Abstract:
Temperature sensing devices and methods for determining downhole fluid temperature at a drill string in a borehole while drilling are disclosed. The device includes a temperature sensor capable of detecting and measuring rapid temperature changes and may be used to sense the temperature of fluid inside or outside the drill string. In addition, the device includes a thermal conductor that receives and secures the temperature sensor; the thermal conductor is in turn received and secured in a thermal insulator that provides a thermal barrier. In an embodiment, the device is disposed in a channel within an outer diameter of the drill string such that the device is protected from the side wall of the borehole and drilling fluid and cuttings can pass through the channel without becoming packed around the temperature sensor.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application claims the benefit of U.S. Provisional Application No. 61/883,578, filed Sep. 27, 2013, entitled “Downhole Temperature Sensing of the Fluid Flow in and Around a Drill String Tool,” which is incorporated herein by reference in its entirety for all purposes. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    Not applicable. 
       BACKGROUND 
       [0003]    The present disclosure relates generally to methods and apparatus for sensing temperature proximate a drill string tool conveyed in a borehole. The present disclosure relates more particularly to methods and apparatus for sensing the temperature of drilling fluid in the inner diameter, or flowbore, of the drill string tool or in the annulus between the outer diameter of the drill string tool and the borehole. 
         [0004]    To recover hydrocarbons from subterranean formations, wells are generally constructed by drilling into the formation using a rotating drill bit attached to the lower end of an assembly of drill pipe sections connected end-to-end to form a drill string. In some cases the drill string and bit are rotated by a drilling table at the surface, and in other cases the drill bit may be rotated by a downhole motor within the drill string above the bit, while remaining portions of the drill string remain stationary. In most cases, the downhole motor is a progressive cavity motor that derives power from drilling fluid (sometimes referred to as mud) pumped from the surface, through the drill string, and then through the motor (hence the motor may also be referred to as a mud motor). 
         [0005]    Modern oil field operations demand a great quantity of information relating to the parameters and conditions encountered downhole. Such information typically includes borehole environmental information, such as temperature, pressure, etc., and drill string operational information. Temperature is a common downhole reading; however, sensors are often not placed optimally for temperature measurements. Sensors are typically disposed on the downhole tools and measure the temperature of the tool housing and do not track temperature changes very well. Alternatively, temperature sensors may be placed at the point of interest; however, the point of interest in a borehole is in the path of the fluid flowing either through the internal diameter (ID) of the drill pipe or through the annulus formed about the outer diameter (OD) of the pipe. In either case, an exposed temperature probe is difficult to handle and subject to erosion from the fluid flowing at hundreds of gallons per minute (GPM). 
         [0006]    There is a need to measure small temperature changes in the borehole while drilling. Temperature changes on the order of tenths of a degree are very informative of the borehole environment and provide a method for predicting the events that will follow. Temperature has an impact on all downhole readings and being able to detect small changes in temperature allows the exact temperature coefficient in every calculation be determined, which helps correctly depict the temperature reading by subtracting the temperature effects from other readings. However, commonly used temperature measuring systems can be inaccurate due to a margin of error from +/−2° C. up to +/−5° C. at higher temperatures, non-optimal sensor positioning as previously discussed, temperature dissipation in the body in which the housing of the downhole tools acts as a shield against rapid temperature changes and delays the sensor&#39;s ability to detect rapid temperature changes, and low precision of the temperature sensor where the sensor resolution is limited to 1.0 or 0.5° C. There is a further need to prevent drilling fluid and cuttings from becoming packed around the temperature sensors. Drilling fluid acts as a thermal insulator and may prevent true temperature measurement readings as the temperature fluctuates. 
       BRIEF SUMMARY OF THE DISCLOSURE 
       [0007]    In one embodiment, a temperature sensing device for determining downhole fluid temperature at a drill string in a borehole includes a resistance temperature sensor coupled with thermally conductive epoxy to an internal surface of a cylindrical thermal conductor and a cylindrical thermal insulator having a cylindrical cavity configured to sealingly house the thermal conductor. In addition, the device includes a plurality of seals disposed between an outer cylindrical surface of the thermal conductor and an inner cylindrical surface of the thermal insulator and between an outer cylindrical surface of the thermal insulator and an inner surface of a cavity in the drill string. The device further includes a first retaining ring disposed in a groove formed in the inner surface of the thermal insulator and a second retaining ring disposed in a groove formed in the inner surface of the cavity in the drill string. In some embodiments, the thermal conductor internal surface is disposed proximate an outer surface of the drill string to sense the fluid temperature outside the drill string. In other embodiments, the thermal conductor internal surface is disposed proximate an inner surface of the drill string to sense the fluid temperature inside the drill string. 
         [0008]    In one embodiment, a method of determining downhole fluid temperature at a drill string in a borehole includes coupling a resistance temperature sensor to an internal surface of a thermal conductor with thermally conductive epoxy and inserting the thermal conductor into a cylindrical cavity of a cylindrical thermal insulator. In addition, the method includes installing a plurality of seals between an outer cylindrical surface of the thermal conductor and an inner cylindrical surface of the thermal insulator and between an outer cylindrical surface of the thermal insulator and an inner surface of a cavity in the drill string. The method further includes installing a first retaining ring in a groove formed in the inner surface of the thermal insulator and installing a second retaining ring in a groove formed in the inner surface of the cavity in the drill string. In some embodiments, the method may further include disposing the thermal conductor internal surface proximate an outer surface of the drill string to sense the fluid temperature outside the drill string. In other embodiments, the method may further include disposing the thermal conductor internal surface proximate an inner surface of the drill string to sense the fluid temperature inside the drill string. 
