prEN ISO 16911-1

SLOVENSKI STANDARD
01-september-2025
Emisije nepremičnih virov - Ročno in avtomatsko določanje hitrosti in
volumenskega pretoka v odvodnikih - 1. del: Ročna referenčna metod (ISO/DIS
16911-1:2025)
Stationary source emissions - Manual and automatic determination of velocity and
volume flow rate in ducts - Part 1: Manual reference method (ISO/DIS 16911-1:2025)
Emissionen aus stationären Quellen - Manuelle und automatische Bestimmung der
Geschwindigkeit und des Volumenstroms in Abgaskanälen - Teil 1: Manuelles
Referenzverfahren (ISO/DIS 16911-1:2025)
Émissions de sources fixes - Détermination manuelle et automatique de la vitesse et du
débit-volume d’écoulement dans les conduits - Partie 1: Méthode de référence manuelle
(ISO/DIS 16911-1:2025)
Ta slovenski standard je istoveten z: prEN ISO 16911-1
ICS:
13.040.40 Emisije nepremičnih virov Stationary source emissions
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

DRAFT
International
Standard
ISO/DIS 16911-1
ISO/TC 146/SC 1
Stationary source emissions —
Secretariat: BIS
Manual and automatic
Voting begins on:
determination of velocity and
2025-08-07
volume flow rate in ducts —
Voting terminates on:
2025-10-30
Part 1:
Manual reference method
Émissions de sources fixes — Détermination manuelle et
automatique de la vitesse et du débit-volume d'écoulement dans
les conduits —
Partie 1: Méthode de référence manuelle
ICS: 13.040.40
THIS DOCUMENT IS A DRAFT CIRCULATED
FOR COMMENTS AND APPROVAL. IT
IS THEREFORE SUBJECT TO CHANGE
AND MAY NOT BE REFERRED TO AS AN
INTERNATIONAL STANDARD UNTIL
PUBLISHED AS SUCH.
This document is circulated as received from the committee secretariat.
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Reference number
ISO/DIS 16911-1:2025(en)
DRAFT
ISO/DIS 16911-1:2025(en)
International
Standard
ISO/DIS 16911-1
ISO/TC 146/SC 1
Stationary source emissions —
Secretariat: BIS
Manual and automatic
Voting begins on:
determination of velocity and
volume flow rate in ducts —
Voting terminates on:
Part 1:
Manual reference method
Émissions de sources fixes — Détermination manuelle et
automatique de la vitesse et du débit-volume d'écoulement dans
les conduits —
Partie 1: Méthode de référence manuelle
ICS: 13.040.40
THIS DOCUMENT IS A DRAFT CIRCULATED
FOR COMMENTS AND APPROVAL. IT
IS THEREFORE SUBJECT TO CHANGE
AND MAY NOT BE REFERRED TO AS AN
INTERNATIONAL STANDARD UNTIL
PUBLISHED AS SUCH.
This document is circulated as received from the committee secretariat.
IN ADDITION TO THEIR EVALUATION AS
BEING ACCEPTABLE FOR INDUSTRIAL,
© ISO 2025
TECHNOLOGICAL, COMMERCIAL AND
USER PURPOSES, DRAFT INTERNATIONAL
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
STANDARDS MAY ON OCCASION HAVE TO
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Published in Switzerland Reference number
ISO/DIS 16911-1:2025(en)
ii
ISO/DIS 16911-1:2025(en)
Contents Page
Foreword .v
Introduction .vi
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Symbols and abbreviated terms. 4
4.1 Symbols .4
4.2 Abbreviated terms .7
5 Principle . 7
5.1 General .7
5.2 Monitoring objectives.8
5.3 Principle of flow velocity determination at a point in the duct .8
5.4 Principle of measurement of volume flow rate .9
5.4.1 General .9
5.4.2 Principle of volume flow rate determination from point velocity measurements .9
5.4.3 Determination of volume flow rate using tracer dilution measurements .10
5.4.4 Determination of volume flow rate from plant thermal input .10
6 Selection of measurement technique. 10
6.1 Monitoring objective .10
6.2 Choice of measurement technique to determine point flow velocity .11
6.3 Choice of the measurement technique for volume flow rate and average flow
determination . . . 12
7 Measuring equipment .13
7.1 General . 13
7.2 Measurement of duct area . 13
8 Performance characteristics and requirements for differential pressure devices and
vane anemometers . 14
8.1 General .14
8.2 Differential pressure devices .14
8.3 Vane anemometers .16
9 Measurement procedure . 17
9.1 Measurement strategy .17
9.1.1 Site survey before measurement .17
9.1.2 Correction for time related flow variation for the characterisation of a velocity
profile .17
9.1.3 Consideration of flow measurement assembly surface area in relation to
measurement plane area .18
9.2 Determination of measurement plane and number of measurement points .18
9.3 Checks before sampling .18
9.3.1 General .18
9.3.2 Pre-test leak check . .19
9.3.3 Check on stagnation and reference pressure taps (S-type Pitot tube) .19
9.3.4 Tests of repeatability at a single point . 20
9.3.5 Swirl or cyclonic flow . 20
9.4 Quality control . 20
9.5 Measurement of flow at locations within the measurement plane .21
9.6 Post-measurement quality control .21
10 Calculation of results .22
10.1 General . 22
10.2 Measurement of velocity. 22
10.3 Determination of the mean velocity . 22

