EN 61161:2007

SLOVENSKI STANDARD
01-februar-2008
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SIST EN 61161:2002
SIST EN 61161:2002/A1:2002
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Ultrasonics - Power measurement - Radiation force balances and performance
requirements (IEC 61161:2006)
Ultraschall - Leistungsmessung - Schallfeldkraft-Waagen und Anforderungen an ihre
Funktionseigenschaften (IEC 61161:2006)
Ultrasons - Mesurage de puissance - Balances de forces de rayonnement et exigences
de fonctionnement (IEC 61161:2006)
Ta slovenski standard je istoveten z: EN 61161:2007
ICS:
17.140.50
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EUROPEAN STANDARD
EN 61161
NORME EUROPÉENNE
April 2007
EUROPÄISCHE NORM
ICS 17.140.50 Supersedes EN 61161:1994 + A1:1998

English version
Ultrasonics -
Power measurement -
Radiation force balances and performance requirements
(IEC 61161:2006)
Ultrasons -  Ultraschall -
Mesurage de puissance - Leistungsmessung -
Balances de forces de rayonnement Schallfeldkraft-Waagen
et exigences de fonctionnement und Anforderungen an ihre
(CEI 61161:2006) Funktionseigenschaften
(IEC 61161:2006)
This European Standard was approved by CENELEC on 2007-03-01. CENELEC members are bound to comply
with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this European Standard
the status of a national standard without any alteration.

Up-to-date lists and bibliographical references concerning such national standards may be obtained on
application to the Central Secretariat or to any CENELEC member.

This European Standard exists in three official versions (English, French, German). A version in any other
language made by translation under the responsibility of a CENELEC member into its own language and notified
to the Central Secretariat has the same status as the official versions.

CENELEC members are the national electrotechnical committees of Austria, Belgium, Bulgaria, Cyprus, the
Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia,
Lithuania, Luxembourg, Malta, the Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia, Spain,
Sweden, Switzerland and the United Kingdom.

CENELEC
European Committee for Electrotechnical Standardization
Comité Européen de Normalisation Electrotechnique
Europäisches Komitee für Elektrotechnische Normung

Central Secretariat: rue de Stassart 35, B - 1050 Brussels

© 2007 CENELEC - All rights of exploitation in any form and by any means reserved worldwide for CENELEC members.
Ref. No. EN 61161:2007 E
Foreword
The text of document 87/325/CDV, future edition 2 of IEC 61161, prepared by IEC TC 87, Ultrasonics,
was submitted to the IEC-CENELEC parallel Unique Acceptance Procedure and was approved by
CENELEC as EN 61161 on 2007-03-01.
This European Standard supersedes EN 61161:1994 + A1:1998.
The main significant changes are:
– the main body of the standard has been restricted to normative statements;
– informative statements on corresponding aspects of ultrasonic power measurement and radiation
force balances have been collected in Annex A;
– Annexes A, D, E and F are new;
– more radiation force balance arrangements are dealt with. The new material relates particularly to
power measurement of ultrasonic physiotherapy devices.
The following dates were fixed:
– latest date by which the EN has to be implemented
at national level by publication of an identical
national standard or by endorsement (dop) 2007-12-01
– latest date by which the national standards conflicting
with the EN have to be withdrawn (dow) 2010-03-01
The following print types are used:
– requirements: roman type;
– notes: small roman type;
– words in bold in the text are defined in Clause 3.
The numbers in square brackets refer to the Bibliography (after the annexes).
Annex ZA has been added by CENELEC.
__________
Endorsement notice
The text of the International Standard IEC 61161:2006 was approved by CENELEC as a European
Standard without any modification.
In the official version, for Bibliography, the following notes have to be added for the standards indicated:
IEC 60601-2-5 NOTE  Harmonized as EN 60601-2-5:2000 (not modified).
IEC 61157 NOTE  Harmonized as EN 61157:1994 (not modified).
__________
- 3 - EN 61161:2007
Annex ZA
(normative)
Normative references to international publications
with their corresponding European publications

The following referenced documents are indispensable for the application of this document. For dated
references, only the edition cited applies. For undated references, the latest edition of the referenced
document (including any amendments) applies.

