Flowmeter

Measuring and testing – Volume or rate of flow – By measuring vibrations or acoustic energy

Reexamination Certificate

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Reexamination Certificate

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06799475

ABSTRACT:

BACKGROUND OF THE INVENTION
This invention relates to a flowmeter incorporating a measuring tube, an ultrasound transducer, an ultrasound waveguide and a seal, said ultrasound transducer connecting outside the measuring tube to the ultrasound waveguide in such fashion that ultrasound waves generated by the ultrasound transducer can be transferred to the ultrasound waveguide and, conversely, ultrasound waves received by the ultrasound waveguide can be transferred to the ultrasound transducer, with the ultrasound waveguide protruding at least partly into the measuring tube.
A flowmeter of this type may be an ultrasound flowmeter or a vortex flowmeter. The ultrasound transducers employed are typically piezoelectric crystals capable of generating and/or detecting ultrasound waves.
There are applications for which it is possible to equip the flowmeter with only an ultrasound transducer without an ultrasound waveguide, the ultrasound transducer serving to generate as well as detect ultrasound waves. In any such design, the ultrasound transducer must be directly positioned at the point where the ultrasound waves are introduced or detected. That, however, tends to create a problem insofar as the piezoelectric crystals which, as pointed out above, are typically used as the ultrasound transducers in flowmeters, cannot be used above a certain temperature, the so-called Curie temperature. This is because above the Curie temperature, the crystal no longer possesses a ferroelectric or ferromagnetic phase, the prerequisite for the piezoelectric properties of the crystal.
Therefore, in cases where for instance the moving fluid whose flow rate is to be measured by the ultrasound flowmeter is so hot that its temperature is above the Curie temperature of the piezoelectric crystal, any reliable operation requires a certain thermal insulation of the ultrasound transducer from the hot fluid. It is for that reason that flowmeters are equipped with ultrasound transducers so configured as to leave a spatial clearance between the ultrasound transducer and the hot fluid. The ultrasound waveguide in that configuration must ensure the best possible thermal insulation between the ultrasound transducer and the hot fluid while at the same time optimizing a loss-free and unimpeded transfer of the ultrasound waves. In other words, the design of an ultrasound waveguide where the ultrasound transducer is located at a distance from the hot fluid or at least thermally insulated from that fluid enables the ultrasound transducer to inject ultrasound waves generated by it into the moving fluid and to receive the ultrasound waves from the hot fluid.
Conventional flowmeters equipped with an ultrasound transducer and an associated ultrasound waveguide employ ultrasound waveguides of the type as described, for instance, in WO 96/41157. The ultrasound waveguide according to that design incorporates multiple, mutually parallel, very thin rods, with the diameter of the individual rods being significantly smaller than the wavelength of the ultrasound signal to be conducted. The rods are typically bundled closely together and fitted into a tube that supports them laterally and constitutes a jacket for the ultrasound waveguide, thus making for a compact ultrasound waveguide design.
WO 96/41157 also describes an ultrasound waveguide design in which metal plates, bent in an essentially circular shape, are interleaved at a distance from one another. These rounded metal plates are again housed in a tube that constitutes the outer enclosure of the ultrasound waveguide.
EP 1 098 295 on its part describes an ultrasound waveguide that consists of a rolled-up foil fitted tightly into a metal tube. For transmitting ultrasound waves in the frequency range from 15 kHz to 20 kHz, the thickness of the foil layers is less than 0.1 mm. The foil typically consists of a metallic material.
According to the generally applied design concept, the ultrasound transducer is positioned at one end of the ultrasound waveguide in such fashion that the ultrasound transducer can feed ultrasound waves into, and receive them from, the ultrasound waveguide. The ultrasound transducer is usually plugged into one end of the ultrasound waveguide and thus is in direct physical contact with it. In the case of the aforementioned ultrasound waveguide with the rolled-up foil according to EP 1 098 295, the ends of the ultrasound waveguide are usually welded up and faced, and the ultrasound transducer is mounted on that welded, flat end face of the ultrasound waveguide.
As stated above, the ultrasound waveguide in flowmeters of the type to which this invention relates protrudes at least partly into the measuring tube. This allows the ultrasound waveguide to make direct contact with the fluid into which ultrasound waves are to be injected and from which ultrasound waves are to be received. That, however, poses a problem insofar as the measuring tube with the inserted ultrasound transducer must be sealed toward the outside. One way to solve that is to fit the jacket of the ultrasound waveguide directly into the measuring tube and seal it for instance by welding. Another way is to fit the ultrasound waveguide into a flange, again sealed for instance by welding, and to then attach that flange to a corresponding flange on the measuring tube, with the seal provided between the two flanges.
If the ultrasound waveguide is at least partly inserted into the measuring tube via a flange, a corresponding adapter may be provided on the measuring tube. In that case the waveguide does not have to extend into the measuring tube past the outer wall of the latter. Nevertheless, as intended by this invention, the waveguide protrudes at least partly into the measuring tube due to the fact that the waveguide is positioned in the cavity created by the adapter that connects directly to the interior of the measuring tube. In other words, the invention is not limited to configurations where the waveguide is at least partly inserted into the measuring tube in a manner whereby it protrudes into the interior of the measuring tube itself. Indeed, the waveguide may even be set back from the measuring tube proper. All that matters is that the waveguide, by virtue of its at least partial insertion in the measuring tube, is at least in indirect contact with the fluid passing through the measuring tube.
What poses a problem, however, is the fact that mounting a flange on the measuring tube, typically via an adapter, creates the cavity referred to above, which could interfere with the flow of the fluid in the measuring tube. In other words, the fluid might penetrate into the cavity created by the adapter and thus all the way to the seal between the two flanges which, in turn, requires a seal that can withstand the temperature of what may be a very hot fluid. This limits the available options for the gasket material while at the same time having a negative effect on the suppression of cross coupling, or a phenomenon referred to as crosstalk.
The problem with the flowmeter designs discussed is that the ultrasound waves generated by the ultrasound transducer, when transmitted, are injected not only into the ultrasound waveguide but also into the jacket encasing the ultrasound waveguide. Conversely, when the ultrasound transducer is intended to detect ultrasound waves, the ultrasound waves reach the ultrasound transducer not only via the ultrasound waveguide but via the jacket as well. It follows that the ultrasound waves transmitted or detected by the ultrasound transducer are waves transferred by both the ultrasound waveguide and the surrounding jacket. Now if it is by way of its jacket that the ultrasound waveguide is installed in and connected to the wall of the measuring tube carrying the fluid whose flow rate is to be determined, not only the ultrasound waves passing through the fluid but also those ultrasound waves that travel through the wall of the tube to and from the ultrasound transducer will be collected. The resulting crosstalk phenomenon may lead to a hetero-dyning or even total disruption of t

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