Measurement of flow by dilution methods depends on the determination of the
degree of dilution of an added tracer solution by the flow. Dilution methods of
measuring discharge have been known since at least 1863 [Ref. 11]. Until
recently, chemical salts [Ref. 8] were the most commonly used tracers. Radioactive
tracers have been used successfully, but handling problems have limited widespread
use [Ref. 9]. The development of stable fluorescent dyes and fluorometers
that can measure them at very low concentrations has greatly enhanced the use of
dilution methods [Refs. 5, 7]. Hence, the use of fluorescent dyes as the tracer is
addressed, although the principles discussed apply to any tracer.

Typical examples of situations where the dilution methods might be used are
turbulent mountain streams, pipes, canals, sewers, ice-covered streams, and sand channel streams.

Theory

Measurement of the degree of dilution of a known quantity of tracer after its
mixing in a flowing stream of water is the basis of dilution gaging. There are two
main approaches: (1) the slug-injection of a known amount of tracer into the flow,
which requires that the dilution of the tracer be accounted for by the complete
measurement of its mass downstream (for this reason, it is sometimes referred to
as the total recovery method) and (2) the constant-rate injection of a tracer solution
into the flow, which requires only the measurement of the plateau level of
concentration that results downstream after equilibrium has been reached. The
principles are simple, yet their successful application in streams, canals, pipes,
and elsewhere requires a good understanding of the dispersion process. No
elaborate theoretical treatment is used in explaining these processes; however, the
reader is urged to heed the following principles, for they can eliminate many of the
problems others have had in performing such measurements.