Detecting thermocouple failure using loop resistance

• US 20040220775A1
• Filed: 04/29/2003
• Published: 11/04/2004
• Est. Priority Date: 04/29/2003
• Status: Active Grant

• Current Assignee: Watlow Electric Manufacturing Company
• Original Assignee: Watlow Electric Manufacturing Company
• Inventors: Schuh, William C.
• Granted Patent: US 6,983,223 B2
• Time in Patent Office: Days
• US Class Current: 702/185
• CPC Class Codes: G01K 7/026 Arrangements for signalling failure or disconnection of thermocouples

Abstract

A system and method is presented for detecting the degradation of a thermocouple circuit prior to failure and the prediction of certain impending thermocouple failures. The detection system monitors the loop resistance of the measurement circuit and isolates the heat affected loop resistance changes from the degraded circuit changes. The monitoring circuit is divided into a sheathed portion, generally exposed to the temperature measurement, and an unsheathed portion of the monitoring system generally at ambient temperature. An algorithm is provided for standard instrumentation systems to predict certain types of impending failure in thermocouple temperature measurement circuits. The algorithm essentially compensates the loop resistance measurements by removing the heat affected sheathed portion loop resistance changes computed from the initial unsheathed portion resistance based on manufacturers parameters specific to a thermocouple and an initial loop resistance measurement, leaving only those changes due to degradations in the unsheathed portion of the circuit.

Claims

1. A method of detecting thermocouple failure in a temperature monitoring circuit, the method comprising:

inputting and storing manufacturers values for an initial resistance R_m0 and a temperature coefficient of resistance α

for a thermocouple based on the thermocouple part number;

measuring a loop resistance LR at an indicated initial temperature T_mo;

computing and storing an initial circuit resistance R_c of the unsheathed portion of the temperature monitoring circuit, based on the LR measurement and initial temperature T_mo;

measuring a current loop resistance LR at a current indicated temperature T_m;

computing a thermocouple confidence level %CL based on comparing a predicted LR, to the current measured LR at the current indicated temperature T_m according to a thermocouple failure detection algorithm;

determining whether the thermocouple confidence level %CL has exceeded a predetermined limit;

measuring another current loop resistance LR at the current indicated temperature T_m, if the %CL has not exceeded the predetermined limit;

generating an alarm for maintenance of the thermocouple, if the %CL has exceeded the predetermined limit; and

wherein the limit of the thermocouple confidence level corresponds to an impending thermocouple failure and the prediction thereof.

2. The method of claim 1, further comprising waiting for a time to make the current measurement of the loop resistance LR at the current indicated temperature T_m.

3. The method of claim 2, wherein the waiting for a time to make the current measurement of the loop resistance LR at the current indicated temperature T_m, comprises waiting for a periodic time to make the measurement.

4. The method of claim 1, further comprising waiting for a time to make another current measurement of the loop resistance LR at the current indicated temperature T_m, if the %CL has not exceeded the predetermined limit.

5. The method of claim 1, wherein the computing and storing an initial circuit resistance R_c of the unsheathed portion of the temperature monitoring circuit, based on the LR measurement and initial temperature T_mo is represented by R_c=LR-R_m0*(1+α

/2*(T_m0−

T_a) where R_mo is the initial resistance of the sheathed portion of the thermocouple, α

is the temperature coefficient of resistance of the thermocouple, LR is the measured loop resistance of the thermocouple monitoring circuit measured at the initial indicated temperature T_m0 with the unsheathed portion of the circuit at an ambient temperature T_a.

6. The method of claim 4, wherein the thermocouple sheathed portion values R_mo, and α

are supplied by the manufacturer of the thermocouple.

7. The method of claim 4, wherein the thermocouple sheathed portion values R_mo, α

, and T_m0 are supplied by the manufacturer of the thermocouple.

8. The method of claim 1, wherein the computing of the thermocouple confidence level %CL in accordance with the thermocouple failure detection algorithm is represented by %CL={R_mo*(1+α

/2*(T_m−

T_a))−

(LR-R_c)}/(LR-R_c) where R_mo is the initial resistance of the sheathed portion of the thermocouple, α

is the temperature coefficient of resistance of the thermocouple, LR is the measured loop resistance of the thermocouple monitoring circuit measured at the current indicated temperature T_m with the unsheathed portion of the circuit at an ambient temperature T_a.

9. The method of claim 1, further comprising creating a time-series history model of the thermocouple confidence levels for predicting variables relating to confidence level and thermocouple failure at a next loop resistance measurement.

12. The method of claim 1, further comprising waiting for a time to make the current measurement of the loop resistance LR at the current indicated temperature T_m.

13. The method of claim 2, wherein the waiting for a time to make the current measurement of the loop resistance LR at the current indicated temperature T_m, comprises waiting for a periodic time to make the measurement.

14. The method of claim 1, further comprising waiting for a time to make another current measurement of the loop resistance LR at the current indicated temperature T_m, if the Z has not exceeded the predetermined limit.

15. The method of claim 1, wherein the computing and storing an initial circuit resistance R_c of the unsheathed portion of the temperature monitoring circuit, based on the LR measurement and initial temperature T_mo is represented by R_c=LR-R_m0*(1+α

/2*(T_m0−

T_a) where R_mo is the initial resistance of the sheathed portion of the thermocouple, α

is the temperature coefficient of resistance of the thermocouple, LR is the measured loop resistance of the thermocouple monitoring circuit measured at the initial indicated temperature T_m0 with the unsheathed portion of the circuit at an ambient temperature T_a.

