DEVICE FOR MEASURING VISCOSITIES

Abstract

An apparatus for measuring viscosities of fluids is described, having a measuring system (1) with at least one measuring tube (5) which, during the measuring operation, is filled with a fluid or is flowed through by the fluid and which has at least one tube portion (4, 7) which is excited to perform vibrations, having an exciter device (3) for exciting at least two useful vibration modes having different frequencies, at each of which at least one of the tube portions (4, 7) is excited to perform vibrations, in particular resonance vibrations, of the respective useful vibration mode, having a measuring device (9) which is designed such that, for each of the useful vibration modes excited during the measuring operation, said measuring device determines a frequency and an attenuation, in particular a frequency, an amplitude and an attenuation, of the resultant vibration of at least one pipe portion (4, 7) excited to perform vibrations in the respective useful vibration mode, and having an evaluating device (15) which is designed such that, on the basis of calibration data stored in a memory (17), the evaluating device determines a measured shear rate value and a measured viscosity value for the individual useful vibration modes excited during the measuring operation on the basis of the frequency determined during the excitation thereof and the attenuation, in particular the frequency, the amplitude and the attenuation, of the resultant vibration, wherein the measured viscosity value corresponds to the dynamic viscosity of the fluid at a steady shear rate corresponding to the shear rate value.

Description

[0001] Device for measuring viscosities

[0002] The invention relates to a device for measuring the viscosity of fluids, with

[0003] - a measuring system with at least one measuring tube filled with or through which a fluid flows during measuring operation, which is used for at least one measuring operation

[0004] has a section of pipe that can be excited by vibrations

[0005] - an excitation device for exciting at least two useful vibration modes of different frequencies, in which at least one of the pipe sections is excited to vibrations, in particular to resonance vibrations, of the respective useful vibration mode, and

[0006] - a measuring device which is designed in such a way that it is suitable for use in measuring operation

[0007] For each of the excited useful vibration modes, a frequency and a damping, in particular a frequency, an amplitude and a damping, of the resulting vibration of at least one pipe section excited to vibrations of the respective useful vibration mode is determined.

[0008] Devices for measuring the viscosity of fluids are used, for example, in industrial metrology. Devices for measuring fluid viscosity are described, for example, in EP 1 158 289 B1 and DE 10 2004 021 690 A1.

[0009] The measurement of the viscosity of a fluid is described in which a pipe section of a measuring tube through which the fluid flows is excited to vibrations of a useful vibration mode and the viscosity of the fluid flowing through it is determined on the basis of the damping of the resulting vibration which depends on the viscosity.

[0010] Furthermore, DE 10 2004 021 690 A1 describes a Coriolis mass flow meter designed for use in applications where the parameters of inhomogeneous fluids, e.g., two-phase or multi-phase fluids, are to be determined. DE 10 2004 021 690 A1 describes a method for this purpose in which a straight

[0011] The measuring tube is simultaneously or sequentially excited to lateral and torsional vibrations of different frequencies, and the damping of the resulting lateral vibration, which depends essentially only on the viscosity of the fluid, and the damping of the resulting torsional vibration, which depends on the viscosity and inhomogeneity of the fluid, are measured. Intermediate values ​​are derived from the two measured damping values ​​and then used to improve the measurement accuracy of quantities measured with the instrument, such as mass flow rate, density, or viscosity. This method is based on the fact that the

[0012] The viscosity of the fluid is identical for the two vibration modes used to determine the intermediate values. This method allows inhomogeneities to be detected and accounted for. However, it cannot answer the crucial question, relevant to fluid flow behavior and rheology, of whether the fluid is a Newtonian fluid. Similarly, any existing...

[0013] The shear rate dependence of the viscosity of a fluid cannot be qualitatively detected or quantitatively determined with the measuring device described in DE 10 2004 021 690 A1.

[0014] It is an object of the invention to provide a device and a method with which shear rate-dependent viscosities of fluids can be determined.

[0015] The invention includes a device for measuring the viscosity of fluids, with

[0016] - a measuring system with at least one measuring tube filled with or through which fluid flows during measuring operation, which has at least one pipe section capable of being excited to vibrations,

[0017] - an excitation device for exciting at least two useful vibration modes of different frequencies, in which at least one of the pipe sections is excited to vibrations, in particular to resonance vibrations, of the respective useful vibration mode,

[0018] - a measuring device which is designed in such a way that it is suitable for use in measuring operation

[0019] for each of the excited useful vibration modes a frequency and a damping, in particular a frequency, an amplitude and a damping, of the resulting vibration of at least one pipe section excited to vibrations of the respective useful vibration mode, and

[0020] - an evaluation unit that is designed to use data from a

[0021] The stored calibration data for each of the useful vibration modes excited during measurement operation is used to determine a shear rate value and a viscosity measurement value based on the frequency and damping determined during their excitation, in particular the frequency, amplitude and damping of the resulting vibration, where the viscosity measurement value corresponds to the dynamic viscosity of the fluid at a static shear rate corresponding to the shear rate value.

[0022] A first advanced training is characterized by the fact that the evaluation device is designed in such a way that it can be used to determine viscosity values ​​based on at least two viscosity measurements determined at different shear rate values.

[0023] - a viscosity profile was created that represents the viscosity of the fluid as a function of the shear rate,

[0024] - detects whether the viscosity of the fluid exhibits a shear rate dependence, and / or

[0025] - based on the values ​​determined at the different shear rate values

[0026] Viscosity measurements determine whether the fluid is a Newtonian fluid, whose viscosity is essentially independent of the shear rate, or a

[0027] The fluid is either a shear-thinning fluid whose viscosity decreases with increasing shear rate, or a shear-thickening fluid whose viscosity increases with increasing shear rate. According to a second embodiment, the device comprises a detection unit for detecting changes in the fluid and / or deviations in the fluid's viscosity from applicable specifications that occur during measurement operation, and which is designed such that

[0028] - they compare the viscosity measurements determined from the measured shear rate values ​​or from the viscosity profile with reference values, in particular predefined target values ​​stored in a memory or predefined

[0029] target value ranges and / or viscosity measurements stored in a memory at one or more earlier times at shear rate values ​​determined by the device, and

[0030] - a change and / or deviation is detected if at least one of the

[0031] Viscosity measurements deviate from the corresponding reference value by more than a predefined tolerance, in particular a tolerance specified depending on the measurement accuracy of the viscosity measurement or a user-defined tolerance. A first embodiment is characterized by the fact that

[0032] - the excitation device includes a control device that regulates the amplitudes of the vibrations resulting from the excitation of the individual useful vibration modes to a constant value, and

[0033] - the measuring device is designed in such a way that it measures the damping based on a measure determined by means of an energy demand measuring device for a given situation.

