Temperature probe with improved response time

Abstract

The temperature probe (100) includes a sheath (104), a temperature sensitive element (102, 122), and an insert member (200). The sheath (104) has a sidewall (108) defining an interior space therein. The temperature sensitive element (102, 122) is disposed within the interior space of the sidewall (108) and has electrical properties that vary with temperature. The insert member (200), formed of silicon carbide, is operatively interposed between the sidewall (108) and the temperature sensitive element (102, 122). A method of manufacturing the temperature probe is also provided. A temperature sensing system using the temperature probe is also provided.

Description

[Technical Field]

[0001] background Temperature probes are used in a variety of industries and environments to indicate the temperature of a material or surface, such as a process fluid flowing within a process fluid conduit, such as a pipe. Temperature probes typically include an outer sheath, made of metal, ceramic, or glass, that protects a temperature-sensitive element located within the sheath from impact and exposure to the process fluid. A non-conductive powder, such as magnesium oxide (MgO) or a ceramic (such as alumina oxide - Al2O3), is typically used to fill any gaps between the inner surface of the sheath and the temperature-sensitive element.

[0002] There are various design considerations that must be taken into account when designing a temperature probe for a specific application. These considerations include accuracy, temperature operating range, and response time. In many high-precision industries, such as pharmaceutical, food and beverage manufacturing, and goods storage and transportation, a fast response time is a critical consideration. Providing a temperature probe with an improved response time would allow such a probe to be used in more applications, especially those requiring a fast response time. Summary of the Invention

[0003] overview The temperature probe includes a sheath, a temperature-sensing element, and an insert. The sheath has a sidewall defining an interior space thereof. The temperature-sensing element is disposed within the interior space of the sidewall and has electrical properties that vary with temperature. The insert, formed of silicon carbide, is operably disposed between the sidewall and the temperature-sensing element. Also provided are methods for manufacturing the temperature probe. Also provided are temperature detection systems using the temperature probe. [Brief explanation of the drawings]

[0004] [Figure 1] FIG. 1 is a schematic diagram illustrating a portion of a prior art RTD-based temperature probe. [Figure 2A]FIG. 1 is a schematic cross-sectional view of a portion of a prior art RTD-based temperature probe. [Figure 2B] FIG. 1 is a schematic cross-sectional view of a portion of a prior art RTD-based temperature probe. [Figure 3] FIG. 1 is a schematic perspective view of a thermal insert for an RTD-based temperature probe according to one embodiment of the present invention. [Figure 4] FIG. 2 is a schematic diagram of a thermal insert disposed within a stainless steel sheath, according to one embodiment of the present invention. [Figure 5] FIG. 1 is a perspective view of an RTD-based temperature probe according to one embodiment of the present invention. [Figure 6A] FIG. 1 is a schematic cross-sectional view of a portion of an RTD-based temperature probe in accordance with one embodiment of the present invention. [Figure 6B] FIG. 1 is a schematic cross-sectional view of a portion of an RTD-based temperature probe in accordance with one embodiment of the present invention. [Figure 7] FIG. 1 is a flow diagram of a method for manufacturing an RTD-based temperature probe in accordance with an embodiment of the present invention. [Figure 8] 1 is a schematic diagram of a thermal insert for application in a thermowell according to one embodiment of the present invention; FIG. DETAILED DESCRIPTION OF THE INVENTION

[0005] DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS FIG. 1 is a schematic diagram illustrating a portion of a prior art RTD-based temperature probe. The probe 100 generally includes an RTD element 102 disposed within a metal sheath 104 having a metal end 106. The sidewall 108 and end 106 together form the end assembly of the temperature probe 100. The end assembly is welded or otherwise bonded to a sheath sidewall 110 at a weld 112. An insulating powder, such as magnesium oxide (MgO), is disposed within the sheath 104 to generally maintain the position of the RTD element 102 within the sheath 104. The RTD element 102 can be formed according to any suitable RTD element formation process, such as thin-film or wirewound techniques. In either case, a circuit is generally provided formed of a metal having a resistance that changes in response to temperature changes. Examples of such metals include platinum, copper, and nickel. Two or more conductors 116, 118 extend through the insulating powder 114 to couple the element 102 to appropriate measurement circuitry (not shown).

