1. General

Tracer methods can be used to determine discharge with accuracies that can vary considerably from about +/-1 percent to over 30 percent, depending on the equipment used and the care in applying the techniques. Closed conduit measurements are typically more accurate than open channel measurements because of better area measurements, better tracer dispersion and mixing control, and better measurement of tracer cloud travel times. However, injection of tracers against pressure in pipelines can be a challenge. Finding sufficient length of pipeline above ground for accurate time measurement may be difficult due to high velocities and pipeline fittings. In contrast, mixing can be a problem at low velocities, which often happens in open channels. Tracer methods are included in the American Society of Mechanical Engineers Performance Test Codes (1992) because of their high accuracy potential.