Method for operating a Coriolis mass flowmeter and corresponding Coriolis mass flowmeter

Abstract

A method for operating a Coriolis mass flowmeter (1) having at least one measuring tube (2) and at least one sensor (3), wherein the sensor (3) emits an electric sensor signal depending on the temperature of the sensor (3), the sensor (3) is mechanically coupled to the rest of the Coriolis mass flowmeter (1) via a connection (5) and the connection (5) has a thermal resistance. To provide a method for operating a Coriolis mass flowmeter that makes recognition of a change in the connection possible an electric excitation signal is generated, the excitation signal is impressed in the sensor (3), the sensor signal influenced by the excitation signal is detected, a change between the detected sensor signal and a reference signal is determined and the change between the detected sensor signal and the reference signal is associated with a change in the thermal resistance.

Claims

1. Method for operating a Coriolis mass flowmeter having at least one measuring tube and at least one sensor that is mechanically coupled to the rest of the Coriolis mass flowmeter via a connection that has a thermal resistance, comprising the steps of: emitting an electric sensor signal from the sensor depending on the temperature of the sensor, generating an electric excitation signal, impressing the excitation signal on the sensor, detecting a change of the sensor signal due to the impressing of the excitation signal, determining a difference between the changed sensor signal and a reference signal, and associating the difference between the changed sensor signal and the reference signal with a change in the thermal resistance.

2. Method according to claim 1, wherein the difference between the detected sensor signal and the reference signal is determined in that a time constant of the changed sensor signal and a change between time constants of the detected sensor signal and a time constant of the reference signal are determined.

3. Method according to claim 2, wherein a decrease of the time constant of the detected signal as compared to the time constant of the reference signal is associated with a increase of the thermal resistance of the connection.

4. Method according to claim 1, wherein the difference between the changed sensor signal and the reference signal is associated with a change of the mechanical coupling via the connection.

5. Method according to claim 2, wherein the difference between the changed sensor signal and the reference signal is associated with a change of the mechanical coupling via the connection and wherein a decrease of the time constant of the detected sensor signal compared to the time constant of the reference signal is associated with a decrease of the mechanical coupling via the connection.

6. Method according to claim 1, wherein the excitation signal is generated in a temporal course with a step from a first excitation signal value to a second excitation signal value and constant excitation signal values are generated temporally before and after the step.

7. Method according to claim 6, wherein the excitation signal is generated so that the first excitation signal value is greater than the second excitation signal value.

8. Method according to claim 1, wherein the excitation signal is impressed in the sensor in the form of an electric current.

9. Method according to claim 1, wherein the Coriolis mass flowmeter has an additional sensor, wherein the additional sensor emits an additional electric sensor signal depending on the temperature of the additional sensor, wherein the additional sensor is mechanically coupled to the rest of the Coriolis mass flowmeter via an additional connection, the additional connection having a additional thermal resistance, wherein a additional electric excitation signal is generated and impressed on the additional sensor, a change in the additional sensor signal due the additional excitation signal is detected and used as the reference signal.

10. Method according to claim 9, wherein, relative to the additional sensor, the additional excitation signal is generated in a temporal course with a step from a first excitation signal value to a second excitation signal value and constant excitation signal values are generated temporally before and after the step.

11. A Coriolis mass flowmeter, comprising: at least one measuring tube, at least one sensor and an evaluation unit, wherein the sensor is adapted to emit an electric sensor signal depending on the temperature of the sensor, wherein the sensor is mechanically coupled to the rest of the Coriolis mass flowmeter via a connection having a thermal resistance, wherein the evaluation unit is adapted for generating an electric excitation signal and impressing the excitation signal on the sensor, detecting a change in the sensor signal due to the excitation signal, determining a change between the changed sensor signal and a reference signal and associating the change between the detected sensor signal and the reference signal with a change of the thermal resistance.

12. The Coriolis mass flowmeter according to claim 11, wherein evaluation unit is adapted for associated the difference between the changed sensor signal and the reference signal with a change of the mechanical coupling via the connection.

13. The Coriolis mass flowmeter according to claim 11, wherein the sensor is a resistive temperature sensor.

14. The Coriolis mass flowmeter according to claim 11, wherein the sensor is arranged on the measuring tube via the connection.

15. The Coriolis mass flowmeter according to claim 11, wherein the connection is made using an adhesive bond.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The sole FIGURE schematically depicts an embodiment of the Coriolis mass flowmeter according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

(2) The Coriolis mass flowmeter 1 shown schematically in the FIGURE has a measuring tube 2, a sensor 3 and an evaluation unit 4.

(3) The sensor 3 is a temperature sensor and emits an electric sensor signal dependent on the temperature .sub.S of the sensor 3. It is mechanically coupled to the rest of the Coriolis mass flowmeter 1 via a connection 5, wherein the temperature of the rest of the Coriolis mass flowmeter 1 is .sub.U. The connection 5 is characterized by a thermal resistance R.sub.SU.

