The control technique with penetrating liquids is based on the “wetting/capillarity” properties of particular liquids and the method is used to detect surface defects on both metallic and non-metallic materials. Schematically, it works by applying a liquid to the surface under examination which penetrate by capillarity inside the emerging discontinuities; and, after removing the excess liquid from the surface itself, the fraction introduced into the discontinuities is highlighted by a particular absorbent agent, thus giving rise to a visible indication corresponding to the surface defect sought.

The indications obtained can be observed in white light or with Wood’s light (UV radiation) depending on whether they are used, respectively, penetrating liquids with COLOR CONTRAST or FLUORESCENT. The latter, by virtue of the resulting high luminous contrast, guarantee a higher detection sensitivity than can be obtained with colour contrast penetrants.

Differently from magnetic particles testing, where the machine is often designed considering the piece that has to be inspected, Penetrant Testing is recommended to be the most flexible and malleable one and it offers a single system thought for an extended range of items. These systems are divided into three large families: