Ultrasonic thickness gauges use sound wave to measure wall thickness. Different types of materials have different inherent acoustic velocities. ( sound travels at different speeds through different materials ).For example the acoustic velocity of steel is 0.2330 inches/microsecond and that of aluminum is 0.2500 inches/microsecond. The unit can be set to any acoustic velocity depending on the material being measured. The instruction book includes a table of acoustic velocities for common materials. Even if you don't know the acoustic velocity or your material,you can measure it with the sample of material of known thickness to start with.