Abstract: Measurement apparatus utilizes a sonar head for sending and receiving sonar signals and the apparatus is calibrated before each measurement by locating the sonar head a fixed, predetermined distance from a stationary target. Movement of the sonar head is accomplished by a stepper motor and lead screw arrangement to position the sonar head the same, predetermined distance from the element to be measured as it was originally placed for calibration, and the difference between the calibration distance and the second distance is from a target motor and lead screw arrangement to position the sonar head the same, predetermined distance from the element to be measured as it was originally placed for calibration, and the difference between the calibration distance and the second distance is translated into a distance representing the desired measurement.