Overview
ISO 2975-2:1975 - "Measurement of water flow in closed conduits - Tracer methods - Part 2: Constant rate injection method using non‑radioactive tracers" specifies a practical, non‑radioactive tracer technique for measuring water flow in closed conduits (pipes, sewers, tunnels) where a tracer solution can be injected and thoroughly mixed. The method determines flow by continuously injecting a known tracer volume rate and comparing downstream sample concentrations after the mixing distance is reached.
Key topics and technical requirements
- Principle: Continuous (constant‑rate) injection of a tracer with known injection flow; downstream sampling after adequate mixing; flow computed from known injection rate and measured dilution.
- Mixing distance: Effective mixing is fundamental. The standard defines mixing distance and gives methods to estimate or measure it; insufficient mixing invalidates results.
- Injection duration and steady state: Injection must continue long enough to establish a stable, constant tracer concentration across the measuring cross‑section for several minutes. The minimum injection time is determined from preliminary instantaneous tracer tests or by monitoring sample concentration vs time.
- Tracer and solution preparation: Tracer solutions must be homogeneous and free of undissolved particles. Preparation in a separate container with vigorous mixing is recommended.
- Injection equipment and measurement of injection rate: Devices include positive displacement pumps (preferably driven by synchronous motors), calibrated tanks, propeller or volumetric meters (when calibrated/used appropriately). Injection flow accuracy and leak‑free systems are required and must be included in error analysis.
- Sampling protocol: Typical practice includes upstream/background checks, multiple injected‑solution samples (3–5), and several downstream diluted solution samples (≥5) taken across the measuring cross‑section and over the passage period.
- Analysis methods: Approved non‑radioactive analysis techniques include colorimetric, fluorimetric, and conductimetric methods. The standard discusses colorimetric analysis (example: sodium dichromate detection with diphenylcarbazide reagent).
- Error analysis and accuracy: When mixing and injection measurement meet the standard’s requirements, accuracies around 1% are achievable; better accuracy is possible under optimal conditions. Clause 7 of Part I is referenced for error estimation.
Practical applications
- Measuring discharge in pipes, culverts, sewer mains and closed conduits where conventional flowmeters are impractical.
- Calibration or verification of in‑line flowmeters, hydraulic modeling validation, leak or inflow studies, and flow assessment during temporary installations.
- Situations requiring non‑radioactive tracers for safety, regulatory, or operational reasons.
Who should use this standard
- Water utilities, environmental and civil engineers, flow‑measurement technicians, laboratory analysts, and contractors responsible for pipeline commissioning, flow monitoring, or tracer testing.
Related standards
ISO 2975 series (Part I: General; Part III/IV: radioactive tracer methods; Parts V–VII: integration and transit time methods) - Part 2 complements these by specifying the constant‑rate, non‑radioactive approach for closed‑conduit flow measurement.
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