ISO 3966:2025

Measurement of fluid flow in closed conduits — Velocity area method using Pitot static tubes

This document specifies a method for the determination in a closed conduit of the volume rate of flow of a regular flow a) of a fluid of substantially constant density or corresponding to a Mach number not exceeding 0,25, b) with substantially uniform stagnation temperature across the measuring cross-section, c) running full in the conduit, and d) under steady flow conditions. In particular, it deals with the technology and maintenance of Pitot static tubes, with the calculation of local velocities from measured differential pressures and with the computation of the flow rate by velocity integration.

Mesurage du débit des fluides dans les conduites fermées — Méthode d'exploration du champ des vitesses au moyen de tubes de Pitot doubles

Le présent document spécifie une méthode de détermination du débit-volume d'un écoulement régulier dans une conduite fermée a) d'un fluide de masse volumique sensiblement constante ou correspondant à un nombre de Mach inférieur ou égal à 0,25; b) dont la température d'arrêt est sensiblement uniforme dans toute la section de mesure; c) remplissant complètement la conduite; et d) en régime permanent. Il traite en particulier de la technologie et de l'entretien des tubes de Pitot doubles, du calcul des vitesses locales à partir des pressions différentielles mesurées et du calcul du débit par intégration de ces vitesses.

General Information

• Status: Published
• Publication Date: 02-Jul-2025
• ICS: 17.120.10 - Flow in closed conduits
• Technical Committee: ISO/TC 30/SC 5 - Velocity and mass methods
• Drafting Committee: ISO/TC 30/SC 5 - Velocity and mass methods
• Current Stage: 6060 - International Standard published
• Start Date: 03-Jul-2025
• Due Date: 02-Nov-2025
• Completion Date: 03-Jul-2025

Relations

• Revises: ISO 3966:2020 - Measurement of fluid flow in closed conduits — Velocity area method using Pitot static tubes
• Effective Date: 05-Nov-2022

Standards Content (Sample)

International
Standard
ISO 3966
Fourth edition
Measurement of fluid flow in closed
2025-07
conduits — Velocity area method
using Pitot static tubes
Mesurage du débit des fluides dans les conduites fermées —
Méthode d'exploration du champ des vitesses au moyen de tubes
de Pitot doubles
Reference number
© ISO 2025
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 on
the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address below
or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: [phone]
Email: [e-mail]
Website: www.iso.org
Published in Switzerland
ii
Contents Page
Foreword .v
1 Scope . 1
2 Normative references . 1
3 Terms, definitions and symbols . 1
3.1 Terms and definitions .1
3.2 Symbols .2
4 Principle . 3
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 Measurement of the measuring cross-section .4
4.2.1 Circular cross-sections .4
4.2.2 Rectangular cross-sections .4
4.3 Measurement of local velocities .5
4.3.1 Method of exploring traverse section .5
4.3.2 Reference measurement .5
4.3.3 Checking of velocity distribution .5
4.4 Location and number of measuring points in the cross-section .6
4.4.1 General requirements .6
4.4.2 Circular cross-sections .6
4.4.3 Rectangular cross-sections .6
5 Design of Pitot tubes . 6
5.1 General description .6
5.2 Criteria to be fulfilled by the Pitot tube .7
6 Requirements for use of Pitot tubes . 8
6.1 Selection of the measuring cross-section .8
6.1.1 Location of the measuring cross-section (of selection) .8
6.1.2 Avoidance of asymmetry, swirl and turbulences .8
6.1.3 Maximum flow deviation .8
6.1.4 Measuring cross-sectional velocity distribution .8
6.1.5 Measuring cross-sectional temperature distribution .10
6.1.6 Preliminary traverse tests .10
6.2 Devices for improving flow conditions .10
6.2.1 Anti-swirl device .10
6.2.2 Profile developer . 12
6.2.3 Turning vanes . . . 13
6.2.4 Static mixer .14
6.2.5 Positioning/Location of devices .16
6.2.6 Provisional guiding installation .16
6.3 Limits of use .16
6.3.1 Nature of the fluid .16
6.3.2 Range of velocities .16
6.3.3 Nature of the flow .17
6.3.4 Dimensional limitations.17
6.3.5 Influence of turbulence . .17
6.4 Performance of measurements .17
6.4.1 Measurement of differential pressure .17
6.4.2 Differential pressure fluctuations .17
6.4.3 Determination of fluid density .18
6.5 Inspection and maintenance of the Pitot tube .18
7 Positioning of Pitot tube .18

iii
8 Velocity computation .18
8.1 Verification of conditions for a measurement .18
8.2 Formulae for velocity computation .19
9 Determination of the discharge velocity by graphical integration of the velocity area .21
9.1 Circular cross-section .21
9.2 Rectangular cross-sections . 23
10 Determination of the discharge velocity by numerical integration of the velocity area .24
10.1 Circular cross-sections. 25
10.2 Rectangular cross-sections .27
11 Determination of the discharge velocity by arithmetic methods .27
11.1 “Log-linear” method . 28
11.1.1 Circular cross-sections . 28
11.1.2 Rectangular cross-sections . 28
11.2 Log-Chebyshev method. 29
11.2.1 Circular cross-sections . 29
11.2.2 Rectangular cross-sections . 30
12 Corrections of local velocity measurements .31
12.1 Correction for stem blockage .31
12.1.1 Case where the correction can be neglected .31
12.1.2 Estimation of the correction of local velocity measurement .31
12.1.3 Estimation of the overall correction of the flow-rate value (application to
arithmetic methods) .32
12.2 Correction for transverse velocity gradient.
...

International
Standard
ISO 3966
Fourth edition
Measurement of fluid flow in closed
conduits — Velocity area method
using Pitot static tubes
Mesurage du débit des fluides dans les conduites fermées —
Méthode d'exploration du champ des vitesses au moyen de tubes
de Pitot doubles
PROOF/ÉPREUVE
Reference number
© ISO 2025
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 on
the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address below
or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: [phone]
Email: [e-mail]
Website: www.iso.org
Published in Switzerland
PROOF/ÉPREUVE
ii
Contents Page
Foreword .v
1 Scope . 1
2 Normative references . 1
3 Terms, definitions and symbols . 1
3.1 Terms and definitions .1
3.2 Symbols .2
4 Principle . 3
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 Measurement of the measuring cross-section .4
4.2.1 Circular cross-sections .4
4.2.2 Rectangular cross-sections .4
4.3 Measurement of local velocities .5
4.3.1 Method of exploring traverse section .5
4.3.2 Reference measurement .5
4.3.3 Checking of velocity distribution .5
4.4 Location and number of measuring points in the cross-section .6
4.4.1 General requirements .6
4.4.2 Circular cross-sections .6
4.4.3 Rectangular cross-sections .6
5 Design of Pitot tubes . 6
5.1 General description .6
5.2 Criteria to be fulfilled by the Pitot tube .7
6 Requirements for use of Pitot tubes . 8
6.1 Selection of the measuring cross-section .8
6.1.1 Location of the measuring cross-section (of selection) .8
6.1.2 Avoidance of asymmetry, swirl and turbulences .8
6.1.3 Maximum flow deviation .8
6.1.4 Measuring cross-sectional velocity distribution .8
6.1.5 Measuring cross-sectional temperature distribution .10
6.1.6 Preliminary traverse tests .10
6.2 Devices for improving flow conditions .11
6.2.1 Anti-swirl device .11
6.2.2 Profile developer . 12
6.2.3 Turning vanes . . .14
6.2.4 Static mixer . 15
6.2.5 Positioning/Location of devices .17
6.2.6 Provisional guiding installation .17
6.3 Limits of use .17
6.3.1 Nature of the fluid .17
6.3.2 Range of velocities .17
6.3.3 Nature of the flow .18
6.3.4 Dimensional limitations.18
6.3.5 Influence of turbulence . .18
6.4 Performance of measurements .18
6.4.1 Measurement of differential pressure .18
6.4.2 Differential pressure fluctuations .18
6.4.3 Determination of fluid density .19
6.5 Inspection and maintenance of the Pitot tube .19
7 Positioning of Pitot tube . 19
PROOF/ÉPREUVE
iii
8 Velocity computation . 19
8.1 Verification of conditions for a measurement .19
8.2 Formulae for velocity computation . 20
9 Determination of the discharge velocity by graphical integration of the velocity area .22
9.1 Circular cross-section . 22
9.2 Rectangular cross-sections .24
10 Determination of the discharge velocity by numerical integration of the velocity area .25
10.1 Circular cross-sections. 26
10.2 Rectangular cross-sections . 28
11 Determination of the discharge velocity by arithmetic methods .28
11.1 “Log-linear” method . 29
11.1.1 Circular cross-sections . 29
11.1.2 Rectangular cross-sections . 29
11.2 Log-Chebyshev method. 30
11.2.1 Circular cross-sections . 30
11.2.2 Rectangular cross-sections .31
12 Corrections of local velocity measurements .32
12.1 Correction for stem blockage .32
12.1.1 Case where the correction can be neglected .32
12.1.2 Estimation of the correction of local velocity measurement .32
12.1.3 Estimation of the overall correction of the flow-rate value (application to
arithmetic methods) . 33
12.2 Correction for transverse velocity gradient.
...

