How long does the dwell time continue for Lipophilic post-emulsifiable penetrant?

• A. Until the surface shows brightness
• B. Indefinitely
• C. Until the emulsifier just reaches the surface and should not diffuse into entrapped penetrant
• D. For exactly 5 minutes

Answer: (C)

Dwell time for lipophilic post-emulsifiable penetrant is governed by the diffusion of the emulsifier to the surface. You continue the dwell until the emulsifier front just reaches the surface, and it should not diffuse further into any penetrant that may be trapped in flaws. If you extend dwell beyond that point, the emulsifier can diffuse into penetrant inside defects and begin washing it out, reducing the indication. So the stopping point is when the emulsifier has reached the surface, not before or after. The other options aren’t appropriate because dwell isn’t tied to a surface brightness reading, it isn’t indefinite, and it isn’t a fixed time like five minutes—the correct criterion depends on diffusion behavior.