Abstract:
A temperature sensor capable of operation in high vibration environment includes a sensor sheath mounted at the distal end of a mineral insulated cable. A resistance temperature detector (RTD) sensing element is connected to leads of the cable within the sheath. The sheath is filled at least partially with a ceramic thermal adhesive.

Description:
BACKGROUND 
       [0001]    The present invention relates to temperature sensors, and in particular to a temperature sensor capable of operating in high vibration environments with improved-accuracy and a high temperature range. 
         [0002]    The temperature of a process fluid in an industrial process is typically measured by a temperature sensor or probe that is positioned in the fluid. The temperature sensor may use a thermocouple or a resistance temperature detector (RTD) to produce an electrical signal that is a function of temperature. 
         [0003]    A thermocouple makes use of two dissimilar metals with different Seebeck coefficients. The; thermocouple generates a voltage based upon a temperature difference between the thermocouple junction and a reference junction. The thermocouple offers a wide temperature operating range (typically from 0° C. to 1450° C.), and does not require a power source to generate an output signal. Thermocouples also are capable of operating in high vibration environments. However, thermocouples are less accurate than RTD devices. 
         [0004]    A resistance temperature detector (RTD) senses temperature by a change in electrical resistance of a metal. The higher the temperature of the RTD, the higher the resistance. An output signal of the RTD sensor is generated by passing a constant electrical current through the RTD and measuring the voltage produced. 
         [0005]    An RTD may be either a wire wound or a thin film device. The RTD may be encapsulated in a temperature probe and used in conjunction with an industrial process transmitter to generate a transmitter output representing the temperature of the fluid in contact with the probe. Platinum is commonly used in wire wound and thin film RTDs, and provides stable and accurate measurement of temperatures up to about 600° C. to 650° C. 
         [0006]    When compared to thermocouples, RTD devices are capable of higher accuracy but have smaller overall temperature range. Also, RTD devices are more susceptible to damage or failure in high vibration environments than are thermocouples. 
         [0007]    There is a need for a temperature sensor capable of operation in high vibration environments with the accuracy of an RTD and with a better high temperature range than is currently available with RTDs designed for high vibration environments. 
       SUMMARY 
       [0008]    A temperature sensor includes a sensor sheath mounted at a distal end of a cable carrying electrical leads. An RTD sensing element positioned within the sheath is connected to leads from the cable. A ceramic thermal adhesive holds the RTD sensing element in place within the sheath. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is a cross-sectional view of a distal portion of the RTD temperature sensor of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0010]      FIG. 1  is a cross-sectional view of a distal portion of RTD temperature sensor  10 , which is capable of operating in high vibration environments, which provides improved high temperature performance. RTD sensor  10  includes mineral insulator (MI) cable  12 , sheath  14 , and RTD sensing element  16 . 
         [0011]    MI cable  12  extends from the proximal end (not shown) of RTD sensor  10  to sheath  14  at the distal end of RTD sensor  10 . MI cable  12  includes outer tube  20 , electrical leads  22   a,    22   b,    22   c,  and  22   d,  and a filling of a mineral insulator powder. In one embodiment, outer tube  20  is a metallic tube made of 321 stainless steel, leads  22   a - 22   d  are nickel leads, and mineral insulator filler  24  is magnesium oxide (MgO) powder. 
         [0012]    Sheath  14  includes extension tube  30  and end cap  32 . Distal end of extension tube  30  is welded to the distal end of tube  20 . End cap  32  is welded to the distal end of extension tube  30  to close the distal end of sheath  14 . In one embodiment, both extension tube  30  and end cap  32  are 316 stainless steel. In other embodiments, extension tube  30  may be formed of 316L, 321, or 316Ti stainless steel. 
