T.O. 33B-1-1be highly visible. In the oil-and-whiting days, it was found that used or dirty oil was much more visible thanclean machine oil. Present penetrants obtain visibility by having highly colored dyes dissolved in thepenetrating vehicle or oil. The type of dye materials provides one means of classifying penetrants.2.2.2.2VisiblePenetrant.CAUTIONDOD prohibits the use of visible-dye penetrant on aircraft, engines andmissiles, except for those parts with specific engineering approval.Visible-dye or color-contrast penetrants contain a red dye dissolved in the penetrating oil. The visibility isfurther enhanced during the penetrant process by the application of a layer of white developer. The whitedeveloper provides a high contrast background for the bright red penetrant when viewed under natural orwhite light.2.2.2.3FluorescentPenetrant.CAUTIONDOD prohibits the use of water washable dye penetrant on aircraft, engines,and missiles, except for those parts with specific engineering approval.Some chemical compounds have the capability of emitting visible light when exposed to near-ultraviolet light(UV-A, energy with a wavelength of 320 to 400 nanometers), commonly called black light. This property istermed f luorescence, and the materials are called f luorescent (see paragraph 2.4.4.2.6). Very smallquantities of f luorescent penetrant will emit highly visible indications when exposed to black light.2.2.2.4DualMode(BothVisibleandFluorescent)Penetrant.CAUTIONDOD prohibits the use of visible / f luorescent (dual mode) penetrant onaircraft, engines, and missiles, except for those parts with specific engineer-ing approval.Dual mode penetrants contain dye materials that are reddish in color under white light and f luorescentunder black light. However, the intensities of the visible red color and the f luorescent color (usually orange)are less than the individual visible dye and f luorescent penetrants. The brilliance of the visible color andbrightness of f luorescence are less than that obtained with the single mode visible-dye and f luorescent-dyepenetrants respectively.2.2.3MethodsofPenetrantRemoval.2.2.3.1General.Penetrant materials are manufactured or formulated for specific removal methods. The removal methodprovides another means of classifying penetrant materials. Each removal method has advantages anddisadvantages, which are covered in later paragraphs.2.2.3.2WaterWashable.The usual liquid base or vehicle for a penetrant is petroleum oil, which is insoluble or immiscible in water.This means that the penetrant cannot be removed with water. However, there are chemical compoundscalled emulsifiers that can mix with oil vehicles to form a mixture that can be removed with water. Whenemulsified, the oil-based penetrant then can be removed with water. The chemical compound forming theemulsifiable mixture is called an emulsifying agent2-8Change 2
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