[0006] 2A and 2B are cross-sectional views of prior art RTD-based temperature probes. As shown in FIG. 2A, a rectangular RTD element 120 is disposed within MgO powder 114 within a sheath 104. The rectangular RTD element 120 can be formed according to thin-film deposition techniques in which metal is sputtered or otherwise deposited on a non-conductive substrate such as silicon. In FIG. 2B, a circular wirewound RTD sensor element 122 is disposed within the MgO powder 114 within the sheath 104. In either case, to detect temperature from a surface or environment outside the sheath 104, thermal energy must flow through the metal sheath 104 (which can be made of stainless steel or Inconel alloy) and through the MgO powder 114 for the RTD element to generate a detectable temperature change. As will be appreciated, thermal energy can flow in either direction, depending on whether the temperature change is to a higher or lower temperature. In either case, the time required for the thermal energy to travel affects the response time of the RTD. 2A and 2B, the thermal conductivity of MgO powder is approximately 18 W / C. The thermal conductivity of MgO powder, along with the distance that heat must flow through the powder, is believed to provide an opportunity to improve thermal response characteristics (i.e., reduce response time) by providing an insulating structure with a higher thermal conductivity than MgO powder.

[0007] FIG. 3 is a perspective view of a silicon carbide insert for a temperature probe according to one embodiment of the present invention. The silicon carbide insert 200 has a generally cylindrical shape with an outer diameter 202 sized to fit within the inner diameter of the stainless steel sheath 104 (shown in FIG. 1). Additionally, the insert 200 also includes an internal bore 204 sized to receive a temperature-sensitive element, such as an RTD sensor element, shown diagrammatically at 120 or 122 (shown in FIGS. 2A and 2B). The temperature-sensitive element has an electrical property that changes with temperature. For an RTD, the property is resistance; for a thermocouple, the property is voltage. When a thin-film RTD sensor element (such as the rectangular thin-film element 120) is used, the bore 204 of the insert 200 is sized to encompass the rectangular shape of the sensor 120. Similarly, if a wire-wound RTD sensor element 122 is used, the bore 204 is sized so that the outer diameter of the wire-wound RTD sensor 122 passes through the inner diameter bore 204 of the insert 200 .

[0008] FIG. 4 is a schematic diagram of a silicon carbide insert 200 disposed within a stainless steel sheath 104 according to one embodiment of the present invention. In the construction of the sheath 104, the end cap portion joined by the end cap 106 is typically welded to the cylindrical side wall 110 at weld 112, which is a potential area of ​​weakness in the sheath. In one aspect of the present invention, the insert 200 extends from the end cap 106 to a location beyond the weld 112. In this manner, the rigidity of the carbide insert 200 provides strength to the temperature probe at the weld 112. This provides a more robust structure, as the weld 112 can be a source of wear and tear in prior art devices.

[0009] 5 is a perspective view of an RTD-based temperature probe according to one embodiment of the present invention. The thin film RTD sensor element 120 is disposed within the bore 204 of the silicon carbide insert 200. Additionally, a quantity of MgO powder 114 is provided between the inner diameter 204 of the silicon carbide insert 200 and the outer surface 205 of the thin film RTD sensor element 120. Additionally, the additional MgO powder 114 is positioned above the end cap 106 and below the lower surface 220 of the RTD sensor element 120 to provide support.

[0010] The selection of silicon carbide as the material for the insert 200 was based on a careful balancing of various design constraints. The material within the temperature probe must be able to withstand moderately high temperatures, not form galvanic cells with the sheath material, and be able to withstand moderate thermal and mechanical shocks. Furthermore, such a material must be available at a price that maintains the economic viability of the overall design. Silicon carbide meets the stringent material property requirements for temperature probes and offers a thermal conductivity of 200 W / m*K, far exceeding materials commonly used in RTD probe construction. For comparison, MgO powder has a thermal conductivity of 60 W / m*K. The specific heat of MgO powder is 0.880 J / g*K, or 10 Ω at 20°C. 14 It has an electrical resistivity greater than Ω*Cm. The density of MgO powder is also about 3.6 g / cm. 3 On the other hand, silicon carbide has a thermal conductivity of 200 W / m*K, a specific heat of 0.67 J / g*K, and an electrical resistivity of 10 8 Ω*Cm. The density of silicon carbide is 3.2g / cm 3 is.

[0011] In the following comparative analysis of response times, the following equations 1 to 3 are useful.

[0012]

number

[0013]

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[0014]

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[0015] For heat flow comparison, in the prior art, heat from the environment typically flows through the thermal resistance of the sheath, then through the thermal resistance of the MgO powder, and into the RTD sensor element. In embodiments of the present invention employing a thin film RTD sensor element, heat flows through the sheath, through the silicon carbide insert, and through the relatively small amount of MgO powder between the inner diameter of the silicon carbide insert and the thin film sensor.

[0016] The wire-wound sensor embodiment of the present invention simply replaces the heat flow through the MgO of the prior art with heat flow directly through the silicon carbide insert.