[0034] The energy required to excite the respective useful vibration mode to maintain the resulting vibrations of constant amplitude is determined.

[0035] A third type of further training is characterized by the fact that

[0036] - the calibration data for each useful vibration mode in a preceding

[0037] Calibration procedures include data from which a shear rate value and a viscosity measurement value are assigned to the damping and frequencies measured during the measurement operation when the useful vibration mode is excited, in particular to the measured damping, frequencies and amplitudes, or

[0038] - the calibration data for each useful vibration mode in a preceding

[0039] The calibration procedure includes data from which a viscosity measurement is assigned to the damping measured during operation when the useful vibration mode is excited, and a shear rate value is assigned to each measured frequency, which is essentially equal to 2π times the frequency of the resulting vibration of the respective pipe section. A second embodiment is characterized in that the device includes a measuring unit connected to the measuring device, which is configured to determine a mass flow rate and / or a density of the fluid during operation based on the vibration resulting from the excitation of at least one of the useful vibration modes.

[0040] A fourth type of advanced training is characterized by the fact that

[0041] - the useful vibration modes exhibit frequencies of less than or equal to 1600 Hz, in particular less than or equal to 1000 kHz, in particular less than or equal to 800 Hz, in particular from 100 Hz to 800 Hz, and / or

[0042] - the pipe section or sections that can be excited to useful vibrations each have a pipe radius that is greater than a penetration depth to which shear waves forming in the fluid from a pipe wall of the respective pipe section when the respective pipe section is excited to vibrations of the respective useful vibration mode penetrate into the fluid.

[0043] A fifth advanced training course is characterized by the fact that

[0044] - the evaluation device is designed in such a way that it can determine the shear rate value and the viscosity measurement value for the individual components excited during the measurement operation

[0045] The useful vibration modes are determined based on the frequency, amplitude, and damping of the resulting vibration, as determined during their excitation, or

[0046] - a pipe radius and a length or partial length of each pipe section deflected during excitation of the respective useful vibration mode are each dimensioned such that the frequencies and the dampings of the individual

[0047] The vibrations resulting from the useful vibration modes exhibit no or only a slight dependence on the amplitude of the resulting vibration, and the evaluation device is designed in such a way that it can determine the shear rate value and the viscosity measurement value for the individual vibrations excited during the measurement operation.

[0048] The useful vibration modes are determined based on the frequency determined during their excitation and the damping of the resulting vibration.

[0049] A sixth advanced training course is characterized by the fact that

[0050] - the fluid has a complex viscosity, comprising a contribution attributable to elastic and a contribution attributable to viscous properties, and the contribution attributable to viscous properties is large compared to the contribution attributable to elastic properties

[0051] properties, whereby the complex viscosity exhibits essentially exclusively elastic properties,

[0052] - the pipe section or sections that can be excited to useful vibrations each have a pipe radius that is greater than or equal to ten times the penetration depth to which shear waves, forming in the fluid when the respective pipe section is excited to vibrations of the respective useful vibration mode, penetrate into the fluid from a pipe wall of the respective pipe section, and

[0053] - the evaluation unit is designed in such a way that it can determine the damping of the resulting vibration based on the damping determined during the excitation of the respective useful vibration mode.

[0054] The vibration of the respective pipe section determines the viscosity measurement value, and the corresponding measured shear rate is determined based on the frequency of the resulting vibration of the respective pipe section. Furthermore, the invention comprises a method for measuring the viscosity of a fluid, in which measurements are performed for at least two useful vibration modes of different frequencies, in which

[0055] - at least one pipe section of at least one measuring tube of a measuring system filled with and / or through which fluid flows, by means of an excitation device to oscillations, in particular to resonance oscillations, of the respective

[0056] The useful vibration mode is excited.

[0057] - a frequency and an attenuation, in particular a frequency, an amplitude and a

[0058] damping, the resulting vibration of at least one pipe section excited to vibrations of the respective useful vibration mode is determined, and - based on calibration data determined by a calibration procedure and the frequency determined during the excitation of the respective useful vibration mode and the

[0059] Damping, in particular the frequency, amplitude and damping, of the resulting vibration, a shear rate value and a viscosity measurement are determined, wherein the viscosity measurement is the dynamic viscosity of the fluid at a given temperature.

[0060] Shear rate value corresponds to the static shear rate.

[0061] A first further development of the procedure is characterized by the fact that

[0062] - the useful vibration modes have frequencies of less than or equal to 1600 Hz, in particular less than or equal to 1000 kHz, in particular less than or equal to 800 Hz, in particular from 100 Hz to 800 Hz, and / or - the pipe section or sections that can be excited to useful vibrations each have a pipe radius that is greater than a penetration depth to which shear waves forming in the fluid when the respective pipe section is excited to the respective useful vibration penetrate into the fluid from a pipe wall of the respective pipe section.

[0063] A second further development of the procedure is characterized by the fact that

[0064] - the fluid has a complex viscosity comprising a contribution attributable to elastic and a contribution attributable to viscous properties, and the contribution attributable to viscous properties is large compared to the contribution attributable to elastic properties, wherein the complex viscosity exhibits, in particular, essentially exclusively elastic properties,

[0065] - the pipe section or sections that can be excited to useful vibrations each have a pipe radius that is greater than or equal to ten times the penetration depth to which, when the respective section is excited,

[0066] Shear waves forming the respective useful vibration in the fluid, originating from a pipe wall of the respective pipe section, penetrate into the fluid, and

[0067] - based on the damping of the resulting vibration of the respective pipe section determined during the excitation of the respective useful vibration mode

[0068] The viscosity measurement and the corresponding shear rate value are determined based on the frequency of the resulting vibration of the respective pipe section.

[0069] A third further development of the procedure is characterized by the fact that

[0070] - in the calibration procedure for each of the individual useful vibration modes

[0071] Reference measurements are performed, in each of which the respective

[0072] The desired vibration mode is excited while the corresponding pipe section is filled with and / or through which a reference fluid flows, and the damping and frequency, in particular the damping, frequency and amplitude, of the vibration resulting from the excitation of the respective desired vibration mode are measured.

