MAGNETIC INDUCTIVE FLOW METER
Description:
Magnetic inductive flow meter
Technical field:
The invention relates to magnetic inductive flow meters according to the preamble of
claim 1.
Background Art:
Magnetic inductive flow meters use a measuring method that is based on Faraday's law
of electromagnetic induction. The first basis for the magnetic inductive measurement of
the flow velocity of fluids was reported in 1832 in a publication by Michael Faraday.
Modern electronic switching technology in conjunction with alternating magnetic fields
made it possible to overcome the separation of the useful signals, proportional to the
flow velocity, from interference signals, which occur in electrochemical processes during
the generation of the magnetic field at the electrodes used for signal decoupling. Thus,
nothing seemed to stand in the way of the wide industrial use of magnetic inductive flow
meters.
The measuring principle of magnetic inductive flow meters utilizes the separation of
moving charges in a magnetic field. The conductive fluid to be measured flows through
a tube which is made of nonmagnetic material and whose interior is electrically
insulated. A magnetic field is applied from the outside by means of coils. The charge
carriers present in the conductive fluid, such as ions and other charged particles, are
deflected by the magnetic field: the positive charge carriers to one side and the negative
charge carriers to another side. A voltage, which is detected with a measuring device,
arises owing to the charge separation at measuring electrodes arranged perpendicular
to the magnetic field. The value of the measured voltage is proportional to the flow

velocity of the charge carriers and thereby proportional to the flow velocity of the
measuring fluid. The flow volume can be determined over time by integration.
In magnetic fields generated by pure alternating voltage, induction of interference
voltages occurs in the electrodes, which must be suppressed by suitable and costly
filters. For this reason, the magnetic field is usually generated by a clocked direct
current of alternating polarity. This assures a stable zero point and makes the
measurement insensitive to effects by multiphase substances and inhomogeneities in
the fluid. In this way, a usable measuring signal can also be achieved at a low
conductivity.
If a measuring fluid moves through the measuring tube, according to the induction law a
voltage is present at both measuring electrodes, which are arranged in the measuring
tube perpendicular to the flow direction and perpendicular to the magnetic field. This
voltage in the case of a symmetric flow profile and a homogeneous magnetic field is
directly proportional to the average flow velocity. The inductive flow measuring method
is capable of generating an electrically usable signal for further processing directly from
the flow. The following equation basically applies:
U = k*B*D*v
where U = voltage, k = proportionality factor, B = magnetic field strength, D = tube
diameter, and v = flow velocity.
The selection of the proper electrode material is critical for the reliable function and
measuring accuracy of the magnetic inductive flow meter. The measuring electrodes
are in direct contact with the medium and must therefore be sufficiently corrosion-
resistant and ensure good electrical transfer to the measuring fluid. The following are
used as electrode materials: stainless steel, CrNi alloys, platinum, tantalum, titanium,
and zirconium. Sintered electrodes are also used in the case of measuring sensors with
ceramic measuring tubes.

EP 1616152 B1, whose entire disclosure is integrated herein by reference, discloses
improved electrodes. These electrodes consist of a metal and a salt of said metal, which
is arranged so that it is located between the metal and the fluid, whereby the salt layer
is either applied electrochemically or sintered on. Silver as the salt silver chloride or
silver fluoride is preferred as the metal. A porous protective element, for example, a
glass frit, can be mounted in front of the silver electrode as protection against dirt.
A possible realization of a magnetic inductive flow meter is disclosed in US 6,626,048
B1, whose entire disclosure is integrated herein by reference. Nevertheless, this
publication presents only the physical and electronic fundamentals but no practical
realization.
It is understood that major problems must be solved in the practical realization of a
magnetic inductive flow meter.
In one respect, this is a matter of the material. The measuring tube must be amagnetic
in order not to interfere with the magnetic field. The measuring tube further must be
electrically insulating in order not to interfere with the picking up of the voltage with use
of the electrodes. Moreover, the tube must consist of a food-safe material, when the
liquid is a food, for example, drinking water.
These requirements can be fulfilled best when a food-safe plastic is used as the
material. Nevertheless, plastics have the disadvantage of a much lower strength
compared with metal. Resistance to internal pressure, however, is an essential
requirement. The attempt to achieve internal pressure resistance with an increased
thickness of the tube wall is not practicable, because otherwise the magnetic field would
be weakened too greatly.
Another problem with plastics is water diffusion. This causes swelling of the plastic, as a
result of which the dimensions particularly of the measuring channel change, which
leads to a deterioration in the measuring accuracy. Water diffusion also greatly reduces

