Method and measuring device for determining a viscosity measurement value and method and measuring arrangement for determining a flow measurement value

Description

[0001] The present invention relates to a method and a measuring device for determining a viscosity measurement value as well as a method and a measuring arrangement for determining a flow measurement value.

[0002] It is generally known to determine the viscosity of a medium conveyed in an oscillating measuring tube based on the damping of the measuring tube vibrations, for example, based on the ratio of excitation current to the resulting oscillation amplitude at the resonance frequency of a vibration mode excited by the exciter. The media-specific damping component must then be identified in order to determine the viscosity on this basis. Viscosity measurement using vibration damping of measuring tubes is described, among others, in US 7,059,176 B2, DE 100 20 606 A1, and DE 10 2017 116 515 A1. US 2005 / 0229719 A1, WO 2018 / 208301 A1, WO 2020 / 108910 A1, and WO 2005 / 095901 disclose other conventional measurement methods.

[0003] For viscosities above 100 cP, the procedure outlined leads to satisfactory results. However, for lower viscosities, significant deviations from the reference values for the viscosity of the medium can be observed between the measured values determined in this way. This is also unsatisfactory insofar as the viscosity measured values are used for Reynolds number correction in flow measurement, and it would be advantageous to determine valid viscosity measured values based on the vibration damping of a measuring tube, which is also used for flow measurement, even for viscosities below 100 cP, especially below 10 cP, which would enable an accurate Reynolds number correction. The object of the present invention is to remedy this situation.

[0004] The method according to the invention for determining a viscosity measurement value of a medium guided in an oscillating measuring tube comprises: exciting at least one oscillation mode of the measuring tube; determining a natural frequency of the oscillation mode; determining the density of the medium based on the natural frequency; determining the damping of the oscillation mode; determining the viscosity measurement value as a function of the density, the natural frequency, and the damping of the oscillation mode, wherein, according to the invention, the viscosity measurement value is determined as a function of a specification of the type of medium using a model corresponding to the specification.

[0005] According to the invention, when determining the viscosity based on a specification of the medium, the type of medium is taken into account. If necessary, the chemical composition of the medium can be used to specify it, whereby the model corresponding to the specification for determining the viscosity measurement value is developed based on media of the same or similar chemical composition.

[0006] According to the invention, the identification comprises assigning the medium to a substance group, with the model comprising a substance group-specific model. This refinement simplifies the method compared to embodiments with a model development for a plurality of chemical compositions.

[0007] According to the invention, the viscosity measurement values determined with a first model for a first substance group are at least 20%, in particular at least 40%, greater than the viscosity measurement values determined with a second model for a second substance group if the viscosity measurement values determined with the second model do not exceed 10 cP.

[0008] For example, the first and second models can have an identical transfer function that differs between the models only in its coefficients, but fundamentally different transfer functions can also be used.

[0009] According to a further development of the invention, the first model and the second model each have a transfer function for determining the viscosity measurement value as a function of the damping and the natural frequency of the vibration mode as well as the density of the medium, wherein the first and the second transfer function each have a basic damping term, wherein the determined damping is to be corrected with the basic damping term in order to determine a media-specific damping value, wherein a first basic damping term for the first transfer function differs from a second basic damping term for the second transfer function.

[0010] The basic damping term describes the damping of the raw measurement vibrations for viscosities approaching zero. For a sensor type or for an individual sensor, a substance-group-specific basic damping value is determined for each of the transfer functions using a sufficiently low-viscosity medium from the substance group. According to the prior art, a basic damping value was determined independently of the substance group by extrapolating viscosity-dependent damping values to low viscosities. Investigations related to the present invention have shown that this approach is not effective for viscosities below approximately 10 cP. Therefore, the invention proposes determining substance-specific or substance-group-specific basic damping values.

[0011] According to a further development of the invention, the first transfer function for the first model has a first sensitivity factor A 11 which differs from a second sensitivity factor A 12 of the second transfer function for the second model, wherein the determined viscosity measurement value is in each case proportional to the sensitivity factor.

[0012] According to a further development of the invention, the determined viscosity measurement value depends on the square of the media-specific damping value and is in particular proportional to the square of the media-specific damping value.

[0013] According to a further development of the invention, the basic attenuation term comprises a product of a basic attenuation factor A 0i and a density correction term, wherein the basic attenuation factor A 0i of the first transfer function deviates from the basic attenuation factor of the second transfer function.

[0014] According to a further development of the invention, the density correction term has a function of a deviation of the determined density of the medium from a reference density.

