Volume 2, Number 1 January 2003
TNT Tips:
Eliminating Retakes in Radiographic Exposures
Calculators and charts help determine radiographic exposure times but many variables affect the result and can cause retakes. This technique can reduce time and cost of establishing techniques and retakes by eliminating calculation errors and unanticipated results caused by variables such as incorrect thickness, material types, or change in radiation sources.Many radiographers take trial exposures (technique shots) and develop the film to ensure calculations are correct. This practice can be eliminated with a self-fading pocket dosimeter. You must know exposure requirements in milliroentgens for the target density of selected film(s). Exposure requirements can be determined from the film manufacturer or by using a known satisfactory exposure technique to determine required roentgens. A 130 µC/kg (500 mR) dosimeter works well for this practice.
Place dosimeter behind film cassette during setup. Expose film for a portion of the calculated exposure time that is within range of dosimeter (such as 25 percent of calculated exposure time). Check dosimeter reading to verify correct calculation or make an adjustment to exposure time.
Example: Exposure time is calculated to be 2 min. and the required exposure for a 2.5 density is 460 µC/kg (1,800 mR). Place dosimeter behind film cassette and expose film for 20 s (one-sixth total exposure). Check dosimeter reading. If reading is close to 80 µC/kg (300 mR), continue exposure for an additional 100 s to complete 2 min. If dosimeter reading is about 50 µC/kg (200 mR), then you need nine times more exposure (9 x 20 s = 180 s = 3 min.). This establishes an acceptable exposure time for that technique.
Bill Plumstead
Greenville, South Carolina
Avoiding Signal Confusion in UT of Welds
Here's help for technicians practicing contact ultrasonic testing on weldments where the item isn't a standard joint. Many weldments use backing strips, unequal material thicknesses, high weld crowns, "T" joint configurations, skewed joints, etc. These conditions can lead to confusion interpreting the signals produced by them. To isolate or eliminate some of these signals:
- Calculate the sound path and skip distance to the area where geometric changes can occur.
- Perform a scan of the joint taking note of changes and where they occur on the screen (mark screen with grease pencil).
- Compare actual path against calculated path to determine accuracy of locations identified as trouble spots (forms from UT equipment suppliers for laying out weld and sound paths help identify geometry problems).
- Snap a chalk line or, if possible, attach a set of straight edges using magnets or clamps if ends of the material are accessible, immediately adjacent to the problem area. This allows scanning the applicable portion of the weld area without continuously trying to interpret irrelevant geometric configuration signals. This will speed up the inspection and eliminate a lot of headaches.
- Finally, realign straight edges up to area of interest and scan the location by the geometric anomaly for indications that may be immediately adjacent to weld joint.
Stuart Kleven
Alloyweld Inspection Company, Inc.
Bensenville, Illinois
[ The NDT Technician ]
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