NOTE  When an international publication has been modified by common modifications, indicated by (mod), the relevant EN/HD
applies.
Publication Year Title EN/HD Year

1)
IEC 60050-801 - International Electrotechnical Vocabulary - -
(IEV) -
Chapter 801: Acoustics and electroacoustics

IEC 60854 1986 Methods of measuring the performance of - -
ultrasonic pulse-echo diagnostic equipment

IEC 60866 1987 Characteristics and calibration of - -
hydrophones for operation in the frequency
range 0,5 MHz to 15 MHz
IEC 61101 1991 The absolute calibration of hydrophones EN 61101 1993
using the planar scanning technique in the
frequency range 0,5 MHz to 15 MHz

IEC 61102 1991 Measurement and characterisation of EN 61102 1993
ultrasonic fields using hydrophones in the
frequency range 0,5 MHz to 15 MHz

IEC 61689 1996 Ultrasonics - Physiotherapy systems - EN 61689 1996
Performance requirements and methods of
measurement in the frequency range
0,5 MHz to 5 MHz
IEC 61846 1998 Ultrasonics - Pressure pulse lithotripters - EN 61846 1998
Characteristics of fields
1)
Undated reference.
NORME CEI
INTERNATIONALE
IEC
INTERNATIONAL
Deuxième édition
STANDARD
Second edition
2006-12
Ultrasons – Mesurage de puissance –
Balances de forces de rayonnement
et exigences de fonctionnement

Ultrasonics – Power measurement –
Radiation force balances and
performance requirements
© IEC 2006 Droits de reproduction réservés ⎯ Copyright - all rights reserved
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61161 © IEC:2006 – 3 –
CONTENTS
FOREWORD.7
INTRODUCTION.11

1 Scope.13
2 Normative references .13
3 Terms and definitions .15
4 List of symbols .17
5 Requirements for radiation force balances.17
5.1 General .17
5.2 Target type.19
5.3 Target diameter.19
5.4 Balance / Force measuring system .21
5.5 System tank .21
5.6 Target support structures .21
5.7 Transducer positioning .21
5.8 Anti-streaming foils.21
5.9 Transducer coupling.21
5.10 Calibration.21
6 Requirements for measuring conditions .23
6.1 Lateral target position.23
6.2 Transducer-target separation .23
6.3 Water .23
6.4 Water contact .23
6.5 Environmental conditions .23
6.6 Thermal drifts .23
7 Measurement uncertainty .25
7.1 General .25
7.2 Balance system including target suspension.25
7.3 Linearity and resolution of the balance system .25
7.4 Extrapolation to the moment of switching the ultrasonic transducer .25
7.5 Target imperfections.27
7.6 Reflecting target geometry .27
7.7 Lateral absorbers in the case of reflecting target measurements .27
7.8 Target misalignment.27
7.9 Ultrasonic transducer misalignment .27
7.10 Water temperature .27
7.11 Ultrasonic attenuation and acoustic streaming.27
7.12 Foil properties .27
7.13 Finite target size.27
7.14 Plane-wave assumption.27
7.15 Environmental influences .29
7.16 Excitation voltage measurement .29
7.17 Ultrasonic transducer temperature.29
7.18 Non-linearity.29
7.19 Other sources.29

61161 © IEC:2006 – 5 –
Annex A (informative) Additional information on various aspects of radiation force
measurements .33
Annex B (normative) Basic formulae .57
Annex C (informative) Other methods of ultrasonic power measurement.63
Annex D (informative) Propagation medium and degassing.65
Annex E (informative) Radiation force measurement with diverging ultrasonic beams .75
Annex F (informative) Limitations associated with the balance arrangements.83