         [0009]    In an embodiment, a temperature sensing device for determining downhole fluid temperature at a drill string in a borehole includes a thermal insulator to be received and secured in a cavity in the drill string, a thermal conductor to be received and secured in the thermal insulator, and a temperature sensor to be received and secured in the thermal conductor and disposed adjacent a first opening in the cavity. In addition, the device includes a thermally insulating plug to be received in a second opening in the cavity and to be secured in the cavity to retain the thermal insulator and the thermal conductor. Moreover, the thermal insulator provides a first thermal barrier between the thermal conductor and the drill string and the thermally insulating plug provides a second thermal barrier between the thermal conductor and the drill string. In some embodiments, the device further includes a thermally insulating ring disposed between the plug and the thermal conductor to provide the second thermal barrier. In some embodiments, the second thermal barrier is disposed in the cavity such that the cavity is separated into a first sensor portion and a second portion. 
         [0010]    In one embodiment, a temperature sensing device for determining downhole fluid temperature at a drill string in a borehole includes a thermal insulator to be received and secured in a cavity in the drill string, a thermal conductor to be received and secured in the thermal insulator, a temperature sensor to be received and secured in the thermal conductor and disposed adjacent a first opening in the cavity, and an inner cavity portion disposed radially inward of the thermal insulator and the thermal conductor. In addition, the thermal insulator provides a first thermal barrier between the thermal conductor and the drill string and the inner cavity portion provides a second thermal barrier between the thermal conductor and the drill string. In some embodiments, air in the inner cavity thermally insulates the thermal conductor from the drill string at the second thermal barrier. In some embodiments, a thermal conduction path to the temperature sensor disposed outside of the inner cavity portion. In some embodiments, the device is disposed in a channel on the drill string and within an outer diameter of the drill string. 
         [0011]    In one embodiment, a temperature sensing device for determining downhole fluid temperature at a drill string in a borehole includes a housing having a cylindrical cavity, a resistance temperature sensor coupled with thermally conductive epoxy to an internal surface of the cavity, and a plurality of stabilizers configured to secure the housing within the drill string. In some embodiments, the resistance temperature sensor is further coupled with potting to the internal surface of the cavity. In some embodiments, the housing may be steel and have a coating to prevent erosion. In some embodiments, the stabilizers have a tapered outer surface. 
         [0012]    Embodiments described herein comprise a combination of features and advantages intended to address various shortcomings associated with certain prior devices, systems, and methods. The foregoing has outlined rather broadly the features and technical advantages of the invention such that the detailed description of the invention that follows may be better understood. The various characteristics described above, as well as other features, will be readily apparent to those skilled in the art upon reading the following detailed description, and by referring to the accompanying drawings. It should be appreciated by those skilled in the art that the conception and the specific embodiments disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    For a detailed description of the disclosure, reference will now be made to the accompanying drawings in which: 
           [0014]      FIG. 1  is a schematic view of a drilling system including an embodiment of a system in accordance with the principles described herein; 
           [0015]      FIG. 2  is an enlarged cross-sectional schematic view of a portion of a first embodiment of the system shown in  FIG. 1 ; 
           [0016]      FIG. 3  is an enlarged schematic view of a portion of the system shown in  FIG. 2 ; 
           [0017]      FIG. 4  is an enlarged schematic view of a first alternative inner diameter sensor of the system shown in  FIG. 3 ; 
           [0018]      FIG. 4A  is an isolated view of a cavity of the inner diameter sensor shown in  FIG. 4 ; 
           [0019]      FIG. 4B  is an isolated view of an insulator of the inner diameter sensor shown in  FIG. 4 ; 
           [0020]      FIG. 4C  is an isolated view of a conductor of the inner diameter sensor shown in  FIG. 4 ; 
           [0021]      FIG. 4D  is an isolated view of a threaded plug of the inner diameter sensor shown in  FIG. 4 ; 
           [0022]      FIG. 5  is an enlarged schematic view of a first alternative outer diameter sensor of the system shown in  FIG. 3 ; 
           [0023]      FIG. 5A  is an isolated view of a cavity of the outer diameter sensor shown in  FIG. 5 ; 
           [0024]      FIG. 5B  is an isolated view of an insulator of the outer diameter sensor shown in  FIG. 5 , 
           [0025]      FIG. 5C  is an isolated view of a conductor of the outer diameter sensor shown in  FIG. 5 ; 
           [0026]      FIG. 6  is an enlarged schematic view of a second alternative inner diameter sensor of the system shown in  FIG. 3 ; 
           [0027]      FIG. 6A  is an isolated view of an insulator of the second alternative inner diameter sensor shown in  FIG. 6 ; 
           [0028]      FIG. 6B  is an isolated view of a conductor of the second alternative inner diameter sensor shown in  FIG. 6 ; 
           [0029]      FIG. 7  is an enlarged schematic view of a second alternative outer diameter sensor of the system shown in  FIG. 3 ; 
           [0030]      FIG. 7A  is an isolated view of a cavity of the second alternative outer diameter sensor shown in  FIG. 7 ; 
           [0031]      FIG. 8  is an enlarged partial cross-sectional schematic view of a portion of a second embodiment of the system shown in  FIG. 1 ; 
           [0032]      FIG. 9  is an enlarged schematic view of a portion of the system shown in  FIG. 8 ; 
           [0033]      FIG. 10A  is an enlarged schematic top view of a portion of an alternative embodiment of the system shown in  FIG. 3 ; 
           [0034]      FIG. 10B  is an enlarged schematic view of the embodiment shown in  FIG. 10A ; and 
           [0035]      FIG. 10C  is an enlarged schematic side view of the embodiment shown in  FIG. 10A . 
