H. Monty O'Connor
Eddy Current Technology Incorporated
201A Horace Avenue
Virginia Beach, VA 23462
USA
Telephone: 1 757 490 1814
Fax: 1 757 490 2778
Web Site: www.eddy-current.com
E-Mail:monty@eddy-current.com
All you do is pull a probe through a tube and you get a unique signal from every defect.
The accompanying figure shows the signals from five defects in an ASME Calibration Tube: a through wall hole, 80 per cent, 60 per cent, 40 per cent, and 20 per cent O.D. pits, and a dent.
In the impedance plane portion on the left of this diagram, the dent is the signal that is almost totally horizontal.
In the strip chart representation to the right, the strip chart on the left is the horizontal and on the right is the vertical.
As the probe is pulled through the tube, it first passes the dent. The resulting signal is seen primarily in the horizontal channel. The next defects that are passed in sequence are the hole, at about 45 degrees in the impedance plane, and then increasing in the clock wise direction, the 80, 60, 40, and 20 per cent pits.
The depth of the defect can be determined by the angle of the signal. Also, the total volume of the defect can be determined approximately by the amplitude of the signal.
It's easy!
A few controls must be set correctly in order to result in the signals looking like this.
The phase control is used to rotate the entire group of signals. The phase control is used to set the dent signal to be horizontal. (It is the author's preference and recommendation to set the dent signal horizontal, but other schemes will work.)
The gain control is used to adjust the amplitude of the signals. A good starting point is to adjust the through wall hole to be approximately half the size of the screen.
Actually, the most critical setting is the selection of an appropriate inspection frequency, and conveniently, this information is available on the World Wide Web at http://www.eddy-current.com/contable.htm
If your specific tube alloy or wall thickness is not in the table, then a formula to calculate the frequency is available at
http://www.eddy-current.com/conform.htm
and a list of electrical conductivities is available at
http://www.eddy-current.com/condmat.htm
Look for the next in this series of articles entitled"Eddy Current Testing of Heat Exchanger Tubes is Not Quite That Easy".
Eddy Current Technology Incorporated
201A Horace Ave., Virginia Beach, Virginia, USA 23462
Phone 1 757 490 1814 Facsimile 1 757 490 2778
email: monty@eddy-current.com