In an electromagnetic flow meter having a ceramic measuring tube,
a housing arrangement is provided which forms a fluid tight capsule
surrounding the measuring tube to prevent the escape of fluid if
the tube should happen to crack or break in the use thereof.
What is claimed is:
1. An electromagnetic flow meter, comprising, a tubularly shaped
ceramic measuring tube, tubularly shaped housing means surrounding
said measuring tube in spaced relation thereto to provide a hollow
space therebetween, a pole shoe and coil arrangement of a magnetic
system mounted in said hollow space in operable relation to said
measuring tube, said measuring tube having two radially outwardly
projecting flanges at opposite ends thereof, said housing means
having radially inwardly projecting flanges with surface in radial
aubtting and sealing engagement with surfaces of said tube flanges,
and annularly shaped sealing disks provided at both ends of said
tube and housing means in axially abutting and sealing engagement
with said tube and said housing means to form a pressure tight capsule,
there being circumferential seals provided between said abutting
tube and housing flange surfaces.
2. A flow meter according to claim 1 characterized in that the
ends of said measuring tube extend axially somewhat beyond the corresponding
ends of said housing means.
The invention relates to an electromagnetic flow meter with a pressure
resistant ceramic measuring tube and a housing surrounding same
to leave a hollow space between them in which there are the pole
shoes and coil arrangement of a magnet system, sealing faces for
the abutment of sealing ring discs being provided at both ends.
In a known flow meter of this kind (EU-OS 80 535), the measuring
tube is of an oxide ceramic so that it will also be suitable for
aggresive and hot media. The measuring tube has such a high mechanical
strength that it is not necessary to provide a metal covering which
extends over the entire length and increases the pressure and bending
resistance (as in the case of the construction of U.S. Pat. No.
3750468). Consequently, the pole shoes and coil arrangement can
be arranged directly beyond the ceramic measuring tube and the metal
housing can be used as the magnetic return path. To form an adequately
large sealing surface, the metal tube has a flange at both ends
to resemble the shape of a yarn bobbin. The peripheral faces of
these flanges are held in a complementary bore of the housing and
can be secured therein by means of shrinkage stresses. Ceramic materials
are highly sensitive to impact and percussion loads, pressure peaks
in the flowing medium, rapid temperature fluctuations etc. If the
ceramic measuring tube breaks, the flow medium can escape over the
entire housing. This can cause extensive damage, especially in the
case of aggressive liquids.
The invention is based on the problem of providing an electromagnetic
flow meter of the aforementioned kind that is pressure tight despite
using a ceramic measuring tube.
This problem is solved according to the invention in that the housing
forms a pressure tight capsule of at least the same pressure resistance
as the measuring tube and its end faces form the outer part of the
In this construction, no measures are taken to strengthen the ceramic
measuring tube. Instead, the measuring tube is enclosed in a pressure
tight chamber. For this purpose, it is necessary on the one hand
for the housing to have an adequate pressure resistance and on the
other hand not only the measuring tube but also the housing should
be sealed at the ends from connecting conduits. If the measuring
tube should now break as a result of exceptional stresses, the flow
medium can simply flow into the hollow space between the measuring
tube and the housing but not reach the outside. Consequently even
aggressive media cannot cause damage. Also, the housing need generally
have only a limited resistance to the flow medium. This is because
a break in the measuring tube is found relatively easily, for example
through a change in the measuring signal, so that replacement of
the flow meter is readily possible within the service life of the
Preferably, the housing has two flanges projecting inwardly up
to the zone of the sealing face. This not only gives the desired
seal between the housing and connecting conduits but also leads
to the measuring tube having no, or no marked, outer flanges at
its ends. This considerably improves the production and strength
of the ceramic body.
Preferably, the housing is formed from a tube and two flanges.
These three parts can be readily sealingly interconnected and provide
a very strong housing. This housing has a comparatively small external
diameter so that it can be readily accommodated within the clamping
screws that interconnect the connecting conduits and the spacing
of which from the central axis is standardised. In a preferred embodiment,
the outer part of the sealing face and the inner part of the sealing
face formed by the end face of the measuring tube have substantially
the same radial width. As a result, the end of the housing provides
a seal of the same quality as the end of the measuring tube.
