ISO 3354:1975

INTERNATIONAL STANDARD @ '*Xe 3354
INTERNATIONAL ORGANIZATION FOR STANDARDIZATION *MEXnYHAPOLXHAR OPrAHHJAUHR il0 CTAHLIAPTH3AUHH.ORGANlSATlON INTERNATIONALE DE NORMALISATION
*
Measurement of clean water flow in closed conduits -
Velocity-area method using current-meters
Mesure du débit d'eau propre dans les conduites fermées - Méthode d'exploration du champ des
vitesses au moyen de moulinets
First edition - 1975-09-15
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UDC 681.121.84 Ref. No. IS0 3354-1975 (E)
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Descriptors : flow measurement, pipe flow, water flow, velocity measurement, flowmeters.
Price based on 26 pages
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FOREWORD
IS0 (the International Organization for Standardization) is a worldwide federation
of national standards institutes (IS0 Member Bodies). The work of developing
International Standards is carried out through IS0 Technical Committees. Every
Member Body interested in a subject for which a Technical Committee has been set
up 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.
Draft International Standards adopted by the Technical Committees are circulated
to the Member Bodies for approval before their acceptance as International
Standards by the IS0 Council.
International Standard IS0 3354 was drawn up by Technical Committee
ISO/TC 30, Measurement of fluid flow in closed conduits, and circulated to the
Member Bodies in May 1974.
It has been approved by the Member Bodies of the following countries :
Switzerland
Australia Germany
Austria India Thailand
Belgium Ireland Turkey
Bulgaria Netherlands United I U.S.S.R.
Czechoslovakia Romania
Yugoslavia
France Spain
The Member Body of the following country expressed disapproval of the document
on technical grounds :
U.S.A.
3 International Organization for Standardization, 1975
Printed in Switzerland
II
5 Description of the current-meter . 4
6 Requirements for use of current-meters . 4
6.1 Selection of the measuring cross-section . 4
6.2 Devices for improving flow conditions . 5
6.3 Calibration of the current-meter . 5
6.4 Limits of use . 5
6.5 Inspection and maintenance of current-meters . 6
7 Setting of current-meters into the conduit . 6
7.1 Setting of current-meters . 6
7.2 Mounting in a circular cross-section . 7
7.3 Mounting in a rectangular cross-section . 7
8 Determination of the discharge velocity by graphical integration of the
.................................
velocity area 8
8.1 Circular cross-sections . 8
8.2 Rectangular cross-sections . 10
9 Determination of the discharge velocity by numerical integration of the
velocity area .
9.1 Circular cross-sections 12
..........................
9.2 Rectangular cross-sections 12
.......................
10 Determination of the discharge velocity by arithmetical methods . 13
10.1 "Log-linear" method . 13
10.2 "Log-Tchebycheff" method . 15
...
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11 Errors 16
11.1 Definition of the error . 16
11.2 Errors in the estimation of local velocity . 16
11.3 Errors in the estimation of flow rate . 16
11.4 Definition of the standard deviation . 17
11.5 Definition of the uncertainty . 17
11.6 Calculation of the standard deviation . 18
Annexes
A : Corrections for blockage effect . 19
6: Recommendations for the selection of the type of current-meter and
mounting rod . 20
C: Example of measuring point distribution along a radius for velocity
measurement in a conduit of circular cross-section in the case of the graphical
and numerical methods . 21
D : Determination of coefficient m for extrapolation near the wall . 23
E : Examples of values of components of the overall error . 24
F : Example of calculation of the uncertainty on the flow rate measurement
by means of current-meters . 25
iv
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IS0 3354-1975 (E)
INTERNATIONAL STANDARD
Measurement of clean water flow in closed conduits -
Velocity-area method using current-meters
3.2 stationary battery : A set of current-meters mounted
1 SCOPE AND FIELD OF APPLICATION
on one or several fixed supports which explore the whole
This International Standard specifies a method for the
measuring section simultaneously.
determination, in a closed conduit, of the volume rate of a
regular flow')
3.3 peripheral flow rate : The volume of liquid which
- of clean waterz) or water considered as clean,
flows, per unit time, in the region located between the pipe
e
wall and the contour defined by the velocity measuring
- running full in this conduit,
points which are the closest to the wall.
