ISO INTERNATIONAL STANDARD 3846 Third edition 2008-02-15 Hydrometry Open channel flow measurement using rectangular broad-crested weirs Hydrometrie-Mesure de debit des liquides dans les canaux decouverts au moyen de déversoirs rectangulaires a seuil épais Reference number ISO 3846:2008(E) @ ISO 2008 ISO 3846:2008(E) Contents Page Foreword 1 Scope 1 2 Normative references 3 Terms and definitions... 4 Symbols 5 Installation 5.1 General. 5.2 Selection of site 5.3 Installationconditions 6 General maintenance requirements... 7 Measurement of head(s) 7.1 General. 7.2 Stilling or float well.... 7.3 Zero setting ...... 8 Rectangular broad-crested weirs.... 8.1 Specification for the standard weir.... 8.2 Location of the head gauge section . 8.3 Provision for modular flow .... 9 Discharge relationships. 9.1 Modular flow discharge equation... 9.2 Modular coefficient of discharge .. 9.3 Limitations for operation in the modular flow range ... 9.4 Drowned flow discharge equation .... 9.5 Limitations for operation in the drowned flow range... 10 10 Uncertainties in flow measurement .... 11 10.1 General.. 11 10.2 Statement of the uncertainty of a flow measurement in a channel 10.3 12 10.4 Uncertainty of discharge coefficient u(C) for the broad-crested weir. 13 10.5 Uncertainty budget .... 13 11 Example 13 11.1 General.. 13 11.2 Characteristics Gauging structure..... 14 11.3 Characteristics - -Gauged head instrumentation. 14 11.4 The discharge coefficient .... 14 11.5 The discharge estimate... 14 11.6 Uncertainty statement .... 15 Annex A (informative) Modular limit.... Annex B (informative) Gauged head discharge coefficient and total head discharge coefficient ... 18 Annex C (informative) Introduction to measurement uncertainty.... Annex D (informative) Sample measurement performance for use in hydrometric worked examples.... .26 Bibliography . ..28 ii ISO 3846:2008(E) Foreword IsO (the International Organization for Standardization) is a worldwide federation of national standards bodies (isO member bodies). The work of preparing International Standards is normally carried out through isO technical committees. Each member body interested in a subject for which a technical committee has been non-governmental,inliaisonwithIsO,alsotakepartinthework.IsOcollaboratescloselywiththe International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization. International Standards are drafted in accordance with the rules given in the ISO/lEC Directives, Part 2. adopted by the technical committees are circulated to the member bodies for voting. Publication as an International Standard requires approval by at least 75 % of the member bodies casting a vote. rights. IsO shall not be held responsible for identifying any or all such patent rights. ISO 3846 was prepared by Technical Committee ISO/TC 113, Hydrometry, Subcommittee SC 2, Flow measurement structures. This third edition cancels and replaces the second edition (IsO 3846:1989), of which it constitutes a technical revision. Iso 2008 - All rights reserved ISO 3846:2008(E) INTERNATIONAL STANDARD Hydrometry Open channel flow measurement using rectangular broad-crested weirs 1 Scope This International Standard lays down requirements for the use of rectangular broad-crested weirs for the accuratemeasurementofflowofclearwaterinopenchannelsunderfreeflowconditions 2 Normative references references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies. Iso 772, Hydrometry - Vocabulary and symbols 3 Terms and definitions For the purposes of this document, the terms and definitions given in isO 772 apply. 4 Symbols A m2 area of approach channel b m width of weir crest perpendicular to flow direction c discharge coefficient (gauged head) drowned flow reduction factor fC combined coefficient of discharge Cd discharge coefficient (total head) C, coefficient of velocity E m encoder height from datum eb m