Abstrict A magnetic-inductive flow meter with a measuring tube, which can
be fitted into a pipeline system by using flange portions at the
ends, with at least two measuring electrodes that are fitted into
the wall of the measuring tube opposite each other in an electrically
isolated manner and are intended for sensing a measuring voltage,
a magnet unit, which is likewise arranged on the outside of the
measuring tube, generating a magnetic field that is aligned substantially
perpendicularly in relation to the direction of flow of the conductive
flow medium to be measured, and the measuring tube being provided
on the inside with means for the electric isolation of the measuring
electrodes from the measuring tube, the means for the electric isolation
taking the form of a thin inner coating of the measuring tube, the
layer thickness of which lies in the range of 0.1-500 .mu.m.
Claims 2. The magnetic-inductive flow meter as claimed in claim 10 wherein
the electrically isolating inner coating extends beyond the inner
region of the measuring tube at least to the region of the flange
portions.
3. The magnetic-inductive flow meter as claimed in claim 10 wherein
the inner coating is applied in an integrally bonding manner by
sputtering or vapor-depositing electrically isolating material on
the measuring tube.
4. The magnetic-inductive flow meter as claimed in claim 10 wherein
the inner coating is applied by spraying electrically isolating
ceramic and/or lacquer materials on the measuring tube.
5. The magnetic-inductive flow meter as claimed in claim 10 wherein
the inner coating is applied by spraying electrically isolating
plastic PTFE or PEEK on the measuring tube.
6. The magnetic-inductive flow meter as claimed in claim 10 wherein
the measuring tube consists of a pressure-resistant plastic.
7. The magnetic-inductive flow meter as claimed in claim 10 wherein
the measuring tube consists of a metal.
8. The magnetic-inductive flow meter as claimed in claim 7 wherein
the metal measuring tube consists of steel, titanium, tantalum,
platinum-iridium or alloys thereof or of a lightweight metal.
10. A magnetic-inductive flow meter comprising: a measuring tube
comprising a wall, an inside and flange portions at each end of
said tube and at least two measuring electrodes that are fitted
into said wall of said measuring tube opposite each other in an
electrically isolated manner, said at least two measuring electrodes
for sensing a measuring voltage, a magnet unit arranged on the outside
of said measuring tube, said magnet unit generating a magnetic field
that is aligned substantially perpendicularly in relation to the
direction of flow of a conductive flow medium to be measured through
said measuring tube; and means provided on said measuring tube inside
for the electric isolation of said measuring electrodes from said
measuring tube, wherein the means for the electric isolation take
the form of a thin inner coating of said measuring tube, the layer
thickness of which lies in the range of 0.1-500 .mu.m.
11. The magnetic-inductive flow meter as claimed in claim 2 wherein
the inner coating is applied in an integrally bonding manner by
sputtering or vapor-depositing electrically isolating material on
the measuring tube.
12. The magnetic-inductive flow meter as claimed in claim 2 wherein
the inner coating is applied by spraying electrically isolating
ceramic and/or lacquer materials on the measuring tube.
13. The magnetic-inductive flow meter as claimed in claim 2 wherein
the inner coating is applied by spraying electrically isolating
plastic PTFE or PEEK on the measuring tube.
14. A method for producing a magnetic-inductive flow meter, said
flow meter comprising: a measuring tube comprising a wall, an inside
and flange portions at each end of said tube and at least two measuring
electrodes that are fitted into said wall of said measuring tube
opposite each other in an electrically isolated manner, said at
least two measuring electrodes for sensing a measuring voltage;
and a magnet unit arranged on the outside of said measuring tube,
said magnet unit generating a magnetic field that is aligned substantially
perpendicularly in relation to the direction of flow of a conductive
flow medium to be measured through said measuring tube; providing
an electrically isolating inner coating on said measuring tube inside
for the electric isolation of said measuring electrodes from said
measuring tube having a layer thickness which lies in the range
of 0.1-500 .mu.m.
15. The method of claim 14 further comprising providing said electrically
isolating inner coating either by spraying, sputtering or vapor-depositing.
