Patent Publication Number: US-11650106-B2

Title: Temperature probe with improved response time

Description:
BACKGROUND 
     Temperature probes are used in a variety of industries and environments to provide an indication of temperature of a substance or surface, such as a process fluid flowing in a process fluid conduit, such as a pipe. A temperature probe generally includes an outer sheath that is formed of metal, ceramic or glass and that protects the temperature sensitive element, located inside the sheath, from impacts and exposure to process fluids or the like. Non-conductive powder, such as Magnesium Oxide (MgO) or ceramic (such as Alumina Oxide—Al 2 O 3 ) is usually used to fill the space between the inner surface of the sheath and the temperature sensitive element. 
     Temperature probes have a variety of design considerations that must be considered for applicability to a particular application. Among these considerations are accuracy, thermal operating range, and response time. A fast response time is a very important consideration in a number of high-precision industries, such as pharmaceuticals, food and beverage production, and custody transfer of goods. Providing a temperature probe with an improved response time would allow such temperature probes to be used in more applications, and particularly applications that require fast response times. 
     SUMMARY 
     A temperature probe includes a sheath, a temperature sensitive element, and an insert. The sheath has a sidewall defining an interior space therein. The temperature sensitive element is disposed within the interior space of the sidewall and has an electrical characteristic that varies with temperature. The insert, which is formed of silicon carbide, is operably interposed between the sidewall and the temperature-sensitive element. A method of manufacturing a temperature probe is also provided. A temperature sensing system employing a temperature probe is also provided. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a diagrammatic view of a portion of an RTD-based temperature probe in accordance with the prior art. 
         FIGS.  2 A and  2 B  are diagrammatic cross-sectional views of portions of RTD-based temperature probes in accordance with the prior art. 
         FIG.  3    is a diagrammatic perspective view of a thermal insert for an RTD-based temperature probe in accordance with an embodiment of the present invention. 
         FIG.  4    is a diagrammatic view of a thermal insert disposed within a stainless-steel sheath in accordance with an embodiment of the present invention. 
         FIG.  5    is a diagrammatic view of an RTD-based temperature probe in accordance with an embodiment of the present invention. 
         FIGS.  6 A and  6 B  are diagrammatic cross-sectional views of portions of RTD-based temperature probes in accordance with embodiments of the present invention. 
         FIG.  7    is a flow diagram of a method of manufacturing an RTD-based temperature probe in accordance with an embodiment of the present invention. 
         FIG.  8    is a diagrammatic view of a thermal insert applied to a thermowell in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
       FIG.  1    is a diagrammatic view of a portion of an RTD-based temperature probe in accordance with the prior art. Probe  100  generally includes an RTD element  102  disposed within a metallic sheath  104  having a metallic end  106 . Together sidewall  108  and end  106  form an end assembly of temperature probe  100 . The end assembly is welded to, or otherwise coupled to sheath sidewall  110  at weld  112 . An insulative powder, such as magnesium oxide (MgO), is disposed within sheath  104  and generally maintains the position of RTD element  102  within sheath  104 . RTD element  102  can be formed in accordance with any suitable RTD element forming processes, such as thin film technology, or wire wound technologies. In either case, a circuit is provided formed of a metal that has a resistance that generally changes in response to temperature changes. Examples of such metals include platinum, copper, and nickel. Two or more conductors  116 ,  118 , extend through insulative powder  114  and couple element  102  to appropriate measurement circuitry (not shown). 
