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Technical Discussions
Don W
Don W
06:51 Dec-01-2004
Optimum probe frequency and sampling rates

I wish to measure a water gap to an accuracy of +/- 20um using a spherically focussed UT immersion probe. What probe frequency, probe characteristics (broad-band/highly dampled etc) and A/D sampling rates are required to achieve this?
Is there an ASME of EN standard that would apply ?


 
 Reply 
 
Pim van Andel
Pim van Andel
02:49 Dec-01-2004
Re: Optimum probe frequency and sampling rates
----------- Start Original Message -----------
: I wish to measure a water gap to an accuracy of +/- 20um using a spherically focussed UT immersion probe. What probe frequency, probe characteristics (broad-band/highly dampled etc) and A/D sampling rates are required to achieve this?
: Is there an ASME of EN standard that would apply ?
------------ End Original Message ------------

Recently I made a system with a 1um resolution for one of our customers. I used a not focussed highly damped 15MHz transducer and a 8bit, 100MSample/sec A/D on a low cost ultrasonic PC instrument (UltraZ). I used FFT resampling to 1GSample/sec and used cross correlation with a 1nsec resolution to find the time delay. Beam divergence and unexpected delays make the relation between time of flight and gap width less trivial. And I had to use various compensations for temperature effects (UltraZ can also measure temperature). Apparently your problem is at least 20 times less complex ;)

Regards,
Pim van Andel


 
 Reply 
 
Ed Ginzel
R & D, -
Materials Research Institute, Canada, Joined Nov 1998, 1286

Ed Ginzel

R & D, -
Materials Research Institute,
Canada,
Joined Nov 1998
1286
04:28 Dec-05-2004
Re: Optimum probe frequency and sampling rates
Don:
I do not think that ASME and ASTM have developed considerations specific to digital applications for thickness determination. I am not so familiar with EN Standards, but in 1989 IIW issued a handy little booklet , "Automated Ultrasonic Inspection of Welds", ISBN 0 903132 15 X

They considered the minimum "recommended" digitisation for an AUT system to be about 5 times the nominal probe frequency. But that was to ensure that amplitude was within 1dB of true (analogue) peak.

Your application has water as the medium to be "timed". At 1500m/s and a 20ns sample interval (50MHz digitisation), the 20ns would represent 15 microns. That is close to your requirement.

If it is only the first arrival over a threshold I suppose any probe frequency would suffice. I would think you could improve results with a broadband probe of a higher (10-20MHz) frequency. I suspect best results would be had if the distance measured was close to the near zone or focal distance if focused. If the system uses peak amplitude to indicate travel time of the signal then a higher digitisation would be better.

20microns is reasonably achievable electronically but a good mechanical positioning will be critical. there are about 100 microns in the thickness of a sheet of paper so you are looking for a very subtle change.

Regards
Ed

----------- Start Original Message -----------
: : I wish to measure a water gap to an accuracy of +/- 20um using a spherically focussed UT immersion probe. What probe frequency, probe characteristics (broad-band/highly dampled etc) and A/D sampling rates are required to achieve this?
: : Is there an ASME of EN standard that would apply ?
: Recently I made a system with a 1um resolution for one of our customers. I used a not focussed highly damped 15MHz transducer and a 8bit, 100MSample/sec A/D on a low cost ultrasonic PC instrument (UltraZ). I used FFT resampling to 1GSample/sec and used cross correlation with a 1nsec resolution to find the time delay. Beam divergence and unexpected delays make the relation between time of flight and gap width less trivial. And I had to use various compensations for temperature effects (UltraZ can also measure temperature). Apparently your problem is at least 20 times less complex ;)
: Regards,
: Pim van Andel
------------ End Original Message ------------




 
 Reply 
 
justin timberlake
justin timberlake
02:33 Jan-19-2005
justin timberlake

----------- Start Original Message -----------
: Don:
: I do not think that ASME and ASTM have developed considerations specific to digital applications for thickness determination. I am not so familiar with EN Standards, but in 1989 IIW issued a handy little booklet , "Automated Ultrasonic Inspection of Welds", ISBN 0 903132 15 X
: They considered the minimum "recommended" digitisation for an AUT system to be about 5 times the nominal probe frequency. But that was to ensure that amplitude was within 1dB of true (analogue) peak.
: Your application has water as the medium to be "timed". At 1500m/s and a 20ns sample interval (50MHz digitisation), the 20ns would represent 15 microns. That is close to your requirement.
: If it is only the first arrival over a threshold I suppose any probe frequency would suffice. I would think you could improve results with a broadband probe of a higher (10-20MHz) frequency. I suspect best results would be had if the distance measured was close to the near zone or focal distance if focused. If the system uses peak amplitude to indicate travel time of the signal then a higher digitisation would be better.
: 20microns is reasonably achievable electronically but a good mechanical positioning will be critical. there are about 100 microns in the thickness of a sheet of paper so you are looking for a very subtle change.
: Regards
: Ed
: : : I wish to measure a water gap to an accuracy of /- 20um using a spherically focussed UT immersion probe. What probe frequency, probe characteristics (broad-band/highly dampled etc) and A/D sampling rates are required to achieve this?
: : : Is there an ASME of EN standard that would apply ?
: : Recently I made a system with a 1um resolution for one of our customers. I used a not focussed highly damped 15MHz transducer and a 8bit, 100MSample/sec A/D on a low cost ultrasonic PC instrument (UltraZ). I used FFT resampling to 1GSample/sec and used cross correlation with a 1nsec resolution to find the time delay. Beam divergence and unexpected delays make the relation between time of flight and gap width less trivial. And I had to use various compensations for temperature effects (UltraZ can also measure temperature). Apparently your problem is at least 20 times less complex ;)
: : Regards,
: : Pim van Andel
------------ End Original Message ------------

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