[0017] For comparison purposes and to illustrate the differences in heat flow and response time, specific prototypes and dimensions were used. In the examples below, a stainless steel sheath with an outer diameter of 5.95 mm, an inner diameter of 5.35 mm, and a length of 28 mm was used. This resulted in a thermal resistance across the sheath, R sheath As a result, 0.0403C / W is obtained.

[0018] For comparison with the thin-film embodiment, a prior art MgO powder with an outer diameter of 5.35 mm, an inner diameter of 3.0 mm, and a length of 28 mm has a thermal resistance of 0.0548 C / W, while a silicon carbide insert with the exact same dimensions has a thermal resistance of 0.0164 C / W, or in other words, a thermal conductance of 60.7934 C. This represents a 70% reduction in overall thermal resistance.

[0019] When using a silicon insert with an inner diameter of 3.0 mm, a small amount of MgO powder is still required to fill the gap between the rectangular sensor element and the inner diameter of the silicon insert. The outer diameter of this MgO is the same as the inner diameter of the insert (3.0 mm), which is 2.95 mm. This results in a thermal resistance of the MgO of 0.0016 C / W, which is 0.0164 C / W and the thermal resistance of the silicon carbide insert (R). sheath The thermal resistance (0.0403) is added to the total thermal resistance R total This provides a 38.67% reduction from the thin film-based RTD sensor using only MgO powder and no silicon carbide insert, as shown in Figure 6A.

[0020] The improvement provided by the silicon carbide insert is even more pronounced when comparing the wire-wound embodiment. A sheath with an outer diameter of 5.95 mm, an inner diameter of 5.35 mm, and a length of 47 mm was used. This sheath had a thermal resistance of 0.0240 C / W. An MgO powder with an outer diameter of 5.35 mm, an inner diameter of 2.60 mm, and a length of 47 mm provides a thermal resistance of 0.0407 C / W. Thus, the total thermal resistance of the prior art system is 0.0647 C / W. If a silicon carbide insert with the same dimensions as the MgO powder were used, the thermal resistance of the insert would be 0.0122 C / W, resulting in a total thermal resistance of 0.0362 C / W. This results in a R, as shown in FIG. 6B. total These reductions in thermal resistance for embodiments of the present invention provide faster response times for RTD-based temperature probes overall.

[0021] Silicon carbide is composed of strongly bonded tetrahedra of carbon and silicon atoms within a crystal lattice. This creates an extremely hard and strong material. Silicon carbide is impervious to acids, alkalis, and molten salts up to 800°C. In air, silicon carbide forms a protective silicon oxide film at 1200°C and can be used up to 1600°C. Its high thermal conductivity, along with its low thermal expansion and high strength, give this material excellent thermal shock resistance. Silicon carbide ceramics, containing little or no grain boundary impurities, maintain their strength at extremely high temperatures approaching 1600°C. This material's chemical purity, resistance to chemical attack at high temperatures, and high-temperature strength make it highly popular for wafer tray supports and paddles in semiconductor furnaces. The material's electrical conductivity leads to its use in resistive heating elements for electric furnaces and as the main component of thermistors and varistors.

[0022] Returning to Figure 5, a silicon carbide insert 200 is typically press-fit into the sheath at the hot end of a temperature probe. The outer diameter of the insert 200 is matched to the inner diameter of the probe sheath, with the inner diameter of the silicon carbide insert being slightly larger than the sensor element (thin film or wirewound). For thin film elements, the remaining volume is filled with MgO powder to secure the sensing element to the center of the assembly. In some embodiments, a solid blank can be selected to provide the opportunity to customize the pocket to fit the shape of the individual element. This customization can be performed later in the manufacturing process. This allows MgO powder discarded during the manufacturing process to be recovered and reused to fill the void. Furthermore, when a solid insert is used, it can be used with a backstop or blind hole to improve the consistency and repeatability of element placement. Filling the remaining void after the RTD element is installed with MgO (standard magnesia powder) not only provides a rigid internal structure but also thermal conductivity. Additionally, MgO compensates for variations in the shape of the detector element.

[0023] FIG. 7 is a flow diagram of a method for manufacturing an RTD-based temperature probe according to one embodiment of the present invention. Method 300 begins at block 302, where an end of a sheath is provided. As shown in FIG. 1, the end has an end cap 106. Next, at block 304, a silicon carbide insert is press-fit or otherwise placed within the sheath. In one embodiment, the length of the silicon carbide insert is selected to extend from the sheath end cap to a location beyond any end cap / sidewall welds within the sheath. Next, at block 306, an RTD element or blank is placed within the silicon carbide insert. As shown in FIG. 7, the RTD element can be a thin-film element 308 or a wire-wound element 310, or a solid blank of an appropriate shape and size, such as either one of elements 308 / 310. In an embodiment using a thin-film sensor 310, MgO powder is dispensed at block 312 to fill the area between the inner diameter of the silicon carbide insert and the outer surface of the thin-film RTD sensor element.

