Hydrometry - Calibration of current-meters in straight open tanks

This document specifies a calibration method for mechanical type, electromagnetic type and acoustic type hydrometric current-meters used for point velocity measurement of flowing water. The method requires towing the instrument through still water in a straight open tank. It includes measuring apparatus, the calibration procedure, the method of presenting the results and the uncertainties associated with the method.

Hydrométrie - Étalonnage des moulinets en bassins découverts rectilignes

Hidrometrija - Kalibracija merilnikov tokov v ravnih odprtih cisternah

General Information

Status
Published
Publication Date
05-Sep-2021
Technical Committee
Current Stage
6060 - National Implementation/Publication (Adopted Project)
Start Date
01-Sep-2021
Due Date
06-Nov-2021
Completion Date
06-Sep-2021

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SLOVENSKI STANDARD
SIST ISO 3455:2021
01-oktober-2021
Nadomešča:
SIST ISO 3455:2013
Hidrometrija - Kalibracija merilnikov tokov v ravnih odprtih cisternah
Hydrometry - Calibration of current-meters in straight open tanks
Hydrométrie - Étalonnage des moulinets en bassins découverts rectilignes
Ta slovenski standard je istoveten z: ISO 3455:2021
ICS:
17.120.20 Pretok v odprtih kanalih Flow in open channels
SIST ISO 3455:2021 en

2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

---------------------- Page: 1 ----------------------
SIST ISO 3455:2021
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SIST ISO 3455:2021
INTERNATIONAL ISO
STANDARD 3455
Third edition
2021-01
Hydrometry — Calibration of current-
meters in straight open tanks
Hydrométrie — Étalonnage des moulinets en bassins découverts
rectilignes
Reference number
ISO 3455:2021(E)
ISO 2021
---------------------- Page: 3 ----------------------
SIST ISO 3455:2021
ISO 3455:2021(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO 2021

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
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: +41 22 749 01 11
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
ii © ISO 2021 – All rights reserved
---------------------- Page: 4 ----------------------
SIST ISO 3455:2021
ISO 3455:2021(E)
Contents Page

Foreword ........................................................................................................................................................................................................................................iv

1 Scope ................................................................................................................................................................................................................................. 1

2 Normative references ...................................................................................................................................................................................... 1

3 Terms and definitions ..................................................................................................................................................................................... 1

4 Principle of calibration .................................................................................................................................................................................. 1

4.1 Statement of the principle ............................................................................................................................................................. 1

4.2 Accuracy of the method ................................................................................................................................................................... 2

4.2.1 Overall uncertainty on the velocity measurement ............................................................................. 2

4.2.2 Requirements for accurate measurements ............................................................................................... 2

5 Infrastructure .......................................................................................................................................................................................................... 2

5.1 Dimensions of the towing tank ................................................................................................................................................. 2

5.1.1 General...................................................................................................................................................................................... 2

5.1.2 Length........................................................................................................................................................................................ 2

5.1.3 Depth and width ............................................................................................................................................................... 2

5.2 Towing cart ................................................................................................................................................................................................. 3

5.2.1 General...................................................................................................................................................................................... 3

5.2.2 Cart track system ............................................................................................................................................................. 3

5.2.3 Types of towing carts ................................................................................................................................................... 3

5.2.4 Cart operation .................................................................................................................................................................... 3

5.2.5 Cart control ........................................................................................................................................................................... 4

5.3 Measuring equipment ....................................................................................................................................................................... 4

5.3.1 General...................................................................................................................................................................................... 4

5.3.2 Distance measurement ............................................................................................................................................... 4

5.3.3 Time measurement ........................................................................................................................................................ 4

5.3.4 Current-meter velocity measurement ........................................................................................................... 4

5.3.5 Sources of error related to infrastructure .................................................................................................. 5

5.4 Data acquisition ..................................................................................................................................................................................... 6

5.5 Data processing ...................................................................................................................................................................................... 6

5.6 Other requirements ............................................................................................................................................................................ 6

6 Calculation of uncertainty .......................................................................................................................................................................... 6

7 Calibration procedure ..................................................................................................................................................................................... 7

7.1 Calibration of rotating element current-meters ........................................................................................................ 7

7.1.1 Suspension of the current-meter ....................................................................................................................... 7

7.1.2 Performance of calibration ..................................................................................................................................... 7

7.1.3 Data analysis ........................................................................................................................................................................ 8

7.1.4 Presentation of results ................................................................................................................................................ 9

7.2 Calibration of electromagnetic current-meters .......................................................................................................10

7.2.1 Instructions for calibration ..................................................................................................................................10

7.2.2 Mounting the electromagnetic current-meter ....................................................................................10

7.2.3 Number of calibration points .............................................................................................................................10

7.2.4 Performance of calibration ..................................................................................................................................11

7.2.5 Data analysis .....................................................................................................................................................................11

7.2.6 Presentation of results .............................................................................................................................................11

7.3 Calibration of acoustic current-meters for point velocity measurement ..........................................11

7.3.1 Instructions for calibration ..................................................................................................................................11

7.3.2 Mounting the acoustic current-meter ........................................................................................................12

7.3.3 Performance of calibration ..................................................................................................................................12

7.3.4 Data analysis .....................................................................................................................................................................12

7.3.5 Presentation of results .............................................................................................................................................12

Bibliography .............................................................................................................................................................................................................................13

© ISO 2021 – All rights reserved iii
---------------------- Page: 5 ----------------------
SIST ISO 3455:2021
ISO 3455:2021(E)
Foreword

ISO (the International Organization for Standardization) is a worldwide federation of national standards

bodies (ISO member bodies). The work of preparing International Standards is normally carried out

through ISO technical committees. Each member body interested in a subject for which a technical

committee has been established has the right to be represented on that committee. International

organizations, governmental and non-governmental, in liaison with ISO, also take part in the work.

ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of

electrotechnical standardization.

The procedures used to develop this document and those intended for its further maintenance are

described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the

different types of ISO documents should be noted. This document was drafted in accordance with the

editorial rules of the ISO/IEC Directives, Part 2 (see www .iso .org/ directives).

Attention is drawn to the possibility that some of the elements of this document may be the subject of

patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of

any patent rights identified during the development of the document will be in the Introduction and/or

on the ISO list of patent declarations received (see www .iso .org/ patents).

Any trade name used in this document is information given for the convenience of users and does not

constitute an endorsement.

For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and

expressions related to conformity assessment, as well as information about ISO's adherence to the

World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT) see www .iso .org/

iso/ foreword .html.

This document was prepared by Technical Committee ISO/TC 113, Hydrometry, Subcommittee SC 5,

Instruments, equipment and data management.

This third edition cancels and replaces the second edition (ISO 3455:2007), which has been technically

revised.
The main changes compared to the previous editions are as follows:

— a subclause for calibration of acoustic current-meters for point velocity measurement has been added;

— clauses referring to outdated tracking systems like track systems using tooth belts have been

removed;

— clauses referring to outdated technique for data acquisition like strip chart recorder or magnetic

tapes have been removed;

— the clause for computerized data acquisition and processing system has been removed;

— the clause discussing the Epper effect has been removed.

