Size and type of defect
Wide shallow defects are most likely to be detected by the post-emulsifiable method. Fine defects are best located by fluorescent methods
Geometry and intricacy
Post emulsifiable methods, whilst more sensitive than water washable, would in all likelihood leave behind excessive background coloration on such samples.
Rough surfaces are difficult to clean fully when tested with post-emulsifiable methods, and components such as sand castings would be best tested by a water washable method.
Fluorescent methods are also less suited to testing rough components than visible methods (colour contrast) due to problems in adequately monitoring penetrant removal.
Solvent removable methods may lead to surface damage due to incompatibility between the penetrant and the material under test.
Size and position of the item to be tested
On-site welds are unlikely to be tested by fluorescent and water washable methods due to the requirement for darkened conditions in the former and a water source in the latter case.
Equipment and expertise available
Fluorescent water washable and post-emulsifiable test methods generally involve use of flow lines and are thus more suited to factory use than on site testing.
Water washable penetrant methods are much cheaper than solvent removable and post-emulsifiable methods due to the availability of the main cleaning fluid.
Number of components to be tested
Fluorescent methods are recommended for batch inspections because of the higher sensitivity of the eye to fluorescent indications over visible colour contrast ones and reduced levels of fatigue suffered by the operator performing the inspections.
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