[0015] According to a further development of the invention, the density correction term is identical for the first model and the second model.

[0016] According to a further development of the invention, the substance group-specific viscosity measurement values determined with the transfer functions for viscosities between 90 cP and 110 cP do not deviate by more than 10% from control viscosity measurement values determined with the following substance group-specific control transfer functions, wherein the substance group-specific viscosity measurement values determined with the transfer functions for viscosities between 1 cP and 90 cP do not deviate by more than 50% from control viscosity measurement values determined with the following substance group-specific control transfer functions: η = A 1 i f 3 ρ D − A 0 i 1 − A 2 i ρ − ρ ref 2 where A 0i is the substance group-dependent basic damping factor that differs between the first and second control transfer function, where A 1i is the substance group-dependent sensitivity factor that differs between the first control transfer function and the second control transfer function, where A 2i is a weighting factor to model an influence of density on the damping, where ρ is the determined density of the medium and ρ ref is a reference density, where D is the determined damping, and where f denotes the frequency of the excited bending vibration mode.

[0017] In a further development of the invention, ρ ref is the same for both control transfer functions.

[0018] In a further development of the invention, A 2i is the same for both control transfer functions.

[0019] According to a further development of the invention, the reference density ρ ref is the same for both transfer functions, wherein in particular the coefficient A 2i of the density correction term is the same for both transfer functions.

[0020] The above control functions are obviously suitable transfer functions for calculating the desired viscosity measurement values based on the specified input variables, as they represent the physical situation in a comprehensible manner. Nevertheless, it is also possible to approximate the desired viscosity measurement values in the specified viscosity range using other functions with the specified accuracy. Other formal mathematical descriptions of the substance-specific models according to the invention can therefore also be used to implement the invention.

[0021] The measuring device according to the invention for determining a viscosity measurement value of a medium comprises: a measuring sensor with an oscillating measuring tube for guiding the medium, an exciter for exciting at least one oscillation mode of the measuring tube, and a oscillation sensor for detecting the measuring tube oscillations; and a measuring and operating circuit with a computing unit for driving the exciter and for detecting signals from the oscillation sensor; wherein the measuring and operating circuit is configured according to the invention to carry out the method according to the invention with the measuring device.

[0022] The inventive method for determining a flow measurement value of a flowing medium comprises: determining a preliminary flow measurement value; determining a viscosity measurement value of the medium by means of the inventive method for determining a viscosity measurement value; determining a Reynolds number of the medium based on the preliminary flow measurement value, the density measurement value, and the viscosity measurement value; and determining a flow measurement value corrected with the Reynolds number based on the preliminary flow measurement value and the Reynolds number.

[0023] The measuring point according to the invention for determining a flow measurement value of a flowing medium by means of the method according to the invention for determining a flow measurement value comprises: a flow measuring device for determining a preliminary flow measurement value and a measuring device according to claim 12, which are functionally connected to one another in order to exchange the necessary data for carrying out the method.

[0024] The Coriolis mass flowmeter according to the invention comprises: at least one measuring tube for conveying a medium; at least one exciter for exciting at least one vibration mode of the measuring tube; at least two vibration sensors for detecting measuring tube vibrations and a flow-dependent deformation of a vibration mode; a measuring and operating circuit with a computing unit configured to carry out the inventive method for determining a viscosity measurement value and a flow measurement value using the Coriolis mass flowmeter.

[0025] The invention will now be explained with reference to the exemplary embodiments illustrated in the drawings. It shows: Fig. 1 : Deviations of state-of-the-art viscosity measurements from reference data for different media; Fig. 2 : a flowchart for an embodiment of the method according to the invention; and Fig. 3 : a schematic representation of an embodiment of a measuring point according to the invention.

[0026] In Fig. 1 Relative deviations from state-of-the-art viscosity measurements for various media are plotted against the respective reference viscosity values. The viscosity measurements were determined based on the vibration damping of a measuring tube using a media-independent model according to: η = A 1 f 3 ρ D − A 0 1 − A 2 ρ − ρ ref 2

[0027] In the above equation, the media-independent coefficients A 1 , A 2 and A 0 describe a device-specific sensitivity factor, a coefficient for a density correction term and a basic damping, where ρ and ρ ref denote a density value of the respective medium and a reference density value, in particular of 1000 kg / m 3<. The solid lines indicate relative deviations of ± 10 % from the respective reference value of the viscosity, including ± 1 cP. The basic damping A 0 is determined using a regression calculation based on the observed damping values and the reference viscosities. With decreasing viscosity, it can be seen that the media-independent basic damping is too large for viscosities below 10 cP for some media and too small for others.

