Liquid penetrant testing (LPT) is another means of enhancing the capability of visual observation. It is suitable for use on smooth surfaces of all materials inclusive of magnetic as well as non-magnetic. It is limited to surface defects. Its impotence to indicate the depth or breadth of flaw should be kept in mind while selecting this methods for particular application.
Use of LPT for porous materials is ruled out since the absorption into the pores would mask the presence of defects. A clean surface is a per-requisite as penetrant cannot enter in to the cracks that is filled with dirt, oil, or other forms of contaminant.Size, shape, weight and number of workpieces to be inspected often influence the selection of a penetrant system. The desired extent of sensitivity and the cost are the most important factors in selecting a system.
LPT can also be used for leak testing. In this type, the component casting is filled with penetrant and developer is applied outside the surface.