US 6,983,223 B2
Detecting thermocouple failure using loop resistance
William C. Schuh, Delavan, Wis. (US)
Assigned to Watlow Electric Manufacturing Company, St. Louis, Mo. (US)
Filed on Apr. 29, 2003, as Appl. No. 10/425,391.
Prior Publication US 2004/0220775 A1, Nov. 04, 2004
Int. Cl. G06F 11/30 (2006.01)
U.S. Cl. 702—185 33 Claims
OG exemplary drawing
 
1. A method of detecting thermocouple failure in a temperature monitoring circuit, the method comprising:
inputting and storing manufacturers values for an initial resistance Rmo 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 Tmo;
computing and storing an initial circuit resistance Rc of the unsheathed portion of the temperature monitoring circuit, based on the LR measurement and initial temperature Tmo;
measuring a current loop resistance LR at a current indicated temperature Tm;
computing a thermocouple confidence level % CL based on comparing a predicted LR, to the current measured LR at the current indicated temperature Tm 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 Tm, 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.