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- since 1996 -

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Technical Discussions
Stefan
Stefan
09:00 Feb-05-2004
couplant influence

Can anybody help me with the question on the influence of the gel layer on the ultrasonic wave propagation? If a transducer is mounted e.g. on a tank wall and the acoustic coupling is done by gel layer, is there additional reflection due to the layer, are there any phase shifts or amplitude modifications.
Many thanks
Stefan


    
 
 
Ethan
Ethan
05:31 Feb-05-2004
Re: couplant influence
Hi Stefan,

This is pretty much dependent on the frequency that you are working with, the couplant material and it's thickness. If this is relatively traditional application (0.5 MHz to say 15 MHz range and a common gel couplant on a relatively smooth surface) you will not see any couplant layer reflections. If it is anything other than this, please provide more information about your application. Being a very 'hands on' person, I usually default to 'try it and see what happens'

Regards

Ethan

----------- Start Original Message -----------
: Can anybody help me with the question on the influence of the gel layer on the ultrasonic wave propagation? If a transducer is mounted e.g. on a tank wall and the acoustic coupling is done by gel layer, is there additional reflection due to the layer, are there any phase shifts or amplitude modifications.
: Many thanks
: Stefan
------------ End Original Message ------------




    
 
 
Philippe Rubbers
Engineering
SCM, South Africa, Joined Nov 1998, 21

Philippe Rubbers

Engineering
SCM,
South Africa,
Joined Nov 1998
21
07:37 Feb-06-2004
Re: couplant influence
Coupling does make a difference, and surprisingly a solid bond between the probe and the material is worse than with gel (L waves). An excellent reference is 'The ultrasonic testing of materials' by Krautkramer and Krautkramer, Appendix A has all the transmission curves for various angles.

Note: Use the english 4th edition 4, or German 5th edition 5, as there are errors in the equations and graphs in previous editions.


----------- Start Original Message -----------
: Can anybody help me with the question on the influence of the gel layer on the ultrasonic wave propagation? If a transducer is mounted e.g. on a tank wall and the acoustic coupling is done by gel layer, is there additional reflection due to the layer, are there any phase shifts or amplitude modifications.
: Many thanks
: Stefan
------------ End Original Message ------------




    
 
 
Gordon E. Smith
Gordon E. Smith
02:18 Feb-07-2004
Re: couplant influence
Almost all competent "shearwave" welding inspectors will tell you that they can easily find both the Transducer Face to Wedge interface and the Wedge to Test Item interface at 2.25 Mhz.

The are many cases of poorly trained "Techs" identifying the Wedge to Test item reflectors as near surface defects.

In AWS UT Weld Inspections there are several safe guards for this, but there are many unqualified persons who cannot do these simple checks and who do not understand what they see on the screen.

Thus these surfaces at conventional low frequencies will produce echoes. And these echoes can look like flaws.


    
 
 
Stefan
Stefan
02:26 Feb-07-2004
Re: couplant influence
Hi,

thanks to all for your help. I am having a traditional
application working with 0.5 mhz frequency to measure the filling level in a steel tank (2meter diameter, 4 m height). I is very difficult to distinguish the liquid surface echo form the noise especially when the level is below 50 cm. So I thought it might help to know something more detailed about the influence of the coupling gel (esp. for the transmission of longitudinal waves). Do you think an increase of frequency might help ?


    
 
 
Neil Burleigh
Sales
Krautkramer Australia Pty Ltd, Australia, Joined Dec 2002, 153

Neil Burleigh

Sales
Krautkramer Australia Pty Ltd,
Australia,
Joined Dec 2002
153
07:25 Feb-09-2004
Re: couplant influence
----------- Start Original Message -----------
: Hi,
: thanks to all for your help. I am having a traditional
: application working with 0.5 mhz frequency to measure the filling level in a steel tank (2meter diameter, 4 m height). I is very difficult to distinguish the liquid surface echo form the noise especially when the level is below 50 cm. So I thought it might help to know something more detailed about the influence of the coupling gel (esp. for the transmission of longitudinal waves). Do you think an increase of frequency might help ?
------------ End Original Message ------------
I have had similar experience trying to measure the level of fuel, petrol, benzine in a fuel tanker. These tankers are approx. 2.5 -3.0 metres in dia and my prospective customer wanted to measure the liquid level as the tank was emptied. The customer wanted an accurate method of the height of the remaining liquid in the tank. Our first problem we had was that the liquid kept on moving inside the tanker, we could actually see the waves on the 'A' scan display. This was solved by using a floating baffle to act as a reflector.
In our initial trials we used the hybrid analog/digital 'A' scan ultrasonic flaw detector (USK7D) and a 2MHz probe (B2S). The probe was coupled to the bottom of the tanker and the sound beam was directed straight up we were able to measure the liquid level to less than 100 mm without a problem. We even got accetable results down to 50 mm but the repeat back wall echoes started to interfere with the readings.
Now my prospective customer let it slip that he was having the same problems you are having. They were using a 0.5 MHz probe and could not get acceptable readings below about 500mm. Well it seems that they had purchased a fish finder from the local sports store with a 500KHz transducer for about 1/10 the price of the equipment that I was using. You will find that a fish finder or depth sounder will have difficulty resolving signals/echoes so close to the transducer (dead zone).

You have not mentioned the type of equipment you are using and the method of test. Please let us know your procedure and we will try and assist you in solving your problem.

Regards

Neil Burleigh




    
 
 

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