ISO/FDIS 16911-1

• Abstract:

  ISO 16911-1:2013 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 EN 15259. Minimum and maximum duct sizes are driven by practical considerations of the measurement devices described within ISO 16911-1:2013. ISO 16911-1:2013 requires all flow measurements to have demonstrable metrological traceability to national or international primary standards. To be used as a standard reference method, the user is required to demonstrate that the performance characteristics of the method are equal to or better than the performance criteria defined in ISO 16911-1:2013 and that the overall uncertainty of the method, expressed with a level of confidence of 95 %, is determined and reported. The results for each method defined in ISO 16911-1:2013 have different uncertainties within a range of 1 % to 10 % at flow velocities of 20 m/s. Methods further to these can be used provided that the user can demonstrate equivalence, based on the principles of CEN/TS 14793.

• Status: Not Published
• ICS: 13.040.40 - Stationary source emissions
• Technical Committee: ISO/TC 146/SC 1 - Stationary source emissions
• Drafting Committee: ISO/TC 146/SC 1 - Stationary source emissions
• Current Stage: 5020 - FDIS ballot initiated: 2 months. Proof sent to secretariat
• Start Date: 16-Sep-2026
• Completion Date: 16-Sep-2026

• Consolidates: FprEN ISO 16911-1 - Stationary source emissions - Manual and automatic determination of velocity and volume flow rate of waste gas in ducts - Part 1: Manual reference method (ISO/FDIS 16911-1:2026)
• Effective Date: 12-Feb-2026
• Revises: ISO 16911-1:2013 - Stationary source emissions — Manual and automatic determination of velocity and volume flow rate in ducts — Part 1: Manual reference method
• Effective Date: 01-Oct-2022

ISO/FDIS 16911-1 is an ISO standard for stationary source emissions. It specifies a manual reference method for determining the axial velocity and volume flow rate of waste gas in ducts and stacks, and it also describes tracer-based and calculation-based alternatives for volume flow rate. It is written for users who need periodic measurements with stated uncertainty and metrological traceability, such as emission monitoring, calibration work, and regulatory reporting.

What does ISO/FDIS 16911-1 specify?

ISO/FDIS 16911-1 specifies how to measure gas velocity and waste gas volume flow rate in circular or rectangular ducts, provided the measurement location meets ISO 15259. The document covers direct point-velocity measurements, plus tracer dilution, tracer transit time, and energy-consumption-based methods for volume flow rate.

The standard is organized into clauses on principle, method selection, equipment, performance characteristics, field procedure, calculations, uncertainty, and method evaluation. It then adds normative annexes for the main measurement methods and informative annexes for supporting topics.

Annex	What it covers
Annex A	Measurement of velocity using differential pressure-based techniques, including Pitot tube designs
Annex B	Vane anemometer
Annex C	Tracer gas dilution method for volume flow rate and average velocity
Annex D	Transit time tracer gas method for average velocity
Annex E	Calculation of waste gas volume flow rate from energy consumption
Annex F	Use of time-of-flight instruments based on modulated laser light
Annex G	Example uncertainty budget for velocity and volume flow rate measurements by Pitot tube
Annex H	Validation studies
Annex I	Check of validity of Pitot tube calibration
Annex J	Differential pressure measurement
Annex K	Example method for degree of swirl determination

What are the key requirements of ISO/FDIS 16911-1?

ISO/FDIS 16911-1 requires the user to choose a measurement method that matches the monitoring objective, the needed uncertainty, and the physical conditions of the duct. In practice, the standard is as much about selecting and controlling the measurement as it is about the arithmetic.

Monitoring objectives drive quality control

ISO/FDIS 16911-1 groups work into two monitoring objectives, MO1 and MO2. MO1 covers periodic measurements for regulatory applications such as pollution inventory reporting and support of isokinetic sampling, while MO2 covers more demanding work such as Emission Trading Schemes and calibration of an automated measuring system (AMS).

This matters because the document ties the amount of quality control, the accepted performance characteristics, and the uncertainty target to the chosen monitoring objective. A user needs to define the objective before measuring, otherwise the result may not be acceptable for the intended use.

Point velocity may be measured by Pitot tubes or vane anemometers

ISO/FDIS 16911-1 allows point flow velocity to be measured with differential pressure devices, including Pitot tubes, or with a vane anemometer. Annex A covers L-type, S-type, 2D and 3D Pitot designs, and Annex B covers vane anemometers.

This matters in practice because the choice depends on duct conditions. The standard notes, for example, that an S-type Pitot tube can suit small ports or dusty gas, while vane anemometers are not recommended where fibres or droplets are present.

Volume flow rate can be obtained by several routes

ISO/FDIS 16911-1 lets users determine volume flow rate either from a grid of point velocity measurements or by alternative methods in Annex C, Annex D, and Annex E. The tracer methods can also support calibration of an AMS, while the energy-consumption approach is for combustion or similar processes where the process inputs are known.

This matters because the document separates methods that measure the duct directly from methods that infer flow from tracer behavior or plant energy data. It also warns that the calculation approach is not for AMS calibration.

The measurement plane and duct area must be controlled

ISO/FDIS 16911-1 requires the measurement plane to comply with ISO 15259 and the internal duct area to be determined by direct dimensions where possible. The expanded uncertainty of the duct area at the measurement plane has to meet the acceptance criterion given in the standard, and port length and wall thickness also have to be measured.

This matters because a flow result is only as good as the area used to turn average velocity into flow rate. The standard also explains when engineering drawings may support direct measurement and when a wall correction factor should be used.

Device performance and calibration are part of the method

ISO/FDIS 16911-1 sets performance checks for differential pressure readout devices, Pitot tubes, temperature and pressure instruments, and vane anemometers. It requires traceable calibration, regular repeatability and linearity checks, and use of equipment ranges that suit the expected flow.

This matters because the standard treats instrument quality as part of the measurement uncertainty. For example, it gives explicit acceptance criteria for readout repeatability, lack-of-fit, calibration uncertainty, and the lowest usable range for the measuring device.

