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NDT liquid penetrant testingLiquid penetrant testing is known by a variety of other names: dye penetrant, fluorescent penetrant inspection (FPI),  red dye, fluorescent check, and blacklight check. It is also known sometimes known as Magnaflux check, however, Magnaflux is a name brand known for specializing in both liquid penetrant and magnetic particle, which are two different testing methods. This unfortunately creates confusion on which method to use when a Liquid Penetrant test is referred to as a Magnaflux test.

Liquid penetrant testing is a form of non-destructive testing. This means that this test is used to examine the properties of a material, system, or part without damaging it. Liquid penetrant testing is used to detect surface cracks and other surface defects on various materials. Metallic materials are most common, but liquid penetrant testing can be performed on many non-porous surfaces. 

Liquid penetrant is a relatively easy and inexpensive test, but it must be performed by a certified inspector. Different NDT codes, specifications and procedures require the inspection to be performed in a particular manner. NDT inspectors performing Liquid penetrant (PT), should be certified to a Level 1, 2, or 3 and required to follow procedures such as SNT-TC-1A or NAS-410.

In order to perform a liquid penetrant test, the part being examined must be thoroughly cleaned beforehand. Then the penetrant is applied to the surface of the part. The penetrant is allowed to sit on the part (this time is called dwell time), and during this time the penetrant will seep into any cracks in the surface. After the dwell time, the penetrant is wiped off the surface (any penetrant that has seeped below the surface is not removed in this process). Then a developer is applied, which draws out the penetrant from any flaws in the material to the surface. The technician examines the penetrant that has been drawn to the surface to determine the extent of the flaw. 

Different materials and situations will have different requirements for performing a liquid penetrant test.  The sensitivity rating of penetrants allows them to be used on casting and machined surfaces by using lower sensitivity level ½ on castings and level III on machined surfaces. Testing of high-temperature applications, such as hot-in-service pressurized pipes, must use a specially formulated penetrant chemical. A trained technician will be able to determine the appropriate penetrant and testing procedure for each specific scenario to provide accurate results. 

Some basic things to consider when performing a liquid penetrant test are:

  1. Whether a visible or fluorescent penetrant is required
  2. The sensitivity requirements
  3. Whether pre-cleaning of the part or component is needed
  4. How accessible the inspection area is
  5. Whether the dark room to be used is capable of the 2fc lighting requirements


If you’re looking for an experienced NDT technician to perform liquid penetrant testing, RRL NDT can help. Our experienced level 2 and 3 technicians can perform PT at your site, or provide NDT training to your technicians.