EN ISO 5167-4:2003

SLOVENSKI STANDARD
01-januar-2004
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SIST EN ISO 5167-1:1997
SIST EN ISO 5167-1:1997/A1:2001
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Measurement of fluid flow by means of pressure differential devices inserted in circular
cross-section conduits running full - Part 4: Venturi tubes (ISO 5167-4:2003)
Durchflussmessung von Fluiden mit Drosselgeräten in voll durchströmten Leitungen mit
Kreisquerschnitt - Teil 4: Venturirohre (ISO 5167-4:2003)
Mesure de débit des fluides au moyen d'appareils déprimogenes insérés dans des
conduites en charge de section circulaire - Partie 4: Tubes de Venturi (ISO 5167-4:2003)
Ta slovenski standard je istoveten z: EN ISO 5167-4:2003
ICS:
17.120.10 Pretok v zaprtih vodih Flow in closed conduits
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EUROPEAN STANDARD
EN ISO 5167-4
NORME EUROPÉENNE
EUROPÄISCHE NORM
March 2003
ICS 17.120.10 Together with EN ISO 5167-1:2003,
EN ISO 5167-2:2003 and EN ISO 5167-3:2003,
supersedes EN ISO 5167-1:1995
English version
Measurement of fluid flow by means of pressure differential
devices inserted in circular cross-section conduits running full -
Part 4: Venturi tubes (ISO 5167-4:2003)
Mesure de débit des fluides au moyen d'appareils Durchflussmessung von Fluiden mit Drosselgeräten in voll
déprimogènes insérés dans des conduites en charge de durchströmten Leitungen mit Kreisquerschnitt - Teil 4:
section circulaire - Partie 4: Tubes de Venturi (ISO 5167- Venturirohre (ISO 5167-4:2003)
4:2003)
This European Standard was approved by CEN on 20 February 2003.
CEN members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this European
Standard the status of a national standard without any alteration. Up-to-date lists and bibliographical references concerning such national
standards may be obtained on application to the Management Centre or to any CEN member.
This European Standard exists in three official versions (English, French, German). A version in any other language made by translation
under the responsibility of a CEN member into its own language and notified to the Management Centre has the same status as the official
versions.
CEN members are the national standards bodies of Austria, Belgium, Czech Republic, Denmark, Finland, France, Germany, Greece,
Hungary, Iceland, Ireland, Italy, Luxembourg, Malta, Netherlands, Norway, Portugal, Slovakia, Spain, Sweden, Switzerland and United
Kingdom.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION
EUROPÄISCHES KOMITEE FÜR NORMUNG
Management Centre: rue de Stassart, 36  B-1050 Brussels
© 2003 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN ISO 5167-4:2003 E
worldwide for CEN national Members.

CORRECTED  2003-09-03
Foreword
This document (EN ISO 5167-4:2003) has been prepared by Technical Committee ISO/TC 30
"Measurement of fluid flow in closed conduits" in collaboration with CMC.
This European Standard shall be given the status of a national standard, either by publication of an
identical text or by endorsement, at the latest by September 2003, and conflicting national
standards shall be withdrawn at the latest by September 2003.
This document, together with EN ISO 5167-1:2003, EN ISO 5167-2:2003 and EN ISO 5167-3:2003,
supersedes EN ISO 5167-1:1995.
According to the CEN/CENELEC Internal Regulations, the national standards organizations of the
following countries are bound to implement this European Standard: Austria, Belgium, Czech
Republic, Denmark, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy,
Luxembourg, Malta, Netherlands, Norway, Portugal, Slovakia, Spain, Sweden, Switzerland and the
United Kingdom.
Endorsement notice
The text of ISO 5167-4:2003 has been approved by CEN as EN ISO 5167-4:2003 without any
modifications.
NOTE Normative references to International Standards are listed in Annex ZA (normative).
Annex ZA
(normative)
Normative references to international publications
with their relevant European publications
This European Standard incorporates by dated or undated reference, provisions from other
publications. These normative references are cited at the appropriate places in the text and the
publications are listed hereafter. For dated references, subsequent amendments to or revisions of any
of these publications apply to this European Standard only when incorporated in it by amendment or
revision. For undated references the latest edition of the publication referred to applies (including
amendments).
NOTE Where an International Publication has been modified by common modifications, indicated by
(mod.), the relevant EN/HD applies.
Publication Year Title EN Year
ISO 4006 1991 Measurement of fluid flow in closed EN 24006 1993
conduits - Vocabulary and symbols
ISO 5167-1 2003 Measurement of fluid flow by means of EN ISO 5167-1 2003
pressure differential devices inserted in
circular cross-section conduits running
full - Part 1: General principles and
requirements
INTERNATIONAL ISO
STANDARD 5167-4
First edition
2003-03-01
Measurement of fluid flow by means of
pressure differential devices inserted in
circular cross-section conduits running
full —
Part 4:
Venturi tubes
Mesure de débit des fluides au moyen d'appareils déprimogènes
insérés dans des conduites en charge de section circulaire —
Partie 4: Tubes de Venturi
Reference number
ISO 5167-4:2003(E)
©
ISO 2003
ISO 5167-4:2003(E)
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ii © ISO 2003 — All rights reserved

ISO 5167-4:2003(E)
Contents Page
Foreword. iv
Introduction . v
1 Scope. 1
2 Normative references . 2
3 Terms and definitions. 2
4 Principles of the method of measurement and computation. 2
5 Classical Venturi tubes. 3
5.1 Field of application . 3
5.2 General shape . 3
5.3 Material and manufacture. 7
5.4 Pressure tappings. 7
5.5 Discharge coefficient, C . 8
5.6 Expansibility [expansion] factor, ε . 9
5.7 Uncertainty of the discharge coefficient C. 10
5.8 Uncertainty of the expansibility [expansion] factor ε. 10
5.9 Pressure loss. 10
6 Installation requirements . 11
6.1 General. 11
6.2 Minimum upstream and downstream straight lengths for installation between various
fittings and the Venturi tube . 11
6.3 Flow conditioners . 15
6.4 Additional specific installation requirements for classical Venturi tubes . 15
Annex A (informative) Table of expansibility [expansion] factor . 17
Annex B (informative) Classical Venturi tubes used outside the scope of ISO 5167-4. 18
Annex C (informative) Pressure loss in a classical Venturi tube . 22
Bibliography . 24

ISO 5167-4:2003(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
established has the right to be represented on that committee. International organizations, governmental and
non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely with the
International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.
The main task of technical committees is to prepare International Standards. Draft International Standards
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.
Attention is drawn to the possibility that some of the elements of this document may be the subject of patent
rights. ISO shall not be held responsible for identifying any or all such patent rights.
ISO 5167-4 was prepared by Technical Committee ISO/TC 30, Measurement of fluid flow in closed conduits,
Subcommittee SC 2, Pressure differential devices.
This first edition of ISO 5167-4, together with the second edition of ISO 5167-1 and the first editions of
ISO 5167-2 and ISO 5167-3, cancels and replaces the first edition of ISO 5167-1:1991, which has been
technically revised, and ISO 5167-1:1991/Amd.1:1998.
ISO 5167 consists of the following parts, under the general title Measurement of fluid flow by means of
pressure differential devices inserted in circular cross-section conduits running full:
 Part 1: General principles and requirements
 Part 2: Orifice plates
 Part 3: Nozzles and Venturi nozzles
 Part 4: Venturi tubes
iv © ISO 2003 — All rights reserved