Field procedure includes leak checks, alignment, and averaging

ISO/FDIS 16911-1 requires checks before sampling, quality control during measurement, and a post-measurement leak check for Pitot tube work. It also requires the probe angle and sensor alignment to stay within the stated limits, and it calls for averaging differential pressure over at least one minute from at least three instantaneous readings.

This matters because the method is not just a single reading at each point. The user has to control leaks, misalignment, and time variation in the gas flow, especially when MO2 applies.

Results may need wall correction and condition conversion

ISO/FDIS 16911-1 calculates mean velocity from equal-area point measurements and then derives volume flow rate from mean velocity times duct area. It also provides wall adjustment factors for circular and rectangular ducts and methods for converting results to standard conditions, dry conditions, and reference oxygen content.

This matters because the reported flow must match the purpose of the measurement. For example, flow used with concentration data should be expressed under the same wet or dry basis to avoid extra uncertainty from unnecessary conversions.

Uncertainty has to be estimated and reported

ISO/FDIS 16911-1 requires an uncertainty budget based on ISO/IEC Guide 98-3 and ISO 20988. The user has to combine uncertainty sources from calibration, duct area, conversion to reported conditions, and, where relevant, wall effects and swirl, then report the overall uncertainty at 95 % confidence.

This matters because the standard is explicit about measurement quality. A result is not complete until its uncertainty is worked out and reported for the actual measurement conditions.

What terms does ISO/FDIS 16911-1 define?

• monitoring objective (MO) - The regulatory framework that sets the measurement task and the uncertainty requirement.
• measurement task - The practical scope of the measurement programme derived from the monitoring objective.
• Pitot tube - A point-velocity device that works by measuring differential pressure between stagnation and static pressure.
• measurement plane - The plane normal to the duct centreline where flow velocity or volume flow rate is measured.
• volume flow rate - The axial flow of gas through the duct, expressed as cubic metres per second or cubic metres per hour.
• expanded uncertainty - The uncertainty interval reported in this document with k = 2 and a 95 % confidence level.
• metrological traceability - The documented calibration chain linking a result to a national or international primary standard.
• automated measuring system (AMS) - A permanently installed system for continuous flow measurement; the document points to ISO 16911-2 for its determination.

Who uses ISO/FDIS 16911-1?

ISO/FDIS 16911-1 is used by laboratories, plant operators, environmental compliance teams, and calibration specialists who measure stack or duct flow. It is relevant in stationary source emissions work, especially where mass emissions, isokinetic sampling support, flow profile characterization, or AMS calibration are involved.

The standard also suits buyers and quality managers who need to specify the acceptable method for field measurements, because it links method choice to uncertainty, equipment checks, and reporting conditions. It is particularly relevant to combustion plants, emission trading work, waste incineration, and other installations with measurable exhaust ducts.

What changed in ISO/FDIS 16911-1 from the previous edition?

ISO/FDIS 16911-1 is the second edition and it replaces ISO 16911-1:2013. The foreword states that the document was technically revised.

The stated changes are:

• monitoring objectives with different uncertainty needs were clarified
• the quality control level was clarified in relation to those uncertainty needs
• the monitoring objectives were grouped by required quality control
• the measurement techniques and their requirements were described in more detail
• performance characteristics for differential pressure devices and vane anemometers were updated to reflect current practice
• the example uncertainty calculations were improved and corrected

Which standards are used with ISO/FDIS 16911-1?

Standard	What it contributes
ISO 15259:2023	Requirements for measurement sections and sites, the measurement objective, plan, and report
ISO 20988:2007	Guidance for estimating measurement uncertainty
ISO/IEC Guide 98-3:2008	The GUM approach used for uncertainty expression and combination
ISO/IEC 17025	Competence framework for testing and calibration laboratories
EN 12952-15:2003	Acceptance tests for water-tube boilers and auxiliary installations
ISO 16911-2	Flow measurement with AMS, referenced for calibration of automated systems
EN 14793	Equivalence principles for using alternative methods
ISO 3966	Pitot-tube-based flow measurement principles cited for differential pressure methods
ISO 10780	Flow measurement methods cited alongside Pitot-tube practice

What does the ISO/FDIS 16911-1 document contain?

ISO/FDIS 16911-1 contains selection guidance, measurement procedures, device performance criteria, and the calculations needed to turn local readings into a reported flow result. It also includes formulas for mean velocity, wall correction, volume flow rate, standard-condition conversion, and reference oxygen correction.

The annexes provide the practical methods behind the main clause text: Pitot tube use, vane anemometer use, tracer dilution, tracer transit time, and energy-based flow calculation. Supporting annexes add a calibration check for Pitot tubes, uncertainty examples, differential pressure guidance, swirl checking, and validation-study summaries.

The document therefore serves both field users and technical reviewers. It tells the operator what to measure, what to check, how to calculate the result, and how to support the uncertainty claim with traceable data.

FINAL DRAFT
International
Standard
ISO/TC 146/SC 1
Stationary source emissions —
Secretariat: BIS
Manual and automatic
Voting begins on:
determination of velocity and
2026-09-16
volume flow rate of waste gas in
Voting terminates on:
ducts —
2026-11-11
Part 1:
Manual reference method
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WITH THEIR COMMENTS, NOTIFICATION OF ANY
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TO BE CONSIDERED IN THE LIGHT OF THEIR POTENTIAL
TO BECOME STAN DARDS TO WHICH REFERENCE MAY BE
MADE IN NATIONAL REGULATIONS.
Reference number
FINAL DRAFT
International
Standard
ISO/TC 146/SC 1
Stationary source emissions —
Secretariat: BIS
Manual and automatic
Voting begins on:
determination of velocity and
volume flow rate of waste gas in
Voting terminates on:
ducts —
Part 1:
Manual reference method
RECIPIENTS OF THIS DRAFT ARE INVITED TO SUBMIT,
WITH THEIR COMMENTS, NOTIFICATION OF ANY
RELEVANT PATENT RIGHTS OF WHICH THEY ARE AWARE
AND TO PROVIDE SUPPOR TING DOCUMENTATION.
© ISO 2026
IN ADDITION TO THEIR EVALUATION AS
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BEING ACCEPTABLE FOR INDUSTRIAL, TECHNO-
ISO/CEN PARALLEL PROCESSING
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ii
Contents Page
Foreword .v
Introduction .vi
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 2
4 Symbols and abbreviated terms. 4
4.1 Symbols .4
4.2 Abbreviated terms .7
5 Principle . 8
5.1 General .8
5.2 Monitoring objectives.8
5.3 Principle of flow velocity determination at a point in the duct .9
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 using transit time tracer measurements .10
5.4.5 Determination of volume flow rate from calculation approach based on energy
consumption . .11
6 Selection of measurement technique.11
6.1 Measurement task .11
6.2 Choice of measurement technique to determine point flow velocity . 12
6.3 Choice of the measurement technique for volume flow rate and average flow
determination . . . 13
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 . 15
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 characterization 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 .19
9.3.1 General .19
9.3.2 Leak check .19
9.3.3 Check on stagnation and reference pressure taps (S-type Pitot tube) . 20
9.3.4 Swirl . 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 .21
10.1 General .21
10.2 Measurement of velocity. 22

