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- since 1996 -
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Technical Discussions
G.R
G.R
14:15 Jan-14-2020
DAC with notches

Can someone clarify the use of notches for building a DAC?
The reference blocks we use are the 19mm & 38mm ASME blocks but it isn’t big enough to fit a half, full skip and a skip and a half?
I’d rather use the 1.5mm SDH’s but then spec asks for notches.

 
 Reply 
 
Paul Holloway
Consultant,
Holloway NDT & Engineering Inc , Canada, Joined Apr 2010, 218

Paul Holloway

Consultant,
Holloway NDT & Engineering Inc ,
Canada,
Joined Apr 2010
218
00:02 Jan-15-2020
Re: DAC with notches
In Reply to G.R at 14:15 Jan-14-2020 (Opening).

If your procedure calls for notches, then it sounds like you're inspecting piping. The ASME code allows for both notches and side-drilled holes (alternate block) with piping.

A flat block like the ASME basic 19mm or 38mm can be used only on diameters greater than 20 inches. And if it's piping, then the pipe has to be within 25% of the calibration block thickness.

Don't use the 2% notches on the basic ASME block, they serve no purpose. They were originally put there just as little markers to help folks identify the OD and ID positions on an analog screen. But of course, that would only be helpful if the part you were inspecting was exactly the thickness of the calibration block, which is probably wasn't, so again... they serve no purpose.

1
 
 Reply 
 
G.R
G.R
01:05 Jan-15-2020
Re: DAC with notches
In Reply to Paul Holloway at 00:02 Jan-15-2020 .

Thanks for that Paul,

I’m just getting started in the ut side and when I read the spec saying a DAC using notches, I knew everyone at work had always been using the SDH’s so wanted to get some more info on it. Thanks again.

 
 Reply 
 
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
01:56 Jan-15-2020
Re: DAC with notches
In Reply to G.R at 14:15 Jan-14-2020 (Opening).

Hi. I am not familiar with the ASME codes, my background is rail flaw detection. However, I have done a lot of work on side drilled holes vs notches for calibration blocks and setting up DAC curves. The general rule is the the variation of echo amplitude with distance is different for side drilled holes as reference reflectors and flat bottomed notches as reference reflectors. They are not interchangeable. So, if the code calls for notches, use notches.
The theory for this can be explained by the Ermolov Equations (with limitations), and several references are available on NDT Net.
John Norman

 
 Reply 
 
G.R
NDT Inspector,
United Kingdom, Joined Jan 2020, 3

G.R

NDT Inspector,
United Kingdom,
Joined Jan 2020
3
10:26 Jan-15-2020
Re: DAC with notches
In Reply to John Norman at 01:56 Jan-15-2020 .

Thanks for the input John.
I agree with following the code, the problem is the ref. blocks aren’t big enough to fit in 3 points to make the DAC with notches, and it’s one of those situations where it’s been done a certain way for 20 odd years and no one has questioned it, I know for a fact if I continue doing it that way it’ll come back on me and I’ll get the blame for not following the spec!

 
 Reply 
 

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