         [0013]    RTD sensing element  16  is positioned within sheath  14  near end cap  32 . Leads  34   a  and  34   b  of RTD sensing element  16  extend in a proximal direction to make connection with leads  22   a - 22   d  of cable  12 . Lead  34   a  of RTD sensing element  16  is connected to the distal ends of leads  22   a  and  22   b  by laser weld  36   a.  Lead  34   b  of RTD sensing element  16  is connected to the distal ends of cable leads  22   c  and  22   d  by laser weld  36   b.  In one embodiment, RTD sensing element  16  is a thin film RTD device, such as the HD-421 sensing element manufactured by Heraeus Sensor GmbH. In that embodiment, lead  34   a  and  34   b  are platinum leads. In other embodiments, wire wound RTD sensing elements may be used. 
         [0014]    The interior of sheath  14  is filled with ceramic adhesive filler  38 . In one embodiment, ceramic adhesive filler  38  is a two-component thermoepoxy Thermoguss 2000, which provides stable temperature performance up to about 450° C. In another embodiment, ceramic adhesive filler  38  is Cerastil V336, a two-component ceramic adhesive, which provides stable operation up to about 600° C. 
         [0015]    Ceramic adhesive filler  38  must provide electrical insulation, stable characteristics up to the desired maximum temperature, and must prevent relative movement of RTD sensing element  16  and sheath  14 . Ceramic adhesive filler  38  prevents relative movement by forming a rigid mass within sheath  14 , so that RTD sensing element  16  cannot move relative to capsule  14  during vibration of RTD sensor  10 . 
         [0016]    In high vibration environments, the vibrational load on sensor  10  can exceed an acceleration of 100 m/s 2  at frequencies in range of 10 Hz to 500 Hz. In some cases, the acceleration can be up to 600 m/s 2  over the frequency range of 10 Hz to 500 Hz. 
         [0017]    For operating temperatures up to about 450° C., Thermoguss 2000 ceramic adhesive provides the necessary vibration resistance and is a very good heat conductor. Cerastil V336 offers a higher operating range (up to 600° C.), but does not have as high a thermal conductivity as Thermoguss 2000. It is possible, however, to achieve enhanced temperature range and response times by using a combination of Cerastil V336 and Thermoguss 2000. In one embodiment, approximately two thirds of the interior of sheath  14  is filled with Cerastil V336, and one third of sheath  14  is filled with Thermoguss 2000. In that embodiment, the portion filled by Thermoguss 2000 is at the distal end, nearest RTD sensing element  16 . Other combinations of layers of ceramic adhesives are also possible. 
         [0018]    RTD sensor  10  is fabricated by laser welding leads  34   a  and  34   b  to leads  22   a - 22   d  that extend from the distal end of MI cable  12 . Extension tube  30  is then placed over leads  22   a - 22   d,  leads  34   a,    34   b,  and RTD sensing element  16  so that the proximal end of extension tube  30  abuts the distal end of tube  20  of cable  12 . A laser welded butt joint is then formed between tube  20  and extension tube  30 . 
         [0019]    Ceramic adhesive filler  38  is then introduced into the interior of sheath  14  as defined by extension tube  30 . End cap  32  has not yet been joined to extension tube  30 , so that ceramic adhesive filler  38  can be introduced through the distal opening of sheath  14 . Ceramic adhesive filler  38  may be allowed to cure and harden before end cap  32  is inserted into the distal opening and welded to extension tube  30 . 
         [0020]    Tests of RTD sensors in which the capsule was filled entirely with Cerastil V336 and in which the capsule was filled two thirds with Cerastil V336 and one third with Thermoguss 2000 showed satisfactory operation over a range from −60° C. to 600° C. The devices worked satisfactorily with loads of acceleration up to 600 m/s 2  in the range of 10 Hz to 500 Hz. 
         [0021]    RTD sensors in which the entire capsule was filled with Thermoguss 2000 also provided satisfactory operation in vibrational loads of acceleration up to 600 m/s 2  in a range of frequency from 10 Hz to 500 Hz. The RTD sensors in which Thermoguss 2000 filled the entire capsule provided satisfactory stable temperature performance up to about 450° C. 
         [0022]    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.