Abstract:
A method of identifying changes in a host material having a thermocouple embedded therein which includes using a Loop Current Step Response (LCSR) test method on a first thermocouple to obtain thermocouple LCSR data for the first thermocouple, recording the obtained thermocouple LCSR data within a storage medium, placing a second thermocouple identical to the first thermocouple at different location within a host material, monitoring sensor response data for second the thermocouple, comparing the sensor response data for the second thermocouple with the thermocouple LCSR data of the first thermocouple stored within the storage medium and identifying changes in the host material based on differences in sensor response data for the second thermocouple based on the stored thermocouple LCSR data of the first thermocouple.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 62/155,208 filed on Apr. 30, 2015, the disclosure of which is hereby incorporated by reference herein in its entirety. 
     
    
     FIELD OF INVENTION 
       [0002]    The present general inventive concept relates to a thermocouple, and more particularly to a balanced-lead thermocouple having a lead temperature ratio of unity when subjected to resistive joule heating. The balanced-lead thermocouple according to the present invention is designed such that the thermocouple bead and leads are at the same initial temperature, when using a Loop Current Step Response (LCSR) test method. The balanced-lead thermocouple, characterized by the LCSR test method, may be configured to measure both temperature and fluid velocity. 
         [0003]    The present general inventive concept also relates to a system and method of monitoring material degradation, and more particularly to a system and method of determining material degradation of a host material based on the thermal response of a balanced-lead thermocouple embedded within the host material. 
         [0004]    The present general inventive concept also relates to a system and method of monitoring the thermal response of a sensor embedded within a host material, and more particularly to an in-situ instrumentation system used in conjunction with a Loop Current Step Response (LCSR) test method used to identify the sensor&#39;s response over time as a function of changes to the interface between the sensor and the host material. 
         [0005]    The method according to the present general inventive concept provides a technique to monitor host material to sensor interface conditions and provides qualitative insight into host material degradation mechanisms. 
       BACKGROUND 
       [0006]    For many high-temperature and transient applications, thermocouples are suitable temperature sensors due to their simple design, fast response times, and ability to accommodate unique installation geometries. However, conventional thermocouples cannot be adequately characterized in-situ to determine the thermocouple&#39;s time constant. 
         [0007]    The time constant is a valuable parameter in correcting for lag associated with the thermal response of a sensor when subjected to highly transient processes or environments. A sensor&#39;s time constant provides a quantitative metric of how fast or slow the sensor responds to a change in ambient conditions. However, conventional thermocouples cannot be adequately tested in-situ and analyzed to obtain the thermocouple&#39;s time constant. 
         [0008]    The conventional method for determining temperature sensor time constant is referred to as the plunge test. Typically, the time constants of resistance temperature detectors (RTD) (i.e., temperature sensors) have been characterized by a single variable called the plunge time constant, which refers to an amount of time required for the sensor output to achieve 63.2% of its final value after a step change in temperature is imposed on the sensor&#39;s surface. A step change in temperature is imposed in a testing environment by suddenly drawing the sensor from one medium at an initial temperature to another medium, usually water flowing at 1 ms-1, at a different temperature. However, the plunge test method is deficient in that this method does not account for an influence of process conditions and/or installation of the sensor on the time constant of the sensor. 
         [0009]    In order to address the problems with the plunge test, the Loop Current Step Response (LCSR) test method was developed. The LCSR in-situ test method is based on heating a temperature sensor internally by applying a step change in current applied to leads of the sensor. The current heats the sensing element of the sensor and the sensor&#39;s temperature rises as a function of the magnitude of the supplied current and the rate of heat transfer between the sensor and its surroundings (e.g., host material). The resulting temperature transient is then analyzed to provide a time constant. As a result, the LCSR test method provides in-situ time constants of the sensors, which are more accurate and precise than time constants determined by the plunge test before sensor installation. 
         [0010]    Therefore, what is desired is a thermocouple designed to be analyzed in-situ and a system and method incorporating such thermocouple which monitors degradation of a material in contact with the thermocouple based on sensor interface conditions. 
       BRIEF SUMMARY 
       [0011]    The present general inventive concept provides a balanced-lead thermocouple having a lead temperature ratio of unity when subjected to resistive joule heating. The balanced-lead thermocouple according to the present invention is designed such that the thermocouple bead (i.e., sensing element) and leads are at the same initial temperature, when using a Loop Current Step Response (LCSR) test method. The balanced-lead thermocouple, characterized by the LCSR test method, may be configured to measure both temperature and fluid velocity. 
         [0012]    The present general inventive concept also provides a method of in-situ thermal response testing of a balanced-lead thermocouple (BTC). 
         [0013]    The present general inventive concept also relates to a system and method of characterizing and monitoring material degradation of a host material based on the thermal response of a BTC embedded within the host material. 
         [0014]    The present general inventive concept also relates to an in-situ BTC instrumentation system used in conjunction with a Loop Current Step Response (LCSR) test method used to identify a sensor&#39;s time constant as a function of changes to the interface between the sensor and the host material. 
