Abstract: A composite probe apparatus exhibiting a high performance and a high efficiency in detecting internal defects of a steel pipe or a steel plate, coupled with offering a stable sensitivity and covering a wide range for flaw detection. A plurality of reception oscillators PR1 to PR4 are disposed in a zigzag fashion in both sides of a transmission oscillator PT. The length l T of the transmission oscillator PT is determined such that a near sound field is established over the entire measuring range, while the width WT of the transmission oscillator PT is determined such that a far sound field takes place over the entire measuring range. The transmission and reception oscillators are constructed with a common piezoelectric plate an electrode of which undergoes division.

Abstract: An apparatus for measuring a dimension of an object by using ultrasonic waves includes: a pair of probes spaced apart one of the probes being to face a top surface of the object and the other of the probes being to face a bottom surface of the object; control for choosing what to be currently measured from three kinds of travelling periods of time which ultrasonic pulses take: to propagate between the probes; to travel between the top surface of the object and the probe facing the top surface; and to travel between the bottom surface of the object and the other probe facing the bottom surface and apparatus responsive to the transmission and reception signals for evaluating the three kinds of travelling periods of time and for calculating the dimension of the object from the evaluated travelling periods, whereby the probes can be out of touch with the surfaces of the object.