ISO 2975-1:1974

Overview

ISO 2975-1:1974, "Measurement of water flow in closed conduits - Tracer methods - Part 1: General," defines the general principles and practical guidance for measuring water flow in pipes and other closed conduits using tracer techniques. The standard applies where a solution can be injected and effective mixing with the conduit flow can be achieved - a fundamental requirement for reliable tracer-based flow measurement. SI units are used throughout.

Key topics and requirements

• Scope and applicability
  • Applies to water flow measurement in closed conduits; other liquids and gases are treated in other standards.
  • Requires the ability to inject a tracer solution and attain adequate mixing before measurements.
• Choice of method
  • Describes three fundamental procedures: dilution methods (constant-rate injection and integration/sudden injection) and transit-time methods.
  • Discusses advantages and limitations of dilution versus transit-time approaches for different field conditions.
• Choice of tracer
  • Reviews non‑radioactive tracers (e.g., sodium dichromate, sodium chloride, rhodamines, fluorescein) and radioactive tracers (e.g., bromine-82, sodium-24, tritium), with pros/cons for detection sensitivity, toxicity, regulatory needs, and operational logistics.
  • Emphasizes tracer properties: mixability, chemical inertness, detectability at low concentration, minimal impact on flow, and cost.
• Mixing distance and measuring length
  • Defines mixing distance as the minimum distance from injection to measurement where cross‑sectional concentration variation is within acceptable limits.
  • Notes dependence of mixing distance on pipe diameter, Reynolds number, friction factor and allowed concentration variation.
  • Provides practical measures to reduce mixing distance, such as multi‑orifice injectors, multi‑point sampling, high‑velocity jets, upstream pumps/turbines, and use of bends or obstructions to promote turbulence.
• Errors and uncertainty
  • Identifies systematic and random errors pertinent to tracer methods and states that, under good conditions, errors can be less than 1%.

Applications and users

ISO 2975-1 is intended for professionals who need accurate water flow measurement in closed conduits:

• Water utilities and pipeline operators for flow gauging and commissioning
• Environmental and hydraulic engineers conducting field studies
• Researchers and laboratories validating flowmeter performance
• Consultants specifying tracer‑based flow surveys for treatment plants, pumped systems, and closed drainage

Practical uses include selecting an appropriate tracer, determining required measuring lengths and mixing distances, designing injection and sampling layouts, and assessing measurement uncertainty.

Related standards

This document is Part 1 of a series by ISO/TC 30. Subsequent parts cover specific methods and tracer types (e.g., Parts II–VII addressing constant‑rate and integration methods, radioactive and non‑radioactive tracers, and transit‑time techniques). Consult the full ISO 2975 series for detailed procedures and test methods.