- under steady flow conditions,
3.4 discharge velocity : The ratio of the volume rate of
this measurement being carried out by means of the
flow (integral of the axial component of local velocities
velocity-area method using propeller-type current-meters. It
with respect to the cross-sectional area) to the area of the
deals particularly with the technology and calibration of
measuring cross-section.
current-meters, the measurement of local velocities and the
computation of the flow rate by velocity integration. The
3.5 relative velocity : The ratio of the flow velocity at the
method of measurement and the requirements defined in
considered point to a reference velocity measured at the
this International Standard aim at reaching, at the 95 %
same time and being either the velocity at a particular point
confidence level, an uncertainty on flow not greater than
(for example, the centre of a circular conduit) or the
L 2 % provided that the correction for blockage effect has
discharge velocity in the measuring section.
been applied. If any of the requirements of this
International Standard are not fulfilled, this method may
3.6 straight length : A portion of a conduit whose axis is
still be applied but the uncertainty on flow rate will be
straight and in which the cross-sectional area and
larger.
cross-sectional shape are constant; the cross-sectional shape
is usually circular or possibly rectangular.
2 REFERENCE
3.7 irregularity : Any component or configuration of a
IS0 3455, Liquid flow measurement in open channels -
conduit which makes it different from a straight length.
Calibration of current-meters in straight Open tanks.3 )
For the purpose of this International Standard, those
irregularities which create the most serious disturbances are
3 DEFINITIONS
bends, valves, gates and sudden widening of the section.
The following definitions are given only for terms used with
a special meaning or for terms the meaning of which might
be usefully recalled :
4 PRINCIPLE
3.1 current-meter : A device provided with a rotor the
4.1 General
rotational speed of which is a function of the local velocity
The principle of the method consists in
of the fluid in which the device is immersed.
a) measuring the dimensions of the gauging section
This International Standard is concerned only with
which shall be chosen as normal to the conduit axis; this
propeller-type current-meters, i.e. current-meters the rotor
measurement is intended for defining the area of the
of which is a propeller rotating around an axis
cross-section (see 4.2);
approximately parallel to the flow.
1) See6.1.
This method may be applied to other single-phase fluids but special precautions shall be taken in this case.
2)
At present at the stage of draft.
3)
IS0 3354-1975 (E)
four diameters at approximately equal angles to one another
b) defining the position of the measuring points in the
in the gauging section. If the difference between the lengths
cross-section;
of two consecutive diameters is greater than 0,5 %, the
c) measuring the axial component of the velocity at a
number of measured diameters shall be doubled.
sufficient number of points in a flow cross-section to
determine the velocity profile adequately (see 4.3);
4.2.2 Rectangular cross-sections
d) determining the discharge velocity from the
The conduit width and height shall both be measured at
preceding measurements;
least on each straight line passing through the measuring
e) calculating the volume rate of flow equal to the points. If the difference between the widths (or heights)
product of the cross-sectional area and the discharge corresponding to two successive measuring lines is greater
velocity. than 1 %, the number of measured widths (or heights) shall
be doubled.
The error made when using the velocity distribution
method is dependent, among other factors, on the shape of
4.3 Measurement of local velocities
the velocity profile and on the number and position of the
measuring points.
The flow velocity at a point of the gauging section is
determined by measuring the rotational speed of a
This International Standard presents three types of method
for determining the discharge velocity, namely current-meter placed at that point and entering this value in
the calibration equation of the current-meter.
- graphical integration of the velocity area (see
clause 8)
The current-meter rotational speed may be obtained either
by counting the number of propeller rotations within a
This method consists in plotting the velocity profile on a
pre-determined period, or by measuring the time required
graph and evaluating the area under the curve which is
by the propeller to perform the specified number of
bounded by the measuring points closest to the wall. To
is the one
rotations. Another method that can be used
the value thus obtained is added a calculated term
whereby the velocity is determined by direct measurement
which allows for the flow in the peripheral zone
of the signal frequency.
(the area between the wall and the curve through the
measuring positions closest to the wall) on the
Various measuring points in one cross-section may be
assumption that the velocity profile in this zone satisfies explored simultaneously or successively (see 4.3.1
the laws of a turbulent boundary layer.
and 4.3.2).
For this method the measuring points may be located at
4.3.1 Simultaneous measurements
whichever positions are required in order to obtain a
satisfactory knowledge of the velocity profile.
When several current-meters are used simultaneously, the
second method given above requires more sophisticated
- numerical integration of the velocity area (see
counting equipment than the first one, but it is more
clause 9)
accurate. The first method actually leeds to an error the
statistical resultant of which is positive, due to time
The difference between this method and the preceding
one lies in the fact that the graphical velocity profile is intervals not being taken into account at the beginning and
replaced by an algebraic curve and the integration is at the end of the counting time; the greater the number of
carried out analytically. counted contacts, the smaller the percentage error.
