The liquid used for a dye-penetrant must both wet the surface of the workpiece and have a high surface tension to cause a sufficient quantity of the liquid to be drawn into the cavity. Viscosity is not a direct measure of the ability of a liquid to enter a cavity, but liquids with a high viscosity flow too slowly to be certain that they will fill cavities in a reasonable time to make them useful penetrants.

The oil-base liquids that are used as the penetrant for solvent-removable dye-penetrant systems can be applied to a surface of the workpiece having any orientation, by spraying from an aerosol can. Sufficient dwell time must be allowed for the penetrant to be drawn into surface flaws in the workpiece. ASME Code, Section V, requires a minimum dwell time of 10 minutes. The sensitivity for detecting very fine surface cracks can be increased by increasing the dwell time. Doubling the dwell time to a minimum of 20 minutes is considered to be a high sensitivity PT procedure, and should be used for tight cracks that can be difficult to detect, such as stress-corrosion cracks in austenitic stainless steel.

It is best to first wipe the excess penetrant from the surface with a dry cloth, followed by a second wiping with a clean cloth dampened with the solvent. The surface can also be flushed by direct application of the solvent to remove the penetrant, but there is danger of inadvertently removing the penetrant from flaws before they are revealed by application of the developer. ASME Code, Section V, prohibits removal of the excess penetrant by flushing with solvent because of this risk. The excess penetrant must be completely removed from the surface before the developer is applied.

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