Abstract: A coextruded zirconium tube and tube liner are monitored along their lengths by well known ultrasonic measurement techniques that are modified by the blocking out of extraneous signals at opposite sides of a timing gate that varies in width according to variations in the width and shape characteristics of successive points along the length of such coextruded tube and liner. The blocking of such extraneous signals enables a normally very weak signal representing the interface between tube and tube liner to be identified and used, together with a group of stronger signals representing the inner surface of the tube liner, in monitoring tube liner thickness and characteristics by a sonic wave traversing the distance across the timing gate.