- arithmetical methods (see clause IO)
As flow is generally subject to long period fluctuations, it is
necessary to provide a sufficient period of measurement for
The arithmetical methods assume that the velocity
determining the mean velocity correctly. This period of
distribution follows a particular law; the mean velocity
time may be determined by measuring the same flow rate
in the conduit is then given by a linear combination of
during gradually increasing intervals of time. The time of
the individual velocities measured at the locations
measurement t to be adopted shall be such that the values
specified by the method.
of the mean velocity in the cross-section obtained for
measuring times equal to t and (t i- At) shall not vary by
For the arithmetical methods described in clause IO,
more than x %. For example, At could be about 30 s and x
assumption is made in the peripheral zone of a
could be chosen equal to 0,l %. Time t may vary according
logarithmic law for velocity distribution as a function of
to the mean flow velocity.
the distance from the wall.
4.3.2 Non-simultaneous measurements
4.2 Measurement of the gauging cross-section
In cases when all velocity measurement points are not
explored simultaneously, it is essential that the shape of the
4.2.1 Circular cross-sections
velocity profile in the gauging cross-section remain stable
The mean diameter of the conduit is taken as equal to the
and be unaffected by possible variations of the flow rate
arithmetical mean of measurements carried out on at least
during the measurement.
IS0 3354-1975 (E)
In such a case the stability of flow may be checked and 4.4 Location and number of measuring points in the
point velocities possibly corrected by means of a cross-section
continuous measurement, during the whole duration of
at a reference point.
gauging, of the velocity
4.4.1 General requirements
If only one measuring device is available, the steadiness of
The rules to be followed for locating the measuring points
the flow shall be checked by frequently repeating
differ according to the methods of determination of the
measurements at the reference point.
discharge velocity as specified in this International
Standard. These rules are given in clauses8, 9 and 10
When the curve of reference velocity variation vr has been
respectively.
plotted against time, this curve is used to relate all traverse
measurements to the same reference flow rate q, Whatever the method, the following dimensional rules shall
(preferably that which corresponds to'the mean of velocity be complied with :
measurements at the fixed point). For comparatively small
- minimum distance between the current-meter axis
changes of the reference velocity, the velocity v,,* measured
and the wall : 0,75 d,
at any point at time t can be transposed by multiplication
by the ratio of velocity v,,, at the reference point
- minimum distance between the axes of two
corresponding to flow rate qo to velocity v,,~ at this
current-meters : d + 0,03 m,
reference point at time t :
where d is the outside diameter of the current-meter
propeller.
The location of the current-meter shall be measured to the
smaller of the following two tolerances :
However, it must be emphasized that velocity profile f 0,001 L, where L is the dimension of the duct parallel
fluctuations may occur without creating flow rate to the direction of measurement of the current-meter
fluctuations. In such a case the use of reference point position,
velocity may lead to errors and it is preferable to check
f 0,OZ y, where y is the distance of the current-meter
flow steadiness by means of any pressure-difference device
from the nearest wall.
(standardized or non-standardized pressure-difference
flow-meter, piezometric control on a convergent, bend,
A minimum number of measuring points applying in
spiral casing, peculiar pressure loss, etc.) even if it is not
particular to small-dimension conduits is prescribed in 4.4.2
calibrated provided that its reliability and adequate
and 4.4.3. As it is necessary that the velocity profile be
sensitivity have been ascertained.
as accurately as possible, the number of measuring
known
points can be advantageously increased provided that this is
allowed by the above-mentioned requirements and that it
does not cause notable blockage effects (see 6.4.3).
4.3.3 Checking of velocity distribution
When a single meter is traversed across the duct, the
Even when discharge velocity is calculated by a method
te
distance between a reference point (from which each
which does not require plotting of the velocity profile, it is
position is measured) and the wall of the duct must first be
recommended, in order to be confident that the velocity
obtained. This may introduce a relatively large systematic
distribution is regular, to carry out this plotting or at least
error in all position measurements. It is therefore
to check its regularity by some other means.
case of a circular cross-section
recommended, in the
conduit, that complete diameters be traversed (rather than
In the same way, when several measurements are made on
opposite radii on each diameter) since the systematic error
the same cross-section at different flow rates it is
will then tend to cancel out on the two halves of the
recommended that the velocity profiles be plotted in a
traverse.
non-dimensional manner (i.e. by using the relative
velocities, see 3.3) to check their consistency with one
another and hence ensure that there are no abnormal features
4.4.2 Circular cross-sections
at particular flow rates (thus, the profiles shall not change
The measuring points shall be located on concentric circles
erratically as the flow rate varies over a wide range of
and on at least two mutually perpendicular diameters in the
Reynolds numbers).
cross-sect ion.
