Advantages and Limitations

T.O. 33B-1-1 3-61 primarily, by which powder will give the best contrast and visibility on the parts being inspected and the degree of sensitivity desired. 3.5.5.2 Advantages and Limitations. The dry powder method has good and bad features. The advantages and disadvantages, which may influence its use for a specific application, are summarized in the following list. a. Excellent for locating defects wholly below the surface and deeper than a few thousandths of an inch. b. Easy to use for large objects with portable equipment. c. Easy to use for field inspection with portable equipment. d. Good mobility when used with AC or half-wave (HW). e. Not as messy as the wet method. f. Equipment may be less expensive. g. Not as sensitive as the wet method for very fine and shallow cracks. h. Not easy to cover all surfaces properly, especially of irregularly shaped or large parts. i. Slower than the wet method for large numbers of small parts. j. Not readily usable for the short, timed shot technique of the continuous method. k. Difficult to adapt to a mechanized test system. 3.5.5.3 Powder Selection by Visibility and Contrast. Selection of the color of particles to use is essentially a matter of obtaining the best possible contrast with the background of the surface of the part being inspected. The differences in visibility among the black, gray, yellow, and red particles are considerable on backgrounds which may be dark or bright, and which may be viewed in various kinds of light. If difficulty is experienced in seeing indications, the inspector should try a different colored powder. Available colors for powders for the dry method are: a. Gray Powder is a general-purpose high contrast powder and by far the most widely used of the dry powders. It is effective on dark surfaces, whether black, gray or rust colored. b. Black Powder is especially designed for use on light colored surfaces. It is dust-free as well as the most sensitive of the dry powders. Its higher sensitivity is because it contains the highest proportion of magnetic material of all the dry powders. c. Red Powder is a dark reddish powder used on light colored surfaces as is the black powder. However, since the black powder on a silvery or polished surface is sometimes hard to see, the red color may offer a better contrast, particularly under incandescent lighting where the red color stands out. d. Yellow Powder is pale yellow powder featuring fair sensitivity and good contrast on dark colored surfaces. 3.5.5.4 Surface Preparation. In general, the smoother the surface of the part and the more uniform its color, the more favorable are the conditions for the formation and the observation of indications. This statement applies particularly to inspections being made on horizontal surfaces. For sloping and vertical surfaces, the dry powder may not be held on a very smooth surface by a