7. Tracer-Dilution Methods

The tracer-dilution method is capable of measuring both open channel and closed conduit flow. However, possible tracer losses may be more of a problem in open channel flow as discussed previously. Either salts or dyes may be used as tracers. The tracer-dilution method consists of adding a known, strong concentration of tracer solution, C1 (equation 12-2) at a constant rate, to the flow (Schuster, 1970; Collins and Wright, 1964; and University of Newcastle on Tyne, 1964). Then, by chemical analysis, the downstream diluted uniformly mixed concentration, C2, is measured. The solution must be added at a known constant discharge, q.

No measurements of flow section geometry or reach distance are required because the total flow is measured directly. The discharge of the channel flow, Q, is measured by determining C0, C1, C2, and the injection rate, q. These required variables and equation 12-2 show that the dilution method does not need measurement of channel geometry or time measurement. Only the final plateau value or C2, the downstream concentration, must be recorded rather than a complete record of the passing cloud that is needed with the salt-velocity-area method.

Because the concentrated tracer solution must be added to the flow at a constant known rate, positive displacement metering pumps are needed for injection. Also, this method requires a sufficient flow travel length with enough turbulence to thoroughly mix the dye. Required mixing lengths can perhaps be reduced by turbulators or injecting the dye simultaneously at a number of points across the stream, but the injection arrays may need prevalidation by analysis and preliminary measurement runs to assure complete mixing.

If salt solutions are used as tracers, then chemical or conductivity measurement methods are used for detection and concentration measurements. Finely ground salt should be purchased for ease in mixing the solution if selected as the tracer. If dyes are used, then visual color intensity comparison standards may be used. Modern fluorometers can measure dye amounts to one part of dye in a million parts of water and can detect one part in a billion. The human eye cannot detect these minute dilutions, but the dye is quite discernible to the instrument.

The color-dilution method may be used for measuring small, medium, or large flows because the cost of the dye is relatively low. The salt-dilution method is applicable to measuring discharges in turbulent streams of moderate or small size where other methods are impracticable. Excessive quantities of salt are required on large streams.

Tracer methods require special equipment and experienced personnel, and its use is relatively expensive. The injection equipment and electrodes or fluorometers for detecting and measuring the tracer concentration of the resulting downstream diluted flow make this method quite costly compared to other measuring methods.