Bibliography.93

Figure 1 – Section through an absorbing target.31
Figure 2 – Linearity check: balance readout as a function of the input quantity .31
Figure D.1 – Example of dissolved oxygen concentration as a function of time for
200 ml of vacuum degassed water in a glass with liquid surface area of 34 cm .69
Figure D.2 – Dissolved oxygen concentration as a function of time for 2 g/l, 4 g/l and
6 g/l of sodium sulphite in de-mineralized water and for different surface areas and
volumes of water.71
Figure E.1 – Piston result (oscillating curve) for P/cF as a function of ka, with "peak"
approximation (unbroken line) and the central, half-way curve (broken line)
representing the correction factor corr. .77
Figure E.2 – P/cF as a function of ka for four different pseudo-trapezoidal amplitude
distributions, with ε = 0 (piston) (solid); ε = 0,1 (dash); ε = 0,25 (dot); ε = 0,6
(dash/dot). .77
Figure E.3 – Ratio of the radiation conductance G as obtained using a convex-conical
reflecting target with a cone half-angle of 45° to an absorbing target versus the value
of ka for 11 different transducers to be used in physiotherapy and 3 different
laboratories [26].81
Figure F.1 – Arrangement A, with absorbing (a) or reflecting (b) target .85
Figure F.2 – Arrangement B, with convex-conical reflecting target .85
Figure F.3 – Arrangement B, with absorbing target .85
Figure F.4 – Arrangement C, with absorbing target .87
Figure F.5 – Arrangement E, with absorbing (a) or concave-conical reflecting (b) target .87
Figure F.6 – Arrangement F, with convex-conical reflecting target .87
Figure F.7 – Arrangement F, with absorbing target .87

Table D.1 – Water degassing methods.67
Table D.2 – Conditions for degassing by boiling.69
Table F.1 – Advantages and disadvantages of different arrangements.91

61161 © IEC:2006 – 7 –
INTERNATIONAL ELECTROTECHNICAL COMMISSION
–––––––––
ULTRASONICS – POWER MEASUREMENT −
RADIATION FORCE BALANCES AND PERFORMANCE REQUIREMENTS

FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
all national electrotechnical committees (IEC National Committees). The object of IEC is to promote
international co-operation on all questions concerning standardization in the electrical and electronic fields. To
this end and in addition to other activities, IEC publishes International Standards, Technical Specifications,
Technical Reports, Publicly Available Specifications (PAS) and Guides (hereafter referred to as “IEC
Publication(s)”). Their preparation is entrusted to technical committees; any IEC National Committee interested
in the subject dealt with may participate in this preparatory work. International, governmental and non-
governmental organizations liaising with the IEC also participate in this preparation. IEC collaborates closely
with the International Organization for Standardization (ISO) in accordance with conditions determined by
agreement between the two organizations.
2) The formal decisions or agreements of IEC on technical matters express, as nearly as possible, an international
consensus of opinion on the relevant subjects since each technical committee has representation from all
interested IEC National Committees.
3) IEC Publications have the form of recommendations for international use and are accepted by IEC National
Committees in that sense. While all reasonable efforts are made to ensure that the technical content of IEC
Publications is accurate, IEC cannot be held responsible for the way in which they are used or for any
misinterpretation by any end user.
4) In order to promote international uniformity, IEC National Committees undertake to apply IEC Publications
transparently to the maximum extent possible in their national and regional publications. Any divergence
between any IEC Publication and the corresponding national or regional publication shall be clearly indicated in
the latter.
5) IEC provides no marking procedure to indicate its approval and cannot be rendered responsible for any
equipment declared to be in conformity with an IEC Publication.
6) All users should ensure that they have the latest edition of this publication.
7) No liability shall attach to IEC or its directors, employees, servants or agents including individual experts and
members of its technical committees and IEC National Committees for any personal injury, property damage or
other damage of any nature whatsoever, whether direct or indirect, or for costs (including legal fees) and
expenses arising out of the publication, use of, or reliance upon, this IEC Publication or any other IEC
Publications.
8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
indispensable for the correct application of this publication.
9) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of
patent rights. IEC shall not be held responsible for identifying any or all such patent rights.
International Standard IEC 61161 has been prepared by IEC technical committee 87:
Ultrasonics
This second edition of IEC 61161 cancels and replaces the first edition published in 1992, and
its Amendment 1 (1998). It constitutes a technical revision. The main significant changes are:
– the main body of the Standard has been restricted to normative statements;
– informative statements on corresponding aspects of ultrasonic power measurement and
radiation force balances have been collected in Annex A;
– Annexes A, D, E and F are new;
– more radiation force balance arrangements are dealt with. The new material relates
particularly to power measurement of ultrasonic physiotherapy devices.

