Measuring and testing – Volume or rate of flow – By measuring electrical or magnetic properties
Reexamination Certificate
1999-07-22
2001-01-30
Fuller, Benjamin R. (Department: 2855)
Measuring and testing
Volume or rate of flow
By measuring electrical or magnetic properties
C073S866500
Reexamination Certificate
active
06178826
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to electrode assemblies for electromagnetic flow sensors.
BACKGROUND OF THE INVENTION
Electromagnetic flowmeters, as is well known, measure the volumetric flow rate of electrically conductive liquids. The measurement is based on Faraday+s law of induction. The liquid is passed through a magnetic field, so that a voltage is induced in it, which is taken off by means of at least two electrode assemblies.
The means for conducting the liquid in the form of a measuring tube, the means for generating the magnetic field, the electrode assemblies, and possibly further components form the electromagnetic flow sensor.
The inner surface of the measuring tube must, at least in the region of the magnetic field, be made of an insulating material in order that the induced voltage will not be short-circuited. Therefore, metallic measuring tubes are lined with a suitable insulating material.
Furthermore, metallic measuring tubes must not be made of ferromagnetic material, so that the magnetic field generated by means of a coil assembly disposed outside the measuring tube can reach and pass through the liquid. They are therefore generally made from high-grade steel, so that their external surface is stainless and generally requires no further treatment.
An electrode assembly comprises a measuring electrode with a head and a shank. The measuring electrode is fixed in a hole of the wall of the measuring tube in such a way that it is electrically insulated from the wall. This insulation must be very good, since the induced voltage has only small values of the order of 1 mV and its internal resistance may be on the order of 100 k&OHgr;.
The measure of the quality of the insulation is usually the insulation resistance between the measuring electrode and the measuring tube. In dry conditions, the insulation resistance is usually of the order of 100 &OHgr;.
It has turned out that during operation of the flow sensors, these values may deteriorate to the order of the internal resistance of the induced voltage, whereby the latter is reduced to approximately one half. Investigations by the inventors have shown that this occurs mainly when the measuring tube is colder than the ambient air. The ambient air usually leaks into a housing that encloses the electrode assemblies, even if the housing is filled with foam.
Thus, moisture will settle on the measuring tube, especially near the measuring electrodes; this may even result in the formation of water droplets. Due to capillary forces, the moisture will creep into gaps that may exist, on the one hand, between the measuring electrodes and the insulation and, on the other hand, between the insulation and the measuring tube. Furthermore, as a result of adhesion, films of moisture may form on the external surfaces of the insulation and the measuring electrodes.
SUMMARY OF THE INVENTION
It is therefore an object of the invention to avoid the reduction of the insulation resistance of the measuring electrodes by a suitable design of the electrode assembly.
To attain this object, the invention provides an electrode assembly for an electromagnetic flow sensor, said electromagnetic flow sensor comprising:
a measuring tube made of a metallic, nonferromagnetic material
which is lined with an insulating material, and
having an external surface which is untreated;
a coil assembly for generating a magnetic field passing through the measuring tube at essentially right angles and having a central axis; and
two electrode assemblies located opposite each other on a diameter of the measuring tube perpendicular to the central axis,
said electrode assembly comprising:
a measuring electrode having a head and a shank provided with an external thread and having a smaller diameter than the head, said measuring electrode being fitted in a hole of a wall of the measuring tube by means of an insulating body so as to be insulated from said wall, said insulating body
having a disk-shaped portion and
a tubular extension fitting the hole; and
a cylindrical part made of an electrically insulating, hydrophobic material and having a bottom and a wall portion,
said bottom having a central opening with a diameter less than the diameter of the disk-shaped portion, and
said wall portion having an inside diameter slightly greater than the diameter of the disk-shaped portion,
the cylindrical part, the insulating body, and a spring part, in that order, being fixed by means of a nut screwed onto the external thread of the shank, with the extension fitted in the hole, and the disk-shaped portion and the bottom being supported on the external surface of the measuring tube.
In a preferred embodiment of the invention, the ex-tension of the disk-shaped portion of the insulating body fits the central opening of the cylindrical part. Preferably, the cylindrical part is made of perfluoralkoxy.
One advantage of the invention is that, since no liquid film can form on the hydrophobic cylindrical part, a reduction of the insulation resistance is avoided and the induced voltage is not attenuated. This also prevents the occurrence of long-time insulation-resistance variations, which would result in interfering voltages that would be superposed on the induced voltage.
REFERENCES:
patent: 3177709 (1965-04-01), Fischer
patent: 3213685 (1965-10-01), Mannhertz et al.
patent: 3397575 (1968-08-01), Ham
patent: 4358964 (1982-11-01), Otsuka
patent: 4517846 (1985-05-01), Harrison et al.
patent: 0 766 069 (1997-04-01), None
patent: 2 047 409 (1980-11-01), None
Graf Oliver
Schoohf Michael
Bose McKinney & Evans LLP
Fuller Benjamin R.
Patel Jagdish
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