ISO 5167-4:2003(E)
Introduction
ISO 5167, divided into four parts, covers the geometry and method of use (installation and operating
conditions) of orifice plates, nozzles and Venturi tubes when they are inserted in a conduit running full to
determine the flowrate of the fluid flowing in the conduit. It also gives necessary information for calculating the
flowrate and its associated uncertainty.
ISO 5167 is applicable only to pressure differential devices in which the flow remains subsonic throughout the
measuring section and where the fluid can be considered as single-phase, but is not applicable to the
measurement of pulsating flow. Furthermore, each of these devices can only be used within specified limits of
pipe size and Reynolds number.
ISO 5167 deals with devices for which direct calibration experiments have been made, sufficient in number,
spread and quality to enable coherent systems of application to be based on their results and coefficients to
be given with certain predictable limits of uncertainty.
The devices introduced into the pipe are called “primary devices”. The term primary device also includes the
pressure tappings. All other instruments or devices required for the measurement are known as “secondary
1)
devices”. ISO 5167 covers primary devices; secondary devices will be mentioned only occasionally.
ISO 5167 is divided into the following four parts.
a) Part 1 of ISO 5167 gives general terms and definitions, symbols, principles and requirements as well as
methods of measurement and uncertainty that are to be used in conjunction with Parts 2 to 4 of ISO 5167.
b) Part 2 of ISO 5167 specifies orifice plates, which can be used with corner pressure tappings, D and D/2
2)
pressure tappings , and flange pressure tappings.
3)
c) Part 3 of ISO 5167 specifies ISA 1932 nozzles , long radius nozzles and Venturi nozzles, which differ in
shape and in the position of the pressure tappings.
4)
d) This part of ISO 5167 specifies classical Venturi tubes .
Aspects of safety are not dealt with in Parts 1 to 4 of ISO 5167. It is the responsibility of the user to ensure
that the system meets applicable safety regulations.

1) See ISO 2186:1973, Fluid flow in closed conduits — Connections for pressure signal transmissions between primary
and secondary elements.
2) Orifice plates with “vena contracta” pressure tappings are not considered in ISO 5167.
3) ISA is the abbreviation for the International Federation of the National Standardizing Associations, which was
succeeded by ISO in 1946.
4) In the USA the classical Venturi tube is sometimes called the Herschel Venturi tube.
INTERNATIONAL STANDARD ISO 5167-4:2003(E)

Measurement of fluid flow by means of pressure differential
devices inserted in circular cross-section conduits running
full —
Part 4:
Venturi tubes
1 Scope
This part of ISO 5167 specifies the geometry and method of use (installation and operating conditions) of
Venturi tubes when they are inserted in a conduit running full to determine the flowrate of the fluid flowing in
the conduit.
This part of ISO 5167 also provides background information for calculating the flowrate and is applicable in
conjunction with the requirements given in ISO 5167-1.
This part of ISO 5167 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, each of these devices
can only be used within specified limits of pipe size, roughness, diameter ratio and Reynolds number. This
part of ISO 5167 is not applicable to the measurement of pulsating flow. It does not cover the use of Venturi
tubes in pipes sized less than 50 mm or more than 1 200 mm, or where the pipe Reynolds numbers are below
2 × 10 .
This part of ISO 5167 deals with the three types of classical Venturi tubes:
a) cast;
b) machined;
c) rough welded sheet-iron.
