This International Standard deals with the measurement of water flow in closed conduits by using tracer methods; the flow of other liquids and of gases will be dealt with in subsequent International Standards. These methods apply to flow measurement in conduits into which a solution can be injected and those cases where effective mixing of this solution with the water flowing in the conduit can be achieved, this last condition being fundamental. Three fundamental procedures are used : Two procedures, known as the constant rate injection method and the integration (sudden injection) method, are based on the dilution principle : a tracer solution is injected into the conduit and the dilution (ratio) of this tracer in the water flowing in the conduit is determined, this dilution being proportional to the rate of flow, The third is a method of measurement of the mean transit time (formerly called Allen velocity method) : the tracer is injected into the conduit and the time taken by the tracer to travel a specified length between two cross-sections in each of which it is detected, is measured. In these three methods, the advantages and disadvantages of which are considered in clause 4, the distance between injection and measuring sections should be great enough to achieve adequate mixing of the tracer with the water flowing in the conduit; the problem of an adequate mixing distance is considered in clause 6. A large number of different tracers may be used, such as radioactive or non-radioactive, mineral or organic materials. The choice of tracer depends on the circumstances of the measurement (clause 5). The error in measurements using tracers can be less than 1 % under good conditions (clause 7).