A penetrant testing method in which an emulsifier, separate from the penetrant, is used is called?

The method in which an emulsifier, separate from the penetrant, is used is referred to as post emulsifying. This technique involves applying a penetrant to the surface, allowing it to seep into any defects, and then using a separate emulsifier to aid in the removal of excess penetrant. The emulsifier interacts with the penetrant, making it easier to wash away from the surface while keeping the penetrant that has entered the flaws intact.

In contrast, other methods like solvent removable, water washable, and self emulsifying do not require a separate emulsifier. Solvent removable systems depend on a solvent to wash away the excess penetrant without the need for an additional emulsification step. Water washable penetrants can be rinsed away with water due to their inherent formulation, and self emulsifying penetrants contain components that allow them to turn into an emulsion upon contact with water, eliminating the need for a separate emulsifier. Thus, the distinction of using a separate emulsifier is what firmly categorizes this method as post emulsifying.