Liquid penetrant testing dwell time is indicated in the approved written procedure. We have two dwell times, one for penetrant, the time we wait for the wet penetrant fluid to soak inside of discontinuities, e.g., cracks, porosity, hot tea, etc. and the second one is after application of the developer. We have to wait for a particular time before performing the evaluation.
The ASME Code Section V article 6 provides a table for penetrant dwell time. Based on this table the dwell time depends on the expected defect and product form. For example, the dwell time for carbon steel material for welding (Liquid Penetrant testing on weld seam) is 5 min and expected discontinuities are cracks, lack of fusion and porosities.
This dwell time is for test object temperature from 50 F to 125 F but if the object temperature is between 40 F to 50 F, the dwell time will be two times of list value. In above example, if the object temperature is 45 F then dwell time will be 10 min.
After application of developer, the technician should wait for 10 min to let reverse capillary action take place and trapped penetrant to bleed out.
The interpretation should be done within of 10 min and 60 min. The interpretation will not be valid if be performed after 60 min (1 hour). The 10 and 60 min requirement is from ASME Section V article 6.