Description [0001] The present invention relates to a magnetic-inductive flow
meter with a measuring tube, which can be fitted into a pipeline
system by using flange portions at the ends, with at least two measuring
electrodes that are fitted into the wall of the measuring tube opposite
each other in an electrically isolated manner and are intended for
sensing a measuring voltage, a magnet unit, which is likewise arranged
on the outside of the measuring tube, generating a magnetic field
that is aligned substantially perpendicularly in relation to the
direction of flow of the conductive flow medium to be measured,
and the measuring tube being provided on the inside with means for
the electric isolation of the measuring electrodes from the measuring
tube.
[0002] The invention also relates to a method for producing such
a magnetic-inductive flow meter.
[0003] A magnetic-inductive flow meter is preferably used as a
flow meter for liquids, slurries and pastes which have a specific
minimum electrical conductivity. This type of flow meter is distinguished
by quite accurate measuring results, without any pressure loss being
caused in the pipeline system by the measurement. Furthermore, magnetic-inductive
flow meters do not have any movable components or components protruding
into the measuring tube, which are particularly liable to wear.
The area of use of the flow meter of interest here extends primarily
to applications in the chemical industry, pharmaceuticals and the
cosmetics industry as well as communal water and waste-water management
and the food industry.
[0004] Faraday's law of induction forms the physical basis for
the measuring method of a magnetic-inductive flow meter. This natural
law states that a voltage is induced in a conductor moving in a
magnetic field. When this natural law is exploited in measuring
technology, the electrically conductive medium flows through a measuring
tube in which a magnetic field is generated perpendicularly in relation
to the direction of flow. The voltage induced in the medium is picked
up by an arrangement of electrodes. Since the measuring voltage
obtained in this way is proportional to the average flow rate of
the flowing medium, the volumetric flow of the medium can be determined
from this. Taking the density of the flowing medium into account,
its mass flow can be ascertained.
[0005] EP 0 869 336 A2 discloses a magnetic-inductive flow meter
of the generic type. Its arrangement of electrodes interacts with
two opposite solenoids, which generate the required magnetic field
perpendicularly in relation to the direction of flow in the measuring
tube. Within this magnetic field, each volume element of the flowing
medium moving through the magnetic field, with the field strength
that is present in this volume element, makes a contribution to
the measuring voltage picked up by means of the measuring electrodes.
The measuring voltage is fed to the input side of downstream evaluation
electronics. Within the evaluation electronics, firstly a signal
amplification takes place by means of an electronic differential
amplifier, the differential amplifier operating here with respect
to the reference potential, which usually corresponds to ground
potential. On the basis of the measuring voltage, the evaluation
electronics produce a value for the volumetric flow of the medium
flowing through the measuring tube.
[0006] If it consists of a conductive material, the measuring tube
of such a magnetic-inductive flow meter is usually lined with a
hollow-cylindrical body consisting of a non-conductive material.
The lining primarily serves for the electric isolation of the measuring
electrodes extending through the wall of the measuring tube with
respect to the conductive measuring tube. So-called liners, that
is thin-walled plastic tubes that are drawn into the usually metallic
measuring tube, are usually used in practice as the lining.
[0007] A disadvantage of this technical solution is that the production
step of drawing the plastic tube into the measuring tube is quite
complex. In order for this to work at all, the plastic tube to be
drawn in must have a certain minimum wall thickness of more than
1 mm, in order to ensure dimensionally stable insertion of the plastic
tube into the measuring tube. Furthermore, the plastic tube must
be closely toleranced in its dimensions with respect to the measuring
tube, in order that the plastic tube comes to bear against the inner
wall of the measuring tube as free from play as possible.
[0008] It is therefore the object of the present invention to further
improve a magnetic-inductive flow meter of the type described above
to the extent that the means for electric isolation required inside
the measuring tube can be easily produced.
[0009] The object is achieved on the basis of a magnetic-inductive
flow meter according to the preamble of claim 1 together with its
defining features. In terms of providing a technical method, the
object is achieved by claim 9. The related claims present advantageous
developments of the invention.
[0010] The invention includes the technical teaching that the means
for the electric isolation of the measuring tube take the form of
a thin inner coating of the measuring tube, the layer thickness
of which lies in the range of 0.1-500 .mu.m.