iii
10.3 Determination of the mean velocity . 22
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 waste gas volume flow rate from energy consumption .57
Annex F (informative) 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 .81
Annex I (informative) Check of validity of the calibration of a Pitot tube .87
Annex J (informative) Differential pressure measurement .89
Annex K (informative) Degree of swirl determination example method .92
Bibliography .93

iv
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 document 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).
ISO draws attention to the possibility that the implementation of this document may involve the use of (a)
patent(s). ISO takes no position concerning the evidence, validity or applicability of any claimed patent
rights in respect thereof. As of the date of publication of this document, ISO had not received notice of (a)
patent(s) which may be required to implement this document. However, implementers are cautioned that
this may not represent the latest information, which may be obtained from the patent database available at
www.iso.org/patents. ISO shall not be held responsible for identifying any or all such patent rights.
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 European Committee for Standardization (CEN) Technical
Committee CEN/TC 264, Air quality, in accordance with the Agreement on technical cooperation between
ISO and CEN (Vienna Agreement).
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 (e.g.
Emission Trading Schemes and calibration of automated flow measuring systems) to less demanding
(e.g. 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
Introduction
For emission monitoring, normally only the mass concentration of the relevant pollutants is measured.
These measurements require in some cases isokinetic sampling on the basis of the local flow velocity at
specific measurement points in a sampling grid.
For many installations, the mass emissions are also determined, which can be calculated as the product of
the pollutant mass concentration and the waste gas volume flow rate.
Therefore, the velocity and volume flow rate of gas in ducts are important parameters. The waste gas volume
flow rate can often be calculated on the basis of known plant parameters, such as fuel consumption or steam
generating capacity. If the plant’s operating parameters fluctuate, the waste gas volume flow rate needs to
be determined directly.
If the cross-section and flow profile of the waste gas flow rate are known, the volume flow rate can be
determined on the basis of the flow velocity. The methods of determining volume flow rate used in emission
measurements are based on local flow velocity measurements taken in the flow cross-section of a waste gas
duct, or by the measurement of tracer dilution, the measurement of tracer transit time or the calculation
from energy consumption.
vi
FINAL DRAFT International Standard ISO/FDIS 16911-1:2026(en)
Stationary source emissions — Manual and automatic
determination of velocity and volume flow rate of waste gas 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 in ducts (e.g. stacks) which uses point measurements of the flow velocity.
This document 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.
[11]
NOTE ISO 15259:2023 is identical to EN 15259:2008 .
This document also specifies methods for the determination of volume flow rate based on the measurement
of tracer dilution, the measurement of tracer transit time and by calculation from energy consumption. This
document provides guidance on when these methods can be used.
This document applies to all flow measurements with demonstrable metrological traceability to national or
international primary standards.
This document specifies two monitoring objectives with different uncertainty requirements ranging from
very stringent (e.g. Emission Trading Schemes and calibration of automated flow measuring systems) to
less demanding (e.g. support of isokinetic sampling). The level of quality control within this document is
determined by the uncertainty requirements of the monitoring objective. This document specifies which
requirements and performance characteristics apply to specified measurement tasks and application areas
which fall under the two monitoring objectives.
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 specified 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
[10]
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.
EN 12952-15:2003, Water-tube boilers and auxiliary installations — Acceptance tests
ISO 15259:2023, Air quality — Measurement of stationary source emissions — Requirements for measurement
sections and sites and for the measurement objective, plan and report

ISO 20988:2007, Air quality — Guidelines for estimating measurement uncertainty
ISO/IEC Guide 98-3:2008, Uncertainty of measurement — Part 3: Guide to the expression of uncertainty in
me a s ur ement (GUM: 1995)
ISO/IEC 17025, General requirements for the competence of testing and calibration laboratories
3 Terms and definitions
For the purposes of this document, the following terms and definitions 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/
3.1
monitoring objective
requirements under which measurement tasks (3.2) are undertaken
3.2
measurement task
scope of the measurement programme deduced from the monitoring objective (3.1) and specific technical
activities carried out under a measurement programme
Note 1 to entry: The term “measurement task” can also refer to a “measurement objective” as defined in ISO 15259:2023,
3.20. The term “measurement task” is used to avoid confusion and to clearly distinguish between a legally specified
monitoring objective and the scope of the measurement programme deduced from the monitoring objective and
specific technical activities carried out under a measurement programme.
3.3
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.
3.4
measurement line
line across the duct, on a measurement plane (3.5), along which flow measurements are made to characterize
the flow velocity profile or to determine the average flow
3.5
measurement plane
plane normal to the centreline of the duct at the measurement location at which flow velocity or volume flow
rate (3.7) is measured
3.6
measurement point
sampling term
position in the measurement plane (3.5) at which the sample stream is extracted or the measurement data
are obtained directly
[SOURCE: ISO 15259:2023, 3.16, modified — Note 1 to entry has been deleted and an admitted term has
been added.]
3.7
volume flow rate
volume flow of gas axially along a duct
Note 1 to entry: Volume flow rate is expressed in cubic metres per second or cubic metres per hour.