iii
ISO/DIS 16911-1:2025(en)
10.4 Correction of average velocity for wall effects . 22
10.5 Calculation of the volume flow rate from the average velocity . 23
10.6 Conversion of results to standard conditions . 23
10.6.1 General . 23
10.6.2 Conversion of the volume flow rate to standard conditions . 23
10.6.3 Dry volume flow rate in standard conditions . 23
10.6.4 Conversion of the volume flow rate to a reference oxygen concentration.24
11 Establishment of the uncertainty of results .24
12 Evaluation of the method .25
Annex A (normative) Measurement of velocity using differential pressure based techniques.26
Annex B (normative) Vane anemometer .38
Annex C (normative) Tracer gas dilution method determination of volume flow rate and
average velocity .43
Annex D (normative) Transit time tracer gas method determination of average velocity.50
Annex E (normative) Calculation of flue gas volume flow rate from energy consumption .57
Annex F (informative) The use of time of flight measurement instruments based on modulated
laser light .65
Annex G (informative) Example of uncertainty budget established for velocity and volume flow
rate measurements by Pitot tube . . .66
Annex H (informative) Description of validation studies .79
Annex I (informative) Check of validity of the calibration of a Pitot tube .86
Annex J (informative) Differential pressure measurement .88
Annex K (informative) Degree of swirl determination example method .91
Bibliography .92

iv
ISO/DIS 16911-1:2025(en)
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.
This document was prepared by Technical Committee ISO/TC 146, Air quality, Subcommittee SC 1, Stationary
source emissions, in collaboration with the Technical Committee CEN/TC 264, Air quality, of the European
Committee for Standardization (CEN).
This second edition cancels and replaces the first edition (ISO 16911-1:2013), which has been technically
revised.
The main changes are as follows:
— The monitoring objectives with different uncertainty requirements, ranging from very stringent
(Emission Trading Schemes and calibration of automated flow measuring systems) to less demanding
(support of isokinetic sampling) have been clarified.
— The level of quality control in relation to the uncertainty requirements of the monitoring objective have
been clarified.
— Monitoring objectives have been grouped based on the required quality control.
— The measurement techniques and the associated requirements have been described in more detail.
— Performance characteristics and requirements for differential pressure devices and vane anemometers
have been adapted to the state of the art.
— The example uncertainty calculations have been improved and corrected.
A list of all parts in the ISO 16911 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/DIS 16911-1:2025(en)
Introduction
This document describes a method for periodic determination of the axial velocity and volume flow rate
of gas within emissions ducts and stacks and a method for the calibration of automated flow measuring
systems permanently installed on a duct or stack.
This document provides a method which uses point measurements of the flow velocity to determine the flow
profile and mean velocity and volume flow rates. It also provides for alternative methods based on tracer gas
injection, which can also be used to provide routine calibration for automated flow measuring systems. A
method based on calculation from energy consumption is also described. This document provides guidance
on when these alternative methods can be used.