[0073] - wherein the reference measurements include reference measurements performed with Newtonian reference fluids of various known dynamic viscosities, and

[0074] - wherein each measured frequency is assigned a shear rate value which is essentially equal to 2π times the frequency of the resulting vibration of the respective pipe section and / or the reference measurements with non-Newtonian reference fluids with viscosity dependent on the shear rate carried out in a known manner,

[0075] - wherein the reference measurements include, in particular, reference measurements performed with reference fluids having elastic properties of varying degrees relative to their viscous properties, and

[0076] - Calibration data are determined for each useful vibration mode, whereby a shear rate value and a viscosity measurement value are assigned to the dampings and frequencies measured during the measurement operation when the respective useful vibration mode is excited, in particular to the measured dampings, frequencies and amplitudes.

[0077] A third further development of the method is characterized by the fact that viscosity measurements are taken from at least two viscosity values ​​determined at different shear rate values.

[0078] - a viscosity profile is created that represents the viscosity of the fluid as a function of the shear rate, - it is determined whether the viscosity of the fluid exhibits a shear rate dependency, and / or

[0079] - based on the viscosity measurements determined at the shear rate values, it is determined whether the fluid is a Newtonian fluid, a shear-thinning fluid, or a

[0080] is a shear-thickening fluid.

[0081] A fourth further development of the method is characterized by the fact that changes in the fluid and / or deviations occurring during the measurement operation are taken into account.

[0082] The viscosity of the fluid is detected according to the applicable specifications by

[0083] - the viscosity measurements determined at the measured shear rate values ​​with

[0084] Reference values, especially with predefined target values, with predefined

[0085] target value ranges or viscosity measurements determined at one or more earlier times at measured shear rate values, and

[0086] - a change and / or deviation is detected if at least one of the

[0087] Viscosity measurements deviate from the corresponding reference value by more than a specified amount.

[0088] Tolerance, in particular a tolerance specified depending on the measurement accuracy of the viscosity measurement or a tolerance specified by the user, deviates.

[0089] The invention and further advantages will now be explained in more detail with reference to the figures in the drawing, in which an exemplary embodiment is shown; identical parts are provided with the same reference numerals in the figures.

[0090] Fig. 1 shows: a device according to the invention;

[0091] Fig. 2 shows: three examples of each of a natural mode of an even wave

[0092] Pipe section corresponding to useful vibration modes;

[0093] Fig. 3 shows: an embodiment of a measuring system;

[0094] Fig. 4 shows: a measuring system with four measuring tubes with curved tube sections; and

[0095] Fig. 5 shows a measuring system with four measuring tubes with straight tube sections. Fig. 1 shows a device according to the invention for measuring the viscosity of fluids. The device comprises a measuring system 1, shown in Fig. 1 only as a functional block, which includes at least one measuring tube filled with a fluid or through which the fluid flows during measurement operation, and which has at least one tube section that can be excited to vibration.

[0096] Furthermore, the devices comprise an excitation unit 3 for exciting at least two useful vibration modes of different frequencies, in each case exciting at least one of the pipe sections to vibrations, in particular to resonant vibrations, of the respective useful vibration mode. For this purpose, the excitation unit 3 is designed such that, during measurement operation, at least one of the intended pipe sections can be excited to vibrations of a first useful vibration mode, and at least one of the intended pipe sections can be excited to vibrations of at least one further useful vibration mode. The individual useful vibration modes preferably correspond to natural vibration modes of the respective pipe section. The specific characteristics depend on the design of the pipe sections to be excited and the shape of the

[0097] The question arises whether the excitation of the individual useful vibration modes and the subsequent evaluation of the resulting vibrations can be carried out simultaneously for the individual useful vibration modes or must be carried out sequentially.

[0098] To carry out the measuring method according to the invention, a measuring system 1 with only a single measuring tube with only a single tube section is sufficient.

[0099] Vibrations of a first and at least one further, differently frequencyed, useful vibration mode can be excited. An example of this is measuring systems with at least one measuring tube, which includes, for example, at least one tube section that can be excited to torsional and / or bending vibrations.

[0100] In this respect, the measuring system 1 can, for example, comprise a straight pipe section 4 fixed at its opposite ends. Such a pipe section 4 can, for example, be excited to natural modes of vibration, such as lateral vibrations. The pipe section 4 is thereby subjected to vibrations along an axis y defined by its longitudinal axis x and an axis y perpendicular to its longitudinal axis x.

[0101] Vibration plane xy is excited, in which it experiences displacements perpendicular to its longitudinal axis x. These natural modes include a fundamental mode shown in Fig. 2 as the first useful mode N1, in which the displacement amplitudes occurring perpendicular to the longitudinal axis x form a single

[0102] They form an antinode. Furthermore, they include a next higher, symmetrical mode, shown in Fig. 2 as the second useful vibration mode N2, and

[0103] Accordingly, this natural mode produces three antinodes. Alternatively or additionally, straight pipe sections 4 can be excited to natural vibrations formed as torsional vibrations around a rotation axis running through the two fixed ends of the pipe section 4.

[0104] Fig. 2 shows, as an example, a third useful vibration mode N3 designed as a pure torsional vibration mode. Alternatively, a measuring system 1 can be used which includes at least one pipe section with a different shape. Examples of this are

[0105] Measuring systems 1, comprising at least one measuring tube having a curved, bent, u-shaped, v-shaped and / or at least one Ω-shaped

[0106] The measuring system 1 comprises a pipe section capable of excitation of vibrations of a predefined useful vibration mode. Alternatively, the measuring system 1 can also comprise two or more measuring pipes connected in parallel or series with each other in terms of flow, at least one of which contains one or more pipes through which flow is series-connected and which can be excited to vibrations.

[0107] has pipe sections, each of which can also have, for example, one of the above shapes.

[0108] Fig. 3 shows, as an embodiment, a measuring system 1 comprising two essentially parallel measuring tubes 5, each of which has a mechanically clamped sensor at its opposite ends, as illustrated.

[0109] In this embodiment, the pipe sections 7 are curved and capable of being excited to vibration. In this embodiment, the useful vibration modes preferably include natural vibration modes of the pipe sections 7, such as a fundamental mode and a higher-order mode of combined torsional-bending vibrations, in which the pipe sections 7 are excited to vibrations around a curve defined by their clamped ends.

[0110] The torsion axis running along the line is excited.

