Ultrasonic flaw sizing is important issue for remaining life prediction of industrial plants. Time-of-Flight Diffraction (TOFD) method is recognized as one of the most accurate flaw sizing method. However, to measure the crack height accurately, the crack must be symmetrically placed between transmitter and receiver. When the crack do not exist such position, the B-scan is required for accurate crack sizing. By the way, if additional transmitter or receiver is added to the system, the crack location and height can be estimated without B-scan. In this study, we examined the 4 sensor TOFD method which utilizes two transmitters and two receivers. In the result, the crack location and height can be estimated by 4 sensor TOFD method. However, the estimation errors were large due to the low SN ratio of received signal. Then we applied the pulse compression technique to the 4 sensor TOFD method for improving SN ratio of received signal. In the result, the SN ratio was enhanced about 11dB by the pulse compression. The average location on error is reduced from 2.49mm to 1.13mm, and the average depth estimation error is reduced from 0.28mm to 0.21mm.
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