Dye Penetrant Testing (PT): Advantages and Processes for NDT

Dye penetration testing or liquid penetrate inspection (LPI) is done to locate surface breaking defects. The method can be used on ceramics, plastics, metals and other non-porous materials. A dye penetration test is a low-cost, non destructive testing method. It is useful in detecting fatigue cracks and leaks in new products.

Principles of dye penetrant inspection (DPI):

As the name implies, this inspection method involves the penetration of dye into the surface defects and faults. The dye (or draw penetrant) is applied through spraying, dipping or brushing. Then, the penetrant is removed and the developer is applied (for visual inspection later). Liquid penetrant examinations identify defects within a part and can be used as a test method to ensure part durability.

The processes – how liquid penetrant testing is done:

Generally, there are 6 steps in dye penetrant testing. These are:

  1. Pre-cleaning
  2. Penetrant application
  3. Removal of excess penetrant
  4. Application of developer
  5. Inspection
  6. Post-cleaning (water washable)


Training and experience are valuable in acquiring accurate results and preventing false indications. Also, removing the surface contaminants is a valuable step in the whole process.

Advantages and disadvantages:

Many engineers and manufacturers choose the dye penetrant inspection method because it’s quick and low cost. The penetration time (dwell time) can be as short as 5 minutes (or up to 30 minutes depending on the application).

In addition, dye penetrant testing allows easy inspection of parts with complex shapes. This method also allows for quick inspection of large areas and large volumes.

However, there are also less advantageous elements of dye penetrant testing. One is that the detection may be only limited to surface flaws. In addition, testing on rough surfaces might lead to false indications (due to difficulty removing the excess penetrant).

There are also concerns about worker safety. The spraying of the penetrant might lead to skin irritations. Therefore, proper training and handling is required for the procedure. It’s also important to perform a proper selection of penetrant and developer.

Dye penetrant testing for NDT:

At NDT Equipment Sales, we can assist you with the selection of all types of penetrant and developer. Our expertise means we can find a solution specific to your inspection service application. Contact us today to place an order for any NDT equipment or to ask any questions about our stock.