It may also be useful to plot the velocity distribution curves
The measurements shall be carried out at at least three
as indicated above in order to detect any error in the
points per radius, so that there shall be a minimum cf
a local velocity. The doubtful
measurement of
twelve points in the cross-section. An additional measuring
measurement shall be repeated whenever possible; when
point at the centre of the conduit is desirable to check the
this cannot be done, it shall be ignored and the velocity
shape of the velocity profile.
profile drawn on the basis of the previously obtained
profiles provided there are independent reasons for
NOTE - When the measurements are carried out by means of a
is false. stationary battery, reference shall be made to 6.4.4 for the
believing that the doubtful measurement
IS0 3354-1975 (E)
5.6 Provision shall be made for fixing the current-meter on
minimum diameter of conduits in which this method can be applied;
but in any case the general requirements above on the minimum
a rod in a well-defined position.
distance between two current-meters forbid the use of a stationary
battery within conduits the diameter of which is less than
(7.5 d + 0.18) m.
4.4.3 Rectangular cross-sections
6 REQUIREMENTS FOR USE OF CURRENT-METERS
The minimum number of measuring points shall be 25.
Unless a special layout of measuring points is adopted for
6.1 Selection of the measuring cross-section
the use of an arithmetical method, their position shall be
6.1 .I The cross-section selected for measurements shall be
defined by the intersections of at least five straight lines
located in a straight pipe length, perpendicular to the
running parallel to each wall of the conduit.
direction of flow and of simple shape, for example, either
NOTE - When the measurements are carried out by means of a
circular or rectangular. It shall be located in an area where
stationary battery, reference shall be made to 6.4.4 for the
the measured velocities fall within the normal working
minimum dimensions of conduits in which the method can be
range of the current-meters used (see 6.4.2).
applied; but in any case the general requirements above on the
minimum distance between two current-meters forbid the use of a
stationary battery within conduits the smaller dimension of which is
6.1.2 Close to the measuring cross-section, flow shall be
less than (5.5 d + 0.12) m.
substantially parallel to and symmetric about the conduit
axis and present neither excessive turbulence nor swirl; the
measuring cross-section shall thus be chosen far enough
away from any disturbances that could create asymmetry,
5 DESCRIPTION OF THE CURRENT-METER swirl or turbulence (see 5. î .4).
5.1 A propeller-type current-meter consists of a propeller, The length of straight pipe that may be required to achieve
these conditions will vary with the flow velocity, upstream
an axis of rotation, bearings and the current-meter body
with the counting mechanism. disturbances and the level of turbulence and the degree of
swirl, if any.
5.2 Each current-meter may be fitted with different types
For guidance, it is normally assumed that to comply with
of propeller (pitch, diameter, etc.). The propeller is made of
these conditions there shall be a length of upstream
two or more blades and may be manufactured out of metal
conduit between the beginning of the working section and
or plastics material.
any important upstream irregularity’ ) of at least twenty
diameters of a circular cross-section (or 80 times the
5.3 Current-meters for site measurements shall be
hydraulic radius of a conduit of any cross-sectional shape).
manufactured out of non-corrosive material only or shall be
at least five diameters of a circular
Similarly, there shall be
effectively protected against corrosion. They shall be of
cross-section (or 20 times the hydraulic radius of a conduit
sufficiently sturdy construction for their calibration to
of any cross-sectional shape) between the measuring
remain valid under normal field operating conditions.
cross-section and any important downstream irregularity.’ )
5.4 Components shall be interchangeable to allow easy
6.1.3 Although measurements with current-meters in
substitution of worn or damaged parts, but interchange
oblique or converging flow shall as far as possible be
shall not increase the uncertainty of the measurement.
avoided, they may be carried out provided that
- the current-meters used are designed to measure
5.5 Signals may be transmitted by mechanical contact or
accurately the true axial component of the velocity, this
any magnetic, electrical or optical device. They are
being checked by an appropriate calibration;
totalized or recorded on an appropriate receiver or
indicated by an acoustic or optical device.
- the maximum flow deviation with respect to the
current-meter axis does not exceed 5’.
