Ultrasonic Thickness Gauge is a widely used nondestructive test technique for measuring the thickness of a material from one side. It is fast, reliable, and versatile, and unlike a micrometer or caliper it requires access to only one side of the test piece. The first commercial ultrasonic gages, using principles derived from sonar, were introduced in the late 1940s. Small, portable instruments optimized for a wide variety of test applications became common in the 1970s. Later advances in microprocessor technology led to new levels of performance in today's sophisticated, easy-to-use miniature instruments.