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On-Site Characterization of Ultrasonic Probe Pauli Särkiniemi Contact

SUMMARY

Many standards and recommended practices demand for characterization of ultrasonic probe a lot of measurements, which are time consuming and expensive. Also specific equipment like immersion tanks with scanning device, hydrophones and spectrum analysers are needed (ASTM-1065-96). This has led to the situation, that many inspection companies perform periodic characterization only partially. Even manufacturers of transducers do not follow all requirements of standards.

This presentation introduces the characterization method which can be carried out with usual ultrasonic instrument. The only requirement for the instrument is that the signal can be examined unrectified i.e. in RF-mode. Characterization can be conducted on-site without any specific tools, but the same method is also convinient for laboratory use.

INTRODUCTION

Few sporadic observations started the development of characterization method for ultrasonic probes. In the following there are two examples. (1) A 5 MHz short pulse straight beam probe provided for thickness measurement didn´t work satisfactorily. RF-signal of ultrasonic instrument was examined and it showed the pulse length to be about 10 cycles. (2) A 10 MHz probe provided for thickness measurement of thin walled tube section didn´t work as expected. The length of one cycle was measured from RF-signal and frequency was calculated. Result was about 5 MHz.

In general the unrectified pulse shape (RF-signal) is measured with an oscilloscope and the pulse is analysed with a spectrum analyser to get frequency spectrum (fig. 1).

Fig 1: Typical RF-signal and frequency spectrum (Krautkrämer).

Measurement carried out by VTT (Särkiniemi et al., 1991) and examination of data sheets of different manufacturers showed close connection between the pulse length in cycles and the band width. This led to the idea that the band width of the probe can be derived with reasonable accuracy from the number of cycles in a pulse. By developing the characterization method it is assumed that the requirements of ASME XI App. VIII for replacing search unit can be fullfilled.

Measurement of variables like probe index, beam angle, beam spread, signal-to-noise ratio and etc. shall be carried out accordig to the usual procedure. From the RF-signal follwing parameters are calculated, measured or derived

• Center frequency
• Pulse length in cycles
• Relative band width

DESCRIPTION OF THE METHOD

The maximum sound path of the ultrasonic instrument is adjusted as usually and RF-mode has to be selected. An echo coming for instance from a curved surface is maximised to 100% of full screen height (FSH). A wide calibration block is recommended in order to avoid side-wall echos.The gate is chosen and adjusted to 10% of FSH. It is usefull to zoom gate to improve read out accuracy (fig. 2).

The length of cycle is read for instance from peak-to-peak and the center frequency is calculated by using equation (1). Better read out accuracy and more precise frequency value can be achieved if the length of cycles is read over two or more cycles.

(1)

f_c = center frequency (1/s)
v = sound velocity (m/s)
x = measured cycle length (m)

Then the number of cycles exceeding 10% gate is counted. Most accurate value can be achieved if the envelope is outlined via peaks and finding points wherethe envelope cuts 10% gate level. This is very important especially in the case of short pulse (£ 2 cycles). The number of cycles P_n shall be recorded.

Fig 2: RF-signal with the gate at 10% of maximum.

Fig 3: Relative band width as a function of number of cycles (preliminary curve).

There is a close connection between the number of cycles and relative band width. The preliminary curve is presented in figure 3. Relative band width is defined as follows

(2)

BW = relative band width (%)
f_u = upper limit (-6 dB, MHz)
f_l = lower limit (-6 dB, MHz)
f_c = center frequency (MHz)

DISCUSSION

The measurement carried out by VTT and examination of data sheets of different manufacturers showed close connection between the pulse length in cycles and the band width. The sample included about 40 probes made by different manufacturers from different crystal materials.

The method described here is simple and therefore can not be very accurate. Nevertheless, by developing the method it is assumed that maximum error of ± 10 % in relative band width can be achieved down to pulse length one cycle. It has to be also emphasized that for the operator the common touch on band width is often enough and more important than high accuracy.

Further work is needed to develop the method including mathematical modelling of the dependency between the pulse length in cycles and the relative band width.

CONCLUSION

A method for characterizing ultrasonic probe on-site has been described. The conventional ultrasonic instrument with RF-signal presentation is used. Even relative band width can be determined with reasonable accuracy. The method is simple, quick and cheap.

The method is emphasized as an on-site method but it is also convinient in laboratory use.

Charactezations required by standards are often expensive and time consuming and therefore they are perfomed only partially or as modified. The on-site method is a good alternative and characterization can be performed completly.

Further work is needed to develop the method. Mathematical modelling of the dependency between the pulse length in cycles and relative band width is one of the most important areas of developement.

REFERENCES

1. ASTM E 1065-96. Guide for Evaluating Characteristics of Ultrasonic Search Units. American Society for Testing and Materials. Volume 03.03, 1997. Pp. 546-474.
2. P. Särkiniemi, P. Kauppinen, R. Tarvainen, J. Sillanpää. Characterization of Ultrasonic Equipment (in Finnish). Research Report STUK-YTO-TR 31. Helsinki, November 1991. 39 pp., 5 Appendices.
3. ASME XI, Appendix VIII, 1998 Edition. Performance Demonstration for Ultrasonic Examination Systems. American Society of Mechanical Engineering. July 1. 1998. Pp. 353-379.

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