ISO INTERNATIONAL STANDARD 3966 Third edition 2020-07 Measurement of fluid flow in closed conduits Velocity area method using Pitot static tubes Mesuragedudebitdesfluidesdanslesconduitesfermees-Methode d'exploration du champdesvitessesaumoyendetubes dePitotdoubles Referencenumber IS0 3966:2020(E) ISO @ISO 2020 IS0 3966:2020(E) COPYRIGHTPROTECTEDDOCUMENT @IS02020 All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting below or Iso's memberbody in the country ofthe requester. ISO copyright office CP 40i : Ch.de Blandonnet 8 CH-1214 Vernier, Geneva Phone: +41 22.749 01 11 Email: [email protected] Website: www.iso.org Published in Switzerland ii @ IS0 2020 - All rights reserved IS0 3966:2020(E) Contents Page Foreword ..V 1 Scope ..1 2 Normative references 3 Terms, definitions and symbols 3.1 Terms and definitions .1 3.2 Symbols 4 Principle 4.1 General principle ..3 4.1.1 Graphical integration of the velocity area (see Clause 9) ..4 4.1.2 Numerical integration of the velocity area (see Clause 10) .4 4.1.3 Arithmetical methods (see Clause 11) .4 4.2 Measurementofthemeasuringcross-section 4 4.2.1 Circular cross-sections .4 4.2.2 Rectangular cross-sections .4 4.3 Measurement of local velocities .5 .5 4.3.1 Method of exploringtraversesection 4.3.2 Referencemeasurement .5 4.3.3 Checking of velocitydistribution 4.4 Location and number of measuring points in the cross-section 6 4.4.1 ..6 Generalrequirements 4.4.2 Circular cross-sections .6 4.4.3 Rectangular cross-sections .6 5 Design of Pitot tubes. ..7 5.1 General description .7 5.2 Criteria to be fulfilled by the Pitot tube .7 6 Requirements foruse of Pitot tubes. ..8 6.1 Selection of the measuring cross-section. .8 6.1.1 Location ofthemeasuringcross-section (of selection) .8 6.1.2 Avoidanceofasymmetry,swirlandturbulences .8 6.1.3 Maximumflowdeviation. .8 6.1.4 Preliminary traverse tests .9 6.2 Devices for improving flow conditions 9 .9 6.2.1 Anti-swirl device. 6.2.2 Profile developer. 9 6.2.3 Positioning/Location of devices ..9 6.2.4 Provisional guiding installation 9 6.3 Limits ofuse .9 6.3.1 Nature of the fluid. .9 6.3.2 Range ofvelocities 9 6.3.3 Nature of theflow .10 6.3.4 Dimensional limitations .10 6.3.5 Influenceof turbulence ..10 6.4 Performanceofmeasurements .10 6.4.1 Measurement of differentialpressure .10 6.4.2 Differentialpressurefluctuations .10 6.4.3 Determination offluid density 6.5 Inspection and maintenance of the Pitot tube ..11 7 Positioning of Pitot tube ..11 8 Velocity computation ..11 8.1 Verification of conditionsfora measurement. ..11 8.2 Formulaefor velocity computation. .12 @IS0 2020 -All rights reserved iii