Counting shall be accurate and reliable for any given
velocity within the operational range specified by the
NOTE - Commonly used propellers may give correct indications up
manufacturer. The number of signals delivered per propeller
to incidences of 5” with an accuracy of 1 % (relative deviation
revolution shall be consistent with the velocities to be between the measured velocity and the axial component of the flow
velocity) There exist self-component propellers which measure
measured, the design of the receiver and an acceptable
directly the axial component of velocity with an error smaller than
measuring period. In some cases it will therefore be
1 % for greater angles of incidence, but attention shall be drawn to
necessary to be able to choose the number of signals per
the particular sensitivity of such propellers to the influence of the
propeller revol iition. current-meter support and to the flow turbulence

IS0 3354-1975 (E)
For guidance, it can be considered that a swirl is small 6.3 Calibration of the current-meter
enough not to increase the confidence limits given in this
6.3.1 The calibration of a current-meter requires the
International Standard on the measured flow rate, if the
empirical determination of the relationship between water
resulting gradient of local velocities to the pipe axis is less
velocity and propeller velocity. This relationship is
than 5'.
generally
represented by one or several straight lines the
is
equation of which
6.1.4 If there is doubt about the flow conditions, it is
v =An+B
necessary to make preliminary traverse tests by which to
ascertain the regularity of flow.
where
If these traverses show that flow is not satisfactory, this v is the velocity of the water, in metres per second;
can, sometimes, be remedied using one of the devices
n is the rotational speed of the propeller, in revolutions
described in 62.
per second;
Once these devices are in place a check shall be carried out
A and B are constants to be determined.
for compliance with the flow requirements of this
International Standard. In the contrary case a more detailed
6.3.2 Calibration shall be carried out in an installation
traverse of the measuring cross-section is necessary, and
0 specially designed for this purpose in conformity with the
reference shall then be made to a separate document which
prescriptions of IS0 3455.
will be published later.
6.3.3 For calibration, the current-meter shall be fitted
with the same support as that used for measurements.
6.2 Devices for improving flow conditions
6.3.4 The calibration report shall include :
6.2.1 If swirl can be observed in the flow, this can be
tentatively suppressed by means of an anti-swirl device
- a graph on which each calibration point is plotted
constituted either by piled up pipes parallel to the flow
and the mean straight lines are set out (with n in abscissa
direction or by a honeycomb with square or hexagonal
and A v = v-A,n in ordinate, A, being a value close to
cells. Whichever type is used, the whole device shall be
the actual values of A);
rigorously symmetrical and the following requirements shall
- the equations v = An + B of calibration straight lines
:
be met
with the limits of the velocity ranges where each
- the maximum transverse dimensiona of a channel
equation is applicable.
shall be less than 0.25 D.
The shapes and dimensions of the current-meter support,
- the length I shall be greater than 10 a.
the lubricant used for current-meter lubrication and the
temperature of the calibrating channel water shall be
indicated as well.
6.2.2 If the velocity distribution is unacceptably
irregular, it can often be remedied by means of a profile
1)
6.3.5 In principle, each current-meter shall be calibrated
developer consisting of, for example, one or more screens,
before and after each series of measurements. However, if
grids or perforated plates. It must be noted, however, that
the calibration shows a hydraulic similitude of the
such devices are effective only at the price of a rather high
propellers and if the propellers are dimensionally consistent
head loss.
and interchangeable, statistical calibration may be carried
out from a sufficient number of individual calibrations
6.2.3 The devices described in 6.2.1 and 6.2.2 shall be
under well-defined conditions. In this case, the calibrating
located at the greatest possible distance upstream from the
organization shall indicate the maximum probable
measuring cross-section and in any case at a distance of at
deviations from the mean calibration formula proposed.
least five diameters of a circular cross-section (or 20 times
the hydraulic radius of a conduit of any cross-sectional
6.4 Limits of use
shape). Furthermore, they shall not be located immediately
downstream of a disturbance.
6.4.1 Nature of the liquid
6.2.4 If the velocity distribution is too irregular or if the
Current-meters shall not be used when their performance
flow is not sufficiently parallel, but if it is known that no
may be disturbed by suspended materials in the conduit
swirl exists in the flow, it is sometimes possible to remedy
water.
those irregularities by means of a guiding installation. The
latter will consist of a slightly converging entrance
6.4.2 Range of velocities
connected, without creating any separation, to a straight
Current-meters shall be used only in their normal range of
pipe length, the length of which will be, if possible, at least
use, i.e. the range of velocities for which they have been
equal to twice the larger dimension of the conduit.
IS0 3354-1975 (E)
calibrated; extrapolation towards higher velocities may In conduits with smaller dimensions, a device which enables
nevertheless be tolerated in the case when calibration non-simultaneous measurements to be made should
cannot be achieved at those higher velocities. However, the preferably be used (see 7.2.2, 7.2.3 or 7.3.2).
calibration curve shall never be extrapolated in the area of
lower velocities where the accuracy and above all the
6.4.5 Influence of turbulence and velocity fluctuations
reliability of current-meters decrease considerably. As a
Although the influence of longitudinal and transvefse
general rule no current-meter shall be used at velocities less
components of flow turbulence on current-meter behaviour
than a certain threshold below which the lack of reliability
is still incompletely defined, attention is drawn to the
may lead to important errors (the threshold is a function of
fundamental difference between current-meter gauging by
the current-meter type).
hauling in stagnant water and use of current-meters in
turbulent flow. Longitudinal fluctuations lead to a positive
error on the velocity measured by means of the
current-meter whereas transverse fluctuations generally lead
6.4.3 Blockage effect
to a negative error. While keeping generally in mind that
The velocity distribution in the conduit is disturbed by the
many factors influence the current-meter response, it may
current-meters and their support and this leads to a positive
be observed that the error will increase as :
error being made on the flow rate measurement.
- fluctuation amplitude and frequency increase,
It will be clear that the magnitude of this error is dependent
- the mean velocity decreases,
upon the types of current-meter being used, their number,
it
and the profile of the support rods. In general, however,
- the moment of inertia of the propeller increases.
has been found that the relative blockage of the main
support cross with respect to the measuring section, i.e. the
6.5 Inspection and maintenance of current-meters
ratio of the frontal area of the main support cross to the
cross-sectional area of the conduit, is the most important
6.5.1 Inspection
geometrical parameter. If this relative blockage is between 2
and 6 %, a correction shall be made (see annex A); if it is
The condition of the current-meter shall be checked before
greater than 6%, the measurement cannot be made in
and after each measurement, in particular for the
accordance with this International Standard.
following :
- free rotation in bearings;
- absence of propeller deformation;
6.4.4 Dimensional restrictions
- correct functioning of the rotational speed detection
The above-mentioned remarks relating to the blockage
device.
effect on the one hand, and to the dimensional
requirements as specified in 4.4.1 on the other hand, lead
Friction inspection in bearings may be carried out by
to the proscription of gauging by means of current-meters
observing how the propeller slows down after having been
in conduits, the dimensions of which are too small
started at a certain speed. In no case shall stopping be
compared with those of the current-meters used.
abrupt.
In general, it can be accepted that a fixed current-meter
The propeller shape may be checked by means of a plaster
battery may be used if the diameter of a circular
mould or by means of a metal profile template.
cross-section conduit is greater than 9 times the propeller
diameter or if the smaller side of a rectangular cross-section
6.5.2 Maintenance
is greater than 8 times the propeller diameter, provided that
the relative blockage as defined in 6.4.3 is less than 6 %.
After each series of measurements the current-meter shall
be dismantled, carefully cleaned, then re-lubricated using
Thus, for example, for those types of current-meter and
the same lubricant as that used for calibration.
support cross that are commonly used for industrial
measurements and have propellers with diameters in the
range 0,lO to 0,125 m (4 to 5 in), it is generally accepted,
7 SETTING OF CURRENT-METERS INTO THE
in practice, taking account of the general requirements on
CON DU IT
the minimum distance between two current-meters on the
(see 4.4.2) and of the blockage due to the support
one hand
7.1 Setting of current-meters
on the other hand, that a stationary battery mounted on
cross-bars may be used only in circular conduits of
Current-meters shall be rigidly fixed on the mounting rod in
diameters greater than 1,4 m (56 in). In a rectangular
such a way that the propeller axis is strictly perpendicular
it is also agreed that the smaller dimension of
cross-section,
to the measuring section plane.
the conduit (the support rods being parallel to that
direction) shall be at least equal to 1 m (40 in), and to the
Mounting rods themselves shall be rigidly connected
furthermore that the other dimension shall be sufficient to conduit walls. They shall be designed to offer sufficient
limit the blockage effect (see 6.4.3). mechanical strength (in particular to avoid any prejudicial
IS0 3354-1975 (E)
vibration), minimum and stable drag, and minimum
made by continuous integration be checked either by
interference with the current-meter operation. To this
rotating the rotating arm in the reverse direction or
effect it is especially recommended that polygonal profile
by making a gauging by positioning the arm at a number
rods such as that described in annex B be used.
of fixed locations.
