Measuring and testing – Volume or rate of flow – By measuring electrical or magnetic properties
Patent
1996-02-27
1997-09-23
Chilcot, Richard
Measuring and testing
Volume or rate of flow
By measuring electrical or magnetic properties
29593, G01F 158
Patent
active
056707231
DESCRIPTION:
BRIEF SUMMARY
The invention relates to a measuring section for an electromagnetic flowmeter having a metallic measuring tube made of a non-magnetic material, a set of magnetic coils for generating a magnetic field in a direction substantially at right angles to the direction of flow, an electrode arrangement which is arranged substantially at right angles to the direction of the magnetic field and substantially at right angles to the direction of flow, and an electrical insulator on the inside of the measuring tube; the invention also relates to a method for the manufacture of a measuring section in which an insulator is arranged on the inside of the measuring tube.
Such measuring sections serve to determine the rate of flow of fluids, for example, in order to be able to dispense the fluids in the correct volume. The range of application extends from very small measuring tubes of a few square millimetres in cross-section to quite large measuring tubes which have a considerably larger flow cross-section. These last-mentioned tubes in particular have to be capable of bearing the weight and pressure of the fluid flowing through them without being adversely affected thereby. For that reason, in many cases a metallic material, such as aluminium or stainless steel, is selected. So that the magnetic field is not disrupted, it is a prerequisite that the material of the measuring tube is non-magnetic, that is to say, does not influence the magnetic field. As most metallic materials are electrically conductive, however, electrical insulation has to be provided between the fluid and the metal tube, otherwise the measuring tube itself would short-circuit the voltage created by the fluid flowing past and the magnetic field and prevent measurement taking place. The insulation must satisfy various requirements here. On the one hand it must have the necessary very high electrical resistance in order to be able to act as an electrical insulator. On the other hand it must, of course, also be resistant to the fluids flowing through. In many cases there is an added requirement for a certain thermal stability, especially when the flowmeter is used in systems which have to be cleaned from time to time, for example in the food industry, where for reasons of hygiene cleaning has to be carried out at temperatures far in excess of 100.degree. C. and with cleaning agents that are in some cases corrosive. Plastics materials that in themselves have the necessary dielectric and insulating properties cannot normally be considered for this purpose because they are not able to withstand the elevated temperatures.
U.S. Pat. No. 3,750,468 discloses a measuring section for an electromagnetic flowmeter in which a tubular insert of aluminium oxide is inserted in a titanium tube. This insert serves electrically as the insulator. It is also sufficiently resistant for the majority of applications, and is in particular resistant to relatively high temperatures and has a certain resistance to corrosive fluids.
The manufacture of such a measuring section is not without problems, however. The insert must lie as closely as possible against the metal tube at all points, that is to say, for its entire length and around its entire circumference. This requires very precise pre-machining so that the outside of the insert and the inside of the metal tube have a cross-section that is as accurately circular as is possible. In addition, the external dimension of the insert and the internal dimension of the tube must be matched to one another exactly, so that the fit of the insert in the tube can be achieved, for example, by shrinking. Even when the shrinking process, in which the metal tube is heated, pushed onto the insert and then cooled, is carried out with the required care, now and then the insert may not be able to withstand the mechanical stresses occurring in the process, and will sustain damage. In many cases, such damage, which can be in the form of hairline cracks, for instance, goes unnoticed, so that the measuring section cannot operate with the required accura
REFERENCES:
patent: 3750468 (1973-08-01), Grauer
patent: 4281552 (1981-08-01), Nissen et al.
patent: 4785672 (1988-11-01), Picone
Gade Niels
Hansen Henning Max
Pedersen Hans J.o slashed.rgen
Voss Frands Wulff
Chilcot Richard
Danfoss A/S
Patel Harshad
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