temperature probe

Abstract

A temperature probe includes a mineral insulated cable having a metal outer sheath surrounding a mineral insulation located therein. The mineral insulated cable has a plurality of conductors extending through the mineral insulation. A temperature sensitive element has a pair of leads. An insert has at least one conduit to receive the pair of leads of the temperature sensitive element. The insert also has a recess configured to receive the temperature sensitive element. An insert sheath is configured to slide over the insert and has a first end configured to connect to the metal 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 push the temperature sensitive element into contact with the end cap.

Description

BACKGROUND

[0001] Temperature probes are used in a variety of industries and environments to provide an indication of the temperature of a substance or surface, such as a process fluid flowing in a process fluid conduit such as a pipe. Temperature probes typically include an outer sheath formed of metal, ceramic, or glass that protects a temperature sensitive element located inside the sheath from impact and exposure to process fluids, etc. Non-conductive powders, such as magnesium oxide (MgO) or ceramic (e.g., aluminum oxide - AI2O3) are often used to fill the space between the inner surface of the sheath and the temperature sensitive element.

[0002] Temperature probes have a variety of design considerations that must be taken into account for a particular application. These considerations include accuracy, thermal operating range, and response time. Fast response time is a very important consideration for many high accuracy industries, such as pharmaceutical, food and beverage production, and transport monitoring of goods. Providing a temperature probe with improved response time would allow such temperature probes to be used in more applications, particularly those requiring fast response times.

[0003] Resistance temperature detectors (RTDs) and thermocouples (TCs) are the most common industrial temperature sensing elements. Each type of temperature detector has advantages. RTDs are generally considered more accurate and have excellent long-term stability. Thermocouples are generally considered less accurate than RTDs and suffer from more drift than RTDs, but thermocouples require less process immersion, have superior response times, and are better resistant to vibration. SUMMARY

[0004] A temperature probe includes a mineral insulated cable having a metal outer sheath surrounding a mineral insulation located in the metal outer sheath. The mineral insulated cable has a plurality of conductors extending through the mineral insulation. A temperature sensitive element has a pair of leads. An insert has at least one conduit to receive the pair of leads of the temperature sensitive element. The insert also has a recess configured to receive the temperature sensitive element. An insert sheath is configured to slide over the insert and has a first end configured to connect to the metal 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 push the temperature sensitive element into contact with the end cap and provide strain relief between the leads and the element. BRIEF DESCRIPTION OF DRAWINGS

[0005] Figures 1A-1F is a schematic view of portions of a known temperature sensor probe design.

[0006] Figure 2is a schematic exploded perspective view of a temperature probe according to an embodiment of the present application.

[0007] Figure 3 is an enlarged cross-sectional view of a temperature probe according to an embodiment of the present application.

[0008] Figure 4 and 5 is a schematic perspective view of a temperature sensing element mounting insert according to an embodiment of the present application.

[0009] Figure 6A and 6B is a schematic cross-sectional view of a portion of a temperature sensing probe according to an embodiment of the present application.

[0010] Figure 7 is a chart illustrating response times of various temperature probes.

[0011] Figure 8 is a flowchart of a method of manufacturing a temperature probe according to an embodiment of the present application. DETAILED DESCRIPTION

[0012] While embodiments of the present application are generally applicable to temperature sensitive elements employing thin film RTDs, those skilled in the art will recognize that embodiments of the present application can be practiced with various types of temperature sensors, such as thermocouples, thermistors, etc.

[0013] A typical industrial RTD design generally suspends the sensing element in a loose MgO powder. The density of the powder fill is important for both vibration resistance and heat transfer. The powder has high thermal conductivity, but it is not completely solid and air gaps and powder will form another layer for heat transfer through. The contact between the thermowell and the sensor capsule is important for time response and accuracy. Spring loaded sensors are the most common capsule assembly and the spring loaded sensor ensures firm contact between the sensor and the thermowell tip. Because the sensing junction is generally very close to the sensor end cap, the thermocouple has superior response time. Because the thermocouple is welded to the capsule wall near or at the end cap, the grounded thermocouple generally provides the fastest response time. A typical sensor capsule assembly is shown in Figures 1A to 1D .

[0014] In Figure 1A , the sensor end cap is removed from the end 100 of the sensor capsule 102. It can be seen that the thermocouple wires 104, 106 extend through the MI cable 108. The thermocouple 110 is formed at the junction of the different thermocouple wires 104, 106. This thermocouple tip is grounded (i.e., attached electrically and mechanically to the sensor capsule sheath wall 112). The space 114 near the end 100 of the sensor capsule is generally filled with insulating powder, such as MgO or ceramic.

