Comparison Between Dye Penetrant and Magnetic Particle Examination

Both Magnetic Particle and Dye Penetrant are widely using non-destructive testing techniques which are applied for the examination of surface related discontinuities. Although used for the same job but there are some key features that differentiate them for certain areas of application. Also there are some features which are common in both techniques.
Described below is a brief comparison between magnetic particle and dye penetrant examination.
1. Cost
Magnetic particle testing offers lower cost equipment and materials.
Dye Penetrant also offers lower cost for materials and equipment.
2. Time
MPE is fast inspection technique.
DPE is relatively slower process.
3. Materials
MPE can only be applied in examination of ferromagnetic materials because they are highly responsive to magnetic principles. It cannot be applied on all metals and non-magnetic alloys.
Dye penetrant provides optimal results in all metals and alloys.
4. Discontinuities
Dye penetrant testing only detects discontinuities which are on surface while magnetic particle testing detects not only surface cracks but also those imperfections that are very near to surface.
5. Surface conditions
Strict pre-cleaning is required before dye penetrant inspection to start. The test surface must be clean adequately, providing smooth and precise inspection. While in case of magnetic particle inspection no such pre-cleaning and post-cleaning processes are necessary because the surface pollutants don’t have any adverse effect on inspection results.
6. Temperature
Dye Penetrant Examination is limited to temperature conditions normally (5-60 degree Celsius). Outside this range DPE is not applicable because at high temperature the used penetrant becomes dry while at low temperature the penetrant liquid becomes highly viscous, thus impeding the penetration phenomenon and reducing the efficiency of testing.
On the other hand Magnetic Particle Inspection is applicable at extreme temperatures and can be carried out over a considerable range of temperature (normally below Curie temperature). Curie temperature is a point at which, change of magnetic properties occurs in materials and this point is different for different for different materials.
7. Equipment
In MPE fluorescently or (colored) visibly pigmented magnetic particles are employed which when apply on the test section, indicate the presence of surface flaws/ imperfections. While in Dye penetrant fluorescent or visible penetrants are employed along with developer, for examination.
8. Safety
Equipment and materials which are employed in both magnetic and dye penetrant examination are safe to use. Less level of risk for any loss or injury to operator.
9. Working Principle
Both non-destructive techniques are used for the same examination purpose but with different working principle. In Dye penetrant technique, liquid penetrants penetrate the surface breakings/ flaws. After that a developer is applied that allows the penetrant to come out from breakings again, making an indication.
In MPE, the test material is magnetized by using equipment. The magnetic flux at surface flaws/cracks tends to form ‘Leakage Fields’. Magnetic particles when employ, gets attracted by these leakage fields over the cracks, providing an instant examination.
10. Manipulation
Both techniques are easy to operate when compare to other non-destructive examination techniques. Least amount of training is required and an operator gets full command after making few practices and exercise. Many institutes for non-destructive examination courses, are working on training programs for interested candidates. 

Dye Penetrant
Magnetic Particle
Fluorescent/visible penetrants.
Fluorescent/visible magnetic particles. Magnetizing systems.
Comparatively slower than MPE
Fast inspection Technique
Minimum Risk of loss or injury
On all metals and alloys
Only on ferromagnetic materials
Minimum required

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