TABLE OF CONTENTS

Introduction The Digitization Technique Data Reduction Conclusion Literature

Introduction

Recently, quasi-gate-free digital ultrasonic instruments were developed for in-service inspection of NPP (Nuclear Power Plants) to improve the inspection results and decrease test and evaluation time.
The signal recognition and the data reduction for inspection visualization are important issues in the application of these new systems.

In particular an optimization of:

• Digitization of the analog signal without missing essential information.
• Sampling rate during generation of the row data as RF signal data in such a way so that defect analysis is possible, for instance with Synthetic Aperture Focusing Technique (SAFT), Time of Flight Diffraction Technique (TOFD) or with ultrasonic holography.
• Number of pixels in the evaluation range or pixel width to ensure a reliable comparison with the test results of a previous in-service inspection as well as description of possible defects.
• Determining the Amplituden Laufzeit Ortskurven (ALOK = Amplitude Time Locus Curves) parameters i and k.

Digital ultrasonic instruments have been in use for some time as components of NPP inspection. So far, guidelines and standards have not contained characterizations of signal recognition or evaluation, as long as the Digitization has been applied for gate (monitor) techniques only. That means there is no documentation concerning the Digitization of ultrasonic echo signals in DIN 54119 (08/1981) "Ultrasonic Testing - Terms (Ultraschallprüfung - Begriffe)" and DIN 25450 (09/1990) "Ultrasonic Test Systems for Manual Testing (Ultraschallprüfsysteme für die manuelle Prüfung)".

Only in DIN 25435-1 is it mentioned that the resolution must be equal to or greater than 1 dB if analog to digital conversion is applied (DIN 25435-1 [11/1987] "Wiederkehrende Prüfungen der Komponenten des Primärkreises von Leichtwasserreaktoren - Mechanisierte Ultraschallprüfung"). For our interpretation a limit of the quantifying error is determined and in particular the applicable echo dynamic range is limited (e.g., for a 8 Bit A/D converter approx. 20 dB).

In ultrasonic testing procedures for defect description certain methods are used which cannot be detailed within the framework of a guideline stipulation. Nevertheless, standards of inspection procedures and instruments must be applied for defect detection, to ensure repeatability and comparison with previous inspections. This must also be a consideration when digital ultrasonic instruments are used.

Therefore, in new or modified ultrasonic testing standards it is necessary to include and define applicable terms, as well to define the characterization of ultrasonic instruments, such as A/D converters, sampling error, sampling frequency, data recognition and reduction, evaluation procedure, etc.