T.O. 33B-1-1
2.4.5.5.2.5
Penetrant Viscosity.
Viscosity has been previously defined as the resistance of a liquid to changing shape. This resistance controls
the f low characteristics of a liquid and is a major factor in the time required to enter and fill a void. Viscosity
of oils, which includes penetrants, changes drastically with temperature. Oils become thin (less viscous) at
high temperatures and thick (more viscous) at low temperatures. Figure 2-12 illustrates how the viscosities
of a number of QPL penetrants change with temperature. The horizontal and vertical scales are spaced to
show the viscosity changes as a straight line function. The chart shows that the viscosity of a high
sensitivity, postemulsifiable (PE) penetrant is about 3 centistokes (cs) at 120oF (49oC) and about 75 cs at
-10oF (-23.4oC), or becomes about 25 times thicker. The viscosity of visible dye is about 2 cs at 120oF (49oC)
and 22 cs at -10oF (-23.5oC), which is an eleven times increase in viscosity. See ASTM D341-43 for viscosity of
several QPL Penetrants.
Figure 2-12.
Viscosity of Several QPL Penetrants at Various Temperatures
2.4.5.5.2.5.1
The part temperature range for applying penetrants is 40oF (4oC) to 120oF (49oC). Actually, most penetrants
are applied at or near a part temperature of 70oF (21.1oC). Therefore, nearly all operating instructions or
procedures specifying dwell times are based on applying penetrant to a part that is at or near a temperature
of 70oF (21.1oC). For high sensitivity PE penetrant, Figure 2-13 shows that the viscosity (7 cs) at 70oF
(21.1oC) is twice the viscosity (14 cs)
2-42
Change 2