[0015] Figure 1B is a schematic view of another type of sensor capsule that is part of a sensor. Sensor capsule 120 is similar to sensor capsule 102, but thermocouple 122, which is formed at the junction of different thermocouple wires, is not grounded to the sheath wall 112. Thus, Figure 1B The design shown in

[0016] Figure 1C is a schematic view of a known RTD-based thin film sensor capsule. Sensor capsule 140 generally provides a thin film RTD sensing element 142 that is disposed within a space 114 near an end 144 of the sensor capsule 140. The sensor capsule 140 employs a known 4-wire RTD measurement configuration in which a plurality of first wires 146 are connected to first leads 148 of the RTD element 142 and a plurality of second wires 150 are connected to second leads 152 of the temperature sensitive element 142. The connections are generally provided by soldering, and then the interior of the space 114 is filled with insulating powder to provide thermal conduction and vibration resistance.

[0017] Figure 1D is a schematic view of another type of known RTD sensor capsule design. Sensor capsule 160 uses a wire-wound RTD temperature sensitive element 162 in place of the thin film element 142 (as shown in Figure 1C Again, a 4-wire connection is used in which wires 146 are soldered to leads 148 and wires 150 are soldered to leads 152.

[0018] For temperature probe designs, fast response time is generally considered important for improved process control and efficiency. Often, when forced to choose a thermocouple to improve response time, the user can sacrifice some of the accuracy and stability of the RTD. In addition, RTDs are generally considered more susceptible to vibration-induced failure. The primary cause of RTD failure in the field is believed to be lead breakage due to mechanical shock or vibration.

[0019] Figure 1E and Figure 1FCross-sectional and magnified cross-sectional views of lead breakage due to mechanical shock or vibration are shown. RTDs are typically composed of a platinum sensing element body encapsulated in ceramic and glass, with leads (typically 0.2 millimeter in diameter) welded to the material insulated cable conductors (146, 150). The temperature sensitive element is suspended in a metal sheath and filled with loose MgO powder to prevent element shorting and to prevent excessive movement in the assembly. The mass of the RTD thin film element body encourages strain on the thin leads during vibration and while the MgO powder provides some damping, it is limited. Thermocouples are considered superior in vibration resistance compared to RTDs because thermocouples have a robust design with two mineral insulated cable conductors (over 0.8 millimeter in diameter) that typically protrude a short distance and then are welded together.

[0020] According to embodiments provided herein, a new RTD sensor capsule design replaces an amount of loose powder with a solid insert. Further, the insert is configured to press the RTD element directly against a metal sensor end cap. Thus, the insert positions the element to improve response time, immersion performance, and provide strain relief to increase vibration resistance. The sensor insert design provides the option of high vibration and time response without sacrificing accuracy and long term stability. The material selected to construct the insert should be electrically insulating to avoid lead problems. Other materials for the insert can be used if the wires are provided with their own coating 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 beneficial to response time if the insert material also has a relatively high thermal conductivity to facilitate heat transfer. Ceramic materials would also provide good performance.

[0021] Figure 2is a schematic view of a portion of a temperature probe according to embodiments of the application. The temperature probe 200 generally has 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 suitable number of conductors 146, 150 for providing the required RTD connections. For example, the number of conductors will typically be between 2 and 4. The conductors extend within a mineral insulation 212 within a metal sheath 214, which can be formed of any 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 over the insert 206 once the leads 148, 152 of the thin film sensor 208 have been welded to the conductors 150, 146, respectively. The insert 206 includes a recess 220 that is sized and shaped to mate with a receiving surface 222 of the thin film sensor 208, and thus press an 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 results in a very vibration and heat resistant RTD based sensor probe. While the embodiments disclosed herein are described with respect to the use of MI cable, it is expressly contemplated that functionally similar structures (i.e., insulated conductors disposed in a tube or cable with an environmental seal at the sensor insert) can be used according to embodiments of the application. Furthermore, while the embodiments disclosed herein are described with respect to the use of a thin film RTD sensor, it is expressly contemplated that other types of temperature sensitive elements can be used according to embodiments of the application. Figure 2 An insert 206 supporting a single temperature sensitive element (sensor 208) is shown, but it is expressly contemplated that in some embodiments the insert 206 is configured to support multiple temperature sensitive elements.

[0022] Figure 3 is an enlarged cross-sectional view of a portion of an assembled sensor probe 200 according to embodiments of the application. As can be seen, the insert 206 is disposed within the insert sheath 204 and provides a surface 230 that urges a surface 224 of the thin film sensor 222 into contact with a surface 226 of the end cap 210. In the example shown, the end cap 210 is sized to be received by the insert sheath 204, and the connection of the end cap 210 to the insert sheath 204 is provided by a weld produced at an interface 232.

