter on your sample, you change the frequency of the oscillating rod. By doing this, and having calibrated using the sample of known hardness, the ultrasonic hardness tester can compare and determine your material hardness.