       FIGS.  2 A and  2 B  are cross sectional views of RTD-based temperature probes in accordance with the prior art. As shown in  FIG.  2 A , a rectangular RTD element  120  is positioned within MgO powder  114  within sheath  104 . Rectangular RTD element  120  may be formed in accordance with thin film deposition techniques where the metal is sputtered or otherwise deposited on a non-conductive substrate, such as silicon. In  FIG.  2 B , a circular wire-wound RTD sensor element  122  is positioned within MgO powder  114  within sheath  104 . In each case, in order for the temperature from the surface or environment outside of sheath  104  to be detected, thermal energy must flow through the metallic sheath  104  (which can be formed of stainless steel or an Inconel alloy), and through MgO powder  114  in order for the RTD element to generate a detectable temperature change. As can be appreciated, the thermal energy may flow in either direction depending on whether the temperature change is hotter or cooler. In either case, the time required for the thermal energy to transfer will affect the response time of the RTD. As shown in  FIGS.  2 A and  2 B , the thermal conductance of MgO powder is approximately 18 W/C. It is believed that the thermal conductance of the MgO powder, coupled with the distance that the heat must flow through the powder, provides an opportunity to improve thermal response characteristics (i.e., reduce response time) by providing an insulative structure having a higher thermal conductivity than the MgO powder. 
       FIG.  3    is a diagrammatic view of a silicon carbide insert for a temperature probe in accordance with an embodiment of the present invention. Silicon carbide insert  200  generally has a cylindrical shape with an outer diameter  202  that is sized to fit within an inner diameter of a stainless-steel sheath  104  (shown in  FIG.  1   ). Additionally, insert  200  also includes an interior bore  204  that is sized to receive a temperature sensitive element, such as an RTD sensor element, indicated diagrammatically at reference numerals  120 , or  122  (shown in  FIGS.  2 A and  2 B ). The temperature sensitive element has an electrical characteristic that varies with temperature. In the case of an RTD, the characteristic is resistance and in the case of a thermocouple, the characteristic is voltage. When a thin film RTD sensor element is used (such as square thin film element  120 ) bore  204  of insert  200  is sized to circumscribe the square shape of sensor  120 . Similarly, when wire-wound RTD sensor element  122  is used, bore  204  is sized such that the outer diameter of wire-wound RTD sensor  122  will pass through inner bore  204  of insert  200 . 
       FIG.  4    is a diagrammatic view of silicon carbide insert  200  disposed within a stainless-steel sheath  104  in accordance with an embodiment of the present invention. In the construction of sheath  104 , the endcap portion bound by end cap  106  is typically welded to the cylindrical sidewall  110  at weld  112 . This is an area of potential weakness in the sheath. In accordance with one aspect of the present invention, insert  200  extends from end cap  106  to a location beyond weld  112 . In this way, the rigidity of carbide insert  200  also provides strength to the temperature probe at the location of weld  112 . This provides a more robust structure in that weld  112  is sometimes the source of wear or breakage in prior art devices. 
       FIG.  5    is a diagrammatic view of an RTD-based temperature probe in accordance with an embodiment of the present invention. Thin film RTD sensor element  120  is disposed within bore  204  of silicon carbide insert  200 . Further, a quantity of MgO powder  114  is provided between inner diameter  204  of silicon carbide insert  200  and the exterior surface  205  of thin film RTD sensor element  120 . Further, additional MgO powder  114  is located below and supports lower surface  220  of RTD sensor element  120  above end cap  106 . 
     The selection of silicon carbide for the material of insert  200  is based on a careful balance of various design constraints. Materials within the temperature probe must withstand reasonably high temperatures, must not create a galvanic cell with the sheath material, and must be able to withstand reasonable thermal and mechanical shock. Further, such materials must be able to be used at prices that maintain the economic feasibility of the overall design. Silicon carbide meets the stringent material property requirements needed in such as a temperature probe, and provides a 200 W/m*K thermal conductivity that far exceeds that of 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 with an electrical resistivity greater than 10 14  ohms*Cm @ 20 degrees C. The density of MgO powder is also approximately 3.6 grams/cm 3 . In contrast, silicon carbide has a thermal conductivity of 200 W/m*K with a specific heat of 0.67 J/g*K and an electrical resistivity of 10 8  ohms*Cm @ 20 degrees C. The density of silicon carbide is 3.2 grams/cm 3 . 
     In the following analysis of response time comparisons, Equations 1-3, set forth below, are useful. 
     
