Wetting ability of a penetrant is measured by:

The wetting ability of a penetrant is effectively measured by the contact angle. When a liquid penetrant is applied to a surface, the extent to which it spreads or wets the surface is characterized by the contact angle formed at the interface between the liquid and the solid. A smaller contact angle indicates better wetting ability, meaning that the penetrant can more easily fill any discontinuities or surface irregularities, which is crucial for effective liquid penetrant testing.

In practice, if the contact angle is low, it generally suggests that the surface has been effectively wetted by the penetrant, allowing for optimal detection of defects. Conversely, a high contact angle indicates poor wetting, which may hinder the penetrant's ability to penetrate cracks or voids effectively.

Understanding this aspect of wetting ability is vital in ensuring that the penetrant can do its job as intended during inspection processes in NDT (Non-Destructive Testing) practices. Therefore, contact angle serves as a direct indicator of how well a penetrant can adhere to and permeate a surface, making it the correct measure for wetting ability.