       
    
    
     DETAILED DESCRIPTION 
       [0036]    The following discussion is directed to various exemplary embodiments. However, one skilled in the art will understand that the examples disclosed herein have broad application, and that the discussion of any embodiment is meant only to be exemplary of that embodiment, and not intended to suggest that the scope of the disclosures, including the claims, is limited to that embodiment. 
         [0037]    Certain terms are used throughout the following description and claims to refer to particular system components. This document does not intend to distinguish between components that differ in name but not function. Moreover, the drawing figures are not necessarily to scale. Certain features of the disclosure may be shown exaggerated in scale or in somewhat schematic form, and some details of conventional elements may not be shown in the interest of clarity and conciseness. Further, some drawing figures may depict vessels in either a horizontal or vertical orientation; unless otherwise noted, such orientations are for illustrative purposes only and is not a required aspect of this disclosure. 
         [0038]    In the following discussion and in the claims, the terms “including” and “comprising” are used in an open-ended fashion, and thus should be interpreted to mean “including, but not limited to . . . . ” Also, the terms “couple,” “attach,” “connect” or the like are intended to mean either an indirect or direct mechanical or fluid connection, or an indirect, direct, optical or wireless electrical connection. Thus, if a first device couples to a second device, that connection may be through a direct mechanical or electrical connection, through an indirect mechanical or electrical connection via other devices and connections, through an optical electrical connection, or through a wireless electrical connection. In addition, as used herein, the terms “axial” and “axially” generally mean along or parallel to a given axis (e.g., central axis of a body or a port), while the terms “radial” and “radially” generally mean perpendicular to the axis. For instance, an axial distance refers to a distance measured along or parallel to the axis, and a radial distance means a distance measured perpendicular to the axis. Any reference to up or down in the description and the claims will be made for purpose of clarification, with “up,” “upper,” “upwardly,” or “upstream” meaning toward the surface of the well and with “down,” “lower,” “downwardly,” or “downstream” meaning toward the terminal end of the well, regardless of the well bore orientation. In some applications of the technology, the orientations of the components with respect to the surroundings may be different. For example, components described as facing “up,” in another application, may face to the left, may face down, or may face in another direction. 
         [0039]    In various embodiments to be described in detail below, a system and process for determining the temperature of the drilling fluid includes the use of resistance temperature detectors (RTD) in accordance with the principles of the present disclosure. In certain embodiments, the temperature of the drilling fluid in the inner diameter (ID) of the drill string tool is determined and in certain other embodiments, the temperature of the drilling fluid in the borehole annulus or outer diameter (OD) of the drill string tool is determined. 
         [0040]    Referring now to  FIG. 1 , which shows a drilling system  10  including sensor assembly  100  in accordance with various embodiments. As shown, the drilling system  10  is a land based drilling system, but could also be water based. A drilling platform  12  supports a drilling rig  14  having a hoisting device  16  for raising and lowering a drill string  18  having a central axis  11 . The drill string  18  comprises a bottom hole assembly  20  having a downhole tool  22  and a drill bit  24  driven by a downhole motor and/or rotation of the drill string  18 . As bit  24  rotates, it creates a borehole  26  that passes through various subsurface formations. A pump  30  circulates drilling fluid  32  through a feed pipe  34 , downhole through the inner diameter of drill string  18 , through orifices in drill bit  24 , back to the ground surface  50  via the annulus  28  around the drill string  18 , and into a drilling fluid reservoir  36 , such as a mud tank or retention pit. The drilling fluid transports cuttings from the borehole into the reservoir  34  and aids in maintaining the borehole integrity. 
         [0041]    In addition to the sensor assembly  100 , there may be one or more additional sensors  101  located proximate to, or at distances from, the sensor assembly  100 . The additional sensors  101  may be any suitable sensor for determining one or more downhole parameters, such as, but not limited to, a gyroscopic sensor, a strain gauge sensor, a pressure sensor, a temperature sensor, a logging tool, a measurement while drilling tool, or other sensor. The additional sensors  101  may be used independently or in combination with the sensor assembly  100 . 
         [0042]    The drilling system  10  may further comprise a memory element  102 , where the data collected by the sensors  100 ,  101  is stored for retrieval at the surface. This stored data may be downloaded from the memory  102  when the downhole tool  22  is brought to the surface  50  at the end of drilling operations. 
         [0043]    Drilling system  10  further comprises a controller  40 , which sends and receives signals about the drilling system  10  via one or more communication links  42 . The communication link  42  may be any communications system known in the art including, but not limited to, a wired pipe system, a mud-pulse system, an electromagnetic telemetry system, a radio frequency transmission system, or an acoustic transmission system. 
