Liquid Penetrant Testing  is a nondestructive examination (NDE) method that utilizes Visible or fluorescent dye to reveal surface flaws on parts and equipment which might not otherwise be visible. The technique works via the principle of “capillary action,” a process where a liquid flows into a narrow space without help from gravity. Because it is one of the easiest and least expensive NDE techniques to perform, LPT is one of the most commonly used inspection techniques in many industries, including oil and gas.