Thermal protection systems material degradation monitoring system

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.

FIELD OF INVENTION

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.

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.

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's response over time as a function of changes to the interface between the sensor and the host material.

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

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's time constant.

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'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's time constant.

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'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.

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'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.

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

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.

The present general inventive concept also provides a method of in-situ thermal response testing of a balanced-lead thermocouple (BTC).

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.

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's time constant as a function of changes to the interface between the sensor and the host material.

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.

DETAILED DESCRIPTION

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.

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.

FIG. 1is a schematic perspective view of a balanced-lead thermocouple100according to an exemplary embodiment of the present general inventive concept.

The balanced-lead thermocouple (BTC)100of 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, and106are identical to the BTC102disclosed below and are provided similar reference numbers.

As illustrated inFIG. 1, the BTC102includes a first lead102aand a second lead102bcoupled to a sensing element102c. In the present exemplary embodiment, the lengths of the first and second leads102a,102bare identical. However, the present general inventive concept is not limited thereto.

In alternative exemplary embodiments, the diameter and material of the leads102a,102bmay be varied in order to compensate for various inherent shortcomings of the LCSR test method.

The first and second leads102aand102bare welded together at a junction to form a spherical bead which is the sensing element102cof the BTC102. The sensing element102cis formed as a composition of material from materials of both the first and second leads102aand102b. The first lead102ahas a first diameter DIA1and the second lead102bhas a second diameter DIA2. In the present embodiment, the first lead102ais 24AWG Chromel wire and the second lead102bis 26 AWG Alumel wire. However, the present general inventive concept is not limited thereto. That is, the first diameter DIA1and the second diameter DIA2may be precisely machined and/or manufactured such that the entire sensor102and the leads are heated uniformly.

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.

In the present general inventive concept, the BTC102uses the same materials as a standard commercially-available thermocouple, and therefore its thermophysical properties of interest have been well documented and tested.

In an exemplary embodiment, an instrumentation system200according to the present general inventive concept connects a plurality of the BTCs100to specialized hardware (i.e., a data acquisition device) that is capable of providing current for thermal response testing of the plurality of BTCs102,104,106which are analyzed using accompanying specialized software, while the BTCs are in actual use.

Based on the type of thermocouple selected, the BTC100may operate in various temperature ranges including about −200oC to about 2320oC. As such, the BTC100combined 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 BTC100may change based on which type of host material it is embedded in.

That is, the system200according 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)100embedded within a host material10with specialized hardware and software to provide a complete instrumentation system200.

The system200is also capable of obtaining accurate in-situ positional temperature data and sensor response data of BTCs102,104,106embedded in the host material10and determining and evaluating host material failure mechanisms based on an analysis of recorded positional temperature data and sensor response data of the BTCs102,104,106.

In an exemplary embodiment, sensor response data of BTCs100is measured and recorded within a testing lab to determine a sensor response profile of each particular BTC100type. The BTC100is then placed within a host material10and the sensor responses are monitored. As the host material10degrades while in use, the monitored sensor response profile of the BTCs100changes and a user may then predict a type of degradation, a location of degradation, and a time when the host material10will completely fail based on the sensor response profiles recorded for each BTC100within the testing lab.

FIG. 2Ais a schematic cross-sectional view of a host material10having a plurality of balanced-lead thermocouples100embedded therein, wherein the host material10is at an initial condition without degradation.

Referring toFIG. 2A, a plurality of BTCs100including a first BTC102, a second BTC104, and a third BTC106are placed along a single axis within the host material10. The first BTC102is embedded at a first depth D1, the second BTC104is embedded at a second depth D2, and the third BTC106is embedded at a third depth D3within the host material10.FIG. 2Bis a graph illustrating BTCs102,104,106response as a function of TPS material degradation therein of the host material10illustrated inFIG. 2A.

FIG. 3Ais a schematic cross-sectional view of the host material10having a plurality of BTCs100embedded therein, wherein the host material10has degraded to a first position A1.FIG. 3Bis a graph illustrating thermocouple response as a function of TPS material degradation therein of the host material illustrated inFIG. 3A.FIG. 4Ais a schematic cross-sectional view of the host material10having a plurality of balanced-lead thermocouples100embedded therein, wherein the host material has degraded to a second position A2.FIG. 4Bis a graph illustrating thermocouple response as a function of TPS material degradation therein of the host material illustrated inFIG. 4A.

Referring now toFIG. 3A, the degradation of the host material10has developed into a region where the first BTC102is embedded. That is, the first position A1refers to a distance in which the host material10has changed and/or degraded. As a result, the thermal response profile102dof the first BTC102changes, thereby alerting the user that the host material10has degraded to at least the first BTC102at the first depth D1.

Similarly, referring now toFIG. 4A, the degradation of the host material10has developed into a region where the first BTC102and the second BTC104are embedded. That is, the second position A2refers to a distance in which the host material10has changed and/or degraded. As a result, the thermal response profiles102dand104dof the first and second BTC102,104changes, thereby alerting the user that the host material10has degraded to at least the second BTC104at the second depth D2. That is, based on known distances of each BTC100within the host material10, a user can determine the amount (or depth) of degradation of the host material10, in real-time.

Referring toFIGS. 4B, the user may determine from the monitored response profiles of BTCs100that the depth of degradation of the host material10is between the depth D2of the second BTC104and the depth D3of the third BTC106. In particular, as illustrated inFIG. 4B, the response profiles102d,104dof BTCs102and104have changed, thereby indicating that the host material10has changed and/or degraded to at least the second BTC104at the second depth D2.

The balanced-lead thermocouple (BTC100) 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.

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.