A Venturi tube is a device which consists of a convergent inlet connected to a cylindrical throat which is in turn
connected to a conical expanding section called the “divergent”. The differences between the values of the
uncertainty of the discharge coefficient for the three types of classical Venturi tube show, on the one hand, the
number of results available for each type of classical Venturi tube and, on the other hand, the more or less
precise definition of the geometric profile. The values are based on data collected many years ago. Venturi
nozzles (and other nozzles) are dealt with in ISO 5167-3.
NOTE 1 Research into the use of Venturi tubes in high-pressure gas [ W 1 MPa ( W 10 bar)] is being carried out at
present (see References [1], [2], [3] in the Bibliography). In many cases for Venturi tubes with machined convergent
sections discharge coefficients which lie outside the range predicted by this part of ISO 5167 by 2 % or more have been
found. For optimum accuracy Venturi tubes for use in gas should be calibrated over the required flowrate range. In high-
pressure gas the use of single tappings (or at most two tappings in each plane) is not uncommon.
NOTE 2 In the USA the classical Venturi tube is sometimes called the Herschel Venturi tube.
ISO 5167-4:2003(E)
2 Normative references
The following referenced documents are indispensable for the application of this document. For dated
references, only the edition cited applies. For undated references, the latest edition of the referenced
document (including any amendments) applies.
ISO 4006:1991, Measurement of fluid flow in closed conduits — Vocabulary and symbols
ISO 5167-1:2003, Measurement of fluid flow by means of pressure differential devices inserted in circular
cross-section conduits running full — Part 1: General principles and requirements
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 4006 and ISO 5167-1 apply.
4 Principles of the method of measurement and computation
The principle of the method of measurement is based on the installation of a Venturi tube into a pipeline in
which a fluid is running full. In a Venturi tube a static pressure difference exists between the upstream section
and the throat section of the device. Whenever the device is geometrically similar to one on which direct
calibration has been made, the conditions of use being the same, the flowrate can be determined from the
measured value of this pressure difference and from a knowledge of the fluid conditions.
The mass flowrate can be determined by the following formula:
C π
qd=∆ερ2p (1)
m 1
1− β
The uncertainty limits can be calculated using the procedure given in Clause 8 of ISO 5167-1:2003.
Similarly, the value of the volume flowrate can be calculated since
q
m
q =
V
ρ
where ρ is the fluid density at the temperature and pressure for which the volume is stated.
Computation of the flowrate, which is a purely arithmetic process, is performed by replacing the different items
on the right-hand side of Equation (1) by their numerical values. Table A.1 gives Venturi tube expansibility
factors (ε). They are not intended for precise interpolation. Extrapolation is not permitted.
The diameters d and D mentioned in Equation (1) are the values of the diameters at working conditions.
Measurements taken at any other conditions should be corrected for any possible expansion or contraction of
the primary device and the pipe due to the values of the temperature and pressure of the fluid during the
measurement.
It is necessary to know the density and the viscosity of the fluid at working conditions. In the case of a
compressible fluid, it is also necessary to know the isentropic exponent of the fluid at working conditions.
2 © ISO 2003 — All rights reserved