61161 © IEC:2006 – 9 –
The text of this standard is based on the following documents:
Enquiry draft Report on voting
87/325/CDV 87/358/RVC
Full information on the voting for the approval of this standard can be found in the report on
voting indicated in the above table.
This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.
NOTE The following print types are used:
• Requirements: roman type
• Notes: in small roman type
• Words in bold in the text are defined in Clause 3.
• The numbers in square brackets refer to the Bibliography (after the annexes).
The committee has decided that the contents of this publication will remain unchanged until
the maintenance result date indicated on the IEC web site under "http://webstore.iec.ch" in
the data related to the specific publication. At this date, the publication will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
61161 © IEC:2006 – 11 –
INTRODUCTION
A number of measuring methods exist for the determination of the total emitted power of
ultrasonic transducers ([1], [2], [3], see also Annex C). The purpose of this International
Standard is to establish standard methods of measurement of ultrasonic power in liquids in
the lower megahertz frequency range based on the measurement of the radiation force using
a gravimetric balance. The great advantage of radiation force measurements is that a value
for the total radiated power is obtained without the need to integrate field data over the cross-
section of the radiated sound beam. This standard identifies the sources of errors and
describes a systematic step-by-step procedure to assess overall measurement uncertainty as
well as the precautions that should be undertaken, and uncertainties that should be taken into
account, while performing power measurements
Basic safety requirements for ultrasonic physiotherapy devices are identified in IEC 60601-2-5
and make reference to IEC 61689, which specifies the need for acoustic power measurements
with an uncertainty better than ± 15 %. Considering the usual degradation of accuracy in the
practical application of this standard, reference measurement methods need to be established
with uncertainties better than ± 7 %. Ultrasonic diagnostic device declaration requirements,
including acoustic power, are specified in other IEC standards, as for example in IEC 61157.
The measurement of acoustic power accurately, precisely and repeatably using a radiation
force balance as defined in this standard is influenced by a number of practical problems. As
a guide to the user, additional information is provided in Annex A using the same section and
clause numbering as the main body.

61161 © IEC:2006 – 13 –
ULTRASONICS – POWER MEASUREMENT –
RADIATION FORCE BALANCES AND PERFORMANCE REQUIREMENTS

1 Scope
This International Standard
• specifies a method of determining the total emitted acoustic power of ultrasonic
transducers based on the use of a radiation force balance;
• establishes general principles for the use of radiation force balances in which an obstacle
(target) intercepts the sound field to be measured;
• establishes limitations of the radiation force method related to cavitation and temperature
rise;
• establishes quantitative limitations of the radiation force method in relation to diverging
and focused beams;
• provides information on assessment of overall measurement uncertainties.
This International Standard is applicable to:
• the measurement of ultrasonic power up to 1 W based on the use of a radiation force
balance in the frequency range from 0,5 MHz to 25 MHz;
• the measurement of ultrasonic power up to 20 W based on the use of a radiation force
balance in the frequency range 0,75 MHz to 5 MHz;
• the measurement of total ultrasonic power of transducers, preferably with well-collimated
beams;
• the use of radiation force balances of the gravimetric type or force feedback type.
NOTE The titles of all publications referred to in this Standard are listed in the Bibliography.
2 Normative references
The following referenced documents are indispensable for the application of this document.
For dated references, only the edition cited applies. For undated references, the latest edition
of the referenced document (including any amendments) applies.
IEC 60050, International Electrotechnical Vocabulary (IEV) – Chapter 801: Acoustics and
Electroacoustics, Chapter 802: Ultrasonics
IEC 60854:1986, Methods of measuring the performance of ultrasonic pulse-echo diagnostic
equipment
IEC 60866:1987, Characteristics and calibration of hydrophones for operation in the
frequency range 0,5 MHz to 15 MHz
IEC 61101:1991, The absolute calibration of hydrophones using the planar scanning
technique in the frequency range 0,5 MHz to 15 MHz
IEC 61102:1991, Measurement and characterisation of ultrasonic fields using hydrophones in
the frequency range 0,5 MHz to 15 MHz
IEC 61689:1996, Ultrasonics – Physiotherapy systems – Performance requirements and
methods of measurement in the frequency range 0,5 MHz to 5 MHz
IEC 61846:1998, Ultrasonics – Pressure pulse lithotripters – Characteristics of fields

