ISO/FDIS 3455

DRAFT INTERNATIONAL STANDARD
ISO/DIS 3455
ISO/TC 113/SC 5 Secretariat: ANSI
Voting begins on: Voting terminates on:
2020-02-05 2020-04-29
Hydrometry — Calibration of current-meters in straight
open tanks
Hydrométrie — Étalonnage des moulinets en bassins découverts rectilignes
ICS: 17.120.20
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ISO/DIS 3455:2020(E)
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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 signal 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 ..................................................................................................................................................................................... 6

7.1 Calibration of rotating element current-meters ........................................................................................................ 6

7.1.1 Suspension of the current-meter ....................................................................................................................... 6

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

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

7.1.4 Presentation of results ................................................................................................................................................ 8

7.2 Calibration of electromagnetic current-meters .......................................................................................................... 9

7.2.1 Instructions for calibration ..................................................................................................................................... 9

7.2.2 Mounting the electromagnetic current-meter ....................................................................................... 9

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

7.2.4 Performance of calibration ..................................................................................................................................10

7.2.5 Data analysis .....................................................................................................................................................................10

7.2.6 Presentation of results .............................................................................................................................................10

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 ........................................................................................................11

7.3.3 Performance of calibration ..................................................................................................................................11

7.3.4 Data analysis .....................................................................................................................................................................11

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

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The committee responsible for this document is ISO/TC 113, Hydrometry, Subcommittee SC 5,

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

This International Standard 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

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 748, Hydrometry — Measurement of liquid flow in open channels using current-meters or floats

JCGM 100:2008, Evaluation of measurement data — Guide to the expression of uncertainty in

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

are based on the International System of Units (SI). ISO and IEC maintain terminological databases for

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 equations, 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

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

positions and time measurements. Provided that the precautions listed in 4.2.2 are taken, this method

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

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

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

The length of the accelerating and braking sections depends 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 need to 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 will, therefore, depend on the type of current-meter being calibrated, type of cart and the way

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 Equation (1):

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

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 the calibration

velocities in higher range are attained before the interference or that they exceed it sufficiently for the

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

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

The cart may run on two parallel rails which must 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

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

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 will be heavier

in construction as it has to carry the driving motors. This results in greater inertia of the cart and

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

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

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

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

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

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

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

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

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

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.

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

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 ten 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

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

Sources of error (sources of uncertainty) shall be treated according to GUM (Guide to the expression of

uncertainty in measurement, JCGM 100:2008). Only the principal sources of systematic and random

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

• 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

• The suspension of the current-meter shall be as specified by the manufacturer/user of the current-

meter. This shall usually be the same as that used during field measurement. If during the field

measurements, the current-meter is attached near the lower end of the rod, it shall be mounted in

the same position on the cart during calibration. Should this not be the case, the current-meter shall

be mounted far enough from the end of the rod to ensure that any influence from this part of the