Ultrasonic thickness gage

An ultrasonic method was developed for making approximate measurements of the thickness of a web moving between parallel guides. Since practical considerations ruled out mechanical contact, a phasesensitive detection scheme for measuring the thickness of the air spaces between the fixed guides and the web was chosen so that: (1) the apparatus was insensitive to variations in surface properties of the web, and (2) a linear response was closely approximated. Unwanted reflections and their influence were reduced through the design of the acoustical geometry and a dummy arrangement. Another set of dummies compensated for changes in sound velocity with temperature changes. Performance curves of the ultrasonic measuring device show that the device performs satisfactorily if all electrical-to-sonic transducers have identical temperature vs. phase-lag characteristics at the operating frequency.