[0111] Fig. 4 shows, as a further embodiment, a measuring system 1 comprising four in

[0112] The flow direction comprises measuring tubes 5 connected parallel to each other, each having a curved tube section 7 mechanically clamped at its opposite ends and capable of being excited to vibration. Fig. 5 further shows

[0113] Exemplary embodiment of a measuring system 1 comprising four measuring tubes connected parallel to each other in the direction of flow, each of which has at least one attached to it

[0114] The measuring systems 1 shown in Figures 4 and 5 are preferably operated such that two of the four measuring tubes form a pair of measuring tubes, the parallel pipe sections 4 and 7 of which are respectively connected to each other.

[0115] Vibrations, preferably antiphase vibrations, are excited by at least one of the useful vibration modes previously described in connection with Fig. 2 for straight pipe sections 4 or in connection with Fig. 3 for curved pipe sections 7. The method is preferably carried out and / or the pipe sections 4 and 7 are preferably designed such that the frequencies of the useful vibration modes of the pipe sections 4 and 7 of the two pairs of measuring tubes are different.

[0116] The excitation of the individual useful vibration modes of the pipe sections 4, 7 of the respective measuring system 1 that can be excited to vibration is preferably effected by one or more drives D of the excitation device 3, each acting on a region of the respective pipe section 4, 7 selected according to the useful vibration mode to be excited. For this purpose, drives D, which are commonly used today in Coriolis mass flow meters or density meters, are particularly suitable for excitation.

[0117] Drives used to excite the desired vibration modes, such as electrostatic, piezoelectric, or magnetic drives D, are employed. The corresponding drives D, as well as their number and arrangement required for exciting the desired vibration modes, are known to those skilled in the art and are therefore not described in detail here. In the embodiment shown in Figures 1 and 3, the excitation device 3 comprises a drive D acting on the center of the arcuate pipe sections 7 in Figure 3, which is preferably designed such that it excites the parallel pipe sections 7 to oscillations out of phase.

[0118] Additionally, the device comprises a measuring device 9, which is configured to determine, for each of the useful vibration modes excited during the measurement operation, a frequency and a damping of the resulting vibration of at least one pipe section 4, 7 excited to vibrations of the respective useful vibration mode. As explained in detail below, there are embodiments of the devices according to the invention in which, in addition to the frequencies of the

[0119] to also detect the amplitude of the resulting vibrations. In this case, the measuring device 9 is designed such that it determines the frequency, amplitude, and damping of the resulting vibrations.

[0120] During measurement operation, the device leads to at least two [missing information] in each measurement cycle.

[0121] Measurements of different frequency modes of useful vibration are performed, in which at least one pipe section 4, 7 is excited to vibrations of the respective useful vibration mode. During the excitation of the respective useful vibration mode, the resulting vibration of the excited pipe section 4 or pipe sections 4, 7 is preferably measured using at least one sensor S of the measuring device 9. Suitable sensors S for this purpose include, for example,

[0122] Electrostatic, piezoelectric, or magnetic sensors, as well as their required number and arrangement in relation to the respective pipe sections 4, 7 for detecting the resulting vibrations, are known from the prior art and are therefore not described in detail here. The sensors S can, for example, be designed and / or arranged in the same way as in Coriolis mass flow meters or density meters known from the prior art. In the embodiment shown in Figures 1 and 3, the measuring device 9 comprises two sensors S arranged on the inlet and outlet sides in the region of the legs of the arc-shaped pipe sections 7 in Figure 3 for the metrological detection of their resulting vibrations. To determine the viscosity of the fluid filling the respective pipe section 4, 7, the sensors S are used to measure the vibrations.The device, in particular its measuring device 9, can be designed, for example, to determine the damping of the vibration resulting from the excitation of the respective useful vibration mode in the manner described in US 2004 / 0255648 A1, based on the vibration quality of the respective pipe section 4, 7, which can be derived, for example, from the width of the resonance peak of the respective useful vibration mode.

[0123] Alternatively, it can be designed in such a way that it adjusts the damping to, for example, a value as specified in EP 1 158 289 B1 or in DE 10 2004 021 690 A1 in conjunction with

[0124] The mass flow meter is used to determine the energy required to maintain the respective useful vibration mode, as described above. This embodiment is shown in Fig. 1. There, the excitation device 3 is preferably controlled via a control device 1 connected to the measuring device 9 and the excitation device 3 such that it adjusts the amplitudes measured by the sensors S of the mass flow meter.

[0125] Excitation of the respective useful vibration mode regulates the resulting vibration of the respective pipe section 4, 7 to a constant value. The resulting vibration is controlled by the system.

[0126] Maintaining the resulting vibration with a constant amplitude requires energy that depends on the damping, which in turn depends on the viscosity of the fluid. Accordingly, a suitably designed [device / system] is used.

[0127] Energy demand measuring device 13 determines a measure for the energy required for this purpose, which corresponds to the viscosity-dependent damping.

[0128] The excitation of the useful vibration modes preferably occurs at a frequency corresponding to the resonance frequency of the respective useful vibration mode. The excitation of the resonance frequency is achieved, for example, at a frequency known from the prior art.

[0129] In this way, the device is preferably designed such that it measures the amplitude of the resulting vibration measured by the sensors S by means of a

[0130] corresponding control of the drive frequency maximized and / or a

[0131] The phase shift between excitation and resulting oscillation is controlled by adjusting the drive frequency to a value corresponding to the resonance.

[0132] According to the invention, the device comprises an evaluation unit 15, which is configured to determine a shear rate value and a viscosity measurement value for each of the useful vibration modes excited during measurement operation, based on calibration data stored in a memory 17. The shear rate value and the viscosity measurement value are determined based on the frequency and damping, and, if necessary, also the amplitude, of the vibration resulting from the excitation of the respective pipe section 4, 7. The shear rate values ​​and the viscosity measurement values ​​are determined based on the calibration data such that the viscosity measurement values ​​correspond to the dynamic viscosity of the fluid at a static shear rate corresponding to the associated shear rate value.

[0133] Excitation of the respective vibration mode causes an oscillatory movement of the excited pipe section 4, 7. This generates shear waves in the fluid flowing through or within the pipe section, which penetrate the fluid to a certain depth h, originating from a pipe wall of the respective pipe section. The penetration depth h is regularly dependent on the frequency of the resulting vibration, as well as the dynamic viscosity and the density of the fluid, and can be determined, for example, according to:

[0134] can be estimated, where η denotes the dynamic viscosity, f the frequency of the resulting vibration, and p the density of the fluid. The viscosity-dependent damping of the vibration resulting from the excitation of the respective useful vibration mode increases with the penetration depth h, which increases with increasing viscosity. Accordingly, viscosities can be measured with devices according to the invention that lie within a range of values ​​in which the penetration depth h is...

