Control of Developer
The function of the developer is very important
in a penetrant inspection. It must draw out of the discontinuity
a sufficient amount of penetrant to form an indication, and it
must spread the penetrant out on the surface to produce a visible
indication. In a fluorescent penetrant inspection, the amount
of penetrant brought to the surface must exceed the dye's thin
film threshold of fluorescence, or the indication will not fluoresce.
Additionally, the developer makes fluorescent indications appear
brighter than indications produced with the same amount of dye
but without the developer.
In order to accomplish these functions, a developer
must adhere to the part surface and result in a uniform, highly
porous layer with many paths for the penetrant to be moved due
to capillary action. Developers are either applied wet or dry, but the desired end result is always a uniform, highly porous,
surface layer. Since the quality control requirements for each
of the developer types is slightly different, they will be covered
Dry Powder Developer
A dry powder developer should be checked daily to
ensure that it is fluffy and not caked. It should be similar to
fresh powdered sugar and not granulated like powdered soap. It
should also be relatively free from specks of fluorescent penetrant
material from previous inspection. This check is performed by
spreading a sample of the developer out and examining it under
UV light. If there are ten or more fluorescent specks in a 10
cm diameter area, the batch should be discarded.
Apply a light coat of the developer by immersing
the test component or dusting the surface. After the development
time, excessive powder can be removed by gently blowing on the
surface with air not exceeding 35 kPa or 5 psi.
Wet Soluble/Suspendable Developer
Wet soluble developer must be completely dissolved
in the water and wet suspendable developer must be thoroughly
mixed prior to application. The concentration of powder in the
carrier solution must be controlled in these developers. The concentration
should be checked at least weekly using a hydrometer to make sure
it meets the manufacturer's specification. To check for contamination,
the solution should be examined weekly using both white light
and UV light. If a scum is present or the solution fluoresces,
it should be replaced. Some specifications require that a clean
aluminum panel be dipped in the developer, dried, and examined
for indications of contamination by fluorescent penetrant materials.
These developers are applied immediately after the
final wash. A uniform coating should be applied by spraying, flowing
or immersing the component. They should never be applied with
a brush. Care should be taken to avoid a heavy accumulation of
the developer solution in crevices and recesses. Prolonged contact
of the component with the developer solution should be avoided
in order to minimize dilution or removal of the penetrant from
Solvent Suspendable (AKA Nonaqueous Wet)
Solvent suspendable developers are typically supplied
in an sealed aerosol spray can. Since the developer solution is
in a sealed vessel, direct check of the solution is not possible.
However, the way that the developer is dispensed must be monitored.
The spray developer should produce a fine, even coating on the
surface of the part. Make sure the can is well shaken and apply
a thin coating to a test article. If the spray produces spatters
or an uneven coating, the can should be discarded.
When applying a solvent suspendable developer, it
is up to the inspector to control the thickness of the coating.
with a visible penetrant system, the developer coating must be
thick enough to provide a white contrasting background but not
heavy enough to mask indications. When using a fluorescent penetrant
system, a very light coating should be used. The developer should
be applied under white light and should appear evenly
Parts should be allowed to develop for a minimum
of 10 minutes and no more than 2 hours before inspecting.