EN ISO 5167-1:2022

SLOVENSKI STANDARD
01-september-2022
Nadomešča:
SIST EN ISO 5167-1:2004
Merjenje pretoka fluida na osnovi tlačne razlike, povzročene z napravo, vstavljeno
v polno zapolnjen vod s krožnim prerezom – 1. del: Splošna načela in zahteve (ISO
5167-1:2022)
Measurement of fluid flow by means of pressure differential devices inserted in circular
cross-section conduits running full - Part 1: General principles and requirements (ISO
5167-1:2022)
Durchflussmessung von Fluiden mit Drosselgeräten in voll durchströmten Leitungen mit
Kreisquerschnitt - Teil 1: Allgemeine Grundlagen und Anforderungen (ISO 5167-1:2022)
Mesurage de débit des fluides au moyen d'appareils déprimogènes insérés dans des
conduites en charge de section circulaire - Partie 1: Principes généraux et exigences
générales (ISO 5167-1:2022)
Ta slovenski standard je istoveten z: EN ISO 5167-1:2022
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.

EN ISO 5167-1
EUROPEAN STANDARD
NORME EUROPÉENNE
June 2022
EUROPÄISCHE NORM
ICS 17.120.10 Supersedes EN ISO 5167-1:2003
English Version
Measurement of fluid flow by means of pressure
differential devices inserted in circular cross-section
conduits running full - Part 1: General principles and
requirements (ISO 5167-1:2022)
Mesurage de débit des fluides au moyen d'appareils Durchflussmessung von Fluiden mit Drosselgeräten in
déprimogènes insérés dans des conduites en charge de voll durchströmten Leitungen mit Kreisquerschnitt -
section circulaire - Partie 1: Principes généraux et Teil 1: Allgemeine Grundlagen und Anforderungen (ISO
exigences générales (ISO 5167-1:2022) 5167-1:2022)
This European Standard was approved by CEN on 14 June 2022.

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 CEN-CENELEC 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 CEN-CENELEC Management
Centre has the same status as the official versions.

CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia,
Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway,
Poland, Portugal, Republic of North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Turkey and
United Kingdom.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION

EUROPÄISCHES KOMITEE FÜR NORMUNG

CEN-CENELEC Management Centre: Rue de la Science 23, B-1040 Brussels
© 2022 CEN All rights of exploitation in any form and by any means reserved Ref. No. EN ISO 5167-1:2022 E
worldwide for CEN national Members.

Contents Page
European foreword . 3

European foreword
This document (EN ISO 5167-1:2022) has been prepared by Technical Committee ISO/TC 30
"Measurement of fluid flow in closed conduits" in collaboration with Technical Committee CEN/SS F05
“Measuring Instruments” the secretariat of which is held by CCMC.
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 December 2022, and conflicting national standards
shall be withdrawn at the latest by December 2022.
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. CEN shall not be held responsible for identifying any or all such patent rights.
This document supersedes EN ISO 5167-1:2003.
Any feedback and questions on this document should be directed to the users’ national standards
body/national committee. A complete listing of these bodies can be found on the CEN website.
According to the CEN-CENELEC Internal Regulations, the national standards organizations of the
following countries are bound to implement this European Standard: Austria, Belgium, Bulgaria,
Croatia, Cyprus, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland,
Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Republic of
North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Turkey and the
United Kingdom.
Endorsement notice
The text of ISO 5167-1:2022 has been approved by CEN as EN ISO 5167-1:2022 without any
modification.
INTERNATIONAL ISO
STANDARD 5167-1
Third edition
2022-06
Measurement of fluid flow by means of
pressure differential devices inserted
in circular cross-section conduits
running full —
Part 1:
General principles and requirements
Mesurage de débit des fluides au moyen d'appareils déprimogènes
insérés dans des conduites en charge de section circulaire —
Partie 1: Principes généraux et exigences générales
Reference number
ISO 5167-1:2022(E)
ISO 5167-1:2022(E)
© ISO 2022
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
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ISO copyright office
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CH-1214 Vernier, Geneva
Phone: +41 22 749 01 11
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
ii
ISO 5167-1:2022(E)
Contents Page
Foreword .v
Introduction . vi
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
3.1 Pressure measurement . 2
3.2 Primary devices . . 2
3.3 Flow . 3
4 Symbols and subscripts . 6
4.1 Symbols . 6
5 Principle of the method of measurement and computation . 7
5.1 Principle of the method of measurement . 7
5.2 Method of determination of the required diameter ratio for the selected standard
primary device . 8
5.3 Computation of flow rate . . 8
5.4 Determination of density, pressure and temperature . 8
5.4.1 General . 8
5.4.2 Density . 9
5.4.3 Static pressure . 9
5.4.4 Temperature . 9
5.5 Differential pressure flow measurement system . 10
5.5.1 General . 10
5.5.2 Primary device . 11
5.5.3 Impulse lines and transmitters .12
5.5.4 Impulse line isolation valves and valve manifolds .12
5.5.5 Flow computer . .12
5.6 Differential pressure flow measurement system design considerations .12
5.6.1 Flow rate turndown and stacked transmitters .12
5.6.2 Meter calibration .12
5.6.3 Permanent pressure loss . 13
5.6.4 Diagnostics and meter verification . 14
5.6.5 Overall uncertainty of differential pressure metering system . 14
6 General requirements for the measurements .14
6.1 Primary device . 14
6.2 Nature of the fluid. 15
6.3 Flow conditions . 15
7 Installation requirements .15
7.1 General . 15
7.2 Minimum upstream and downstream straight lengths . 17
7.3 General requirement for flow conditions at the primary device . 17
7.3.1 Requirement . 17
7.3.2 Swirl-free conditions . 17
7.3.3 Good velocity profile conditions . 17
7.4 Flow conditioners . 17
7.4.1 Compliance testing . 17
7.4.2 Specific test .20
8 Uncertainties on the measurement of flow rate .20
8.1 General . 20
8.2 Definition of uncertainty .20
8.3 Practical computation of the uncertainty . 21
8.3.1 Component uncertainties . 21
iii
ISO 5167-1:2022(E)
8.3.2 Practical working formula . 21
Annex A (informative) Iterative computations .24
Annex B (informative) Examples of values of the pipe wall uniform equivalent roughness, k .26
a
Annex C (informative) Flow conditioners and flow straighteners .27
Annex D (informative) Differential pressure transmitters, flow range and turndown .29
Annex E (informative) Example of uncertainty calculation for a differential pressure device .36
Annex F (informative) Permanent pressure loss example .40
Bibliography .42
iv
ISO 5167-1:2022(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.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www.iso.org/directives).
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. Details of
any patent rights identified during the development of the document will be in the Introduction and/or
on the ISO list of patent declarations received (see www.iso.org/patents).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and
expressions related to conformity assessment, as well as information about ISO's adherence to
the World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT), see
www.iso.org/iso/foreword.html.
ISO 5167-1 was prepared by Technical Committee ISO/TC 30, Measurement of fluid flow in closed conduits,
Subcommittee SC 2, Pressure differential devices, in collaboration with the European Committee for
Standardization (CEN) Technical Committee CEN/SS F05, Measuring instruments, in accordance with
the Agreement on technical cooperation between ISO and CEN (Vienna Agreement).
This third edition cancels and replaces the second edition (ISO 5167-1:2003), which has been technically
revised
The main changes are as follows:
— improved consistency between ISO 5167-1 to ISO 5167-6 (some items that were new in ISO 5167-5
and ISO 5167-6 have been moved to this document);
— a primary element has been set as part of a differential pressure metering system;
— a short section on diagnostics and CBM (Condition Based Monitoring) has been included;
— a limitation on the use of the 5 % 2° rule for an acceptable profile has been noted;
— improved text about uncertainty calculation and an example in Annex E has been provided;
— annexes on turndown and permanent pressure loss have been included.
A list of all parts in the ISO 5167 series can be found on the ISO website.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.
v
ISO 5167-1:2022(E)
Introduction
ISO 5167, consisting of six parts, covers the geometry and method of use (installation and operating
conditions) of orifice plates, nozzles, Venturi tubes, cone meters and wedge meters when they are
inserted in a conduit running full to determine the flow rate of the fluid flowing in the conduit. It also
gives necessary information for calculating the flow rate and its associated uncertainty.
ISO 5167 (all parts) 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 uncalibrated within specified limits of pipe size and Reynolds number, or alternatively they can be
used across their calibrated range.
ISO 5167 (all parts) 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. ISO 5167 also
provides methodology for bespoke calibration of differential pressure meters.
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 to facilitate the instrument readings
are known as secondary devices, and the flow computer that receives these readings and performs
the algorithms is known as a tertiary device. ISO 5167 covers primary devices; secondary devices (see
ISO 2186) and tertiary devices will be mentioned only occasionally.
Aspects of safety are not dealt with in ISO 5167-1 to ISO 5167-6. It is the responsibility of the user to
ensure that the system meets applicable safety regulations.
Additional documents that may provide assistance include:
— ISO/TR 3313;
— ISO/TR 9464;
— ISO/TR 12767;
— ISO/TR 15377.
vi
INTERNATIONAL STANDARD ISO 5167-1:2022(E)
Measurement of fluid flow by means of pressure
differential devices inserted in circular cross-section
conduits running full —
Part 1:
General principles and requirements
1 Scope
This document defines terms and symbols and establishes the general principles for methods of
measurement and computation of the flow rate of fluid flowing in a conduit by means of pressure
differential devices (orifice plates, nozzles, Venturi tubes, cone meters, and wedge meters) when they
are inserted into a circular cross-section conduit running full. This document also specifies the general
requirements for methods of measurement, installation and determination of the uncertainty of the
measurement of flow rate.
ISO 5167 (all parts) is applicable only to flow that remains subsonic throughout the measuring section
and where the fluid can be considered as single-phase. It is not applicable to the measurement of
pulsating flow.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements 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, Measurement of fluid flow in closed conduits — Vocabulary and symbols
ISO 5167 (all parts), Measurement of fluid flow by means of pressure differential devices inserted in circular
cross-section conduits running full
ISO 5168, Measurement of fluid flow — Procedures for the evaluation of uncertainties
ISO/IEC Guide 98-3, Uncertainty of measurement — Part 3: Guide to the expression of uncertainty in
measurement (GUM: 1995)
3 Terms and definitions
For the purposes of this document, the terms, definitions and symbols given in ISO 4006 and the
following apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
ISO 5167-1:2022(E)
3.1 Pressure measurement
3.1.1
wall pressure tapping
annular slot or circular hole drilled in the wall of a conduit in such a way that the edge of the hole is
flush with the internal surface of the conduit
Note 1 to entry: The pressure tapping is usually a circular hole but in certain cases may be an annular slot.
3.1.2
static pressure
p
pressure which can be measured by connecting a pressure-measuring device to a wall pressure tapping
(3.1.1)
Note 1 to entry: Only the value of the absolute static pressure is considered in ISO 5167 (all parts).
3.1.3
differential pressure
DP
Δp
difference between the (static) pressures measured at the wall pressure tappings, one of which is on
the upstream side and the other of which is on the downstream side of a primary device [or in the
throat for a throat-tapped nozzle, a Venturi nozzle (3.2.4) or a Venturi tube (3.2.5)], inserted in a straight
pipe through which flow occurs, when any difference in height between the upstream and downstream
tappings has been taken into account
Note 1 to entry: In ISO 5167 (all parts) the term “differential pressure” is used only if the pressure tappings are in
the positions specified for each standard primary device.