ISO 5167-4:2003(E)
5 Classical Venturi tubes
5.1 Field of application
5.1.1 General
The field of application of the classical Venturi tubes dealt with in this part of ISO 5167 depends on the way in
which they are manufactured.
Three types of standard classical Venturi tube are defined according to the method of manufacture of the
internal surface of the entrance cone and the profile at the intersection of the entrance cone and the throat.
These three methods of manufacture are described in 5.1.2 to 5.1.4 and have somewhat different
characteristics.
There are limits to the roughness and Reynolds number for each type which shall be addressed.
5.1.2 Classical Venturi tube with an “as cast” convergent section
This is a classical Venturi tube made by casting in a sand mould, or by other methods which leave a finish on
the surface of the convergent section similar to that produced by sand casting. The throat is machined and the
junctions between the cylinders and cones are rounded.
These classical Venturi tubes can be used in pipes of diameter between 100 mm and 800 mm and with
diameter ratios β between 0,3 and 0,75 inclusive.
5.1.3 Classical Venturi tube with a machined convergent section
This is a classical Venturi tube cast or fabricated as in 5.1.2 but in which the convergent section is machined
as are the throat and the entrance cylinder. The junctions between the cylinders and cones may or may not be
rounded.
These classical Venturi tubes can be used in pipes of diameter between 50 mm and 250 mm and with
diameter ratios β between 0,4 and 0,75 inclusive.
5.1.4 Classical Venturi tube with a rough-welded sheet-iron convergent section
This is a classical Venturi tube normally fabricated by welding. For larger sizes it may not be machined if the
tolerance required in 5.2.4 can be achieved, but in the smaller sizes the throat is machined.
These classical Venturi tubes can be used in pipes of diameter between 200 mm and 1 200 mm and with
diameter ratios β between 0,4 and 0,7 inclusive.
5.2 General shape
5.2.1 Figure 1 shows a section through the centreline of the throat of a classical Venturi tube. The letters
used in the text refer to those shown on Figure 1.
The classical Venturi tube is made up of an entrance cylinder A connected to a conical convergent section B,
a cylindrical throat C and a conical divergent section E. The internal surface of the device is cylindrical and
concentric with the pipe centreline. The coaxiality of the convergent section and the cylindrical throat is
assessed by visual inspection.
5.2.2 The minimum cylinder length, measured from the plane containing the intersection of the cone
frustum B with the cylinder A, may vary as a result of the manufacturing process (see 5.2.8 to 5.2.10). It is,
however, recommended that it be chosen to be equal to D.
ISO 5167-4:2003(E)
The entrance cylinder diameter D shall be measured in the plane of the upstream pressure tappings. The
number of measurements shall be at least equal to the number of pressure tappings (with a minimum of four).
The diameters shall be measured near each pair of pressure tappings, and also between these pairs. The
arithmetic mean value of these measurements shall be taken as the value of D in the calculations.
Diameters shall also be measured in planes other than the plane of the pressure tappings.
No diameter along the entrance cylinder shall differ by more than 0,4 % from the value of the mean diameter.
This requirement is satisfied when the difference in the length of any of the measured diameters complies with
the said requirement with respect to the mean of the measured diameters.
Key
1 conical convergent E
2 cylindrical throat, C
3 conical convergent B
4 entrance cylinder A
5 connecting planes
a
7° u ϕ u 15°
b
Flow direction
c
See 5.4.7
Figure 1 — Geometric profile of the classical Venturi tube
4 © ISO 2003 — All rights reserved