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.
iv © ISO 2021 – All rights reserved
---------------------- Page: 6 ----------------------
SIST ISO 3455:2021
INTERNATIONAL STANDARD ISO 3455:2021(E)
Hydrometry — Calibration of current-meters in straight
open tanks
1 Scope

This document specifies a calibration method for mechanical type, electromagnetic type and acoustic

type hydrometric current-meters used for point velocity measurement of flowing water. The method

requires towing the instrument through still water in a straight open tank. It includes measuring

apparatus, the calibration procedure, the method of presenting the results and the uncertainties

associated with the method.
2 Normative references

The following documents are referred to in the text in such a way that some or all of their content

constitutes requirements of this document. For dated references, only the edition cited applies. For

undated references, the latest edition of the referenced document (including any amendments) applies.

ISO 772, Hydrometry — Vocabulary and symbols
ISO 2537, Hydrometry — Rotating-element current-meters

ISO/IEC Guide 98-3Uncertainty of measurement — Part 3: Guide to the expression of uncertainty in

me a s ur ement (GUM: 1995)
3 Terms and definitions

For the purposes of this document, the terms and definitions given in ISO 772 apply.

ISO and IEC maintain terminological databases for use in standardization at the following addresses:

— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at http:// www .electropedia .org/
4 Principle of calibration
4.1 Statement of the principle

Calibration of a current-meter means experimental determination of the relationship between water

velocity and either the rate of revolution of the rotating element or the velocity directly indicated

by the current-meter. For this purpose, the current-meter is mounted on a towing cart and drawn

through still water contained in a straight tank with a uniform cross section at a number of steady

speeds of the towing cart. Simultaneous measurements of the speed of the towing cart and the rate

of revolution of the rotating element or the velocity indicated by the current-meter are made. In the

case of rotating-element current-meters, the two parameters are related by one or more formula(e), the

limits of validity of which are stated. In the case of stationary-sensor type current-meters, containing

no rotating elements, the velocity indicated by its display unit is compared with the corresponding cart

speed to know the error in measurement.
© ISO 2021 – All rights reserved 1
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SIST ISO 3455:2021
ISO 3455:2021(E)
4.2 Accuracy of the method
4.2.1 Overall uncertainty on the velocity measurement

The towing method gives an absolute measurement of water speed, which in principle only requires

position and time measurements. This method can be considered as very accurate if the precautions

listed in 4.2.2 are taken.
4.2.2 Requirements for accurate measurements
The towing method gives an accurate measurement of water speed provided that:

a) the position, the timing and means for starting and stopping it achieve the necessary accuracy;

b) residual currents in the water are small.
5 Infrastructure
5.1 Dimensions of the towing tank
5.1.1 General

The dimensions of the tank and the number and relative position of current-meters in the tank cross

section shall be chosen so that their effects on the test result are minimized.
5.1.2 Length

The length of a rating tank comprises of accelerating, stabilizing, measuring and braking sections.

The length of the accelerating and braking sections depend on the design of the cart, the maximum

acceleration and deceleration achievable at maximum payload, and the maximum speed at which the

payload is to be towed along the tank. Safety requirements of the cart should be taken into account

while working out the length of the braking section. The length of the measuring section shall be such

that the calibration error, which is composed of inaccuracies in the measurement of time, distance

covered and rate of revolution, does not exceed the desired tolerance at any velocity. The required

length, therefore, depends on the type of current-meter being calibrated, type of cart and the way the

signals are produced and transmitted.
5.1.3 Depth and width

The depth of the tank can have an influence on the test results which cannot be regarded as negligible,

more particularly when the towing speed coincides with the velocity of propagation of the surface

wave. The dependence of this critical velocity, v , on tank depth is given by the Formula (1):

vg= d (1)
where
g is the acceleration due to gravity;
d is the depth of water.

Depending on the size of the current-meter(s) and the cross section of the suspension equipment

relative to the cross-sectional area of the tank, the wave crest produced by the current-meter and its

means of suspension may cause an error in calibration within a narrow band in the velocity range from

0,5 v to 1,5 v . It is a systematic and not a random error.
c c
2 © ISO 2021 – All rights reserved
---------------------- Page: 8 ----------------------
SIST ISO 3455:2021
ISO 3455:2021(E)

The depth and width of the tank shall therefore be chosen to suit the size and the maximum velocity

limits of the current-meters to be calibrated. Care shall be taken to ensure that either high calibration

velocities are attained before the interference or that they exceed it sufficiently for the critical zone to

be bridged without extrapolation.
5.2 Towing cart
5.2.1 General

During calibration, the current-meter is suspended below the cart and immersed in the water at a

specified depth and the cart travels along the length of the tank at known and accurate speeds in the

measuring section.
5.2.2 Cart track system

The cart may run on two parallel rails which shall be accurately aligned with both the length of the

tank and the surface of the water in the tank. It is essential that the rails are straight and that both the

rails and the wheels of the cart are free of irregularities in order to avoid irregular motions of the cart.

The material and hardness of the rails and the driving wheels should be chosen so that there shall not

be undue wear and tear of the wheels. In the case of rubber tire wheels, provision shall be made to lift

the wheels above the rail surface when not in use for a long time.
5.2.3 Types of towing carts
The following types of towing carts are in common usage.

a) The towed cart which is moved along the track by a cable driven from a constant speed motor

standing apart from the moving cart. The towed cart may be lighter in construction with the

consequent advantage of high acceleration and quick braking, but the elasticity of the towing cable

can cause irregularities in the running of the cart thereby affecting the accuracy of current-meter

calibration.

b) The self-propelled cart which is moved along the track by internally mounted electric motor(s). The

power to the cart may be fed by a trailing wire track system, by an overhead conductor system or

other systems specially designed for the purpose. The self-propelled cart is heavier in construction

as it carries the driving motors. This results in greater inertia of the cart and assists in smoothing

out the running irregularities of the cart.
5.2.4 Cart operation

The cart shall travel smoothly and at constant speed in the measuring section of the towing tank

ensuring that oscillatory motion is not transmitted to the current-meters under calibration.

The cart shall have smooth operational capability. Vibration of the tow cart should be avoided.

The cart shall remain stable during acceleration, deceleration and braking. There shall not be any

forward/backward or sideways rocking, or slippage during peak acceleration/deceleration and during

normal operation at any speed in specified range.

During calibration, the measuring equipment, sensors and other instruments shall not be affected

by noise and spurious signals induced by the main power supply or cart drive and control system or

otherwise by electrical equipment installed in the rating tank building and vicinity.

In addition to normal braking, an alternate brake system shall also be provided on the cart which would

automatically activate during an emergency.
© ISO 2021 – All rights reserved 3
---------------------- Page: 9 ----------------------
SIST ISO 3455:2021
ISO 3455:2021(E)
5.2.5 Cart control

The cart may be manned or unmanned. In the case of a manned cart, an operator on-board controls

various functions of the cart.
The unmanned cart is operated remotely without any operator on board.
5.3 Measuring equipment
5.3.1 General

The calibration of a current-meter calls for the simultaneous recording of the following three

parameters:
a) distance covered by the cart;
b) time; and

c) signal (pulses) delivered by the current-meter or velocity processed by the meter control unit.