3.1.4
pressure ratio
τ
ratio of the absolute (static) pressure at the downstream pressure tapping to the absolute (static)
pressure at the upstream pressure tapping
3.1.5
vena contracta
location in a fluid stream where the diameter of the stream is smallest
3.2 Primary devices
3.2.1
orifice
throat opening of minimum cross-sectional area of a primary device
3.2.2
orifice plate
thin plate in which a circular opening has been machined
Note 1 to entry: Standard orifice plates are described as “thin plate” and “with sharp square edge”, because the
thickness of the plate is small compared with the diameter of the measuring section and because the upstream
edge of the orifice (3.2.1) is sharp and square.
3.2.3
nozzle
device which consists of a convergent inlet connected to a cylindrical section generally called the
“throat”
ISO 5167-1:2022(E)
3.2.4
Venturi nozzle
device which consists of a convergent inlet which is a standardized ISA 1932 nozzle connected to a
cylindrical part called the “throat”, which is itself connected to an expanding section called the
“divergent” which is conical
3.2.5
Venturi tube
device which consists of a convergent inlet which is conical connected to a cylindrical part called the
“throat”, which is itself connected to an expanding section called the “divergent” which is conical
3.2.6
cone meter
device which consists of a cone-shaped restriction held in the centre of the pipe with the nose of the
cone upstream
3.2.7
wedge meter
device which consists of a wedge-shaped restriction
3.2.8
diameter ratio
β
square root of the ratio of the area of the throat of the
primary device to the internal area of the measuring pipe upstream of the primary device
Note 1 to entry: In ISO 5167-2 and ISO 5167-3 the diameter ratio is the ratio of the diameter of the throat of the
primary device to the internal diameter of the measuring pipe upstream of the primary device.
Note 2 to entry: In ISO 5167-4, where the primary device has a cylindrical section upstream, having the same
diameter as that of the pipe, the diameter ratio is the ratio of the throat diameter to the diameter of this cylindrical
section at the plane of the upstream pressure tappings.
3.2.9
carrier ring
device which is used to hold the primary element in the centre of the pipe and may incorporate the
pressure tappings
3.3 Flow
3.3.1
flow rate
rate of flow
q
mass or volume of fluid passing through the primary device per unit time
3.3.1.1
mass flow rate
rate of mass flow
q
m
mass of fluid passing through the primary device per unit time
3.3.1.2
volume flow rate
rate of volume flow
q
V
volume of fluid passing through the primary device per unit time
Note 1 to entry: In the case of volume flow rate, it is necessary to state the pressure and temperature at which the
volume is referenced.
ISO 5167-1:2022(E)
3.3.2
Reynolds number
Re
dimensionless parameter expressing the ratio between the inertia and viscous forces
3.3.2.1
pipe Reynolds number
Re
D
dimensionless parameter expressing the ratio between the inertia and viscous forces in the upstream
pipe
VD 4q
1 m
Re ==
D
νμπ D
3.3.2.2
throat Reynolds number
Re
d
dimensionless parameter expressing the ratio between the inertia and viscous forces in the orifice or
throat of the primary device
Re
D
Re =
d
β
Note 1 to entry: When an orifice plate is used the throat Reynolds number is sometimes called the orifice
Reynolds number.
3.3.3
isentropic exponent
κ
ratio of the relative variation in pressure to the corresponding relative variation in density under
elementary reversible adiabatic (isentropic) transformation conditions
Note 1 to entry: The isentropic exponent κ appears in the different formulae for the expansibility [expansion]
factor ε and varies with the nature of the gas and with its temperature and pressure.
Note 2 to entry: There are many gases and vapours for which no values for κ have been published so far,
particularly over a wide range of pressure and temperature. In such a case, for the purposes of ISO 5167 (all parts),
the ratio of the specific heat capacity at constant pressure to the specific heat capacity at constant volume of
ideal gases can be used in place of the isentropic exponent.
