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This document specifies the geometry and method of use (installation and operating conditions) of Venturi tubes[1] when they are inserted in a conduit running full to determine the flow rate of the fluid flowing in the conduit. This document also provides background information for calculating the flow rate and is applicable in conjunction with the requirements given in ISO 5167-1. This document is applicable only to Venturi tubes in which the flow remains subsonic throughout the measuring section and where the fluid can be considered as single-phase. In addition, Venturi tubes can only be used uncalibrated in accordance with this standard within specified limits of pipe size, roughness, diameter ratio and Reynolds number, or alternatively they can be used across their calibrated range. This document is not applicable to the measurement of pulsating flow. It does not cover the use of uncalibrated Venturi tubes in pipes sized less than 50 mm or more than 1 200 mm, or where the pipe Reynolds numbers are below 2 × 105. This document deals with the three types of classical Venturi tubes: a) “as cast”; b) machined; c) fabricated (also known as “rough-welded sheet-iron”). A Venturi tube consists of a convergent inlet connected to a cylindrical throat which is in turn connected to a conical expanding section called the divergent section (or alternatively the diffuser). Venturi nozzles (and other nozzles) are dealt with in ISO 5167-3. NOTE       In the USA the classical Venturi tube is sometimes called the Herschel Venturi tube.   [1]   In the USA the classical Venturi tube is sometimes called the Herschel Venturi tube.