Maximum Dwell Time.
The maximum dwell times specified are based on small discontinuities. Medium or
large discontinuities, which develop faster, will be blurred at these maximum dwell
times. However, medium or large discontinuities contain enough penetrant to form
an observable indication even though it is blurred. Indications from small
discontinuities may be missed if the maximum dwell times are exceeded. To
increase penetrant system capability, parts should be viewed periodically during
Over-development, i.e., too long a development time, is possible and must be avoided. Developer action starts when
the developer is completely dry and continues until all of the available penetrant is extracted. An indication will
gradually form, reach a maximum resolution point (bright and sharp), and then begin to degrade. The lateral diffusion
of penetrant over a period of time can be so great that the indication becomes indistinct. Medium size or large
discontinuities will appear as a smear or blob of penetrant. Small indications are especially critical, since the small
amount of penetrant may not be observed when it diffuses. The maximum developer dwell time SHALL NOT exceed
the following, unless otherwise specified in a written inspection procedure:
Nonaqueous developer 30 minutes
Aqueous developer 1 hour
Dry developer 2 hours
Comparison of Developers.
The relative sensitivities of penetrant inspection with various forms of developer are influenced by a number of factors.
The method of applying the developer produces a range of sensitivities for each of the developer forms. Table 2-5 lists
some of the common forms of developer, plus the application method, arranged in order of decreasing sensitivity. This
is the sensitivity order most generally accepted. It is recognized that solvent suspended developers applied by spraying
produce the highest sensitive penetrant system. Industry agreement on the developer sensitivity order ends at this
point. The type of test sample, type of flaw, flaw size and shape, type of penetrant, method of removal, and drying
procedures will affect the sensitivity of the penetrant system. The number of variables involved has resulted in
conflicting reports on the relative performance of dry versus water based (suspended and soluble) developers. It is
agreed that, when properly applied, the water base developers form a coating with a finer matrix of developer particles
that are in more intimate contact with the part surface, than is possible with dry developers. The opposing argument is
that an uneven coating of water based developers can mask indications. There is agreement that water soluble
developers should not be used on water washable penetrant. Figure 2-25 contains photographs of a single cracked
chrome plated panel that has been processed with four forms of developer using application methods available to base
level NDI laboratories.