3.3.4
Joule Thomson coefficient
isenthalpic temperature-pressure coefficient
μ
JT
rate of change of temperature with respect to pressure at constant enthalpy:
∂T
μ =
JT
∂p
H
Note 1 to entry: The Joule Thomson coefficient varies with the nature of the gas and with its temperature and
pressure and can be calculated.
Note 2 to entry: An approximation for the Joule Thomson coefficient for some natural gases is given in
ISO/TR 9464:2008, 5.1.5.4.4.
ISO 5167-1:2022(E)
3.3.5
discharge coefficient
C
coefficient, defined for an incompressible fluid flow, which relates the actual flow rate to the theoretical
flow rate through a device, and is given by the formula for incompressible fluids
q 1−β
m
C =
Ap2Δ ρ
t 1
Note 1 to entry: Calibration of standard primary devices by means of incompressible fluids (liquids) shows
that the discharge coefficient is dependent only on the Reynolds number for a given primary device in a given
installation.
The numerical value of C for any individual differential pressure meter is the same for different installations
whenever such installations are geometrically similar and the flows are characterised by identical Reynolds
numbers.
The formulae for the numerical values of C given in ISO 5167 (all parts) are based on data determined
experimentally.
The uncertainty in the value of C can be reduced by flow calibration in a suitable laboratory.
Note 2 to entry: The quantity 11/ −β is called the “velocity of approach factor”, and C is called the
1−β
“flow coefficient”.
3.3.6
expansibility [expansion] factor
ε
coefficient used to take into account the compressibility of the fluid
q 1−β
m
ε =
AC 2Δpρ
t 1
Note 1 to entry: Calibration of a given primary device by means of a compressible fluid (gas) shows that the
following ratio is dependent on the value of the Reynolds number as well as on the values of the pressure ratio
and the isentropic exponent of the gas:
q 1−β
m
Ap2Δ ρ
t 1
The method adopted for representing these variations consists of multiplying the discharge coefficient, C, of the
primary device considered, as determined by direct calibration carried out with liquids for the same value of the
Reynolds number, by the expansibility [expansion] factor, ε.
The expansibility factor, ε, is equal to unity when the fluid is considered incompressible (liquid) and is less than
unity when the fluid is compressible (gaseous).
This method is possible because experiments show that ε is practically independent of the Reynolds number
and, for a given diameter ratio of a given primary device, ε only depends on the pressure ratio and the isentropic
exponent.
The numerical values of ε for orifice plates given in ISO 5167-2 and for cone meters given in ISO 5167-5 are based
on data determined experimentally. For nozzles (see ISO 5167-3), Venturi tubes (see ISO 5167-4) and wedge
meters (see ISO 5167-6) they are based on the thermodynamic general formula applied to isentropic expansion.
ISO 5167-1:2022(E)
3.3.7
arithmetical mean deviation of the roughness profile
Ra
arithmetical mean deviation from the mean line of the profile being measured
Note 1 to entry: The mean line is such that the sum of the squares of the distances between the effective surface
and the mean line is a minimum. In practice, Ra can be measured with standard equipment for machined surfaces
but can only be estimated for rougher surfaces of pipes. See also ISO 21920-3.
Note 2 to entry: For pipes, the uniform equivalent roughness k may also be used. This value can be determined
a
experimentally (see 7.1.5) or taken from tables (see Annex B).
