Penetrant are classified based on their the strength of the indication that is produced for a number flaw very small in size & tight.

Level 1/2 : Ultra low sensitivity.

Level 1 :  low sensitivity.

Level 2 : medium sensitivity.

Level 3 : High sensitivity.

Level 4 : Ultra high sensitivity.

These rating depend on the detectability of fatigue crack & brightness of indications.

Penetrant materials based of light requirement classified in two types.

• Type 1 – Fluorescent Penetrants
• Type 2 – Visible Penetrants

Fluorescent penetrants contain a dye or several dyes that fluoresce when exposed to ultraviolet radiation.  Fluorescent penetrant systems are more sensitive than visible penetrant systems because the eye is drawn to the glow of the fluorescing indication.

Visible penetrants contain a red dye that provides high contrast against the white developer background.   However, visible penetrants do not require a darkened area and an ultraviolet light in order to make an inspection. Visible penetrants are also less vulnerable to contamination from things such as cleaning fluid that can significantly reduce the strength of a fluorescent indication.

Inspection steps:

1. Surface Cleaning or Pre cleaning: The test surface is cleaned to remove any dirt, paint, oil, grease or any loose scale that could either keep penetrant out of a defect, or cause irrelevant or false indications. Cleaning methods may include solvents, alkaline cleaning steps, vapor de-greasing, or media blasting. Purpose of cleaning to remove dirt from defect open to surface.
2. Application of Penetrant: The penetrant is then applied to the surface of the item being tested. The penetrant is usually a contrast coloured mobile fluid with high wetting capability. The penetrant is allowed “dwell time” to soak into any flaws (generally 5 to 30 minutes). The dwell time mainly depends upon the type of penetrant being used, material being tested and the type & size of flaws.
3. Removal of Penetrant: First of all clean he surface with dry lint free cloth as best as possible. Take moist or damped cloth with solvent & clean entire surface until it no sign of penetrant on cleaning cloth. Important  not spray the solvent on the test surface directly, because this can remove the penetrant from the flaws. If excess penetrant is not properly removed, once the developer is applied, it may leave a background in the developed area that can mask indications or defects. In addition, this may also produce false indications severely hindering the ability to do a proper inspection. Also, the removal of excessive penetrant is done towards one direction either vertically or horizontally as the case may be.
4. Application of developer: After excess penetrant has been removed, a white developer is applied to the sample.Apply Non aqueous developer over entire surface of item under examination.

Portable kit for Penetrant testing :
1. For field use penetrant materials are packed in aerosol spray cans.
2. A set of cleaner / remover, penetrant and developer are all that required to carry out the field examination.
3. Aerosol cans provide uncontaminated liquids for the examination and can have a shelf life of 5 years. However, if the pressure inside the can drops significantly, the can becomes useless.