To determine the viscosity of a liquid, a Zahn cup is dipped and completely filled with the liquid to be measured. After lifting the cup out of the liquid, the user measures the time it takes for the liquid streaming from the opening in the bottom of the cup to begin “breaking up.” This is the corresponding "efflux time."
(Quick note: Efflux is defined as “the flowing out of a particular substance or particle.” Cups of this nature may be more recognizable by names like Zahn Cup, Shell Cup, or Ford Cup. For purposes of this blog, I am using the Zahn cup to refer to all efflux cups). As I am fond of saying, it's not rocket science it's fluid science, (which requires a degree of education).
Sounds simple right?