Method for determining and/or monitoring viscosity and corresponding apparatus

Abstract

A method for determining and/or monitoring the viscosity of a medium, wherein a mechanically oscillatable unit is excited to execute oscillations based on an exciter signal, and wherein oscillations are received from the mechanically oscillatable unit and transduced into a received signal. The eigenfrequency and/or resonance frequency of the mechanically oscillatable unit and/or phase relationship between the exciter signal and the received signal are/is ascertained and/or monitored, and from changes in the eigenfrequency and/or resonance frequency and/or phase relationship, a change in viscosity is deduced and/or, based on dependencies of the oscillations on the viscosity of the medium, from the eigenfrequency and/or resonance frequency and/or phase relationship, viscosity is ascertained. In a second variant of the method, decay behavior of the mechanically oscillatable unit is evaluated. An apparatus for determining and/or monitoring viscosity is also presented.

Claims

1. An apparatus for determining and/or monitoring at least viscosity of a medium, comprising: at least one mechanically oscillatable unit; and at least one evaluation unit, which supplies said mechanically oscillatable unit with an exciter signal with oscillations of the mechanically oscillatable unit and receives a received signal from said mechanically oscillatable unit, wherein said evaluation unit is embodied to ascertain and/or monitor the eigenfrequency of the mechanically oscillatable unit and/or the phase relationship between the exciter signal and the received signal, wherein said evaluation unit is embodied to deduce a change in viscosity from changes in the eigenfrequency and/or from changes in the phase relationship between the exciter signal and the received signal, with oscillations of the mechanically oscillatable unit at the resonance frequency, and/or wherein said evaluation unit is embodied to ascertain the viscosity, based on dependencies of the oscillations of the mechanically oscillatable unit on the viscosity of the medium, from the eigenfrequency and/or the phase relationship between the exciter signal and the received signal, with oscillations of the mechanically oscillatable unit at the resonance frequency; and wherein said mechanically oscillatable unit is embodied in such a manner, that the geometry of the mechanical oscillatable unit is optimized such that effects of changes in density on the mechanical oscillations of said mechanically oscillatable unit are negligible.

2. The apparatus as claimed in claim 1, wherein: said mechanically oscillatable unit includes at least one membrane, or diaphragm, and two fork tines; and said fork tines are essentially cylindrically embodied.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will now be explained in greater detail on the basis of the appended drawing, the figures of which show as follows:

(2) FIG. 1 is a schematic representation of measuring with an oscillatory fork;

(3) FIG. 2 shows dependence of eigenfrequency on viscosity of the medium in the case of constant density of the medium;

(4) FIG. 3 shows dependence of change of eigenfrequency on viscosity of the medium in the case of constant density of the medium;

(5) FIG. 4 shows dependence of change of resonance frequency on viscosity of the medium in the case of constant density of the medium;

(6) FIG. 5 is a schematic representation of a variant of the mechanically oscillatable unit;

(7) FIG. 6 shows dependence of phase of received signal relative to exciter signal at resonance frequency on viscosity of the medium in the case of constant density of the medium;

(8) FIG. 7 shows dependence of quotient of resonance frequency and eigenfrequency on viscosity of the medium;

(9) FIG. 8 shows dependence of amplitude in the case of the decay behavior of the mechanically oscillatable unit in the case of constant density of the medium; and

(10) FIG. 9 shows dependence of Lehr's damping ratio on viscosity.

DETAILED DISCUSSION IN CONJUNCTION WITH THE DRAWINGS

(11) FIG. 1 shows the measuring method, in principle, with a mechanically oscillatable unit 1. The mechanically oscillatable unit 1 is, in this example, a so-called oscillatory fork. Alternative embodiments include membrane, or diaphragm, oscillators, or so-called single rods. In the case of the here illustrated, oscillatory fork, the mechanically oscillatable unit 1 is composed of two fork tines, which are secured on a membrane, or diaphragm.

(12) On the inside of the membrane, or diaphragm, is located a driving/receiving unit (not shown), in the form, for example, of a piezoelectric element. The driving/receiving unit transduces, for example, an electrical alternating voltage as exciter signal into mechanical movements of the membrane, or diaphragm, and, thereby, the fork tines and, thus, the mechanically oscillatable unit 1 as a whole. Conversely, the driving/receiving unit serves also for transducing the mechanical oscillations of the mechanically oscillatable unit 1 into an electrical signal, which is, here, likewise, an electrical, alternating voltage. This is the received signal.

(13) The mechanically oscillatable unit 1 is placed on the wall of the container 3 in such a manner, that it comes in contact with the medium 2 at a certain fill level thereof, or in such a manner that the mechanically oscillatable unit 1 is covered to a certain degree by the medium 2 at a desired fill level of the medium 2. In an embodiment, the mechanically oscillatable unit 1 is completely covered by the medium 2. Medium 2 is, in such case, especially, a liquid.

(14) From the characteristic variables of the oscillations of the mechanically oscillatable unit 1, such as special frequencies, amplitude or phase relationship of the received signal relative to the exciter signal, in given cases, as a function of the particular frequency, process variables of the medium 2 can be determined, or changes of these process variables monitored.

(15) Thus, for example, fill level can be monitored by the fact that the frequency, or the amplitude, is reduced, when the mechanically oscillatable unit 1 comes in direct contact with the medium 2, or, conversely, from an increasing of the amplitude, or the frequency, it can be deduced, that the medium 2 has a fill level below the mechanically oscillatable unit 1.

(16) For determining, or monitoring, such process variables, for instance viscosity, or density, of the medium 2, in most cases, a certain fill level of the medium 2, i.e. a certain degree of covering of the mechanically oscillatable unit 1 by the medium 2, is chosen.

(17) The driving of the mechanically oscillatable unit and, respectively, the evaluation the measuring signals is performed here by an evaluation unit 4. Furnished, e.g. stored, in this evaluation unit 4 are the dependencies of the variables of the oscillations of the mechanically oscillatable unit to be measured, or to be monitored, on viscosity. These dependencies can be stored, for example, in the form of tables, selected value pairs (hash tables) or in the form of functional relationships.

(18) Since density and fill level, or degree of covering, are disturbance variables in determining, or monitoring, viscosity, these are kept constant, for example, by the boundary conditions of the measuring or, for example, a measuring, or monitoring, of the disturbance variables is performed by additional measuring devices. In an alternative embodiment, the density of the medium is measured by the same mechanically oscillatable unit as for used viscosity, by tuning a phase between the excitation signal and the received signal set, at which the oscillations are essentially independent of viscosity, or a change in viscosity. I.e., density and viscosity are measured with one measuring device. When the disturbance variables are held constant, knowledge of the dependence of a characteristic variable, or a number of characteristic variables, of the oscillations on viscosity of the medium is sufficient, in order to make statements concerning viscosity. If the disturbance variables change and the disturbance variables are measured, then knowledge concerning this, thus, multidimensional dependence is required, or the data must be suitably furnished, or stored in the evaluation unit 4.

(19) FIG. 2 shows the viscosity dependent behavior of the eigenfrequency .sub.0 of the oscillations of the mechanically oscillatable unit. Represented is the dependence of phase of the oscillations of the received signal relative to the exciter signal as a function of the frequency of the oscillations in the case of known, or fixed, density of the medium. The eigenfrequency .sub.0 is, in such case, characterized in that it is the oscillation frequency, at which the phase of the oscillations and, thus, the received signal, relative to the exciter signal is 90. This phase of 90 is shown in the figure by the horizontal, dot dashed line. I.e., from the intersection of the line with the curve, there results the eigenfrequency .sub.0 present in the case of the particular viscosity of the medium. The individual curves belong in such case to media with equal density, however, different viscosity.

(20) As clearly recognizable, as a function of viscosity, in each case, a clearly different eigenfrequency .sub.0 is present. Thus, it is possible, based on the eigenfrequency .sub.0, at known density, to deduce viscosity. In the case, wherein only a change in viscosity should be detected, it is already sufficient to detect a change of eigenfrequency .sub.0 in the case of unchanged density, or unchanged degree of covering, or process conditions otherwise generally kept constant.

(21) The eigenfrequency .sub.0 is in such case ascertained in such a manner, that a frequency range is run through and the phases are evaluated. The frequency, at which a phase difference of 90 occurs, is, thus, the eigenfrequency .sub.0. Another opportunity is to set a phase difference of 90 and to measure the resulting frequency. This is then the eigenfrequency .sub.0.

(22) FIG. 3 details again the dependence of change of eigenfrequency .sub.0 on the dynamic viscosity of the medium. The dynamic viscosity describes in such case the viscous behavior of the medium without taking into consideration the density of the medium. Taking density into consideration yields the kinematic viscosity.

(23) The frequencies of FIG. 3 are given in such case relative to the eigenfrequency of a mechanically oscillatable unit, for example, as illustrated in FIG. 1 in the case of oscillations in water (density=1 and viscosity=1).

(24) The eigenfrequency .sub.0 and resonance frequency .sub.res of the oscillations of the mechanically oscillatable unit are related to one another as a function of the damping D: .sub.res=.sub.0.Math.{square root over (12.Math.D.sup.2)}.

(25) In the case of resonance frequency .sub.res, such is that frequency, at which the amplitude of the oscillations has its local maximum. In the case, that no damping is present, or that the damping is negligible, then the resonance frequency .sub.res and the eigenfrequency .sub.0 are the same.

(26) If one considers an almost density insensitive oscillatory system (compare FIG. 5), then the eigenfrequency .sub.0 can be assumed to be constant and then alone the damping effect of viscosity on the oscillatory system of the mechanically oscillatable unit is measured by the evaluation of frequency.

(27) FIG. 4 shows the dependence of change of resonance frequency .sub.res on the dynamic viscosity in the case of constant density of the medium. The resonance frequency .sub.res is in such case measured in such a manner, that a certain frequency band is run through, and the amplitude evaluated. Thus, also the resonance frequency .sub.res permits monitoring, or in the case of suitably furnished values, or dependencies, the ascertaining of viscosity.