the strength of the plastic. In fiber-reinforced plastics, the adhesion between the plastic
and fiber is also partially lost.
During measurement of warm and hot fluids, the plastic softens and the strength also
declines.
Chemicals, e.g., chlorine, in the measuring fluid can also attack the plastic. This also
applies to UV radiation.
Furthermore, the meter housing must be tension-resistant, because considerable tensile
stress can occur when a meter is screwed into existing tubing, e.g., in the screw thread.
Tensile stress, particularly long-term tensile stress, is damaging to plastics, however, in
particular the thinner the plastic material.
During installation on-site, other forces can act on the plastic, which lead to damage,
when no provisions have been made by the design engineers and manufacturers.
Disclosure of the invention:
The present invention has as its object to provide a magnetic inductive flow meter,
which overcomes the aforementioned problems and whose plastic housing is resistant
both to the internal pressure originating from the measuring fluid and to tensile stress,
as well as to other thermal and mechanical loads.
This object is attained by magnetic inductive flow meters with the features of claim 1.
The described contrary conditions can be met optimally owing to the features of the
invention. In the area of magnetic field lines, the wall of the measuring channel is
optimally thin, so that a homogeneous magnetic field of optimal strength is achieved.
The internal pressure of the measuring fluid is absorbed by the inner reinforcement
cage, consisting of two inner transverse partitions and at least two inner longitudinal
ribs.

The inner reinforcement cage is supported in addition by an outer reinforcement cage,
consisting of at least two first outer longitudinal ribs. The main task of the outer
reinforcement cage, however, is to protect the measuring section of the housing and
particularly the area with the minimum wall thickness from tensile stress originating from
the connecting pieces.
A further advantage of this form is that the housing can be produced by an injection
molding process.
Advantageously, the measuring channel has a rectangular cross section. A
homogeneous magnetic field can be realized optimally in this way.
According to an embodiment of the invention, the inner longitudinal ribs have recesses
in the area of the reduced channel wall for mounting the magnetic poles.
In addition, the inner longitudinal ribs may have in areas additional recesses for
mounting additional structural components or for anchoring materials for the protection
of the measuring electronics.
To achieve a high pressure resistance, the inner transverse partitions are preferably
positioned directly before and behind the reduced channel wall.
According to a refinement of the invention and to reinforce the housing, the outer
reinforcement cage comprises in addition at least two second outer longitudinal ribs
oriented perpendicular to the first outer longitudinal ribs.
For further bracing in the transverse direction, at least two outer transverse partitions
can also be provided.

For the optimal transfer of tensile forces from the outer reinforcement cage to the
connecting pieces, wedge-shaped reinforcement ribs are good options, which pass the
magnetic flux to the inlet connecting pieces and outlet connecting pieces.
An optimal construction is then present when the relative linear expansion of the
measuring section reinforced by the inner and outer reinforcement cage, said expansion
which is caused by tension at the inlet connecting pieces and outlet connecting pieces,
is not greater than the relative linear expansion of the connecting pieces themselves. It
is prevented in this way that individual parts of the housing can be overexpanded.
According to a refinement of the invention, the measuring section, including the
magnetic poles and electrodes, is enclosed with an insulating layer. This can occur, for
example, by molding, whereby the material also penetrates into the aforementioned
recesses.
Advantageously, an electrical and/or magnetic shielding surrounds the entire measuring
section. In this case, the shielding can be mechanically connected to the connecting
pieces or the outer transverse partitions. In this way, the shielding can supplement the
function of the outer reinforcement cage.
Advantageously, the housing consists of a suitable reinforced plastic, particularly fiber-
reinforced thermoplastic.
According to a variant of the invention, the housing consists of two separately produced
individual parts, the actual pressure-resistant housing and an external measuring
module. The housing has a recess for the separate module, preferably insertable by
raising/lowering. The module comprises at least the inner transverse partitions, the
measuring channel, the electrodes, and the magnetic poles.
According to another variant of the invention, the housing consists of three separately
produced individual parts, connected to one another in a tension-resistant manner. The

