Overview
ISO 1438:2017 - Hydrometry - Open channel flow measurement using thin-plate weirs specifies requirements and best practices for measuring flow of clear water in open channels using thin‑plate weirs. The standard covers both rectangular and triangular (V‑notch) thin‑plate weirs under free‑flow conditions and includes provisions for full‑width rectangular weirs in submerged (drowned) flow. It defines installation, head measurement, discharge formulae, uncertainty evaluation and maintenance needed to achieve reliable flow gauging.
Key Topics and Technical Requirements
- Scope and principle: Discharge depends on upstream head, geometry of the weir and approach channel, and an experimentally determined discharge coefficient (C).
- Installation requirements: Site selection, approach channel and downstream conditions, stilling or float well design, and flow straightener guidance to ensure valid hydraulic conditions.
- Head measurement: Specifications for head‑measuring devices, head‑measurement sections, determination of gauge zero, and placement distance upstream of the crest.
- Weir specifications:
- Rectangular thin‑plate weirs - standard and full‑width forms, installation details, and practical limits.
- Triangular (V‑notch) thin‑plate weirs - notch angle, installation and head‑measurement rules.
- Discharge formulae: Includes established empirical formulae such as Kindsvater–Carter (rectangular) and Kindsvater–Shen (triangular) approaches, with guidance for evaluating coefficients and applicable ranges.
- Submerged flow: Requirements and correction factors for drowning/submergence effects on full‑width rectangular weirs.
- Uncertainty and accuracy: Methods to quantify measurement uncertainty, combine component uncertainties (instrumentation, geometric, coefficient), and sample uncertainty budgets.
- Maintenance and verification: Regular inspection, cleaning, and calibration procedures to maintain measurement integrity.