ISO 5167-4:2003(E)
5.2.3 The convergent section B shall be conical and shall have an included angle of 21°± 1° for all types of
classical Venturi tube. It is limited upstream by the plane containing the intersection of the cone frustum B with
the entrance to cylinder A (or their prolongations) and downstream by the plane containing the intersection of
the cone frustum B with the throat C (or their prolongations).
The overall length of the convergent B measured parallel to the centreline of the Venturi tube is therefore
approximately equal to 2,7(D − d).
The convergent section B is blended to the entrance cylinder A by a curvature of radius R , the value of which
depends on the type of classical Venturi tube.
The profile of the convergent section shall be checked by means of a template. The deviation between the
template and the conical section of the convergent section shall not exceed, in any place, 0,004D.
The internal surface of the conical section of the convergent section is taken as being a surface of revolution if
two diameters situated in the same plane perpendicular to the axis of revolution do not differ from the value of
the mean diameter by more than 0,4 %.
It shall be checked in the same way that the joining curvature with a radius R is a surface of revolution.
5.2.4 The throat C shall be cylindrical with a diameter d. It is limited upstream by the plane containing the
intersection of the cone frustum B with the throat C (or their prolongations) and downstream by the plane
containing the intersection of the throat C with the cone frustum E (or their prolongations). The length of the
throat C, i.e the distance between those two planes, shall be equal to d ± 0,03d whatever the type of classical
Venturi tube.
The throat C is connected to the convergent section B by a curvature of radius R and to the divergent section
E by a curvature of radius R . The values of R and R depend on the type of classical Venturi tube.
3 2 3
The diameter d shall be measured very carefully in the plane of the throat pressure tappings. The number of
measurements shall be at least equal to the number of pressure tappings (with a minimum of four). The
diameters shall be measured near each pair of pressure tappings and also between these pairs. The
arithmetic mean value of all these measurements shall be taken as the value of d in the calculations.
Diameters shall also be measured in planes other than the plane of the pressure tappings.
No diameter along the throat shall differ by more than 0,1 % of the value of the mean diameter. This
requirement is satisfied when the difference in the length of any of the measured diameters complies with the
said requirement in respect of the mean of the measured diameters.
The throat of the classical Venturi tube shall be machined or be of equivalent smoothness over the whole of its
length to the surface roughness specified in 5.2.7.
It shall be checked that the joining curvatures into the throat with radii R and R are surfaces of revolution as
2 3
described in 5.2.3. This requirement is satisfied when two diameters, situated in the same plane perpendicular
to the axis of revolution, do not differ from the value of the mean diameter by more than 0,1 %.
The values of the radii of curvature R and R shall be checked by means of a template.
2 3
The deviation between the template and the classical Venturi tube shall evolve in a regular way for each
curvature so that the single maximum deviation that is measured occurs at approximately midway along the
template profile. The value of this maximum deviation shall not exceed 0,02d.
5.2.5 The divergent section E shall be conical and may have an included angle, ϕ, of between 7° and 15°. It
is, however, recommended that an angle between 7° and 8° be chosen. Its smallest diameter shall not be less
than the throat diameter.
5.2.6 A classical Venturi tube is called “truncated” when the outlet diameter of the divergent section is less
than the diameter D and “not truncated” when the outlet diameter is equal to diameter D. The divergent portion
ISO 5167-4:2003(E)
may be truncated by about 35 % of its length without significantly modifying the pressure loss of the device or
its discharge coefficient.
5.2.7 The roughness criterion Ra, of the throat and that of the adjacent curvature shall be as small as
−4
possible and shall always be less than 10 d. The divergent section is rough cast. Its internal surface shall be
clean and smooth. Other parts of the classical Venturi tube have specified roughness limits depending on the
type considered.
5.2.8 The profile of the classical Venturi tube with an “as cast” convergent section has the following
characteristics.
The internal surface of the convergent section B is sand cast. It shall be free from cracks, fissures,
−4
depressions, irregularities and impurities. The roughness criterion Ra for the surface shall be less than 10 D.
The minimum length of the entrance cylinder A shall be equal to the smaller of the following two values:
 D, or
 0,25D + 250 mm (see 5.2.2).
The internal surface of the entrance cylinder A may be left “as cast” provided that it has the same surface
finish as the convergent section B.
The radius of curvature R shall be equal to 1,375D ± 0,275D.
The radius of curvature R shall be equal to 3,625d ± 0,125d.
The length of the cylindrical part of the throat shall be no less than d/3. In addition, the length of the cylindrical
part between the end of the joining curvature R and the plane of the pressure tappings, as well as the length
of the cylindrical part between the plane of the throat pressure tappings and the beginning of the joining
curvature R , shall be no less than d/6 (see also 5.2.4 for the throat length).
The radius of curvature R shall lie between 5d and 15d. Its value shall increase as the divergent angle
decreases. A value close to 10d is recommended.
5.2.9 The profile of the classical Venturi tube with a machined convergent section has the following
characteristics.
The minimum length of the entrance cylinder A shall be equal to D.
The radius of curvature R shall be less than 0,25D and preferably equal to zero.
The radius of curvature R shall be less than 0,25d and preferably equal to zero.
The length of the throat cylindrical part between the end of the curvature R and the plane of the throat
pressure tappings shall be no less than 0,25d.
The length of the throat cylindrical part between the plane of the throat pressure tappings and the beginning of
the joining curvature R shall be no less than 0,3d.
The radius of curvature R shall be less than 0,25d and preferably equal to zero.
The entrance cylinder and the convergent section shall have a surface finish equal to that of the throat (see
5.2.7).
5.2.10 The profile of the classical Venturi tube with a rough-welded sheet-iron convergent section has the
following characteristics.
6 © ISO 2003 — All rights reserved