       
         
           
             
               
                 
                   Q 
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                         t 
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                       R 
                       total 
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   1 
                 
               
             
           
         
       
     
     In Equation 1, Q represents heat flow across a total thermal resistance R total  with a temperature differential t 2 −t 1 . 
     
       
         
           
             
               
                 
                   
                     R 
                     cylinder 
                   
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                             1 
                           
                         
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                       Lk 
                     
                   
                 
               
               
                 
                   Equation 
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                   2 
                 
               
             
           
         
       
     
     In Equation 2, R cylinder  is the thermal resistance through the walls of a cylinder having an inner radius r 1  and an outside radius r 2  where L is the length of the cylinder and k is the thermal conductivity of the material. 
     
       
         
           
             
               
                 
                   
                     C 
                     total 
                   
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                         1 
                         
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                         total 
                       
                     
                   
                 
               
               
                 
                   Equation 
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                   3 
                 
               
             
           
         
       
     
     Equation 3 defines total conductance C total  and the inverse of total thermal resistance R total . 
     For heat flow comparisons, in the prior art, the heat from the environment generally flows through the thermal resistance of the sheath then flow through the thermal resistance of the MgO powder into the RTD sensor element. For embodiments of the present invention that employ thin film RTD sensor element, the thermal heat flow passes through the sheath, through the silicon carbide insert, and through a relatively small amount of MgO powder between the inner bore of the silicon carbide insert and the thin film sensor. 
     For wire-wound sensor embodiments of the present invention, the heat flow through the prior art MgO is simply replaced with heat flow directly through the silicon carbide insert. 
     For comparison sake, specific prototypes and dimensions are used in order to illustrate the differences in heat flow and response time. In the following examples, a stainless-steel sheath was used having an outside diameter of 5.95 mm and an inside diameter of 5.35 mm with a length of 28 mm. This provides a thermal resistance across the sheath R sheath  of 0.0403 C/W. 
     For comparing thin film embodiments, the prior art MgO powder also has an outside diameter of 5.35 mm and an inside diameter of 3.0 mm and a length of 28 mm for a thermal resistance of 0.0548 C/W. In contrast, a silicon carbide insert having precisely the same dimensions has a thermal resistance of 0.0164 C/W or in other words a thermal conductance of 60.7934. This provides a 70% decrease in overall thermal resistance. 
     Using a silicon insert with a 3.0 mm inside diameter will still require a small amount of MgO powder to fill the space between the rectangular sensor element and the inside diameter of the silicon insert. The outside diameter of this MgO is the same and the inside diameter of the insert (3.0 mm) and the inside diameter of the MgO is 2.95 mm. This yields an MgO thermal resistance of 0.0016 C/W which is added to the thermal resistance of the silicon carbide insert (0.0164 C/W) and R sheath  (0.0403) provides a total thermal resistance R total  of 0.0583 C/W. This is a 38.67% reduction from thin film-based RTD sensors that employ only MgO powder and do not use a silicon carbide insert, as shown in  FIG.  6 A   
     Comparing wire-wound embodiments, the improvement provided by a silicon carbide insert is more pronounced. A sheath with an outside diameter of 5.95 mm, inside diameter of 5.35 mm and length of 47 mm was used. This sheath had a thermal resistance of 0.0240 C/W. MgO powder having an outside diameter of 5.35 mm, and inside 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. When a silicon carbide insert is used having the same dimensions as the MgO powder, the thermal resistance of the insert is 0.0122 C/W for a total thermal resistance of 0.0362 C/W. This provides a 44.05% decrease in R total , as shown in  FIG.  6 B . These decreases in thermal resistance of embodiments of the present invention provide quicker response times for the overall RTD-based temperature probe. 
