Overview
ISO 2975-6:1977 - Measurement of water flow in closed conduits - Tracer methods - Part 6: Transit time method using non-radioactive tracers specifies the transit time (Allen velocity) method for measuring water flow in closed pipes by tracking the time delay of a non-radioactive tracer between two detector cross-sections. The standard addresses practical requirements for tracer selection (commonly sodium chloride), injection, detector arrangement (electrodes/flow-cells), data recording and calculation of transit time and flow rate.
Key topics and technical requirements
- Principle: Determine flow rate from the transit time of labelled fluid particles between two detectors a known distance apart. The tracer concentration vs. time curves at each detector are used to compute mean arrival times.
- Tracer: Non-radioactive tracers are specified; sodium chloride is recommended because conductivity is linear with concentration over a wide range and simple electrodes can be used.
- Mixing and injection:
- The tracer must be effectively mixed at the first detector position; the injection-to-first-detector distance should be at least the calculated mixing distance (defined by <0.5% cross-section concentration variation in examples).
- Injection must be rapid to minimize pulse duration and avoid tailing (spring-loaded valves, ampoules, pressure injection or symmetric central injection are suggested).
- Measuring length and detector spacing:
- The section between detectors should preferably be a straight pipe of uniform cross-section and free of fittings that create dead zones.
- Recommendations for detector spacing account for longitudinal dispersion and recorder type: for single-channel recording, detectors must be spaced so recorded pulses do not overlap (practical guidance: p > 1; multi-channel recorders allow shorter spacing, with guidance p > 0.5).
- Example guidance: keep injection-to-first-detector spacing such that 99.7% of the tracer pulse passes.
- Detection and data processing:
- Electrode detectors or flow-cells produce concentration/time distributions. Transit time may be derived from centres of gravity, mid-area (half-area) points, half-height or other characteristic points.
- Corrections may be required for detector response differences and flow-cell transit differences.
- Volume measurement: Accurate internal volume of the measuring section is critical; direct capacity measurement is preferred over relying on drawings.
- Test practice: At least five successive injections are recommended to assess random uncertainty.
Applications
- On-site flow measurement in water distribution, hydraulic machine testing, irrigation systems, and industrial process piping where invasive flowmeters are impractical.
- Situations requiring non-invasive, short-length measurements in closed conduits.
- Useful for calibration, commissioning, troubleshooting and acceptance testing of pipe systems.
Who uses this standard
- Hydraulic engineers, flow-measurement specialists, water utilities, plant operators, calibration laboratories and testing contractors performing tracer-based flow-rate measurements.
Related standards
- ISO 2975 series (Parts I–VII) - Part I: General; Parts II–V: injection/integration methods (radioactive and non-radioactive); Part VII: transit time with radioactive tracers.