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How Did This Come Up?
by William E. Mooz*
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I am glad that there are people out in our NDT world who like to make sure that things work the way they should. Communication is one area that always needs improving, it seems. When I write something, the meaning is clear as a bell - to me. When someone reads what I wrote, the interpretation may be very different. Here is a clarification of penetrant standard use that I think is important. It is better written than I could ever do. Hope it works for you.
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Introduction
Recently, there have been problems with the interpretation of specifications used by various companies regarding the use of PSM-5 panels. The problem seems to be twofold. Part of the problem concerns the use and purpose of the panel itself; the other part of the problem concerns the specifications that are written concerning the use of these panels.
THE PURPOSE OF PSM-5 PANELS
ASTM E 1417: Standard Practice for Liquid Penetrant Examination is the document which most facilities use (ASTM International, 1999). Paragraph 7.8.3 is titled "System Performance" and reads, in part, "The penetrant system shall be checked daily for performance. The check shall be made with known defect standards." Then, paragraph 7.8.3.1, titled "Known Defect Standards," reads, "The selection and procedures for the maintenance of known defect standards shall be approved by the contracting agency. The defects in the standard shall be capable of demonstrating unsatisfactory system performance."
The creators of the panel do not imply anywhere that the PSM-5 panel can be used to measure sensitivity.
This is pretty clear. The objective is to have a discontinuity standard that contains discontinuities which are of such size that if the penetrant testing system is not performing properly, the discontinuities will not be found or that it will be obvious that something is wrong with the system performance. This discontinuity standard, used for this purpose (whatever it may be), must be approved by the customer, as pointed out in paragraph 7.8.3.1 - which makes very good sense.
Enter the PSM-5 Panel
Pratt & Whitney, which deals with hundreds of contractors and subcontractors, decided years ago to develop a known discontinuity standard which they could require to be used by all those folks who did work for them. Uniformity was the objective and the result was the PSM-5 panel.While they developed the panel for use in work related to their company, it also became an easy tool for others to use and, as a result, is found in many penetrant testing installations today where no work for that particular company is done.
HOW IT IS USED
First, let's talk about the company which developed the panel. When a panel is originally received by them, it is tested with level four penetrant and the crack indications must meet certain minimum and maximum size requirements. If the panel passes this test, the panel is then serialized and calibrated to all of the sensitivity levels in use at the facility where the panel is to be used. In the calibration, the panel must show three indications for level one, four indications for levels two and three and five indications for level four. Following this, the panel is processed through the system in which it is to be used and it must detect no fewer indications than required in the calibration process. If the panel passes this test, it is retained as a production tool for use on that particular penetrant line.The creators of the panel do not imply anywhere that the PSM-5 panel can be used to measure sensitivity. However, the fact that they calibrate the panels with different sensitivity penetrants probably has mistakenly led some NDT personnel to believe that the panels are a tool which can measure sensitivity. This is not at all true.
Other companies use the panel differently. For example, Bombardier's specifications (1997) state that "The penetrant test shall reveal all 5 cracks of PSM-5 panel....." This requirement is made without reference to the sensitivity level of the penetrant in use. There are still other examples, where the specification states that the panel shall find one crack for level one, two for level two and so forth. Uniformity among specifications is notably absent and this must confuse auditors, making it very difficult for a testing laboratory that does work for several different companies, each of which has a different specification or procedure for the use of the panel.
To What End?
It is instructive to go back to ASTM E 1417. What is important is that the known discontinuity standard is capable of showing that the testing line either is or is not working as desired. It just has to be able to give an indication of how the system is performing.
Panel Calibration
Will a PSM-5 panel which is out of calibration work? The answer to this depends on whether it meets the important criteria of ASTM E 1417. This specification does not specify what kind of discontinuity standard must be used to demonstrate this. It simply says that it must be approved by the customer. There is no requirement for a specific type of discontinuity standard. The answer comes in two parts. The first part is that if you are working to the specifications set up by the inventing company, it will not be allowed. But will it work? If it can discriminate between a properly functioning system and one that is not functioning properly, it will do the job. An inspector simply tests a new panel and notes what indications are found on the panel and what they look like. Then the inspector uses this panel regularly, as specified. If the results differ from the initial test, one can suspect and investigate a possible problem. Does it make any difference that the cracks are not as specified on the PSM drawing? PSM-5 panels are each different. Our recommendations are always the same. The user processes a new panel through a system that is known to be operating correctly. The user then notes and records how many indications show on the panel. This is then used as a guide when testing the system periodically. Does it make a difference if the cracks on the panel are not as specified on the drawing? Only if the user is working for the original company does it make a difference; otherwise, probably not.
SPECIFICATION
Our advice to specification writers is to understand what is to be accomplished by the use of a known discontinuity standard, whatever kind is to be used. Then, understand that a testing facility may be doing work for several companies. Differing specifications about the use of a known discontinuity standard can only cause confusion and complicate audits. Try to understand this and to keep sight of the initial purpose - to be able to tell when things are not under control and the testing line is not performing as desired. There is little need to make a simple objective more complicated.
REFERENCES
ASTM International, ASTM E 1417: Standard Practice for Liquid Penetrant Examination, West Conshohocken, Pennsylvania, ASTM International, 1999.Bombardier, Inc., "Materials and Processes Specification," No. MPS 176-2, November 1997, p. 23.
* Met-L-Chek Company, 1639 Euclid St., Santa Monica, CA 90404; (310) 450-1111; fax (310) 452-4046; e-mail <info@met-l-chek.com>.
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