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Technical Discussions
Carlos Correia
R & D, - -
UCV & EGROUP, Venezuela, Joined Oct 2008, 120

Carlos Correia

R & D, - -
UCV & EGROUP,
Venezuela,
Joined Oct 2008
120
17:14 Feb-11-2012
combined POD

Hello, regard POD:

Assuming the following expression is valid for two differents NDT methods:
PODcombined = 1 - (1 - POD1) x (1 - POD2);

Could be the same expression valid for two differents techniques? (eg TOFD and UTPA)
Thanks

Oliviero
NDT Inspector,
Quality Control srl, Italy, Joined Oct 2008, 414

Oliviero

NDT Inspector,
Quality Control srl,
Italy,
Joined Oct 2008
414
21:24 Feb-11-2012
Re: combined POD
In Reply to Carlos Correia at 17:14 Feb-11-2012 (Opening).

I think yes

Frank Lund
R & D,
United Kingdom, Joined Apr 2005, 222

Frank Lund

R & D,
United Kingdom,
Joined Apr 2005
222
15:08 Feb-12-2012
Re: combined POD
In Reply to Carlos Correia at 17:14 Feb-11-2012 (Opening).

You can visualise the combination of two independent PODs if you draw a simple square map with a horizontal division representing the POD of Method A (e.g. 95%) and a vertical division representing the POD of Method B (e.g. 90%)

Thus you have:-

An area 95% by 90% = 85.5% representing anomally indicated by both methods
An area 5% by 10% = 0.5% representing anomally missed by both method
An area 95% by 10% = 9.5% representing anomally detected by Method A but nissed by Method B
An area 5% by 90% = 4.5% representing anomally missed by Method A but detected by Method B

This gives you the total 100% and the detail for all four combinations of detection and missing for the two methods.

However, you need to be sure that the PODs that you are combining are truely independent, not just variants on the same basis (e.g. AC mag versus permanent magnet, daylight versus UV mag)

David Forsyth
R & D
TRI/Austin, USA, Joined Nov 2001, 41

David Forsyth

R & D
TRI/Austin,
USA,
Joined Nov 2001
41
17:55 Feb-15-2012
Re: combined POD
In Reply to Carlos Correia at 17:14 Feb-11-2012 (Opening).

The ability to combine probabilities in this way ONLY applies to random processes, like flipping a coin. An NDT process is not random, in fact it is a combination of deteministic processes such as ultrasonic signal propagation with random processes such as instrument noise, probe-part positioning, coupling, etc.

So basically this idea does not work, you will OVER estimate your final POD. See for reference:
Kimberly Erland
Quantifying the benefit of redundant fluorescent penetrant inspection
in Review of Progress in Quantitative NDE, Vol 8B, 1989
pp 2221 - 2228

also

David S Forsyth, Abbas Fahr
On the Independence of Multiple Inspections and the Resulting POD
in the Review of Progress in QNDE, Vol. 19B, 2000 pp 2159-2166.

Regards, Dave.

Carlos Correia
R & D, - -
UCV & EGROUP, Venezuela, Joined Oct 2008, 120

Carlos Correia

R & D, - -
UCV & EGROUP,
Venezuela,
Joined Oct 2008
120
00:39 Feb-16-2012
Re: combined POD
In Reply to David Forsyth at 17:55 Feb-15-2012 .

Thank you very much.
I will search both works to carefully reading.

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