Dye tracing is tracking and tracing various flows

Fluid dynamics

In physics, fluid dynamics is a sub-discipline of fluid mechanics that deals with fluid flow—the natural science of fluids in motion. It has several subdisciplines itself, including aerodynamics and hydrodynamics...

 using dye

Dye

A dye is a colored substance that has an affinity to the substrate to which it is being applied. The dye is generally applied in an aqueous solution, and requires a mordant to improve the fastness of the dye on the fiber....

 added to the liquid

Liquid

Liquid is one of the three classical states of matter . Like a gas, a liquid is able to flow and take the shape of a container. Some liquids resist compression, while others can be compressed. Unlike a gas, a liquid does not disperse to fill every space of a container, and maintains a fairly...

 in question. The purpose of tracking may be an analysis of the flow itself, of the transport of something by the flow of the objects that convey the flow. It is an evolution of the ages-known float tracing method, which basically consists of throwing a buoyant object into a waterflow to see where it goes or where it emerges.

Dye tracking may be either qualitative, i.e., the presence of particular flow and its estimate, or quantitative, when the amount of the traced dye is measured by special instruments.
Often fluorescent dyes are used for this purpose, especially in the following circumstances

• Insufficient lighting (e.g., sewers or cave waters)
• Precise quantitative data are required, measured by a fluorometer
  Fluorometer
  A fluorometer or fluorimeter is a device used to measure parameters of fluorescence: its intensity and wavelength distribution of emission spectrum after excitation by a certain spectrum of light. These parameters are used to identify the presence and the amount of specific molecules in a medium...
  
  
• Very small amounts of the dye is allowed to be added (1 part per trillion
  Parts-per notation
  In science and engineering, the parts-per notation is a set of pseudo units to describe small values of miscellaneous dimensionless quantities, e.g. mole fraction or mass fraction. Since these fractions are quantity-per-quantity measures, they are pure numbers with no associated units of measurement...
  
   may be detected).

Fluorescein

Fluorescein

Fluorescein is a synthetic organic compound available as a dark orange/red powder soluble in water and alcohol. It is widely used as a fluorescent tracer for many applications....

 is among the first fluorescent dyes, developed in 1871.