(4) The evaluation unit 4 generates an excitation signal with the following characteristics. In the temporal course, there is a step from a first excitation value to a second excitation value. The first excitation value is thereby greater than the second excitation value. The excitation values are constant in terms of time before and after the step. The excitation signal is thus a step function. Since the first excitation signal is greater than the second excitation signal, then sensor 3 cools down after the step.

(5) The evaluation unit 4 impresses the excitation signal in the sensor 3 as electric current. The excitation signal increases the temperature .sub.S of the sensor 3 up to the point in time of the step of the excitation signal, wherein the temperature .sub.S of the sensor 3 is .sub.S,0 at the point in time of the step.

(6) The evaluation unit 4 detects the sensor signal influenced by the excitation signal at at least two different points in time t.sub.1 and t.sub.2 after the step and determines the temperature .sub.S(t.sub.1) and .sub.S(t.sub.2) of the sensor 3 from the detected sensor signal at these points in time. Then, the evaluation unit 4 forms the difference temperature .sub.D(t.sub.1)=.sub.S(t.sub.1).sub.U and .sub.D(t.sub.2)=.sub.S(t.sub.2).sub.U between the sensor 3 and the rest of the Coriolis mass flowmeter 1.

(7) The determination of a change between the detected sensor signal and a reference signal forms the basis of the energy balance for the cooling of the sensor 3, from which a differential equation is derived:

(8) $\frac{{\mathrm{dW}}_S}{\mathrm{dt}}=-\frac{{}_S-{}_U}{R_{\mathrm{SU}}},\mathrm{with}\text{:}{C}_S=\frac{{\mathrm{dW}}_S}{d{}_S}.Math.C_S\frac{d{}_S}{dt}+\frac{{}_S}{R_{\mathrm{SU}}}=\frac{{}_U}{R_{\mathrm{SU}}},\mathrm{with}\text{:}=C_S{R}_{\mathrm{SU}}.Math.\frac{d{}_S}{\mathrm{dt}}+\frac{1}{}{}_S=\frac{1}{}{}_U\frac{d\left({}_S-{}_U\right)}{\mathrm{dt}}+\frac{1}{}\left({}_S-{}_U\right)=0,\mathrm{since}\text{:}\frac{d{}_U}{\mathrm{dt}}=0,\mathrm{with}\text{:}{}_D={}_S-{}_U.Math.\frac{d{}_D}{\mathrm{dt}}+\frac{1}{}{}_D=0$

(9) In the equations, in the order according to their first occurrence, W.sub.S is the thermal energy of the sensor 3, t is the time, C.sub.S is the thermal capacity of the sensor 3, and is the time constant of the sensor 3. The energy balance is based on the knowledge that the exchange of heat between the sensor 3 and an ambient atmosphere 6 is negligible compared to the exchange of heat between the sensor 3 and the rest of the Coriolis mass flowmeter 1 and the temperature .sub.U of the rest of the Coriolis mass flowmeter 1 is constant.

(10) ${}_D={}_{D,0}{e}^{-\frac{t}{}},\mathrm{with}\text{:}{}_{D,0}={}_{S,0}-{}_U$
is found as solution of the differential equation.

(11) In the solution of the differential equation, .sub.D,0 is the difference temperature at the point in time of the step of the excitation signal. After the step, thus, the temperature .sub.S of the sensor 3 approaches the temperature .sub.U of the rest of the Coriolis mass flowmeter 1 again over time. It has been thereby recognized that the increase of the temperature .sub.S of the sensor 3 by supplying the sensor 3 with electric energy required by the sensor 3 for measuring the physical variables, for which it is designed, is negligible.

(12) Determining the change between the detected sensor signal and the reference signal is carried out in that the evaluation unit 4 first determines the time constant of the detected sensor signal and then the change between the time constant of the detected signal and the time constant of the reference signal.

(13) Thereby, the evaluation unit 4 determines the time constant of the sensor signal as follows:

(14) ${}_D\left(t_1\right)={}_{D,0}{e}^{-\frac{t_1}{}},{}_D\left(t_2\right)={}_{D,0}{e}^{-\frac{t_2}{}}.Math.\frac{{}_D\left(t_2\right)}{{}_D\left(t_1\right)}=\frac{e^{-\frac{t_2}{}}}{e^{-\frac{t_1}{}}}=e^{-\left(t_2-t_1\right)}=\frac{t_1-t_2}{\ln \left(\frac{{}_D\left(t_2\right)}{{}_D\left(t_1\right)}\right)}$

(15) The evaluation unit 4 associates the change between the time constant of the detected sensor signal and the time constant of the reference signal with the thermal resistance R.sub.SU and emits a signal when the change exceeds a predetermined threshold value.