Alerting a user of a material inspection device to a change in thickness of a material being inspected is disclosed. A thickness offset is determined from calibration information. The calibration information identifies a time of flight of a pulse through a reference sample similar in composition to a material to be inspected. The thickness offset indicates when a thickness of a material being inspected differs from a thickness of the reference sample. A calibration thickness alarm is set, the calibration thickness alarm corresponding to the thickness offset. A change in thickness of the material being inspected is detected. The calibration thickness alarm is engaged to alert the user of the inspection device of a detected change in thickness of the material being inspected.