         [0044]    The controller  40  may be used to control the equipment at the drilling system  10 , such as, but not limited to, the downhole tool  22 , the hoisting device  16 , one or more pumps  30 , the sensor assembly  100 , and the additional sensors  101 . Further, the controller  40  may receive data from the sensor assembly  100 , the additional sensors  101 , and/or the memory  102  at a data transmission rate of 0.4 Hz to 800 Hz depending upon the speed of the communications link  42 . The data received by the controller  40  may be used to evaluate and/or manipulate drilling system operations. 
         [0045]    In the present embodiment, the sensor assembly  100  is shown and described as being located within the drill string  18 . The sensor assembly  100  may be located at any suitable downhole location including, but not limited to, in or about a drill collar, in an annulus of a drill collar, in a sub, in or about a tool body, or other downhole locations. Further, the sensor assembly  100  may be located in more than one downhole location, as will be described in more detail below. 
         [0046]    Referring now to  FIG. 2 , which shows an enlarged schematic view of a portion of a first embodiment of the drill string  18  of drilling system  10  shown in  FIG. 1  having sensor assembly  100 . The sensor assembly  100  may comprise either one sensor  200  configured to measure the temperature of drilling fluid  32   a  flowing down the inner diameter of the drill string  18  (“ID sensor  200 ”) or one sensor  300  configured to measure the temperature of the drilling fluid  32   b  flowing up the annulus  28  or outer diameter of the borehole  26  (“OD sensor  300 ”); or sensor assembly  100  may comprise two sensors  200 ,  300  configured to measure the temperature of both the drilling fluid  32   a  flowing down the inner diameter of the drill string  18  (ID sensor  200 ) and the drilling fluid  32   b  flowing up the annulus  28  (OD sensor  300 ) as shown in the present embodiment. Further, more than one sensor assembly  100  may be employed in a drilling system  10  at various locations to measure the temperature of the drilling fluid  32  at different locations within the drill string  18  and/or in the annulus  28 . It should be understood that other downhole fluids can take the place of the drilling fluid in the embodiments described herein, including but not limited to, completion fluids, servicing fluids, formation fluids, production fluids, and other downhole fluids. 
         [0047]    Referring now to  FIG. 3 , which shows an enlarged view of section  3  depicted in  FIG. 2  and includes sensor assembly  100  having an ID sensor  200  with central axis  211  and an OD sensor  300  with central axis  311 . Central axes  211 ,  311  are orthogonally positioned in relation to the central axis  11  of the drill string  18 . In the present embodiment, and for simplicity and ease of illustration, ID sensor  200  is positioned axially proximate OD sensor  300 . However, in other embodiments, ID sensor  200  may be positioned an axial distance away from OD sensor  300 . Each sensor  200 ,  300  comprises a resistance temperature detector (RTD)  250 ,  350 , respectively, as shown in the enlarged views of sensors  200 ,  300 . In general, RTDs  250 ,  350  can be any resistance temperature detector known in the art including, but not limited to, the Leaded Platinum Temperature Sensor available from Vishay Intertechnology, Inc. 
         [0048]    Referring now to  FIGS. 4 and 4   a , an enlarged schematic view of a first alternative ID sensor  200  installed in drill string  18  is shown. Drill string  18  further comprises a through bore or cavity  215  that extends from the OD  201  of drill string  18  to the ID  202  of drill string, where cavity  215  has a central axis coaxial with the central axis  211  of sensor  200 . The diameter of cavity  215  generally decreases from the OD  201  to the ID  202  of the drill string  18  and comprises a tapered opening or sloped portion  215   a  that angles radially inward toward central axis  211  from OD  201  to outer shoulder  215   b . Upper cylindrical portion  215   c  of cavity  215  extends axially from the outer shoulder  215   b  toward ID  202  to inner shoulder  215   d . Lower cylindrical portion or opening  215   e  extends axially from ID  202  to inner shoulder  215   d . Drill string  18  further comprises a conduit  216  extending away from cavity  215  toward controller  40 . At least a portion of upper cylindrical portion  215   c  of cavity  215  below outer shoulder  215   b  and above conduit  216  is threaded. 
         [0049]    Referring now to  FIGS. 4 ,  4   a , and  4   b , sensor  200  comprises a thermal insulator  220 , thermal conductor  230 , seals  243 ,  245 ,  247 , a RTD  250 , thermally conductive epoxy  257 , and a retention assembly  260 . Thermal insulator  220  is generally cylindrical, has a central axis  211 , an upper end  220   a  opposite a lower end  220   b , an external cylindrical surface  220   c  coaxial with an internal cylindrical surface  220   d  and with central axis  211 , a through hole  220   e  coaxial with central axis  211 , an internal shoulder  220   f , and two circumferential channels or grooves  225 . External cylindrical surface  220   c  extends axially from upper end  220   a  to lower end  220   b . Internal cylindrical surface  220   d  with internal shoulder  220   f  form a cavity  227  that is coaxial with central axis  211 , and extends axially from internal shoulder  220   f  to upper end  220   a . Through hole  220   e  extends axially from internal shoulder  220   f  to lower end  220   b  and has a diameter less than the diameter of internal cylindrical surface  220   d . The two grooves  225 , axially spaced apart from each other, are disposed on and coaxial with external cylindrical surface  220   c  of thermal insulator  220 . Thermal insulator  220  may be made of any suitable thermally insulative material known in the art, including but not limited to ceramics, rubber, polymers, polyetheretherketone (PEEK), and thermoplastics. 
