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Technical Discussions
Michael Trinidad
Consultant,
LMATS Pty Ltd , Australia, Joined Jan 2003, 138

Michael Trinidad

Consultant,
LMATS Pty Ltd ,
Australia,
Joined Jan 2003
138
03:16 May-09-1998
Ultrasonic coating thickness measurement

We have an engineer doing research for us and is developing some new equipment. A brilliant electronics man however the practical UT side I give him what I want the equipment to do. He has manufactured a prototype ut set from scratch for further research now he has been told that we want the instrument to disregard coating thicknesses.

My understanding of this aspect of ut is that sets capable of doing this gate the multiple back wall echoes and then average out the results. My question is am I basically correct?

Kindest Regards

Michael Trinidad
International Refinery Services Thailand


 
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Rolf Diederichs
Director,
NDT.net, Germany, Joined Nov 1998, 612

Rolf Diederichs

Director,
NDT.net,
Germany,
Joined Nov 1998
612
09:13 May-11-1998
Re: Ultrasonic coating thickness measurement

: We have an engineer doing research for us and is developing some new equipment. A brilliant electronics man however the practical UT side I give him what I want the equipment to do. He has manufactured a prototype ut set from scratch for further research now he has been told that we want the instrument to disregard coating thicknesses.

: My understanding of this aspect of ut is that sets capable of doing this gate the multiple back wall echoes and then average out the results. My question is am I basically correct?

: Kindest Regards

: Michael Trinidad
: International Refinery Services Thailand

Michael is basically correct.
The evaluation of multiple backwall echoes is commonly used to measure only the
parent material thickness while ignoring the coating thickness.

Many commercial thickness gauges can measure the time interval between the
first (or later) backwall echo (n), thus measuring only the material transit
time. However be aware that this option is mostly provided for other reasons -
the multiple echo measurement is applied for increasing measurement accuracy.

It might be possible to apply those gauges for coated components also.
But there are a lot of problems which can cause error readings (see figure).
For that reason an instrument with A-scan display is recommended to find proper
adjustments for the following situations:

1. Absolute thickness of coating and parent material.
2. Ratio of coating and parent material thickness.
3. Acoustic impedance of the materials.
4. Material attenuation.
5. Interferences caused by multiple reflections of the coating path.

The adjustment of a correct gate position is mainly a problem if the gauge is
applied in a wide range of applications. Adjustment of thresholds, trigger
phase are functions which are needed to consider the specific acoustic situation.

You may find commercial gauges without an A-scan display for this application.
For a limited range of components (applications, geometries) a flexible
instrument adjustment may not be needed.
However, in generally I would prefer the use of an A-scan display for adjustment
an verification of the measurement.

Rolf Diederichs

BTW: This topic was posted also on the newsgroup.
So far it received answers by Joe Buckley (jmb34@student.open.ac.uk)
Doug Breeze (isl@inspection.co.uk).
Click on 'Show posts' at http://www.ndt.net/newsweb/newsweb.htm



 
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