ISO 2975-3:1976

Overview

ISO 2975-3:1976 - Measurement of water flow in closed conduits - Tracer methods - Part 3: Constant rate injection method using radioactive tracers - specifies a procedure for measuring flow in pipes and other closed conduits by continuously injecting a radioactive tracer at a known rate and comparing its concentration at the injection point with concentrations sampled downstream. This method applies where a solution can be injected and effective mixing across the pipe cross-section is achieved.

Key topics and technical requirements

• Principle: Flow rate is computed from the known tracer injection volumetric rate and the ratio between tracer concentration in the injected solution and in downstream samples.
• Tracer selection & handling: Radioisotopes must meet general tracer requirements (see Part 1), and handling, storage and transport must comply with statutory radiation regulations.
• Mixing distance & homogeneity: The standard emphasizes determining a mixing distance where cross‑sectional concentration variation is acceptably small (example threshold cited: 0.5%). Preliminary instantaneous injections (dye or tracer) are recommended to estimate mixing and required injection duration.
• Injection system: Use of a positive-displacement pump (often with synchronous motor) is recommended. The injection system must be leak‑free and its injection rate calibrated before and after tests - mean value used if calibrations agree within required accuracy (e.g., ~1%).
• Duration of injection: Long enough to establish a stable concentration at the measuring cross‑section for several minutes; minimal duration determined from preliminary tests.
• Sampling & dilution: Samples required from the conduit (multiple points/times) and from the injected solution. Concentrated injection solutions are often diluted gravimetrically or volumetrically with conduit water so detector count rates fall within the instrument linear range. Density and temperature effects on dilution should be corrected.
• Counting & corrections: Measure background count rate, correct for radioactive decay if NI and N2 are measured at different times, ensure detector thermal equilibrium and account for electronic dead time. Use multiple dilutions and repeat samples to assess random errors.

Practical applications

• Accurate flow measurement in pipelines, culverts and closed conduits where conventional flowmeters cannot be installed or calibrated.
• Situations with opaque or turbulent flows (industrial wastewater, sewer trunks, hydropower penstocks) where tracer dispersion is achievable.
• Short-term field studies, hydrographic investigations, leak detection and calibration checks of existing flow measurement systems.

Who uses this standard

• Water and wastewater utilities
• Environmental and hydrologic engineers
• Pipeline operators and consultants
• Research laboratories and flow measurement specialists

Related standards

ISO 2975 series:

• Part 1: General
• Part 2: Constant rate injection with non‑radioactive tracers
• Part 4/5: Integration (sudden injection) methods (non‑radioactive / radioactive)
• Parts 6/7: Transit time methods (non‑radioactive / radioactive)

Keywords: ISO 2975-3:1976, tracer methods, radioactive tracers, constant rate injection, measurement of water flow in closed conduits, pipe flow, mixing distance, injection rate, sampling, count rate, flow measurement.