ISO 9213:2004

INTERNATIONAL ISO
STANDARD 9213
Second edition
2004-04-01
Measurement of total discharge in open
channels — Electromagnetic method
using a full-channel-width coil
Mesurage du débit total dans les canaux découverts — Méthode
électromagnétique à l'aide d'une bobine d'induction couvrant toute la
largeur du chenal

Reference number
ISO 9213:2004(E)
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ISO 9213:2004(E)
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ISO 9213:2004(E)
Contents Page
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Principles of operation and practice . 1
5 Applications . 5
6 Selection of site . 6
7 Design and construction . 6
8 Uncertainties in flow measurement . 12
9 Gauge calibration and verification . 13
Annex A (informative) Site survey for electrical interference . 14
Annex B (informative) Design aspects of the electromagnetic coil . 15
Annex C (informative) Numerical example of the calculation of uncertainty . 16
Annex D (normative) Gauge calibration procedure . 17
Bibliography . 19
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ISO 9213:2004(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.
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 9213 was prepared by Technical Committee ISO/TC 113, Hydrometry, Subcommittee SC 1, Velocity area
methods.
This second edition cancels and replaces the first edition (ISO 9213:1992), which has been technically revised.
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INTERNATIONAL STANDARD ISO 9213:2004(E)
Measurement of total discharge in open channels —
Electromagnetic method using a full-channel-width coil
1Scope
This International Standard specifies procedures for the establishment and operation of a gauging station,
equipped with an electromagnetic flow meter, in an open channel or a closed conduit with a free water surface.
This International Standard is applicable to configurations where an artificial magnetic field is generated
through which the entire body of water flows. The induced voltage is sensed in such a way that all elements of
the moving water contribute. The equipment described normally requires an electrical mains power supply.
This International Standard is not applicable to devices sampling only part of the flowing body of water (e.g.
velocity meters) or to flow meters which operate by using the Earth's magnetic field.
2 Normative references
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 748, Measurement of liquid flow in open channels — Velocity-area methods
ISO 772, Hydrometric determinations — Vocabulary and symbols
ISO 1100-2, Measurement of liquid flow in open channels — Part 2: Determination of the stage-discharge
relation
1)
ISO 5168:— , Measurement of fluid flow — Evaluation of uncertainties
ISO/TR 7066-1, Assessment of uncertainty in calibration and use of flow measurement devices — Part 1:
Linear calibration relationships
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 772 apply.
4 Principles of operation and practice
4.1 This is a velocity-area method of discharge determination. The electromagnetic gauge operates on
Faraday's principle of electromagnetic induction. If a length of conductor moves through a magnetic field, a
voltage is generated between the ends of the conductor. In the electromagnetic gauge, a vertical magnetic field
is generated by means of an insulated coil which is located either above or beneath the channel. The conductor
is formed by the water which moves through the magnetic field; the ends of the conductor are represented by
the channel walls or riverbanks. The voltage generated is sensed by electrodes on the channel extremities and
these are connected to the input of a sensitive voltage-measuring device. The faster the velocity of the water,
the greater is the voltage which is generated.
1) To be published. (Revision of ISO/TR 5168:1998)
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ISO 9213:2004(E)
4.2 The principle is widely applied to flow meters in circular pipes running full and in this case approximate
formulae may be generated theoretically and refinement made by calibration through factory produced models.
The open channel flow meter however does not lend itself to such treatment and hence in situ calibration is
always necessary.
4.3 Bevir's formula for the potential generated between electrodes placed in a body of conducting fluid moving
in a magnetic field is given by Equation (1) and illustrated in Figure 1.
�
E= B×j×v×dτ (1)
where
E is the potential between the electrodes;
B is the vector magnetic induction;
j is the vector virtual current between the electrodes;
v is the vector velocity function;
dτ is an element of volume.
Key
1electrode
Figure 1 — Illustration of Bevir's formula
This involves integration between the electrodes over the entire space occupied by the fluid. In practice, the
general case is not solvable since the spatial functions are unknown or difficult to determine. In the simple case
of a rectangular horizontal channel of width wv, expressed in metres, with water flowing with a mean velocity ,
expressed in metres per second, in a uniform vertical magnetic field H, expressed in amperes per metre, the
induced potential E, expressed in microvolts, is measured at electrodes at the sides and is calculated using
Equation (2).
