SIST ISO 3455:2013

INTERNATIONAL ISO
STANDARD 3455
Second edition
2007-06-01
Hydrometry — Calibration of current-
meters in straight open tanks
Hydrométrie — Étalonnage des moulinets en bassins découverts
rectilignes
Reference number
©
ISO 2007
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ii © ISO 2007 – All rights reserved

Contents Page
Foreword. iv
1 Scope . 1
2 Normative reference . 1
3 Terms and definitions. 1
4 Principle of calibration . 1
5 Design criteria for calibration stations. 1
5.1 Dimensions of rating (calibration) tank. 1
5.2 Rating carriage (trolley). 3
5.3 Measuring equipment. 4
5.4 Other requirements. 5
6 Computerized data acquisition and processing system . 6
6.1 Data acquisition . 6
6.2 Data processing. 6
7 Calibration procedure. 7
7.1 Calibration of rotating-element current-meters. 7
7.2 Calibration of electromagnetic meters . 13

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.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.
The main task of technical committees is to prepare International Standards. Draft International Standards
adopted by the technical committees are circulated to the member bodies for voting. Publication as an
International Standard requires approval by at least 75 % of the member bodies casting a vote.
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.
ISO 3455 was prepared by Technical Committee ISO/TC 113, Hydrometry, Subcommittee SC 5, Instruments,
equipment and data management.
This second edition cancels and replaces the first edition (ISO 3455:1976), which has been technically revised.

iv © ISO 2007 – All rights reserved

INTERNATIONAL STANDARD ISO 3455:2007(E)

Hydrometry — Calibration of current-meters in straight open
tanks
1 Scope
This International Standard specifies the procedure of calibration of current-meters of rotating-element type as
well as stationary-sensor type (electromagnetic type) in straight open tanks. It also specifies the types of tank,
rating carriage and equipment to be used and the method of presenting the results.
The procedure does not take into account any possible difference existing between the behaviour of a current-
meter moving in motionless water and that of a fixed current-meter in turbulent flow.
2 Normative reference
The following referenced documents are indispensable for the application of this document. For dated
references, only the edition cited applies. For undated references, the latest edition of the referenced
document (including any amendments) applies.
ISO 772, Hydrometric determinations — Vocabulary and symbols
ISO 2537, Hydrometry — Rotating-element current-meters
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 772 apply.
4 Principle of calibration
Calibration of a current-meter means experimental determination of the relationship between liquid 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 carriage and drawn through still water contained in
a straight tank of a uniform cross section at a number of steady speeds of the towing carriage. Simultaneous
measurements of the speed of the towing carriage 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, the velocity indicated by its display unit is compared with the
corresponding carriage speed to know the error in measurement.
5 Design criteria for calibration stations
5.1 Dimensions of rating (calibration) 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 the test results are not affected.
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 depends on the design of the carriage, 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 carriage 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 carriage and the way the signals are produced and transmitted.
For example, if measured times both for distance covered by the carriage and for the revolutions counted are
accurate to 0,01 s in order to limit the error in time measurement to 0,1 % at the 95 % confidence level, the
duration of the test shall be at least 10 s at maximum speed. If the maximum speed is 6 m/s, the measuring
section of the tank would be 60 m long. The total length of the tank would be about 100 m of which about
20 m would be for acceleration and stabilizing and 20 m for braking.
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 Equation (1):
c
vg= d (1)
c
where
g is the acceleration due to gravity;
d is the depth of water.
The wave crest produced by the current-meter and its means of suspension, which moves forward with the
instrument, causes an increase in the height of the wetted cross section and thus, in accordance with the
continuity equation, a reduction of the relative velocity. This phenomenon, known as the Epper effect, may
cause an error in calibration within a narrow band in the velocity range from 0,5 v to 1,5 v . The magnitude of
c c
the Epper effect depends on the size of the current-meter(s) and suspension equipment, relative to the cross-
sectional area of the tank. It may be little more than the uncertainty of a single calibration point. It is a
systematic and not a random error. It may be negligible when a very small current-meter is being calibrated.
The depth 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 critical zone to be bridged without
extrapolation.
The width of the tank is of importance because the Epper effect is more pronounced in a narrower tank. The
width also limits the number of instruments that can be calibrated simultaneously and has an effect on the
stilling characteristics (time taken for the water to become reasonably still).
For example, when a field type current-meter on rod suspension is being calibrated in a tank 1,83 m wide in
which the depth of water is 1,83 m, the Epper effect is greater at a speed of about 4 m/s ( 9,81×1,83 ) and
amounts to 0,3 %. The size of the effect dies away on either side of the critical velocity, but is detectable at
velocities between 3 m/s and 5 m/s.
2 © ISO 2007 – All rights reserved

5.2 Rating carriage (trolley)
5.2.1 General
During calibration, the current-meter is suspended from the carriage and immersed in the water at specified
depth and the carriage travels along the length of the tank at known and accurate speeds in the measuring
section.
5.2.2 Carriage track system
The carriage 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 carriage are free of irregularities, otherwise the carriage will move with irregular motion and
cause vibration which may be transmitted to the current-meter(s) and disturb the rating. 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.
Another track system may have toothed belts, mounted on both sides of the tank, driven by the guiding rollers
and used for transportation of the carriage. If this system is adopted, it shall be ensured that there is no
slippage or sag of the belt and in case the slippage or sag occurs, it shall be possible to remove it manually.
The belts shall be strong enough to withstand the load of the carriage they have to transport during calibration
and the climatic conditions prevailing at the rating laboratory.
5.2.3 Types of rating carriages
The following types of rating carriages are in common usage.
a) The towed carriage which is moved along the track by a cable driven from a constant speed motor
standing apart from the moving carriage. The towed carriage 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 carriage thereby affecting the accuracy of current-meter
calibration.
b) The self-propelled carriage which is moved along the track by internally mounted electric motor(s). The
power to the carriage 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 carriage will be heavier in
construction as it has to carry the driving motors. This results in greater inertia of the carriage and assists
in smoothing out the running irregularities of the carriage.
The weight of the rating carriage can substantially be reduced by using light but rugged material for its
construction. The lightweight carriage can achieve a high rate of acceleration/deceleration resulting in
relatively smaller length of tank.
5.2
...