3.8
point flow velocity
local gas velocity at a point in the duct
Note 1 to entry: Unless otherwise specified, the term can be taken to mean the axial velocity at the measurement
location.
Note 2 to entry: Point flow velocity is expressed in metres per second.
3.9
average flow velocity
velocity which, when multiplied by the area of the measurement plane (3.5) of the duct, gives the volume flow
rate (3.7) in that duct
3.10
standard conditions
reference values for pressure (101,3 kPa) and for temperature (273,15 K)
3.11
uncertainty
uncertainty of measurement
parameter, associated with the result of a measurement, that characterizes the dispersion of the values that
can reasonably be attributed to the measurand
[SOURCE: ISO/IEC Guide 98-3:2008, B.2.18, modified — "could" has been replaced with "can" in the definition
and NOTES 1, 2 and 3 have been deleted.]
3.12
uncertainty budget
statement of a measurement uncertainty (3.11) 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 defined according to ISO 14956 or
ISO/IEC Guide 98-3.
3.13
standard uncertainty
uncertainty (3.11) of the result of a measurement expressed as a standard deviation
[SOURCE: ISO/IEC Guide 98-3:2008, 2.3.1]
3.14
expanded uncertainty
quantity defining an interval about the result of a measurement that can be expected to encompass a large
fraction of the distribution of values that can reasonably be attributed to the measurand
[SOURCE: ISO/IEC Guide 98-3:2008, 2.3.5, modified — "may" and "could" have been replaced with "can" in
the definition.]
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 %.
3.15
overall uncertainty
expanded uncertainty (3.14) attached to the measurement result
Note 1 to entry: The overall uncertainty is calculated according to ISO/IEC Guide 98-3.
3.16
swirl
cyclonic flow
tangential component of the flow vector providing a measure of the non-axial flow at the measurement plane
(3.5)
3.17
automated measuring system
AMS
measuring system permanently installed on site for continuous measurement of flow
Note 1 to entry: The determination of velocity and volume flow rate with AMS is described in ISO 16911-2.
3.18
portable automated measuring system
P-AMS
automated measuring system (3.17) which is in a condition or application to be moved from one to another
measurement site to obtain measurement results for a short measurement period
Note 1 to entry: The measurement period is typically 8 h for a day.
Note 2 to entry: The P-AMS can be configured at the measurement site for the special application but can be also set-up
in a van or mobile container. The probe and the sample gas lines are installed often just before the measurement task
is started.
[12]
[SOURCE: EN 15267-4:2023, 3.3 ]
3.19
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 uncertainty of measurement (3.11)
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, a documented measurement procedure, metrological traceability to the SI units, and calibration intervals.
4 Symbols and abbreviated terms
4.1 Symbols
A internal cross-sectional area of the duct at the measurement plane m
B number of component B
a , a angle between sensing holes °
1 2
c constant
d outer tube diameter mm
dl measuring rod length change m
d inner duct diameter mm
d
e net specific energy (NSE) of the fuel as received MJ/kg
(N)
e reproducibility of output quantities
P
F force acting on the vane wheel N
−1
f vane frequency s
f mass specific fuel factor m /kg
ms
f normalization factor
n,i
f velocity correction factor at measurement point i
v,cor,i
f wall adjustment factor
WA
h corrected height of the indicating fluid of a liquid manometer to standard tem-
s
perature
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
nonlinear calibration factor dependent on density ρ , and viscosity η
0 dyn
K 

0,
dyn
k coverage factor
L length of the measurement 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 waste gas kg/mol
M molar mass of component B kg/mol
B
n number of measurement points
P energy production MW
p waste 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
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 measure- Pa
∆p
i
ment plane
maximum value of dynamic pressure measured Pa
∆p
i,max
∆p minimum values of dynamic pressure measured Pa
i,min
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
dry volume flow rate, under standard conditions of temperature and pressure and m /s
q
Vd,0 ,O
on actual oxygen concentration
dry volume flow rate, under standard conditions of temperature and pressure, m /s
q
Vd,,0 O,ref
and reference oxygen concentration
volume flow rate at sample oxygen content and moisture under standard conditions m /s
q
V,0,O
q volume flow rate under the conditions of temperature and pressure of the duct, m /s
V,w
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 waste 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
v axial velocity m/s
c
ν local velocity at measurement point i m/s
i
v measured velocity m/s
m
v peripheral velocity, v = ϖr m/s
t t Sp
v average velocity at the fixed measurement point m/s
f,av
v velocity of fixed measurement device when traverse measurement device is at m/s
f,i
measurement point i
v corrected velocity of traverse measurement device at measurement point i m/s
t,cor,i
v velocity of traverse measurement device at measurement point i m/s
t,i
axial approach velocity m/s
v
∞
v
mean velocity m/s
mean velocity corrected for wall effects m/s
v
WA
average of the point velocity measurements m/s
v
p
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
moisture mass fraction in solid fuel as received
w
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 1/K
α pitch of blade
η thermal efficiency
η dynamic viscosity Pa s
dyn
θ measured angle °
m
ρ density of the waste gas under duct conditions of temperature and pressure of kg/m
wet gas
ρ density of the indicating fluid of a liquid manometer at standard temperature
s
ρ density of the indicating fluid of a liquid manometer at the temperature when read
t
standard deviation of the m dynamic pressure measurements in the measurement
()p
i
point i
Φ thermal input MW
(N)F
φ volume concentration of component B %
B
CO volume concentration in the gas stream in wet gas %
ϕ
CO ,w
water vapour volume concentration in the gas stream in wet gas %
ϕ
HO
oxygen volume concentration in the gas stream in dry gas %
ϕ
O
oxygen volume concentration measured in the duct during the exploration of the %
ϕ
O,d
duct on dry gas
reference oxygen volume concentration %
ϕ
O,ref
oxygen volume concentration in the gas stream in wet gas %
ϕ
Ow,
−1
ω angular frequency s
4.2 Abbreviated terms
AMCA Air Movement and Control Association
AMS automated measuring system
CETIAT Centre Technique des Industries Aérauliques et Thermiques
MO monitoring objective
NSE net specific energy
NPL National Physical Laboratory
P-AMS portable automated measuring system