7.2 Mounting in a circular cross-section
7.2.1 Stationary battery
/I
Current-meters are generally used as stationary batteries.
Mounting rods shall therefore be arranged along the conduit
radius so as to constitute at least two diameters (see 4.4.2);
an example of this arrangement is given in figure 1. As far
as possible, no measuring arm shall be located in the vertical
plane of the pipe axis.
I ItC
v,/ ,, , /3, ,, , ,,, ,/,/,//, ,,/,//,~
d C
a) Direction of flow
b) Current-meters
c) Rotating arm
d) Tripod support
e) Attachment at wall
f) Driving gear of rotating arm
g) To the recorder
I
FIGURE 2 - Diametrical arm device rotating in a circular conduit
FIGURE 1 -Stationary battery of current-meters
mounted on cross-bars in a circular conduit
7.2.3 Exploration by means of a single current-meter
7.2.2 Rotating arm
A current-meter may be used in isolation by placing it
Current-meters may be fixed along a diameter rotating
successively at each measuring point. This method requires
around the cross-section axis. (An example of this
special arrangements (lock-vanes) which enable the
arrangement is given in figure 2.) This sophisticated device
current-meter support rod to be guided along the explored
(rotation shall be controlled from the outside and vibration
diameter and to be transferred from one measuring
risks shall be avoided) allows the exploration of a far
diameter to the next one, while maintaining the water-
greater number of measuring points.
tightness. in other respects, account shall be taken of 4.3.2.
It may, if required, allow direct measurement of the mean
velocity per circle, the integration along each circle being
obtained by making the diameter rotate at a constant
7.3 Mounting in a rectangular cross-section
speed. The maximum tangential speed of the rotating
diameter shall not exceed 5 % of the mean flow velocity.
7.3.1 Stationary battery
In order to ensure that the measurement is not falsified by
an excessive gradient of the apparent velocity with respect Current-meters may be used as a stationary battery
to the current-meter, due to the superposition of the mounted on a number of parallel rods. This method may
current-meter movement and a pre-existing obliqueness or
present the disadvantage of having the cross-section
swirl of the flow, it is recommended that the measurements significantly obstructed by the supports.
IS0 3354-1975 (E)
8.1 Circular cross-sections
7.3.2 Exploration of a section by means of a row of
curren t-me ters
If v is the flow velocity at a point of polar co-ordinates r
Another method consists in using a sliding rest bearing one and a, and if R is the mean radius of the measuring section,
the discharge velocity U is given by the formula :
(or two) row(s) of current-meters, travelling in such a way
that the current-meters are successively placed on all
horizontal measuring lines (or alternatively all vertical
lines). This device requires external control with watertight
sealing of certain parts. In other respects, account shall be
taken of 4.3.2.
This process may allow direct measurement of the mean
= f’ U
velocity along one vertical line (or one horizontal line), the
.O
integration along each vertical (or horizontal) line being
obtained by constant speed displacement of the slide rest.
This speed shall not exceed 5 % of the mean flow velocity.
In order to ensure that the measurement is not falsified by
an excessive gradient of the apparent velocity with respect
to the current-meter, due to the superposition of the
current-meter movement and a pre-existing obliqueness or where
it is recommended that the measurements
swirl of the flow,
made by continuous integration be checked either by
U is the mean space velocity along the circumference of
moving the slide in the reverse directioq or by making a
radius r;
gauging by positioning the slide at a number of fixed
I ocati ons.
is the radius of the circle relative to the measuring
r,
points closest to the wall.
However, in the case of a flow where the maximum
water-level is only slightly above the upper boundary of the
The method used consists in :
is simplified : the
conduit, the application of this procedure
slide device can be moved in wells or grooves (for example
bulkhead grooves) opening to atmosphere and its control
a) taking U, (arithmetical mean of the velocities at the
device can be located above the maximum water-level.
measuring points located on a circle of radius r,) as the
value of U;’ )
b) plotting the variation curve of U, with (rc/R)2
8 DETERMINATION OF THE DISCHARGE VELOCITY between r = O and r = rn;2)
BY GRAPHICAL INTEGRATION OF THE VELOCITY
AREA
c) determining graphically the value of the included
area below this curve between r= O and r= r,, (see
The general principle of this method is specified in 4.1.
figure 3);
The measuring points shall be located along straight lines,
d) adding to this value the following terms)
and in order to determine m accurately, two measuring
corresponding to the peripheral zone :
points shall be placed on each straight line as close as
possible to the wall.
