temperature probe

Abstract

The temperature probe (200) includes a mineral insulated cable (202) having a metallic outer sheath (214) that encloses a mineral insulation (212) therein. The mineral insulated cable (202) has a plurality of conductors (146, 150) extending through the mineral insulation (212). A temperature sensing element (208) has a pair of leads (148, 152). An insert (206) has at least one conduit for receiving the pair of leads (148, 152) of the temperature sensing element (208). The insert (206) also has a recess (220) configured to receive the temperature sensing element (208). An insert sheath (204) is configured to slide over the insert (206) and has a first end configured to couple to the metallic outer sheath (214) of the mineral insulated cable (202) and a second end. An end cap (210) is attached to the second end of the insert sheath (204). The insert (206) is configured to bring the temperature sensing element (208) into contact with the end cap (210).

Description

[Technical Field]

[0001] background Temperature probes are used in a variety of industries and environments to provide temperature readings of materials or surfaces, such as process fluids flowing in process fluid conduits, such as pipes. Temperature probes typically include an outer sheath, made of metal, ceramic, or glass, that protects a temperature-sensing element located inside the sheath from impact and exposure to the process fluid. Typically, a non-conductive powder, such as magnesium oxide (MgO) or a ceramic (such as aluminum oxide—Al2O3), is used to fill the space between the inner surface of the sheath and the temperature-sensing element.

[0002] Temperature probes have a variety of design considerations that must be taken into account regarding their suitability for a particular application. Among these considerations are accuracy, thermal operating range, and response time. In several high-precision industries, such as pharmaceutical, food, and beverage manufacturing, and custody transfer of goods, fast response time is a critical consideration. Providing a temperature probe with improved response time would enable such temperature probes to be used in more applications, particularly those requiring fast response times. Summary of the Invention [Problem to be solved by the invention]

[0003] Resistance temperature detectors (RTDs) and thermocouples (TCs) are the most common industrial temperature sensing elements. Each type of temperature detector has its advantages. RTDs are generally considered to be more accurate and have better long-term stability. Thermocouples are generally considered to be less accurate and subject to more drift than RTDs, but require less immersion in the process, have better response times, and are more vibration resistant. [Means for solving the problem]

[0004] The temperature probe includes a mineral-insulated cable having a metallic outer sheath surrounding mineral insulation therein. The mineral-insulated cable has multiple conductors extending through the mineral insulation. A temperature sensing element has a pair of leads. An insert has at least one conduit for receiving the pair of leads of the temperature sensing element. The insert also has a recess configured to receive the temperature sensing element. An insert sheath is configured to slide over the insert and has a first end configured to couple to the metallic outer sheath of the mineral-insulated cable and a second end. An end cap is attached to the second end of the insert sheath. The insert is configured to contact the temperature sensing element with the end cap and provide strain relief between the leads and the element. [Brief explanation of the drawings]

[0005] [Figure 1A] 1 is a diagram of a portion of a known temperature sensor probe design. [Figure 1B] 1 is a diagram of a portion of a known temperature sensor probe design. [Figure 1C] 1 is a diagram of a portion of a known temperature sensor probe design. [Figure 1D] 1 is a diagram of a portion of a known temperature sensor probe design. [Figure 1E] 1 is a diagram of a portion of a known temperature sensor probe design. [Figure 1F] 1 is a diagram of a portion of a known temperature sensor probe design. [Figure 2] FIG. 1 is an exploded perspective view of a temperature probe according to an embodiment of the present invention. [Figure 3] FIG. 2 is an enlarged cross-sectional view of a temperature probe according to an embodiment of the present invention. [Figure 4] 1 is a perspective view of a temperature sensing element mounting insert according to one embodiment of the present invention; [Figure 5] 1 is a perspective view of a temperature sensing element mounting insert according to one embodiment of the present invention; [Figure 6A] 1 is a cross-sectional view of a portion of a temperature sensing probe according to an embodiment of the present invention. [Figure 6B] 1 is a cross-sectional view of a portion of a temperature sensing probe according to an embodiment of the present invention. [Figure 7] 1 is a chart showing the response times of various temperature probes. [Figure 8] 1 is a flow diagram of a method for manufacturing a temperature probe, according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0006] DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS Although embodiments of the present invention are generally applicable to temperature sensing elements using thin film RTDs, those skilled in the art will recognize that embodiments of the present invention may be implemented with various types of temperature sensors, such as thermocouples, thermistors, etc.