QAL2 quality assurance level 2
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 given 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.
— 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 can be reported at duct conditions or expressed at standard conditions (273,15 K and
101,3 kPa) on either the wet or dry basis.
This document applies to two different 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
Measurements are grouped based on the required quality control. The grouping is as follows:
— MO1: periodic measurements with uncertainty requirements according to ISO 15259 or pollution
inventory reporting which involves the determination of mass emissions and for the control of isokinetic
conditions during manual extractive sampling;
— MO2: periodic measurements with more stringent uncertainty requirements, e.g. measurements under
the requirements of Emission Trading Schemes or calibration of an AMS under ISO 16911-2 or flow profile
characterization either to meet the requirements of Emission Trading Schemes.

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
[3]
ISO 3966 . 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] [18]
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.
[20]
If 2D Pitot tubes are to be used, information on the necessary QA/QC can be found in US EPA Method 2G .
[19]
The usage of 3D Pitot tubes is described in US EPA Method 2F .
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. If an alternative approach is applied, the requirements of the corresponding Annex C, Annex D
or Annex E shall be met.
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 shall
be used to determine the measurement points for circular or rectangular ducts. The tangential methodology
provided in ISO 15259 shall be 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
characterizing the flow profile, but it is not recommended for determining the average flow in the duct.

The measurement plane is selected to be representative of the required waste gas 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 internal cross-sectional
V
area of the duct at the measurement plane according to Formula (1):
qvA (1)
Vp
where
v is the average of the point velocity measurements;
p
A is the internal cross-sectional area of the duct at the measurement pla
...

ISO/TC 146/SC 1/WG 36 N XXX
revised ISO/DIS 16911-1:2025(en)
ISO TC 146/SC 1/WG 36
Version 10black (N XXX)
Previous version: Version 09 (N XXX)
Secretariat: BIS
Date: 2026-09-01
Stationary source emissions — Manual and automatic
determination of velocity and volume flow rate of waste gas in
ducts —
—
Part 1:
Manual reference method
Second edition
Date: 2026-05-07
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FDIS stage
ISO/DISFDIS 16911-1:20252026(en)
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication
may be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying,
or posting on the internet or an intranet, without prior written permission. Permission can be requested from either ISO
at the address below or ISO’s member body in the country of the requester.
ISO copyright office
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CH-1214 Vernier, Geneva
Phone: + 41 22 749 01 11
EmailE-mail: [e-mail]
Website: www.iso.org
Published in Switzerland
iii
ISO/WD FDIS 16911-1:2025(E2026(en)
Contents
Foreword . vi
Introduction . viii
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 2
4 Symbols and abbreviated terms . 5
4.1 Symbols . 5
4.2 Abbreviated terms . 7
5 Principle . 8
5.1 General . 8
5.2 Monitoring objectives . 9
5.3 Principle of flow velocity determination at a point in the duct . 9
5.4 Principle of measurement of volume flow rate . 10
6 Selection of measurement technique . 11
6.1 Measurement task . 11
6.2 Choice of measurement technique to determine point flow velocity . 12
6.3 Choice of the measurement technique for volume flow rate and average flow
determination . 13
7 Measuring equipment . 14
7.1 General . 14
7.2 Measurement of duct area . 14
8 Performance characteristics and requirements for differential pressure devices and
vane anemometers . 15
8.1 General . 15
8.2 Differential pressure devices . 16
8.3 Vane anemometers . 18
9 Measurement procedure . 19
9.1 Measurement strategy . 19
9.2 Determination of measurement plane and number of measurement points . 20
9.3 Checks before sampling . 20
9.4 Quality control . 22
9.5 Measurement of flow at locations within the measurement plane . 22
9.6 Post-measurement quality control . 23
10 Calculation of results . 23
10.1 General . 23
10.2 Measurement of velocity . 23
10.3 Determination of the mean velocity . 23
10.4 Correction of average velocity for wall effects . 24
10.5 Calculation of the volume flow rate from the average velocity . 24
10.6 Conversion of results to standard conditions . 24
11 Establishment of the uncertainty of results . 25
12 Evaluation of the method . 26
Annex A (normative) Measurement of velocity using differential pressure based techniques . 27
Annex B (normative) Vane anemometer . 44
iv
ISO/DISFDIS 16911-1:20252026(en)
Annex C (normative) Tracer gas dilution method determination of volume flow rate and
average velocity . 49
Annex D (normative) Transit time tracer gas method determination of average velocity . 57
Annex E (normative) Calculation of waste gas volume flow rate from energy consumption . 66
Annex F (informative) Use of time of flight measurement instruments based on modulated laser
light . 75
Annex G (informative) Example of uncertainty budget established for velocity and volume flow
rate measurements by Pitot tube . 76
Annex H (informative) Description of validation studies . 93
Annex I (informative) Check of validity of the calibration of a Pitot tube . 100
Annex J (informative) Differential pressure measurement . 102
Annex K (informative) Degree of swirl determination example method . 106
Bibliography . 107