ISO 5167-4:2003(E)
The minimum length of the entrance cylinder A shall be equal to D.
There shall be no joining curvature between the entrance cylinder A and the convergent section B other than
that resulting from welding.
There shall be no joining curvature between the convergent section B and the throat C other than that
resulting from welding.
There shall be no joining curvature between the throat C and the divergent section E.
The internal surface of the entrance cylinder A and the convergent section B shall be clean and free from
−4
encrustation and welding deposits. It may be galvanized. Its roughness criterion Ra shall be about 5 × 10 D.
The internal welded seams shall be flush with the surrounding surfaces. They shall not be located in the
vicinity of the pressure tappings.
5.3 Material and manufacture
5.3.1 The classical Venturi tube may be manufactured from any material, provided that it is in accordance
with the foregoing description and will remain so during use.
5.3.2 It is also recommended that the convergent section B and the throat C be joined as one part. It is
recommended that in the case of a classical Venturi tube with a machined convergent, the throat and the
convergent section be manufactured from one piece of material. If, however, they are made in two separate
parts they shall be assembled before the internal surface is finally machined.
5.3.3 Particular care shall be given to the centring of the divergent section E on the throat. There shall be
no step in diameters between the two parts.
This can be established by touch before the classical Venturi tube is installed, but after the divergent section
has been assembled with the throat section.
5.4 Pressure tappings
5.4.1 The upstream and throat pressure tappings shall be made in the form of separate pipe wall pressure
tappings interconnected by annular chambers, piezometer rings or, if there are four tappings, a “triple-T”
arrangement (see 5.4.3 of ISO 5167-1:2003).
5.4.2 If d is greater than or equal to 33,3 mm, the diameter of these tappings shall be between 4 mm and
10 mm and moreover shall never be greater than 0,1D for the upstream tappings and 0,13d for the throat
pressure tappings.
If d is less than 33,3 mm, the diameter of the throat pressure tappings shall be between 0,1d and 0,13d and
the diameter of the upstream pressure tappings shall be between 0,1d and 0,1D.
It is recommended that pressure tappings be as small as compatible with the fluid be used (for example, with
its viscosity and cleanness).
5.4.3 At least four pressure tappings shall be provided for the upstream and throat pressure measurements.
The centrelines of the pressure tappings shall meet the centreline of the classical Venturi tube, shall form
equal angles with each other and shall be contained in planes perpendicular to the centreline of the classical
Venturi tube.
5.4.4 At the point of break-through, the hole of the pressure tapping shall be circular. The edges shall be
flush with the pipe wall and free from burrs. If joining curvatures are required, the radius shall not exceed
one-tenth of the diameter of the pressure tapping.
ISO 5167-4:2003(E)
5.4.5 The pressure tappings shall be cylindrical over a length at least 2,5 times the internal diameter of the
tapping, measured from the inner wall of the pipeline.
5.4.6 Conformity of the pressure tappings with the two foregoing requirements is assessed by visual
inspection.
5.4.7 The spacing of a pressure tapping is the distance, measured on a straight line parallel to the
centreline of the classical Venturi tube, between the centreline of the pressure tapping and the reference
planes defined below.
For the classical Venturi tube with an “as cast” convergent section, the spacing between the upstream
pressure tappings situated on the entrance cylinder and the plane of intersection between the prolongations of
the entrance cylinder A and the convergent section B shall be
 0,5D + 0,25D for 100 mm < D < 150 mm, and
 0,5D for 150 mm < D < 800 mm.
−0,25D
For classical Venturi tubes with a machined convergent section and with a rough-welded sheet-iron
convergent, the spacing between the upstream pressure tappings and the plane of intersection between the
entrance cylinder A and the convergent section B (or their prolongations) shall be
0,5D + 0,05D
For all types of classical Venturi tube, the spacing between the plane containing the axes of the points of
break-through of the throat pressure tappings and the intersection plane of the convergent section B and the
throat C (or their prolongations) shall be
0,5d + 0,02d
5.4.8 The area of the free cross-section of the annular chamber of the pressure tappings shall be greater
than or equal to half the total area of the tapping holes connecting the chamber to the pipe.
It is recommended, however, that the chamber section mentioned above be doubled when the classical
Venturi tube is used with a minimum upstream straight length from a fitting causing non symmetrical flow.
5.5 Discharge coefficient, C
5.5.1 Limits of use
Whatever the type of classical Venturi tube, a simultaneous use of extreme values for D, β and Re shall be
D
avoided as otherwise the uncertainties given in 5.7 are likely to be increased.
For installations outside the limits defined in 5.5.2, 5.5.3 and 5.5.4 for D, β and Re it remains necessary to
D
calibrate separately the primary element in its actual conditions of service.
The effects of Re , Ra/D and β on C are not yet sufficiently known for it to be possible to give reliable values of
D
C outside the limits defined for each type of classical Venturi tube. (See Annex B.)
5.5.2 Discharge coefficient of the classical Venturi tube with an “as cast” convergent section
Classical Venturi tubes with an “as cast” convergent section can only be used in accordance with this part of
ISO 5167 when
100 mm u D u 800 mm
0,3 u β u 0,75
5 6
2 × 10 u Re u 2 × 10
D
8 © ISO 2003 — All rights reserved

