In-Service Inspection of Nuclear Power Plants - gnssn - International ...

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This publication is no longer valid Please see http://www-ns.iaea.org/standards/ a. Impedance : the impedance of the transducer, provided with its own cable and air coupled, is as a general rule evaluated at the resonance frequency (series). In the case of high damping probes, whose curve does not present extremes, the impedance is measured at the nominal frequency. b. Echo analysis : the signal reflected by a flat perpendicular to the incidence plane (fig.6) is submitted to a time and frequency analysis, which gives the following characteristics (flg.7) . sensitivity (S) : peak-to-peak amplitude of the signal . internal reflection (IR) : amplitude ratio of a possible internal reflection echo and the main signal . nominal freqeuncy . quality factor (Q), evaluated at -3 and -12 dB. At the present time, all these measurements are carried out automatically : the analog signal is converted in a digital oscilloscope and transmitted by GP-IB interface to a controller, which performs very rapidly the digital processing. As an example, fig. 8 illustrates the presentation of the results. c. Dead zone : this measurement is carried out on an air coupled probe. The signals generated by the internal reflections and the vibrations of the piezoelectric crystal are compared with a reference level, which is the echo reflected by a 2 mm diameter hole, side-drilled in a steel block at a depth giving a 60 mm path. The dead zone is then defined as the length of the steel path during which the ratio of these signals remains greater than 20 dB (fig.9). d. Cross-talk : this parameter, which applies to send-receive probes only, is given by the amplitude ratio of a reference echo (backwall echo at the beam crossing depth) and the residual echo of the probe. e. Acoustic energy distribution : the transducer is put on a triangular section block, as shown on fig. 10. The opposite wall, which is at first supposed to be perpendicular to the beam axis, is scanned by an electro-magnetic transducer, and the signals received are transmitted through a wideband amplifier to an X-Y plotter. The bench is thus made of different triangular blocks, whose angles are the most commonly used refraction angles in ultrasonic testing. The repetition of this measurement with different transducer positions (and thus different paths) allows the visualization of the acoustic energy distribution either in transverse sections of the beam (fig.11) or along its axis (fig.12). Text continued on p. 114. 108
This publication is no longer valid Please see http://www-ns.iaea.org/standards/ SIMULATED ECHO FOR AMPLIFIER CHARACTERISATION FIG. 5 ( Pulse Generator Analysis System V_j •—i Transducer \ Carbon Steel Block ECHO TIME AND FREQUENCY ANALYSIS FIG. 6 109
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