The towing speed is calculated from the simultaneous measurement of distance and time. In case of

a rotating-element current-meter, the rate of current-meter revolutions (rotations) is obtained by the

simultaneous measurement of the number of signals (pulses) and the time.
5.3.2 Distance measurement

Different methods are available for measurement of distance to the specified measurement uncertainty

(see 5.3.5). Two of the most common methods are as follows:

a) the establishment of light barriers (markers) at regular intervals along the length of the tank which

actuate mechanical or optical pulse transmitters fitted to the cart;

b) the use of measuring wheels with mechanical or photoelectric pulse transmitters/optical encoders

which are drawn along the track by the cart.

In the case of using a measuring wheel, it shall be ensured that there is no slippage during travel. An

additional method of precise speed measurement shall also be provided to check the accuracy of the

measuring wheel on a regular basis.
5.3.3 Time measurement

The time of travel of the cart is normally measured by an electronic counter with an in-built accurate

time reference, for example a quartz crystal. A period can thus be read to 1 ms or better. This equipment

should be checked periodically against a reference device traceable to a national time standard.

5.3.4 Current-meter velocity measurement

The cart shall be provided with a suitable recording device for the measurement of current-meter

signals.

In the case of rotating-element current-meters, the sensor of the current-meter shall generate a clear

and positive signal corresponding to the rotor revolutions. Normally, as per the design of the system,

the signals are generated once per revolution, twice per revolution, or in some cases, once for five

revolutions or even once for 10 revolutions. The signals received from the current-meter(s) may be

counted using the counting device of the current-meter. In measuring the number of current-meter

revolutions in a given time, it is important to measure between identical points on the current-meter

signal. It shall be ensured that none of the signals are missed.

In the case of an electromagnetic or acoustic current-meter, the electrical signals from its sensor are

processed by its control unit.
4 © ISO 2021 – All rights reserved
---------------------- Page: 10 ----------------------
SIST ISO 3455:2021
ISO 3455:2021(E)
5.3.5 Sources of error related to infrastructure
5.3.5.1 General

Sources of error (sources of uncertainty) shall be treated according to ISO/IEC Guide 98-3 (GUM). Only

the principal sources of systematic and random errors are considered below.
5.3.5.2 Error due to the distance measurement

The following influence factors can contribute to the measurement uncertainty of the distance

measurement method:
— systematic uncertainty of the reference distance measuring method;
— thermal expansion on measuring wheels;
— repeatability of the distance measurement method;
— detection/counting of impulses from pulse transmitter or encoders.
5.3.5.3 Error due to timing measurement

The following influence factors can contribute to the measurement uncertainty of the timing

measurement method:
— systematic uncertainty of the reference timing measuring method;
— drift of the time reference.
5.3.5.4 Error due to residual currents in the water

The water in the tank is never completely still and residual currents from various origins are always

present. The following influence factors can contribute to the upper limit on residual currents:

— standing waves (surface gravity waves) linked to the dimensions of the tank and due to previous

measurements or disturbances in the water;
— thermal convection currents due to temperature gradients.
5.3.5.5 Error due to environmental conditions

The ambient temperature fluctuation should be as low as possible, and direct sunlight should be

avoided, to minimize the influence on the tank's measurement system and to reduce the creation of

thermal convection currents.

Electromagnetic interference generated by drive units and drive unit controls such as frequency

converters and power rails, especially if these are located directly on the tow carts, may have an

influence on the device under test and sensitive electronic devices.

Magnetic inductive flow meters may be affected by ferro magnetic structures like steel beams or

reinforced concrete in the near surrounding.
5.3.5.6 Random errors

The repeatability with which the cart speed can be determined depends on the repeatability of the

distance measurement method and on the timing accuracy. For any installation, this may be determined

experimentally by setting the cart speed to a fixed value and performing a series of runs to provide a

series of estimations of the cart speed.
© ISO 2021 – All rights reserved 5
---------------------- Page: 11 ----------------------
SIST ISO 3455:2021
ISO 3455:2021(E)

This is repeated for several different cart speeds and from the standard deviation of each series of

measurements, the 95 % confidence limits may be evaluated.

The repeatability of the rate of revolution of the rotating element or the velocity indicated by the

current-meter for different cart speeds can be determined in a similar way.
5.4 Data acquisition

In order to facilitate automation and greater accuracy in measurement, a computerized data acquisition

system may be provided.

The data acquisition software shall accept any relevant information related to calibration.

Velocity measurement devices use a wide range of data collection and transfer methods. The calibration

facility should ensure that their equipment does not interfere with the data transfer signal.

5.5 Data processing

The data processing software shall be customized to process the calibration curves, calibration

formula(e) and calibration tables as described in 7.1.4.

Generic functions/tools like least square estimation, statistical functions, and tabulation for rating

tables shall be provided in the processing software.
5.6 Other requirements

In addition, provision of the following ancillary equipment at the towing tank facility is recommended:

a) artificial beaches, stilling devices or other similar devices to reduce the reflection of disturbance in

the water by the end walls of the tank;

b) means for checking the orientation of the stationary-sensor type current-meters;

c) one or more thermometers to measure the temperature of the water in the tank;

d) conductivity measurement device if magnetic inductive instruments are calibrated.

6 Calculation of uncertainty

The uncertainty associated with a measurement is obtained by combining the uncertainties arising

from the sources described in 5.3.5 in accordance to GUM.

As an example, Formula (2) can be used for estimating the combined uncertainty of velocity.

22 2
Uu=+ uu+ 2 (2)
 d tc 
where
...

INTERNATIONAL ISO
STANDARD 3455
Third edition
2021-01
Hydrometry — Calibration of current-
meters in straight open tanks
Hydrométrie — Étalonnage des moulinets en bassins découverts
rectilignes
Reference number
ISO 3455:2021(E)
ISO 2021
---------------------- Page: 1 ----------------------
ISO 3455:2021(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO 2021

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
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: +41 22 749 01 11
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
ii © ISO 2021 – All rights reserved
---------------------- Page: 2 ----------------------
ISO 3455:2021(E)
Contents Page

Foreword ........................................................................................................................................................................................................................................iv

1 Scope ................................................................................................................................................................................................................................. 1

2 Normative references ...................................................................................................................................................................................... 1

3 Terms and definitions ..................................................................................................................................................................................... 1

4 Principle of calibration .................................................................................................................................................................................. 1

4.1 Statement of the principle ............................................................................................................................................................. 1

4.2 Accuracy of the method ................................................................................................................................................................... 2

4.2.1 Overall uncertainty on the velocity measurement ............................................................................. 2

4.2.2 Requirements for accurate measurements ............................................................................................... 2

5 Infrastructure .......................................................................................................................................................................................................... 2

5.1 Dimensions of the towing tank ................................................................................................................................................. 2

5.1.1 General...................................................................................................................................................................................... 2

5.1.2 Length........................................................................................................................................................................................ 2

5.1.3 Depth and width ............................................................................................................................................................... 2

5.2 Towing cart ................................................................................................................................................................................................. 3

5.2.1 General...................................................................................................................................................................................... 3

5.2.2 Cart track system ............................................................................................................................................................. 3

5.2.3 Types of towing carts ................................................................................................................................................... 3

5.2.4 Cart operation .................................................................................................................................................................... 3

5.2.5 Cart control ........................................................................................................................................................................... 4

5.3 Measuring equipment ....................................................................................................................................................................... 4

5.3.1 General...................................................................................................................................................................................... 4

5.3.2 Distance measurement ............................................................................................................................................... 4

5.3.3 Time measurement ........................................................................................................................................................ 4

5.3.4 Current-meter velocity measurement ........................................................................................................... 4

5.3.5 Sources of error related to infrastructure .................................................................................................. 5

5.4 Data acquisition ..................................................................................................................................................................................... 6

5.5 Data processing ...................................................................................................................................................................................... 6

5.6 Other requirements ............................................................................................................................................................................ 6

6 Calculation of uncertainty .......................................................................................................................................................................... 6

7 Calibration procedure ..................................................................................................................................................................................... 7

7.1 Calibration of rotating element current-meters ........................................................................................................ 7

7.1.1 Suspension of the current-meter ....................................................................................................................... 7

7.1.2 Performance of calibration ..................................................................................................................................... 7

7.1.3 Data analysis ........................................................................................................................................................................ 8

7.1.4 Presentation of results ................................................................................................................................................ 9

7.2 Calibration of electromagnetic current-meters .......................................................................................................10

7.2.1 Instructions for calibration ..................................................................................................................................10

7.2.2 Mounting the electromagnetic current-meter ....................................................................................10

7.2.3 Number of calibration points .............................................................................................................................10

7.2.4 Performance of calibration ..................................................................................................................................11

7.2.5 Data analysis .....................................................................................................................................................................11

7.2.6 Presentation of results .............................................................................................................................................11

7.3 Calibration of acoustic current-meters for point velocity measurement ..........................................11

7.3.1 Instructions for calibration ..................................................................................................................................11

7.3.2 Mounting the acoustic current-meter ........................................................................................................12

7.3.3 Performance of calibration ..................................................................................................................................12

7.3.4 Data analysis .....................................................................................................................................................................12

7.3.5 Presentation of results .............................................................................................................................................12

Bibliography .............................................................................................................................................................................................................................13

© ISO 2021 – All rights reserved iii
---------------------- Page: 3 ----------------------
ISO 3455:2021(E)
Foreword

ISO (the International Organization for Standardization) is a worldwide federation of national standards

bodies (ISO member bodies). The work of preparing International Standards is normally carried out

through ISO technical committees. Each member body interested in a subject for which a technical

committee has been established has the right to be represented on that committee. International

organizations, governmental and non-governmental, in liaison with ISO, also take part in the work.

ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of

electrotechnical standardization.

The procedures used to develop this document and those intended for its further maintenance are

described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the

different types of ISO documents should be noted. This document was drafted in accordance with the

editorial rules of the ISO/IEC Directives, Part 2 (see www .iso .org/ directives).

Attention is drawn to the possibility that some of the elements of this document may be the subject of

patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of

any patent rights identified during the development of the document will be in the Introduction and/or

on the ISO list of patent declarations received (see www .iso .org/ patents).

Any trade name used in this document is information given for the convenience of users and does not

constitute an endorsement.

For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and

expressions related to conformity assessment, as well as information about ISO's adherence to the

World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT) see www .iso .org/

iso/ foreword .html.

This document was prepared by Technical Committee ISO/TC 113, Hydrometry, Subcommittee SC 5,

Instruments, equipment and data management.

This third edition cancels and replaces the second edition (ISO 3455:2007), which has been technically

revised.
The main changes compared to the previous editions are as follows:

— a subclause for calibration of acoustic current-meters for point velocity measurement has been added;

— clauses referring to outdated tracking systems like track systems using tooth belts have been

removed;

— clauses referring to outdated technique for data acquisition like strip chart recorder or magnetic

tapes have been removed;

— the clause for computerized data acquisition and processing system has been removed;

— the clause discussing the Epper effect has been removed.

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.
iv © ISO 2021 – All rights reserved
---------------------- Page: 4 ----------------------
INTERNATIONAL STANDARD ISO 3455:2021(E)
Hydrometry — Calibration of current-meters in straight
open tanks
1 Scope

This document specifies a calibration method for mechanical type, electromagnetic type and acoustic

type hydrometric current-meters used for point velocity measurement of flowing water. The method

requires towing the instrument through still water in a straight open tank. It includes measuring

apparatus, the calibration procedure, the method of presenting the results and the uncertainties

associated with the method.
2 Normative references

The following documents are referred to in the text in such a way that some or all of their content

constitutes requirements of this document. For dated references, only the edition cited applies. For

undated references, the latest edition of the referenced document (including any amendments) applies.

ISO 772, Hydrometry — Vocabulary and symbols
ISO 2537, Hydrometry — Rotating-element current-meters

ISO/IEC Guide 98-3Uncertainty of measurement — Part 3: Guide to the expression of uncertainty in

me a s ur ement (GUM: 1995)
3 Terms and definitions

For the purposes of this document, the terms and definitions given in ISO 772 apply.

ISO and IEC maintain terminological databases for use in standardization at the following addresses:

— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at http:// www .electropedia .org/
4 Principle of calibration
4.1 Statement of the principle

Calibration of a current-meter means experimental determination of the relationship between water

velocity and either the rate of revolution of the rotating element or the velocity directly indicated

by the current-meter. For this purpose, the current-meter is mounted on a towing cart and drawn

through still water contained in a straight tank with a uniform cross section at a number of steady

speeds of the towing cart. Simultaneous measurements of the speed of the towing cart and the rate

of revolution of the rotating element or the velocity indicated by the current-meter are made. In the

case of rotating-element current-meters, the two parameters are related by one or more formula(e), the

limits of validity of which are stated. In the case of stationary-sensor type current-meters, containing

no rotating elements, the velocity indicated by its display unit is compared with the corresponding cart

speed to know the error in measurement.
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ISO 3455:2021(E)
4.2 Accuracy of the method
4.2.1 Overall uncertainty on the velocity measurement

The towing method gives an absolute measurement of water speed, which in principle only requires

position and time measurements. This method can be considered as very accurate if the precautions

listed in 4.2.2 are taken.
4.2.2 Requirements for accurate measurements
The towing method gives an accurate measurement of water speed provided that:

a) the position, the timing and means for starting and stopping it achieve the necessary accuracy;

b) residual currents in the water are small.
5 Infrastructure
5.1 Dimensions of the towing tank
5.1.1 General

The dimensions of the tank and the number and relative position of current-meters in the tank cross

section shall be chosen so that their effects on the test result are minimized.
5.1.2 Length

The length of a rating tank comprises of accelerating, stabilizing, measuring and braking sections.