ISO 5167-4:2003(E)
Under these conditions the value of the discharge coefficient C is
C = 0,984
5.5.3 Discharge coefficient of the classical Venturi tube with a machined convergent section
Classical Venturi tubes with a machined convergent section can only be used in accordance with this part of
ISO 5167 when
50 mm u D u 250 mm
0,4 u β u 0,75
5 6
2 × 10 u Re u 1 × 10
D
Under these conditions the value of the discharge coefficient C is
C = 0,995
5.5.4 Discharge coefficient of the classical Venturi tube with a rough-welded sheet-iron convergent
section
Classical Venturi tubes with a rough-welded sheet-iron convergent section can only be used in accordance
with this part of ISO 5167 when
200 mm u D u 1 200 mm
0,4 u β u 0,7
5 6
2 × 10 u Re u 2 × 10
D
Under these conditions the value of the discharge coefficient C is
C = 0,985
5.6 Expansibility [expansion] factor, ε
The expansibility [expansion] factor, ε, is calculated by means of Equation (2):
2/κκ4 ( −1)/κ
  
κτ 11−−β τ
ε=    (2)
42/κ
  
κτ−−11
1−βτ
  
Equation (2) is applicable only for values of β, D and Re as specified in 5.5.2, 5.5.3 or 5.5.4 as appropriate.
D
Test results for determination of ε are only known for air, steam and natural gas. However, there is no known
objection to using the same formula for other gases and vapours for which the isentropic exponent is known.
However, the formula is applicable only if p /p W 0,75.
2 1
Values of the expansibility [expansion] factor for a range of isentropic exponents, pressure ratios and diameter
ratios are given for convenience in Table A.1. These values are not intended for precise interpolation.
Extrapolation is not permitted.
ISO 5167-4:2003(E)
5.7 Uncertainty of the discharge coefficient C
5.7.1 Classical Venturi tube with an “as cast” convergent section
The relative uncertainty of the discharge coefficient as given in 5.5.2 is equal to 0,7 %.
5.7.2 Classical Venturi tube with a machined convergent section
The relative uncertainty of the discharge coefficient as given in 5.5.3 is equal to 1 %.
5.7.3 Classical Venturi tube with a rough-welded sheet-iron convergent section
The relative uncertainty of the discharge coefficient as given in 5.5.4 is equal to 1,5 %.
5.8 Uncertainty of the expansibility [expansion] factor ε
The relative uncertainty of ε is equal to
∆p
41+ 00β %
()
p
5.9 Pressure loss
5.9.1 Definition of the pressure loss (see Figure 2)
The pressure loss caused by a classical Venturi tube may be determined by pressure measurements made
prior and subsequent to the installation of the Venturi tube in a pipe through which there is a given flow.
If ∆p' is the difference in pressure, measured prior to the installation of the Venturi tube, between two pressure
tappings one of which is situated at least D upstream of the flanges where the Venturi tube will be inserted
and the other of which is 6D downstream of the same flanges, and if ∆p" is the difference in pressure
measured between the same pressure tappings after installation of the Venturi tube between these flanges,
then the pressure loss caused by the Venturi tube is given by ∆p" − ∆p'.
5.9.2 Relative pressure loss
The relative pressure loss, ξ, is the ratio of the pressure loss ∆p" − ∆p' to the differential pressure ∆p:
∆−p'' ∆p'
ξ =
∆p
It depends, in particular, on
 the diameter ratio (ξ decreases when β increases);

 the Reynolds number (ξ decreases when Re increases);
D
 the manufacturing characteristics of the Venturi tube: angle of the divergent, manufacturing of the
convergent, surface finish of the different parts, etc (ξ increases when ϕ and Ra/D increase);
 the installation conditions (good alignment, roughness of the upstream conduit, etc).
For guidance, the value of the relative pressure loss can be accepted as being generally between 5 % and
20 %.
10 © ISO 2003 — All rights reserved

ISO 5167-4:2003(E)
Annex C gives, for guidance only, some information on the effect of these different factors on the values the
pressure loss ξ is likely to have.

a
Pressure loss
b
Direction of flow
Figure 2 — Pressure loss across a classical Venturi tube
6 Installation requirements
6.1 General
General installation requirements for pressure differential devices are contained in Clause 7 of
ISO 5167-1:2003 and should be followed in conjunction with the additional specific installation requirements
for Venturi tubes given in this clause. The general requirements for flow conditions at the primary device are
given in 7.3 of ISO 5167-1:2003. The requirements for use of a flow conditioner are given in 7.4 of
ISO 5167-1:2003. For some commonly used fittings as specified in Table 1 the minimum straight lengths of
pipe indicated may be used. Detailed requirements are given in 6.2. Many of the lengths given in 6.2 are
based on data included in Reference [4] of the Bibliography.
6.2 Minimum
...