[0023] Figure 4 and Figure 5are top and bottom perspective views, respectively, of an RTD temperature sensitive element insert 206 according to embodiments of the present application. As shown, the insert 206 is generally cylindrical so that it can be housed in a sliding manner within the insert sheath 204, which is also cylindrical in shape so that it can be effectively connected to the cable 202. The recess 220 has a shape formed by two semi-cylindrical side notches 240, 242, which are spaced apart by a protrusion 244. The flat surface 230 is generally sized to match the shape and thickness of the thin film RTD sensor used. Additionally, the recess 220 includes a tapered portion 246 that extends into a small cutout 248 in the side wall of the insert 206. However, in other embodiments, the tapered portion 246 need not extend into the side wall of the insert 206.

[0024] Figure 5 A bottom perspective view of the insert 206 is shown, including a pair of apertures 250, 252 through which the leads 148, 152 of the thin film RTD 142 pass. These leads are then connected to the MI wires 146, 150 in the slot area 254.

[0025] Figure 6A and Figure 6B are schematic front and side x-ray images of a prototype RTD sensor employing the insert 206 according to embodiments of the present application. RTD based temperature probes using the designs and embodiments provided herein were tested for response time and compared to known configurations, and the results are listed in Figure 7 Some assemblies were made with the temperature sensitive element oriented with the flat side facing the sensor end cap, while some were made with the temperature sensitive element oriented with the flat side facing away from the sensor end cap. For reference, Figure 3 an embodiment is shown in which the flat side of the temperature sensitive element is oriented to face the sensor end cap.

[0026] The thin film sensing element is generally positioned flat and in contact with the sensor tip. This positions the sensor optimally to reduce immersion error. This also greatly improves response time. Because spring loaded sensors are generally considered to have optimal contact at the tip, positioning the element at the tip of the sensor also provides significant thermal coupling to the thermowell.

[0027] Prototypes of the embodiments were tested for response time with and without a thermowell, and the results are shown in Figure 7 “TF-Jumo Insert-D” is a prototype with the flat portion of the sensing element facing the end cap. As Figure 7The design was shown to have 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. "TF-Jumo Insert-U" is a prototype with the elements oriented in the opposite direction. As can be seen in Table 2, Figure 7 the prototype had 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. "TF-RTD Heraeus Standard" is a standard sensor capsule constructed according to known technology and provided for purposes of reference. As can be seen in Table 2, Figure 7 the sensor had 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. "TF-RTD Jumo Standard" is a standard sensor capsule constructed using Jumo elements for testing. The sensor had 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, "TF-Jumo Insert-D" outperformed all of the sensors tested. The results are discussed using T63, which is the time required for a sensor to reach 63% of the temperature differential. T63 is commonly used in the industry when defining response time. Insert-D was tested as a bare sensor (i.e., without a thermowell) and reduced the response time by 40% compared to a standard sensor capsule using the same thin film RTD element. Furthermore, "TF-RTD Jumo Insert-D" was tested in a standard 3 / 4 inch stainless steel thermowell and provided a response time that was nearly 50% shorter compared to a known sensor capsule construction ("TF-RTD Heraeus Standard" in a 3 / 4 inch stainless steel thermowell).

[0028] Figure 7 The data provided in Table 2 also shows testing of a flat hole thermowell. Typically, thermowells have a W geometry formed by a deep drilled diameter. Flat hole thermowells have a geometry that is machined flat so as to better contact the sensor. This thermowell characteristic provides an additional improvement in thermal response time because it increases the contact area between the end of the thermowell and the sensor capsule. The flat hole works well with the sensor insert designs described above. "TF-RTD Jumo Insert-D in a 3 / 4 inch stainless steel thermowell flat hole" is approximately three times faster (66% reduction) than a standard sensor with a standard hole thermowell.

[0029] The embodiments provided herein generally provide greatly 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 presses the thin film element against the sensor end cap, thereby providing stress relief by isolating the element mass from the fine wires. The solid state design also eliminates powder that can move during vibration.