v
ISO/WD FDIS 16911-1:2025(E2026(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 document 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).
ISO draws attention to the possibility that the implementation of this document may involve the use of (a)
patent(s). ISO takes no position concerning the evidence, validity or applicability of any claimed patent rights
in respect thereof. As of the date of publication of this document, ISO had not received notice of (a) patent(s)
which may be required to implement this document. However, implementers are cautioned that this may not
represent the latest information, which may be obtained from the patent database available at
www.iso.org/patents. ISO shall not be held responsible for identifying any or all such patent rights.
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 European Committee for Standardization (CEN) Technical
Committee CEN/TC 264, Air quality, in accordance with the Agreement on technical cooperation between ISO
and CEN (Vienna Agreement).
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 (e.g.
Emission Trading Schemes and calibration of automated flow measuring systems) to less demanding (e.g.
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.
vi
ISO/DISFDIS 16911-1:20252026(en)
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.
vii
ISO/WD FDIS 16911-1:2025(E2026(en)
Introduction
For emission monitoring, normally only the mass concentration of the relevant pollutants is measured. These
measurements require in some cases isokinetic sampling on the basis of the local flow velocity at specific
measurement points in a sampling grid.
For many installations, the mass emissions are also determined, which can be calculated as the product of the
pollutant mass concentration and the waste gas volume flow rate.
Therefore, the velocity and volume flow rate of gas in ducts are important parameters. The waste gas volume
flow rate can often be calculated on the basis of known plant parameters, such as fuel consumption or steam
generating capacity. If the plant’s operating parameters fluctuate, the waste gas volume flow rate needs to be
determined directly.
If the cross-section and flow profile of the waste gas flow rate are known, the volume flow rate can be
determined on the basis of the flow velocity. The methods of determining volume flow rate used in emission
measurements are based on local flow velocity measurements taken in the flow cross-section of a waste gas
duct, or e.g. by the measurement of tracer dilution, the measurement of tracer transit time, or bythe calculation
from energy consumption.
viii
DRAFT International Standard ISO/DIS 16911-1:2025(en)

