Web caliper measuring system

A non-contact caliper-type gauge particularly suited for measuring the thickness of a running web. In its preferred form, the system includes a pair of opposed scanning heads (12,13) defining a gap (14) through which the running web (16) passes, the heads being adapted to traverse across the width of the running web. Each of the heads includes an ultrasonic transducer (20,21) for directing energy at the web, detecting energy reflected from the face of the web nearest the respective transducer and determining the distance between a reference in the head and the facing surface of the web. One of the heads (12) also includes a transducer (50), preferably an eddy current transducer, which operates on energy to which the web is transparent. The second transducer senses the distance between the heads thereby to determine any variations in the distance between the heads during traverse. The distances determined by each of the ultrasonic transducers to the respective surfaces of the web are subtracted from the total gap distance to produce a corrected measure of web thickness which is substantially independent of mechanical variations in gap between the heads.