vi
DRAFT International Standard ISO/DIS 16911-1:2025(en)
Stationary source emissions — Manual and automatic
determination of velocity and volume flow rate in ducts —
Part 1:
Manual reference method
1 Scope
This document specifies a method for periodic determination of the axial velocity and volume flow rate of gas
within emissions ducts and stacks. It is applicable for use in circular or rectangular ducts with measurement
locations meeting the requirements of ISO 15259. Minimum and maximum duct sizes are driven by practical
considerations of the measurement devices described within this document.
[12]
NOTE ISO 15259 is identical to EN 15259 .
This document requires all flow measurements to have demonstrable metrological traceability to national
or international primary standards.
This document applies to a range of monitoring objectives with different uncertainty requirements, ranging
from very stringent (Emission Trading Schemes and calibration of automated flow measuring systems)
to less demanding (support of isokinetic sampling). The level of quality control within this document is
determined by the uncertainty requirements of the monitoring objective. Monitoring objectives are grouped
based on the required quality control. The document specifies which requirements and performance
characteristics apply to specified measurement objectives and application areas.
The methods specified in this document can be used as a standard reference method, if the user demonstrates
that the performance characteristics of the methods are equal to or better than the performance criteria
specified in this document and that the expanded uncertainty of the measurement results obtained by the
methods, expressed with a level of confidence of 95 %, is determined and reported. The results for each
method defined in this document have different uncertainties within a range of 1 % to 10 % at flow velocities
of 20 m/s.
Other methods can be used provided that the user can demonstrate equivalence, e.g. based on the principles
[11]
of EN 14793 .
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 15259, Air quality — Measurement of stationary source emissions — Requirements for measurement
sections and sites and for the measurement objective, plan and report
ISO 20988, Air quality — Guidelines for estimating measurement uncertainty
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 following terms and definitions apply.

ISO/DIS 16911-1:2025(en)
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/
3.1
Pitot tube
device to measure flow velocity at a point, operating on the principle of differential pressure measurement
Note 1 to entry: A number of designs of Pitot tube can be used, including standard L-type, S-type, 2D and 3D Pitot
tubes. Annex A describes a number of Pitot designs currently in use in Europe.
3.2
measurement line
line across the stack, on a measurement plane, along which flow measurements are made to characterize the
flow velocity profile or to determine the average flow
3.3
measurement plane
plane normal to the centreline of the duct at the measurement location at which the measurement of flow
velocity or volume flow rate is required
3.4
measurement point
sampling point
position in the measurement plane at which the sample stream is extracted or the measurement data are
obtained directly
3.5
volume flow rate
volume flow of gas axially along a duct
Note 1 to entry: If not specifically stated, the term may be taken to mean the mean volume flow passing through the
measurement plane.
Note 2 to entry: Volume flow rate is expressed in cubic metres per second or cubic metres per hour.
3.6
point flow velocity
local gas velocity at a point in the duct
Note 1 to entry: Unless otherwise specified, the term may be taken to mean the axial velocity at the measurement
location.
Note 2 to entry: Point flow velocity is expressed in metres per second.
3.7
average flow velocity
<1> velocity which, when multiplied by the area of the measurement plane of the duct, gives the volume flow
rate in that duct
<2> quotient of the volume flow rate in the duct and the area of the measurement plane of the duct
3.8
standard conditions
conditions for reference values for pressure (101,3 kPa) and temperature (273,15 K)
3.9
uncertainty (of measurement)
parameter, associated with the result of a measurement, that characterizes the dispersion of the values that
could reasonably be attributed to the measurand

ISO/DIS 16911-1:2025(en)
3.10
uncertainty budget
statement of a measurement uncertainty, of the components of that measurement uncertainty, and of their
calculation and combination
[5]
Note 1 to entry: For the purposes of this document, the sources of uncertainty are according to ISO 14956 or
ISO/IEC Guide 98-3.
3.11
standard uncertainty
uncertainty of the result of a measurement expressed as a standard deviation
3.12
expanded uncertainty
quantity defining an interval about the result of a measurement that may be expected to encompass a large
fraction of the distribution of values that could reasonably be attributed to the measurand
Note 1 to entry: In this document, the expanded uncertainty is calculated with a coverage factor of k = 2, and with a
level of confidence of 95 %.
3.13
overall uncertainty
expanded uncertainty attached to the measurement result
Note 1 to entry: The overall uncertainty is calculated according to ISO/IEC Guide 98-3.
3.14
swirl
cyclonic flow
tangential component of the flow vector providing a measure of the non-axial flow at the measurement plane
3.15
automated measuring system
AMS
measuring system permanently installed on site for continuous monitoring of flow
Note 1 to entry: See ISO 16911-2.
3.16
metrological traceability
property of a measurement result whereby the result can be related to a reference through a documented
unbroken chain of calibrations, each contributing to the measurement uncertainty
Note 1 to entry: The elements for confirming metrological traceability are an unbroken metrological traceability
chain to an international measurement standard or a national measurement standard, a documented measurement
uncertainty, documented measurement procedure, accredited technical competence, metrological traceability to the
SI units, and calibration intervals.

ISO/DIS 16911-1:2025(en)
4 Symbols and abbreviated terms
4.1 Symbols
A area of the measurement plane m
A internal area of the measurement plane m
I
B number of component B
a , a angle between sensing holes °
1 2
c constant
f velocity correction factor at measurement point i
v,cor,i
d outer tube diameter mm
dl measuring rod length change m
d stack diameter mm
s
e net specific energy (NSE) of the fuel as received MJ/kg
(N)
e absolute error of measurement
P
F force acting on the vane wheel N
−1
f vane frequency s
f velocity factor
v
f wall adjustment factor
WA
h corrected height of the indicating fluid of a liquid manometer to standard
s
temperature
h height of the indicating fluid at the temperature when read
t
i number of the measurement point
K coefficient of the Pitot tube which includes the Pitot calibration factor and
constant values relating to the Pitot design
non-linear calibration factor dependent on density ρ , and viscosity η
0 dyn
K ρ
()0,η
dyn
k coverage factor
L length of the measuring section, i.e. the stack length between the two meas- m
urement levels
L probe length m
p
L measuring rod initial length m
M molar mass of wet gas effluent kg/mol
M molar mass of component B kg/mol
B
n number of measurement points

ISO/DIS 16911-1:2025(en)
P energy production MW
p flue gas pressure kPa
p . p pressures at points P . P
1 5 1 5
p stagnation point pressure Pa
p static pressure Pa
p atmospheric pressure Pa
atm
p absolute pressure in the duct in the measurement plane Pa
c
p dynamic pressure on the vane wheel Pa
dyn
p density of the indicating fluid of a liquid manometer at standard temperature
s
p density of the indicating fluid of a liquid manometer at the temperature when
t
read
average static pressure in the measurement section Pa
p
stat
Δp differential pressure Pa
average dynamic pressure measured at the measurement point i of the meas- Pa
Δp
i
urement plane
q tracer mass flow rate kg/s
m,t
q volume flow rate m /s
V
q dry volume flow rate, under standard conditions of temperature and pressure m /s
V,0d
q dry volume flow rate, under standard conditions of temperature and pressure m /s
V,0d,O
and on actual oxygen concentration
q dry volume flow rate, under standard conditions of temperature and pressure, m /s
V ,,0dO ,ref
and reference oxygen concentration
q stack gas flow rate at sample oxygen content and moisture under standard m /s
V,0,O
conditions
q volume flow rate under the conditions of temperature and pressure of the m /s
V,w
duct, on wet gas
r geometry of the vane wheel
Sp
R gas constant 8,314 J/(K mol)
t transit time of the tracer pulse between the two measurement points s
T flue gas temperature K
T temperature of gas in the measurement plane K
c
T initial temperature of the rod at the start of the measurement K
T final temperature of the rod at the end of the measurement K
v start-up velocity m/s
ISO/DIS 16911-1:2025(en)
v velocity corrected for flow direction m/s
c
ν local velocity at measurement point i m/s
i
v measured velocity m/s
meas
v peripheral velocity, v = ϖr
t t Sp
v average velocity of fixed device measurements m/s
f,av
v velocity of fixed measurement device at measurement point i m/s
f,i
v corrected velocity at measurement point i m/s
t,cor,i
v velocity of traverse measurement device at measurement point i m/s
t,i
v axial approach velocity m/s
∞
v
mean velocity m/s
v corrected mean velocity m/s
c
v average of the point velocity measurements m/s
p
v mean axial velocity m/s
v
w ash yield mass fraction of solid fuel as received
ash
w carbon mass fraction in fuel as received
C
w fuel mass fraction in fuel as received
f
w hydrogen mass fraction in fuel as received
H
w moisture mass fraction in solid fuel as received
HO
w nitrogen mass fraction in fuel as received
N
w oxygen mass fraction in fuel as received
O
w sulfur mass fraction in fuel as received
S
α linear expansion coefficient m/(m K)
α pitch of blade
η thermal efficiency
η dynamic viscosity Pa s
dyn
θ measured angle °
meas
ρ density of the gas effluent under ambient conditions of temperature and pres- kg/m
sure of wet gas
σ standard deviation of the m dynamic pressure measurements in the measure-
Δp
i
ment point i
Φ process heat release MW
(N)F
φ volume concentration of component B %
B
ISO/DIS 16911-1:2025(en)
ϕ CO volume concentration in the gas stream in wet gas %
CO ,w 2
ϕ flue gas water vapour volume concentration, wet %
HO
ϕ flue gas oxygen volume concentration, dry %
O
ϕ oxygen volume concentration measured in the duct during the exploration of %
O,d
the duct on dry gas
ϕ reference oxygen volume concentration %
O,ref
ϕ oxygen volume concentration in the gas stream in wet gas %
Ow,
−1
ω angular frequency s
−1
ϖ pulsatance s
4.2 Abbreviated terms
AMS automated measuring system
MO monitoring objective
NSE net specific energy
QAL2 second quality assurance level
SRM standard reference method
QA quality assurance
WAF wall adjustment factor
5 Principle
5.1 General
This document provides a method for the determination of gas velocity and volume flow rate within an
emissions duct. It describes a method to determine the velocity profile of the gas flow across a measurement
