Compensating for frequency change in flowmeters

1. A method comprising:

inducing motion in a conduit such that the conduit oscillates in a first mode of vibration and a second mode of vibration, the first mode of vibration having a corresponding first frequency of vibration and the second mode of vibration having a corresponding second frequency of vibration, wherein the first and the second frequencies vary with temperature, and the conduit contains a fluid;

determining a first temperature that influences the first mode of vibration;

determining a second temperature that influences the second mode of vibration; and

determining a property of the fluid based on the first temperature and the second temperature, wherein the property depends on at least one of the first frequency or the second frequency, and the property is determined based on the first and second temperatures such that the property is compensated for at least one of the variation of the first frequency with temperature or the variation of the second frequency with temperature.

2. The method of claim 1 , wherein:

the property depends on both the first and second frequencies; and

the property is determined such that it is compensated for both the variation of the first frequency with temperature and the variation of the second frequency with temperature.

3. The method of claim 1 , wherein the property of the fluid is the density of the fluid.

4. The method of claim 3 further comprising:

determining density calibration constants based on physical properties of the conduit;

determining a reference temperature;

compensating the density calibration constants based on the first temperature, the second temperature, and the reference temperature; and

determining the density based on the compensated density calibration constants.

5. The method of claim 4 , wherein the density is determined based on the calibration constants and the first frequency or the second frequency.

6. The method of claim 4 , wherein the density is determined based on the calibration constants, the first frequency, and the second frequency.

7. The method of claim 1 , wherein the property of the fluid is the mass flow rate of the fluid.

8. The method of claim 7 , wherein the mass flow rate of the fluid depends on the first frequency during a zero-flow condition and the second frequency during the zero-flow condition.

9. The method of claim 1 , wherein the first temperature is a temperature of the fluid contained in the conduit, and the second temperature is a temperature of the conduit.

10. The method of claim 1 , wherein the first temperature is a temperature of the fluid contained in the conduit, and the second temperature is an ambient temperature in the vicinity of the conduit.

11. The method of claim 1 , wherein the first temperature is a temperature of the fluid contained in the conduit, and the second temperature is a temperature of a housing of the conduit.

12. The method of claim 1 , wherein the first temperature is a temperature of one location on the conduit, and the second temperature is a temperature of another location on the conduit.

13. The method of claim 1 , wherein

the first mode is a Coriolis mode, and

the second mode is a driven mode.

14. A flowmeter comprising:

a vibratable flowtube, the flowtube containing a fluid;

a driver connected to the flowtube and configured to impart motion to the flowtube such that the conduit oscillates in a first mode of vibration and a second mode of vibration, the first mode of vibration having a corresponding first frequency of vibration and second mode of vibration having a corresponding second frequency of vibration, wherein the first and the second frequencies vary with temperature;

a sensor connected to the flowtube and configured to sense the motion of the flowtube and generate a sensor signal; and

a controller to receive the sensor signal and configured to:

determine a first temperature that influences the first mode of vibration;

determine a second temperature that influences the second mode of vibration; and

determine a property of the fluid based on the first temperature and the second temperature, wherein the property depends on at least one of the first frequency or the second frequency, and the property is determined based on the first and second temperatures such that the property is compensated for the at least one of the variation of the first frequency with temperature or the variation of the second frequency with temperature.

15. The flowmeter of claim 14 , wherein

the property depends on both the first and second frequencies, and

the property is determined such that it is compensated for both the variation of the first frequency with temperature and the variation of the second frequency with temperature.

16. The flowmeter of claim 14 , wherein the property is the density of the fluid.

17. The flowmeter of claim 16 , wherein the controller is further configured to:

determine density calibration constants based on physical properties of the conduit;

determine a reference temperature;

compensate the density calibration constants based on the first temperature, the second temperature, and the reference temperature; and

determine the density based on the compensated density calibration constants.

18. The flowmeter of claim 14 , wherein the property of the fluid is the mass flow rate of the fluid.

19. The flowmeter of claim 14 , wherein the first temperature is a temperature of the fluid contained in the flowtube, and the second temperature is a temperature of the flowtube.

20. A flowmeter transmitter comprising:

at least one processing device; and

a storage device, the storage device storing instructions for causing the at least one processing device to:

induce motion in a conduit such that the conduit oscillates in a first mode of vibration and a second mode of vibration, the first mode of vibration having a corresponding first frequency of vibration and the second mode of vibration having a corresponding second frequency of vibration, wherein the first and second frequencies vary with temperature and the conduit contains a fluid;

determine a first temperature that influences the first mode of vibration;

determine a second temperature that influences the second mode of vibration; and

determine a property of the fluid based on the first temperature and the second temperature, wherein the property depends on at least one of the first frequency or the second frequency, and the property is determined based on the first and second temperatures such that the property is compensated for the at least one of the variation of the first frequency with temperature or the variation of the second frequency with temperature.

21. The transmitter of claim 20 , wherein

the property depends on both the first and second frequencies; and

the property is determined such that it is compensated for both the variation of the first frequency with temperature and the variation of the second frequency with temperature.

22. The transmitter of claim 20 , wherein the property of the fluid is the density of the fluid.

23. The transmitter of claim 22 , wherein the storage device further includes instructions for causing the at least one processor to:

determine density calibration constants based on physical properties of the conduit;

determine a reference temperature;

compensate the density calibration constants based on the first temperature, the second temperature, and the reference temperature; and

determine the density based on the compensated density calibration constants.

24. The transmitter of claim 20 , wherein the property of the fluid is the mass flow rate of the fluid.

25. The transmitter of claim 20 , wherein the first temperature is a temperature of the fluid, and the second temperature is a temperature of the conduit.