Abstract:
A self-diagnostic thermocouple ( 10 ) having a first ( 12 ), a second ( 14 ) and a reference ( 16 ) thermoelement enclosed in a sheath ( 20 ) filled with an insulator ( 18 ) separating the thermoelectric elements from each other and the sheath. The three thermoelements ( 12,14,16 ) are joined at a common location to form three separate thermocouple junctions ( 22 ) having three distinct electrical outputs as a function of temperature. The reference thermoelement ( 16 ) is made from a pure metal or alloy which has stable thermoelectric characteristics. Deterioration of the electrical output of the self-diagnostic thermocouple ( 10 ) is capable of being detected by a programed microprocessor ( 100 ).

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a continuation-in-part of application(s) Serial No. 08/086,150 filed on Jul. 1, 1993, now U.S. Pat. No. 6,020,551, issued Jan. 1, 2000, entitled Multi-Wire Self-Diagnostic Thermocouple which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention is related to the field of thermocouples and in particular to a three-wire self-diagnostic thermocouple which permits detection of calibration changes. 
     2. Description of the Related Art 
     A change in the calibration of a thermocouple can cause an error in the detection of the temperature to be determined or controlled. In particular, where equipment is being controlled by a thermocouple, this error could result in the equipment being damaged. Additionally, when the thermocouple controls a process, this error could result in the failure of the process to produce the desired product. 
     Currently, the accuracy of an installed thermocouple can be assured by placing a second thermocouple of known calibration beside the installed thermocouple and comparing the measured temperatures. This method is not always easy or possible to do and requires periodic checking to be effective. Additionally, it is not conveniently possible to know with certainty which of the two thermocouples is faulty when the two thermocouples indicate different temperatures. 
     The use of two thermocouple elements within a metal sheath is taught by Burley in U.S. Pat. No. 4,909,855, by Thom et al. in U.S. Pat. No. 4,778,537 and by Hollander in U.S. Pat. No. 5,111,002. Synder Jr. et al. in U.S. Pat. 4,224,461 teach a two wire thermocouple which includes a third wire as a ground wire. Petry in U.S. Pat. No. 2,696,118 teaches a temperature indicating device having two thermocouples in which the two thermocouples have a common element. Further, Bock in Canadian Patent 675,473 teaches that at least four thermocouple junctions are required to eliminate ambiguity from the measured temperature. 
     SUMMARY OF THE INVENTION 
     A self-diagnostic thermocouple is disclosed, consisting of a first thermoelement having first thermoelectric output properties, a second thermoelement having second thermoelectric output properties, and a reference thermoelement having stable reference thermoelectric output characteristic. A common junction of the first, second, and reference thermoelement, forms three discrete thermocouple junctions. Preferably, the first, second, and reference thermoelements are enclosed in a sheath and are separated from each other and the sheath by insulating materials. 
     A first object of the invention is to provide a self-diagnostic thermocouple which can be used to identify when an output of the thermocouple has deteriorated. 
     A second object of the invention is to provide a three wire thermocouple in which one of the three thermoelements is a reference thermoelement having thermoelectric output characteristics which are stable over time. 
     Another object of the invention is to provide a self diagnostic thermocouple in which the reference thermoelement is a pure metal such as gold, platinum, iron or copper. 
     Still another object of the invention is to provide a thermocouple system having a self diagnostic thermocouple and a microprocessor for detecting the deterioration of the thermoelectric properties. 
     These and other objects will become more apparent from a reading of the specification in conjunction with the appended drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of the self-diagnostic thermocouple. 
     FIG. 2 is a longitudinal cross-sectional view showing the thermocouple junction fused to the sheath. 
     FIG. 3 is a graph showing the thermoelectric output characteristics of a first, second, and reference thermoelectric element. 
     FIG. 4 is a graph illustrating the change in the output of the thermocouple junctions as a function of temperature. 
     FIG. 5 is a graph corresponding to FIG. 4 in which one of the thermoelements has deteriorated. 
     FIG. 6 is a block diagram of a detection system embodying a self-diagnostic thermocouple and a microprocessor. 