The length of the accelerating and braking sections depend on the design of the cart, the maximum

acceleration and deceleration achievable at maximum payload, and the maximum speed at which the

payload is to be towed along the tank. Safety requirements of the cart should be taken into account

while working out the length of the braking section. The length of the measuring section shall be such

that the calibration error, which is composed of inaccuracies in the measurement of time, distance

covered and rate of revolution, does not exceed the desired tolerance at any velocity. The required

length, therefore, depends on the type of current-meter being calibrated, type of cart and the way the

signals are produced and transmitted.
5.1.3 Depth and width

The depth of the tank can have an influence on the test results which cannot be regarded as negligible,

more particularly when the towing speed coincides with the velocity of propagation of the surface

wave. The dependence of this critical velocity, v , on tank depth is given by the Formula (1):

vg= d (1)
where
g is the acceleration due to gravity;
d is the depth of water.

Depending on the size of the current-meter(s) and the cross section of the suspension equipment

relative to the cross-sectional area of the tank, the wave crest produced by the current-meter and its

means of suspension may cause an error in calibration within a narrow band in the velocity range from

0,5 v to 1,5 v . It is a systematic and not a random error.
c c
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ISO 3455:2021(E)

The depth and width of the tank shall therefore be chosen to suit the size and the maximum velocity

limits of the current-meters to be calibrated. Care shall be taken to ensure that either high calibration

velocities are attained before the interference or that they exceed it sufficiently for the critical zone to

be bridged without extrapolation.
5.2 Towing cart
5.2.1 General

During calibration, the current-meter is suspended below the cart and immersed in the water at a

specified depth and the cart travels along the length of the tank at known and accurate speeds in the

measuring section.
5.2.2 Cart track system

The cart may run on two parallel rails which shall be accurately aligned with both the length of the

tank and the surface of the water in the tank. It is essential that the rails are straight and that both the

rails and the wheels of the cart are free of irregularities in order to avoid irregular motions of the cart.

The material and hardness of the rails and the driving wheels should be chosen so that there shall not

be undue wear and tear of the wheels. In the case of rubber tire wheels, provision shall be made to lift

the wheels above the rail surface when not in use for a long time.
5.2.3 Types of towing carts
The following types of towing carts are in common usage.

a) The towed cart which is moved along the track by a cable driven from a constant speed motor

standing apart from the moving cart. The towed cart may be lighter in construction with the

consequent advantage of high acceleration and quick braking, but the elasticity of the towing cable

can cause irregularities in the running of the cart thereby affecting the accuracy of current-meter

calibration.

b) The self-propelled cart which is moved along the track by internally mounted electric motor(s). The

power to the cart may be fed by a trailing wire track system, by an overhead conductor system or

other systems specially designed for the purpose. The self-propelled cart is heavier in construction

as it carries the driving motors. This results in greater inertia of the cart and assists in smoothing

out the running irregularities of the cart.
5.2.4 Cart operation

The cart shall travel smoothly and at constant speed in the measuring section of the towing tank

ensuring that oscillatory motion is not transmitted to the current-meters under calibration.

The cart shall have smooth operational capability. Vibration of the tow cart should be avoided.

The cart shall remain stable during acceleration, deceleration and braking. There shall not be any

forward/backward or sideways rocking, or slippage during peak acceleration/deceleration and during

normal operation at any speed in specified range.

During calibration, the measuring equipment, sensors and other instruments shall not be affected

by noise and spurious signals induced by the main power supply or cart drive and control system or

otherwise by electrical equipment installed in the rating tank building and vicinity.

In addition to normal braking, an alternate brake system shall also be provided on the cart which would

automatically activate during an emergency.
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ISO 3455:2021(E)
5.2.5 Cart control

The cart may be manned or unmanned. In the case of a manned cart, an operator on-board controls

various functions of the cart.
The unmanned cart is operated remotely without any operator on board.
5.3 Measuring equipment
5.3.1 General

The calibration of a current-meter calls for the simultaneous recording of the following three

parameters:
a) distance covered by the cart;
b) time; and

c) signal (pulses) delivered by the current-meter or velocity processed by the meter control unit.

The towing speed is calculated from the simultaneous measurement of distance and time. In case of

a rotating-element current-meter, the rate of current-meter revolutions (rotations) is obtained by the

simultaneous measurement of the number of signals (pulses) and the time.
5.3.2 Distance measurement

Different methods are available for measurement of distance to the specified measurement uncertainty

(see 5.3.5). Two of the most common methods are as follows:

a) the establishment of light barriers (markers) at regular intervals along the length of the tank which

actuate mechanical or optical pulse transmitters fitted to the cart;

b) the use of measuring wheels with mechanical or photoelectric pulse transmitters/optical encoders

which are drawn along the track by the cart.

In the case of using a measuring wheel, it shall be ensured that there is no slippage during travel. An

additional method of precise speed measurement shall also be provided to check the accuracy of the

measuring wheel on a regular basis.
5.3.3 Time measurement

The time of travel of the cart is normally measured by an electronic counter with an in-built accurate

time reference, for example a quartz crystal. A period can thus be read to 1 ms or better. This equipment

should be checked periodically against a reference device traceable to a national time standard.

5.3.4 Current-meter velocity measurement

The cart shall be provided with a suitable recording device for the measurement of current-meter

signals.

In the case of rotating-element current-meters, the sensor of the current-meter shall generate a clear

and positive signal corresponding to the rotor revolutions. Normally, as per the design of the system,

the signals are generated once per revolution, twice per revolution, or in some cases, once for five

revolutions or even once for 10 revolutions. The signals received from the current-meter(s) may be

counted using the counting device of the current-meter. In measuring the number of current-meter

revolutions in a given time, it is important to measure between identical points on the current-meter

signal. It shall be ensured that none of the signals are missed.

In the case of an electromagnetic or acoustic current-meter, the electrical signals from its sensor are

processed by its control unit.
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ISO 3455:2021(E)
5.3.5 Sources of error related to infrastructure
5.3.5.1 General

Sources of error (sources of uncertainty) shall be treated according to ISO/IEC Guide 98-3 (GUM). Only

the principal sources of systematic and random errors are considered below.
5.3.5.2 Error due to the distance measurement

The following influence factors can contribute to the measurement uncertainty of the distance

measurement method:
— systematic uncertainty of the reference distance measuring method;
— thermal expansion on measuring wheels;
— repeatability of the distance measurement method;
— detection/counting of impulses from pulse transmitter or encoders.
5.3.5.3 Error due to timing measurement

The following influence factors can contribute to the measurement uncertainty of the timing

measurement method:
— systematic uncertainty of the reference timing measuring method;
— drift of the time reference.
5.3.5.4 Error due to residual currents in the water

The water in the tank is never completely still and residual currents from various origins are always

present. The following influence factors can contribute to the upper limit on residual currents:

— standing waves (surface gravity waves) linked to the dimensions of the tank and due to previous

measurements or disturbances in the water;
— thermal convection currents due to temperature gradients.
5.3.5.5 Error due to environmental conditions

The ambient temperature fluctuation should be as low as possible, and direct sunlight should be

avoided, to minimize the influence on the tank's measurement system and to reduce the creation of

thermal convection currents.