[0007] A typical industrial RTD design generally suspends the sensing element in loose MgO powder. The density of the powder filler is important for both vibration resistance and heat transfer. While the powder is highly thermally conductive, it is not completely solid, and voids and powder form another layer for heat conduction. Contact between the thermowell and the sensor capsule is critical for time response and accuracy. Spring-loaded sensors are the most common capsule assembly, ensuring solid contact between the sensor and thermowell tip. Thermocouples have excellent response times because their sensing junctions are typically located very close to the sensor end cap. Grounded thermocouples, typically fused to the end cap or to the capsule wall near the end cap, offer the fastest response times. A typical sensor capsule assembly is shown in Figures 1A-1D.

[0008] 1A, the sensor end cap has been removed from the end 100 of the sensor capsule 102. As can be seen, thermocouple wires 104, 106 run through an MI cable 108. A thermocouple 110 is formed at the junction of the different thermocouple wires 104, 106. The tip of this thermocouple is grounded (i.e., electrically and mechanically attached to the sensor capsule sheath wall 112). A space 114 near the end of the sensor capsule 100 is typically filled with an insulating powder, such as MgO or ceramic.

[0009] 1B is a diagram of a portion of another type of sensor capsule. Sensor capsule 120 is similar to sensor capsule 102, except that thermocouple 122, formed at the junction of different thermocouple wires, is not grounded to sheath wall 112. Thus, the design shown in FIG. 1B is considered a single ungrounded tip configuration. Again, a mineral insulating powder (e.g., MgO) or ceramic is provided within space 114 to reduce response time and provide vibration resistance.

[0010] 1C is a diagram of a known RTD-based thin film sensor capsule. The sensor capsule 140 generally provides a thin film RTD sensing element 142 disposed within a space 114 near an end 144 of the sensor capsule 140. The sensor capsule 140 uses a known four-wire RTD measurement configuration in which a first plurality of wires 146 is coupled to a first lead of the RTD element 142 and a second plurality of wires 150 is coupled to a second lead 152 of the temperature sensing element 142. This coupling is typically provided by welding, after which the interior of the space 114 is filled with insulating powder to provide thermal conductivity and vibration resistance.

[0011] Figure 1D is a diagram of another type of known RTD sensor capsule design. Sensor capsule 160 uses a wire-wound RTD temperature sensing element 162 instead of thin-film element 142 (shown in Figure 1C). Again, a four-wire connection is used, with wire 146 welded to lead 148 and wire 150 welded to lead 152.

[0012] For temperature probe designs, fast response times are generally considered important for improved process control and efficiency. Typically, users are forced to select a thermocouple for improved response time, at the expense of some RTD accuracy and stability. In addition, RTDs are often considered more susceptible to vibration-induced failure. Lead wire breakage due to mechanical shock or vibration is considered a major cause of RTD failure in the field.

[0013] Figures 1E and 1F show cross-sectional and enlarged cross-sectional views of lead wire fracture due to mechanical shock or vibration. RTDs often consist of a platinum sensing element body encapsulated in ceramic and glass, with lead wires (typically 0.2 mm in diameter) welded to material-insulated cable conductors (146, 150). The temperature sensing element is suspended in a metal sheath and filled with loose MgO powder to prevent shorting of the element and excessive movement during assembly. The mass of the RTD thin-film element body strains the thin lead wires during vibration, and the MgO powder provides some, but limited, damping. Thermocouples are considered more vibration-resistant than RTDs due to their rugged design of two mineral-insulated cable conductors (greater than 0.8 mm in diameter) that typically extend a very short distance and are then welded together.