Stationary source emissions — Manual and automatic determination
of velocity and volume flow rate of waste gas 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
in ducts (e.g. stacks) which uses point measurements of the flow velocity. It
This document 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.
[11][10]
NOTE ISO 15259:2023 is identical to EN 15259:2008 .
This document also specifies methods for the determination of volume flow rate based on the measurement
of tracer dilution, the measurement of tracer transit time, and by calculation from energy consumption. This
document provides guidance on when these methods can be used.
This document requiresapplies to all flow measurements to havewith demonstrable metrological traceability
to national or international primary standards.
This document specifies two monitoring objectives with different uncertainty requirements ranging from very
stringent (e.g. Emission Trading Schemes and calibration of automated flow measuring systems) to less
demanding (e.g. support of isokinetic sampling). The level of quality control within this document is
determined by the uncertainty requirements of the monitoring objective. This document specifies which
requirements and performance characteristics apply to specified measurement tasks and application areas
which fall under the two monitoring objectives.
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 specified 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
[10][9]
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.
EN 12952--15:2003, Water-tube boilers and auxiliary installations — Acceptance tests
ISO 15259:2023, Air quality — Measurement of stationary source emissions — Requirements for measurement
sections and sites and for the measurement objective, plan and report
ISO 20988:2007, Air quality — Guidelines for estimating measurement uncertainty
ISO/IEC Guide 98--3:2008, Uncertainty of measurement — Part 3: Guide to the expression of uncertainty in
measurement (GUM:1995)
ISO/IEC 17025, General requirements for the competence of testing and calibration laboratories
3 Terms and definitions
For the purposes of this document, the following terms and definitions 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/
3.1 3.1
monitoring objective
regulatory requirements under which measurement tasks (3.2) are undertaken
3.2 3.2
measurement task
scope of the measurement programme deduced from the monitoring objective (3.1(3.1)) and specific technical
activities carried out under a measurement programme
Note 1 to entry: The term “measurement task (3.2) used in this document” can also refer to a “measurement objective”
as defined in ISO 15259:2023, 3.20. The term “measurement task (3.2)” is used to avoid confusion and to clearly
distinguish between a legally specified monitoring objective (3.1) and the scope of the measurement programme deduced
from the monitoring objective (3.1) and specific technical activities carried out under a measurement programme.
3.3 3.3
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 AAnnex A describes a number of Pitot designs currently in use in Europe.
3.4 3.4
measurement line
line across the duct, on a measurement plane (3.5(3.5),), along which flow measurements are made to
characterize the flow velocity profile or to determine the average flow
3.5 3.5
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 (3.7(3.7)) is requiredmeasured
3.6 3.6
measurement point
sampling term
ISO/DISFDIS 16911-1:20252026(en)
position in the measurement plane (3.5(3.5)) at which the sample stream is extracted or the measurement data
are obtained directly
Note 1 to entry: Measurement point is also known as sampling point.
[SOURCE: ISO 15259:2023, 3.16], modified — Note 1 to entry has been deleted and an admitted term has been
added.]
3.7 3.7
volume flow rate
volume flow of gas axially along a duct
Note 1 to entry: Volume flow rate is expressed in cubic metres per second or cubic metres per hour.
3.8 3.8
point flow velocity
local gas velocity at a point in the duct
Note 1 to entry: Unless otherwise specified, the term can be taken to mean the axial velocity at the measurement location.
Note 2 to entry: Point flow velocity is expressed in metres per second.
3.9 3.9
average flow velocity
velocity which, when multiplied by the area of the measurement plane (3.5(3.5)) of the duct, gives the volume
flow rate (3.7(3.7)) in that duct
3.10 3.10
standard conditions
conditions for reference values for pressure (101,3 kPa) and for temperature (273,15 K)
3.11 3.11
uncertainty (
uncertainty of measurement)
parameter, associated with the result of a measurement, that characterizes the dispersion of the values that
couldcan reasonably be attributed to the measurand
[SOURCE: ISO/IEC Guide 98-3:2008, B.2.18]
, modified — "could" has been replaced with "can" in the definition and NOTES 1, 2 and 3.12 have been
deleted.]
3.12
uncertainty budget
statement of a measurement uncertainty (3.11,) of the components of that measurement uncertainty, and of
their calculation and combination
[5][5]
Note 1 to entry: For the purposes of this document, the sources of uncertainty are defined according to ISO 14956 or
ISO/IEC Guide 98--3.
3.13 3.13
standard uncertainty
uncertainty (3.11) of the result of a measurement expressed as a standard deviation
[SOURCE: ISO/IEC Guide 98-3:2008, 2.3.1]
3.14 3.14
expanded uncertainty
quantity defining an interval about the result of a measurement that maycan be expected to encompass a large
fraction of the distribution of values that couldcan reasonably be attributed to the measurand
[SOURCE: ISO/IEC Guide 98-3:2008, 2.3.5], modified — "may" and "could" have been replaced with "can" in
the definition.]
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 %.
3.15 3.15
overall uncertainty
expanded uncertainty (3.14(3.14)) attached to the measurement result
Note 1 to entry: The overall uncertainty is calculated according to ISO/IEC Guide 98-3.
-3.
3.16 16
swirl
cyclonic flow
tangential component of the flow vector providing a measure of the non-axial flow at the measurement plane
(3.5(3.5))
3.17 3.17
automated measuring system
AMS
measuring system permanently installed on site for continuous measurement of flow
Note 1 to entry: See The determination of velocity and volume flow rate with AMS is described in ISO 16911--2.
3.18 3.18
portable automated measuring system
P-AMS
automated measuring system (3.17) which is in a condition or application to be moved from one to another
measurement site to obtain measurement results for a short measurement period
Note 1 to entry: The measurement period is typically 8 h for a day.
Note 2 to entry: The P-AMS can be configured at the measurement site for the special application but can be also set-up
in a van or mobile container. The probe and the sample gas lines are installed often just before the measurement task is
started.
[12]
[SOURCE: EN 15267-4:2023, 3.3 ]]
3.19 3.19
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 uncertainty of measurement (3.11(3.11))
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,
a documented measurement procedure, accredited technical competence, metrological traceability to the SI units, and
calibration intervals.
ISO/DISFDIS 16911-1:20252026(en)
4 Symbols and abbreviated terms
4.1 Symbols
A internal cross-sectional area of the duct at the measurement plane m
B number of component B
a , a angle between sensing holes °
1 2
c constant
d outer tube diameter mm
dl measuring rod length change m
d inner duct diameter mm
d
e(N) net specific energy (NSE) of the fuel as received MJ/kg
e reproducibility of output quantities
P
F force acting on the vane wheel N
−1
f vane frequency s
fms mass specific fuel factor m /kg
f normalization factor
n,i
f velocity correction factor at measurement point i
v,cor,i
fWA wall adjustment factor
h corrected height of the indicating fluid of a liquid manometer to standard
s
temperature
ht height of the indicating fluid at the temperature when read
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
𝐾𝐾(𝜌𝜌 ) non-linearnonlinear calibration factor dependent on density ρ , and viscosity
0,𝜂𝜂
dyn
ηdyn
k coverage factor
L length of the measurement section, i.e. the stack length between the two m
measurement levels
L probe length m
p
L measuring rod initial length m
M molar mass of wet waste gas kg/mol
M molar mass of component B kg/mol
B
n number of measurement points
P energy production MW
p waste 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
𝑝𝑝 ¯ average static pressure in the measurement section Pa
stat
Δp differential pressure Pa
¯
𝛥𝛥𝑝𝑝 average dynamic pressure measured at the measurement point i of the Pa
𝚤𝚤
measurement plane
𝛥𝛥𝑝𝑝 maximum value of dynamic pressure measured Pa
𝑖𝑖,max
𝛥𝛥𝑝𝑝 minimum values of dynamic pressure measured Pa
𝑖𝑖,min
q tracer mass flow rate kg/s
m,t
qV volume flow rate m /s
q dry volume flow rate, under standard conditions of temperature and pressure m /s
V,0d
𝑞𝑞 dry volume flow rate, under standard conditions of temperature and pressure m /s
V,0d,O
and on actual oxygen concentration
𝑞𝑞 dry volume flow rate, under standard conditions of temperature and pressure, m /s
V,0d,O ,ref
and reference oxygen concentration
𝑞𝑞 volume 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 duct, m /s
V,w
on wet gas
rSp geometry of the vane wheel
R gas constant 8,314 J/(K mol)
t transit time of the tracer pulse between the two measurement points s
T waste gas temperature K
Tc temperature of gas in the measurement plane K
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
v0 start-up velocity m/s
v axial velocity m/s
c
ν local velocity at measurement point i m/s
