Penetrant dwell times, which are normally based on application at room temperatures, must be adjusted to compensate
for viscosity changes at other temperatures. The following factors SHALL be used to correct penetrant dwell time for
a. Temperatures between 60°F (15.5°C) and 100°F (37.2°C) shall be considered room temperature, so the
specified dwell time SHALL be used over that range.
b. When the part ambient temperature is between 40°F (4°C) and 60°F (15.5°C), the specified dwell time
SHALL be doubled.
c. When the part or ambient temperature is between 100°F (37.2°C) and 120°F (49°C), the specified dwell
time MAY be reduced by half, but SHALL NOT be less than 5 minutes.
d. The minimum dwell time for service-induced flaws SHALL NOT be less than 30 minutes.
18.104.22.168.2.6 Cleanliness of the Discontinuity
Penetrant dwell times are based on clean parts without entrapped contaminants. Inspection of parts that have been in
service can be complicated by the difficulty of removing all of the entrapped soil from the discontinuities. The effect of
the entrapped soil on the penetrant dwell time depends upon the type and amount of soil involved.
If the discontinuity is full of soil that is not soluble in penetrant, penetration cannot occur. A change in penetrant
sensitivity or dwell time will not help since penetrant can not enter such flaws. A discontinuity that is only partially
filled with insoluble soil will produce a smaller and less visible indication. Increasing the dwell time will not improve
the indication. However, a more sensitive penetrant with its higher dye content will produce a more visible indication.
When discontinuities contain soils that are soluble in penetrants, such as unpigmented grease, oils, cleaning solutions
and other soluble organics, the situation is different. The penetrant will fill any vacant space in the discontinuity and
then stop. Diffusion then begins between the penetrant and soluble soil (paragraph 22.214.171.124.1 describes the diffusion
process). In a short time, the penetrant and soil become evenly mixed. This mixture will fluoresce much less and may
not give a useful indication. An increase in dwell time will improve the visibility of the indication. With increased
dwell time some of the soil diffuses out of the discontinuity and is replaced with pure penetrant. Using a more sensitive
penetrant will improve the visibility of the indication since the higher dye content can withstand more dilution.
In summary, when a flaw is partially filled with an insoluble soil, an increase in dwell time will not improve the
visibility of the indication. However, if the soil is soluble in the penetrant, the visibility will improve with increases in
dwell time. In both cases, with insoluble and soluble soil contamination, a higher sensitivity penetrant should improve
Penetrant Dwell Characteristics.
There are two basic penetrant dwell modes, immersion and drain.
126.96.36.199.1.1 Immersion Dwell.
In this mode the part remains submerged in a tank of liquid penetrant for the entire dwell period. Immersion dwell can
also be performed by continuously brushing with fresh penetrant throughout the dwell period.