plane in the duct, and a method to determine the total volume flow rate at a measurement plane in the duct
based on a grid of point velocity measurements made across the measurement plane. In addition, alternative
methods are described for the determination of volume flow rate based on the measurement of tracer
dilution, tracer transit time, and by calculation from energy consumption.
Techniques for determining gas velocity at a point include a calibrated differential pressure device (Pitot
tube, see Annex A) and a calibrated vane anemometer (see Annex B). Selection criteria for the use of different
types of Pitot and the vane anemometer are given in Clause 6. However, it is up to the user to ensure the
method selected for a given application meets the performance criteria defined by this document. The
volume flow rate within a duct is determined by measuring the duct axial gas velocity at a series of points
along measurement lines across the duct on a single measurement plane. The number of measurement lines
and measurement points required depends on the duct shape and size. The spacing of the measurement
points is based on the principle of equal areas as defined in ISO 15259. The volume flow rate is calculated
from the average axial velocity and the duct area at the measurement plane. If required a correction is
applied to account for wall effects (see 10.4).
Three alternative methods are also described to determine volume flow rate and average flow velocity:
— Annex C describes a method based on tracer dilution measurements. In this method, the volume flow
rate is determined from the dilution of a known concentration of injected tracer.

ISO/DIS 16911-1:2025(en)
— Annex D describes a method based on a tracer transit time measurement technique. The volume flow rate
is determined from the time for a pulse of tracer gas to traverse between two measurement locations.
— Annex E describes a method to determine the volume flow rate using a calculation-based approach to
derive the flow from the energy consumption of a combustion process.
The volume flow rate may be reported at stack conditions or may be expressed at standard conditions
(273,15 K and 101,3 kPa) on either the wet or dry basis.
This document applies to a range of monitoring objectives (MO) with different uncertainty requirements,
and it provides quality control checks to enable these to be met. The level and extent of quality control checks
and the selection of performance characteristics and their criteria have been established and specified
based on the monitoring objective uncertainty requirements.
5.2 Monitoring objectives
Monitoring objectives are grouped based on the required quality control. The grouping of monitoring
objectives is as follows:
MO1: periodic monitoring for regulatory compliance purposes according to ISO 15259 and/or pollution
inventory reporting which involves the determination of mass emissions and for the control of isoki-
netic conditions during manual extractive sampling;
MO2: periodic measurements under the requirements of Emission Trading Schemes and/or calibration of
an AMS under ISO 16911-2 and/or flow profile characterization either to meet the requirements of
Emission Trading Schemes or any other regulatory requirements.
For simplicity any reference throughout this document to MO1 or MO2 refers to the above list.
This document can be used for other monitoring objectives, but the user has to specify the required level of
quality control based on the uncertainty requirement of the monitoring objective.
5.3 Principle of flow velocity determination at a point in the duct
The axial flow velocity at a point in the duct is determined using one of two techniques described in this
document:
— differential pressure based measurement using Pitot tubes, and
— vane anemometry.
Annex A provides details for the use of differential pressure based techniques. Annex B describes the vane
anemometer in detail.
The flow velocity is determined as the duct axial velocity at each point determined according to ISO 15259.
The differential pressure based techniques are based on the principle of the Pitot tube as defined in ISO 3966.
[3]
A probe with one or more pressure taps is inserted into the flow. The basic principle is that one pressure
tap is impacted by the flowing gas, and one or more other pressure taps are exposed to the static pressure in
the duct. The probe assembly allows the resultant pressure difference between these to be measured by an
external differential pressure measuring device.
Different implementations of the differential pressure approach are available. These include standard
L-type, S-type, and multi-axis Pitot tubes (3D and 2D Pitot tubes). Each has their own specific advantages
and disadvantages, and these are described in this document. The methods used are based on those specified
[4] [3] [16]
in ISO 10780, ISO 3966, and US EPA Method 2. Performance requirements and quality assurance
procedures are applied to achieve the uncertainties defined in this document.
[18]
If 2D Pitot tubes are to be used, information on the necessary QA/QC can be found in US EPA Method 2G .