inlet and outlet connecting pieces are formed identical. The production of the plastic
parts can be streamlined in this way. The ends of the measuring unit are sealed fluid-
tight by seals in recesses in the connecting pieces.
The magnetic poles abut the channel wall not only externally but can also be integrated
into the channel wall.
Brief description of the drawings:
The invention will be described in the form of an exemplary embodiment with use of the
drawing. In the drawing, in each case not true to scale,
FIG. 1 shows a plan view of a pressure-resistant, one-part plastic housing, cut away on
half of a side, for a magnetic inductive flow meter;
FIG. 2 shows a cross section through the housing of FIG. 1 along the line ll-ll;
FIG. 3 shows a two-part housing as an exploded view; and
FIG. 4 shows a partial section of a three-part housing, also as an exploded view.
Modes for carrying out the invention and industrial applicability:
FIG. 1 shows purely schematically and not true to scale a plan view of a pressure-
resistant, one-part plastic housing, cut longitudinally on half of a side, for a magnetic
inductive flow meter. Three functional units are evident: an inlet connecting piece 10, an
outlet connecting piece 20, and a measuring section 30 between these, which has
measuring channel 31, through which measuring fluid flows, with a channel wall 32, two
opposing magnetic poles 2 outside on channel wall 32, and two opposing measuring
electrodes 1, oriented perpendicular to magnetic poles 2, in channel wall 32. The
thickness of channel wall 32 is reduced in the area of magnetic poles 2 to the extent
permissible with consideration of the maximum inner pressure of the measuring fluid in

measuring channel 31, so that a homogeneous magnetic field generated by magnetic
poles 2 is sufficiently strong in the area of measuring channel 31.
To absorb the internal fluid pressure, an inner reinforcement cage is provided,
consisting of at least two inner transverse partitions 37 and at least two inner
longitudinal ribs 38. Inner transverse partitions 37 are located directly before and behind
channel wall 32 of minimal thickness to absorb the deformations of channel wall 32. The
inner longitudinal ribs 38, which brace the long channel walls 32, have the same task
but must be cut out at least in the area of magnetic poles 2.
Inner longitudinal ribs 38 may have in areas additional recesses 39 for mounting
possible additional structural parts or for anchoring insulating sealing compound.
In addition to the inner reinforcement cage, an outer reinforcement cage is provided
consisting of two first outer longitudinal ribs 40, two second outer longitudinal ribs 41,
oriented perpendicular thereto, and two outer transverse partitions 42. The outer
reinforcement cage and in particular first outer longitudinal ribs 40 support the inner
reinforcement cage in addition against internal fluid pressure.
The main task of the outer reinforcement cage, however, is to absorb tensile stress,
which arises during the exertion of tensile forces on the inlet and outlet connecting
pieces 10, 20. Without the inner and outer reinforcement cage, this tensile stress would
damage the housing in the area of the reduced channel wall 32. This is prevented by
the outer reinforcement cage.
An optimal transfer of tensile stress from the outer cage and in particular its transverse
partitions 42 to the inlet and outlet connecting pieces 10, 20 is achieved by wedge-
shaped reinforcement ribs 14, 24.
FIG. 2 shows a cross section along the line ll-ll through the housing of FIG. 1. The
rectangular measuring channel 31 can be seen, bounded on the right and left by the

reduced channel wall 32, whose outer side is abutted by magnetic poles 2. A transverse
partition 37 of the inner reinforcement cage is visible; of the outer reinforcement cage,
the first and second outer longitudinal ribs 40, 41 are evident in section, as well as an
outer transverse partition 42.
Finally, measuring electrodes 1, which are oriented perpendicular to magnetic poles 2,
are evident in FIG. 2. Measuring electrodes 1 are held and protected by a housing collar
33.
The one-part design shown in FIGS. 1 and 2 is not the only one possible.
FIG. 3 shows a two-part design. Inlet connecting piece 10, outlet connecting piece 20,
outer reinforcement cage 40, 41, 42, and the first inner longitudinal ribs 38 of the inner
reinforcement cage form a unit. A recess 50' is located in the center of the housing. The
measuring chamber, formed as an independent module 50, can be placed by raising/
lowering and sealed in this recess with electrodes 1, magnetic poles 2, and inner
transverse partitions 37.
FIG. 4 shows a three-part design in a partially cut view. Inlet connecting piece 10 and
outlet connecting piece 20 are formed identical. Their reinforced flanges 15, 25 each
have a recess 16, 26, in which measuring section 30 is inserted with the aid of seals 3.
Bored holes 17, 27 make it possible to use lag screws for tension-resistant connection
of the three housing parts 10, 20, 30.
In the example of FIG. 4, magnetic poles 2 are molded in the wall of the measuring
channel. As a result, it is possible to bring magnetic poles 2 extremely close to the
measuring channel. The inner and outer longitudinal ribs are formed on measuring unit
30; reinforced flanges 15, 25 also take over the function of the outer transverse
partitions.