[0028] According to the invention, it is therefore proposed to take the type of medium into account when determining a viscosity measurement value, for example by creating a specific model for the medium or media type that describes the relationship between viscosity and damping, or by assigning the medium or media type to a group of substances for which a model already exists that describes the relationship between viscosity and damping with sufficient accuracy. For a media type, for example, media-type-specific coefficients A 1m , A 2m and A 0m can be determined for the above equation, which are then used to determine the viscosity, particularly for viscosities below approximately 10 cP. In the present context, a media type is understood to mean, for example, a binary or possibly ternary mixture, whereby the viscosity depends on the mixing ratio of the mixture components.For example, with a mixture of water and glycerin, viscosities ranging from less than 1 cP to 1000 cP can be represented, for which damping values can be determined. Based on the damping values for the mixtures and the known viscosity reference values for the mixtures, a model specific to the water-glycerin media type can then be developed, for example, by determining appropriate coefficients A 1m , A 2m , and A 0m .

[0029] Another example of a media type is a substance whose viscosity can be adjusted via molecular weight. This is the case, for example, with silicone oils, for which viscosity values from less than 10 cP to over 10,000 cP can be achieved. The procedure for setting up a media-type-specific model for silicone oils is similar to the previously described procedure for mixtures.

[0030] The diagram in Fig 1 It can also be seen that for viscosities below 10 cP the relative deviations of the viscosity measurements from the reference values for viscosity can be summarized in two groups of substances, as indicated by the ellipses in Fig. 1 is indicated. The dashed ellipse groups together into a first substance group those media or media types for which the viscosity measurement value based on the damping with the model according to equation I that is independent of the media type is too large. For this first substance group, deviations can be significantly reduced with a first substance group-specific model that uses a larger, first substance group-specific basic damping value A 01. Furthermore, the solid ellipse groups together into a second substance group those media or media types for which the viscosity measurement value based on the damping with the model according to equation I that is independent of the media type is too small. For this second substance group, deviations can be significantly reduced with a second substance group-specific model that uses a smaller, second substance group-specific basic damping value A 02.The precise substance-group-specific damping values can be determined based on the observed damping values for the media in a substance group, for example, using regression calculations. If a media type that has not yet been classified needs to be analyzed after the models have been established, it can, in the simplest case, be assigned to one of the substance groups by performing a few damping measurements at low viscosities, especially below 10 cP. Determining viscosity measurements based on the damping of a measuring tube oscillation using a substance-group-specific model leads to significantly improved accuracy compared to the state of the art.

[0031] In the measurement operation, therefore, substance group-specific models according to the invention or their transfer functions, for example in the form of η = A 1 i f 3 ρ D − A 0 i 1 − A 2 i ρ − ρ ref 2 used to calculate the viscosity measurements.

[0032] The first group of substances includes, for example, the following media or media types: water-glycerol mixtures, dextrose and polytungstate-water solutions

[0033] The second group of substances includes, for example, the following media or media types: ethanol-water mixtures, acetone-isooctane mixtures and silicone oils.

[0034] To implement the method according to the invention, at least one media type-specific model or at least one substance group-specific model is provided to a computing unit of a measuring and operating circuit, as well as information as to which model is to be used, which can be done, for example, by entering the media type, wherein the computing unit is configured to select a suitable model on the basis of stored assignments.

[0035] The implementation of an embodiment 100 of the method according to the invention will now be described with reference to Fig. 2 explained. The method can be carried out, for example, with a Coriolis mass flowmeter and begins with the acquisition of the input variables required for viscosity determination. For this purpose, in a first step 110, at least one vibration mode of the measuring tube or measuring tube pair of the sensor is excited; this can be, for example, a bending vibration mode or a torsional mode of a straight measuring tube. In a second step 120, a natural frequency of the vibration mode is determined. Based on the natural frequency, the density of the medium is determined in a next step 130.

[0036] The temperatures of a support tube and the measuring tube of the measuring device are also taken into account in determining the density in order to calculate the temperature-dependent elastic modulus and to take thermally induced mechanical stresses into account.

[0037] In a further step 140, the damping of the oscillation mode is determined. This damping is determined, for example, from the ratio of the excitation current required to maintain the oscillation in the oscillation mode and the resulting oscillation amplitude, or from the decay behavior of the oscillation amplitude after the excitation power is switched off.

[0038] The determination of the damping also takes into account the temperatures and, if applicable, the support tube and the measuring tube of the measuring device in order to compensate for the temperature dependence of the efficiency of the exciter and the sensitivity of the sensors.