[0014] In accordance with the embodiments provided herein, a novel RTD sensor capsule design replaces a certain amount of loose powder with a solid insert. Furthermore, the insert is configured to press the RTD element directly against the metal sensor end cap. Therefore, the insert not only positions the element for improved response time and immersion performance, but also provides strain relief for increased vibration resistance. The sensor insert design offers options for high vibration resistance and time response without sacrificing accuracy and long-term stability. To avoid issues with the lead wires, the material selected for insert construction should be electrically insulating. Other materials can also be used for the insert, provided the lead wires have their own coated insulation. The thin-film portion of the RTD element typically has an electrically insulating layer that allows direct contact with the metal sensor end cap. It would be advantageous in terms of response time if the insert material had a relatively high thermal conductivity to facilitate heat transfer. Ceramic materials would also provide good performance.

[0015] FIG. 2 is a diagram of a portion of a temperature probe according to an embodiment of the present invention. The temperature probe 200 generally includes a length of metal-sheathed, mineral-insulated (MI) cable 202, an insert sheath 204, an insert 206, a thin-film RTD sensor 208, and an end cap 210. The metal-sheathed, mineral-insulated cable 202 can have any number of conductors 146, 150 appropriate for providing the desired RTD connection. For example, the number of conductors typically ranges from two to four. The conductors extend within mineral insulation 212 within a metal sheath 214, which may be formed of a suitable metal such as stainless steel. The insert sheath 204 is preferably formed of the same metal as the sheath 214 to facilitate welding or other suitable joining methods. The insert sheath 204 is sized to slide onto the insert 206 once the leads 148, 152 of the thin-film sensor 208 are welded to the wires 150, 146, respectively. The insert 206 includes a recess 220 that is sized and shaped to receive a surface 222 of the thin-film sensor 208 and thus press the opposing flat surface 224 against a surface 226 of the end cap 210. Construction of the probe 200 is completed by welding or otherwise attaching the sensor sheath 204 to the MI cable 202 and welding or otherwise attaching the end cap 210 to the insert sheath 204. This creates a highly vibration-resistant and thermally responsive RTD-based sensor probe. While the embodiments disclosed herein are described with respect to the use of an MI cable, it is clearly contemplated that functionally similar structures (i.e., insulated conductors disposed within a tube or cable with an environmental seal on the sensor insert) may be used in accordance with embodiments of the present invention. Additionally, while FIG. 2 shows the insert 206 supporting a single temperature-sensing element (sensor 208), it is clearly contemplated that in some embodiments, the insert 206 may be configured to support multiple temperature-sensing elements.

[0016] 3 is an enlarged cross-sectional view of a portion of an assembled sensor probe 200 in accordance with an embodiment of the present invention. As can be seen, the insert 206 is disposed within the insert sheath 204 and provides a surface 230 that brings the surface 224 of the thin-film sensor 222 into contact with the surface 226 of the end cap 210. In the illustrated example, the end cap 210 is sized to be received by the insert sheath 204, and coupling of the end cap 210 to the insert sheath 204 is provided by a weld formed at an interface 232.

[0017] 4 and 5 are top and bottom perspective views, respectively, of an RTD temperature sensing element insert 206 in accordance with an embodiment of the present invention. As shown, the insert 206 is generally cylindrical in shape for slidable reception within the insert sheath 204, which in turn is cylindrical in shape for efficient coupling to the cable 202. The recess 220 has a shape formed by two semi-cylindrical sidewalls separated by a protrusion 244. The flat surface 230 is generally sized to accommodate the shape and thickness of the thin-film RTD sensor being used. Additionally, the recess 220 includes a tapered portion 246 that extends to a small notch 248 in the sidewall of the insert 206. However, in other embodiments, the tapered portion 246 need not extend all the way to the sidewall of the insert 206.

[0018] 5 shows a bottom perspective view of the insert 206, which includes a pair of openings 250, 252 through which the leads 148, 152 of the thin film RTD 142 pass. These leads, in this case, are coupled to the MI wires 146, 150 in the trough region 254.

[0019] 6A and 6B are front and side x-ray images of a prototype RTD sensor using an insert 206 according to an embodiment of the present invention. RTD-based temperature probes utilizing the designs and embodiments provided herein were tested and compared with known configurations for response time. The results are shown in FIG. 7. Some assemblies were fabricated with the flat side of the temperature sensing element facing toward the sensor end cap, and some were fabricated with the flat side facing away from the sensor end cap. For reference, FIG. 3 shows an embodiment in which the flat side of the temperature sensing element faces toward the sensor end cap.