61161 © IEC:2006 – 15 –
3 Terms and definitions
For the purposes of this document, the following terms and definitions as well as the
definitions of IEC 60050-801 and IEC 60050-802 apply.
3.1
acoustic streaming
bulk fluid motion initiated by a sound field
3.2
free field
sound field in a homogeneous isotropic medium whose boundaries exert a negligible effect on
the sound waves
[IEV 801-23-28, modified]
3.3
output power
time-average ultrasonic power emitted by an ultrasonic transducer into an approximately
free field under specified conditions in a specified medium, preferably water
Symbol: P
Unit: watt, W
3.4
radiation force
acoustic radiation force
time-average force acting on a body in a sound field and caused by the sound field, excluding
the component due to acoustic streaming; or, more generally: time-average force (excluding
the component due to acoustic streaming) in a sound field, appearing at the boundary surface
between two media of different acoustic properties
Symbol: F
Unit: Newton, N
3.5
radiation pressure
acoustic radiation pressure
radiation force per unit area
NOTE This term is widely used in the literature. However, strictly speaking, the radiation force per unit area is a
tensor quantity [4] and it should be referred to as the acoustic radiation stress tensor when a strict scientific
terminology is to be used. The integral quantity "acoustic radiation force" is generally preferred in this International
Standard. Whenever at some places the term "acoustic radiation pressure" appears, it is to be understood as the
negative value of the normal radiation stress in the direction of the field axis.
Unit: Pascal, Pa
3.6
target
device specially designed to intercept substantially all of the ultrasonic field and to serve as
the object which is acted upon by the radiation force
3.7
ultrasonic transducer
device capable of converting electrical energy to mechanical energy within the ultrasonic
frequency range and/or reciprocally of converting mechanical energy to electrical energy

61161 © IEC:2006 – 17 –
3.8
radiation conductance
ratio of the acoustic output power and the squared RMS transducer input voltage. It is used
to characterize the electrical to acoustical transfer of ultrasonic transducers
Symbol: G
Unit: siemens, S
4 List of symbols
a radius of a source ultrasonic transducer
c speed of sound (usually in water)
d geometrical focal length of a focused ultrasonic transducer
F radiation force on a target in the direction of the incident ultrasonic wave
g acceleration due to gravity
G radiation conductance
k circular wavenumber (2π/λ)
P output power of an ultrasonic transducer
s normalized distance from an ultrasonic transducer (s = z λ / a )
z distance between a target and an ultrasonic transducer
α amplitude attenuation coefficient of plane waves in a medium (usually water)
γ focus (half-)angle of a focused ultrasonic transducer (arc sin a/d)
θ angle between the direction of the incident ultrasonic wave and the normal to a reflecting
surface of a target
λ ultrasonic wavelength
ρ (mass) density of the sound-propagating medium (usually water).
NOTE The direction of the incident wave mentioned above under F and θ is understood to be the direction of the
field axis, i.e., it is understood in a global sense rather than in a local sense.
5 Requirements for radiation force balances
5.1 General
The radiation force balance shall consist of a target which is connected to a balance. The
ultrasonic beam shall be directed vertically upwards or downwards or horizontally on the
target and the radiation force exerted by the ultrasonic beam shall be measured by the
balance. The ultrasonic power shall be determined from the difference between the force
measured with and without ultrasonic radiation, in accordance with the formulae given in
Annex B. Calibration can be carried out by means of small precision weights of known mass.
NOTE Different possible radiation force measurement set-ups are presented in Figures F.1 to F.7. Each
measurement set-up has its own merits, which are also summarised in Annex F.

