Abstrict In the case of an inductive flow meter for electrically conductive
liquids having an insulating flow channel section of circular cross
section, a pair of electrodes situated diametrically opposite one
another and electrically coupled to the electrically conductive
liquid, and having a magnetic field generation system that surrounds
the flow channel section at a specific angle of wrap and generates
a magnetic field with a field line direction substantially perpendicular
to the connecting straight line between the measuring electrodes
and perpendicular to the flow channel longitudinal axis, an increased
insensitivity to measured value falsifications on the basis of variations
that are dependent on flow rate, in the flow profile in the flow
channel section, or on the basis of asymmetries in the flow profile
relative to the flow channel middle axis is achieved by virtue of
the fact that active area arrangements, specifically areal formations,
onto which the field lines of the magnetic field generation system
penetrates the flow channel inner wall in a restricted fashion are
dimensioned in a particular way, specifically in such a way that,
in a developed presentation of the flow channel inner wall, bounding
straight lines that are laid tangentially against the active area
arrangements and touch the active area arrangements at two points
from outside, converge in pairs in the direction of the respective
location of the measuring electrodes, and are located between the
respective two points of contact concave regions in which the boundary
lines of the active area arrangements have no points of contact
of any sort with the tangential bounding straight lines.
Claims 1. An inductive flow meter for electrically conductive liquids,
having a flow channel section (1) of essentially circular cross
section that is electrically insulating at least on its inner side;
having at least one pair of electrodes (2 3) that are situated
diametrically opposite one another and are electrically coupled
to the electrically conductive liquid; and having a magnetic field
generation system (7) that is formed either from a magnetic closed
circuit (20) with a field coil and with pole shoes (7P) that surround
the flow channel section (1) over a specific axial length (b) and
at a specific angle of wrap (2.phi..sub.0), or by a field coil arrangement
(7L) that surrounds the flow channel section (1) over a specific
axial length (b) and at a specific angle of wrap (2.phi..sub.0),
and which system generates a magnetic field that penetrates the
flow channel interior in the region upstream and downstream of the
measuring electrodes (2 3) as well as between the latter and is
oriented substantially perpendicular to the connecting straight
line between the measuring electrodes and perpendicular to the flow
channel longitudinal axis (Z), and whose field lines respectively
penetrate the flow channel inner wall in a restricted fashion on
active area arrangements (Fa, Fb, Fc, Fd, Fe, Ff) that are positioned
between the measuring electrodes (2 3) and extend circumferentially
in accordance with said angle of wrap (2.phi..sub.0) and axially
in accordance with said axial length (b); characterized in that,
in a developed presentation of the flow channel inner wall, bounding
straight lines (T1 T2) that are laid tangentially against the active
area arrangements (Fa, Fb, Fc, Fd, Fe, Ff) and touch the active
area arrangements at two points from outside, converge in pairs
in the direction of the respective location of the measuring electrodes
(2 3), and there are located between the respective two points
of contact concave regions (K) in which the boundary lines of the
active area arrangements have no points of contact of any sort with
the tangential bounding straight lines.
2. The inductive flow meter as claimed in claim 1 characterized
in that the circumferential extent (2.phi..sub.0) of the active
area arrangement is 125.degree. to 145.degree., or at least 120.degree.,
preferably more than 140.degree..
3. The inductive flow meter as claimed in claim 1 or 2 characterized
in that each active area arrangement (Fa-Ff) has at least 65%, preferably
more than 75% of its areal content in a circumferential region from
the circumferential center, positioned symmetrically between the
measuring electrodes (2 3) in accordance with .phi.=0 up to .phi.=.+-..phi..sub.0/2
and respectively correspondingly has at most 35%, preferably less
than 25% of its areal content in the circumferential region from
.phi.=.+-..phi..sub.0/2 to .phi.=.+-..phi..sub.0.
4. The inductive flow meter as claimed in one of claims 1 to 3
characterized in that the active area arrangements are each inherently
closed formations.
5. The inductive flow meter as claimed in one of claims 1 to 3
characterized in that the active area arrangements are each not
inherently closed areal formations.
6. The inductive flow meter as claimed in claim 5 characterized
by interposed partial active areas (Fu) through which there penetrate
field lines of magnetic fields that are oriented opposite to the
field lines penetrating the active area arrangements, and are generated
by additional magnetic field generation systems that include additional
magnetic closed circuits or additional field coil arrangements (FIG.
7F).
7. The inductive flow meter as claimed in one of claims 1 to 6
characterized in that the density of the magnetic field lines in
the active area arrangements is substantially constant throughout
the latter.
Description [0001] The invention relates to inductive flow meters having the
features of the preamble of patent claim 1.
[0002] Flow meters of the type considered here are used to determine
the flow of electrically conducting liquids through channels or
tubes of circular cross section, in particular. In a channel section
having an electrically insulating channel wall or tube wall, there
are provided at mutually opposite points of a channel cross section
punctiform electrodes that are electrically coupled to the electrically
conducting liquid, are exposed to the channel interior, in particular,
and are connected to the conducting liquid in a conducting fashion.
Running perpendicular to the connecting line between the electrodes,
and perpendicular to the flow lines of the conducting liquid flowing
through the channel or the tube are the field lines of a magnetic
field that is generated by a permanent magnet arrangement or, in
particular by a coil arrangement. Conductor paths which run from
one punctiform electrode to the other punctiform electrode and which
penetrate the entire tube cross section or channel cross section
of the tube section or channel section containing the electrodes
can, when the conducting liquid is moving along the channel or tube,
be understood as conductors which are moving in the magnetic field
and in which liquid flow induces voltages that are tapped by the
punctiform electrodes via connections led through the insulating
channel wall or tube wall, and are a measure of the flow of the
conducting liquid through the tube or the channel.
[0003] Upon closer scrutiny, the output signal of an inductive
flow meter of the type described above that can be tapped by the
electrodes may be specified as follows: S .about. .intg. ( Vol )
.times. ( B -> .times. W -> ) v -> .times. .times. d (
Vol ) .
[0004] The integral over the volume is formed by the respective
product value of vectors of three vector fields, of which {right
arrow over (B)} is the magnetic induction in the cylindrical space
surrounding the flow channel having the channel inner cross section
and having a specific length upstream and downstream of the radial
plane containing the electrodes, and {right arrow over (W)} denotes
a weight vector field that is to be understood as a field of vectors
in the previously defined cylindrical space and which characterizes
the configuration of the conductor paths between the electrodes
in the cylindrical space. Finally, {right arrow over (v)} denotes
the vector field in said cylindrical space with vectors corresponding
to the speeds of the particles of the conductive liquid.
[0005] If the values of {right arrow over (B)} were constant in
modulus and direction (homogeneous magnetic field), and if the values
of the weight vector field {right arrow over (W)} were corresponding
flow paths, constant in modulus and direction, running parallel
to one another between parallel electrodes, the partial product
{right arrow over (B)}.times.{right arrow over (W)} would be constant
in such a way that non-uniform and/or asymmetric velocity distributions
of the flow, which is to be investigated, of the electrically conductive
liquid through the flow channel section do not lead to measured
value falsifications.
[0006] It is true that with little effort the magnetic field of
the magnetic field generation system can be formed such that it
is substantially homogeneous in the region of the interior of the
flow channel section, whereas the weight vector field is in no way
homogeneous given a flow channel section of round cross section
and given essentially punctiform electrodes situated diametrically
opposite one another. This is shown directly by the following consideration:
[0007] If these essentially entirely overlapping conductor paths
are drawn in tube cross sections or channel cross sections, it will
be seen that a conductor path concentration is present in the region
near the punctiform electrodes in such a way that movements of the
conductor paths because of the flow of the conducting liquid in
these regions exert a particularly strong influence on the signal
that can be tapped from the electrodes.
[0008] If they are laminar, characteristic flows in the flow channel
section of an inductive flow meter can have a velocity profile that
is rotationally symmetrical with reference to the flow channel middle
axis in the undisturbed state, or can, given an asymmetric disturbance,
exhibit a flow profile whose maximum is offset laterally in the
radial direction from the flow channel middle axis. For high flow
velocities, the flow can become turbulent in such a way that the
flow profile exhibits a plateau area with reference to the flow
channel cross section, and areas of lower flow velocity near the
edge.