         [0050]    Thermal insulator  220  is disposed in cavity  215  of the drill string  18  such that lower end  220   b  of insulator  220  is in contact with inner shoulder  215   d  of cavity  215 , and external cylindrical surface  220   c  of insulator  220  is sealingly coupled to a portion of upper cylindrical portion  215   c  of cavity  215 . The thermal insulator  220  acts as a thermal barrier, resisting or blocking heat transfer from the drill string  18  to the interior or cavity  227  of the thermal insulator  220 . A seal  243  is disposed in each groove  225  to seal the internal components from the pressure and fluid of the drilling fluid  32  during operation. In general, seals  243  can be any O-ring seal and/or back up ring known in the art. 
         [0051]    Referring now to  FIGS. 4 and 4   a - 4   c , thermal conductor  230  is generally cylindrical, has a central axis  211 , an upper end  230   a  opposite a lower end  230   b , an upper external cylindrical surface  230   c  coaxial with an upper internal cylindrical surface  230   d  and with central axis  211 , a lower external cylindrical surface  230   e  coaxial with a lower internal cylindrical surface  230   g  and with central axis  211 , an internal bottom surface  220   h , an external shoulder  230   f , and two circumferential channels or grooves  235 . Upper external cylindrical surface  230   c  extends axially from upper end  230   a  to external shoulder  230   f . External shoulder  230   f  extends radially inward toward central axis  211  from upper external cylindrical surface  230   c  to lower external cylindrical surface  230   e . The intersection of upper external cylindrical surface  230   c  and external shoulder  230   f  may follow any geometry including but not limited to orthogonal, rounded, curved, or slanted (shown). Lower external cylindrical surface  230   e  extends axially from external shoulder  230   f  to lower end  230   b.    
         [0052]    Upper external cylindrical surface  230   c  has a diameter greater than the diameter of lower external cylindrical surface  230   e , and upper internal surface  230   d  has a diameter greater than the diameter of lower internal surface  230   g . Internal cylindrical surfaces  230   d ,  230   g  with internal bottom surface  230   h  form a cavity or inner bore  237  that is coaxial with central axis  211 , and extends from internal bottom surface  230   h  upward to upper end  230   a  while flaring outward such that lower internal cylindrical surface  230   g  forms the portion of bore  237  that has a smaller diameter than upper internal surface  230   d , which forms the portion of bore  237  that has a larger diameter. The two grooves  235 , axially spaced apart from each other, are disposed on and coaxial with upper external cylindrical surface  230   c  of thermal conductor  230 . Thermal conductor  230  may be made of any suitable thermally conductive material known in the art, including but not limited to metals. The thermal conductance of the thermal conductor  230  material is preferably higher than the thermal conductance of the main tool body. Furthermore, the thickness of the lower end  230   b  of conductor  230  to the internal bottom surface  230   h  can be adjusted based on the erosion testing results of the material selected for the conductor  230 . Materials more resistant to erosion may allow for a thinner lower end  230   b  of conductor  230 . The thinner the lower end  230   b  can be, the less time it will take to see the accurate temperature reading. Further, the more surface area that can be provided by the thermal conductor  230  to be in contact with the drilling fluid  32   a , the more the drilling fluid  32   a  flow can affect the sensors reading. 
         [0053]    Thermal conductor  230  is coupled to the thermal insulator  220  such that external shoulder  230   f  of conductor  230  is in contact with internal shoulder  220   f  of insulator  220 ; upper external cylindrical surface  230   c  of conductor  230  is sealingly coupled to internal cylindrical surface  220   d  of insulator  220 ; and upper end  220   a  of insulator  220  is flush with upper end  230   a  of conductor  230 . Further, thermal conductor lower end  230   b  and a portion of lower external surface  230   e , and thus a portion of inner bore  237 , extend through hole  220   e  of thermal insulator  220 . The thermal insulator  220  acts as a thermal barrier, resisting or blocking heat transfer between the drill string  18  and thermal conductor  230 . A seal  245  is disposed in each groove  235  to seal the internal components from the pressure and fluid of the drilling fluid  32  during operation. In general, seals  245  can be any O-ring seal and/or back up ring known in the art. Further, through hole  220   e  of insulator  220  may be in contact with lower external surface  230   e  of conductor  230 , but need not be. 
         [0054]    A recessed portion or circular channel  218  is formed between lower cylindrical portion  215   e  of cavity  215  and lower external cylindrical surface  230   e  of conductor  230  and connected by lower end  220   b  of insulator  220 . Lower end  230   b  of conductor  230  may protrude beyond the surface of ID  202  of drill string  18 ; lower end  230   b  more preferably is flush with or below the ID  202  of drill string  18 . During operation, the drilling fluid  32   a  flowing down the inner diameter  202  of the drill string  18  flows into and around channel  218  as well as over lower end  230   b  of conductor  230 . The channel  218  and protruding lower end  230   b  of conductor  230  provide an increased surface area for the drilling fluid  32   a  to contact on the conductor  230  and subsequently, the RTD  250 . The increased surface area allows the RTD  250 , via the conductor  230 , to respond quickly to changes in drilling fluid  32   a  temperature. Further, the small profile of the conductor  230  minimizes the amount of conductor material and in addition to the insulation (i.e., insulator  220 ) surrounding the conductor  230 , prevents the dissipation of heat from the drilling fluid  32   a  to the rest of the drill string component  18 . 