61161 © IEC:2006 – 19 –
5.2 Target type
5.2.1 General
The target shall have known acoustic properties, these being relevant to the details of the
relation between ultrasonic power and radiation force. (See also A.5.2.)
If the target is chosen so as to closely approach one of the two extreme cases, i.e. perfect
absorber or perfect reflector, the appropriate formula of Annex B shall be used depending on
the field structure and the following requirements shall apply:
5.2.2 Absorbing target
An absorbing target (see Figure 1, F.1a, F.3, F.4, F.5a and F.7) shall have:
ƒ an amplitude reflection factor of less than 3,5 %;
ƒ an acoustic energy absorption within the target of at least 99 %.
(See also A.5.2.2.)
5.2.3 Reflecting target
A reflecting target (see Figure F.1b, F.2, F.5b and F.6) shall have:
ƒ an amplitude reflection factor of greater than 99 %.
A conical reflecting target should not be used for power measurements of transducers where
ka < 30. A convex-conical reflector with a cone half-angle of 45° shall not be used for power
measurements of transducers where ka < 17,4, which follows from theoretical consideration of
the effects of beam divergence. See also A.5.3.
NOTE 1 The exact meaning of the quantity a depends on circumstances. For practical transducers, this is the
effective transducer radius in accordance with the particular definition in the field of application. In model
calculations using a piston approach, it is the geometrical piston radius.
In addition, a convex-conical reflector with a cone half-angle of 45° should not be used for
power measurements of focused transducers where d < 32a. If the geometrical focal length d
is not known, then a convex-conical reflector with a cone half-angle of 45° should not be used
when the distance z of the pressure maximum from the transducer is z < 1/((1/32a) + (λ/a )).
f f
NOTE 2 This condition recommends to restrict the use of convex-conical reflectors to the unfocused case or the
case of weak focusing, and this is in line with the restriction to "preferably well-collimated beams" in the Scope. If,
nevertheless, a convex-conical reflector is used in strongly focused fields and formula (B.5) is applied, additional
uncertainties that are not covered by Clause 7 need to be taken into account.
(See also A.5.2.3 and B.6.)
5.3 Target diameter
The target diameter shall be large enough to intercept all significant parts of the field. As a
general requirement, it shall in no case be lower than 1,5 times the appropriate dimension
(e.g. the diameter) of the ultrasonic transducer.
Whether or not the target diameter should be more than 1,5 times the transducer diameter,
depends on the beam diameter of the field at the particular location of the target. The beam
dimensions shall be measured or calculated from theoretical estimation as given, for example
in A.5.3.
61161 © IEC:2006 – 21 –
5.4 Balance / Force measuring system
The radiation force balance may be a gravimetric balance with, therefore, the beam
orientation vertical. Alternatively the balance may be of a force feed-back design, allowing the
beam to be horizontal. If the balance has been calibrated against mass units, a correct
conversion of the balance readings to force values shall be ensured by the manufacturer of
the radiation force device or by the user.
NOTE Vertical beam orientation allows traceability to national mass standards (calibrated weights). Set-ups with
horizontal beam orientation exist in practice using either a reflecting target [5,6] or an absorbing target [7].
Calibration may be carried out using an appropriate balance arm attachment, or by calibration against sources of
known acoustic power.
The balance used shall have sufficient resolution for the magnitude of the ultrasonic power to
be measured. (See A.5.4)
5.5 System tank
If a reflecting target is used, an absorbing lining of the measuring vessel shall be used so that
returning reflections do not contribute to more than 1 % of the overall measured power. (See
also A.5.5.)
5.6 Target support structures
In static-force balances, the structural members supporting the target and carrying the
radiation force across the air-water interface shall be designed to limit the effect of surface
tension to less than 1 % of the overall measured power. (See also A.5.6.)
5.7 Transducer positioning
The ultrasonic transducer mount shall allow stable and reproducible positioning of the
ultrasonic transducer with respect to the target in a way that related changes in overall
measured power do not exceed 1 %.
5.8 Anti-streaming foils
If an anti-streaming foil is used it shall be positioned close to the target and shall not be
oriented parallel to the surface of the ultrasonic transducer [8]. Its transmission coefficient
shall be known from measurement and a correction shall be applied, if its influence is more
than 1 % of the overall measured power. (See also A.5.8.)
NOTE In practice a tilt angle of 5° to 10° has been found to be adequate.
5.9 Transducer coupling
The ultrasonic transducer shall be coupled to the measurement device such that the impact
on the overall measured power is less than 1 %, otherwise a correction shall be applied. (See
