Method to provide a quality measure for meter verification results

1. A method for evaluating a Coriolis mass flowmeter having a flowtube, a driver for driving the flowtube, a pair of sensors for sensing a vibrational response of the flowtube at different locations and providing signals representative of the vibrational response, and a processor configured to determine a mass flow rate of a fluid flowing through the flowtube based on a phase difference between the sensor signals, the method comprising:

performing a diagnostic test for detecting a physical change in the flowtube that could impair operation of the Coriolis mass flowmeter;

outputting a result of the diagnostic test;

evaluating a reliability of the diagnostic test using information about process conditions; and

outputting an indication of the reliability of the diagnostic test.

2. The method of claim 1 wherein the evaluating comprises using information about at least one of the following process conditions: an amount of energy supplied to the driver, a frequency of the vibrational response of the flowtube, a damping characteristic of the fluid, a temperature of the fluid, a temperature of the flowtube, a gas void fraction of the fluid, the mass flow rate of the fluid, a viscosity of the fluid, a density of the fluid, a pressure of the fluid, a control state of the flowtube, and an event in a distributed control system.

3. The method of claim 1 wherein the evaluating comprises using information about the frequency of the vibrational response of the flowtube.

4. The method of claim 3 wherein the evaluating comprises detecting a change in the frequency of the vibrational response of the flowtube.

5. The method of claim 1 wherein said performing the diagnostic test comprises:

driving the flowtube in a first vibrational mode;

sensing a first vibrational response of the flowtube to the first vibrational mode;

driving the flowtube in a second vibrational mode;

sensing a second vibrational response of the flowtube to the second vibrational mode;

determining a resonant frequency of the first and second vibrational responses; and

computing a ratio of the resonant frequency of the first vibrational response to the resonant frequency of the second vibrational response.

6. The method of claim 5 wherein the first vibrational mode comprises a symmetrical mode and the second vibrational mode comprises an anti-symmetrical mode.

7. The method of claim 1 wherein said performing the diagnostic test comprises at least one of the following: (i) detecting an amount of energy in a noise portion of a frequency spectrum of the signals from the sensors while driving the flowtube and monitoring changes in the amount energy in the noise portion of the frequency spectrum over time; (ii) monitoring the signals from the sensors after de-energizing the driver to detect a decay characteristic of the signals; (iii) and calculating a stiffness of the flowtube to detect any structural changes in the flowtube that affect stiffness.

8. The method of claim 1 wherein that method comprising calculating a stiffness of the flowtube to detect any structural changes in the flowtube that affect stiffness by sensing at least three vibrational responses of the flowtube using the sensors, one of the at least three vibrational responses being responsive to a vibration of the flowtube at a substantially resonant frequency of the flowtube and the others of the at least three vibrational responses being responsive to vibrations of the flowtube at non-resonant frequencies of the flowtube.

9. The method of claim 1 wherein said outputting the indication of the reliability of the diagnostic test comprises displaying on a display a first indication when the reliability is low, a second indication when the reliability is medium, and a third indication when the reliability is high, the first, second, and third indications being different from one another.

10. The method of claim 1 further comprising using the diagnostic test to detect at least one of corrosion of the flowtube, erosion of the flowtube, and fatigue of the flowtube.

11. A system for evaluating a mass flow rate measurement, the system comprising:

a Coriolis flowmeter comprising:

a flowtube;

a driver for driving the flowtube; and

a pair of sensors for sensing a vibrational response of the flowtube at different locations and providing sensor signals representative of the vibration; and

a diagnostic system configured to:

perform a diagnostic test for detecting a physical change in the flowtube that could impair operation of the Coriolis mass flowmeter;

output a result of the diagnostic test;

evaluate a reliability of the diagnostic test using information about a process condition; and

output an indication of the reliability of the diagnostic test.

12. The system for evaluating a mass flow rate measurement of claim 11 wherein the diagnostic system is configured to use information about at least one of the following process conditions to evaluates the reliability of the diagnostic test: an amount of energy supplied to the driver, a frequency of the vibrational response of the flowtube, a damping characteristic of the flowtube, a temperature of the fluid, a temperature of the flowtube, a gas void fraction of the fluid, the mass flow rate of the fluid, a viscosity of the fluid, a density of the fluid, a pressure of the fluid, a control state of the flowtube, and an event in a distributed control system.

13. The system for evaluating a mass flow rate measurement of claim 11 wherein the diagnostic system is configured to use information about the frequency of the vibrational response of the flowtube to evaluate the reliability of the diagnostic test.

14. The system for evaluating a mass flow rate measurement of claim 13 wherein the diagnostic system is configured to detect a change in the frequency of the vibrational response of the flowtube to evaluate the reliability of the diagnostic test.

15. The system for evaluating a mass flow rate measurement of claim 11 wherein, the diagnostic system is configured to:

drive the flowtube in a first drive mode;

sense a first vibrational response of the flowtube to the first vibrational mode;

drive the flowtube in a second drive mode;

sense a second vibrational response of the flowtube to the second vibrational mode;

determine a resonant frequency of the first and second vibrational responses; and

compute a ratio of the resonant frequency of the first vibrational response to the resonant frequency of the second vibrational response.

16. The system for evaluating a mass flow measurement of claim 15 wherein the first vibrational mode comprises a symmetrical mode and the second vibrational mode comprises an anti-symmetrical mode.

17. The system for evaluating a mass flow rate measurement of claim 11 wherein the diagnostic system is configured to do at least one of the following: (i) detect an amount of energy in a noise portion of a frequency spectrum of the signals from the sensors while driving the flowtube and to monitor changes in the amount of energy in the noise portion of the frequency spectrum over time; (ii) monitor the signals from the sensors after de-energizing the driver to detect a decay characteristic of the signals; and (iii) calculate a stiffness of the flowtube to detect any structural change in the flowtube that affects stiffness.

18. The system for evaluating a mass flow rate measurement of claim 11 wherein the sensors sense at least three vibrational responses of the flowtube, one of the at least three vibrational responses being responsive to a vibration of the flowtube at a substantially resonant frequency of the flowtube and the others of the at least three vibrational responses being responsive to vibrations of the flowtube at non-resonant frequencies of the flowtube.

19. The system for evaluating a mass flow rate measurement of claim 11 wherein the diagnostic system is further configured to display on a display a first indication when the reliability is low, a second indication when the reliability is medium, and a third indication when the reliability is high, the first, second, and third indications being different from one another.

20. The system for evaluating a mass flow rate measurement of claim 11 wherein the diagnostic system is configured to detect at least one of corrosion of the flowtube, erosion of the flowtube, and fatigue of the flowtube.