An electromagnetic flow meter having a magnet system with pole
shoes on opposite sides of a measuring tube, a magnetic coil arrangement
with a yoke for the magnetic return circuit, and two measuring electrodes
between the respective pole shoes. The magnetic coil is coaxial
with the measuring tube and surrounds the pole shoes and the pole
shoes are connected to the yoke by radially extending webs at opposite
ends of the magnetic coil.
1. An electromagnetic flow meter, comprising, a measuring tube
of an electrically insulating material, a magnet system including
two pole shoes on diametrically opposite sides of said tube, a magnetic
coil surrounding said shoes in coaxial relation to said tube, two
measuring electrodes circumferentially between said pole shoes,
a yoke for the magnetic return circuit surrounding said magnetic
coil, each said pole shoe having at only one end thereof a radially
and circumferentially extending web portion with said web portions
being adjacent opposite ends of said coil connected to said yoke.
2. An electromagnetic flow meter according to claim 1 characterized
in that said yoke has a cylindrical inner periphery and said radially
extending web portions are formed by a segmented ring having at
least two circumferential sections.
3. An electromagnetic flow meter according to claim 2 characterized
in that at least one of said circumferential sections is made in
one piece with one of said pole shoes.
4. An electromagnetic flow meter according to claim 2 characterized
in that said circumferential sections are divided substantially
in a radial plane halving said pole shoes.
5. An electromagnetic flow meter according to claim 1 characterized
in that said yoke is formed by a cylindrical sleeve.
6. An electromagnetic flow meter according to claim 1 including
flanges for said measuring tube end in straddling relation to said
magnetic coil and said web portions.
7. An electromagnetic flow meter according to claim 1 characterized
in that said pole shoe radially extending web portions have cross
sections which increases radially outwardly.
8. An electromagnetic flow meter according to claim 1 characterized
in that said pole shoe radially extending web portions have axially
extending extensions at the outer peripheries thereof.
9. An electromagnetic flow meter according to claim 1 characterized
in that one of said radially extending web portions is formed to
provide an annular space surrounding said tube, leads for said measuring
electrodes disposed in said space and extending axially between
said shoes up to one or the other of said radially extending web
portions and then outwardly.
10. An electromagnetic flow meter according to claim 9 characterized
in that at least one of said pole shoes has a circumferentially
extending groove into which is inserted said measuring electrode
11. An electromagnetic flow meter according to claim 1 including
cylindrical screening of electrically conductive material provided
at the inner periphery of said magnetic coil.
The invention relates to an electromagnetic flow meter of electrically
insulating material, comprising a magnet system having on each of
opposite sides of the measuring tube a pole shoe, a magnetic coil
arrangement and a yoke for the magnetic return circuit, and two
measuring electrodes disposed between the respective pole shoes.
In a known flow meter of this kind (EU OS 80 535), a ceramic measuring
tube is provided with a flange at both ends. Adjacent to each pole
shoe there is a magnetic coil of which the axis is radial and perpendicular
to the axis of the measuring electrodes. A steel housing which receives
the measuring tube in a bore under radial prestressing serves as
a magnetic return circuit for the magnet system.
With such a construction, the magnet system projects considerably
radially beyond the flanges of the measuring tube. This results
in comparatively large external dimensions. In addition, the magnetic
coils in many cases obstruct the passage of clamping bolts serving
to clamp the flow meter tight between the flanges of two connecting
tubes. The position and number of such clamping bolts is prescribed
by the appropriate Standard Specification.
The invention is based on the problem of providing an electromagnetic
flow meter of the aforementioned kind which has smaller dimensions
under otherwise the same conditions and is much less hinderance
to the application of clamping bolts.
This problem is solved according to the invention in that the magnetic
coil arrangement is formed by a magnetic coil which is coaxial with
the measuring tube and surrounds the pole shoes and that the pole
shoes are connected to the yoke by radial webs at opposite ends
of the magnetic coil.
In this construction, a single magnetic coil is coaxial with the
measuring tube. An adequate number of windings can therefore be
accommodated over a comparatively small radial extent. The special
connection of the pole shoes to the yoke ensures that the effective
field will pass radially through the measuring tube despite the
tangentially wound magnetic coil. The smaller radial extent also
results in shorter magnetic paths. This leads to lower magnetic
voltage drops and to less temperature dependence of the permeability
of the iron. Since the flow meter has smaller radial dimensions
as a whole (in most cases it is not necessary to exceed the usual
flange diameter of the measuring tube, or to exceed it only slightly),
clamping bolts can be arranged around the flow meter in any desired
position. In particular, the magnet system and the entire flow meter
can be disposed entirely radially within the clamping bolts.
With particular advantage, the yoke has a cylindrical inner periphery
and the radial webs are each formed by a segmented ring consisting
of at least two circumferential sections. Because of the segmenting,
these radial web rings are easily arranged on the measuring tube
and then retained by the cylindrical inner circumference of the
yoke in a manner such that the magnetic circuit is also closed.
The ring form results in a large circumferential surface which minimises
the magnetic resistance in the return path.
At least one of the radial web ring sections should be made in
one piece with a pole shoe. This leads to a corresponding reduction
in the number of components and to simplified assembly.
It is particularly favourable if the radial web rings are segmented
substantially in the radial plane halving the pole shoes. There
are no or only very few magnetic field lines in this plane that
might have to be led away across the separation.
Advantageously, the yoke is formed by a cylindrical sleeve. This
sleeve has a small radial extent and contributes to keeping the
external dimensions to a minimum.
When using a measuring tube with end flanges, it is advisable for
the radial web rings to have an external diameter equal to or larger
than the flange diameter. The upper limit is dictated by the fact
that the yoke does not obstruct the clamping bolts. In this way,
the cylindrical inner circumference of the yoke can be axially pushed
without effort over the measuring tube and the rest of the magnet
system. If both diameters are equal, the yoke or the cylindrical
sleeve may at the same time serve to exert radial prestressing on
the flanges of the measuring tube.
With particular advantage, the radial webs have a cross section
which increases radially outwardly. This gives a large circumferential
surface and correspondingly low losses in the transition of the
magnetic field lines from the radial web to the yoke.
Alternatively, the radial webs have an axial extension at the outer
circumference. This saves magnetic material. In addition, a cavity
for accommodating conductors is formed between the radial web and
flange. In a preferred embodiment, the measuring electrode leads
are guided in the space remaining between the pole shoes beyond
the measuring tube up to one of the radial webs and then outwardly.
In this case, one will require only one outlet aperture in the region
of the radial webs without influencing the magnet system. In particular,
if a free annular space is in any case available lengthwise of the
radial webs, the conductors will be led in a very simple manner.
Further, at least one pole shoe may have a circumferential groove
for inserting a measuring electrode lead. This lead can then be
passed to the opposite electrode and both leads can be installed
It is particularly favourable if the magnetic coil is provided
at the inner periphery with a cylindrical screen of electrically
conductive material. This screen prevents capacitative coupling
between the electrodes and the magnetic coil so that the tapped
signal is not influenced in a capacitative sense. In addition, the
screening can serve to hold the pole shoes in position at least
Preferred examples of the invention will now be described in more
detail with reference to the drawing, wherein:
FIG. 1 is a longitudinal section through a flow meter according
to the invention;
FIG. 2 is a section on the line A--A in FIG. 1;
FIG. 3 is a perspective view of a different embodiment of measuring
tube, pole shoes and radial web rings, partly in section;
FIG. 4 is a part-sectional perspective view of the FIG. 3 embodiment
after all the parts have been assembled, and
FIG. 5 is a view similar to FIG. 3 of a further modification.
According to FIGS. 1 and 2 a measuring tube 1 has an axial flow
passage 2 and a flange 3 or 4 at each end. This measuring tube is
of electrically insulating plastics material or preferably ceramic.
An annular groove 5 is therefore left between the flanges.
Two pole shoes 6 and 7 lying against the measuring tube 1 are provided
at opposite sides in the annular groove 5. The pole shoe 6 is continued
by a radial web ring 8 and the pole shoe 7 by a radial web ring
9. Both the rings 8 and 9 are magnetically conductive material and
are segmented in a radial plane passing through the measuring tube
axis perpendicular to the plane of the drawing. Wound about both
pole shoes 6 and 7 there is a magnetic coil 10 having its axis coincident
with the measuring tube axis. Between the magnetic coil 10 and measuring
tube 1 there is a screen 11 of electrically conductive material
such as aluminium. A cylindrical sleeve serving as a yoke 12 for
the magnetic return circuit has a cylindrical inner periphery equal
to the outer periphery of the flanges 3 and 4 and the outer periphery
of the radial web rings 8 and 9.
Two measuring electrodes 13 and 14 are disposed at the inner wall
of the measuring tube 1 at opposite sides. Their axis is perpendicular
to the plane of symmetry of the pole shoes 6 7.
When assembling this flow meter, the pole shoes 6 and 7 with the
associated radial web rings 8 and 9 are positioned and temporarily
held by the screening 11. The magnetic coil 10 is then wound and
finally the sleeve-like yoke 12 is pushed axially thereover.
In operation, the magnetic coil 10 concentric with the measuring
tube 1 produces a magnetic field that passes radially through the
passage 2 because the pole shoes 6 and 7 are connected at opposite
ends of the magnetic coil 10 to the sleeve-like return yoke by way
of the radial web rings 8 and 9. The flow meter is clamped between
two connecting conduits 17 and 18 with interposed seals 15 and 16.
Clamping bolts 19 arranged in a circle about the measuring tube
axis pass through flanges of the connecting conduits. The sleeve-like
yoke 12 has an external diameter which lies entirely radially within
the clamping bolts.
In the FIGS. 3 and 4 embodiment, corresponding elements are given
reference numerals increased by 20. It will be seen that the radial
web rings 28 and 29 are segmented into two parts 28a, 28b or 29a,
29b along the dividing groove F, the part 28a being made in one
piece with the pole shoe 26 and the part 29a in one piece with the
pole shoe 27. At its radial flange, each radial web ring section
has an axial extension 35 to give a large abutment face against
the cylindrical sleeve serving as the yoke 32. In this case, the
outer diameter of the radial web rings 28 and 29 is larger than
that of the flanges 23 24. Each flange is surrounded by a clamping
ring 36 exerting radial prestressing and applied for example by
being pushed on. The two clamping rings 36 are interconnected by
a protecting sleeve 37. The two measuring electrodes 33 and 34 are
each provided with a lead 38. The lead 38 passes through a circumferential
groove 40 on the inside of the pole shoe 27. Both leads then pass
through a space 41 kept free between the two pole shoes beyond the
measuring tube 21. They also pass through a free space 42 formed
between the radial web ring 28 and the flange 24 and through outlet
apertures 43 in the ring 28 in the sleeve-like yoke 32 and in the
protecting sleeve 37. The leads 44 for the magnetic coil 30 are
passed through the same outlet apertures.
Assembly and construction in this embodiment are similar to FIGS.
1 and 2.
In the FIG. 5 modification, corresponding parts have reference
numerals increased by 50 over those of FIGS. 1 and 2 and by 30 over
those of FIGS. 3 and 4. The main difference is that the radial web
ring 58 is subdivided into two parts 58a and 58b of which the divisions
F pass through the central plane of the pole since 56 so that the
latter also consists of two parts 56a and 56b each connected to
one half of the ring 58. The same applies to the radial web ring
59 having the two halves 59a and 59b with a dividing line F lying
in the same plane.