Under optimum conditions, commercial ultrasonic gages can achieve accuracies as high as +/- 0.001 mm (0.00004"), with accuracies of +/- 0.025 mm (0.001") or better possible in most common engineering materials. Factors affecting accuracy include the uniformity of sound velocity the test material, the degree of sound scattering or absorption, the surface condition, and the accuracy and care with which the instrument has been calibrated for the application at hand.