[0020] The thin film sensing element is typically placed flat in contact with the sensor tip. This optimally positions the sensor and reduces immersion errors. It also provides a significant improvement in response time. Also, since spring-loaded sensors are generally considered to have the best contact at the tip, placing the element at the tip of the sensor provides significant thermal coupling with the thermowell.

[0021] Prototypes of the embodiment were tested for response time both with and without a thermowell. The results are shown in FIG. 7. The "TF-Jumo Insert-D" is a prototype with the flat portion of the sensing element facing the end cap. As shown in FIG. 7, this design exhibited a T90 time of 16.82 seconds, a T63 time of 5.55 seconds, a T50 time of 3.72 seconds, and a T10 time of 0.81 seconds. The "TF-Jumo Insert-U" is a prototype with the element facing the opposite direction. As can be seen in FIG. 7, this prototype exhibited a T90 time of 21.07 seconds, a T63 time of 7.71 seconds, a T50 time of 5.20 seconds, and a T10 time of 0.87 seconds. The "TF-RTD Heraeus standard" is a standard sensor constructed according to known techniques and is provided for reference. As shown in Figure 7, this sensor exhibited a T90 time of 18.28 seconds, a T62 time of 8.88 seconds, a T50 time of 6.70 seconds, and a T10 time of 2.09 seconds. The "TF-RTD Jumo Standard" is a standard sensor capsule design tested using the Jumo element. This sensor exhibited a T90 time of 19.12 seconds, a T63 time of 9.36 seconds, a T50 time of 7.07 seconds, and a T10 time of 2.14 seconds. As can be seen, the "TF-Jumo Insert-D" outperformed all sensors tested. Results are discussed using T63, the time it takes the sensor to reach 63% of the temperature difference. T63 is commonly used in the industry to measure response time. Tested as a bare sensor (i.e., without a thermowell), the Insert-D exhibited a 40% reduction in response time compared to a standard sensor capsule using the same thin-film RTD element. Additionally, when the TF-RTD Jumo Insert-D was tested in a standard 3 / 4" stainless steel thermowell, it demonstrated a nearly 50% reduction in response time compared to a known sensor capsule design (TF-RTD Heraeus Standard in 3 / 4" SST Twell).

[0022] The data provided in Figure 7 also shows testing of a flat bore thermowell. Typically, thermowells have a W-shape due to gun drilling the hole diameter. Flat bore thermowells have a flat machined shape for better contact with the sensor. This thermowell feature provides a further improvement in thermal response time due to increased contact area between the end of the thermowell and the sensor capsule. The flat bore works well with the sensor insert design described above. The TF-RTD Jumo Insert-D in 3 / 4" SST Twell flat bore is approximately three times faster (66% faster) than a standard sensor with a standard bore thermowell.

[0023] The embodiments provided herein generally provide significantly improved response times for RTD-based temperature probes. In addition, the embodiments provided herein generally improve the vibration resistance of RTD-based temperature probes. The solid-state sensor insert is believed to improve the vibration resistance of the RTD element. The insert provides strain relief by pressing the thin-film element against the end cap of the sensor, thereby separating the element mass from the thin lead wires. This solid-state design also eliminates powder that may move during vibration cycles.

[0024] FIG. 8 is a flow chart of a method for manufacturing a temperature probe according to an embodiment of the present invention. Method 300 begins at block 302, where an MI cable having a sufficient number of conductors extending therethrough is provided. Next, at block 304, a temperature sensing element, such as a thin-film RTD sensor, is provided, with leads threaded through multiple holes in a sensor insert, such as insert 206 (shown in FIG. 5). Next, at block 306, the leads of the temperature sensing element are coupled (e.g., by welding, brazing, or soldering) to the conductors of the MI cable. At block 308, a metal sheath is slid onto the insert. Then, at block 310, the metal sheath is attached to the MI cable. Such attachment can be performed by any suitable method, but it is preferred that the metal sheath be formed of the same metal as the outer metal sheath of the MI cable, and that the attachment be, for example, a continuous weld that not only physically couples the metal sheath to the MI cable but also forms an effective seal. Finally, at block 312, an end cap is attached to the metal sheath. Preferably, the end cap is also attached using welding to seal the end cap to the metal sheath.

