Page 45 - VAK_Broschuere_II_18.indd

This wall measurement represents a reading within 0.001 inch (0.025 mm) for this nominal 0.019 inch (0.048 mm) wall tube. The main concern using this approach is to select a transducer with a much shorter wavelength than the thickness of the tube wall.
What if you are not able to nd a 20 MHz center
frequency transducer?
We discussed the idea of a burst pulse earlier. Consider that we have a transducer that is broadband with center frequency a center fre- quency that 16 MHZ. Yet we would like to work with frequency at 20 MHz. Looking at the broadband characteristics, we note that it shows a 20MHz frequency at a point between center frequency 16MHz and the upper –3dB down frequency position. With a pulser that has an adjust- able 20MHz burst frequency we can force the transducer to operate at this 20MHz position on the curve. This would enable us to operate a 16MHz at the same frequency as the earlier transducer with its center frequency at 20MHz using the simple spike pulser. Because, the burst pulser that we have cannot provide a burst frequency above 10MHz, we are unable to demonstrate the affects of burst frequency shifting away from the center frequency to a higher point , above the –3 dB on the transducer’s broad band spectral curve.
However, if you look back to the early 1970s, you will nd that a company called Nutronik GmbH produced and sold ultrasonic reso- nant frequency instrumentation. Instead of using a burst frequency, they swept the frequencies, from 7.5 MHz to 15MHz,
across the bandwidth of broadband transducers. When- ever the wavelength ( Lambda/2) of the sweeping fre- quency matched the thickness it produced a spike at the calibrated thickness for the frequency. This was the forerunner to the burst pulsers of today. So using high frequency wavelengths for ultra-thin wall is not actually a new technique. Instead it is merely a new resurrection of something old with new electronic techniques.
A Final Comment:
Fig. 8
The tools to perform ultra-thin wall measurements using ultrasonic methods are available. Nevertheless if you are looking for a system to perform these measurements on super thin wall, small diameter tubing, you need to be aware of the need for working with high frequencies in the range of 17.5 MHz to 22.0 MHz if you expect to avoid long drawn out changeover times.
Technical Papers
Terrance R Banach
Vanishing Point Education LLC 29 Cedar Lane Ext IUS-CT-06784 Sherman
USA
Tel.: +18603509863 tbanach@snet.net www.vanishingpoint.com.au
Fig. 9
ITAtube Journal No2/July 2018
45