Claims^
r
1. A magnetic inductive flow meter having a pressure-resistant plastic housing,
comprising
an inlet connecting piece (10),
an outlet connecting piece (20),
and a measuring unit (30) between these having
- a measuring channel (31), through which the measuring fluid flows, with a
channel wall (32),
- two opposing magnetic poles (2) at the measuring channel (31),
- and two opposing measuring electrodes (1), oriented perpendicular to the
magnetic poles (2), in the channel wall (32),
characterized by the features:
with consideration of the maximum permissible internal pressure for the selected
plastic, the channel wall (32) is reduced to a still permissible extent in the area of
the magnetic poles (2),
an inner reinforcement cage, consisting of at least two inner transverse partitions
(37) and at least two inner longitudinal ribs (38), stabilizes the channel wall (32),
an outer reinforcement cage, consisting of at least two first outer longitudinal ribs
(40), holds and stabilizes the inner reinforcement cage and connects the
measuring unit (30) in a tension-resistant manner to the inlet connecting piece
(10) and outlet connecting piece (20).
2. The flow meter according to claim 1, characterized by the feature:
the measuring channel (31) has a rectangular cross section.
3. The flow meter according to claim 1 or 2, characterized by the feature:
the inner longitudinal ribs (38) have recesses in the area of the reduced channel
wall (32) for mounting the magnetic poles (2).
4. The flow meter according to claim 1, 2, or 3, characterized by the feature:

the inner longitudinal ribs (38) have in areas additional recesses (39) for
mounting additional structural parts and/or for anchoring insulating sealing
compound.
5. The flow meter according to any one of claims 1 through 4, characterized by the
feature:
the inner transverse partitions (37) are positioned directly before and behind the
reduced channel wall (32).
6. The flow meter according to any one of claims 1 through 5, characterized by the
feature:
the outer reinforcement cage also comprises at least two outer transverse
partitions (42).
7. The flow meter according to any one of claims 1 through 6, characterized by the
feature:
the outer reinforcement cage also comprises at least two second outer
longitudinal ribs (41) oriented perpendicular to the first outer longitudinal ribs
(40).
8. The flow meter according to any one of claims 1 through 7, characterized by the
feature:
wedge-shaped reinforcement ribs (14, 24) pass the magnetic flux of the first
and/or second outer longitudinal ribs (40, 41) to the inlet connecting piece (10)
and outlet connecting piece (20).
9. The flow meter according to any one of claims 1 through 8, characterized by the
feature:
the relative linear expansion of the measuring unit (30) reinforced by the inner
and outer reinforcement cage, said expansion which is caused by tension at the

inlet connecting piece (10) and outlet connecting piece (20), is not greater than
the relative linear expansion of the connecting pieces (10, 20) themselves.
10. The flow meter according to any one of claims 1 through 9, characterized by the
feature:
the measuring unit (30) is enclosed on the outside with a covering.
11. The flow meter according to any one of claims 1 through 10, characterized by the
feature:
an electrical and/or magnetic shielding surrounds the measuring unit (30),
the shielding is mechanically connected to the connecting pieces (10, 20) or the
outer transverse partitions (42).
12. The flow meter according to any one of claims 1 through 11, characterized by the
feature:
the magnetic poles (2) are integrated into the channel wall (32).
13. The flow meter according to any one of claims 1 through 12, characterized by the
feature:
the housing consists of reinforced plastic, preferably fiber-reinforced
thermoplastic.
14. The flow meter according to any one of claims 1 through 13, characterized by the
feature:
the housing consists of two separately produced individual parts (10, 20, 30; 50),
the measuring unit (30) has a recess (50') for a separate module (50) preferably
insertable by raising/lowering,
the module (50) comprises at least the inner transverse partitions (37), the
measuring channel, the electrodes (1), and the magnetic poles (2).

15. The flow meter according to any one of claims 1 through 13, characterized by the
feature:
the housing consists of three separately produced individual parts (10, 20, 30)
connected to one another in a tension-resistant manner,
the ends of the measuring unit (30) are sealed fluid-tight by seals (3) in recesses
(16, 26) in the connecting pieces (10, 20).

The invention relates to magnetic-inductive flow meters
comprising a pressure-resistant plastic housing that includes an
inlet port (10), an outlet port (20), and an intermediate
measuring unit (30) . The measuring unit (30) comprises a
measurement duct (31), through which the fluid to be measured
flows and which has a duct wall (32). The measuring unit (30)
further comprises two opposite measuring electrodes (1) which
are arranged in the duct wall (32) and are oriented
perpendicular to the magnet poles (2). Taking into account the
maximum admissible pressure for the selected plastic, the duct
wall (32) is reduced to a minimum admissible thickness in the
region of the magnet poles (2) . An internal reinforcement cage
that includes at least two internal transverse bulkheads (37)
and at least two internal longitudinal ribs (38) stabilizes the
duct wall (32) . An external reinforcement cage that includes at
least two first external longitudinal ribs (40) retains and
stabilizes the internal reinforcement cage and connects the
measuring unit (30) in a tension-proof manner to the inlet port
(10) and the outlet port (20).