[0135] The dependence on viscosity changes. This is the case as long as the penetration depth h is less than a pipe radius of pipe section 4, 7. The pipe sections 4, 7 of the measuring tubes 5 regularly have radii in the centimeter range or

[0136] The penetration depth h is in the decimeter range. Accordingly, even at low frequencies, e.g., frequencies in the range of 100 Hz to 1000 Hz, the penetration depth h of the respective pipe section is significantly less.

[0137] In devices according to the invention, the dimensions of the or the

[0138] Pipe sections 4, 7, and their lengths or partial lengths deflected during excitation of the respective useful vibration mode as a function of the viscous or viscous and elastic properties of the fluids whose shear rate-dependent viscosity is to be measured, preferably dimensioned such that the frequencies and the dampings of the individual useful vibration modes during excitation

[0139] The resulting vibrations exhibit no or only a slight dependence on the amplitude of the resulting vibration of the respective pipe section 4, 7. This offers the advantage that the amplitude of the resulting vibrations is neither measured during operation nor affected when determining the shear rate values ​​and the

[0140] Viscosity measurements must be taken into account. Where this is not possible, the amplitudes of the resulting vibrations must be measured using measuring device 9 and used in determining the shear rate values ​​and the

[0141] Viscosity measurements are taken into account based on calibration data that reflect the existing amplitude dependence of the measurements.

[0142] The calibration data is preferably stored in a preceding...

[0143] Calibration methods are determined. According to a first variant, reference measurements are taken for each of the individual useful vibration modes using Newtonian reference fluids of various known dynamic viscosities and non-Newtonian reference fluids.

[0144] Reference fluids with a viscosity dependent on the shear rate, as is known, are used. During these reference measurements, the individual useful vibration modes are excited while the respective pipe section 4, 7 is filled with the reference fluid or while the reference fluid flows through the respective pipe section 4, 7. For each of the useful vibrations, the frequency and damping, and if necessary also the amplitude of the resulting vibration, are measured using the measuring device 9. The calibration data are then derived from these measurements and stored in the memory 17 assigned to the evaluation unit 15.

[0145] Devices according to the invention allow for the dynamic viscosities of in

[0146] The damping is determined for fluids exhibiting essentially exclusively viscous properties, such as oils. For these fluids, the measured damping represents a direct measure of the dynamic viscosity. For this application, the reference measurements are preferably performed with fluids exhibiting essentially exclusively viscous properties.

[0147] The measurements were performed using reference fluids exhibiting specific properties. The reference measurements performed with these Newtonian reference fluids yield calibration data that reflect the dependence of the measured damping on the dynamic viscosity of the fluid. Based on this calibration data, the corresponding viscosity values ​​can be assigned to the damping values ​​measured during the measurement process.

[0148] Furthermore, the reference measurements performed with the non-Newtonian reference fluids yield calibration data from which a shear rate value can be assigned to each of the measured frequencies or frequencies and amplitudes, corresponding to the shear rate at which the reference fluid degrades the shear rate associated with the respective frequency.

[0149] Viscosity measurement value exhibits corresponding dynamic viscosity.

[0150] Furthermore, dynamic devices according to the invention can also be used.

[0151] Viscosities of fluids are determined that, in addition to purely viscous properties, also exhibit elastic properties. In these fluids, damping comprises both a component dependent on the dynamic viscosity and a component attributable to energy losses caused by the fluid's elastic properties. Even in Newtonian fluids, damping can sometimes depend on the frequency or on both the frequency and amplitude. This dependence will be stronger the greater the degree of energy loss due to the elastic properties.

[0152] The damping component is attributable to the properties of the fluid. Conversely, in these fluids, the shear rate value associated with a viscosity measurement can also depend on the dynamic viscosity of the fluid and / or the ratio of its viscous to elastic properties. This is taken into account when recording the

[0153] Calibration data is preferably taken into account by performing reference measurements with reference fluids that have elastic properties of varying degrees relative to their viscous properties. Based on the reference measurements performed with these reference fluids, the

[0154] Dependencies of the damping values ​​measured during operation on the dynamic viscosity and the frequency or the frequency and the amplitude, as well as the dependence of the shear rate values ​​on the measured damping and the frequency or the frequency and the amplitude as a function of the ratio of viscous and elastic

[0155] Determine the properties of the reference fluids. Alternatively or additionally, the calibration data for this application preferably include data obtained using

[0156] Reference measurements were determined using at least one reference fluid that is identical to the fluid whose shear rate-dependent viscosity is to be determined with the device according to the invention, or is as similar as possible to it, especially with regard to its viscoelastic properties.

[0157] In devices according to the invention, where the penetration depth h of the shear wave is significantly smaller, preferably at least ten times smaller, than the tube radius of the respective tube section 4, 7, it can be approximately assumed that shear forces develop in the fluid to which the rule established by W.P. Cox and E.H. Merz for oscillation viscometers is applicable, at least approximately. This rule states that the magnitude of a complex viscosity |η measured with an oscillation viscometer at a specific oscillation frequency ω is *| of a fluid the magnitude of the dynamic viscosity |η| of the fluid at a static shear rate f corresponding to the oscillation frequency ω s This corresponds to the complex viscosity η measured in oscillation viscometers. * It consists of a contribution attributable to the elastic properties and a contribution attributable to the viscous properties of the fluid.

[0158] When this rule is applied to devices according to the invention, the damping values ​​measured with devices according to the invention are each interpreted as a measured quantity corresponding to the complex viscosity measured in an oscillation viscometer and are used as a measure of the dynamic viscosity of the fluid at a static shear rate corresponding to the frequency of the resulting oscillation.

[0159] In this respect, the evaluation device 15 is preferably designed such that it uses the damping of the resulting vibration of the respective pipe section 4, 7, determined during the excitation of the respective useful vibration mode, to determine the

[0160] Viscosity measurement value and the corresponding shear rate value is determined based on the frequency of the resulting vibration of the respective pipe section 4, 7.