[0011] The advantage of the solution according to the invention
is that an inner coating can be realized more easily in production
engineering terms than the previously customary lining. Tests have
shown that an inner coating with the desired electric isolating
properties can be realized for example by sputtering or vapor-depositing
an electrically isolating material on the measuring tube. Alternatively,
an effectively electrically isolating inner coating can also be
applied by spraying electrically isolating ceramic and/or lacquer
materials onto the inside of the measuring tube. Spraying the inner
coating with an electrically isolating plastic--such as PTFE or
PEEK--is also possible.
[0012] In this way, the solution according to the invention departs
from the previously customary liner technique and takes a new approach.
To do so, it was necessary to overcome the preconceived idea that
such a thin-walled coating would not be capable of ensuring the
desired electric isolating properties in continuous operation of
the magnetic-inductive flow meter.
[0013] A further advantage of the solution according to the invention
is that the small thickness of the inner coating according to the
invention allows a corresponding material saving to be achieved.
Furthermore, the inner coating according to the invention of the
measuring tube serves not only for electric isolation but at the
same time also for corrosion protection of the measuring tube.
[0014] According to a further measure improving the invention,
it is provided that the electrically isolating inner coating extends
beyond the inner region of the measuring tube at least also to the
region of the flange portions. In this way, the inner coating according
to the invention can be extended--in particular for the purpose
of better corrosion protection--also to external functional elements--such
as housing parts and process connections--of the magnetic-inductive
flow meter.
[0015] Within the scope of the present invention, the inner coating
is suitable in particular in connection with a measuring tube which
consists of a metal. Metallic measuring tubes of magnetic-inductive
flow meters preferably consist of steel, titanium, tantalum, platinum-iridium
or alloys thereof or of a lightweight metal. The inner coating according
to the invention serves in this case in particular for electric
isolation. However, it is also conceivable to produce the measuring
tube from a pressure-resistant plastic, the inner coating according
to the invention serving here primarily as corrosion protection
with respect to aggressive media.
[0016] Further measures improving the invention are described in
more detail below together with the description of a preferred exemplary
embodiment of the invention on the basis of the single figure. The
figure shows a schematic longitudinal section through a magnetic-inductive
flow meter.
[0017] According to the FIGURE, the magnetic-inductive flow meter
has a measuring tube 1 which can be fitted into a pipeline--not
represented any further here--by using flange portions 2a and 2b
at the ends and is connected by screws to said pipeline using corresponding
flange portions. The measuring tube 1 is flowed through by a flowable
flow medium 3. To conform to the magnetic-inductive flow measuring
principle, the flow medium 3 has at least slight electrical conductivity.
Also provided, on the outside of the measuring tube 1 is a magnet
unit 4a, 4b, which comprises magnets lying opposite each other and
serves for generating a magnetic field extending perpendicularly
in relation to the axis of the measuring tube. The magnet unit 4a,
4b corresponds with two measuring electrodes 5 arranged lying opposite
each other on the measuring tube (of which only one measuring electrode
can be seen in this sectional representation). The measuring electrodes
5 are aligned perpendicularly in relation to the axis of the magnetic
field and serve for measuring measuring voltage induced as a consequence
of the flow of the flow medium 3. The measuring signal is fed to
a downstream electronics unit 6 which serves as an electrical interface
with further signal-processing devices.
[0018] In the case of this exemplary embodiment, the measuring
tube 1 consists of an electrically conductive metal. In order to
create an electric isolation with respect to the measuring electrodes
5 penetrating through the measuring tube 1 and also with respect
to the flow medium 3 the measuring tube has a thin inner coating
7 the layer thickness of which lies in the range of 0.1-500 .mu.m.
[0019] This inner coating 7 is realized here by vapor-depositing
a suitable electrically isolating material. This results in a solid,
integral bond of the inner coating 7 to the measuring tube 1.
[0020] However, here the electrically isolating inner coating 7
does not just extend along the hollow-cylindrical inner region of
the measuring tube 1. Rather, the inner coating 7 also extends in
the region of the flange portions 2a and 2b on both sides into radially
outwardly extending edge regions 8 in order in particular to ensure
corrosion protection at this location. |