i
v measured velocity m/s
m
vt peripheral velocity, vt= = ϖrSp m/s
v average velocity at the fixed measurement point m/s
f,av
v velocity of fixed measurement device when traverse measurement device is at m/s
f,i
measurement point i
v corrected velocity of traverse measurement device at measurement point i m/s
t,cor,i
v velocity of traverse measurement device at measurement point i m/s
t,i
𝑣𝑣 axial approach velocity m/s
∞
ISO/DISFDIS 16911-1:20252026(en)
𝑣𝑣¯ mean velocity m/s
𝑣𝑣¯ mean velocity corrected for wall effects m/s
WA
𝑣𝑣¯ average of the point velocity measurements m/s
p
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
wH hydrogen mass fraction in fuel as received
𝑤𝑤 moisture mass fraction in solid fuel as received
H O
w nitrogen mass fraction in fuel as received
N
w oxygen mass fraction in fuel as received
O
wS sulfur mass fraction in fuel as received
α linear expansion coefficient 1/K
α pitch of blade
η thermal efficiency
ηdyn dynamic viscosity Pa s
θ measured angle °
m
ρ density of the waste gas under duct conditions of temperature and pressure of kg/m
wet gas
ρ density of the indicating fluid of a liquid manometer at standard temperature
s
ρ density of the indicating fluid of a liquid manometer at the temperature when
t
read
𝜎𝜎(𝛥𝛥𝑝𝑝 ) standard deviation of the m dynamic pressure measurements in the
𝑖𝑖
measurement point i
Φ thermal input MW
(N)F
φ volume concentration of component B %
B
𝜑𝜑 CO2 volume concentration in the gas stream in wet gas %
CO ,w
𝜑𝜑 water vapour volume concentration in the gas stream in wet gas %
H O
𝜑𝜑 oxygen volume concentration in the gas stream in dry gas %
O
𝜑𝜑 oxygen volume concentration measured in the duct during the exploration of the %
O ,d
duct on dry gas
𝜑𝜑 reference oxygen volume concentration %
O ,ref
𝜑𝜑 oxygen volume concentration in the gas stream in wet gas %
O ,w
−1
ω angular frequency s
4.2 Abbreviated terms
AMCA Air Movement and Control Association
AMS automated measuring system
CETIAT Centre Technique des Industries Aérauliques et Thermiques
MO monitoring objective
NSE net specific energy
NPL National Physical Laboratory
P-AMS portable automated measuring system
QAL2 quality assurance level 2
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 AAnnex A)) and a calibrated vane anemometer (see Annex BAnnex B).). Selection criteria for the
use of different types of Pitot and the vane anemometer are given in Clause 6Clause 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 10.410.4).).
Three alternative methods are also describedgiven to determine volume flow rate and average flow velocity:
— Annex C— 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.
— Annex D— 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— 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 can be reported at duct conditions or expressed at standard conditions (273,15 K and
101,3 kPa) on either the wet or dry basis.
This document applies to two different 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.
ISO/DISFDIS 16911-1:20252026(en)
5.2 Monitoring objectives
Measurements are grouped based on the required quality control. The grouping is as follows:
MO1: periodic measurements with uncertainty requirements specified under regulatory applications
according to ISO 15259 or pollution inventory reporting which involves the determination of mass
emissions and for the control of isokinetic conditions during manual extractive sampling;
MO2: periodic measurements with more stringent uncertainty requirements, e.g. measurements under
the requirements of Emission Trading Schemes or calibration of an AMS under ISO 16911-2 or
flow profile characterization either to meet the requirements of Emission Trading Schemes or any
other regulatory requirements.
— MO1: periodic measurements with uncertainty requirements according to ISO 15259 or pollution
inventory reporting which involves the determination of mass emissions and for the control of isokinetic
conditions during manual extractive sampling;
— MO2: periodic measurements with more stringent uncertainty requirements, e.g. measurements under
the requirements of Emission Trading Schemes or calibration of an AMS under ISO 16911-2 or flow profile
characterization either to meet the requirements of Emission Trading Schemes.
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 AAnnex A provides details for the use of differential pressure based techniques. Annex BAnnex 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
[3] [3]
ISO 3966 . . 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 in
[4] [4] [3] [3] [18][14]
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.
[20][16]
If 2D Pitot tubes are to be used, information on the necessary QA/QC can be found in US EPA Method 2G .
[19][15]
The usage of 3D Pitot tubes is described in US EPA Method 2F .
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 CAnnex C, Annex D, Annex D and Annex EAnnex E provide details of these
alternative approaches. If an alternative approach is applied, the requirements of the corresponding
Annex CAnnex C, Annex D, Annex D or Annex EAnnex E shall be met.
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 shall be
used to determine the measurement points for circular or rectangular ducts. The tangential methodology
provided in ISO 15259 shall be 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
characterisingcharacterizing the flow profile, but it is not recommended for determining the average flow in
the duct.
The measurement plane is selected to be representative of the required waste gas 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
10.410.4.
The volume flow rate q is determined by multiplying the average velocity by the internal cross-sectional area
V
of the duct at the measurement plane according to Formula (1)Formula (1)::
𝑞𝑞 =𝑣𝑣¯𝐴𝐴 (1)
V p
where
𝑣𝑣¯ is the average of the point velocity measurements;
p
A is the internal cross-sectional area of the duct at 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)Formula (2) which is equivalent to
Formula (1)Formula (1)::
𝑛𝑛
𝐴𝐴
𝑞𝑞 =� 𝑣𝑣 (2)
V 𝑖𝑖
𝑛𝑛
𝑖𝑖=1
ISO/DISFDIS 16911-1:20252026(en)
where
vi is the local velocity at measurement point i;
A is the internal cross-sectional area of the duct at the measurement plane;
n is the number of measurement points.
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 waste gas flow. A known traceable flow rate of a tracer gas with specified composition is injected into
the duct. 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 waste gas has occurred. Adequate mixing can
be achieved when sampling from a test location that meets the homogeneity requirements of ISO 15259 for
the tracer gas concentration. Guidance for achieving good tracer mixing quality are provided in
Annex CAnnex C.
The dilution of the tracer gas by the waste gas provides a measurement of the volume flow rate, provided that:
— — the tracer gas is well mixed in the waste gas;
— — there is no tracer gas present in the waste gas prior to injection or the background concentration can
be measured and subtracted accurately.
5.4.4 Determination of volume flow rate using transit time tracer measurements
A small amount of tracer material is injected rapidly into the waste gas flow, to produce a short pulse of tracer.
After the tracer pulse has moved over the cross-section of the duct, its transit time between two measurement
points placed on a suitable straight duct section is measured. The volume flow rate is calculated by dividing
the duct volume between the measurement points by the transit time. The flow determined using this
technique is representative of a region of the duct defined by the pulse measurement locations, and these are
chosen to be representative of the required measurement plane.
5.4.5 Determination of volume flow rate from calculation approach based on energy consumption
For most combustion sources the volume flow rate can be calculated from the stoichiometric waste gas
volume, determined from the fuel composition and the thermal input rate. The possible calculation methods
are described in EN 12952-15, which includes both direct and indirect methods. In a direct method the fuel
flow is measured and the thermal input is calculated from the specific energy ("(“calorific value")”) of the fuel
and the fuel flow. Use of an indirect method includes measurement of the energy produced and the thermal
efficiency of the plant. Especially for heat generation, or combined heat and power plants, with a high net
thermal efficiency of typically 90 %, the uncertainty of the indirect method to calculate the thermal input is
very low.
To later determine the actual volume flow rate, the oxygen volume concentration at the measurement plane
in many cases shall be used to take account of the excess air. The oxygen concentration can be determined
[8][7]
using EN 14789 . However, the calculation method can also provide results at reference oxygen volume
concentration values without requiring the determination of the oxygen composition in the duct. The
calculation approach determines the volume flow rate on a dry gas basis. It also can be used to determine the
wet waste gas flow but the uncertainty in such cases increases.
6 Selection of measurement technique
6.1 Measurement task
This document provides methods that can be used for two different monitoring objectives. The user of these
methods shall understand the monitoring objective and the associated maximum permissible expanded
uncertainty before undertaking any measurements as required by ISO 15259. The selection of the
measurement technique can depend on the monitoring objective. The monitoring object
...