Note: Dye penetrant is relatively simple to use but can be very easily done inappropriately making the inspection useless if performed by an untrained person.



Liquid penetrant is primarily used in two types: Fluorescent (Type I) and Visible (Type II). The key is that fluorescent penetrant is rated in sensitivity values and the visible is not. So, if you are inspecting a critical aircraft component the procedure dictating the inspection usually calls out a fluorescent method. When we say fluorescent we are saying that the inspection needs to be performed in a darkened room with a black light because the chemicals are fluorescent in nature and will light up like a groovy poster. Visible dye is simply visible and the inspection can be performed under normal lighting or a flashlight. A key component of both methods is lighting requirements.

A critical mistake performed with liquid penetrant is following fluorescent after using visible. If you do this the visible dye usually quenches the fluorescent dye rendering it useless. This is one of the reasons why you should always visibly inspect a part looking for traces of previous processes prior to performing fluorescent penetrant.

For visual testing and fluorescent liquid penetrant lighting is crucial. For visible dye penetrants you usually need 100 fc (foot candle) of white light at the surface of the part that you are inspecting. This is to assure that you can see the flaw adequately. Many people overlook this requirement and miss flaws.

For fluorescent penetrant testing the light requirements are tougher. Hence, the reason most people try to accomplish visible even if it is not appropriate. You usually have to be in a dark area not to exceeding 2 fc (foot candle) and the black light has to be able to produce 1000 µW/cm² (microwatts centimeter squared). In simple terms you have to be in a very dark room with a black light much stronger than what you see at a dance club. If you do not meet these minimums your sensitivity will be off and you will probably miss what you are looking for. The flip side to this statement is that your black light can be too bright and you can miss cracks because of glare which it can produce.

Remember that when we are talking about missing a crack we are referencing the possibility of an inspector missing a crack on an airplane, railroad wheel, fuel tank, ship hull, pipeline, etc. We are usually talking about the possibility of people getting hurt if the technician is doing their job improperly. This is why NDT liquid penetrant technicians should be properly qualified and certified to a Level II when performing examinations. Qualification is referencing classroom training and experience hours. Certification is referencing that the individual has passed a program by testing and demonstration of their capabilities in accordance with a code such as SNT-TC-1A or NAS-410.