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Teletest
Teletest Focus+ was the first system to use long-range guided wave ultrasonic testing to detect corrosion in pipelines.
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Technical Discussions
Jaeseok Park
R & D,
DOOSAN, South Korea, Joined Jun 2009, 10

Jaeseok Park

R & D,
DOOSAN,
South Korea,
Joined Jun 2009
10
08:49 Aug-10-2018
UT SN ratio definition

Dear UT professionals,

(1) Does anybody knows the origin of equation in the middle of linked page below?
(2) Is that the definition of "UT S/N ratio", generally accepted by industry?
(3) please reommend any text which explains physics behind the equation or similar.

http://www.nde-ed.org/EducationResources/CommunityCollege/Ultrasonics/Physics/signaltonoise.htm

I thought the definition SNR is later part of the equation; echo amp devided by noise amp; RMS or peak. However, according to the equation, SNR shall be larger even though the ratio of echo to noise amp is identical, if the material density is lower. Is it true? I really don't understand the sqrt part of the equation.

Thank you.

 
 Reply 
 
Rick Lopez
R & D,
John Deere - Moline Technology Innovation Center, USA, Joined Jul 2011, 190

Rick Lopez

R & D,
John Deere - Moline Technology Innovation Center,
USA,
Joined Jul 2011
190
18:06 Aug-10-2018
Re: UT SN ratio definition
In Reply to Jaeseok Park at 08:49 Aug-10-2018 (Opening).

If I were to guess, I would look to published papers written by the now-retired Frank Margetan. He collaborated with the late R. Bruce Thompson for many years as they explored signal-to-noise ratio, typically using immersion UT on aerospace materials. However, I'm not sure which paper (assuming that I'm on the right trail) that particular equation came from.

examples:
https://aip.scitation.org/doi/10.1063/1.4940530
https://lib.dr.iastate.edu/cgi/viewcontent.cgi?referer=https://www.google.com/&httpsredir=1&article=4078&context=qnde

 
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John Norman
Consultant, owner of business
NTS Ultrasonics Pty Ltd, Australia, Joined Oct 2012, 116

John Norman

Consultant, owner of business
NTS Ultrasonics Pty Ltd,
Australia,
Joined Oct 2012
116
02:49 Aug-13-2018
Re: UT SN ratio definition
In Reply to Jaeseok Park at 08:49 Aug-10-2018 (Opening).

Hi Jaeseok.

I think the formula in the nde-ed page is referring to a different signal to noise ratio from signal amplitude divided by noise amplitude. In the general case the noise amplitude would be noise from all sources including electronic noise. The "electronic" signal to noise ratio is a special case not specific to defect amplitudes, and the noise is electronically generated: thermal noise, flicker noise and other types of electronic noise. In flaw detector specifications, the signal to noise ratio is often given. This is the electronic signal to noise ratio and may be given referenced to the input of the amplifier circuit or to the output, and sometime given as a "noise figure" rather than a signal to noise ratio. This is all esoteric electronic stuff, but from an NDT point of view, lower noise numbers (i.e. higher signal to noise ratio) are generally better.

The formula in the nde-ed page refers to the signal to noise ratio as the ratio of the defect echo amplitude to the amplitude of background scatterers. Nothing to do with electronic noise or noise from other sources. The background scatterers would be the metallurgical grains in a metal, for example. In this case the signal to noise ratio depends on physical factors as seen in the formula where the S/N ratio depends on the ultrasonic speed in the metal, the density, the beam dimensions, etc. Note also that this defect S/N ratio is frequency dependent. Once again, lower noise numbers are generally better.

While the technician cannot do much about the electronic noise, except change the bandwidth filters of the flaw detector if it has any, the technician may be able to modify the inspection to improve the defect signal to noise ratio, and the formula on the nde-ed page gives hints on how to do this. For example smaller beam dimensions (i.e. focussing) increase the S/N. So does a shorter pulse. Also, a different ultrasonic frequency might help.

The Krautkramer book "Ultrasonic Testing of Materials" and the ASNT book "Non-Destructive Testing Handbook: Ultrasonic Testing" should both have information on noise ratios and scattering, etc.

Hope that helps.

John Norman

 
 Reply 
 

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