[0161] In these devices, the rule derived by W.P. Cox and E.H. Merz for oscillation viscometers is preferably applied as described above when deriving the calibration data. In this case, the calibration data for each

[0162] For each useful vibration mode, data are assigned to the damping values ​​of the resulting vibration of the respective pipe section 4, 7 determined during excitation of the respective useful vibration mode, along with the corresponding viscosity measurements and the shear rate values ​​to the frequencies of the resulting vibration of the respective pipe section 4, 7. The calibration data can be determined, for example, in a significantly simplified calibration procedure in which only the dependencies of the

[0163] Damping values ​​are determined from the associated viscosity measurements, and the shear rate values ​​are essentially equal to 2π times the frequency of the

[0164] Resulting vibrations are set. For this purpose, for example, only...

[0165] Reference measurements with Newtonian reference fluids with known,

[0166] Measurements can be performed using shear rate-independent viscosity. Reference measurements with

[0167] Reference fluids with viscosity that is known to depend on the shear rate are not required in this case.

[0168] By applying the rule of WP Cox and EH Merz, acceptable measurement accuracies can be achieved for fluids with predominantly viscous properties, especially when the device is designed in such a way that the damping and frequencies of the vibrations resulting from the excitation of the respective useful vibration modes are essentially independent of the amplitude of the resulting vibrations.

[0169] Devices according to the invention are preferably designed by appropriately dimensioning the dimensions of the pipe sections, in particular their cross-sectional areas, their wall thicknesses and their lengths or partial lengths deflected to excite the individual useful vibration modes, such that the frequencies of the useful vibration modes relevant for the measured shear rate values ​​cover a frequency range that makes it possible to measure dynamic viscosities in a shear rate range in which viscosities of non-Newtonian fluids typically change depending on the shear rate.

[0170] This frequency range can be estimated at least approximately using the above-mentioned application of the rule by W.P. Cox and E.H. Merz. In order to be able to measure dynamic viscosities in a shear rate range of less than or equal to 10,000 1 / s with devices according to the invention, vibration modes with frequencies below 1600 Hz are preferably excited. Preferably,

[0171] Excited modes of useful vibration whose frequencies are less than or equal to 1000 Hz,

[0172] preferably even less than or equal to 800 Hz. For example, based on...

[0173] The useful vibration modes, whose frequencies lie in a frequency range of 100 Hz to 800 Hz, are measured with dynamic viscosities in a shear rate range on the order of 600 1 / s to 5000 1 / s.

[0174] The devices according to the invention offer the advantage that, based on the viscosity measurements they determine, it can be established whether the fluid exhibits a shear rate-dependent viscosity. Furthermore, any shear rate dependence of the viscosity that may be present can be quantitatively determined and / or evaluated. The associated additional information can be determined by the operator of the device based on the viscosity measurements and the corresponding shear rate values. Alternatively, this information can also be determined by the device and made available via an output unit 19. For this purpose, the evaluation unit 15 is preferably designed such that, based on the viscosity measurements determined at least two different shear rate values, it recognizes whether the fluid exhibits a shear rate-dependent viscosity.For this purpose, it is sufficient to check whether at least two viscosity measurements taken at different shear rate values ​​differ from each other by more than a specified minimum deviation.

[0175] Alternatively or additionally, the evaluation unit 15 is preferably designed such that it can be used to determine the values ​​measured at least two different times.

[0176] A viscosity profile representing the viscosity of the fluid as a function of the shear rate is created using viscosity measurements determined by shear rate values. If a sufficient number of viscosity measurements are available at sufficiently different shear rate values, the profile can be created directly from the viscosity measurements and their corresponding shear rate values. Alternatively or additionally, the profile can be determined using a mathematical function that describes the functional dependence of the viscosity of non-Newtonian fluids on the shear rate. In this case, just two viscosity measurements taken at different shear rates are sufficient to create a meaningful profile from which the shear rate dependence of the fluid's viscosity can be extrapolated even for shear rate ranges where no viscosity measurements have been determined.

[0177] Alternatively or additionally, the evaluation unit 15 is preferably designed such that it can be determined based on the values ​​determined at the different shear rate values.

[0178] Viscosity measurements or the profile derived from them determine whether the fluid is a Newtonian fluid whose viscosity is essentially independent of the shear rate, or a shear-thinning fluid whose viscosity decreases with increasing shear rate, or a shear-thickening fluid whose viscosity increases with increasing shear rate.

[0179] Furthermore, the measurements taken with the device according to the invention can be

[0180] Viscosity measurements can also be used during ongoing operations.

[0181] Changes in the viscosity of the fluid occurring during measurement operation and / or

[0182] to detect deviations in the fluid's viscosity from the applicable specifications. Just like the previously described determination of additional information, the operator of the device can also detect these changes and / or deviations using the viscosity measurements. Alternatively, this detection can also be performed by a detection device 21 connected to or integrated into the device and displayed accordingly or output in another way. The function of the detection device 21 can be executed, for example, by a PC equipped with appropriate software, a microprocessor, or similar device. Alternatively, the detection device 21 can be designed as part of the evaluation unit 15, as shown here.

[0183] Detection is preferably carried out by the device performing viscosity measurements at successive times, in each case including at least two

[0184] Viscosity measurements and the associated shear rate values ​​are determined. Subsequently, the values ​​determined for the shear rates are analyzed.

[0185] Viscosity measurements were compared with reference values ​​stored in one of the memory locations 23 assigned to the detection unit 21.

[0186] The reference values ​​can, for example, include predefined target values ​​or target value ranges for the fluid, specifying the viscosity the fluid should exhibit at certain shear rates. In this case, the detection device 21 detects a deviation from the specifications defined by the target values ​​if at least one of the

[0187] Viscosity measurements deviate from the corresponding target value or target value range by more than a predefined tolerance. The predefined tolerance can be, for example, a tolerance determined by the measurement accuracy of the viscosity measurements or a tolerance defined in another way, such as a user-defined tolerance. Alternatively or additionally, the comparison can, of course, also be based on a comparison using the measured

[0188] Viscosity measurements and the associated shear rate values ​​are used to create a viscosity profile. This allows for the detection of deviations even when the target values ​​or ranges are only known for shear rates or shear rate ranges that differ from the shear rate values ​​at which the viscosity measurements were determined.

[0189] Alternatively or additionally, the reference values ​​can be used during one or more measurements previously performed by the device at measured times.

[0190] Shear rate values ​​include certain viscosity measurements. In this case, the detection device 21 detects a change in the viscosity of the fluid over time if at least one of the current viscosity measurements deviates from the corresponding reference value measured at an earlier time and stored in memory 23 by more than a tolerance dependent on the measurement accuracy of the viscosity measurements or a tolerance specified by the user.