It is advisable for the inner part of the sealing face formed by
the end face of the measuring tube to project somewhat axially beyond
the outer part of the sealing face. This results in the main seal
during normal operation being provided between the end of the measuring
tube and the connecting conduits.
Desirably, a circumferential seal is provided between the housing
and measuring tube at each end. It has been found that ceramic measuring
tubes break intermediate their ends so that the flow medium first
penetrates the hollow space of the housing. The circumferential
seal is in series with the outer sealing portion at the end so that
the hollow space is particularly tightly sealed.
Further, the electric terminals for the coil arrangement and for
the electrodes should be led out of the housing through pressure
Another way of increasing the pressure tightness and possibly dispensing
with pressure tight fittings is for the free zone of the hollow
space to be filled with a sealing mass.
A preferred example of the invention will now be described in more
detail with reference to the drawing in which the single FIGURE
shows a longitudinal section through an electromagnetic flow meter
according to the invention.
An electromagnetic flow meter 1 is clamped between the flanges
5 and 6 of two connecting conduits 7 and 8 with the interpositioning
of two sealing ring discs 3 and 4. Clamping bolts 9 with clamping
nuts 10 and 11 are used for this purpose.
The flow meter comprises a ceramic measuring tube 12 with two weakly
formed flanges 13 and 14 of which the end faces constitute an inner
portion 15 and 16 of the sealing faces formed by the sealing ring
discs 3 and 4.
A housing 17 is formed by a tube 18 and two inwardly projecting
flanges 19 and 20. The ends of the housing define the outer part
21 and 22 of the sealing faces formed by the sealing ring discs
3 and 4. The tube 18 has such a wall thickness and is so connected
to the flanges 19 and 20 that the housing 17 has at least the same
pressure resistance as the ceramic measuring tube 12. The connection
between the tube 18 and flanges 19 and 20 is effected by welding,
adhesion or a pressure fit, possibly by using additional sealing
rings to result in a pressure tight joint. Between the peripheral
faces of the inwardly directed flanges 19 and 20 as well as the
outer flanges 13 and 14 of the measuring tube 12 there is a circumferential
seal 23 and 24 in the form of two O-rings. Alternatively, there
can be some other seal provided for example by putty, adhesive or
a pressure fit. A hollow space 25 remaining between the measuring
tube 12 and housing 17 contains pole shoes 26 and 27 which abut
the measuring tube 12 from opposite sides, as well as associated
coils 28 and 29. The tube 18 of the housing forms the yoke, i.e.
the magnetic return path of the magnet system. The remaining free
space of the hollow space 25 is filled with a sealing mass 30. Two
measuring electrodes, of which only the electrode 31 is illustrated,
are disposed in an axis which is perpendicular to the plane of symmetry
of the pole shoes 26 and 27. The leads to the coils 28 and 29 and
to the measuring electrodes extend through pressure tight fittings
of which the fittings 32 and 33 disposed circumferentially between
the pole shoes 26 and 27 are illustrated in a position that is offset
from their actual position.
When clamping the flow meter 1 between the flanges 5 and 6 a seal
is produced not only in the region of the inner part 15 and 16 of
the sealing face but also in the zone of the outer part 21 and 22
of the sealing face, even though the inner sealing face forms the
main seal because the end face of the measuring tube 12 projects
somewhat, for example 1 mm, axially beyond the end face of the housing
17. Should the ceramic measuring tube 12 break for some reason,
the flow medium can penetrate into the hollow space 25. However,
the housing 17 forms a tight capsule which is sealed from the outside
at both ends by the series disposition of the circumferential seals
23 24 and the outer part 20 21 of the sealing face. For this reason,
the flow medium in the hollow space 25 cannot reach the outside.
Even in the case of aggressive media, this protection suffices to
enable replacement of the flow meter because the housing, which
maybe of steel, has an adequately long service life.
As soon as the medium penetrates the hollow space 25 the magnet
system is influenced. The measuring signal will change characteristically
so that the error can be recognised rapidly.