Subscribe to our newsletter to receive the latest news and events from TWI:

Subscribe >
Skip to content

Penetrant systems for carrying out dye penetrant inspection


When considering a penetrant system, the different types of penetrant, excess penetrant removers and developers have all to be taken into account.

According to BS EN 571, there are -

three basic types of penetrant:

  1. Colour contrast
  2. Fluorescent
  3. Dual purpose (fluorescent / colour contrast)

five methods of excess penetrant removal:

  1. Water
  2. Lipophilic emulsifier
    i oil-based emulsifier
    ii rinse with running water
  3. Solvent
  4. Hydrophilic emulsifier
    i optional pre-rinse (water)
    ii emulsifier (water-diluted)
  5. Water and solvent

Note that A, B, D, and E all involve a final water rinse

five forms of developer:

  1. Dry powder
  2. Aqueous (water soluble)
  3. Aqueous (water suspendable)
  4. Non-Aqueous (solvent-based)
  5. Peelable (water or solvent-based for special applications, e.g. peelable developer)

The different types of penetrant, remover and developer are all interchangeable, except for dry powder, which must not be used with the colour contrast penetrant.

You can, therefore, use a fluorescent penetrant with a solvent, water-washable or post-emulsifiable remover in conjunction with either a dry powder, aqueous or non-aqueous developer.

The choice of system depends on the nature of the discontinuities (size and type) to be detected, the surface condition, geometry and intricacy of the component and the number of components to be tested.

Here are some examples to illustrate the use of a particular combination of penetrant, excess penetrant remover and developer -

1. Inspection of a large number of threaded components

Penetrant: Fluorescent
Excess penetrant removal: Water-washable
Developer: Dry powder

Fluorescent penetrant is ideal for mass inspections of this kind because it is easily detected (therefore less tiring for operators) and is more sensitive than the colour contrast system. Water-washable is more suitable than solvents for this task because it is more effective at removing excess penetrant from threads. Components have to be oven dried prior to application of the dry powder.

2. Inspection of a repair weld on the crane of a pipe laying barge

Penetrant: Colour contrast
Excess penetrant removal: Solvent removable
Developer: Solvent-based

On-site inspections often take place in difficult operating conditions. Colour contrast penetrant is good for use in daylight. Solvent-removable is ideal because it is applied from an aerosol and is, therefore, highly portable and convenient. A solvent-based developer gives good contrast and therefore clear identification of flaws.

3. Inspection of turbine blades for fatigue cracks

Penetrant: Fluorescent
Excess penetrant removal: Post-emulsifiable
Developer: Non-aqueous

This inspection task involves a critical component and requires detection of small flaws on smooth surfaces. Fluorescent penetrant is inherently more sensitive than colour contrast. Post-emulsifiable is more sensitive than water washable since there is less danger of over-washing of the penetrant. Non-aqueous developer is the most sensitive.

Further information

Please contact us for more information.

For more information please email: