The invention relates to an electromagnetic flow meter with a magnetic
system having two magnetic poles which are attachable to a measuring
tube from the outside, an outer yoke, and at least one separating
gap. Each pole has a pole shoe and a core section carrying a winding
which is connected at the radially outward end thereof without a
gap to the adjoining magnetic system portion and the separating
gap is disposed beyond the magnetic path for the stray flux.
What is claimed is:
1. An electromagnetic flow meter, comprising a measuring tube of
an electrically insulating material having a cylindrically shaped
section, a magnetic system having two magnetic poles with pole shoes
thereto attachable externally to diametrically opposite sides of
said measuring tube, said poles including core sections carrying
windings connected to said shoes, outer yoke means surrounding said
poles, said poles including radially outward plate means connected
to said core sections which extend circumferentially beyond said
windings to receive stay flux from said pole shoes, and said yoke
means having separating gap means associated therewith disposed
beyond the magnetic path of said stray flux emanating from either
of said pole shoes, said gap means being disposed between said plate
means and said poles means.
2. A flow meter according to claim 1 characterized in that said
separating gap means extend over a larger area than does the cross-section
of either of said core sections.
3. A flow meter according to claim 1 characterized in that said
core sections are connected without a gap to said yoke means and
said yoke means is segmented to form said separating gap means.
4. A flow meter according to claim 1 characterized in that said
core sections are made in one piece with said plate means.
5. A flow meter according to claim 1 characterized in that said
core sections are made in one piece with said yoke means.
6. A flow meter according to claim 1 characterized in that said
yoke means comprise laminae perpendicular to the axis of said measuring
7. A flow meter according to claim 1 wherein an adjoining magnetic
system portion surround said core sections is magnetically conductively
interconnected with said core sections.
8. A flow meter according to claim 1 characterized in that said
windings are disk windings.
The invention relates to an electromagnetic flow meter with a magnetic
system consisting of two magnetic poles which are attachable to
a measuring tube from the outside and each comprise a pole shoe
and a core section carrying a winding, an outer yoke, and at least
one separating gap.
In a known flow meter of this kind (EU-OS 80 535), the pole shoes
of two magnetic poles lie against a ceramic measuring tube and are
diametrally opposed. A steel housing serves as the yoke for the
magnetic return flux. This is a separating gap between the radially
outer end of the core section of each magnetic pole and the associated
yoke. By means of the separation produced along the gap, it is possible
to mount the magnetic system to function properly even though the
end of the measuring tube has connecting flanges of larger diameter.
In this construction, the magnetic system considerably projects
radially beyond the flanges of the measuring tube. This results
in comparatively larger external dimensions. In addition, the magnetic
system in many cases obstructs the passage of throughbolts which
serve to clamp the flow meter between the flanges of two connecting
tubes. The disposition and number of such clamping bolts is prescribed
by the appropriate Standard.
The invention is based on the problem of reducing the magnetic
system in an electromagnetic flow meter of the aforementioned kind
so that under otherwise equal conditions there will be smaller overall
dimensions and it will in particular be possible to accommodate
it within the space defined by the clamping bolts.
According to the invention, this problem is solved in that the
core section is connected at the radially outer end without a gap
to the adjoining magnetic system portion and the separating gap
is disposed beyond the magnetic path for the stray flux.
In an electromagnetic flow meter, the total magnetic flux is divided
into a useful flux flowing from one pole shoe to the other and passing
through the measuring tube and a stray flux flowing from the pole
shoe directly to the portion of the magnetic system adjoining the
core section. Since the stray flux has to travel through nonmagnetic
material along a much shorter path, it is generally larger than
the useful flux, for example 3 times as large. Hitherto, the total
flux had to be led across the separating gap and this caused very
high induction in the gap. In contrast, if one follows the invention
by passing only the useful flux across the gap instead of the stray
flux, the induction is lower. Consequently, a much smaller magnetic
potential difference will suffice for producing an adequate useful
flux. The number of ampere turns is less. The dimensions of the
winding can be kept smaller.
In a further development of the invention, the separating gap should
extend over a larger area than does the cross-section of the core
section. A considerably larger area should be aimed at, for example
from four to twenty times larger. In this way, the conduction in
the separating gap is again considerably reduced. Further, the magnetic
potential difference and thus the number of ampere turns can again
If one employs both features simultaneously, the number of ampere
turns can be reduced by 20 to 30%. This corresponds to a reduction
in coil volume to about one half. The reason is that on the one
hand the longer outer convolutions can be dispensed with and on
the other hand the wire cross-section can be smaller because of
the shorter wire length while maintaining the resistance of the
That part of the magnetic potential difference required for bringing
the useful flux across the gap can be readily reduced to 1% or less,
preferably to even less than 0.3% of the entire magnetic flux. In
this case, the separating gap can even be about 0.2 mm wide. This
value can be maintained even in the case of mass production. If,
by reason of unavoidable tolerances, the separating gap has different
dimensions, the resulting errors are so small that they do not affect
the measuring result.
In a preferred embodyment, the core section is connected without
a gap to an intermediate plate projecting beyond the winding to
receive the stray flux and defining the gap between itself and the
yoke. In this case, the stray flux travels from the rim of the pole
shoe direct to the intermediate plate and need therefore not traverse
any separating gap.
In an alternative construction, the core section is connected to
the yoke without a gap and the yoke is segmented to form the separating
gap. In this case, the stray flux travels from the rim of the pole
shoe directly into the yoke. Again, it need not traverse a separating
To avoid gaps between the core section and the adjoining portion
of the magnetic system, there are various possibilities. Thus, both
parts may be made in one piece. One may also use laminae extending
perpendicular to the measuring tube axis and covering the respective
core section and the adjoining portion of the magnetic system. It
is also possible for the core sections and adjoining portion of
the magnetic system to be interconnected in a magnetically conductive
manner. This occurs, for example, by joining them with magnetically
conductive solder. Ferrous parts could also be welded together,
whether by electron beam welding or pressure welding.
With particular advantage, the winding is a disc winding. Since
the number of turns can be considerably reduced while maintaining
a relatively low effective resistance, it is possible to make the
winding very flat and thereby keep the diameter of the appliance
extraordinarily small. In addition, the diameter of the core section
can be reduced.
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 an electro magnetic flow
meter constructed in accordance with the invention;
FIG. 2 is a cross-section along the line A--A in FIG. 1;
FIG. 3 is a cross-section corresponding to FIG. 2 through the magnetic
system of a modified embodiment;
FIG. 4 is a cross-section through a further embodiment; and
FIG. 5 is a side elevation of a modified form of magnetic pole.
According to FIGS. 1 and 2 a measuring tube 1 is provided having
an axial flow passage 2 and a flange 3 4 at each end. This measuring
tube is of electrically insulating plastics material or preferably
Magnetic poles 6 and 7 are provided on opposite sides. These magnetic
poles each possess a pole shoe 8 or 9 lying against the outside
of the measuring tube 1 and a core section 10 or 11 surrounded by
a disc winding 12 or 13. Connected in one piece therewith there
is an intermediate plate 14 or 15 which projects axially as well
as circumferentially beyond the winding 12 or 13 and the pole shoe
8 or 9. A cylindrical yoke 16 made of magetically conductive material
in the same way as the elements 8 9 10 11 14 15 surrounds the
flanges 3 and 4 as well as the magnetic poles 6 and 7. Consequently,
one obtains a first separating gap 17 between the yoke 16 and intermediate
plate 14 and a second separating gap 18 between the yoke 16 and
intermediate plate 15.
Two measuring electrodes 19 and 20 are disposed at the inner wall
of the measuring tube 1 on opposite sides. Their axis is perpendicular
to the plane of symmetry of the magnetic poles 6 and 7. In operation,
a current passing through the windings 12 and 13 produces a magnetic
field which passes radially through the passage 2. A voltage signal
depending on the flow velocity can then be tapped between the measuring
electrodes 19 and 20.
The flow meter is clamped between two connecting conduits 23 and
24 with the interpositioning of seals 21 and 22. Clamping bolts
25 disposed in a circle about the axis of the measuring tube pass
through flanges of the connecting conduits. The sleeve-like yoke
16 has an external diameter which is entirely disposed radially
within the clamping bolts.
FIG. 3 differs from the embodiment of FIGS. 1 and 2 in that the
yoke 26 is formed by a clamping strap which can be tightened by
means of a clamping screw 27. The clamping strap is axially shorter
than the yoke 16 and only acts on the two magnetic poles 6 and 7
not on the flanges 3 and 4. Consequently, the separating gaps 17
and 18 between the yoke and the intermediate plates 14 and 15 of
the magnetic poles 6 and 7 can be reduced.
The magnetic flux has been entered in FIG. 3. There is the useful
magnetic flux .phi..sub.n, which passes through the passage 2 and
is closed by the yoke 26 and the two separating gaps 17 and 18.
The stray flux .phi..sub.s1 goes direct from the pole shoe 8 to
the intermediate plate 15 i.e. it does not pass through the separating
gap 17. The stray flux .phi..sub.s2 goes direct from the intermediate
plate 15 to the pole shoe 9 and therefore likewise does not pass
through the gap 18. The induction in the two separating gaps 17
and 18 is reduced by a multiple as compared with the known cases
because the much larger stray flux does not pass through the separating
gap and because the area bounding the separating gap is considerably
larger than the cross-sectional area of the core section 10 or 11.
Consequently, a very small part of the magnetic potential difference
will suffice to overcome the magnetic gaps. The reduced number of
ampere turns leads to very small cross-sectional dimensions for
the coils 12 and 13 and thus to a very small outer circumference
for the yoke.
In the FIG. 4 embodiment, the magnetic system is composed of laminae
35. The magnetic poles 36 or 37 again consist of poles shoes 38
or 39 and core sections 40 or 41 respectively. The latter are made
in one piece with yoke segments 42 or 43 together defining the yoke
44. The separating gaps 47 and 48 are disposed at circumferential
portions offset by about 90% from the core sections 40 41. To increase
their bounding surfaces, the separating gaps 47 and 48 lie obliquely
to the radial direction and in a zone of increased radial width.
FIG. 5 shows that a magnetic pole 56 can also be built up from
individual parts, namely a pole shoe 58 a core section 60 and an
intermediate plate 64 if one ensures that a magnetically conductive
seam 65 or 66 is provided at the junctions. This seam can be produced
with the aid of a magnetically conductive solder or by welding.
The yoke need not be cylindrical but could, for example, be polygonal,
e.g. square, if space permits.