Penetrant enters a discontinuity by capillary action. The driving pressure is 2 sigma cos(theta) / r – surface tension divided by the half-width of the opening – and the rate of filling is set by that pressure against the viscous drag of the liquid moving along a very narrow channel. Both terms work against speed: the tighter the crack, the higher the driving pressure but the far greater the drag, so tight discontinuities fill slowly. Dwell time is the time allowed for that filling to happen, and it is the single step most often cut short in practice.

The code tables key dwell to material and product form because those predict the geometry of the discontinuity. Castings and welds contain relatively open defects – gas porosity, shrinkage, cold shuts, lack of fusion – which fill quickly, so 5 min is enough. Wrought material – forgings, extrusions, plate, bar – contains laps and tight forging cracks that have been squeezed nearly closed by the working process, so 10 min is required. Service-induced cracking such as fatigue and stress corrosion is tighter still and routinely needs far longer than any table minimum.

Temperature enters through viscosity. Penetrant thickens as it cools and fills more slowly, which is why the standard qualified range is 40 to 125 °F (5 to 52 °C); outside it the technique must be qualified on a cracked comparator block against a standard-temperature run. In the cold end of that range, extending dwell beyond the table minimum is normal practice.

The penetrant must not be allowed to dry on the part. A dried film cannot be removed cleanly and leaves background that hides indications, so long dwells on hot or windy sites need the surface replenished rather than simply left.

Worked example

Material and product form	metal_wrought
Part and penetrant temperature	20 °C
Procedure minimum dwell	0 min
Table minimum penetrant dwell	10 min
Penetrant dwell to apply	10 min
Minimum developer dwell	10 min
Total dwell before interpretation	20 min
Standard range minimum	5 °C
Standard range maximum	52 °C

A carbon steel forging examined at 20 °C. Wrought product form, so Table T-672 gives a 10 min minimum penetrant dwell and a 10 min minimum developer dwell. No longer procedure minimum is imposed, so the dwell applied is 10 min and the earliest interpretation is 10 + 10 = 20 min after penetrant application. 20 °C sits inside the 5 to 52 °C standard range, so no temperature qualification is needed.

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