Phase II+ Ultrasonic Thickness Gages are on sale at Gaging.com and are an excellent combination of quality and value! Excellent for non-destructive thickness tests on all types of materials, the Phase 2+ UTG measures thickness by sound velocity. Specifically, the time it takes to leave the probe, reflect on a parallel surface and return to the probe. Surfaces of these materials must be parallel to each other. Please let us know if you have any questions. Measure Smarter with Gaging.com!