         [0015]    According to the present invention, the method of in-situ thermal response testing provides a technique to monitor host material to sensor interface conditions. As such, the method provides qualitative insight into host material degradation mechanisms. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0016]    The above-mentioned features of the present general inventive concept will become more clearly understood from the following detailed description of the invention read together with the drawings in which: 
           [0017]      FIG. 1  is a schematic perspective view of a balanced-lead thermocouple according to an exemplary embodiment of the present general inventive concept; 
           [0018]      FIG. 2A  is a schematic cross-sectional view of a host material having a plurality of balanced-lead thermocouples embedded therein, wherein the host material is at an initial condition without degradation; 
           [0019]      FIG. 2B  is a graph illustrating thermocouple response as a function of TPS material degradation therein of the host material illustrated in  FIG. 2A ; 
           [0020]      FIG. 3A  is a schematic cross-sectional view of a host material having a plurality of balanced-lead thermocouples embedded therein, wherein the host material has degraded to a first position; 
           [0021]      FIG. 3B  is a graph illustrating thermocouple response as a function of TPS material degradation therein of the host material illustrated in  FIG. 3A ; 
           [0022]      FIG. 4A  is a schematic cross-sectional view of a host material having a plurality of balanced-lead thermocouples embedded therein, wherein the host material has degraded to a second position; and 
           [0023]      FIG. 4B  is a graph illustrating thermocouple response as a function of TPS material degradation therein of the host material illustrated in  FIG. 4A . 
       
    
    
     DETAILED DESCRIPTION 
       [0024]    The method and system according to the present general inventive concept is capable of evaluating failure mechanisms of a host material based on trending BTC thermal response data over time as the host material degrades when it is subjected to hostile environments that preclude other conventional sensing techniques. 
         [0025]    Based on a given thermocouple type, the thermocouple lead wire diameters are selected to accommodate for joule heating to thereby create a balanced thermocouple according to the present invention. That is, one challenge of performing an LCSR test method on a thermocouple is that when the current supplied to the sensor is removed, the leads and the sensing element are not at the same elevated temperature because the leads are composed of two metals with dissimilar electrical resistivities. 
         [0026]      FIG. 1  is a schematic perspective view of a balanced-lead thermocouple  100  according to an exemplary embodiment of the present general inventive concept. 
         [0027]    The balanced-lead thermocouple (BTC)  100  of the present invention provides a modified thermocouple construction which accounts for heating discrepancies by adjusting dimensions of the lead wires to balance lead temperatures so that the entire sensor is heated uniformly. The balanced-lead thermocouples (BTC)  102 ,  104 , and  106  are identical to the BTC  102  disclosed below and are provided similar reference numbers. 
         [0028]    As illustrated in  FIG. 1 , the BTC  102  includes a first lead  102   a  and a second lead  102   b  coupled to a sensing element  102   c . In the present exemplary embodiment, the lengths of the first and second leads  102   a ,  102   b  are identical. However, the present general inventive concept is not limited thereto. 
         [0029]    In alternative exemplary embodiments, the diameter and material of the leads  102   a ,  102   b  may be varied in order to compensate for various inherent shortcomings of the LCSR test method. 
         [0030]    The first and second leads  102   a  and  102   b  are welded together at a junction to form a spherical bead which is the sensing element  102   c  of the BTC  102 . The sensing element  102   c  is formed as a composition of material from materials of both the first and second leads  102   a  and  102   b . The first lead  102   a  has a first diameter DIA 1  and the second lead  102   b  has a second diameter DIA 2 . In the present embodiment, the first lead  102   a  is 24AWG Chromel wire and the second lead  102   b  is 26 AWG Alumel wire. However, the present general inventive concept is not limited thereto. That is, the first diameter DIA 1  and the second diameter DIA 2  may be precisely machined and/or manufactured such that the entire sensor  102  and the leads are heated uniformly. 
         [0031]    The present invention provides a modified thermocouple construction which accounts for heating discrepancies by adjusting all dimensions of the lead wires to balance lead temperatures so that the entire sensor is heated uniformly. 
         [0032]    In the present general inventive concept, the BTC  102  uses the same materials as a standard commercially-available thermocouple, and therefore its thermophysical properties of interest have been well documented and tested. 
         [0033]    In an exemplary embodiment, an instrumentation system  200  according to the present general inventive concept connects a plurality of the BTCs  100  to specialized hardware (i.e., a data acquisition device) that is capable of providing current for thermal response testing of the plurality of BTCs  102 ,  104 ,  106  which are analyzed using accompanying specialized software, while the BTCs are in actual use. 
         [0034]    Based on the type of thermocouple selected, the BTC  100  may operate in various temperature ranges including about −200oC to about 2320oC. As such, the BTC  100  combined with the LCSR test method can yield more accurate in-situ positional temperature measurements and response data that may provide qualitative insight into host material degradation. The time constant of the BTC  100  may change based on which type of host material it is embedded in. 