ISO/DIS 16911-1:2025(en)
5.4 Principle of measurement of volume flow rate
5.4.1 General
Volume flow rate can be determined from a series of measurements of the point velocity in a duct made
across the measurement plane or by alternative techniques including tracer dilution, tracer transit time or
calculation from energy consumption. Annex C, Annex D and Annex E provide details of these alternative
approaches.
5.4.2 Principle of volume flow rate determination from point velocity measurements
Volume flow rate is determined from a number of point measurements of the axial flow velocity over a
measurement plane. Sufficient point measurements are made to characterize non-uniformities in the
flow profile. The measurement points across the measurement plane are selected to be representative of
regions of equal area. The average velocity passing through the measurement plane is calculated with good
approximation as equal to the average of the point flow measurements. The procedures in ISO 15259 are
used to determine the measurement points for circular or rectangular ducts. The tangential methodology
provided in ISO 15259 is used for circular ducts as described in this document.
The reason why for circular ducts, the tangential methodology is preferred to the general method for
determining equal areas (as specified in ISO 15259), is that in the tangential method the points provide a
measure of the average flow in each equal area. The centre point in the general method does not provide a
measure of the average flow in the centre area, but rather the maximum flow value. This can be useful for
characterising the flow profile, but is not recommended for determining the average flow in the duct.
The measurement plane is selected to be representative of the required duct volume flow rate, and also
to be in a region where it is uniform and stable. If non-axial flow (swirl or cyclonic flow) is expected at
the measurement plane due to geometry of the duct or other upstream conditions, then the degree of swirl
is determined using S-type, 3D or 2D Pitot tube measurements and if it is significant, as defined in this
document, then it is taken into account through the use of additional measurement procedures, or a different
measurement plane is selected.
If required, improved uncertainty in the results is achieved by taking wall effects into account following 10.4.
The volume flow rate q is determined by multiplying the average velocity by the area of the measurement
V
plane (i.e. the internal area of the duct at the measurement plane) according to Formula (1):
qv= A (1)
Vp
where
v is the average of the point velocity measurements;
p
A is the area of the measurement plane.
It is also possible to determine an array of volume flow rates, determined from the point measurements at
each equal area multiplied by the area represented by each measurement point. Each measurement point
area is, by definition, equal to the area of the measurement plane divided by the number of measurement
points. The volume flow rate is then calculated according to Formula (2) which is equivalent to Formula (1):
n A
qv= (2)
Vi∑
i=1
n
where
v is the velocity at measurement point i;
i
A is the area of the measurement plane;
n is the number of measurement points.

ISO/DIS 16911-1:2025(en)
5.4.3 Determination of volume flow rate using tracer dilution measurements
Tracer gas injection is used to measure the volume flow rate by determining the dilution of the injected tracer
by the stack gas flow. A known traceable flow rate of a tracer gas with specified composition is injected into
the stack. The concentration of this tracer gas is measured at a location downstream, representative of the
measurement plane, after adequate mixing of the tracer with the stack gas has occurred. Adequate mixing
can be achieved when
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