Fluid contaminant detection using noise levels in magnetic flow meter sensor signals

1. A magnetic flow meter comprising:

electrode sensors generating a sensor signal indicative of flow of a conductive liquid through a conduit;

a noise identification module identifying a noise level in the sensor signal;

a contaminant identification module using the noise level to determine whether a contaminant is in the conductive liquid in the conduit,

wherein:

the noise identification module identifies the noise level in a frequency-domain representation of the sensor signal; and

the frequency-domain representation of the sensor signal is a filtered representation, in which harmonics of a frequency of a drive signal used to generate a magnetic field in the conduit have been filtered.

2. The magnetic flow meter of claim 1 wherein the sensor signal comprises a periodic square wave and the magnetic flow meter further comprises a data collection module that selects samples of the sensor signal by limiting the selected samples to samples that occur at an end portion of each half-cycle of the square wave.

3. The magnetic flow meter of claim 1 , wherein:

the sensor signal comprises a multi-step pulsed DC wave having a first frequency square wave superimposed on a second frequency square wave;

the first frequency is higher than the second frequency; and

the magnetic flow meter further comprises a data collection module that selects samples of the sensor signal by limiting the selected samples to samples that occur at an end portion of each half-cycle of the first frequency square wave.

4. The magnetic flow meter of claim 1 wherein the sensor signal comprises a sine wave and the magnetic flow meter further comprises a data collection module that applies samples of the sensor signal to a high pass filter.

5. The magnetic flow meter of claim 1 wherein the noise identification module identifies a plurality of noise levels.

6. The magnetic flow meter of claim 5 wherein the contaminant identification module further comprises a filter that filters the plurality of noise levels to produce a filtered noise level and wherein the contaminant identification module uses the filtered noise levels to determine whether a contaminant is in the conductive liquid.

7. The magnetic flow meter of claim 1 wherein a portion of the noise level is due to a gas contaminant.

8. The magnetic flow meter of claim 7 wherein the conductive liquid is water.

9. The magnetic flow meter of claim 1 wherein the conductive liquid is water and a portion of the noise level is due to an oil contaminant.

10. The magnetic flow meter of claim 1 wherein a portion of the noise level is due to a solid contaminant.

11. The magnetic flow meter of claim 1 , wherein harmonics of line noise frequencies are filtered in the frequency-domain representation of the sensor signal.

12. A method comprising:

applying a drive signal to generate a magnetic field in a conduit carrying a conductive liquid;

receiving a sensor signal from electrodes positioned along the conduit;

converting the sensor signal to a frequency-domain representation;

filtering the frequency-domain representation including filtering values for frequencies that are harmonics of a frequency of the drive signal using a filter;

determining a noise level in the filtered frequency-domain representation; and

determining whether the conductive liquid contains a contaminant using the noise level.

13. The method of claim 12 wherein determining the noise level comprises selecting samples of the sensor signal at the end of half-cycles in the sensor signal and using the selected samples to determine the noise level.

14. The method of claim 13 wherein determining the noise level further comprises determining a difference between a maximum and a minimum of the selected samples.

15. The method of claim 12 wherein determining the noise level comprises determining separate noise levels for each of a plurality of sections of the sensor signal to form a plurality of noise levels and filtering the plurality of noise levels.

16. The method of claim 12 wherein determining whether the conductive liquid contains a contaminant comprises determining whether the conductive liquid contains a gas.

17. The method of claim 12 wherein determining whether the conductive liquid contains a contaminant comprises determining whether the conductive liquid contains an oil.

18. The method of claim 12 wherein determining whether the conductive liquid contains a contaminant comprises determining whether the conductive liquid contains a solid particulate.

19. The method of claim 12 , wherein filtering the frequency-domain representation includes filtering values for frequencies that are harmonics of line noise frequencies.

20. A process transmitter comprising:

a magnetic coil;

electrode sensors configured to generate a sensor signal;

a noise identification module configured to convert a section of the sensor signal to a frequency-domain representation, filter the frequency-domain representation including filtering values associated with harmonics of a frequency of a drive signal applied to the magnetic coil, and determine a level of noise in the filtered frequency-domain representation; and

a contaminant identification module configured to identify that a conductive liquid contains a contaminant based on the determined level of noise.

21. The process transmitter of claim 20 wherein the noise identification module is further configured to select samples of the sensor signal to use to determine the level of noise in the sensor signal, wherein the selected samples exclude samples at the beginning of half-cycles of a period signal in the sensor signal.

22. The process transmitter of claim 20 wherein the contaminant comprises gas.

23. The process transmitter of claim 20 wherein the contaminant comprises oil.

24. The process transmitter of claim 20 wherein the contaminant comprises sand.

25. The process transmitter of claim 20 , wherein the noise identification module is configured to filter the frequency-domain representation to filter values associated with harmonics of line noise frequencies.

26. A magnetic flow meter comprising:

electrode sensors generating a sensor signal indicative of flow of a conductive liquid through a conduit in a process system;

a noise identification module identifying a noise level in the sensor signal; and

a valve chatter identification module using the noise level to determine whether to issue an alert indicating valve chatter in the process system.