     FIG. 7 is a flow diagram of the process executed by the microprocessor. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1, there is shown a multi-wire self-diagnostic thermocouple  10  having three thermoelements  12 ,  14  and  16  enclosed in a single sheath  20  preferably made of a metal or other electrically conductive material. The thermoelements  12 ,  14 , and  16  are insulated from each other and from the sheath  20  by a ceramic or mineral insulating material  18 . One end of the three thermoelements  12 ,  14  and  16  may be fused to the ends of the other thermoelements, as indicated by bead  22 , to form three thermocouple junctions. The bead  22  is preferably a weld bead fusing the ends of the three thermoelements to each other. Alternatively, the ends of the three thermoelements may be fused to each other and to the sheath  20  as indicated in FIG. 2 to form and seal the end  24  of the self-diagnostic thermocouple. Two of the three thermoelements  12  and  14 , respectively, may be made from any of the dissimilar metals or alloys known in the thermocouple art, such as ALUMEL®, a nickel-base alloy containing Si, Al and Mn CHROMEL®, a nickel-chromium alloy constantan or platinum alloys, such as platinum-10% rhodium. The third thermoelement  16  is a reference thermoelement made from a pure noble or base metal and alloys such as 310 stainless steel or other alloys having a stable EMF (electromotive force) output verses temperature characteristic, such as pure gold, platinum, rhodium, iron and copper. 
     Preferably, the EMF output versus temperature characteristics of the first two thermoelements are of opposite polarity with reference to the reference thermoelement  16 , as shown on FIG.  3 . However, it is possible that the EMF output versus temperature characteristics of the first two thermoelements may have the same polarity with reference to the reference thermoelement. Still referring to FIG. 3, the EMF output versus temperature characteristics curve  26  of platinum is used as a zero reference. The characteristics of a first thermoelement  12 , such as CHROMEL®, are indicated by curve  28 . The emf versus temperature characteristics of the second thermoelement  14 , such as ALUMEL®, is indicated by curve  30  and the EMF output versus temperature of the reference thermoelement  16 , such as pure iron (Fe) is indicated by curve  32 . CHROMEL® and ALUMEL® are registered trade marks of the Hoskins Manufacturing Co. of Detroit, Mich. The compositions of CHROMEL® and ALUMEL® are well known in the art and may be found in numerous publications such as the  Manual On the Use of Thermocouples in Temperature Measurements , published by the American Society of Testing and Materials (ASTM), Special Technical Publication 470B, printed in July 1981. These compositions are incorporated herein by reference, and for brevity are not repeated herein. 
     The connected ends of the three thermoelements form three thermocouple junctions. The electrical outputs at any temperature θ are represented by the difference between the electrical outputs of the corresponding thermoelements. For example, as shown in FIG. 4, an electrical output A (curves  28 ,  32 ) emanates from the thermocouple junction formed by the CHROMEL® thermoelement  12  and the iron reference thermoelement  16 . An electrical output B (curves  32 ,  30 ′) is the output of the thermocouple junction formed between the ALUMEL® thermoelement  14  and the reference thermoelement  16 . A third electrical output C (curves  28 ,  30 ′) is the output of the junction formed between the ALUMEL® thermoelement  14  and the CHROMEL® thermoelement  12  as shown in FIG.  4 . 
     Table 1 gives the results of a drift test of a type K thermocouple junction formed between an ALUMEL® thermoelement and a CHROMEL® thermoelement as a function of time at a temperature of approximately 2300° F. A type S platinum-platinum 10% rhodium reference thermocouple was used to make an accurate temperature measurement in degrees Fahrenheit (° F.). An initial offset of 12.6° F. was measured at the beginning of the tests which is the result of departure of the CHROMEL®/ALUMEL® thermocouple from the standardized table for the type K thermocouple. 
     
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                 TABLE I 
               
               
                   
               
               
                   
                 Actual Test Temp. 
                   
                   
               
               
                   
                 ° F. 
               
               
                 Time 
                 Type S Reference 
                 Test T/C 
                 Test T/C Error (° F.) 
               