         [0035]    That is, the system  200  according to the present invention incorporates the in-situ sensor thermal response testing of the Loop Current Step Response (LCSR) test method on balanced-lead thermocouples (BTCs)  100  embedded within a host material  10  with specialized hardware and software to provide a complete instrumentation system  200 . 
         [0036]    The system  200  is also capable of obtaining accurate in-situ positional temperature data and sensor response data of BTCs  102 ,  104 ,  106  embedded in the host material  10  and determining and evaluating host material failure mechanisms based on an analysis of recorded positional temperature data and sensor response data of the BTCs  102 ,  104 ,  106 . 
         [0037]    In an exemplary embodiment, sensor response data of BTCs  100  is measured and recorded within a testing lab to determine a sensor response profile of each particular BTC  100  type. The BTC  100  is then placed within a host material  10  and the sensor responses are monitored. As the host material  10  degrades while in use, the monitored sensor response profile of the BTCs  100  changes and a user may then predict a type of degradation, a location of degradation, and a time when the host material  10  will completely fail based on the sensor response profiles recorded for each BTC  100  within the testing lab. 
         [0038]      FIG. 2A  is a schematic cross-sectional view of a host material  10  having a plurality of balanced-lead thermocouples  100  embedded therein, wherein the host material  10  is at an initial condition without degradation. 
         [0039]    Referring to  FIG. 2A , a plurality of BTCs  100  including a first BTC  102 , a second BTC  104 , and a third BTC  106  are placed along a single axis within the host material  10 . The first BTC  102  is embedded at a first depth D 1 , the second BTC  104  is embedded at a second depth D 2 , and the third BTC  106  is embedded at a third depth D 3  within the host material  10 .  FIG. 2B  is a graph illustrating BTCs  102 ,  104 ,  106  response as a function of TPS material degradation therein of the host material  10  illustrated in  FIG. 2A . 
         [0040]      FIG. 3A  is a schematic cross-sectional view of the host material  10  having a plurality of BTCs  100  embedded therein, wherein the host material  10  has degraded to a first position A 1 .  FIG. 3B  is a graph illustrating thermocouple response as a function of TPS material degradation therein of the host material illustrated in  FIG. 3A .  FIG. 4A  is a schematic cross-sectional view of the host material  10  having a plurality of balanced-lead thermocouples  100  embedded therein, wherein the host material has degraded to a second position A 2 .  FIG. 4B  is a graph illustrating thermocouple response as a function of TPS material degradation therein of the host material illustrated in  FIG. 4A . 
         [0041]    Referring now to  FIG. 3A , the degradation of the host material  10  has developed into a region where the first BTC  102  is embedded. That is, the first position A 1  refers to a distance in which the host material  10  has changed and/or degraded. As a result, the thermal response profile  102   d  of the first BTC  102  changes, thereby alerting the user that the host material  10  has degraded to at least the first BTC  102  at the first depth D 1 . 
         [0042]    Similarly, referring now to  FIG. 4A , the degradation of the host material  10  has developed into a region where the first BTC  102  and the second BTC  104  are embedded. That is, the second position A 2  refers to a distance in which the host material  10  has changed and/or degraded. As a result, the thermal response profiles  102   d  and  104   d  of the first and second BTC  102 ,  104  changes, thereby alerting the user that the host material  10  has degraded to at least the second BTC  104  at the second depth D 2 . That is, based on known distances of each BTC  100  within the host material  10 , a user can determine the amount (or depth) of degradation of the host material  10 , in real-time. 
         [0043]    Referring to  FIGS. 4B , the user may determine from the monitored response profiles of BTCs  100  that the depth of degradation of the host material  10  is between the depth D 2  of the second BTC  104  and the depth D 3  of the third BTC  106 . In particular, as illustrated in  FIG. 4B , the response profiles  102   d ,  104   d  of BTCs  102  and  104  have changed, thereby indicating that the host material  10  has changed and/or degraded to at least the second BTC  104  at the second depth D 2 . 
         [0044]    The balanced-lead thermocouple (BTC  100 ) may be used as a temperature sensor and/or a fluid velocity sensor in various applications and environments. The BTC is specifically modified to be used with the LCSR test method and accounts for inherent shortcomings of this test method. 
         [0045]    The instrumentation system monitors material degradation and may be modified to accommodate various other applications. The method according to the present invention incorporates well-known instrumentation technologies and testing methods to yield a robust system that is capable of determining accurate sensor time constants and trending thermal response data over time. The BTC LCSR data obtained by using the system and method according to the present invention yields a response which allows for proper response parameter estimation. 
         [0046]    While the present invention has been illustrated by description of several embodiments and while the illustrative embodiments have been described in detail, it is not the intention of the applicant to restrict or in any way limit the scope of the appended claims to such detail. Additional modifications will readily appear to those skilled in the art. The invention in its broader aspects is therefore not limited to the specific details, representative apparatus and methods, and illustrative examples shown and described. Accordingly, departures may be made from such details without departing from the spirit or scope of applicant&#39;s general inventive concept.