Mass flow rate measuring device

Details Mass flow rate measuring device Mass flow rate measuring device

2002-01-25

2003-05-06

Williams, Hezron (Department: 2855)

Measuring and testing

Volume or rate of flow

Mass flow by imparting angular or transverse momentum to the...

C073S861355

Reexamination Certificate

active

06557422

ABSTRACT:

RELATED APPLICATION
Not Applicable
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a mass flowmeter operating by the Coriolis principle, incorporating a fluid-conducting Coriolis conduit, at least two oscillators associated with and exciting the Coriolis conduit, as well as at least two detectors associated with the Coriolis conduit for capturing Coriolis forces and/or Coriolis-force-derived Coriolis oscillations, said Coriolis conduit permitting excitation in a first oscillation pattern and in an oscillation pattern independent of the first oscillation pattern. The invention further relates to a method for determining the mass flow employing a mass flowmeter operating by the Coriolis principle and incorporating a fluid-conducting Coriolis conduit, at least two oscillators associated with and exciting the Coriolis conduit, as well as at least two detectors associated with the Coriolis conduit for capturing Coriolis forces and/or Coriolis-force-derived Coriolis oscillations, said Coriolis conduit permitting excitation in a first oscillation pattern and in an oscillation pattern independent of the first oscillation pattern.
The above states that the mass flowmeter here discussed incorporates, inter alia, at least two oscillators “associated with” the Coriolis conduit and at least two detectors “associated with” the Coriolis conduit. As a rule, however, the oscillators or at least part thereof and the detectors or at least part thereof are connected to the Coriolis conduit. But since this is not an absolute must, the term “associated with” is being used instead of “connected to”.
There is a fundamental differentiation between Coriolis-type mass flowmeters whose Coriolis conduit is essentially straight and those whose Coriolis conduit is looped. As another differentiating feature in mass flowmeters of the type being discussed, one design concept provides for only one Coriolis conduit while other models come with two Coriolis conduits. In the design versions incorporating two Coriolis conduits, the latter may be laid out in an in-line or a parallel flow configuration.
In more recent times, flowmeters with only one essentially straight Coriolis conduit have gained predominance. In terms of their mechanical architecture, flowmeters with one straight Coriolis conduit are simpler and can be produced at a comparatively lower cost, the inner surfaces of their Coriolis conduit are easily accessible for finishing processes such as polishing, they are subject to only minor pressure drops, and they are self-purging.
Yet their many advantages notwithstanding, mass flowmeters with only one straight Coriolis conduit also pose problems in certain respects.
Mass flowmeters operating by the Coriolis principle produce direct mass-flow readings representing a true mean value across the flow profile of the fluid traveling through the Coriolis conduit. Under ideal conditions, Coriolis-type mass flowmeters, by virtue of their very operating principle, deliver readings which are not influenced by the physical characteristics of the fluid passing through the Coriolis conduit, such as thermal conductivity and thermal capacity, or process parameters such as pressure and temperature. Nevertheless, asymmetries in the mass flowmeter, attributable for instance to unavoidable production tolerances, or changes encountered over time in terms of the operating conditions such as temperature fluctuations or varying clamping parameters lead to a reduction of the measuring precision of the mass flowmeter which, due its design concept, would otherwise be very high. These problems are particularly pronounced in the case of mass flowmeters with only one straight Coriolis conduit.
2. Description of the Prior Art
Various attempts have been made to compensate for the effects of changing operating conditions such as temperature fluctuations or the clamping parameters of the Coriolis conduit when measuring the mass flow. For example, in the case of the above-mentioned mass flowmeter of this type (ref. DE 44 13 239 Al) the design includes provisions which, during operation, measure the intrinsic frequency of the Coriolis conduit in the Coriolis mode, by exciting the Coriolis conduit not only in its operating mode but in the Coriolis mode as well. That, however, does not completely compensate for the above-mentioned varying environmental operating parameters. Specifically, in the case of the mass flowmeter referred to only the effect of temperature changes on the density of the fluid traveling through the Coriolis conduit and the attendant change of the resonant frequency of the fluid-conducting Coriolis in the Coriolis mode are factored in. However, the parameters of the mass flowmeter such as its zero point and its sensitivity cannot be determined that way.
Other problems of mass flowmeters with only one straight Coriolis conduit derive from the fact that the oscillating components, which essentially means the Coriolis conduit proper, transfer oscillating forces or torque to the points at which the Coriolis conduit is clamped, forces which especially in the case of mass flowmeters with only one straight Coriolis conduit cannot be absorbed without a reactive effect. This can cause oscillatory energy to be asymmetrically drained from the Coriolis conduit into the environment and to be unevenly reflected back and coupled into the Coriolis conduit, or vibrations emanating from the surrounding area can be unevenly coupled into the input or output half of the Coriolis conduit, leading to correspondingly corrupted measuring results.
Attempts have been made in the prior art to solve this problem with the aid of counter-oscillations. These generally serve to compensate for housing vibrations. In this context, reference is made for instance to DE 198 40 782 A1, EP 0 598 287 A1, EP 0 759 542 A1, EP 0 831 306 and EP 0 849 568 A1. In spite of these mechanically complex counter-oscillators, the oscillatory separation of Coriolis-type mass flowmeters from the environment has so far remained unsatisfactory. There are also other problems such as the effects of pulsations, meaning continuum fluctuations of the fluid in the Coriolis conduit, on the measuring results, or irregular oscillations of the Coriolis conduit in the case of major inhomogeneities of the conducted fluid. For these reasons, parameters such as zero point and sensitivity change during operation.
SUMMARY OF THE INVENTION
It is therefore the objective of this invention to introduce both a mass flowmeter and a method for the determination of the mass flow by means of which greater accuracy and zero-point stability are attainable.
The mass flowmeter according to this invention which solves the above-described problem and meets the stated objective is typified in that characteristic values of the mass flowmeter can be determined on the basis of a model.
As stated further above, the Coriolis conduit can be excited in mutually independent oscillation patterns. Of course, this excitation in mutually independent oscillatory patterns applies not only to the Coriolis conduit alone but also to the Coriolis conduit as it conducts the fluid whose mass flow is to be measured.
It is fundamentally possible to perform the model-based determination of the characteristic values of the mass flowmeter prior to the start-up of the device or during operating stoppages. However, according to a preferred enhancement of the invention, provisions are made permitting the determination of the characteristic values of the mass flowmeter while it is in operation. This not only obviates the need for interrupting the measuring operation in order to define the characteristic values but it also makes these characteristic values available in “on-line” fashion, i.e. in real time, allowing them to be continuously factored into the determination of the mass flow.
In a preferred, enhanced version of this invention, the Coriolis conduit can be-excited in a third oscillation pattern which is independent of the first oscillation pattern and of the second oscillation pattern, and the res

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