[0191] The additional information as well as the detection of deviations and / or

[0192] Changes in the viscosity of the fluid are particularly advantageous when the

[0193] The device according to the invention is also used to measure other measured quantities, such as mass flow and / or density, and / or to use the viscosity measurements determined by it to compensate for viscosity-dependent

[0194] Measurement errors of density and / or mass flow measurements can be used.

[0195] If a pipe section 4, 7 of a device according to the invention, through which fluid flows, is excited to oscillate by means of the excitation device 3, Coriolis forces arise in the pipe section 4, 7 through which fluid flows, and these forces affect the resulting vibration mode of the pipe section 4, 7. In the embodiments shown here, this leads to the vibrations detected by the sensors S provided on the inlet and outlet sides of the respective pipe section 4, 7 exhibiting frequencies corresponding to the resulting vibration mode, which are phase-shifted relative to each other by a phase shift that depends on the mass flow rate. This phase shift can thus be determined, for example, by means of a measuring unit 25 connected to the corresponding sensors S, which can then use the measured phase shift to determine the

[0196] Mass flow rate is determined. Alternatively or additionally, the device can be used for density measurement. In this case, at least one pipe section 4, 7 of the measuring system 1 is excited to oscillate at a resonance frequency by means of the excitation device 3 and the control device 1. The density-dependent resonance frequency is determined by means of the vibration sensors S connected to the system.

[0197] The connected measuring unit 25 determines the density of the fluid and its density is calculated from this. If the measuring device is also operated as a density measuring device, this offers the advantage that the viscosity measurements corresponding to the dynamic viscosities can be derived from the measured density in the associated kinematic viscosity.

[0198] Corresponding viscosity measurements can be converted. At the same time, density and viscosity measurements can be used to correct density-dependent measurement errors in viscosity measurements and / or viscosity-dependent measurement errors in measured densities. Reference symbol list

[0199] Measuring system 15 Evaluation unit Excitation device 17 Storage

[0200] Pipe section output unit

[0201] Measuring tube 21 Detection device Pipe section Storage

[0202] Measuring device 25 Measuring unit Control device

[0203] Energy demand measuring device

Claims

Patent claims 1. Device for measuring the viscosity of fluids, with - a measuring system (1 ) with at least one measuring tube (5) filled with or through which fluid flows during measuring operation, which has at least one tube section (4, 7) that can be excited to vibrations, - an excitation device (1 ) for exciting at least two useful vibration modes of different frequencies, in each case at least one of the pipe sections (4, 7) is caused to vibrate, in particular to resonant vibrations, of the respective The useful vibration mode is excited. - a measuring device (3) which is designed in such a way that it determines for each of the useful vibration modes excited in the measuring operation a frequency and a damping, in particular a frequency, an amplitude and a damping, of the resulting vibration of at least one pipe section (4, 7) excited to vibrations of the respective useful vibration mode, and - an evaluation device (15) which is designed in such a way that, on the basis of calibration data stored in a memory (17), it determines a shear rate value and a viscosity measurement value for each of the useful vibration modes excited during the measurement operation, based on the frequency and damping determined during their excitation, in particular the frequency, amplitude and damping, of the resulting vibration, wherein the viscosity measurement value corresponds to the dynamic viscosity of the fluid at a static shear rate corresponding to the shear rate value.

2. Device according to claim 1, wherein the evaluation device (15) is configured such that it determines the value based on at least two values ​​determined at different shear rate values. viscosity measurements - a viscosity profile was created that represents the viscosity of the fluid as a function of the shear rate, - detects whether the viscosity of the fluid exhibits a shear rate dependency, and / or - based on the values ​​determined at the different shear rate values Viscosity measurements determine whether the fluid is a Newtonian fluid, whose viscosity is essentially independent of shear rate, or a a shear-thinning fluid whose viscosity decreases with increasing shear rate, or a shear-thickening fluid whose viscosity increases with increasing shear rate.

3. Device according to claims 1 to 2, comprising a detection device (21) for detecting changes in the fluid occurring during the measurement operation and / or Deviations in the viscosity of the fluid from the applicable specifications, which is such that - they determine the viscosity measurements at the measured shear rate values ​​or viscosity measurements determined on the basis of the viscosity profile with reference values, in particular predetermined setpoint values ​​or predetermined setpoint ranges stored in a memory (23) and / or predetermined setpoint ranges stored in a memory (23) at one or more earlier times at shear rate values ​​determined by the device viscosity measurements, compares, and - a change and / or deviation is detected if at least one of the Viscosity measurements deviate from the corresponding reference value by more than a specified tolerance, in particular a tolerance specified depending on the measurement accuracy of the viscosity measurement or a tolerance specified by the user.

4. Device according to claims 1 to 3, wherein - the excitation device (3) comprises a control device (1 1 ) which regulates the amplitudes of the vibrations resulting from the excitation of the individual useful vibration modes to a constant value, and - the measuring device (3) is designed in such a way that it determines the damping on the basis of a measure determined by means of an energy demand measuring device (13) for an energy demand required to maintain the resulting vibrations of constant amplitude when exciting the respective useful vibration mode.

5. Device according to claims 1 to 4, wherein - the calibration data for each useful vibration mode in a preceding Calibration procedures include data from which a shear rate value and a viscosity measurement value are assigned to the dampings and frequencies measured during the measurement operation when the useful vibration mode is excited, in particular to the measured dampings, frequencies and amplitudes, or - the calibration data for each useful vibration mode in a preceding The calibration procedure includes data from which a viscosity measurement value is assigned to the damping measured during the measurement operation when the useful vibration mode is excited, and a shear rate value is assigned to each of the measured frequencies, which is essentially equal to 2π times the frequency of the resulting vibration of the respective pipe section (4, 7).

6. Device according to claims 1 to 5, with a measuring device (9) connected to the measuring device connected measuring unit (25), which is configured such that, during measurement operation, it determines a mass flow rate and / or a density of the fluid based on the vibration resulting from the excitation of at least one of the useful vibration modes.

7. Device according to claims 1 to 6, wherein - the useful vibration modes exhibit frequencies of less than or equal to 1600 Hz, in particular less than or equal to 1000 kHz, in particular less than or equal to 800 Hz, in particular from 100 Hz to 800 Hz, and / or - the pipe section (4) or pipe sections (7) that can be excited to useful vibrations each have a pipe radius that is greater than a penetration depth (h) up to which vibrations of the respective useful vibration mode are formed in the fluid when the respective pipe section (4, 7) is excited to vibrations Shear waves originating from a pipe wall of the respective pipe section (4, 7) penetrate into the fluid.