(28) FIG. 5 shows another embodiment of the mechanically oscillatable unit 1, in the case of which the two fork tines are round rods. Advantageous with these round rods is that the oscillations are almost independent of the density of the medium. In the case that, in determining, or monitoring, viscosity, a geometry of the mechanically oscillatable unit 1 is used, which leads to the fact that this unit 1, or its oscillations, are density dependent, either it must be assumed, that the density remains constant within a certain range, or the dependence of oscillations on density must be previously known. The membrane, or diaphragm, is, here, circular and also the fork tines have a circularly shaped cross section.

(29) Since the resonance frequency .sub.res and the eigenfrequency .sub.0 of the oscillatory system differ from one another in the case, in which the influences of viscosity and density are not negligible, it will become evident in the case of resonance frequency .sub.res that phase relationships between the exciter signal and the received signal do not equal 90. It can be observed, that the phase difference in the case of resonance frequency .sub.res sinks with increasing viscosity. This dependence, or this relationship, permits, thus, also a determining, or monitoring, of viscosity. Density is, in such case, a disturbance variable.

(30) FIG. 6 shows such a relationship between the phase angle in the case of resonance frequency .sub.res and the dynamic viscosity of the medium. Shown is that, in the case of a viscosity of the medium of zero, the phase angle amounts to 90. If, thus, in the case of known or constant density and in the case of known or constant degree of covering, with oscillations of the mechanically oscillatable unit at the resonance frequency .sub.res, the phase between the received signal and the exciter signal is ascertained, then, therefrom, a change in viscosity, or even the measure of viscosity itself, can be deduced.

(31) FIG. 7 shows the ratio between resonance frequency, .sub.res, and eigenfrequency, .sub.0, of the mechanically oscillatable unit as a function of viscosity. If both the resonance frequency .sub.res as well as also the eigenfrequency .sub.0 of the mechanically oscillatable unit is ascertained, then these two measured values can be divided one by the other to form their ratio. As FIG. 7 indicates, also evaluation of this quotient permits monitoring, or determining, of viscosity of the medium.

(32) The methods described in the preceding figures for determining, or monitoring, viscosity assume, in each case, that at least one frequency of the mechanically oscillatable unit, or, generally, the oscillatory system, is ascertained, or monitored, and that the viscosity dependence of the particular frequency, or, associated therewith, the phase, is used. In the following figures, a further method is presented, namely the second variant, in the case of which the dependence of the oscillatory behavior of the oscillatory system on viscosity in another way is utilized. In the previous methods, it is assumed, that the oscillatory system is excited continually to execute oscillations and that, thus, continuously, oscillation parameters are evaluated.

(33) FIG. 8 shows the decay behavior of the mechanically oscillatable unit in various media. The oscillatable unit is, in such case, excited to execute oscillations and the decrease of the amplitude after the one-time exciting is plotted versus time and evaluated. As can be seen, the curves are functionally dependent on the viscosity of the medium. In such case, it is to be observed that, with increasing viscosity, i.e. with increasing damping, the decay is faster, so that, in a high viscosity medium, the oscillations go faster to zero than in a low viscosity medium. Alternatively to the decay of the oscillatory system, also transient response can be evaluated.

(34) For ascertaining the decay, or the transient response, of the oscillatory system, the system should preferably be operated in resonance, in order to enable a highest possible amplitude and, thus, a good evaluation. The exciting of the oscillatory system operated in resonance is, in such case, switched off, in order to record, or measure, the decay. This can be described, for example, by a step function: (1(t)).Math.A.Math.sin(.Math.t).

(35) The decay curves of the amplitudes A(t) can, in such case, be described, for example, by an exponential function of the following form:

(36) $A\left(t\right)=\hat{A}.Math.e^{-\frac{1}{T}.Math.t}=\hat{A}.Math.e^{-.Math.t}.$

(37) Such curves are shown in FIG. 8, in the case of which, in each case, the density of the medium remains the same. The amplitude is, in each case, given in percentage referenced to the first measured, amplitude value.

(38) For determining the time constant , the time is ascertained, at which the amplitude has declined to about 36.8%, corresponding to 1/e. FIG. 8 shows, in such case, this threshold value by a thicker line. It is to be recognized that, with sinking viscosity, the measuring, or determining, of the decay time and, thus, the determining of the decay constant , can always occur more exactly, since, on the one hand, higher amplitudes of the oscillations can be expected and since, on the other hand, the decay times are always larger and, thus, are more exactly ascertainable.

(39) As an add-on to the determining of the decay time constant , an opportunity is to normalize this constant on the particular eigenfrequency .sub.0 of the oscillatory system, in order, in this way, to obtain Lehr's damping ratio. Lehr's damping ratio is defined as follows:

(40) $=\frac{}{{}_0}.$

(41) FIG. 9 shows the viscosity dependence of Lehr's damping ratio. In such case, it is clearly recognizable that, based on decay as a function of eigenfrequency .sub.0, the determining of viscosity is possible. Thus, ascertained, on the one hand, is the eigenfrequency .sub.0, and, on the other hand, the time constant of the decay, and the two variables are taken into consideration for establishing Lehr's damping ratio, wherein the actual, or measured, value permits, via comparison with suitably stored data, the determining of viscosity of the medium.