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Technical Discussions
Harald germ
, Joined ,

Harald germ

,
Joined

00:03 Aug-21-2002
Calibration for DAC (UT)

I would like to use the standardized DAC-diagrams for UT-inspection of welds.
Therefore, for the sensitivity calibration I need to establish the dB value of the back wall. For angle beam probes, it is recommended to use the radii of calibration blocks.
My intention is to perform the sensitivity calibration at the test piece (sheet metal with parallel surfaces)by performing a V-or W- transmission similar to the first part of the transfer correction procedure.
Here is my question:
I am not sure if I can use the obtained signal for a back wall signal of the corresponding sound path (i.e. I set the dB to 80% FSH and add the neccesdary dB-values defined in the standard DAC-diagram to get the neccesary search sensitivity).
If not, is there a possibilty to use correction factors similar to the correction factors for the radii on the calibration blocks. If so, what are the values of these factors.



    
 
 
Udo Schlengrmann
Consultant, -
Standards Consulting, Germany, Joined Nov 1998, 176

Udo Schlengrmann

Consultant, -
Standards Consulting,
Germany,
Joined Nov 1998
176
04:37 Aug-21-2002
Re: Calibration for DAC (UT)
Reply by Udo Schlengermann:
You obviously mean DGS-method, resp. DGS-diagram and not DAC-curves.
If you talk about V- or W-path transmission I guess you mean through transmission using two separate angle-beam probes of the same type, one transmitting and one receiving.
This through transmission in most cases does not give a signal equal to a backwall echo, because reflection at the surfaces of the plate is not total (especially for beam angles away from 45°) and because a plate is not a solid semi-infinite body compared to the beam size. Additional reflections cause mode conversion, i.e. transfer of energy to other wave modes than to transverse waves used for evalution.

For weld testing with the DGS-method you have to use a single probe and and a reference echo from a known reflector. It has not necessarily to be a backwall, it can be a disk-shaped reflector of known size.
There are recommendations for the minimum sizes of reference blocks and reflector positions.
European Standard EN 583-2(UT - Sensitivity and range setting) gives guidance on this matter.
The V- and W-path calibration is not recommended in standards.

Kind regards
Udo Schlengermann
Agfa NDT GmbH
+++
: I would like to use the standardized DAC-diagrams for UT-inspection of welds.
: Therefore, for the sensitivity calibration I need to establish the dB value of the back wall. For angle beam probes, it is recommended to use the radii of calibration blocks.
: My intention is to perform the sensitivity calibration at the test piece (sheet metal with parallel surfaces)by performing a V-or W- transmission similar to the first part of the transfer correction procedure.
: Here is my question:
: I am not sure if I can use the obtained signal for a back wall signal of the corresponding sound path (i.e. I set the dB to 80% FSH and add the neccesdary dB-values defined in the standard DAC-diagram to get the neccesary search sensitivity).
: If not, is there a possibilty to use correction factors similar to the correction factors for the radii on the calibration blocks. If so, what are the values of these factors.
.



    
 
 
Harald Germ
, Joined ,

Harald Germ

,
Joined

05:35 Aug-21-2002
Re: Calibration for DAC (UT)
Reply by Harald Germ
Thank you for your reply.
Your are right, I meant the DGS-method. The reason why I want to use the method I discribed below for setting the sensitivity is that with the standard method with reference blocks, you have to make a transfer correction get reasonable results. Additionally, in my understanding, the calibration with a reference block such as K1 or K2 has also some range of error.
Therefore, I thought that if I use the arrangement for the transfer correction, I would have taken into account the couplinmg losses and will not run into problems with different accoustic properties between the reference block and the test piece. So, although the signal I use for calibration is not ideally from a backwall, I think that the error might be in the same range as for a calibration with a reference block.
Could you please give a short comment if I am wrong with this assumption.

Thank you in advance!

: Reply by Udo Schlengermann:
: You obviously mean DGS-method, resp. DGS-diagram and not DAC-curves.
: If you talk about V- or W-path transmission I guess you mean through transmission using two separate angle-beam probes of the same type, one transmitting and one receiving.
: This through transmission in most cases does not give a signal equal to a backwall echo, because reflection at the surfaces of the plate is not total (especially for beam angles away from 45°) and because a plate is not a solid semi-infinite body compared to the beam size. Additional reflections cause mode conversion, i.e. transfer of energy to other wave modes than to transverse waves used for evalution.
.
: For weld testing with the DGS-method you have to use a single probe and and a reference echo from a known reflector. It has not necessarily to be a backwall, it can be a disk-shaped reflector of known size.
: There are recommendations for the minimum sizes of reference blocks and reflector positions.
: European Standard EN 583-2 (UT - Sensitivity and range setting) gives guidance on this matter.
: The V- and W-path calibration is not recommended in standards.
.
: Kind regards
: Udo Schlengermann
: Agfa NDT GmbH
: +++
: : I would like to use the standardized DAC-diagrams for UT-inspection of welds.
: : Therefore, for the sensitivity calibration I need to establish the dB value of the back wall. For angle beam probes, it is recommended to use the radii of calibration blocks.
: : My intention is to perform the sensitivity calibration at the test piece (sheet metal with parallel surfaces)by performing a V-or W- transmission similar to the first part of the transfer correction procedure.
: : Here is my question:
: : I am not sure if I can use the obtained signal for a back wall signal of the corresponding sound path (i.e. I set the dB to 80% FSH and add the neccesdary dB-values defined in the standard DAC-diagram to get the neccesary search sensitivity).
: : If not, is there a possibilty to use correction factors similar to the correction factors for the radii on the calibration blocks. If so, what are the values of these factors.
: .
.



    
 
 

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