Volume 7, Number 4
October 2008
Q. I'm trying to find information regarding probability of defect detection using NDT methods in the aerospace industry. Has ASNT ever published an article about this subject? G.B., Cudahy, California. A. Probability of detection (POD) uses statistics to predict the reliability of a method in a specific application. The topic of POD appears in three NDT Handbook volumes.
- NDT Handbook, third edition: Vol. 2, Liquid Penetrant Testing. Ch. 9, p 275-285.
- NDT Handbook, third edition: Vol. 5, Electromagnetic Testing, p 260-261, 486-487.
- NDT Handbook, third edition: Vol. 7, Ultrasonic Testing, p 530-534.
Entries in the PT and UT volumes are relevant to aerospace. You may also find the “Complete Materials Evaluation Index” helpful. It lists every article published in Materials Evaluation from 1947 through 2007. A search for your topic turns up a number of articles at Materials Evaluation Complete Index.
Reader response to yoke dragging query (Vol. 7, No. 1). I was reading the “Q&A Inbox” on yokes in the 01/2008 issue. If a yoke operates on 60 Hz AC, the skin depth of field may cause it to be insufficient for demagnetizing a plate above a certain thickness. Further, though dragging yokes down oilfield pipe is a common practice, this action wears down the corners of the yoke legs and, in some cases, results in a yoke that cannot then lift the 10 lb test mass. RKS, Houston, Texas. Q. Can you direct me in how the Standards Development Committee (SDC) found/established equivalent eye examinations? Refer to Draft BSR/ASNT CP-106:200X. para. 7.2.3. M.D. Cheney, Kansas. A. Initially drafted in early 2004 and based on ISO 9712:1999, “CP-106: Nondestructive Testing Qualification and Certification of Personnel” had the term "or equivalent" in the near vision acuity section. Because other ASNT certification documents use the same term, ASNT’s Technical Services Department received numerous calls asking for clarification for the Jaeger number equivalents. As a result, the SDC, ASNT’s consensus committee for developing American National Standards, agreed that a note clarifying equivalents could be added. The following organizations were consulted in drafting the text.
- Snellen values and the Times Roman points: The American Optometric Association, the National Optometric Association, the Ohio State University Department of Optometry and several Columbus, Ohio optometrists.
- OrthoRater values: Reichert, Inc.
- Titmus values: Sperian
On points two and three above, both companies gave the values listed in “CP-106”. Regarding point one above, most responses referenced Clinical Refraction, by Irvin Borish. Dr. Timothy Wingert, Acting Director of the Clinical Care Group of the American Optometric Association, specified the third edition, Vol. 1, (1975): pp 391 and 418. Noting the book was now out of print, Dr. Wingert also stated the information was still valid and summarized the comparison with the following table:
It should be noted that most optometrists record values in even numbers, 20/20 is usually accepted for J-1 and 20/25 for J-2. However, a company should describe whatever convention they use in their written practice. If you research this subject yourself, you may notice that some international NDT certification documents such as ISO 9712, EN 473 and the Canadian CAN/CGSB 48.9712 all list near visual acuity requirement as Jaeger J-1, Times Roman N4.5 or equivalent. But, according to the table shown in Clinical Refraction, N4.5 is equivalent to Jaeger J-2, not J-1, so the SDC chose to keep Times Roman 3.5 as the equivalent listed in “CP-106.”Finally, when using a reading card, note that the distance at which the card is to be read changes with the font size. Using the card at other distances will lead to a false eye test. James W. Houf, Senior Manager, ASNT Technical Services Dept.
TNT
E-mail, fax or phone questions for the Inbox to the Editor:
hhumphries@asnt.org
(800) 222-2768 X 206
(614) 274-6899 fax
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