[0039] In a next step 150, the media-specific damping is determined. A base damping value is subtracted from the determined damping, whereby the base damping value is media-type-specific or substance-group-specific. Finally, in a step 160, the viscosity measurement value is determined. In particular, steps 150 for determining the media-specific damping and 160 for determining the viscosity measurement value can be integrated into the evaluation of a transfer function, for example, the transfer function according to equation II. In this way, the relative measurement error in viscosity measurement is significantly reduced compared to the prior art.

[0040] The viscosity measurement can be output or used as an auxiliary variable for determining other measured variables, such as, for example, for correcting a flow measurement as a function of the Reynolds number, as described in EP 1 281 938 B1 for a mass flow rate. Since the present invention provides a more accurate viscosity measurement, a more accurate Reynolds number can also be determined, which ultimately enables a more accurate correction of the flow measurement.

[0041] This procedure is supplemented by optional procedural steps in Fig. 2 , wherein, in a first step 170, a preliminary flow measurement value is determined using a flow sensor. In the next step 180, the Reynolds number is determined based on the preliminary flow measurement value, the density, the viscosity measurement value, and a characteristic variable of the flow sensor, for example, a measuring tube diameter of the flow sensor. In a final step 190, a corrected flow measurement value is determined according to the principles described in EP 1 281 938 B1.

[0042] The flow sensor can be the same sensor used to determine the viscosity measurement, but this is not mandatory. Likewise, a preliminary flow measurement determined using a different measuring principle can be corrected, for example, with an ultrasonic flow sensor or a magnetic inductive sensor.

[0043] The Fig. 3The illustrated embodiment of a measuring point 50 according to the invention comprises a Coriolis mass flow meter 1, which is designed as an embodiment of a measuring device according to the invention, i.e., is configured to carry out the method according to the invention and to determine a viscosity measurement value. The Coriolis mass flow meter 1 comprises an oscillator 10 which comprises a pair of parallel, oscillatable measuring tubes 14 which extend between an inlet-side flange 11 and an outlet-side flange 12, wherein the flanges each comprise a flow divider or collector into which the measuring tubes 14 open. The flow dividers are connected to one another by a rigid housing 15, so that vibrations of the flow dividers accommodating the measuring tubes are effectively suppressed in the range of vibration frequencies of useful bending vibration modes of the oscillator.The measuring tubes 10 are rigidly connected to a node plate 20 on the inlet side and a node plate 21 on the outlet side. The node plates define vibration nodes of the oscillator 10 formed by the two measuring tubes 14, and thus largely determine the range of possible frequencies of the useful bending vibration mode. The oscillator 10 is excited to oscillate by an electrodynamic exciter 17 acting between the two measuring tubes 14, with the oscillations being detected by two electrodynamic vibration sensors 18, 19 that detect relative movements of the measuring tubes 14. The exciter 17 is operated by a first operating and evaluation circuit 30 with a first computing unit 32, wherein the operating and evaluation circuit is configured to carry out the method according to the invention.The measuring point further comprises a pipeline 70, the nominal diameter of which is a multiple of the nominal diameter of the Coriolis mass flowmeter 1. The Coriolis mass flowmeter is arranged in the bypass, specifically parallel to a flowmeter 60 arranged in the pipeline, for example an ultrasonic flowmeter 60. The ultrasonic flowmeter 60 has a second operating and evaluation circuit 62 with a second computing unit 64 for operating the ultrasonic flowmeter. The second computing unit 64 is connected to the first computing unit 34 for data exchange. The second computing unit 64 is configured to determine a preliminary flow measurement value and, based on the density and viscosity values provided by the first computing unit 34, to provide a flow measurement value corrected for the Reynolds number.

[0044] Of course, the first computing unit 34 of the Coriois mass flow meter 1 can also be configured to provide a flow measurement value corrected with respect to the Reynolds number, either on the basis of a preliminary flow measurement value determined by another flow meter or on the basis of a preliminary flow measurement value determined with the Coriois mass flow meter 1.

Claims

1. A method (100) for determining a measured viscosity value (η) of a medium conducted in a measuring tube that can oscillate; comprising: Initiating at least one oscillation mode (110) of the measuring tube; Determining a natural frequency (120) of the oscillation mode; Determining the density (130) of the medium based on the natural frequency; Determining the damping (140) of the oscillation mode; Determining the measured viscosity value (160) as a function of the density, the natural frequency, and the damping of the oscillation mode, characterized in that the measured viscosity value (η) is determined as a function of a specification of the type of medium with a model corresponding to the specification, the specification comprises the allocation of the medium to a substance group, wherein the model comprises a first substance group-specific model and a second substance group-specific model, and with a given density, natural frequency, and damping, the measured viscosity values (η) determined with a first model for a first substance group are at least 20%, in particular at least 40%, greater than the measured viscosity values (η) determined with a second model for a second substance group, if the measured viscosity values (η) determined with the second model are not more than 10 cP.