also A.5.9.)
5.10 Calibration
The radiation force balance shall be calibrated by the use of small weights of known mass.
Also, the radiation force balance should be calibrated by use of an ultrasonic source of
known output power. In this case, the calibration shall be undertaken once every two years or
more frequently if there is any indication that the balance sensitivity to ultrasonic power has
changed.
61161 © IEC:2006 – 23 –
6 Requirements for measuring conditions
6.1 Lateral target position
The lateral position of the target during measurement shall be constant and reproducible to an
extent that related changes in overall measured power do not exceed 1 %. (See also A.6.1.)
6.2 Transducer-target separation
The distance between the ultrasonic transducer surface and the target, or foil (if used) and
target, shall be known and reproducible to an extent that possible changes in overall
measured power do not exceed 1 %. (See also A.6.2.)
6.3 Water
When using a radiation force balance, the liquid used for the measurements shall be water.
For determining output powers above 1 W, only degassed water shall be used.
Degassing of water shall be accomplished in a well-defined process such as described in
Annex D. Where degassed water is required, the amount of dissolved oxygen in the water
shall be <4 mg/l during all measurements.
(See also A.6.3.)
6.4 Water contact
Before starting the measurements, it shall be ensured that all air bubbles are removed from
the active faces. After measurements are completed, the active faces shall again be
inspected, and the measurements shall be discarded if any air bubbles are found. (See also
A.6.4.)
6.5 Environmental conditions
For measurements in the milliwatt and microwatt region, either the measuring device shall be
provided with thermal isolation or the measurement process, including data acquisition, shall
be performed in such a way that thermal drift and other disturbances during the measurement
cause no more than a 1 % effect on the overall measured power.
The measuring device shall be protected against environmental vibrations and air flow.
(See also A.6.5.)
6.6 Thermal drifts
When using an absorbing target, an estimate of the thermal effects due to the absorbed
sound energy (expansion and buoyancy change) shall be made by recording the measured
signal before and after the switch-on and switch-off of the ultrasonic transducer. (See also
A.6.6.)
61161 © IEC:2006 – 25 –
7 Measurement uncertainty
7.1 General
An estimation of the overall measurement uncertainty or accuracy assessment shall be
determined individually for each set-up used. This assessment should include the following
elements.
The uncertainty shall be assessed using the ISO Guide [9].
7.2 Balance system including target suspension
The balance system shall be checked or calibrated using small weights of known mass with
the whole system prepared for radiation force measurements, including with the target
suspended in water.
This procedure shall be repeated several times with each weight to obtain an indication of the
random scatter of results. An uncertainty estimate for the balance calibration factor shall be
derived from the results of this calibration and from the mass uncertainty of the weights used.
The results of these checks should be filed in order to enable a judgement of the long-term
stability of the balance calibration factor.
(See also A.7.2.)
7.3 Linearity and resolution of the balance system
The linearity of the balance system shall be checked at least every six months as follows.
The measurements described in 7.2 shall be made with at least three weights of different
masses within the balance output range of interest. The balance readout as a function of input
mass can be represented as a graph in accordance with Figure 2. The resulting points of this
graph should ideally be on a straight line starting at the origin of the coordinates. If deviations
from this line occur, an additional uncertainty contribution shall be derived from them.
Since weights of less than 10 mg are difficult to handle, the balance linearity can also be
checked by means of an ultrasonic transducer with known properties, activated by various
levels of voltage amplitude and thus producing radiation forces of various magnitudes. In this
case the input quantity at the abscissa of Figure 2 is the ultrasonic output power of the
transducer, and its uncertainty shall be taken into account.
The limited resolution of the balance leads to a power uncertainty contribution that needs to
be taken into account in the uncertainty analysis.
7.4 Extrapolation to the moment of switching the ultrasonic transducer
In the case of an electronic balance, to obtain the radiation force value, the balance output
signal is typically recorded as a function of time and extrapolated back to the moment of
switching the ultrasonic transducer. This extrapolation involves an uncertainty, depending
mainly on the amount of scatter in the balance output signal (signal-to-noise ratio). The
uncertainty of the extrapolation result shall be estimated by means of standard mathematical
procedures in utilizing the regression algorithm.