16. The method of claim 4, wherein the thermocouple sheathed portion values R_mo, and α

are supplied by the manufacturer of the thermocouple.

17. The method of claim 4, wherein the thermocouple sheathed portion values R_mo, α

, and T_m0 are supplied by the manufacturer of the thermocouple.

18. The method of claim 1, wherein the computing of the thermocouple figure of merit Z in accordance with the thermocouple failure detection algorithm is represented by Z=1/{R_mo*(1+α

/2*(T_m−

T_a))−

(LR-R_c)}/(LR-R_c) where R_mo is the initial resistance of the sheathed portion of the thermocouple, α

is the temperature coefficient of resistance of the thermocouple, LR is the measured loop resistance of the thermocouple monitoring circuit measured at the current indicated temperature T_m with the unsheathed portion of the circuit at an ambient temperature T_a.

19. The method of claim 1, further comprising creating a time-series history model of the thermocouple figure of merit Z values for predicting variables relating to the figure of merit Z and the thermocouple failure at a next loop resistance measurement.

10. A method of detecting thermocouple failure in a temperature monitoring circuit, the method comprising:

inputting and storing manufacturers values for an initial resistance R_m0 and a temperature coefficient of resistance α

for a thermocouple based on the thermocouple part number;

measuring a loop resistance LR at an indicated initial temperature T_mo;

computing and storing an initial circuit resistance R_c of the unsheathed portion of the temperature monitoring circuit, based on the LR measurement and initial temperature T_mo;

measuring a current loop resistance LR at a current indicated temperature T_m;

computing a thermocouple figure of merit Z based on comparing a predicted LR, to the current measured LR at the current indicated temperature T_m according to a thermocouple failure detection algorithm;

determining whether the thermocouple figure of merit Z has exceeded a predetermined limit;

measuring another current loop resistance LR at the current indicated temperature T_m, if the Z has not exceeded the predetermined limit;

generating an alarm for maintenance of the thermocouple, if the Z has exceeded the predetermined limit; and

wherein the limit of the thermocouple figure of merit Z corresponds to an impending thermocouple failure and the prediction thereof.

20. A system for detecting thermocouple failure, comprising:

a temperature measuring component having one or more thermocouples coupled thereto;

a memory storage component;

an analyzer operably coupled to the temperature measuring component and the storage component, the analyzer having a prediction algorithm used by the analyzer to detect thermocouple degradations and to make thermocouple failure predictions;

wherein temperature signals generated by respective thermocouples are provided to the analyzer and utilized within the prediction algorithm within the analyzer to generate a thermocouple degradation detection, the level of which provides one of an indication of thermocouple health and an indication of imminent thermocouple failure; and

wherein the prediction algorithm is performed by the analyzer and conveyed by a computer readable medium.

21. The system of claim 20, wherein the temperature measuring component is operable to monitor one or more thermocouples and the loop resistance of the thermocouple monitoring circuit, and forward the results to the analyzer.

22. The system of claim 20, wherein the analyzer is operable to receive one or more thermocouple parametric inputs provided by the manufacturer, and the results of the temperature measuring component.

23. The system of claim 22, wherein the analyzer is further operable to analyze the results of the temperature monitor component, and use the algorithm together with the thermocouple parametric inputs to compute and store the unsheathed constant resistance R_c to the memory storage component.

24. The system of claim 23, wherein the analyzer is further operable to direct the measurement component to make additional loop resistance measurements of each thermocouple and to analyze and determine using the failure detection algorithm, the confidence level of the thermocouple, make a failure prediction of the thermocouple, and issue an alarm condition if a predetermined limit has been achieved.

25. The prediction algorithm of claim 20, wherein the thermocouple degradation detection indication generated by the prediction algorithm is based on a measurement of the loop resistance of a thermocouple monitoring circuit.

26. wherein the thermocouple degradation detection indication generated by the prediction algorithm is based on a measurement of the loop resistance of a thermocouple monitoring circuit and an indicated temperature.

27. A system for detecting thermocouple failure, comprising:

one or more thermocouples to monitor temperature within the system; and

a computer coupled to the one or more thermocouples;

wherein temperature signals generated by respective thermocouples are provided to the computer and utilized within a prediction algorithm that is embedded within the computer to generate a thermocouple confidence level %CL, which level provides one of an indication of thermocouple health and an indication of imminent thermocouple failure;

wherein the prediction algorithm is performed by the computer and conveyed by a computer readable medium.

28. The prediction algorithm of claim 27, wherein the thermocouple confidence level %CL indication generated by the prediction algorithm is based on a measurement of the loop resistance of a thermocouple monitoring circuit.

29. The prediction algorithm of claim 27, wherein the thermocouple confidence level %CL indication generated by the prediction algorithm is based on a measurement of the loop resistance of a thermocouple monitoring circuit and an indicated temperature.

30. The system of claim 27, wherein the thermocouple confidence level %CL is assigned a predetermined confidence level limit corresponding to a thermocouple failure and the prediction thereof.