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ISO/PRF 3966:2025(en)
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ISO /TC 30/SC 5/WG 8
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Secretariat: SNV
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Date: 2025-04-21xx
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Measurement of fluid flow in closed conduits — Velocity area
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method using Pitot static tubes
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Mesurage du débit des fluides dans les conduites fermées — Méthode d'exploration du champ des vitesses au
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moyen de tubes de Pitot doubles
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PROOF
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ISO/PRF 3966:2025(en)
Formatted: Font: Bold
Formatted: HeaderCentered
Commented [eXtyles1]: The reference "ISO 2025" is to a
withdrawn standard
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication
Formatted: Default Paragraph Font
may be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying,
or posting on the internet or an intranet, without prior written permission. Permission can be requested from either ISO Formatted: Default Paragraph Font
at the address below or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: + 41 22 749 01 11
Formatted: English (United Kingdom)
Formatted: English (United Kingdom)
Fax: [phone]
EmailE-mail: [e-mail]
Formatted: English (United Kingdom)
Website: www.iso.orgwww.iso.org
Formatted: zzCopyright address, Adjust space between
Latin and Asian text, Adjust space between Asian text
Published in Switzerland
and numbers
Formatted: FooterPageRomanNumber
ii
ISO/PRF 3966:2025(en)
Formatted: Font: 11 pt
Formatted: HeaderCentered, Left, Space After: 0 pt,
Line spacing: single, Tab stops: Not at 0.71 cm
Contents
Formatted: Adjust space between Latin and Asian text,
Adjust space between Asian text and numbers
Foreword . vii
1 Scope . 1
2 Normative references . 1
3 Terms, definitions and symbols . 1
3.1 Terms and definitions . 1
3.2 Symbols . 2
4 Principle . 4
4.1 General principle . 4
4.2 Measurement of the measuring cross-section . 5
4.3 Measurement of local velocities . 5
4.4 Location and number of measuring points in the cross-section . 6
5 Design of Pitot tubes . 7
5.1 General description. 7
5.2 Criteria to be fulfilled by the Pitot tube . 8
6 Requirements for use of Pitot tubes . 9
6.1 Selection of the measuring cross-section . 9
6.2 Devices for improving flow conditions . 13
6.3 Limits of use . 18
6.4 Performance of measurements . 19
6.5 Inspection and maintenance of the Pitot tube . 20
7 Positioning of Pitot tube . 20
8 Velocity computation . 21
8.1 Verification of conditions for a measurement . 21
8.2 Formulae for velocity computation . 22
9 Determination of the discharge velocity by graphical integration of the velocity area . 25
9.1 Circular cross-section . 25
9.2 Rectangular cross-sections . 27
10 Determination of the discharge velocity by numerical integration of the velocity area . 29
10.1 Circular cross-sections . 29
10.2 Rectangular cross-sections . 32
11 Determination of the discharge velocity by arithmetic methods . 33
11.1 “Log-linear” method . 33
11.2 Log-Chebyshev method . 35
12 Corrections of local velocity measurements . 37
12.1 Correction for stem blockage . 37
12.2 Correction for transverse velocity gradient . 40
12.3 Correction for turbulence . 43
12.4 Correction for head loss . 43
13 Errors . 43
13.1 Definition of the error . 43
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13.2 Errors in the estimation of the local velocity . 44
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13.3 Errors in the estimation of flow rate . 45
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13.4 Definition of the standard uncertainty . 46
13.5 Definition of the expanded uncertainty . 47 Formatted: Font: 11 pt
13.6 Calculation of standard uncertainty . 47
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iii
ISO/PRF 3966:2025(en)
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Annex A (normative) Pitot tubes . 50
Annex B (normative) Correction to the measuring position of Pitot tubes used in a transverse
velocity gradient . 57
Annex C (normative) Study concerning turbulence correction . 60
Annex D (normative) Damping of pressure gauges . 64
Annex E (normative) Measurements with a Pitot tube in a compressible fluid . 66
Annex F (normative) Determination of coefficient m for extrapolation near the wall . 4
Annex G (informative) Example of calculation of the uncertainty on the flow-rate measurement
by means of Pitot tubes . 6
Bibliography . 10

Foreword . vi
1 Scope . 1
2 Normative references . 1
3 Terms, definitions and symbols . 1
3.1 Terms and definitions . 1
3.2 Symbols . 2
4 Principle . 4
4.1 General principle . 4
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) . 5
4.2 Measurement of the measuring cross-section . 5
4.2.1 Circular cross-sections . 5
4.2.2 Rectangular cross-sections . 5
4.3 Measurement of local velocities . 5
4.3.1 Method of exploring traverse section . 5
4.3.2 Reference measurement . 5
4.3.3 Checking of velocity distribution . 6
4.4 Location and number of measuring points in the cross-section . 6
4.4.1 General requirements . 6
4.4.2 Circular cross-sections .
...