m
The number and position of the other points shall be
selected in such a way that the velocity profile can
be determined satisfactorily. Usually, they should be where
distributed in the cross-section in such a way as to divide it
into areas, ?ach having the same flow rate in order to attach
U, is the value of the arithmetical mean of the
approximately the same importance to all measuring points.
velocities at the measuring points located on the
circle of radius ï, (i.e. the closest to the wall);
An example of measuring point distribution for a circular
section for which no indication on the velocity distribution
m is a coefficient depending on the wall roughness
is available will be found in annex C.
and on the flow conditions, the value of which can be
determined in accordance with the indications given
In general, reference will be made to 4.4 to determine the in annex D and is generally between 4 (rough wall)
number and location of measuring points. and 10 (smooth wail).
d(;)’
IS0 3354-1975 (E)
FIGURE 3 - Computation of the discharge velocity in a circular
conduit - Graphical integration in the area explored
by the current-meters
1, Methods exist which allow direct measurement of the mean velocity along a circumference (see 7.2.2).
2) To help plotting in the vicinity of the measuring point closest to the wall, the tangent line to the curve for r = r, will be drawn with a slope
equal to
denoting (r/R)* as A',
The slope of the curve is derived from Karman's conventional law for the variation of the fluid velocities in the peripheral zone
This simplified expression omits the other term
3)
-- rn
(rn+îii2rn+î)unc-$
in the result of the integration (within the peripheral zone) derived from Karman's conventional law : this latter term represents only about
1 - (r,/R)
4m + 2
times the flow in the peripheral zone.
c) adding to this value two terms corresponding to
8.2 Rectangular cross-sections
peripheral zones and respectively equal to
The computation of the discharge velocity requires a
mc:
double integration across both dimensions of the conduit.
Measurement shall be started either on the vertical lines’)
ïva
m+l
or on the horizontal lines.’) The matter is developed here
where
starting with horizontal line measurements.
va is the velocity at the considered extreme
The formula for the discharge velocity is :
measuring point (at a distance a from the nearest
wall);
m is a coefficient depending on the wall roughness
and on the flow conditions, the value of which can be
determined in accordance with the indications given
in annex D and is generally between 4 (rough wall)
where
and 10 (smooth wall).
L is the conduit width in the measuring cross-section
(the sum so obtained is the mean velocity U, on the
(arithmetical mean of the widths measured on at least
horizontal measuring line concerned3));
each horizontal measuring line;
d) plotting the variation curve of U, between the
H is the conduit height in the measuring cross-section
extreme horizontal measuring lines as a function of the
(arithmetical mean of the heights measured on at least
relative height h,/H of the corresponding horizontal line
each vertical measuring line);
(see figure 4)4);
I is the distance from the considered point to the
e) determining graphically the value of the included
side-wall chosen as origin;
area below this curve (see figure 4) between the extreme
horizontal measuring lines;
h is the height of the considered point above the
bottom.
f) adding to this latter value two terms corresponding
to the peripheral zones in order to obtain the discharge
The method used consists in :
velocity. Both terms are respectively equal to :
a) plotting the variation curve of the velocity on each
m a‘
horizontal line between the extreme measuring points, as
- -Ua,
m+îH
a function of the relative distance llL (see figure 4);*)
I.
where U, IS the mean velocity on the horizontal
b) determining graphically the value of the included
area below this curve between the extreme measuring measuring line closest to the wall (at a distance a’ from
points (see figure 4); the wall).
Throughout this sub-clause, a ”vertical line” will mean a line parallel to the conduit height and a “horizontal line’’ will mean a line parallel
1)
to the conduit width.
2) To facilitate plotting in the vicinity of these extreme measuring points, the tangent line to the curve for each of them will be drawn with a
slope the absolute value of which is equal to
VaL
-
ma
The slope of this curve is derived from Karman‘s conventional law for the variation of the fluid velocities in the peripheral zone
= .(yrn
3) Methods exist which allow direct measurement of the mean velocity along a horizontal (or vertical) line (see 7 3.21.
4) To facilitate plotting in the vicinity of the peripheral zones, the same procedure shall be followed as for the determination of the mean
velocity along each horizontal line (see 8.2a)).
v,
IS0 3354-1975 (E)
h
.
I I
FIGURE 4 - Computation of the discharge velocity in a rectangular conduit -
Graphical integration in the area explored by current-meters
9 DETERMINATION OF THE DISCHARGE VELOCITY
conduits, or in /IL or hlH for rectangular cross-section
BY NUMERICAL INTEGRATION OF THE VELOCITY
condu
...