E∝v×w×H (2)
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ISO 9213:2004(E)
where
H =B/µ
µ is the magnetic permeability of the fluid.
In practice, numerically,
∼
E=v×w×H
4.4 In this simple case, if the water depth is dq metres, the flow , expressed in cubic metres per second, is
given by the following equation:
q=v×w×d=E×d/H
If the field H is produced by an electromagnet in the form of an arrangement of coil(s), then for a given situation,
HI is proportional to the electrical current , expressed in amperes, in the coil. Therefore
q=K×E×d/I
where K is a constant.
In practice, this is an oversimplification and a more generally applicable form of the flow formula, taking account
of non-uniformities, is
q=K×E×f(d)/I
where (d) is a polynomial function of d.
f
Usually, a close approximation is obtained where the polynomial is a quadratic, i.e.
2
q= (E/I)× (K +K d+K d ) (3)
1 2 3
4.5 However, there is a sensitivity to non-uniform velocity distribution in the presence of non-uniformities in
other parameters. Though the mathematical treatment is complex, for the purposes of this International
Standard, it may be stated that if the vertical magnetic field is not uniform then changes to the velocity profile for
given flow and depth values will produce an apparent change in the measured induced voltage. This will have
the effect of producing an uncertainty in the flow value determined by the flow meter.
The designer of the flow meter should strive to produce a vertical magnetic field as uniform as possible to
minimize this uncertainty. A single coil above or below a channel may be sufficient if it is wide compared with the
depth of water. Alternatively, better uniformity may be obtained by “saddle-shaped” coils or a pair of coils
deployed above and below the channel. The design of coil systems is not covered in this International Standard
although some design considerations are given in Annex B.
4.6 With most channels, the material comprising the bed and sides will have an electrical conductivity which
cannot be ignored compared with that of the water flowing in the channel (even if the material is concrete). The
apparent induced potential is thus reduced in the same way that voltage at the terminals of a battery is less if
measured whilst a load is connected. Though attempts have been made to determine the effect and allow for it,
these have generally proved unsuccessful. The recommendation is always to line the channel with an
electrically insulating material which substantially removes the conduction path through the channel material
(see 7.2.3).
Depending on the material used for lining the channel, some form of protection is often required to prevent
physical damage by debris being transported along the channel by the flowing water. This protective layer is
usually concrete and this itself will have a conductivity when wet which may be different from that of the water.
The effect of this is much the same as a layer of silt which may settle on the bed and is described in 4.7.
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ISO 9213:2004(E)
4.7 A layer of silt settling on the bed (or the protective layer described in 4.6) may have an effect on the
induced voltage and hence the flow calculated by the flow meter.
Assuming the magnetic field is fairly uniform then the effect described in 4.5 is negligible.
If the wet silt has a similar conductivity to that of the water, it will be seen as a non-moving (or slowly moving)
layer of water. This is similar to a step change in velocity profile and the flow meter should be programmed with
an effective bed level beneath the silt (at the level of the insulating liner). If, however, the layer has a very low
conductivity (packed clay for example), it will behave like an extension of the liner. In this case, the flow meter
should be programmed with an effective bed position at the top of the silt.
In practice the effective bed level should be taken as the level of the insulating liner. However, sometimes an
offset (D ) to the depth (dD) measured from the surface to the liner should be applied. will depend on the
0 0
thickness and conductive properties of the silt and will have a value between zero and the thickness of the silt.
D is obtained by calibration. It is possible, due to thickness or conductivity variations, that the offset may not be
0
constant and this is a source of uncertainty (see Clause 8).
4.8 The value of induced voltage in a practical application is generally in the range of a few tens to hundreds
of microvolts.
In comparison, the electrodes will be subject to various other effects which produce voltages unrelated to the
flow-induced signal. These interfering voltages (or noise) will have different magnitudes and frequencies and
may be far greater than the induced voltage.
Table 1 gives some indication of the magnitude and frequency of sources of interference that are commonly
encountered. Differential magnitude is that measured between the electrodes; common mode is between either
electrode and ground.
Table 1 — Sources, frequency or rate of change and interference magnitude
Frequency or rate of Interference magnitude Interference magnitude
Source
change (differential) (common mode)
Electrical power distribution 50 Hz or 60 Hz 5 mV 1,5 V
High (radio) frequency Much greater than 1 kHz 5 mV 50 mV
Polarization (electrochemical) 0,01 V/min 1 mV 1 V
It is necessary to create a recognizable pattern for the induced voltage to enable it to be detected in the
presence of this larger interference. In practice, this is usually done by alternating the direction of the coil current
which causes the induced signal to alternate in synchronism. The signal detection circuit is designed to detect
this signal and reject the interference. The choice of the alternating frequency is limited on the one hand by
considerations of inductive power loss in the coil and on the other by the need to avoid the frequency of
interference, particularly 50 Hz or 60 Hz. This is often a problem in the vicinity of power distribution systems
using protective multiple earthing (PME). High frequency (HF) interference is not a problem because it is
normally significantly higher than the alternating frequency of the coil. A simple input filter removes it.
Polarization is due to electrochemical action between the water and the electrodes. Though large voltages may
occur, they are easily removed electronically unless they are fluctuating at a similar frequency to the alternating
coil current. This may be the case in foul sewers when wave motion against the electrodes can occur.
In addition, lightning can produce voltages of thousands of volts, which should be withstood to prevent damage
to the flow meter input circuitry.
4.9 Since the coil current is alternating, the possibility exists of electrical “breakthrough” or “coupling” to the
electrodes or their connecting cables. Since this coupling would be at the same frequency as the coil-switching
signal, it would combine with the flow signal in the synchronous detection circuit to produce an offset voltage.
The mechanism for this coupling may be capacitive, inductive or conductive. Care should be taken in the layout
of the coil and electrode elements of the flow gauge to ensure symmetry to minimize the capacitive and
inductive effects. The quality of insulation of cables and joints should be good to avoid conductive coupling
between the elements.
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ISO 9213:2004(E)
Perfect symmetry is difficult to achieve in practice and a small residual voltage (E ) often occurs adding to or
0
subtracting from the induced flow signal E.
The value of E is usually constant and may be determined by calibration, see Clause 9. However, the purpose
0
of the calibration procedure is to determine the number of coefficients and much complication may be avoided
if the value of E can be determined directly. One way of doing this is to perform a zero check with static water
0
in the gauging section. This may be difficult to achieve in a river but can often be done in an artificial channel.
If this is not possible, an indication of whether a significant value of E exists may be obtained by shorting out
0
the electrodes with a wire placed directly between them. This effectively reduces the induced flow signal E to
zero. The residual signal may be less than E since the electrode shorting may also partially reduce the
0
coupled signal.
4.10 From Equation (3), the flow formula therefore becomes
2
q= [(E+E )/I]× [K +K (d−D )+K (d−D ) ] (4)
0 1 2 0 3 0
5 Applications
5.1 The electromagnetic gauge is particularly suited to the measurement of flow in channels where no well-
defined stage-discharge relationship exists, for example where weed growth in a natural river channel causes
variable backwater effects, and in artificial channels of effluent discharge where little head loss occurs. Other
applications could be in measuring the flow of potable water in treatment works or the flow of cooling water in
power stations.
5.2 Different versions of the electromagnetic gauge are suitable for measuring flow in rivers, partly filled pipes
or culverts carrying storm water, raw effluent or foul sewage.
5.3 The advantages of the method include the following:
a) tolerates weed growth;
b) tolerates entrained air;
c) tolerates temperature stratification;
d) tolerates suspended sediment or floating debris in the water;
e) tolerates deposited sediment or other accretion on the channel bed;
f) tolerates variable backwater;
g) tolerates upstream inflows; however, if the inflow conductivity is significantly different from that of the main
channel, there shall be sufficient distance for adequate mixing;
h) can be designed to detect a minimum velocity of about 0,001 m/s;
i) tolerates irregular velocity profiles, depending on the shape of the magnetic field, including skew flow and
severe eddy currents in the measurement area;
j) can be suitable for gauging shallow water provided D can be accurately defined (see 4.7);
0
k) inherently integrates the velocity profile over the entire channel cross-section;
l) affords a wide range of discharge measurements to a typical dynamic range of 1:1 000;
m) does not constrict the flow;
n) can measure reverse flow;
o) does not increase upstream water levels;
p) does not inhibit the passage of migratory fish.
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ISO 9213:2004(E)
5.4 There are however some disadvantages associated with the method, such as
a) the complexity of the construction, which may involve temporary diversion of flow and lining of the channel;
b) the need for calibration, which may prove substantially complex and may take a significant period of time to
complete satisfactorily over the range of measurement required;
c) the need to derive a solution to the formula involved in computing the flow which is mathematically complex;
d) the effects of electrical interference from other sources (see 4.8);
e) the requirement for a reliable electricity supply;
f) the speed of response which is not instantaneous, which precludes its use where fast control operations are
required;
g) the effect on accuracy of spatial differences in water conductivity, e.g. caused by saline intrusion.
6 Selection of site
6.1 A site survey should be carried out if necessary as outlined in Annex A to measure any external electrical
interference (e.g. power cables, radio stations or electric railways). Areas of high electrical interference should
be avoided.
In some cases, the current from the electrical power supply flowing to ground may cause excessive voltages to
be detected by the electrodes.
6.2 Owing to the high power consumption of the coil, equipment intended to measure flow continuously
cannot reasonably be operated continuously from batteries.
6.3 The site shall afford adequate on-bank working space for handling the membrane and cables during
construction (or the preformed coil and rigid liner in the case of a smaller channel) and good access for
operation and maintenance.
6.4 The site characteristics shall be such that the calibration of the station can be determined by an alternative
method, e.g. current-meter gauging.
6.5 Sites shall be selected where there is no spatial variation in water conductivity. The accuracy of the
method will be reduced if the spatial conductivity is not uniform across the section. Gradual variations with time
are unimportant provided that the spatial uniformity of the conductivity is maintained. This requirement makes
an electromagnetic gauge unsuitable for channels in which fresh water flows over saline water, which often
occurs in estuaries. Provided that these requirements are met, the quality of the water will not affect the
operation of the gauge. Similarly the conductivity of the water will not affect the operation of a gauge in an
insulated channel provided that it exceeds 50µS/m.
7 Design and construction
7.1 General
The electromagnetic gauging station should consist of the following elements (see Figures 2 and 3):
a) a field coil installed beneath or above the channel, or both;
b) a pair of electrodes, one on each side of the channel;
c) an insulating membrane; it may be necessary; this may be necessary to protect the liner with a covering
material such as concrete or stone blockwork;
d) an instrumentation unit, including a coil power supply unit;
e) equipment housing;
f) a water-level measuring device (see 7.2.5).
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ISO 9213:2004(E)
These elements can be separate but some systems combine a number of these elements into one unit.
Key
1 field coil
2 electrodes
3 insulating membrane
4 hut containing instrumentation unit
Figure 2 — Buried coil configuration
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ISO 9213:2004(E)
Key
1 level sensor
2 field coil
3flow
4 channel
5 insulating membrane
6electrodes
7 coil current
8 depth data
9 electrode potential
10 instrumentation unit
11 displays
Figure 3 — Bridged coil configuration
7.2 System equipment
7.2.1 Coil
7.2.1.1 The sensitivity of the equipment to the flow is improved by increasing the strength of the field. This is
proportional to the number of turns in the coil and also to the current flowing through the coil. The energy
required to produce the magnetic field in a coil of a certain size, number of turns and current is inversely
proportional to the cross-sectional area of the conductors which make up the coil. It is also proportional to the
electrical resistivity of the material used for the conductors. A compromise should be made, therefore, between
the capital cost of the cable, electricity running costs and strength of electrical interference, and the resolution
required in the determination of flow.
In practice, a coil with a square configuration slightly larger than the channel width and of some 200 ampere
turns to 1 000 ampere turns, should cover most practical situations.
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ISO 9213:2004(E)
7.2.1.2 Any electrical leakage between the coil and the water in the channel will create voltages across the
width of the channel. These voltages cannot be separated from those generated by the movement of water
through the magnetic field and will produce an apparent offset in the readings of the equipment.
If the coil is located beneath the channel, the use of a polyethylene-insulated cable with a polyethylene outer
sleeve is recommended. In all cases, the insulation between the coil and earth (or the water surrounding the
8
coil) shall exceed 5× 10 Ω.
7.2.1.3 The coil shall be installed in ducting (normally of about 250 mm diameter) to afford access for
maintenance of the cable. Construction constraints normally require the coil to be square in plan.
For a bridged coil, a lesser grade of insulation such as polyvinyl chloride (PVC) is acceptable. The coil shall
span the full width of the river above the maximum stage at which measurements are required. If the coil is likely
to be submerged, it shall be able to withstand impact by floating debris. If meaningful measurements are
8
required in this condition, the insulation shall exceed 5× 10 Ω when submerged.
7.2.1.4 For convenience, the coil may be wound with a multi-core cable and it should not be armoured.
7.2.1.5 In cases where installation is in potentially explosive atmospheres, the design should be approved by
an appropriate body. The design should be such that it limits the energy that could be transferred to the
explosive atmosphere if a fault were to occur.
7.2.1.6 The frequency at which the magnetic field is reversed shall be low enough to permit a stable field to be
established, but not so low as to permit polarization effects to become significant. Typical frequencies would be
0,5 Hz 1 Hz
in the range 0,5cycle/s or 1cycle/s ( to ). The coil current should be either measured or,
alternatively, stabilized at a fixed value.
7.2.1.7 A typical coil design is described in Annex B.
7.2.2 Electrodes
7.2.2.1 It is recommended that the electrodes be made from stainless steel strip or tube. Typically, the width of
flat electrodes may be in the range 50 mm to 100 mm. Tubular electrodes should be of the order of 10 mm to
20 mm
diameter.
7.2.2.2 In channels containing foul water which is liable to putrify, the electrode mounting shall not permit such
water to become trapped in pockets or crevices near the electrode, and no mechanical filter shall be used.
7.2.2.3 In the event of a lightning strike in the vicinity of the gauge, high voltages may be generated. The gauge
may require protection to avoid being damaged.
7.2.2.4 The inductive or capacitive coupling between the signal cable and the coil shall be a minimum in order
to reduce the effects described in 4.9. This can be achieved by the feed from the electrode on the far bank
passing in a straight line through the coil centre to bisect the plan area of the coil. An alternative arrangement is
to take two signal cables from the far bank electrode: one cable passes through the same ducting as the
upstream coil cable and the second electrode cable passes through the downstream coil ducting. The signals
from these two cables are added together using a resistance network. Ducting for the electrode cables either
shall cross the channel beneath the insulating membrane (if used) or shall be bridged across the channel.
7.2.2.5 In open channels, the electrodes may be supported in guides mounted on the walls or banks on either
side of the channel in order that they can easily be removed for maintenance. Such mountings shall extend
throughout the full depth of flow. The guides may consist of slotted plastic rods for flat electrodes or perforated
plastic tubing for tubular electrodes. Alternatively the electrodes may be moulded into glass-reinforced plastic
units, with only one face of the metal electrode exposed. The guides shall be secured to the channel walls or
banks, but the membrane shall not be punctured (except as specified in 7.2.3.4 ) (see Figure 2).
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ISO 9213:2004(E)
In closed conduits the electrodes may be installed as part of the preformed pipe section (see 7.2.3.7 and
Figure 4).
Key
1 concrete benching
2 level sensor
3 U-section channel
4electrodes
5 coil supported on benching
6 glass-reinforced plastic pipe section
7 concrete pipe
Figure 4 — Coil configuration for closed conduits
7.2.3 Insulating membrane
7.2.3.1 An insulating membrane shall be used which is strong enough to withstand the stresses involved. A
high-density polyethylene sheet 2mm or 3mm thick, or equivalent material, is recommended. The resistivity of
12
the material shall be greater than 10 Ω· m. Alternatively glass-reinforced plastic (GRP) laid up on the walls or
fixed in sheets or in preformed shapes could be use
...