CHAPTER 12 - DISCHARGE MEASUREMENTS USING TRACERS
5. Common Sources of Errors
Tracers should be quite stable as previously mentioned. They should not deposit or react with chemicals in the water or with the pipe walls and their encrustations. Selected tracers should neither fade in sunlight nor be absorbed by open channel beds and their biological growths. These losses of tracer are a common source of discharge measurement error. In open channels, large backflow eddies can delay the dye and impede mixing. It is best to select a reach where large eddies or stagnant pools cannot significantly delay the tracers or affect mixing. The concentration of tracer solutions should be determined relative to needed visual observation or equipment detection sensitivity by careful analysis and verified by trial runs before a program of discharge measurements is undertaken.
Accuracy is also sensitive to how well the center of mass of the tracer clouds is determined with respect to time. First and last visual observations of a tracer cloud are difficult, and the mass center may not be located in the time center of the cloud. With elaborate equipment such as multiport pop valves, turbulators (turbulence-creating devices), complex electrodes, and fluorometers, accuracy can approach +/-1 percent. This degree of accuracy requires using the procedures included in American Society of Mechanical Engineers Performance Test Codes (1992).
For irrigation water, the strict code procedures, quality of procedures, equipment, and instrumentation can be relaxed to produce lower levels of accuracy. The selected accuracy target governs the complexity of needed injection equipment, detection equipment, and the quality of recorded data analysis.
The least accurate method would involve breaking a bottle of dye contained in wire mesh at an upstream station of a long reach at time zero and visually observing and estimating the time that the center of mass of the dye cloud passes the exit. Any simplified procedure must be evaluated for effect on mixing. Prior to a measurement program, equations 12-1 and 12-2 should be used for error analyses in terms of proposed equipment and procedures because they affect the equation variables. These analyses will determine if the simplified measurement procedures produce the selected accuracy target.