4 Symbols and subscripts
4.1 Symbols
Table 1 — Symbols
a
Symbol Quantity Dimension SI unit
2 2
A Area of throat L m
t
C Discharge coefficient dimensionless —
2 −2 −1 −1
C Molar-heat capacity at constant pressure ML T Θ mol J/(mol⋅K)
m,p
Diameter of orifice (or throat) of primary device under
d L m
working conditions
Upstream internal pipe diameter (or upstream diameter of a
D L m
classical Venturi tube) under working conditions
2 −2 −1
H Enthalpy ML T mol J/mol
k Coverage factor dimensionless —
k Uniform equivalent roughness L m
a
Pressure loss coefficient (the ratio of the pressure loss, Δϖ,
K to the dynamic pressure, ρV /2), also known as the minor dimensionless —
loss coefficient
l Pressure tapping spacing L m
L Relative pressure tapping spacing: L = l/D dimensionless —
−1 −2
p Absolute static pressure of the fluid ML T Pa
−1
q Mass flow rate MT kg/s
m
3 −1 3
q Volume flow rate L T m /s
V
R Radius L m
2 −2 −1 −1
R Universal gas constant ML T Θ mol J/(mol⋅K)
u
Ra Arithmetical mean deviation of the (roughness) profile L m
Re Reynolds number dimensionless —
Re Throat Reynolds number dimensionless —
d
Re Pipe Reynolds number dimensionless —
D
t Temperature of the fluid Θ °C
T Absolute (thermodynamic) temperature of the fluid Θ K
c c
u Standard uncertainty
a
M = mass, L = length, T = time, Θ = temperature
b
γ is the ratio of the specific heat capacity at constant pressure to the specific heat capacity at constant volume.
For ideal gases, the ratio of the specific heat capacities and the isentropic exponent have the same value (see 3.3.3). These
values depend on the nature of the gas.
The dimensions and units are those of the corresponding quantity.
c
ISO 5167-1:2022(E)
Table 1 (continued)
a
Symbol Quantity Dimension SI unit
′ Relative standard uncertainty dimensionless —
u
c c
U Expanded uncertainty
′ Relative expanded uncertainty dimensionless —
U
−1
V Mean axial velocity of the fluid in the pipe LT m/s
Z Compressibility factor dimensionless —
β Diameter ratio dimensionless —
b
γ Ratio of specific heat capacities dimensionless —
−1 −2
Δp Differential pressure: Δp = p − p ML T Pa
1 2
−1 −2
Δp Pressure loss across a flow conditioner ML T Pa
c
−1 −2
Δϖ Pressure loss across a primary device ML T Pa
ε Expansibility [expansion] factor dimensionless —
b
κ Isentropic exponent dimensionless —
λ Friction factor dimensionless —
−1 −1
μ Dynamic viscosity of the fluid ML T Pa⋅s
−1 2
μ Joule Thomson coefficient M LT Θ K/Pa
JT
2 −1 2
v Kinematic viscosity of the fluid: v = μ/ρ L T m /s
Relative pressure loss (the ratio of the pressure loss to the
ξ dimensionless —
differential pressure)
−3 3
ρ Density of the fluid ML kg/m
τ Pressure ratio: τ = p /p dimensionless —
2 1
ϕ Total angle of the divergent section dimensionless rad
a
M = mass, L = length, T = time, Θ = temperature
b
γ is the ratio of the specific heat capacity at constant pressure to the specific heat capacity at constant volume.
For ideal gases, the ratio of the specific heat capacities and the isentropic exponent have the same value (see 3.3.3). These
values depend on the nature of the gas.
The dimensions and units are those of the corresponding quantity.
c
5 Principle of the method of measurement and computation
5.1 Principle of the method of measurement
The principle of the method of measurement is based on the installation of a primary device into a
pipeline in which a fluid is running full. The installation of the primary device causes a static pressure
difference between the upstream side and the throat or downstream side of the device. The flow rate
can be determined from the measured value of this pressure difference and the knowledge of the
thermodynamic conditions, fluid properties, meter geometry and meter characteristics. It is assumed
that an uncalibrated differential pressure meter is within the geometric and Reynolds number range
required for the ISO discharge coefficient prediction to be valid. Alternatively, it is assumed that a
bespoke calibrated differential pressure meter is to be used within its calibration range.
The mass flow rate can be determined, since it is related to the differential pressure within the
uncertainty limits stated in ISO 5167, using Formula (1):
C
q = ερAp2Δ (1)
mt 1
1−β
NOTE For practical implementation, this formula is expanded upon as Formula (1) of ISO 5167-2, ISO 5167-3,
ISO 5167-4, ISO 5167-5 and ISO 5167-6.
