Liquid Penetrant Inspection (PT), or dye penetrant inspection, is used to find surface connected discontinuities on non-porous materials.

PT can be performed on any non-porous material but MT is often the preferred test method on ferromagnetic materials when looking for cracks. PT uses a low surface tension fluid with a contrasting dye to penetrate into surface discontinuities. The liquid penetrant dye is cleaned off the surface of the part. A developer is then applied to the part drawing out the dye from the surface connected discontinuities.

Liquid penetrant comes in several types and methods:


  • A. Water Washable
  • B. Post-emulsifiable - lipophilic
  • C. Solvent Removable
  • D. Post-emulsifiable - hydrophilic


  • 1. Fluorescent
  • 2. Visible

In addition, developer comes in several different forms. Dry powder, water soluble, water suspendable, and non-aqueous. 

The surface of the part being inspected must be free of any coatings. Cleanliness of the part is also an important factor to the quality of the inspection.

Wikipedia Article