[0024] While embodiments of the present invention are particularly applicable to providing a silicon carbide insert within a conventional stainless steel sheath, given the strength of the silicon carbide insert, it is expressly contemplated that the wall thickness of the stainless steel or other suitable metal may be reduced, thereby further reducing the response time of the temperature probe.

[0025] Although embodiments of the present invention have been described with respect to temperature probes, they can also be used to improve the thermal conductivity and response time of a thermowell. This can be achieved by replacing a segment of material in the thermowell with a silicon carbide insert at the bottom of the thermowell and implementing the idea on the outer diameter of the inserted probe.

[0026] 8 is a schematic diagram of a thermal insert applied to a thermowell according to one embodiment of the present invention. The thermowell system 400 includes a thermowell 402 having a distal portion 404 that extends into a process fluid conduit or other suitable structure to measure temperature. The distal portion is generally cylindrical and has an interior that can accept a temperature probe assembly 408, such as the prior art probe assembly shown in FIG. 1 or the silicon carbide-based arrangement described herein. In accordance with a further aspect of the present invention, the distal portion 404 of the thermowell 402 can also include a silicon carbide insert 406 to further reduce the response time of the thermowell system 400.

[0027] Additionally, the embodiments described herein can be implemented in hygienic sensors having similar insert sensor arrangements at the distal end of the sensor. Furthermore, improvements to conventional sensors can be provided with minimal effort and can be used in a significant number of sensor configurations and elements.

[0028] Although the present invention has been described with reference to preferred embodiments, those skilled in the art will recognize that changes can be made in form and detail without departing from the spirit and scope of the invention. For example, while the embodiments are generally described with reference to RTDs, the embodiments described herein are applicable to any type of temperature sensitive device, including, but not limited to, thermocouples, thermistors, and semiconductor-based integrated circuits.

Claims

1. A temperature probe, a metal sheath having a sidewall defining an interior space therein; a thin film RTD element disposed within the interior space of the sidewall, the thin film RTD element having a rectangular shape and electrical characteristics that vary with temperature; an insert operably inserted to fit between the sidewall and the thin film RTD element, the insert being formed of silicon carbide; and an insulating powder disposed in a gap between the inner diameter of the insert and the rectangular surface of the thin film RTD element; Includes a temperature probe.

2. The temperature probe of claim 1 , wherein the insert member has a cylindrical shape with an outer diameter located where the outer diameter provides an inner diameter of the sidewall of the metal sheath.

3. The temperature probe of claim 1 , wherein the insulating powder spaces the thin film RTD element from the distal end of the metallic sheath.

4. 3. The temperature probe of claim 2, wherein the metal sheath includes an end cap portion welded to the side wall by a weld, and the insert member has a length greater than a distance from a distal end of the metal sheath to the weld.

5. 1. A method for manufacturing a temperature probe, comprising: providing a metal sheath having an end; disposing a silicon carbide insert within the metal sheath, the silicon carbide insert having a bore extending at least partially therethrough; inserting an RTD element into the bore of a silicon carbide insert, said RTD element being a thin film RTD element; filling a gap between the rectangular shape of the thin film RTD element and the inner diameter of the bore of the insert member with insulating powder; A manufacturing method comprising:

6. The method of claim 5 , wherein the step of placing the silicon carbide insert comprises press-fitting the silicon carbide insert into the metal sheath.

7. The method of claim 5 , wherein the insulating powder is magnesium oxide (MgO) powder.

8. 1. A temperature measurement system comprising: a thermowell having a distal end and a cylindrical sidewall extending therefrom; an RTD temperature probe disposed within the thermowell and having a metallic sheath, The RTD temperature probe is a metal sheath having a sidewall defining an interior space; an RTD element disposed within the interior space of the sidewall, the RTD element having an electrical resistance that varies with temperature; and an insert operably inserted to fit between the sidewall and the RTD element, the insert being formed of silicon carbide; and a silicon carbide insert disposed within the thermowell and positioned around the temperature probe; A temperature measurement system including:

9. 9. The temperature measurement system of claim 8, wherein the RTD element is a thin-film RTD element.

10. 10. The temperature measurement system of claim 9, further comprising insulating powder disposed in a gap between a rectangular surface of the thin film RTD element and an inner diameter of the silicon carbide insert.

11. 9. The temperature measurement system of claim 8, wherein the RTD element is a wire-wound RTD element.

12. The temperature measurement system of claim 8 , wherein an end cap of the temperature probe is positioned in contact with a distal end of the thermowell.

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