An ultrasonic system to plot the thickness profile of a fuel-element coolant channel was developed to measure the ETR and ATR fuel elements in a canal after being removed from the reactor. It has an accuracy of 0.5 mil, a resolution of 0.1 mil, and a range from 50 to 175 mils. The heart of the system is a probe holding two 8-MHz, 10-mil-thick lead zirconate titanate piezoelectric crystals. Distance between channel walls is obtained by measuring the time for an 8-MHz sound burst to travel between the crystals which are held next to the walls. The signal received by one crystal, transmitted from the other crystal, is used as a trigger for a time-to-analog converter that is connected to the y axis of a recorder. Longitudinal distance in the channel is converted to an analog output to drive the x axis of the same recorder.