Claims

1. a mineral insulated cable having a metallic outer sheath surrounding mineral insulation therein, the mineral insulated cable having a plurality of conductors extending therethrough; a temperature sensing element having a pair of leads; an insert having at least one conduit for receiving the pair of leads of the temperature sensing element, the insert having a recess configured to receive the temperature sensing element; an insert sheath configured to slide over the insert, the insert sheath having a first end configured to couple to a metallic outer sheath of the mineral insulated cable and further having a second end; an end cap attached to the second end of the insert sheath; Including; The temperature probe, wherein the insert is configured to bring the temperature sensing element into contact with the end cap.

2. The temperature probe of claim 1 , wherein the temperature sensing element is an RTD.

3. The temperature probe of claim 2 wherein said RTD is a thin film RTD.

4. 4. The temperature probe of claim 3, wherein the RTD has a first surface received in the recess and a second surface opposite the first surface that is in contact with the end cap.

5. The temperature probe of claim 4 , wherein the second surface of the RTD is flat.

6. 2. The temperature probe of claim 1, wherein the metallic outer sheath and the insert sheath of the mineral insulated cable are formed from the same material.

7. The temperature probe of claim 1 , wherein the metallic outer sheath is welded to the insert sheath.

8. The temperature probe of claim 1 , wherein the insert sheath and the end cap are formed from the same material.

9. The temperature probe of claim 1 , wherein the insert sheath is welded to the end cap.

10. 2. The temperature probe of claim 1, wherein the end cap is a disk having a first surface in contact with the temperature sensing element and an opposite, flat second surface.

11. 10. The temperature probe of claim 1, wherein the mineral insulated cable has a first plurality of conductors coupled to a first lead of the temperature sensing element and a second plurality of conductors coupled to a second lead of the temperature sensing element.

12. 12. The temperature probe of claim 11, wherein the first plurality of conductors are welded to the first lead of the temperature sensing element and the second plurality of conductors are welded to the second lead of the temperature sensing element.

13. 2. The temperature probe of claim 1, wherein the temperature sensing element is a thermistor.

14. An insert for a temperature probe, comprising: a cylindrical insert body; a recess defined in the cylindrical insert body, the recess configured to receive a temperature sensing element; at least one conduit passing through the cylindrical body, the at least one conduit configured to carry at least one lead of the temperature sensing element; a trough region configured to allow coupling of leads of the temperature sensing element to a conductor of a metal-sheathed, mineral-insulated cable; Including; The recess is configured to bring the temperature sensing element into contact with an end cap.

15. 15. The insert of claim 14, formed from a non-conductive material.

16. The insert of claim 15 , wherein the non-conductive material is a ceramic.

17. The insert of claim 14 , wherein the at least one conduit comprises a pair of conduits, each configured to carry a lead of the temperature sensing element.

18. 1. A method of manufacturing a temperature probe, comprising: providing a metallic sheathed cable having a plurality of conductors disposed therein and separated by mineral insulation; providing an insert and threading a plurality of leads of a thin film RTD through said insert; coupling each lead of the thin film RTD to at least one conductor of the metallic sheathed cable; sliding a metal sheath over the insert; attaching the metal sheath of the insert to the metal-sheathed cable; attaching an end cap to the metal sheath of the insert; and The insert contacts the thin film RTD with the end cap. A method comprising:

19. The method of claim 18, further comprising the step of welding the metal sheath of the insert to the metal sheathed cable and the end cap.

20. a mineral insulated cable having a metallic outer sheath surrounding mineral insulation therein, the mineral insulated cable having a plurality of conductors extending therethrough; a temperature sensing element having a pair of leads; an insert having at least one conduit for receiving the pair of leads of the temperature sensing element, the insert having a recess configured to receive the temperature sensing element; an insert sheath configured to slide over the insert, the insert sheath having a first end configured to couple to a metallic outer sheath of the mineral insulated cable and further having a second end; The insert is configured to bring the temperature sensing element into contact with the insert sheath near the second end.

21. The temperature probe of claim 20 , wherein the insert is configured to support a plurality of temperature sensing elements.

22. The temperature probe of claim 20 , wherein the insert is configured to bring the temperature sensing element into contact with a sidewall of the insert sheath.

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