Overview
SIST ISO 1438:2018 (ISO 1438:2017) specifies requirements for measuring flow in open channels using thin-plate weirs - principally rectangular and triangular (V‑notch) weirs. It covers use under free flow conditions and includes provisions for full‑width rectangular weirs operating in submerged (drowned) conditions. The standard provides guidance on installation, head measurement, discharge formulae, uncertainty assessment and maintenance for accurate gauging of clear water flows.
Key topics and technical requirements
- Weir types and specifications
- Rectangular thin‑plate weirs (including full‑width forms)
- Triangular (V‑notch) thin‑plate weirs with specified notch‑angle ranges (document includes treatment for notch angles between ≈20° and 100°)
- Installation and site selection
- Requirements for approach and downstream channel geometry, weir mounting and flow straightening
- Guidance on stilling wells, head‑measurement sections and gauge zero determination
- Head measurement
- Instrumentation and placement rules for upstream and downstream gauges; use of stilling/float wells
- Discharge formulae and coefficients
- Standard discharge relations (e.g., Kindsvater‑Carter for rectangular weirs; Kindsvater‑Shen for triangular weirs)
- Treatment for modular and non‑modular flow; special provisions for weir plate heights (e.g., supplemented modular formula for p in the 1.0–2.5 m range)
- Uncertainty and performance
- Methods for combining measurement uncertainties, evaluation of uncertainty of discharge coefficients, and sample uncertainty budgets
- Maintenance and practical considerations
- Routine inspection, calibration and limits on geometrical/operational parameters (head, submergence, approach conditions)
Practical applications and users
This standard is intended for professionals needing reliable open‑channel flow measurement, including:
- Hydrometrists and water resources engineers - gauging rivers, streams and service channels
- Environmental agencies and regulators - compliance monitoring and environmental flow assessments
- Irrigation managers and reservoir/dam operators - allocation, intake monitoring and spill measurements
- Consultants and researchers - design, calibration and uncertainty analysis of gauging structures
Typical applications: permanent gauging stations, temporary weir installations, small‑weir tanks for calibration, and monitoring sites requiring traceable, standardized discharge estimates.
Related standards
- ISO 772 - Hydrometry: vocabulary and symbols (normative reference used by ISO 1438)
- Other hydrometry and flow‑measurement standards and national guidance documents on gauging station design and measurement uncertainty.
Keywords: ISO 1438, SIST ISO 1438, thin‑plate weir, open channel flow measurement, V‑notch weir, rectangular weir, discharge coefficient, head measurement, hydrometry, flow measurement standard, measurement uncertainty.