ISO/TR 12765:1998

TECHNICAL ISO/TR
REPORT 12765
First edition
1998-12-15
Measurement of fluid flow in closed
conduits — Methods using transit-time
ultrasonic flowmeters
Mesure de débit des fluides dans les conduites fermées — Méthodes
utilisant des débitmètres à ultrasons à temps de transit
A
Reference number
Contents Page
1 Scope .1
2 Normative references .1
3 Definitions .1
4 Symbols and subscripts .8
5 General principles of measurements.9
5.1 Generation of ultrasonic signals.9
5.2 Transit-time method .11
5.3 Calculation of volume flowrate q .14
v
6 Types of design.15
6.1 Ultrasonic transducer.15
6.2 Control unit.20
7 Uncertainty of measurement .20
7.1 Calculation procedure.20
7.2 Influence factors .22
8 Calibration .24
8.1 Dry calibration.24
8.2 Flow calibration.25
Annex A (informative) Calculation of volume flowrate by transit-time measurement using pulse
techniques.26
Annex B (informative) Recommendations for use and installation .35
Annex C (informative) Information to be supplied by the manufacturer .40
Bibliography.43
©  ISO 1998
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized in any form or by any means, electronic
or mechanical, including photocopying and microfilm, without permission in writing from the publisher.
International Organization for Standardization
Case postale 56 • CH-1211 Genève 20 • Switzerland
Internet iso@iso.ch
Printed in Switzerland
ii
© ISO
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 main task of technical committees is to prepare International Standards, but in exceptional circumstances a
technical committee may propose the publication of a Technical Report of one of the following types:
 type 1, when the required support cannot be obtained for the publication of an International Standard, despite
repeated efforts;
 type 2, when the subject is still under technical development or where for any other reason there is the future
but not immediate possibility of an agreement on an International Standard;
 type 3, when a technical committee has collected data of a different kind from that which is normally published
as an International Standard (“state of the art”, for example).
Technical Reports of types 1 and 2 are subject to review within three years of publication, to decide whether they
can be transformed into International Standards. Technical Reports of type 3 do not necessarily have to be
reviewed until data they provide are considered to be no longer valid or useful.
ISO/TR 12765, which is a Technical Report of type 2, was prepared by Technical Committee ISO/TC 30,
Measurement of fluid flow in closed conduits.
This document is being isssued in the type 2 Technical Report series of publications (according to subclause
G.4.2.2 of part 1 of the ISO/IEC Directives, 1992) as a “prospective standard for a provisional application” in the
field of ultrasonic flowmeters because there is an urgent need for guidance on how standards in this field should be
used to meet an identified need.
This document is not to be regarded as an “International Standard”. It is proposed for provisional application so that
information and experience of its use in practice may be gathered. Comments on the content of this document
should be sent to the ISO Central Secretariat.
A review of this type 2 Technical Report will be carried out not later than three years after its publication with the
options of: extension for another three years; conversion into an International Standard; or withdrawal.
Annexes A, B and C of this Technical Report are for information only.
iii
TECHNICAL REPORT  © ISO ISO/TR 12765:1998(E)
Measurement of fluid flow in closed conduits — Methods
using transit-time ultrasonic flowmeters
1 Scope
This Technical Report gives guidance on the principles and main design features of ultrasonic flowmeters based on
the measurement of the difference in transit time for volume flowrate measurement of fluids. It covers their
operation, performance and calibration. It primarily covers wetted transducers but briefly refers to clamp-on
transducer arrangements.
Annex A of this Technical Report shows the calculation of volume flowrate by transit-time measurement using pulse
techniques.
Annex B covers the recommendations for use and installation.
Annex C gives a list of information to be supplied by the manufacturers.
2 Normative references
The following standards contain provisions which, through reference in this text, constitute provisions of this
Technical Report. At the time of publication, the editions indicated were valid. All standards are subject to revision,
and parties to agreements based on this International Standard are encouraged to investigate the possibility of
applying the most recent editions of the standards indicated below. Members of IEC and ISO maintain registers of
currently valid International Standards.
ISO 4006:1991, Measurement of fluid flow in closed conduits — Vocabulary and symbols.
ISO 4185:1980, Measurement of liquid flow in closed conduits — Weighting method.
ISO 8316:1987, Measurement of liquid flow in closed conduits — Method by collection of the liquid in a volumetric
tank.
ISO 9300:1990, Measurement of gas flow by means of critical flow Venture nozzles.
ISO 9951:1993, Measurement of gas flow in closed conduits — Turbine meters.
International Vocabulary of Basic and General Terms in Metrology (VIM), BIPM, IEC, IFCC, ISO, IUPAC, IUPAP,
OIML, 1993.