Electromagnetic interference generated by drive units and drive unit controls such as frequency

converters and power rails, especially if these are located directly on the tow carts, may have an

influence on the device under test and sensitive electronic devices.

Magnetic inductive flow meters may be affected by ferro magnetic structures like steel beams or

reinforced concrete in the near surrounding.
5.3.5.6 Random errors

The repeatability with which the cart speed can be determined depends on the repeatability of the

distance measurement method and on the timing accuracy. For any installation, this may be determined

experimentally by setting the cart speed to a fixed value and performing a series of runs to provide a

series of estimations of the cart speed.
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ISO 3455:2021(E)

This is repeated for several different cart speeds and from the standard deviation of each series of

measurements, the 95 % confidence limits may be evaluated.

The repeatability of the rate of revolution of the rotating element or the velocity indicated by the

current-meter for different cart speeds can be determined in a similar way.
5.4 Data acquisition

In order to facilitate automation and greater accuracy in measurement, a computerized data acquisition

system may be provided.

The data acquisition software shall accept any relevant information related to calibration.

Velocity measurement devices use a wide range of data collection and transfer methods. The calibration

facility should ensure that their equipment does not interfere with the data transfer signal.

5.5 Data processing

The data processing software shall be customized to process the calibration curves, calibration

formula(e) and calibration tables as described in 7.1.4.

Generic functions/tools like least square estimation, statistical functions, and tabulation for rating

tables shall be provided in the processing software.
5.6 Other requirements

In addition, provision of the following ancillary equipment at the towing tank facility is recommended:

a) artificial beaches, stilling devices or other similar devices to reduce the reflection of disturbance in

the water by the end walls of the tank;

b) means for checking the orientation of the stationary-sensor type current-meters;

c) one or more thermometers to measure the temperature of the water in the tank;

d) conductivity measurement device if magnetic inductive instruments are calibrated.

6 Calculation of uncertainty

The uncertainty associated with a measurement is obtained by combining the uncertainties arising

from the sources described in 5.3.5 in accordance to GUM.

As an example, Formula (2) can be used for estimating the combined uncertainty of velocity.

22 2
Uu=+ uu+ 2 (2)
 d tc 
where
u is the uncertainty of distance measurement;
u is the uncertainty of the timing measurement;
u is the uncertainty due to residual currents.

Formula (2) is an example and is not claiming to be complete. It does not include all uncertainty factors.

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ISO 3455:2021(E)
7 Calibration procedure
7.1 Calibration of rotating element current-meters
7.1.1 Suspension of the current-meter
Attention shall be paid to the following points.

— Before the current-meter is immersed in water, it shall be checked for cleanliness, lubrication and

for its mechanical and electrical functioning. It shall also be ensured that oil, as recommended by

the manufacturer or used by the customer in the field, is used for the current-meters with oil-filled

contact systems and bearings d
...

INTERNATIONAL ISO
STANDARD 3455
Third edition
Hydrometry — Calibration of current-
meters in straight open tanks
Hydrométrie — Étalonnage des moulinets en bassins découverts
rectilignes
PROOF/ÉPREUVE
Reference number
ISO 3455:2020(E)
ISO 2020
---------------------- Page: 1 ----------------------
ISO 3455:2020(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO 2020

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
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: +41 22 749 01 11
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
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ISO 3455:2020(E)
Contents Page

Foreword ........................................................................................................................................................................................................................................iv

1 Scope ................................................................................................................................................................................................................................. 1

2 Normative references ...................................................................................................................................................................................... 1

3 Terms and definitions ..................................................................................................................................................................................... 1

4 Principle of calibration .................................................................................................................................................................................. 1

4.1 Statement of the principle ............................................................................................................................................................. 1

4.2 Accuracy of the method ................................................................................................................................................................... 2

4.2.1 Overall uncertainty on the velocity measurement ............................................................................. 2

4.2.2 Requirements for accurate measurements ............................................................................................... 2

5 Infrastructure .......................................................................................................................................................................................................... 2

5.1 Dimensions of the towing tank ................................................................................................................................................. 2

5.1.1 General...................................................................................................................................................................................... 2

5.1.2 Length........................................................................................................................................................................................ 2

5.1.3 Depth and width ............................................................................................................................................................... 2

5.2 Towing cart ................................................................................................................................................................................................. 3

5.2.1 General...................................................................................................................................................................................... 3

5.2.2 Cart track system ............................................................................................................................................................. 3

5.2.3 Types of towing carts ................................................................................................................................................... 3

5.2.4 Cart operation .................................................................................................................................................................... 3

5.2.5 Cart control ........................................................................................................................................................................... 4

5.3 Measuring equipment ....................................................................................................................................................................... 4

5.3.1 General...................................................................................................................................................................................... 4

5.3.2 Distance measurement ............................................................................................................................................... 4

5.3.3 Time measurement ........................................................................................................................................................ 4

5.3.4 Current-meter velocity measurement ........................................................................................................... 4

5.3.5 Sources of error related to infrastructure .................................................................................................. 5

5.4 Data acquisition ..................................................................................................................................................................................... 6

5.5 Data processing ...................................................................................................................................................................................... 6

5.6 Other requirements ............................................................................................................................................................................ 6

6 Calculation of uncertainty .......................................................................................................................................................................... 6

7 Calibration procedure ..................................................................................................................................................................................... 7

7.1 Calibration of rotating element current-meters ........................................................................................................ 7

7.1.1 Suspension of the current-meter ....................................................................................................................... 7

7.1.2 Performance of calibration ..................................................................................................................................... 7

7.1.3 Data analysis ........................................................................................................................................................................ 8

7.1.4 Presentation of results ................................................................................................................................................ 9

7.2 Calibration of electromagnetic current-meters .......................................................................................................10

7.2.1 Instructions for calibration ..................................................................................................................................10

7.2.2 Mounting the electromagnetic current-meter ....................................................................................10

7.2.3 Number of calibration points .............................................................................................................................10

7.2.4 Performance of calibration ..................................................................................................................................11

7.2.5 Data analysis .....................................................................................................................................................................11

7.2.6 Presentation of results .............................................................................................................................................11

7.3 Calibration of acoustic current-meters for point velocity measurement ..........................................11

7.3.1 Instructions for calibration ..................................................................................................................................11

7.3.2 Mounting the acoustic current-meter ........................................................................................................12

7.3.3 Performance of calibration ..................................................................................................................................12

7.3.4 Data analysis .....................................................................................................................................................................12

7.3.5 Presentation of results .............................................................................................................................................12

Bibliography .............................................................................................................................................................................................................................13

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ISO 3455:2020(E)
Foreword

ISO (the International Organization for Standardization) is a worldwide federation of national standards

bodies (ISO member bodies). The work of preparing International Standards is normally carried out

through ISO technical committees. Each member body interested in a subject for which a technical

committee has been established has the right to be represented on that committee. International

organizations, governmental and non-governmental, in liaison with ISO, also take part in the work.

ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of

electrotechnical standardization.

The procedures used to develop this document and those intended for its further maintenance are

described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the

different types of ISO documents should be noted. This document was drafted in accordance with the

editorial rules of the ISO/IEC Directives, Part 2 (see www .iso .org/ directives).

Attention is drawn to the possibility that some of the elements of this document may be the subject of

patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of

any patent rights identified during the development of the document will be in the Introduction and/or

on the ISO list of patent declarations received (see www .iso .org/ patents).

Any trade name used in this document is information given for the convenience of users and does not

constitute an endorsement.

For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and

expressions related to conformity assessment, as well as information about ISO's adherence to the

World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT) see www .iso .org/

iso/ foreword .html.

This document was prepared by Technical Committee ISO/TC 113, Hydrometry, Subcommittee SC 5,

Instruments, equipment and data management.

This third edition cancels and replaces the second edition (ISO 3455:2007), which has been technically

revised.
The main changes compared to the previous editions are as follows:

— a subclause for calibration of acoustic current-meters for point velocity measurement has been added;

— clauses referring to outdated tracking systems like track systems using tooth belts have been

removed;

— clauses referring to outdated technique for data acquisition like strip chart recorder or magnetic

tapes have been removed;

— the clause for computerized data acquisition and processing system has been removed;

— the clause discussing the Epper effect has been removed.

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.
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INTERNATIONAL STANDARD ISO 3455:2020(E)
Hydrometry — Calibration of current-meters in straight
open tanks
1 Scope

This document specifies a calibration method for mechanical type, electromagnetic current-meters and

acoustic type hydrometric current-meters used for point velocity measurement of flowing water. The

method requires towing the instrument through still water in a straight open tank. It deals in particular

with the measuring apparatus, the calibration procedure, the method of presenting the results and the

uncertainties associated with the method.
2 Normative references

The following documents are referred to in the text in such a way that some or all of their content

constitutes requirements of this document. For dated references, only the edition cited applies. For

undated references, the latest edition of the referenced document (including any amendments) applies.

ISO 772, Hydrometry — Vocabulary and symbols
ISO 2537, Hydrometry — Rotating-element current-meters

ISO/IEC Guide 98-3Uncertainty of measurement — Part 3: Guide to the expression of uncertainty in

me a s ur ement (GUM: 1995)
3 Terms and definitions

For the purposes of this document, the terms and definitions given in ISO 772 apply.

ISO and IEC maintain terminological databases for use in standardization at the following addresses:

— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at http:// www .electropedia .org/
4 Principle of calibration
4.1 Statement of the principle

Calibration of a current-meter means experimental determination of the relationship between water

velocity and either the rate of revolution of the rotating element or the velocity directly indicated

by the current-meter. For this purpose, the current-meter is mounted on a towing cart and drawn

through still water contained in a straight tank with a uniform cross section at a number of steady

speeds of the towing cart. Simultaneous measurements of the speed of the towing cart and the rate

of revolution of the rotating element or the velocity indicated by the current-meter are made. In the

case of rotating-element current-meters, the two parameters are related by one or more formula(e), the

limits of validity of which are stated. In the case of stationary-sensor type current-meters, containing

no rotating elements, the velocity indicated by its display unit is compared with the corresponding cart

speed to know the error in measurement.
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ISO 3455:2020(E)
4.2 Accuracy of the method
4.2.1 Overall uncertainty on the velocity measurement

The towing method gives an absolute measurement of water speed, which in principle only requires

positions and time measurements. This method can be considered as very accurate if the precautions

listed in 4.2.2 are taken.
4.2.2 Requirements for accurate measurements
The towing method gives an accurate measurement of water speed provided that:

a) the position, the timing and means for starting and stopping it achieve the necessary accuracy;

b) residual currents in the water are small.
5 Infrastructure
5.1 Dimensions of the towing tank
5.1.1 General

The dimensions of the tank and the number and relative position of current-meters in the tank cross

section shall be chosen so that their effects on the test result are minimized.
5.1.2 Length

The length of a rating tank can be considered as comprising accelerating, stabilizing, measuring and

braking sections.

The length of the accelerating and braking sections depend on the design of the cart, the maximum

acceleration and deceleration achievable at maximum payload, and the maximum speed at which the

payload is to be towed along the tank. Safety requirements of the cart should be taken into account

while working out the length of the braking section. The length of the measuring section shall be such

that the calibration error, which is composed of inaccuracies in the measurement of time, distance

covered and rate of revolution, does not exceed the desired tolerance at any velocity. The required

length, therefore, depends on the type of current-meter being calibrated, type of cart and the way the

signals are produced and transmitted.
5.1.3 Depth and width

The depth of the tank can have an influence on the test results which cannot be regarded as negligible,

more particularly when the towing speed coincides with the velocity of propagation of the surface

wave. The dependence of this critical velocity, v , on tank depth is given by the Formula (1):

vg= d (1)
where
g is the acceleration due to gravity;
d is the depth of water.

Depending on the size of the current-meter(s) and the cross section of the suspension equipment

relative to the cross-sectional area of the tank, the wave crest produced by the current-meter and its

means of suspension may cause an error in calibration within a narrow band in the velocity range from

0,5 v to 1,5 v . It is a systematic and not a random error.
c c
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ISO 3455:2020(E)

The depth and width of the tank shall therefore be chosen to suit the size and the maximum velocity

limits of the current-meters to be calibrated. Care shall be taken to ensure that either high calibration

velocities are attained before the interference or that they exceed it sufficiently for the critical zone to

be bridged without extrapolation.
5.2 Towing cart
5.2.1 General

During calibration, the current-meter is suspended below the cart and immersed in the water at a

specified depth and the cart travels along the length of the tank at known and accurate speeds in the

measuring section.
5.2.2 Cart track system

The cart may run on two parallel rails which shall be accurately aligned with both the length of the

tank and the surface of the water in the tank. It is essential that the rails are straight and that the rails

and the wheels of the cart are free of irregularities in order to avoid irregular motions of the cart. The

material and hardness of the rails and the driving wheels should be chosen so that there shall not be

undue wear and tear of the wheels. In the case of rubber tire wheels, provision shall be made to lift the

wheels above the rail surface when not in use for a long time.
5.2.3 Types of towing carts
The following types of towing carts are in common usage.

a) The towed cart which is moved along the track by a cable driven from a constant speed motor

standing apart from the moving cart. The towed cart may be lighter in construction with the

consequent advantage of high acceleration and quick braking, but the elasticity of the towing cable

can cause irregularities in the running of the cart thereby affecting the accuracy of current-meter

calibration.

b) The self-propelled cart which is moved along the track by internally mounted electric motor(s). The

power to the cart may be fed by a trailing wire track system or by an overhead conductor system or

other systems specially designed for the purpose. The self-propelled cart is heavier in construction

as it carries the driving motors. This results in greater inertia of the cart and assists in smoothing

out the running irregularities of the cart.
5.2.4 Cart operation

The cart shall travel smoothly and at constant speed in the measuring section of the towing tank

ensuring that oscillatory motion is not transmitted to the current-meters under calibration.

