Coriolis mass flowmeter and method for monitoring a coriolis mass flowmeter
Monitoring a mass flowmeter includes ascertaining a resonant frequency of a bending oscillation, wanted mode, and a density measured value of a medium as a function of the frequency. A bending oscillation is excited outside of resonance with an excitation signal having an amplitude and a frequency (μ times the resonant frequency of the bending oscillation, wanted mode). An amplitude of a sensor signal of the bending oscillation outside of resonance is ascertained. A value of an integrity function of the measuring tube depending on a ratio of the sensor signal amplitude to the excitation signal amplitude of the bending oscillation is ascertained. The integrity function depends further on a density term of a transfer function that models contributions of a plurality of oscillation modes to the sensor signal. This function is reduced to reference conditions, and/or transformed to an integrity value, which has no cross sensitivities for media density.
1 . A method for monitoring a Coriolis mass flowmeter having at least one oscillatable measuring tube for conveying a medium, comprising:
ascertaining a resonant frequency of a bending oscillation, wanted mode of the Coriolis mass flowmeter;
ascertaining a density measured value of a medium conveyed in the measuring tube as a function of the resonant frequency;
exciting a bending oscillation outside of resonance with an excitation signal, which has an excitation signal amplitude and an excitation frequency, which amounts to u times the resonant frequency of the bending oscillation, wanted mode;
registering a sensor signal and ascertaining a sensor signal amplitude of the bending oscillation outside of resonance;
ascertaining a value of an integrity function of the measuring tube depending on a ratio of the sensor signal amplitude of the bending oscillation to the excitation signal amplitude of the bending oscillation,
wherein
the integrity function depends further on a density dependent term of a transfer function,
wherein the density dependent term of the transfer function models contributions of a plurality of oscillation modes to the sensor signal, wherein the integrity function by means of the density dependent term of the transfer function is reduced to reference conditions, and/or transformed to an integrity value, which has no cross sensitivities to media density.
2 . The method as claimed in claim 1 , wherein the transfer function models a ratio of sensor signal amplitude and excitation signal amplitude as a function of the excitation frequency and the media density.
3 . The method as claimed in claim 1 , wherein the density dependent term of the transfer function comprises summands, which are proportional to the contribution of individual oscillation modes to the sensor signal, wherein the contribution of an oscillation mode depends on the ratio of the excitation frequency to the eigenfrequency of the oscillation mode.
4 . The method as claimed in claim 3 , wherein the density dependent term of the transfer function models the eigenfrequencies of the oscillation modes as proportional to one another.
5 . The method as claimed in claim 4 , wherein the density dependent term of the transfer function models a mode specific proportionality factor between the eigenfrequencies of two oscillation modes as a function of density measured value.
6 . The method as claimed in claim 5 , wherein the function is a linear function of density measured value.
7 . The method as claimed in claim 3 , wherein the density dependent term of the transfer function models contributions of individual oscillation modes to the sensor signal as proportional to an effective modal excitability.
8 . The method as claimed in claim 7 , wherein the density dependent term of the transfer function relates the modal excitabilities of two oscillation modes by a proportionality factor.
9 . The method as claimed in claim 1 , wherein the density dependent term of the transfer function models contributions of individual oscillation modes to the sensor signal, in each case, free of damping.
10 . The method as claimed in claim 1 , wherein the density dependent term of the transfer function models besides the contribution of the bending oscillation, wanted mode the contribution to the sensor signal of only one additional oscillation mode.
11 . A Coriolis mass flowmeter, comprising:
at least one oscillatable measuring tube for conveying a medium;
at least one exciter for exciting bending oscillations of the measuring tube as a function of an excitation signal;
at least one oscillation sensor for registering the bending oscillations of the at least one measuring tube and for outputting an oscillation dependent sensor signal; and
at least one measuring- and operating circuit for supplying the exciter with an excitation signal and for registering the sensor signal, wherein the measuring- and operating circuit is adapted to perform the following method:
ascertaining a resonant frequency of a bending oscillation, wanted mode of the Coriolis mass flowmeter;
ascertaining a density measured value of a medium conveyed in the measuring tube as a function of the resonant frequency;
exciting a bending oscillation outside of resonance with an excitation signal, which has an excitation signal amplitude and an excitation frequency, which amounts to u times the resonant frequency of the bending oscillation, wanted mode;
registering a sensor signal and ascertaining a sensor signal amplitude of the bending oscillation outside of resonance;
ascertaining a value of an integrity function of the measuring tube depending on a ratio of the sensor signal amplitude of the bending oscillation to the excitation signal amplitude of the bending oscillation,
wherein
the integrity function depends further on a density dependent term of a transfer function,
wherein the density dependent term of the transfer function models contributions of a plurality of oscillation modes to the sensor signal, wherein the integrity function by means of the density dependent term of the transfer function is reduced to reference conditions, and/or transformed to an integrity value, which has no cross sensitivities to media density.