     Silicon carbide is composed of tetrahedra of carbon and silicon atoms with strong bonds in the crystal lattice. This produces a very hard and strong material. Silicon carbide is not attacked by any acids or alkalis or molten salts up to 800° C. In air, silicon carbide forms a protective silicon oxide coating at 1200° C. and is able to be used up to 1600° C. The high thermal conductivity coupled with low thermal expansion and high strength give this material exceptional thermal shock resistant qualities. Silicon carbide ceramics with little or no grain boundary impurities maintain their strength to very high temperatures approaching 1600° C. with no strength lost. Chemical purity, resistance to chemical attack at temperature, and strength retention at high temperatures has made this material very popular as wafer tray supports and paddles in semiconductor furnaces. The electrical conduction of the material has led to its use in resistance heating elements for electric furnaces and as a key component in thermistors and varistors. 
     Returning to  FIG.  5   , silicon carbide insert  200  is generally pressed into the hot end sheath of a temperature probe. The outside diameter of insert  200  is matched to the inside diameter of the probe sheath, and the inside diameter of the silicon carbide insert is sized to be slightly larger than the sensor element (thin film or wire-wound). For thin film elements, the remaining volume is filled with MgO powder to secure the sensing element in the center of the assembly. In some embodiments, a solid blank may be chosen so that there is an opportunity to customize the pocket to each individual element&#39;s geometry. This customization could be implemented in a late stage of the manufacturing process. This would enable manufacturing to collect waste MgO powder and reuse it in filling of the cavity. Further, if a solid insert is used, it could be employed with a backstop or blind hole to improve the consistency and repeatability of element placement. Providing the MgO (standard magnesia powder) in the remaining void after the RTD element is installed allows for the thermal continuity as well as rigid internal construction. MgO will also compensate for any geometry variation of the sensing element 
       FIG.  7    is a flow diagram of a method of manufacturing an RTD-based temperature probe in accordance with an 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 pressed or otherwise positioned within the sheath. In one embodiment, the length of the silicon carbide insert is selected to extend from an end cap of the sheath to a location beyond any end cap/sidewall weld in the sheath. Next, at block  306 , an RTD element or a blank is disposed within the silicon carbide insert. As shown in  FIG.  7   , the RTD element may be a thin film element  308  or a wire-wound element  310 , or a suitable solid blank shaped and sized like one of elements  308 / 310 . In embodiments that employ the thin film sensor  310 , MgO powder is provided at block  312  to fill the area between the inside diameter of the silicon carbide insert and the outside surface of the thin film RTD sensor element. 
     While embodiments of the present invention are particularly applicable to providing a silicon carbide insert within legacy stainless-steel sheaths, given the strength of the silicon carbide insert, it is also expressly contemplated that the wall thickness of the stainless-steel or other suitable metal may be able to be reduced thus further reducing the response time of the temperature probe. 
     While embodiments of the present invention have been described with respect to temperature probes, embodiments could also be used to improve thermal conductivity and response time of thermowells. This could be accomplished by replacing a material segment of the thermowell with a silicon carbide insert at the bottom of the thermowell and implementing the idea on the outside diameter of inserted probe. 
       FIG.  8    is a diagrammatic view of a thermal insert applied to a thermowell in accordance with an embodiment of the present invention. 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 a temperature. The distal portion is generally cylindrical and has an interior that is able to receive a temperature probe assembly  408 , such as the prior art probe assembly shown in  FIG.  1   , or the silicon carbide-based arrangements described herein. In accordance with a further aspect of the present invention, the distal portion  404  of thermowell  402  may also include a silicon carbide insert  406  to further reduce response time of the thermowell system  400 . 
     Further, embodiments described herein could also be implemented for hygienic sensors with a similar insert sensor placement at the end of a tip of the sensor. Further still, improvements to legacy sensors can be provided with minimal efforts and could be used with a significant number of sensor configurations and elements. 
     Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention. For example, while embodiments are generally described with respect to RTDs, embodiments described herein are applicable to any type of temperature sensitive element including, without limitation, thermocouples, thermistors, and semiconductor-based integrated circuits.