               
                 (Hrs.) 
                 T/C (° F.) 
                 Type K (° F.) 
                 Type K Error (° F.) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 0.0 
                 2291.5 
                 2278.9 
                 −12.6 
               
               
                 5.0 
                 2295.1 
                 2275.1 
                 −20.0 
               
               
                 10.0 
                 2293.7 
                 2271.3 
                 −22.3 
               
               
                 15.0 
                 2299.6 
                 2276.1 
                 −23.5 
               
               
                 20.0 
                 2288.6 
                 2266.2 
                 −22.3 
               
               
                 25.0 
                 2272.7 
                 2248.6 
                 −23.9 
               
               
                 30.0 
                 2288.4 
                 2259.0 
                 −29.4 
               
               
                 35.0 
                 2304.1 
                 2277.6 
                 −26.3 
               
               
                 40.0 
                 2290.3 
                 2267.0 
                 −23.3 
               
               
                   
               
             
          
         
       
     
     As indicated by the test data presented in Table I, the temperature measured by the K type thermocouple had a temperature error which changed from −12.6° F. to −29.4° F. in 30 hours. 
     In many control applications, a temperature measurement error of this magnitude may be unacceptable. Since the individual thermoelements are alloys of different composition, oxidation and evaporation of minor elements will change the thermoelectric properties of each thermoelement in a unique direction and magnitude. As a function of time, the calibration of the thermoelements may change such that the error in the measured temperature may exceed desired limits. 
     Absent a stable reference thermocouple such as the S type thermocouple used in this test, the error in the temperature measurement by the K type thermocouple will go undetected. 
     Table II was obtained by simultaneously reading the three outputs of the self-diagnostic thermocouple according to the invention. In this test, the reference thermoelectric element was a platinum element and the actual temperature was measured using a type S thermocouple. The first column is test time in hours, the second, third and fourth columns are the standardized table values in millivolts (Mv) for CHROMEL®-platinum (Kp) thermocouple, the ALUMEL®-platinum (Kn) thermocouple, and the CHROMEL®-ALUMEL® (type K) thermocouple respectively for the temperature measured by the S type reference thermocouple. The next two columns give the measured EMF output of the Kp type thermocouple and the error between the measured output and the standardized table value. In a like manner, the next two columns give the measured EMF output of the Kn type thermocouple and the error between the measured output and the standardized table output. The penultimate and last columns give the type K thermocouple error in millivolts and ° F., respectively. 
     
       
         
               
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE II 
               
               
                   
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                 Type K 
                   
               
               
                   
                 Std. Kp 
                 Std. Kn 
                 Std. K 
                 Test Kp 
                 Error 
                 Test Kn 
                 Error 
                 T/C 
               
               
                 Time 
                 EMF 
                 EMF 
                 EMF @ S 
                 EMF 
                 EMF 
                 EMF 
                 EMF 
                 Error 
                 Error 
               
               
                 (hrs.) 
                 (Mv) 
                 (Mv) 
                 Temp ° F. 
                 (Mv) 
                 (Mv) 
                 (Mv) 
                 (Mv) 
                 (Mv) 
                 ° F. 
               
               
                   
               
             
             
               
                  0 
                 40.104 
                 10.728 
                 50.832 
                 39.989 
                 −0.115 
                 10.59  
                 0.138 
                 −0.253 
                 −12.6 
               
               
                  5 
                 40.161 
                 10.742 
                 50.903 
                 39.92  
                 −0.241 
                 10.585 
                 0.157 
                 −0.398 
                 −20   
               
               
                 10 
                 40.139 
                 10.737 
                 50.876 
                 39.883 
                 −0.256 
                 10.547 
                 0.19  
                 −0.446 
                 −22.3 
               
               
                 15 
                 40.232 
                 10.761 
                 50.992 
                 39.978 
                 −0.254 
                 10.545 
                 0.216 
                 −0.47  
                 −23.5 
               
               
                 20 
                 40.058 
                 10.717 
                 50.775 
                 39.823 
                 −0.235 
                 10.505 
                 0.212 
                 −0.447 
                 −22.3 
               
               
                 25 
                 39.805 
                 10.654 
                 50.459 
                 39.554 
                 −0.251 
                 10.427 
                 0.227 
                 −0.478 
                 −23.9 
               