         [0055]    Referring to  FIG. 4 , an RTD  250  is adhered to the internal bottom surface  230   h  of conductor  230  with thermally conductive epoxy  257 . A thermal conduction path is formed between the drilling fluid  32   a  and the RTD  250  through the thermal conductor  230  and the thermally conductive epoxy  257 . Epoxy  257  allows sensor  200  to withstand vibrations of the drill string  18  during operations; further strain relief may be added to the RTD  250  using a potting. The thermal epoxy  257  further allows the RTD  250 , via the conductor  230 , to respond quickly to changes in drilling fluid  32   a  temperature. The RTD  250  comprises leads or wires  255 , which are routed up through inner bore  237  of the thermal conductor  230  forming a hollow annulus  231  between the wires  255  and the thermal conductor inner cylindrical surfaces  230   d ,  230   g , then through a passage  265   e  in split ring  265  (to be described in more detail below), and then into the conduit  216 . The RTD wire  255  is in communication with controller  40 . 
         [0056]    Referring now to  FIGS. 4 and 4   d , retention assembly  260  comprises a thermally insulating split ring  265  and a threaded plug  270 . Split ring  265  is generally cylindrical, has a central axis  211 , an upper end  265   a  opposite a lower end  265   b , an external surface  265   c  coaxial with an internal surface  265   d  and with central axis  211 , and a passage  265   e . Passage  265   e  of split ring  265  is aligned with conduit  216  and allows the RTD wires  255  to pass through the split ring  260  and out through conduit  216 . Split ring  265  may be made of any suitable thermally insulative material known in the art, including but not limited to ceramic, polymers, or metals. The split ring  265  is disposed in cavity  215  such that upper end  265   a  of split ring  265  is aligned and in contact with the upper ends  220   a ,  230   a  of the thermal insulator  220  and thermal conductor  230 , respectively, and external surface  265   c  of split ring  265  is in contact with a portion of outer cylindrical portion  215   c  of cavity  215 . The thermally insulating split ring  265  acts as a thermal barrier, resisting or blocking heat transfer between the thermal conductor  230  and the plug  270  as well as between the thermal conductor  230  and the drill string  18 . 
         [0057]    Threaded plug  270  is generally cylindrical, has a central axis  211 , an upper end  270   a  opposite a lower end  270   b , an external cylindrical surface  270   c  coaxial with an internal cylindrical surface  270   d  and with central axis  211 , an internal top surface  270   e , an external shoulder  270   f , an indentation  270   g , and a circumferential channel or groove  275 . At least a portion of external cylindrical surface  270   c  is threaded (not shown). Internal cylindrical surface  270   d  with internal top surface  270   e  form a pocket or cavity  277  that is coaxial with central axis  211 , and extends from internal top surface  270   e  downward to lower end  270   b . The diameter D 270e  of internal top surface  270   e  is preferably between 0.25 and 2.0 inches and the height H 270d  of internal cylindrical surface  270   d  is preferably between 0.25 and 1.0 inch. Internal cylindrical surface  270   d  of threaded plug  270  is coaxial with and approximately aligned with upper internal cylindrical surface  230   d  of conductor  230 . Indentation  270   g  allows the threaded plug  270  to be turned and tightened during installation. The groove  275  is disposed on and coaxial with external cylindrical surface  270   c  of threaded plug  270 . Threaded plug  270  may be made of any suitable material known in the art, including but not limited to metals. 
         [0058]    Referring now to  FIGS. 4 ,  4   a , and  4   d , threaded plug  270  is disposed in cavity  215  such that lower end  270   b  of plug  270  is above and in contact with upper end  265   a  of split ring  265 , external cylindrical surface  270   c  of plug  270  is threadedly engaged with a portion of outer cylindrical portion  215   c  of cavity  215 , and external shoulder  270   f  is in contact with outer shoulder  215   b . A seal  247  is disposed in groove  275  to seal the internal components from the pressure and fluid of the drilling fluid  32  during operation. In general, seal  247  can be any O-ring seal and/or back up ring known in the art. Though shown with a split ring and threaded plug in the present embodiment, any suitable retention means may be used including, but not limited to, retention rings, locking pins, or friction-based retention means. In an alternative embodiment, the threaded plug  270  is thermally insulating and acts as a thermal barrier, resisting or blocking heat transfer between the thermal conductor  230  and the drill string  18 . In this alternative embodiment, the thermally insulating threaded plug  270  may be made from any suitable thermally insulative material known in the art, including by not limited to ceramics, rubber, and polymers, or plug  270  may be coated with a thermally insulative coating. 
         [0059]    Referring now to  FIGS. 5 and 5   a , showing an enlarged schematic view of a first alternative OD sensor  300  installed in drill string  18 . Like numbers are used to designate like parts. Drill string  18  further comprises a bore or cavity  315  that extends from the OD  201  of drill string  18  toward the ID  202  of drill string, where cavity  315  has a central axis coaxial with the central axis  311  of sensor  300 . The diameter of cavity  315  generally decreases from the OD  201  toward ID  202  of the drill string  18  and comprises a tapered opening or sloped portion  315   a  that angles radially inward toward central axis  311  and axially downward from OD  201  to channel or groove  315   b . Upper cylindrical portion  315   c  of cavity  315  extends axially downward from the channel  315   b  toward ID  202  to lower sloped portion  315   d , which extends radially inward toward central axis  311  and axially downward to middle cylindrical portion  315   e . Middle cylindrical portion  315   e  extends axially downward from lower sloped portion  315   d  to internal shoulder  315   f . Lower cylindrical portion  315   g  extends axially from internal shoulder  315   f  to internal bottom surface  315   h . The diameter D 315h  of internal bottom surface  315   h  is preferably between 0.25 and 2.0 inches and the height H 315g  of lower cylindrical portion  315   g  is preferably between 0.25 and 1.0 inch. Due to mechanical properties, these dimensions D 315h , H 315g  depend on the type of material used for the drill string  18  body. Drill string  18  further comprises a conduit  316  extending away from lower cylindrical portion  315   g  of cavity  315  toward controller  40 . 