2. The method as claimed in claim 1, wherein the first model and the second model each have a transfer function, wherein the first and the second transfer functions each have a basic damping term (A01, A02), wherein the determined damping is to be corrected with the basic damping term in order to determine a media-specific damping value, wherein a first basic damping term for the first transfer function differs from a second basic damping term for the second transfer function.

3. The method as claimed in claim 2, wherein the first transfer function has a first sensitivity factor (A11) for the first model, which differs from a second sensitivity factor (A12) of the second transfer function for the second model, wherein the determined measured viscosity value (η) is proportional to each sensitivity factor.

4. The method as claimed in claim 3, wherein the determined measured viscosity value depends on the square of the media-specific damping value and, in particular, is proportional to the square of the media-specific damping value.

5. The method as claimed in claim 3 or 4, wherein the basic damping term has a product of a basic damping factor A0i and a density correction term, wherein the basic damping factor A0i of the first transfer function differs from the basic damping factor of the second transfer function.

6. The method as claimed in claim 5, wherein the density correction term has a function of a deviation of the determined density of the medium from a reference density.

7. The method as claimed in claim 5 or 6, wherein the density correction term is identical for both models.

8. The method as claimed in one of claims 2 to 7, wherein the substance group-specific measured viscosity values determined with the transfer functions for viscosities between 90 cP and 110 cP do not differ by more than 10% from control measured viscosity values which are determined using the following substance group-specific control transfer functions, and wherein the substance group-specific measured viscosity values determined with the transfer functions for viscosities between 1 cP and 90 cP do not differ by more than 50% from control measured viscosity values which are determined using the following substance group-specific control transfer functions: η = A 1 i f 3 ρ D − A 0 i 1 − A 2 i ρ − ρ ref 2 wherein A0i is the substance group-dependent basic damping factor, which is different for the first and second control transfer functions, wherein A1i is the substance group-dependent sensitivity factor, which is different for the first control transfer function and the second control transfer function, wherein A2i is a weighting factor in order to model an effect of the density on the damping, wherein p is the determined density of the medium and ρref is a reference density, wherein D is the determined damping, and wherein f designates the frequency of the initiated bending oscillation mode.

9. The method as claimed in claim 8, wherein ρref is the same for both control transfer functions.

10. The method as claimed in claim 8 or 9, wherein A2i is the same for both control transfer functions.

11. A measuring device (1) for determining a measured viscosity value of a medium, comprising: A sensor with a measuring tube (14) that can oscillate for conducting the medium, an exciter (17) for initiating at least one oscillation mode of the measuring tube (14), and an oscillation sensor (18, 19) for detecting the measuring tube oscillations; and A measuring and operating circuit (32) with a calculation unit (34) for driving the exciter (17) and for detecting signals from the oscillation sensor (18, 19); characterized in that the measuring and operating circuit (32) is configured to perform the method (100) as claimed in one of the preceding claims with the measuring device (1).

12. A method for determining a measured flow value of a flowing medium, comprising: Determining a provisional measured flow value (170); Determining a measured viscosity value (160) of the medium using the method as claimed in one of claims 1 to 10; Determining a Reynolds number (180) of the medium based on the provisional measured flow value, the measured density value, and the measured viscosity value; and Determining a measured flow value (190) corrected with the Reynolds number based on the provisional measured flow value and the Reynolds number.

13. A measuring point (50) for determining a measured flow value of a flowing medium using the method as claimed in claim 12, comprising: A flowmeter (60) for determining a provisional measured flow value and a measuring device (1) as claimed in claim 11, which are functionally connected to each other in order to exchange the required measured values for performing the method.

14. A Coriolis mass flow meter (1), comprising: At least one measuring tube (14) for conducting a medium; At least one exciter (17) for initiating at least one oscillation mode of the measuring tube; At least two oscillation sensors (18, 19) for detecting measuring tube oscillations and a flow-dependent displacement of an oscillation mode; A measuring and operating circuit (32) with a calculation unit (34) which is configured to perform the method as claimed in claims 1 to 10 and 12 with the Coriolis mass flow meter (1).