Specific Gravity

The density or specific gravity of a penetrant material has a slight to negligible effect on the performance of a penetrant. The gravitational force acting on the penetrant liquid can be working either with or against the capillary force depending on the orientation of the flaw during the dwell cycle. When the gravitational pull is working against the capillary rise, the strength of the force is given by the following equation:

$$\text{Force}=\pi r^{2}\rho g h$$

Where:

r = radius of the crack opening
h = height of penetrant above its free surface
$\rho$ = density of the penetrant
g = acceleration due to gravity

When the direction of capillary flow is in the same direction as the force of gravity, the added force driving the penetrant into the flaw is given by the formula shown below:

$$\text{Force}=\rho A h$$

Where:

h = height of penetrant above its free surface
A = cross-sectional area of the opening
$\rho$ = density of the penetrant

Increasing the specific gravity by decreasing the percent of solvent (by volume) in the solution will increase the penetration speed.

Reference:

• Tugrul, A. B., Capillarity Effect Analysis for Alternative Liquid Penetrant Chemicals, NDT & E International, Volume 30 Number 1, Published by Elsevier Science Ltd., Oxford England, February 1997, pp. 19-23.