[0030] Figure 8 is a flowchart of a method of manufacturing a temperature probe according to embodiments of the present application. The method 300 begins at block 302, where an MI cable is provided having a sufficient number of conductors therethrough. Next, at block 304, a temperature sensitive element, such as a thin film RTD sensor, is provided having wires that pass through a plurality of holes of a sensor insert, such as the insert 206 Figure 5 Next, at block 306, the wires of the temperature sensitive element are connected (e.g., by soldering, brazing, or soldering) to the conductors of the MI cable. At block 308, a metal sheath is slid over the insert. The metal sheath is then attached to the MI cable at block 310. This attachment can be made in any suitable manner, but preferably the metal sheath is formed of the same metal as the outer metal sheath of the MI cable and the attachment is a continuous weld, thus not only physically connecting the metal sheath to the MI cable, but also creating an effective seal. Finally, at block 312, an end cap is attached to the metal sheath. Preferably, the attachment of the end cap also uses a weld to seal the end cap to the metal sheath.

[0031] While the application has been described with reference to preferred embodiments, it will be appreciated by those skilled in the art that changes can be made in form and detail without departing from the spirit and scope of the application.

Claims

1. A temperature probe comprising: a mineral insulated cable having a metal outer sheath surrounding a mineral insulation in the metal outer sheath, the mineral insulated cable having a plurality of conductors extending through the mineral insulation; a temperature sensitive element having a pair of leads; an insert having at least one conduit to receive the pair of leads of the temperature sensitive element, the insert having a recess configured to receive the temperature sensitive element; an insert sheath configured to slide over the insert, the insert sheath having a first end configured to connect to the metal outer sheath of the mineral insulated cable, the insert sheath further having a second end; an end cap attached to the second end of the insert sheath; and wherein the insert is configured to urge the temperature sensitive element into contact with the end cap.

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

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

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

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

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

7. The temperature probe of claim 1, wherein, The metal outer sheath is welded to the mineral insulated cable.

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

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

10. The temperature probe of claim 1, wherein, The end cap is a disc having a first surface in contact with the temperature sensitive element and a second, opposite surface configured to contact a flat well thermowell.

11. The temperature probe of claim 10, wherein, The second surface of the disc is flat.

12. The temperature probe of claim 1, wherein, The mineral insulated cable has a first plurality of conductors connected to a first lead of the temperature sensitive element and a second plurality of conductors connected to a second lead of the temperature sensitive element.

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

14. The temperature probe of claim 1, wherein, The temperature sensitive element is a thermistor.

15. An insert for a temperature probe, the insert comprising: a cylindrical insert body; a recess defined in the cylindrical insert body, the recess configured to receive a temperature sensitive element; at least one conduit through the cylindrical insert body, the at least one conduit configured to pass through at least one lead of the temperature sensitive element; and an attachment portion configured to allow connection of leads of the temperature sensitive element to conductors of a mineral insulated cable having a metal sheath; and wherein the recess is configured to urge the temperature sensitive element into contact with an end cap attached to an end of an insert sheath distal from the mineral insulated cable, wherein the insert sheath is configured to slide over the insert. ​ 16. The insert of claim 15, wherein, The cylindrical insert is formed of a non-conductive material.

17. The insert of claim 16, wherein, The non-conductive material is ceramic.

18. The insert of claim 15, wherein, The at least one conduit includes a pair of conduits, each configured to pass a lead of the temperature sensitive element therethrough.

19. The insert of claim 18, further comprising a protrusion in the recess spacing the pair of conduits apart.

20. A method of manufacturing a temperature probe, the method comprising: providing a metal sheathed cable having a plurality of conductors disposed therein and spaced apart by a mineral insulation; providing an insert and passing a plurality of leads of a thin film RTD through the insert; connecting each lead of the thin film RTD to at least one conductor of the metal sheathed cable; sliding a metal sheath over the insert; attaching the metal sheath to the metal sheathed cable; attaching an end cap to the metal sheath; and urging the insert to push the thin film RTD into contact with the end cap.

21. The method of claim 20, further comprising welding the metal sheath to the metal sheathed cable and the end cap.

22. A temperature probe comprising: a mineral insulated cable having a metal outer sheath surrounding a mineral insulation in the metal outer sheath, the mineral insulated cable having a plurality of conductors extending through the mineral insulation; a temperature sensitive element having a pair of leads; an insert having at least one conduit to receive the pair of leads of the temperature sensitive element, the insert having a recess configured to receive the temperature sensitive element; an insert sheath configured to slide over the insert, the insert sheath having a first end configured to connect to the metal outer sheath of the mineral insulated cable, the insert sheath further having a second end; and wherein the insert is configured to push the temperature sensitive element into contact with the insert sheath proximate the second end. The insert is configured to support a plurality of temperature sensitive elements.

23. The temperature probe of claim 22, wherein, The insert is configured to push the temperature sensitive element into contact with a sidewall of the insert sheath.

24. The temperature probe of claim 22, wherein, ​

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