[0009] Both deformations of the flow profile as a function of the
flow velocity, and asymmetries in the flow profile influence a measurement
result obtained with an inductive flow meter of the type considered
here in a way that falsifies measured values.
[0010] An attempt has already been made in the technical teaching
of German patent specification 1 295 223 to inhomogeneously form
the magnetic field in a flow channel section of an inductive flow
meter in such a way as thereby to counteract the influence of the
necessarily present inhomogeneity of the weight vector field on
the measurement result given a non-uniform flow distribution over
the flow channel cross section. German patent specification 1 295
223 proposes for this purpose that the magnet arrangement, that
is to say the magnetic field generation system, be formed such that
the field component in the radial plane containing the electrodes,
and in planes parallel thereto decreases from the inside to the
outside in the direction of the connecting line between the electrodes.
[0011] In order to reduce the falsification of measured values
owing to non-uniform flow distribution, an attempt has already been
made to compensate for the increased influence of the region of
the flow cross section near the electrodes on the magnitude of the
measurement signal by virtue of the fact that, for example in accordance
with the German laid-open patent application 26 22 943 provision
was made when generating the magnetic field by means of energized
coils of additional compensation coils that generated in the cross-sectional
plane of the flow channel section containing the electrodes, or
else upstream or downstream thereof, magnetic fields that permeated
the flow for generating the induced voltages in the conductor paths
in those regions that are to be assigned to the regions located
in the immediate vicinity of the electrodes, the orientation of
these magnetic fields being directed opposite to the main magnetic
field.
[0012] The structure of the overall device resulting from this
is comparatively complicated, the parts of the magnet system located
in the vicinity of the electrodes and which act directly on the
region of very high conducting path density requiring very exact
mounting and extremely fine focusing.
[0013] An inductive flow meter that acts in a similar manner to
the device previously considered and has a simplified structure
of the magnetic field generation system is described in the German
laid-open patent application 400 20 30.
[0014] Finally, European patent application, publication number
41 80 33 discloses an inductive flow meter of the type considered
here having a main magnetic field generation system which is offset
by 90.degree. with reference to the pair of measuring electrodes
situated opposite one another and is equipped in each case with
pole shoes bearing against the outer surface of the flow channel
wall over a restricted angular range of said wall and with auxiliary
coils that wrap around these pole shoes and bear closely against
the outer surface of the flow channel section over a relatively
large angular range in such a way that an approximately sinusoidal
magnetic flux distribution of the magnetic field generation system
is achieved over a lateral surface, spanning less than 180.degree.
in a circumferential direction, of the flow channel wall between
the measuring electrodes.
[0015] It emerges that with this known inductive flow meter, as
well, it is not possible to achieve entirely satisfactory insensitivity
of the measurement result to variations, dependent on flow velocity,
in the flow profile, and to asymmetric distortions of the flow profile
of the flow in the flow channel section.
[0016] It is therefore the object of the present invention to configure
an inductive flow meter of the general type considered here so as
to achieve in conjunction with a comparatively simple design and
simple production of the magnetic field generation system a substantially
improved insensitivity to measured value falsifications owing to
variations, dependent on flow velocity, of the flow profile in the
flow channel cross section, or owing to asymmetries in the flow
profile relative to the flow channel middle axis.
[0017] This object is achieved according to the invention by means
of an inductive flow meter having the features in accordance with
claim 1. It may be stressed here that an insensitivity to measured
value falsifications that is improved by up to an order of magnitude
is achieved according to the invention, the designer being offered
a surprisingly simple concept for achieving the object for manifold
applications.
[0018] Advantageous refinements and developments are the subject
matter of the patent claims dependent on claim 1 the content of
which is hereby expressly rendered a constituent of the description
without repeating the wording at this juncture.
[0019] Some exemplary embodiments are explained below in more detail
with the aid of the drawings, a schematic mode of representation
that chiefly illustrates the mode of operation being selected in
the drawings, and no value being placed on being true to scale.
In the drawings:
[0020] FIG. 1 shows a partly sectional perspective illustration
of an inductive flow meter of the general type considered here for
the purpose of explaining terms and geometric relationships;
[0021] FIG. 2 a perspective view of a part of an inductive flow
meter for the purpose of explaining how an asymmetric distortion
of the velocity vector field in the flow channel cross section relative
to the flow channel middle axis influences measured value falsifications;
[0022] FIG. 3 shows a perspective view of a part of an inductive
flow meter for the purpose of explaining how a change in the flow
velocity vector field, which is symmetrical in relation to the flow
channel middle axis, upon transition from a laminar flow to a turbulent
flow influences measured value falsifications;
[0023] FIG. 4 shows a perspective view of a part of an inductive
flow meter through whose flow channel cross section a laminar flow
of the electrically conducting liquid passes;
[0024] FIG. 5 shows a schematic perspective view of an inductive
flow meter of the type specified here, having a magnetic field generation
system formed from field coils;
[0025] FIG. 6 shows a schematic perspective view of an inductive
flow meter of the type specified here, having a magnetic field generation
system formed from a magnetic closed circuit with pole shoes; and
[0026] FIGS. 7A to 7F show developments of the flow channel inner
wall in a circumferential region between the midpoint between the
measuring electrodes up to a measuring electrode, for various embodiments
of an inductive flow meter of the type specified here.
[0027] The inductive flow meter of the general type considered
here consists in its basic constituents as illustrated in FIG. 1
of a flow channel section 1 in the shape of a tube made from an
electrically insulating material. The central longitudinal axis
of the flow channel section 1 is denoted by Z. Located in the middle
of the longitudinal extent of the flow channel section 1 at locations
which are situated diametrically opposite one another over the relevant
flow channel cross section are measuring electrodes 2 and 3 which,
are, for example, approximately punctiform and are connected to
a voltmeter 6 via measuring lines 4 and 5 respectively, that reach
through the wall of the electrically insulating flow channel section
1. On the inside of the flow channel section 1 the measuring electrodes
2 and 3 can make direct contact with the electrically conducting
liquid flowing through the flow channel section 1 or else can be
coupled capacitively to the electrically conducting liquid where
a magnetic field generation system of the inductive flow meter is
used in a way known to the person skilled in the art to excite alternating
current such that, in this case, the measuring electrodes need not
lie exposed on the inside of the flow channel section 1. The distance
of the cross-sectional plane of the flow channel section 1 containing
the measuring electrodes 2 and 3 from the end of said section lying
upstream, and from the end thereof lying downstream may be denoted
respectively by z.
[0028] Finally, a magnetic field generation system 7 is indicated
by block symbols in FIG. 1. This system generates an induction vector
field {right arrow over (B)} represented by vectors of the magnetic
induction, the magnetic field lines permeating the wall of the flow
channel section 1 and its interior and being oriented substantially
perpendicular to the central axis Z and perpendicular to the diametrical
line, connecting the measuring electrodes 2 and 3 of the flow channel
section.
[0029] The length of 2z considered here for the interior of the
flow channel section 1 is selected here to be approximately equal
to the diameter of the flow channel cross section. A magnetic field
is generated by the magnetic field generation system 7 may firstly
be assumed for the purposes of explanation in conjunction with FIG.
1 to be homogeneous throughout the interior of the flow channel
section 1. If an electrically conducting liquid is now guided through
the interior of the flow channel section 1 the flow particles of
the liquid have velocities corresponding to the individual velocity
vectors of a vector field {right arrow over (v)} that are parallel
to the central longitudinal axis Z.
[0030] A multiplicity of conductor paths that permeate the entire
interior of the flow channel section 1 both over the channel cross
section and over the length of the flow channel section 1 are indicated
by dashed lines w in FIG. 1. If the electrically conducting liquid
moves in accordance with the velocity vector field {right arrow
over (v)} through the flow channel section 1 the conductor paths
are then to be understood in accordance with the lines w as conductors
moved in the magnetic field and in which electromotive forces are
respectively induced because of the movement of the conductor paths
in such a way that a resulting induced measuring voltage is finally
present between the measuring electrodes 2 and 3 and is measured
by the measuring instrument 6 and bears a relationship to the flow
rate per time unit of the electrically conducting liquid.
[0031] Because of the orientation and the course of the conductor
paths, adopted in the electrically conducting liquid, in accordance
with the lines w, the electromotive forces induced in the individual
conductor paths contribute to a different extent to the measuring
signal S that can finally be read off at the measuring instrument
6. This results because, at least in specific sections of their
course between the measuring electrodes 2 and 3 the conductor paths
have an orientation differing from the course perpendicular to the
central longitudinal axis Z and perpendicular to the field lines
of the magnetic field, and also exhibit different lengths in each
case.
[0032] It is justifiable for this reason to consider the conductor
path configuration as a conductor path configuration weight vector
field {right arrow over (W)}, this vector system, denoted below
for short as weight vector field, taking account of the orientation
components of the conductor path course that are responsible for
the induction of electromotive forces.
[0033] The signal S that can be read out at the voltmeter 6 may
be expressed as follows: S .about. .intg. ( Vol ) .times. ( B ->
.times. W -> ) v -> .times. .times. d ( Vol )
[0034] If all the vectors of the flow velocity vector field {right
arrow over (v)} that are parallel to the central longitudinal axis
2 are of the same length, that is to say if the flow velocity is
constant over the flow channel cross section, there is a linear
dependence of the measuring signal S on the flow velocity, since
the product ({right arrow over (B)}.times.{right arrow over (W)})
is essentially as an apparatus constant determined by the geometric
arrangement in the flow meter.
[0035] However, for specific cases of operation of the inductive
flow meter the velocity vector field {right arrow over (v)} suffers
specific distortions in practice that will be treated briefly in
a purely qualitative fashion with reference to FIGS. 2 to 4.
[0036] FIG. 2 shows a vector field {right arrow over (v)} of the
velocity distribution over the flow channel cross section, in which
no rotational symmetry of the flow profile is present with reference
to the central longitudinal axis Z of the flow channel cross section
1. The region of maximum velocity vectors of the vector field {right
arrow over (v)} is offset asymmetrically downward with reference
to the central longitudinal axis Z. This velocity distribution can
result, for example, from the fact that there are located in the
channel sections upstream of the flow channel cross section 1 flow
obstacles, for example valve spools, tube bends and the like, the
effect of which is that the maximal flow vectors of the flow distribution
are located, for example, in the lower quadrant of the tube cross
section. However, the region of the maximum can also lie in other
quadrants, for example in a cross-sectional quadrant adjoined by
the measuring electrode 2 or in a cross-sectional quadrant adjacent
to the zenith of the flow channel cross section 1 or else in the
cross-sectional quadrant adjacent to the measuring electrode 3.
[0037] FIG. 3 shows a situation in which a transition has taken
place from laminar flow (see FIG. 4) to a turbulent flow because
of high flow velocities in the flow channel section 1. The flow
profile is approximated in an axial section by a trapezoidal shape,
edge sections of low flow velocity having a relatively low radial
thickness. In the region of a laminar flow in accordance with FIG.
4 the flow profile of the vector field {right arrow over (v)} has
the shape of a paraboloid of revolution that is symmetrical in relation
to the central longitudinal axis Z.
[0038] Both the position and the size of the asymmetry of the flow
profile relative to the central longitudinal axis Z according to
FIG. 2 and the basic shape of a flow profile, symmetrical relative
to the central longitudinal axis Z, according to FIGS. 3 and 4
and, finally, also a flow profile in the shape of a paraboloid of
revolution in the laminar flow region influence the measuring signal
that can be read off at the measuring instrument 6 according to
FIG. 1 in the sense of a measured value falsification, assuming
a homogeneous magnetic field B, since deviations in the practical
velocity vector fields {right arrow over (v)} from a uniform distribution
over the flow channel cross section respectively signify different
movements of the conductor paths, illustrated by the linear field
w in FIG. 1 of the weight vector field {right arrow over (W)},
and thus different contributions to the signal S.
[0039] It has now been found that the compensation of the influence,
acting to falsify the measured value, of the distortion of the flow
velocity distribution over the flow channel cross section by a particular
shaping of active area arrangements is very successful, these active
area arrangements being those areal formations, on which the field
lines of the magnetic field generation system respectively penetrate
the flow channel inner wall in a restricted fashion. These active
area arrangements lie on the flow channel inner wall between the
measuring electrodes, and extend circumferentially in accordance
with the angle of wrap of pole shoes or field coil arrangements
with reference to the flow channel circumference, as well as in
an axial direction corresponding to the axial extent of pole shoes
or of field coil arrangements, doing so symmetrically upstream and
downstream of the flow channel section radial cross section containing
the measuring electrode points.
[0040] The following may be stated in this regard with the aid
of FIGS. 5 and 6:
[0041] FIG. 5 shows an embodiment in which the magnetic field generation
system assigned to the flow channel section 1 and the measuring
electrodes 2 and 3 in the way illustrated is formed by two field
coils 7L that closely fit the outer circumferential surface of the
flow channel section 1 in the way illustrated, are respectively
situated in regions diametrically opposite one another and between
the measuring electrodes 2 and 3 and which span an angle of wrap
denoted in FIG. 5 by 2.phi..sub.0. The axial extent of the field
coils 7L will in each case amount to b in a position that is symmetrical
in relation to the radial plane containing the measuring electrodes
2 and 3.
[0042] FIG. 6 shows in a wholly corresponding way a highly schematic
perspective view of an inductive flow meter with a closed circuit
20 forming the magnetic field generation system, which supports
a field coil (not illustrated), and which has pole shoes 7P that
are located opposite one another across the flow channel section
1 and, with reference to the axial extent, are in turn situated
symmetrically relative to the radial plane containing the measuring
electrode points and have an axial extent b, while in the circumferential
direction they surround the flow channel section 1 of round cross
section with an angle of wrap of 2.phi..sub.0 in each case, as is
made plain by dashed and dotted lines in FIG. 6.
[0043] If a field current is applied to the field coils 7L of the
embodiment according to FIG. 5 or if the field coil of the magnetic
closed circuit 20 of the embodiment according to FIG. 6 are/is excited,
then magnetic field lines of the magnetic fields generated by the
field coils or the pole shoes 7 in the interior of the flow channel
section 1 permeate the bounding inner wall of the flow channel section
1 in a fashion respectively most largely restricted to cylindrical
active area arrangements situated diametrically opposite one another
and whose cylindrical shape is essentially in accordance with the
teaching given here in order in a surprisingly simple way to compensate
for influences, which falsify measured values, of distortions in
the flow velocity distribution over the flow channel cross section.
[0044] The possible shapes of the active area arrangements can
be seen from developments of the cylindrical inner wall of the flow
channel section 1 in the circumferential region between the cylinder
surface line situated centrally between the measuring electrodes
2 and 3 and the cylinder surface line passing through one each
of the measuring electrodes, for example through the measuring electrode
3 in accordance with the illustrations of FIGS. 7A to 7F. In FIGS.
7A to 7F, a dashed and dotted horizontal line R denotes the track
of the intersection between the flow channel inner wall and a radial
plane containing the measuring electrodes 2 and 3. All the developed
views shown extend over a circumferential angular range of .phi.=90.degree.,
and thus respectively constitute a quarter of the inner circumferential
surface of the flow channel section 1.
[0045] In the case of the embodiment in accordance with FIG. 7A,
the active area arrangement is denoted by Fa and corresponds qualitatively
to that magnetic field line penetration surface that is generated
by the field coil arrangement 7L according to FIG. 5 in a square
of the flow channel inner surface.
[0046] If bounding straight lines T1 and T2 are drawn in a fashion
tangential to the active area arrangement Fa and touch the active
area arrangement Fa at two points from outside, the shape of the
active area arrangement ensures that these bounding straight lines
T1 and T2 converge in pairs in the direction of the respective location
of the measuring electrode, that is to say the measuring electrode
3 in the illustrated selected, this condition not being, however,
sufficient on its own for the targeted purpose. It is of essential
importance that the active area arrangements are shaped such that
there are located between the respective two points of contact concave
regions K in which the boundary lines of the active area arrangements
Fa have no points of contact of any sort with the tangential bounding
straight lines T1 and T2 respectively.
[0047] In the case of the embodiment according to FIG. 7B, the
active area arrangements Fb are provided by means of constrictions
on the basis of corresponding shaping of the field coils 7L in the
regions of greater angle of wrap, such that the concave regions
K are enlarged. The active area arrangements Fb are, however, still
respectively inherently closed.
[0048] In accordance with the embodiment shown in FIG. 7C, the
active area arrangements can also be formed by mutually separate
areal formations Fc, which means that there is provided in a specific
range of angle of wrap of small extent a main field coil at which,
in each case in relatively large ranges of angle of wrap directed
toward the measuring electrodes 2 and 3 small auxiliary field coils
with the same winding senses as that of the main field coil are
located. The concave regions K between the boundary lines of the
active area arrangement Fc and the tangential bounding straight
lines T1 and T2 can be enlarged further by such a formation if this
is desirable in specific cases.
[0049] It has emerged that the circumferential extent of the active
area arrangements Fa and Fb and Fc in accordance with the angle
of wrap 2.phi..sub.0 of the associated field coil arrangement 7L
is in the region of at least 120.degree., preferably more than 140.degree.,
ranges of more than 140.degree. for the angle of wrap leading to
surprisingly good results.
[0050] If the embodiments according to FIGS. 7A to 7C are considered,
it is seen that the active area arrangements Fa and Fb and Fc have
their area centroid lying closer to the circumferential center of
the flow channel inner surface between the measuring electrodes,
while those parts of the active area arrangements that extend further
out to the measuring electrodes 2 and 3 respectively, have a lower
areal weight. It is to be stated in this regard that very advantageous
results are achieved in the sense of the achievement of the object
set when each active area arrangement has in a circumferential region
from the circumferential center, situated symmetrically between
the measuring electrodes, in accordance with .phi.=0 up to .phi.=.+-..phi..sub.0/2
at least 65%, preferably more than 75% of its areal content, and
respectively correspondingly has at most 35%, preferably less than
25% of its areal content in the circumferential region from .phi.=.+-..phi..sub.0/2
to .phi.=.+-..phi..sub.0.
[0051] Shown in FIG. 7D, in a way similar to FIG. 7A, in a quadrant
of the active area arrangements in the developed view as they are
defined by field coil arrangements 7L in accordance with FIG. 5
is the developed view of a quadrant of an active area arrangement
Fd such as is defined, for example, by the pole shoe arrangement
7P of the magnetic closed circuit 20 of FIG. 6. Here, as well, tangential
bounding straight lines T1 and T2 which are laid at the active area
arrangement Fd and respectively touch the active area arrangement
Fd at two points from outside converge in pairs in the direction
of the respective location of the measuring electrodes, that is
to say in the present case to the location of the measuring electrode
3. Lying between the respective two points of contact here, as well,
are concave regions K in which the circumferential lines of the
active area arrangements have no points of contact of any sort with
the tangential bounding straight lines T1 and T2.
[0052] FIG. 7E shows an active area arrangement Fe in which mutually
separate active area elements cooperate. It is evident to the person
skilled in the art that such an active area arrangement can be defined
by pole shoes that, in departure from the arrangement in accordance
with FIG. 6 has, instead of the integrally attached relatively
narrow pole shoe webs, separate auxiliary pole shoes that respectively
bear on both sides of main pole shoes, in a fashion situated between
the latter and the measuring electrodes, against the respective
circumferential surfaces of the flow channel section 1 and are
identically permeated with reference to the main pole shoes. By
comparison with the embodiment according to FIG. 7D, the concave
regions K have a larger extent in the case of the design of the
active area arrangement Fe.
[0053] Finally, the illustration of FIG. 7F shows the possibility,
in the case of active areas not inherently closed, for example in
the manner of FIGS. 7C or 7E to arrange between the active area
parts symmetrically in relation to both sides of a principle active
area element partial surface elements Fu through which field lines
of magnetic fields pass that are oriented oppositely to the field
lines penetrating the active area arrangements, and which are generated
by additional magnetic field generation systems that include additional
magnetic closed circuits or additional field coil arrangements.
A further improvement in the compensation can be achieved here for
specific characteristic distortions of the flow velocity field in
the flow channel cross section. However, it is essential that the
active area arrangement element lying next to the respective measuring
electrode be permeated by field lines that correspond in orientation
to those of the main active area arrangement element.
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