Quality Control of Dwell
The dwell step involves allowing the test component to sit for a sufficient period of time for the penetrant to fill any surface breaking defects that happen to be present. There are basically two dwell modes, immersion-dwell and drain-dwell. The drain-dwell mode has been shown to produce the most sensitive results. The only real quality control required in the dwell step of the process is to ensure that a minimum dwell time is reached. Dwell times are usually recommended by the penetrant producer or required by the specification being followed. There is no harm in allowing a penetrant to dwell longer than the minimum time as long as the penetrant is not allowed to dry on the part.