ISO 5167-1:2022(E)
Similarly, the value of the volume flow rate can be calculated using Formula (2):
q
m
q = (2)
V
ρ
where ρ is the fluid density at the temperature and pressure for which the volume is stated.
5.2 Method of determination of the required diameter ratio for the selected standard
primary device
In practice, when determining the diameter ratio of a primary element to be installed in a given pipeline,
C and ε used in Formula (1) are, in general, not precisely known. Hence the following shall be selected a
priori:
— the type of primary device to be used;
— a flow rate and the corresponding desired value of the differential pressure.
The related values of q and Δp are then inserted in Formula (1), rewritten in the form of Formula (3):
m
4q
Cεβ
m
= (3)
πΔDp2 ρ
1−β
in which the diameter ratio of the selected primary device can be determined by iteration (see Annex A).
For a given flow rate, the uncertainty of the discharge coefficient and that of the predicted differential
pressure are directly linked, because the discharge coefficient is proportional to the reciprocal of the
square root of the differential pressure. Consequently, care shall be taken when determining β that
the maximum differential pressure does not exceed the upper range limit of the transmitter. This is of
particular importance where the uncertainty of the discharge coefficient is large.
5.3 Computation of flow rate
Computation of the flow rate, which is a purely arithmetic process, is performed by replacing the
different terms on the right-hand side of Formula (1) by their numerical values.
C may be dependent on Re, which is itself dependent on q . In such cases the final value of C, and hence
m
of q , is obtained by iteration. See Annex A for guidance regarding the choice of the iteration procedure
m
and initial estimates.
The dimensions used in the formulae are the values of the dimensions at the 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.
NOTE For corrections due to thermal expansion or contraction see ISO/TR 9464:2008 5.1.6.1.3 and 5.2.6.4.2.
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.
5.4 Determination of density, pressure and temperature
5.4.1 General
Any method of determining reliable values of the density, static pressure and temperature of the fluid is
acceptable if it does not interfere with the distribution of the flow in any way at the cross-section where
measurement is made.
ISO 5167-1:2022(E)
5.4.2 Density
It is necessary to know the density of the fluid at the upstream pressure tapping; it can either be
measured directly or be calculated from an appropriate equation of state from a knowledge of the
absolute static pressure, absolute temperature and composition of the fluid at that location.
NOTE ISO/TR 9464:2008, 6.4.2 provides a method for correcting density measured downstream of a device
to upstream conditions.
5.4.3 Static pressure
The static pressure of the fluid shall be measured by means of an individual wall pressure tapping, or
several such tappings interconnected, or by means of carrier ring tappings if they are permitted for the
measurement of differential pressure in that tapping plane for the particular primary device.

a
Flow.
b
Section A-A (upstream) also typical for section B-B (downstream).
Figure 1 — “Triple-T” arrangement
Where four pressure tappings are connected together to give the pressure upstream, downstream or
in the throat of the primary device, it is best that they should be connected together in a “triple-T”
arrangement as shown in Figure 1. The “triple-T” arrangement is often used for measurement with
large Venturi tubes.
It is permissible to link simultaneously one pressure tapping with differential pressure measuring
device(s) and static pressure measuring device(s), provided that these connections do not lead to any
distortion of the differential pressure measurement.
5.4.4 Temperature
5.4.4.1 Temperature measurement requires particular care. The thermometer well or pocket shall
take up as little space as possible to avoid reducing the cross-sectional area of the pipe. Thermometer
probes should have adequate immersion depth to ensure the fluid temperature is measured accurately.
ISO 5167-1:2022(E)
Except where a cone meter is used, the temperature of the fluid shall preferably be measured
downstream of the primary device to avoid disturbance to the flow profile affecting the primary device.
If the thermometer well or pocket is located downstream of the primary device, the distance between it
and the primary device shall be at least equal to 5D (and at most 15D when the fluid is a gas). In the case
of a Venturi tube this distance is measured from the throat
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