The cart shall have smooth operational capability. Vibration of the tow cart should be avoided.

The cart shall remain stable during acceleration, deceleration and braking. There shall not be any

forward/backward or sideways rocking, or slippage during peak acceleration/deceleration and during

normal operation at any speed in specified range.

During calibration, the measuring equipment, sensors and other instruments shall not be affected

by noise and spurious signals induced by the mains power supply or cart drive and control system or

otherwise by electrical equipment installed in the rating tank building and vicinity.

In addition to normal braking, an alternate brake system shall also be provided on the cart which would

automatically activate during an emergency.
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ISO 3455:2020(E)
5.2.5 Cart control

The cart may be manned or unmanned. In the case of a manned cart, an operator on-board controls

various functions of the cart.
The unmanned cart is operated remotely without any operator on board.
5.3 Measuring equipment
5.3.1 General

The calibration of a current-meter calls for the simultaneous recording of the following three

parameters:
a) distance covered by the cart;
b) time; and

c) signal (pulses) delivered by the current-meter or velocity processed by the meter control unit.

The towing speed is calculated from the simultaneous measurement of distance and time. In case of

rotating-element current-meter, the rate of current-meter revolutions (rotations) is obtained by the

simultaneous measurement of the number of signals (pulses) and the time.
5.3.2 Distance measurement

Different methods are available for measurement of distance to the specified measurement uncertainty

(see 5.3.5). Two of the most common methods are as follows:

a) the establishment of light barriers (markers) at regular intervals along the length of the tank which

actuate mechanical or optical pulse transmitters fitted to the cart;

b) the use of measuring wheels with mechanical or photoelectric pulse transmitters/optical encoders

which are drawn along the track by the cart.

In the case of use of a measuring wheel, it shall be ensured that there is no slippage during travel. An

additional method of precise speed measurement shall also be provided to check the accuracy of the

measuring wheel on a regular basis.
5.3.3 Time measurement

The time of travel of the cart is normally measured by an electronic counter with an in-built accurate

time reference, for example a quartz crystal. A period can thus be read to 1 ms or better. This equipment

should be checked periodically against a reference device traceable to a national time standard.

5.3.4 Current-meter velocity measurement

The cart shall be provided with a suitable recording device for the measurement of current-meter

signals.

In the case of rotating-element current-meters, the sensor of the current-meter shall generate a clear

and positive signal corresponding to the rotor revolutions. Normally, as per the design of the system,

the signals are generated once per revolution, twice per revolution or in some cases once for five

revolutions or even once for 10 revolutions. The signals received from the current-meter(s) may be

counted using the counting device of the current-meter. In measuring the number of current-meter

revolutions in a given time, it is important to measure between identical points on the current-meter

signal. It shall be ensured that none of the signals are missed.

In the case of an electromagnetic or acoustic current-meter, the electrical signals from its sensor are

processed by its control unit.
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ISO 3455:2020(E)
5.3.5 Sources of error related to infrastructure
5.3.5.1 General

Sources of error (sources of uncertainty) shall be treated according to ISO/IEC Guide 98-3 (GUM). Only

the principal sources of systematic and random errors are considered below.
5.3.5.2 Error due to the distance measurement

The following influence factors can contribute to the measurement uncertainty of the distance

measurement method:
— systematic uncertainty of the reference distance measuring method;
— thermal expansion on measuring wheels;
— repeatability of the distance measurement method;
— detection/counting of impulses from pulse transmitter or encoders.
5.3.5.3 Error due to timing measurement

The following influence factors can contribute to the measurement uncertainty of the timing

measurement method:
— systematic uncertainty of the reference timing measuring method;
— drift of the time reference.
5.3.5.4 Error due to residual currents in the water

The water in the tank is never completely still and residual currents from various origins are always

present. The following influence factors can contribute to the upper limit on residual currents:

— standing waves (surface gravity waves) linked to the dimensions of the tank and due to previous

measurements or disturbances in the water;
— thermal convection currents due to temperature gradients.
5.3.5.5 Error due to environmental conditions

The ambient temperature fluctuation should be as low as possible and direct sunlight should be avoided

to minimize the influence on the tank's measurement system and to reduce the creation of thermal

convection currents.

Electromagnetic interference generated by drive units and drive unit controls such as frequency

converters and power rails, especially if these are located directly on the tow carts, may have an

influence on the device under test and sensitive electronic devices.

Magnetic inductive flow meters may be affected by ferro magnetic structures like steel beams or

reinforced concrete in the near surrounding.
5.3.5.6 Random errors

The repeatability with which the cart speed can be determined depends on the repeatability of the

distance measurement method and on the timing accuracy. For any installation, this may be determined

experimentally by setting the cart speed to a fixed value and perform a series of runs to provide a series

of estimations of the cart speed.
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ISO 3455:2020(E)

This is repeated for several different cart speeds and from the standard deviation of each series of

measurements, the 95 % confidence limits may be evaluated.

The repeatability of the rate of revolution of the rotating element or the velocity indicated by the

current-meter for different cart speeds can be determined in a similar way.
5.4 Data acquisition

In order to facilitate automation and greater accuracy in measurement, a computerized data acquisition

system may be provided.

The data acquisition software shall accept any relevant information related to calibration.

Velocity measurement devices use a wide range of data collection and transfer methods. The calibration

facility should ensure that their equipment does not interfere with the data transfer signal.

5.5 Data processing

The data processing software shall be customized to process the calibration curves, calibration

formula(e) and calibration tables as described in 7.1.4.

Generic functions/tools like least square estimation, statistical functions, and tabulation for rating

tables shall be provided in the processing software.
5.6 Other requirements

In addition, provision of the following ancillary equipment at the towing tank facility is recommended:

a) artificial beaches, stilling devices or other similar devices to reduce the reflection of disturbance in

the water by the end walls of the tank;

b) means for checking the orientation of the stationary-sensor type current-meters;

c) one or more thermometers to measure the temperature of the water in the tank;

d) conductivity measurement device if magnetic inductive instruments are calibrated.

6 Calculation of uncertainty

The uncertainty associated with a measurement is obtained by combining the uncertainties arising

from the sources described in 5.3.5 in accordance to GUM.
As an example, Formula (2) can be used for uncertainty estimation.
22 2
Uu =+ uu+ 2 (2)
 d tc 
where
u is the uncertainty of distance measurement
u is the uncertainty of the timing measurement
u is the uncertainty due to residual currents

Formula (2) is an example and is not claiming to be complete. It does not include all uncertainty factors.

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ISO 3455:2020(E)
7 Calibration procedure
7.1 Calibration of rotating element current-meters
7.1.1 Suspension of the current-meter
Attention shall be paid to the following points.

— Before the current-meter is immersed in water, it shall be checked for cleanliness, lubrication and

for its mechanical and electrical functioning. It shall also be ensured that oil, as r

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