T.O. 33B-1-12-442.4.5.5.2.5.4Penetrant dwell times, which are normally based on application at room temperatures, must be adjusted to compensatefor viscosity changes at other temperatures. The following factors SHALL be used to correct penetrant dwell time fortemperature changes:a. Temperatures between 60°F (15.5°C) and 100°F (37.2°C) shall be considered room temperature, so thespecified dwell time SHALL be used over that range.b. When the part ambient temperature is between 40°F (4°C) and 60°F (15.5°C), the specified dwell timeSHALL be doubled.c. When the part or ambient temperature is between 100°F (37.2°C) and 120°F (49°C), the specified dwelltime MAY be reduced by half, but SHALL NOT be less than 5 minutes.d. The minimum dwell time for service-induced flaws SHALL NOT be less than 30 minutes.2.4.5.5.2.6 CleanlinessoftheDiscontinuityPenetrant dwell times are based on clean parts without entrapped contaminants. Inspection of parts that have been inservice can be complicated by the difficulty of removing all of the entrapped soil from the discontinuities. The effect ofthe entrapped soil on the penetrant dwell time depends upon the type and amount of soil involved.2.4.5.5.2.6.1If the discontinuity is full of soil that is not soluble in penetrant, penetration cannot occur. A change in penetrantsensitivity or dwell time will not help since penetrant can not enter such flaws. A discontinuity that is only partiallyfilled with insoluble soil will produce a smaller and less visible indication. Increasing the dwell time will not improvethe indication. However, a more sensitive penetrant with its higher dye content will produce a more visible indication.2.4.5.5.2.6.2When discontinuities contain soils that are soluble in penetrants, such as unpigmented grease, oils, cleaning solutionsand other soluble organics, the situation is different. The penetrant will fill any vacant space in the discontinuity andthen stop. Diffusion then begins between the penetrant and soluble soil (paragraph 2.5.5.3.1 describes the diffusionprocess). In a short time, the penetrant and soil become evenly mixed. This mixture will fluoresce much less and maynot give a useful indication. An increase in dwell time will improve the visibility of the indication. With increaseddwell time some of the soil diffuses out of the discontinuity and is replaced with pure penetrant. Using a more sensitivepenetrant will improve the visibility of the indication since the higher dye content can withstand more dilution.2.4.5.5.2.6.3In summary, when a flaw is partially filled with an insoluble soil, an increase in dwell time will not improve thevisibility of the indication. However, if the soil is soluble in the penetrant, the visibility will improve with increases indwell time. In both cases, with insoluble and soluble soil contamination, a higher sensitivity penetrant should improvethe results.2.4.5.6 PenetrantDwellCharacteristics.2.4.5.6.1 DwellModes.There are two basic penetrant dwell modes, immersion and drain.2.4.5.6.1.1 ImmersionDwell.In this mode the part remains submerged in a tank of liquid penetrant for the entire dwell period. Immersion dwell canalso be performed by continuously brushing with fresh penetrant throughout the dwell period.
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