3 Definitions
For the purposes of this Technical Report, the definitions given in the International Vocabulary of Basic and General
Terms in Metrology (VIM), ISO 4006 and the following definitions apply.
© ISO
3.1
transit-time difference method
time-of-flight method
method of flowrate measurement in which the average fluid velocity along the acoustic path v is determined from
the transit-time difference of two ultrasonic signals, one travelling upstream and one downstream, over the same
distance in the flowing fluid
3.2
leading-edge method
method of flowrate measurement in which the transit times of ultrasonic pulses are measured based on triggering at
a predetermined amplitude level of the received signal
See Figure 1.
Key
1 Trigger point at leading edge
Figure 1 — Principle of transit-time measurement using leading-edge method
3.3
pulse-repetition frequency method
sing-around method
frequency-difference method
method of flowrate measurement used in ultrasonic flowmeters whereby two independent streams of pulses are
transmitted in opposite directions, each pulse being emitted immediately after the detection of the preceding pulse in
the stream, and the difference between the pulse-repetition frequencies in the two directions is measured
NOTE The difference between the pulse-repetition frequencies in the two directions is a function of the fluid velocity.
3.4
phase control method
lambda-locked-loop method
method of flowrate measurement in which a measure of the average fluid velocity along the acoustic path v is
derived from the difference in frequency of sound with the same wavelength travelling in opposite directions through
the flowing fluid
3.5
zero-crossing method
method of flowrate measurement in which transit times of ultrasonic pulses are measured using the first (or another
predetermined) "zero-crossing" of the received signal following the first half alternance
See Figure 2.
© ISO
Key
1 Trigger point at zero crossing
Figure 2 — Principle of transit-time measurement using zero-crossing method
3.6
multi-path method
method of flowrate measurement in which the average fluid velocity over a number of different paths is determined
3.7
simultaneous pulse method
method of flow measurement by which the transit times and transit-time difference are determined from signals
which are transmitted simultaneously upstream and downstream over the same acoustic path
3.8
phase shift method
method of flow measurement in which the average fluid velocity along the acoustic path v is determined from the
phase shift of ultrasonic signals in a fluid flow
3.9
ultrasonic flowmeter
USM
flowmeter which generates ultrasonic signals and receives them again after they have been influenced by the flow
in such a way that the observed result can be used as a measure of the flowrate
NOTE An ultrasonic flowmeter normally consists of the ultrasonic transducers and equipment which evaluates the flowrate
measurement from the emitted and received ultrasonic signals and converts these signals to a standard output signal
proportional to the flowrate
3.10
flowrate integrator
device for volume measurement by time-integration of volume flowrate
3.11
ultrasonic transducer
element that converts acoustic energy into electrical signals and/or vice versa
NOTE Ultrasonic transducers used in transit-time flowmeters usually work as both transmitter and receiver.
3.12
clamp-on arrangement
arrangement by which the transducers are attached to the outside wall of the conduit in which the flowrate is to be
measured
© ISO
3.13
meter tube
specially fabricated section of conduit containing the ultrasonic transducers and conforming in all respects to the
specification of the standard
3.14
measurement section
section of conduit consisting of the meter tube, the inlet section and the outlet section
3.15
acoustic path
actual path of the ultrasonic signal between both transducers
3.16
path length
L
p
length of acoustic path, in fluid at rest, from the faces of both transducers
See Figure 3 a) and b).
3.17
interrogation length
L
length of that part of the acoustic path, in fluid at rest, inside the conduit
See Figure 3 a) and b).
3.18
interrogation distance
d
projection of the interrogation length on the line parallel to the axis of the conduit or of the flow
See Figure 3 a) and b).
3.19
inclination angle
f
angle between the axes of the ultrasonic transducers and a line parallel to the axis of the conduit
See Figure 3 a).
3.20
phase angle
phase position of an oscillation
3.21
propagation velocity
c
velocity of acoustic signals relative to an observer at rest
3.22
velocity of sound
c
velocity of acoustic signals in the fluid at rest
3.23
average fluid velocity along the acoustic path
v
fluid velocity in the plane which is formed by the acoustic path and the direction of flow
© ISO
3.24
mean axial fluid velocity
v
A
ratio of the volume flowrate (q ) [the integral over a cross-section of the meter tube of the axial components of the
v
local fluid velocities (v)] to the area of the measurement cross-section (A)
3.25
velocity distribution correction factor
k
h
ratio of the mean axial fluid velocity v in the meter run to the average axial flow velocity v along the acoustic path
A
3.26
ultrasonic pulse
signal generated by finite-duration electrical excitation of an ultrasonic transducer
3.27
continuous-wave ultrasound
signal generated by continuous electrical excitation of an ultrasonic transducer
3.28
transit time
t
time needed by an ultrasonic pulse to traverse the acoustic path
3.29
transit-time difference
Dt
differe
...