         [0060]    Referring now to  FIGS. 5 and 5   b , sensor  300  comprises a thermal insulator  320 , thermal conductor  330 , seals  343 ,  345 ,  347 , a RTD  350 , thermally conductive epoxy  357 , and retention rings  360 ,  361 . Thermal insulator  320  is generally cylindrical, and includes a central axis  311 , an upper end  320   a  opposite a lower end  320   b , an upper external cylindrical surface  320   c  coaxial with an upper internal cylindrical surface  320   d  and with central axis  311 , an outer sloped portion  320   h , a lower external cylindrical surface  320   e  coaxial with a lower internal cylindrical surface  320   g  and with central axis  311 , an inner sloped portion  320   i , a through hole  320   j  coaxial with central axis  311 , an internal shoulder  320   f , two outer circumferential channels or grooves  325 , and an inner circumferential channel or groove  323 . Upper external cylindrical surface  320   c  extends axially downward from OD  201  to outer sloped portion  320   h  and upper internal cylindrical surface  320   d  extends axially downward from OD  201  to inner sloped portion  320   i . The intersection of upper end  320   a  and upper internal cylindrical surface  320   d  may follow any geometry including but not limited to orthogonal, rounded, curved, or slanted (shown). Disposed on and coaxial with internal cylindrical surface  320   d  of thermal insulator  320  is an inner circumferential channel or groove  323 . 
         [0061]    Outer sloped portion  320   h  angles radially inward toward central axis  311  and axially downward from upper external cylindrical surface  320   c  to lower external cylindrical surface  320   e , and inner sloped portion  320   i  angles radially inward toward central axis  311  and axially downward from upper internal cylindrical surface  320   d  to lower internal cylindrical surface  320   g . Lower external cylindrical surface  320   e  extends axially from outer sloped portion  320   h  to lower end  320   b , and lower internal cylindrical surface  320   g  extends axially from inner sloped portion  320   i  to internal shoulder  320   f . The two outer circumferential channels or grooves  325 , axially spaced apart from each other, are disposed on and coaxial with lower external cylindrical surface  320   e  of thermal insulator  320 . Internal shoulder  320   f  extends radially from lower internal cylindrical surface  320   g  to through hole  320   j . Through hole  320   j  extends axially from internal shoulder  320   f  to lower end  320   b . Upper internal cylindrical surface  320   d , inner sloped portion  320   i , and lower internal cylindrical surface  320   g  form a cavity  327  coaxial with central axis  311  and having a diameter greater than the diameter of through hole  320   j . Thermal insulator  320  may be made of any suitable thermally insulative material known in the art, including but not limited to ceramics and polymers (e.g., elastomers or thermoplastics). 
         [0062]    Thermal insulator  320  is disposed in cavity  315  of the drill string  18  such that lower end  320   b  of insulator  320  is in contact with internal shoulder surface  315   f  of cavity  315 , lower external cylindrical surface  320   e  of insulator  320  is sealingly coupled with middle cylindrical portion  315   e  of cavity  315 , outer sloped portion  320   h  of insulator  320  is in contact with lower sloped portion  315   d , and external surface  320   c  of insulator  320  is in contact with upper cylindrical portion  315   c  of cavity  315 . The thermal insulator  320  acts as a thermal barrier, resisting or blocking heat transfer from the drill string  18  to the interior or cavity  327  of the thermal insulator  320 . A seal  343  is disposed in each groove  325  to seal the internal components from the pressure and fluid of the drilling fluid  32  during operation. In general, seals  343  can be any O-ring seal and/or back up ring known in the art. 
         [0063]    Referring now to  FIGS. 5 and 5   c , thermal conductor  330  is generally cylindrical, and includes a central axis  311 , an upper end  330   a  opposite a lower end  330   b , an upper external cylindrical surface  330   c  coaxial with central axis  311 , an internal cylindrical surface  330   d , a middle external cylindrical surface  330   e , a lower external cylindrical surface  330   g , a sloped outer portion  330   i , an internal top surface  330   h , an external shoulder  330   f , and two circumferential channels or grooves  335 . Upper external surface  330   c  extends axially downward from upper end  330   a  to external shoulder  330   f . The intersection of upper end  330   a  and upper external cylindrical surface  330   c  may follow any geometry including but not limited to orthogonal, curved, slanted, or rounded (shown). External shoulder  330   f  extends radially outward from upper external cylindrical surface  330   c  to middle external cylindrical surface  330   e . Middle external cylindrical surface  330   e  extends axially downward from external shoulder  330   f  to sloped outer portion  330   i . Sloped portion  330   i  angles radially inward toward central axis  311  and extends axially downward from middle external cylindrical surface  330   e  to lower external cylindrical surface  330   g . Lower external cylindrical surface  330   g  extends axially downward from sloped outer portion  330   i  to lower end  330   b.    
