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1076 views
09:01 Jun-06-2005

Arianna Macchi

Consultant
Italy,
Joined Apr 2005
6
Couplant for porous stones

Dear colleagues

I'm trying to perform simple UT attenuation measurements on porous stones with a 500 KHz contact transducer in pulse-echo configuration. I'm using vaseline as couplant but I've noticed that when the couplant begins to melt (i.e. becomes more fluid) the sensitivity decrease significantly. By suppyling a new amount of fresh (more solid) substance, things get better, but this happens every 10-15 seconds.

Can you suggest me some possible alternative to vaseline??


 
03:13 Jun-06-2005

John Brunk

Engineering, NDT Level III
Self employed, part-time,
USA,
Joined Oct 1999
158
Re: Couplant for porous stones ----------- Start Original Message -----------
: Dear colleagues
: I'm trying to perform simple UT attenuation measurements on porous stones with a 500 KHz contact transducer in pulse-echo configuration. I'm using vaseline as couplant but I've noticed that when the couplant begins to melt (i.e. becomes more fluid) the sensitivity decrease significantly. By suppyling a new amount of fresh (more solid) substance, things get better, but this happens every 10-15 seconds.
: Can you suggest me some possible alternative to vaseline??
------------ End Original Message ------------

Your data will be altered by couplant entering pores in the samples. You should use self-coupling (dry coupling)transducers. I have used these successfully to examine a variety of porous materials. One source for these is Ultran. If you can't obtain new transducers you might be able to use sheet dental adhesive as a coupling layer. Attenuation will then vary with pressure but maybe you could measure the difference between the first and second back reflections. You should try to get the vaseline out of your samples and re-test them.


 
03:16 Jun-06-2005

Tom Nelligan

Engineering,
retired,
USA,
Joined Nov 1998
390
Re: Couplant for porous stones Several companies (including mine) offer very high viscosity ultrasonic couplants for use with normal incidence shear wave transducers. That sort of medium also works well for coupling longitudinal wave transducers on porous materials, because the very high viscosity limits absorption. I'd suggest that you try a commercial shear wave couplant.

Tom Nelligan
Senior Applications Engineer
Panametrics-NDT, a business of R/D Tech
www.panametrics-ndt.com


----------- Start Original Message -----------
: Dear colleagues
: I'm trying to perform simple UT attenuation measurements on porous stones with a 500 KHz contact transducer in pulse-echo configuration. I'm using vaseline as couplant but I've noticed that when the couplant begins to melt (i.e. becomes more fluid) the sensitivity decrease significantly. By suppyling a new amount of fresh (more solid) substance, things get better, but this happens every 10-15 seconds.
: Can you suggest me some possible alternative to vaseline??
------------ End Original Message ------------




 
09:16 Jun-07-2005

Udo Schlengermann

Consultant, -
Standards Consulting,
Germany,
Joined Nov 1998
174
Re: Couplant for porous stones Reply

Hello,
coming back to the original task: Determination of sound attenuation:
On porous stones, I suppose, it can only be measured for longitudinal waves, and only using the through-transmission technique, with separate transmitter and receiver probes on opposite sides of the sample.
And this can only be a relative measurement compared to a reference sample, using always the same equipment and same set-up.
Losses by changing coupling conditions and by changing divergence losses of the sound beams from sample to sample are much bigger than the losses by attenuation (scattering) in the objects itself.
As mentioned in the replies from collegues before, the change of the elastic properties of couplant during measurement will change the sound transfer conditions (decreasing transmissivity at the boundaries. But filling the pores of your object with liquid will improve transmission in the object. So depending on the measurement conditions (time) the calculated attenuation coefficient will bea random figure.
On porous materials a valid relative attenuation coefficient can only be achieved by dry coupling conditions i.e. using dry coupling probes in a rig (probes aligned controlled pressure) or very thin foils between probes and sample to avoid pentration of a fluid couplant.

Kind regards
Udo Schlengermann
Applications Laboratory
GE Inspection Technologies GmbH
Huerth, Germany


----------- Start Original Message -----------
: Dear colleagues
: I'm trying to perform simple UT attenuation measurements on porous stones with a 500 KHz contact transducer in pulse-echo configuration. I'm using vaseline as couplant but I've noticed that when the couplant begins to melt (i.e. becomes more fluid) the sensitivity decrease significantly. By suppyling a new amount of fresh (more solid) substance, things get better, but this happens every 10-15 seconds.
: Can you suggest me some possible alternative to vaseline??
------------ End Original Message ------------




 


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