8. Device according to claims 1 to 7, wherein - the evaluation device (15) which is designed to measure the shear rate value and the viscosity measurement value for the individual components excited during the measurement operation The useful vibration modes are determined based on the frequency, amplitude, and damping of the resulting vibration determined during their excitation, or - a pipe radius and a frequency determined during the excitation of the respective useful vibration mode. The deflected length or partial length of each pipe section (4, 7) is dimensioned such that the frequencies and dampings of the vibrations resulting from the excitation of the individual useful vibration modes exhibit no or only a slight dependence on the amplitude of the resulting vibration, and the evaluation device (15) is designed such that it determines the shear rate value and the Viscosity measurement value for the individual components excited during measurement operation The useful vibration modes are determined based on the frequency determined during their excitation and the damping of the resulting vibration.

9. Device according to claims 1 to 7, wherein - the fluid has a complex viscosity comprising a contribution attributable to elastic and a contribution attributable to viscous properties, and the contribution attributable to viscous properties is large compared to the contribution attributable to elastic properties, wherein the complex viscosity exhibits, in particular, essentially exclusively elastic properties, - the pipe section (4) or pipe sections (4, 7) that can be excited to useful vibrations each have a pipe radius that is greater than or equal to ten times the penetration depth (h) up to which shear waves form in the fluid when the respective pipe section (4, 7) is excited to vibrations of the respective useful vibration mode, originating from a pipe wall of the respective pipe section (4, 7) penetrate into the fluid, and - the evaluation unit (15) is designed such that it uses the data collected during the Excitation of the respective useful vibration mode, specific damping of the resulting vibration of the respective pipe section (4, 7) the viscosity measurement value and the corresponding measured shear rate is determined based on the frequency of the resulting vibration of the respective pipe section (4, 7).

10. Method for measuring the viscosity of a fluid, in which measurements are performed for at least two useful vibration modes of different frequencies, wherein - at least one pipe section (4, 7) of at least one measuring tube (5) of a measuring system (1) filled with a fluid and / or through which the fluid flows, is excited to vibrations, in particular to resonance vibrations, of the respective useful vibration mode by means of an excitation device (3), - a frequency and an attenuation, in particular a frequency, an amplitude and a damping, the resulting vibration of at least one pipe section (4, 7) excited to vibrations of the respective useful vibration mode is determined, and - based on calibration data determined by a calibration procedure and the frequency determined during the excitation of the respective useful vibration mode and the Damping, in particular of the frequency, amplitude and damping, of the resulting vibration, a shear rate value and a viscosity measurement are determined, wherein the viscosity measurement is the dynamic viscosity of the fluid at a given temperature. Shear rate value corresponds to the static shear rate. 1 1 . Method according to claim 10, wherein - the useful vibration modes exhibit frequencies of less than or equal to 1600 Hz, in particular less than or equal to 1000 kHz, in particular less than or equal to 800 Hz, in particular from 100 Hz to 800 Hz, and / or - the pipe section (4) or pipe sections (4, 7) that can be excited to useful vibrations each have a pipe radius that is greater than a The penetration depth (h) is the depth to which shear waves, forming in the fluid during excitation of the respective pipe section (4, 7) to the respective useful vibration, penetrate into the fluid from a pipe wall of the respective pipe section (4, 7).

12. Method according to claims 10 to 11, wherein - the fluid has a complex viscosity comprising a contribution attributable to elastic and a contribution attributable to viscous properties, and the contribution attributable to viscous properties is large compared to the contribution attributable to elastic properties, wherein the complex viscosity exhibits, in particular, essentially exclusively elastic properties, - the pipe section (4) or pipe sections (4, 7) that can be excited to useful vibrations each have a pipe radius that is greater than or equal to ten times a penetration depth (h) up to which the respective useful vibration develops in the fluid when the respective pipe section (4, 7) is excited to the respective useful vibration Shear waves originating from a pipe wall of the respective pipe section (4, 7) penetrate into the fluid, and - the viscosity measurement value is determined based on the damping of the resulting vibration of the respective pipe section (4, 7) determined during the excitation of the respective useful vibration mode and the corresponding shear rate value is determined based on the frequency of the resulting vibration of the respective pipe section (4, 7).

13. Method according to claims 10 to 12, wherein - in the calibration procedure for each of the individual useful vibration modes Reference measurements are performed, in each of which the respective The useful vibration mode is excited while the corresponding pipe section (4, 7) is filled with and / or through which a reference fluid flows, and the damping and frequency, in particular the damping, frequency and amplitude, of the vibration resulting from the excitation of the respective useful vibration mode are measured. - where the reference measurements are performed using Newtonian reference fluids of various types Reference measurements performed with known dynamic viscosity include, and - wherein each of the measured frequencies is assigned a shear rate value which is essentially equal to 2π times the frequency of the resulting vibration of the respective pipe section (4, 7) and / or the reference measurements with non-Newtonian reference fluids with viscosity dependent on the shear rate carried out in a known manner, - wherein the reference measurements include, in particular, reference measurements performed with reference fluids having elastic properties of varying degrees relative to their viscous properties, and - calibration data are determined for each useful vibration mode, which are used to determine the During measurement operation, when the respective useful vibration mode is excited, the measured dampings and frequencies, in particular the measured dampings, frequencies and amplitudes, are assigned a shear rate value and a viscosity measurement value.

14. Method according to claims 10 to 13, wherein viscosity measurements determined at different shear rate values ​​are used to determine the method. - a viscosity profile is created that represents the viscosity of the fluid as a function of the shear rate, - it is determined whether the viscosity of the fluid exhibits a shear rate dependence, and / or - based on the viscosity measurements determined at the shear rate values, it is determined whether the fluid is a Newtonian fluid, a shear-thinning fluid, or a is a shear-thickening fluid.

15. Method according to claims 10 to 14, wherein changes in the fluid and / or deviations in the viscosity of the fluid from applicable specifications occurring during the measurement operation are detected by - the viscosity measurements determined at the measured shear rate values ​​with Reference values, especially with predefined target values, with predefined target value ranges or viscosity measurements determined at one or more earlier times at measured shear rate values, and - a change and / or deviation is detected if at least one of the viscosity measurements deviates from the corresponding reference value by more than a predetermined amount Tolerance, in particular a tolerance specified depending on the measurement accuracy of the viscosity measurement or a tolerance specified by the user, deviates.