ISO 2975-7:1977

Overview

ISO 2975-7:1977 specifies the transit time method using radioactive tracers for measuring water flow rate in closed conduits (pipe flow). Part VII of the ISO 2975 series, it describes the principle of labelling fluid parcels with a radioactive tracer, recording concentration/time distributions at two detector cross‑sections spaced a known distance apart, and deriving flow rate from the tracer transit time. The method applies where a tracer solution can be injected and effectively mixed with the conduit flow - adequate mixing being fundamental to valid results.

Key topics and technical requirements

• Principle: Measure the transit time of labelled fluid particles between two detector positions; flow rate Q = V / t where V is volume between detectors and t is transit time.
• Mixing requirement: Tracer must be sufficiently mixed at the first detector so recorded distributions represent mean flow. At low Reynolds numbers (Re < 5 000) mixing is generally inadequate and measurements are not valid.
• Measuring section design:
  • Detector section should be straight, uniform cross‑section and free of fittings, branches, leaks or dead zones.
  • Length between detectors must exceed dispersion criteria to avoid signal overlap; recommended guidelines and graphical relations are provided in the standard.
• Tracer selection:
  • Radioactive tracers are subject to the general tracer criteria (see Part I).
  • Gamma (γ)-emitters are generally preferred to beta (β) because detection through pipe walls is easier and fluid self‑absorption is lower, though β-emitters are easier to handle.
• Injection and detection:
  • Rapid, homogeneous injection is essential (pop‑valves, simultaneous or ampoule breakage, or gas‑pressure injection).
  • Detectors should be matched in geometry and response; background and linear range must be characterized.
  • Transit time is determined from concentration/time curves (centres of gravity, mid‑area, or part‑height characteristic points).
• Procedural controls:
  • Determine internal volume of measuring section accurately.
  • Perform multiple injections (recommended at least five) to quantify random uncertainty.
  • Correct for detector transfer times if using flow cells.

Applications and users

Practical uses include:

• Flow measurement and calibration in water distribution and process pipelines
• Hydrometry and flow metrology laboratories
• Pipeline leak and dynamics studies where non‑intrusive tracer methods are required
• Environmental and research settings requiring accurate volumetric flow determinations in closed conduits

Typical users: municipal water utilities, pipeline operators, fluid flow metrology teams, environmental engineers and research laboratories.

Related standards

ISO 2975 is a multi‑part series. Relevant parts include:

• Part I: General
• Part II/III: Constant rate injection methods (non‑radioactive / radioactive)
• Part IV/V: Integration (sudden injection) methods (non‑radioactive / radioactive)
• Part VI: Transit time method using non‑radioactive tracers

Keywords: ISO 2975-7, transit time method, radioactive tracers, water flow measurement, closed conduits, pipe flow, tracer methods, mixing distance, detector, injection.