               
                 30 
                 40.054 
                 10.716 
                 50.771 
                 39.835 
                 −0.219 
                 10.358 
                 0.358 
                 −0.577 
                 −29.1 
               
               
                 35 
                 40.304 
                 10.778 
                 51.081 
                 40.061 
                 −0.243 
                 10.496 
                 0.282 
                 −0.525 
                 −26.3 
               
               
                 40 
                 40.085 
                 10.723 
                 50.808 
                 39.823 
                 −0.262 
                 10.519 
                 0.204 
                 −0.466 
                 −23.3 
               
               
                   
               
             
          
         
       
     
     Logging or measuring the Kp, Kn and K outputs simultaneously can qualify the thermoelectric stability of the K type thermocouple and indicate whether calibration changes are causing high or low errors in the measured temperature. 
     As indicated in FIG. 5, for example, if the EMF output of ALUMEL® thermoelement  14  deteriorates, as indicated by dashed curve  30 ′, the output B of the thermocouple junctions between the ALUMEL® thermoelement  14  and the pure iron thermoelement  16  will be decreased from the calibration value B shown in FIG.  4 . In a like manner, the output C of the thermocouple junction formed between the CHROMEL® and ALUMEL® thermoelements will also decrease. However, the output A from the thermocouple junction formed between the CHROMEL® thermoelement  12  and the reference iron thermoelement  16  may remain relatively unchanged. 
     Since the reference iron thermoelement  16  is very stable, a deterioration of either or both of the other two thermoelements can readily be detected by comparing the outputs A and B. The deterioration of one thermoelement of the self-diagnostic thermocouple may be detected using a programmed microprocessor, as shown in FIG.  6 . In this example, the self-diagnostic thermocouple consists of three thermocouple junctions depicted as thermocouple junctions  102 ,  104  and  106 . Thermocouple junction  102  consists of the junction of thermoelements  12  and  16 , while thermocouple junction  104  consists of the junction between thermoelements  14  and  16  and thermocouple junction  106  consists of the junction between thermoelements  12  and  14 . In the arrangement shown in FIG. 6, the thermoelement  16  is a reference thermoelement. The three thermocouple junctions  102 ,  104  and  106  and their associated thermoelements are shown separately in FIG. 6 for simplicity, even though the self-diagnostic thermocouple according to the invention consists of only three thermoelements. 
     The microprocessor  100  may be a general purpose programed microprocessor used to control an oven or possibly to control a process in which the electrical outputs of the self-diagnostic thermocouple are included as measured parameters. Alternatively, the microprocessor may be a processor dedicated to the detection and determination of a temperature. The microprocessor  100  includes at least one look-up table, data interpolation capabilities and comparison capabilities. 
     The operation of the microprocessor will be discussed relative to the flow diagram shown in FIG.  7 . The microprocessor  100  will first measure the outputs of each of the thermocouple junctions  102 ,  104  and  106  identified as junctions A, B and C respectively in block  108 . The microprocessor will then look up the value B′ corresponding to the expected value of thermocouple junction  104  based on the actual output value A. Next, the microprocessor  100  will compare the actual or measured value B of thermocouple junction  104  with the expected value B′ of thermocouple junction  104  as derived from a look-up table (indicated in decision block  112 ). If B=B′, the microprocessor will check to assure that the value of A is greater than a predetermined value K (indicated in block  114 ) to assure that the diagnostic thermocouple is not broken or disconnected. If the value of A is equal to or greater than K, the microprocessor will return to block  108  and the process will be repeated. If A is not greater than K, the microprocessor will signify a failure of the self-diagnostic thermocouple as indicated by block  116 . 
     If the value of B is not equal to the value of B′ within predetermined limits, the microprocessor will again signify a failure of the self diagnostic thermocouple (as indicated in block  116 ). The microprocessor may signify a failure by energizing a visual alarm such as a steady or blinking red light or actuate an audio alarm or both. 
     It is recognized that the microprocessor may execute different programs from those discussed above to detect a malfunction of the diagnostic thermocouple. 
     Having disclosed the structure and method of operation of the self-diagnostic thermocouple, it is recognized that those skilled in the art may make certain changes or improvements within the scope of the appended claims.