Abstract: An ultrasonic system for measuring dimensional change in a structural member, and particularly change in a fastener due to tensile stress, utilizes means for double pulsing a transducer to transmit an acoustic pulse into the member at one end for reflection from its other end with a period between paired pulses selected to cause the second echo received of the first pulse to coincide with the first echo of the second pulse. A VCO is employed with a digital counter to time the period between paired pulses, the interval between successive paired pulses, and the time of a predetermined number of pulse pairs. The latter timing is used to alternatively shift the frequency of the VCO high and low to cause the first echo of the second pulse to be offset in phase from the coincidence position it might have at the central frequency.