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Posted on mar set 23, 2008 05:47
Post Subject: comparing ECA with simple coil
Dear Sir,
To scan large area of surface by the eddy current testing there are twoo possibilities: either to use simple probe and XY scanner or ECA probe and one axis scanner. Theoretically it is not the same. When using ECA probe, the coils are multiplexed and at a time only one pair of them is active. With some simplification we can look at the inactive coils like an additional conductive material placed on the tested surface near the active coil. It means that the wires of inactive probes can have some effect on eddy furrent field distribution in tested material. The question is to which extent. Is the influence of inactive coils on the signal negligible? Will be the resolution, sensitivity and test results the same regardless of what type of probe was used (simple coil or ECA) ? When not should we prefer using simple probes in cases where high resolution is required?
In case of tube testing the situation is very similar. We have a choice : rotational probe or X probe. X probe is faster but can it be regarded as an equivalent to rotational probe?
Posted on gio set 25, 2008 07:29
Post Subject: RE: comparing ECA with simple coil
Milos,
Thank you for your comments. This is something that may deserve some additional study and verification as your thoughts about the unused ECA coils affecting the active firing coils is in theory possible, however, as you rightly point out the real question is how much effect they will actually have. While there may be some effect form the unused coils as you suggest, this highlights the importance of using a representative calibration standard for ensuring the correct calibration and sensitivity is used for any ECA inspection. In most cases, ONDT has shown that the beneifits of using multiple coils and imaging technology far outweigh any issue such as the effect of unused coils in the probe. We have also done considerable work using high frequency arrays to detect small defects where resolution is paramount, all of this work has shown excellent results.
I hope that this may begin to answer your questions.