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7.5 Target imperfections
The influence of the target imperfections shall be estimated using a plane-wave approach
such as described in A.7.5.
7.6 Reflecting target geometry
The influence of the reflecting target geometry shall be estimated and incorporated into the
overall system uncertainty. (See A.7.6)
7.7 Lateral absorbers in the case of reflecting target measurements
The imperfections of the lateral absorbers in the arrangement of Figure F.1b, F.2, F.5b and
F.6 shall be estimated and incorporated into the overall system uncertainty. (See also A.7.7.)
7.8 Target misalignment
The influence of target misalignment shall be estimated and incorporated into the overall
system uncertainty. (See A.7.8)
7.9 Ultrasonic transducer misalignment
The influence of ultrasonic transducer misalignment shall be estimated and incorporated
into the overall system uncertainty. (See A.7.9)
7.10 Water temperature
The uncertainty caused by water temperature shall be estimated and incorporated into the
overall system uncertainty. (See A.7.10)
7.11 Ultrasonic attenuation and acoustic streaming
The uncertainty caused by ultrasonic attenuation and acoustic streaming shall be estimated
and incorporated into the overall system uncertainty. (See A.7.11)
7.12 Foil properties
If a coupling foil or a shielding foil is used during the radiation force measurements, the foil
transmission loss as measured or estimated shall be taken into account, as well as any
possible effect of the reflected wave on the ultrasonic transducer. The uncertainty
introduced by these effects shall be assessed individually and incorporated into the overall
system uncertainty.
7.13 Finite target size
The effect on uncertainty of the finite target size shall be determined and included in the
overall system uncertainty. (See A.7.13)
7.14 Plane-wave assumption
The uncertainty contribution due to the use of a plane-wave assumption shall be determined
and included in the overall system uncertainty. (See A.7.14)

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7.15 Environmental influences
The uncertainties caused by environmental vibrations, air flow or temperature variations shall
be estimated and incorporated into the overall system uncertainty. (See A.7.15)
7.16 Excitation voltage measurement
If the excitation voltage applied to the ultrasonic transducer is measured and its value is of
relevance to the result of the ultrasonic power measurement, its measurement uncertainty
shall be estimated and incorporated into the overall system uncertainty. (See also A.7.16.)
7.17 Ultrasonic transducer temperature
If ultrasonic power values measured at different temperatures are to be compared, the
dependence of the power on the temperature shall be checked and its influence be taken into
account. (See also A.7.17.)
7.18 Non-linearity
The potential influence of non-linearities regarding the following shall be assessed and, if
necessary, included in the overall system uncertainty:
a) the linearity of the balance system including the target suspension;
b) non-linear contributions due to improperly degassed water;
c) ultrasonic attenuation and acoustic streaming;
d) the theoretical radiation force relations themselves.
(See A.7.18)
7.19 Other sources
Checks should be performed periodically to determine whether the overall uncertainty as
specified in 7.2 to 7.18 using the above guidelines is not influenced
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