General A&P Practice Test

Dwell time is about how long the penetrant must flow into surface-breaking flaws. The geometry of those flaws—their size and shape—controls how capillary action fills them. Narrow, deep, or irregular defects take longer to fill, while large, open defects fill more quickly, so the dwell time is set by how difficult it is for the penetrant to access the defect geometry. Other factors like the penetrant’s color, surface roughness, or coating thickness influence visibility or surface conditions, but don’t determine how long penetration needs. Temperature and viscosity affect penetration rate, but the defect geometry ultimately dictates the appropriate dwell time.