         [0064]    Middle external surface  330   e  has a diameter greater than the diameter of upper external surface  330   c , lower external surface  330   g , and internal surface  330   d . Internal surface  330   d  with internal top surface  330   h  form a cavity or inner bore  337  that is coaxial with central axis  311 , and extends from internal top surface  330   h  downward toward lower end  330   b . The two grooves  335 , axially spaced apart from each other, are disposed on and coaxial with the lower external surface  330   g  of thermal conductor  330 . Thermal conductor  330  may be made of any suitable thermally conductive material known in the art, including but not limited to metals. The thermal conductance of the thermal conductor  330  material is preferably higher than the thermal conductance of the main tool body. Furthermore, the thickness of the upper end  330   a  of conductor  330  to the internal top surface  330   h  can be adjusted based on the erosion testing results of the material selected for the conductor  330 . Materials more resistant to erosion may allow for a thinner upper end  330   b  of conductor  330 . The thinner the upper end  330   a  can be, the less time it will take to see the accurate temperature reading. Further, the more surface area that can be provided by the thermal conductor  330  to be in contact with the drilling fluid  32   b , the more the drilling fluid  32   b  flow can affect the sensor&#39;s reading. 
         [0065]    Referring now to  FIGS. 5 ,  5   b , and  5   c , thermal conductor  330  is coupled to thermal insulator  320  such that external shoulder  330   f  of conductor  330  is in contact with lower end  320   b  of insulator  320 , lower external cylindrical surface  330   g  of conductor  330  is sealingly coupled to the lower internal cylindrical surface  320   g  of insulator  320 , sloped outer portion  330   i  of conductor  330  is in contact with inner sloped portion  320   i  of insulator  320 , and middle external cylindrical surface  320   e  of conductor  330  is in contact with upper internal cylindrical surface  320   d . The thermal insulator  320  acts as a thermal barrier, resisting or blocking heat transfer between the drill string  18  and thermal conductor  330 . A seal  345  is disposed in each groove  335  to seal the internal components from the pressure and fluid of the drilling fluid  32  during operation. In general, seals  345  can be any O-ring seal and/or back up ring known in the art. Further, through hole  320   j  of insulator  320  may be flush with internal cylindrical surface  330   d  of conductor  330 , but need not be. 
         [0066]    Referring still to  FIG. 5 , an RTD  350  is adhered to the internal top surface  330   h  of conductor  330  with thermally conductive epoxy  357 . A thermal conduction path is formed between the drilling fluid  32   b  and the RTD  350  through the thermal conductor  330  and the thermally conductive epoxy  357 . Epoxy  357  allows sensor  300  to withstand vibrations of the drill string  18  during operations; further strain relief may be added to the RTD  350  using a potting. The thermal epoxy  357  further allows the RTD  350 , via the conductor  330 , to respond quickly to changes in drilling fluid  32   b  temperature. The RTD  350  comprises leads or wires  355 , which are routed through inner bore  337  of the thermal conductor  330  forming a hollow annulus  331  between the wires  355  and the thermal conductor internal cylindrical surface  330   d , then through bore  320   j  of insulator  320 , through lower cylindrical portion  315   g  of cavity  315 , and then into the conduit  316 . The RTD wire  355  is in communication with controller  40 . 
         [0067]    Referring now to  FIGS. 5 ,  5   a - 5   c , retention ring  360  is disposed in and extends radially inward beyond groove  315   b  of cavity  315 ; retention ring  360  is also disposed above and in contact with top end  320   a  of insulator  320  to retain insulator  320  in cavity  315 . Retention ring  361  is disposed in and extends radially inward beyond groove  323  of insulator  320 ; retention ring  361  is also disposed above and in contact with external shoulder  330   f  of conductor  330  to retain conductor  330  in cavity  327  of insulator  320 . Though shown with retention rings in the present embodiment, any suitable retention means may be used including, but not limited to, threaded components, locking pins, or friction-based retention means. 
         [0068]    A circular channel  318  is formed with sloped portion  315   a  and upper cylindrical portion  315   c  of cavity  315 , retention rings  360 ,  361 , and upper end  320   a  and upper internal cylindrical surface  320  of insulator  320  comprising the channel&#39;s outer sides. The conductor&#39;s external shoulder  330   f  defines the channel&#39;s bottom. The conductor&#39;s upper external cylindrical surface  330   c  defines the channel&#39;s inner side. Further, upper end  330   a  of conductor  330  may protrude beyond the surface of OD  201  of drill string  18 ; upper end  330   a  more preferably is flush with or below the OD  201  of drill string  18 . During operation, the drilling fluid  32   b  flowing up the annulus  28  or outer diameter of the borehole  26  up the outer diameter  202  of the drill string  18  flows into and around channel  318  as well as over upper end  330   a  of conductor  330 . The channel  318  and protruding upper end  330   a  of conductor  330  provides an increased surface area for the drilling fluid  32   b  to contact on the conductor  330  and subsequently, the RTD  350 . The increased surface area allows the RTD  350 , via the conductor  330 , to respond quickly to changes in drilling fluid  32   b  temperature. Further, the small profile of the conductor  330  minimizes the amount of conductor material and in addition to the insulation (i.e., insulator  320 ) surrounding the conductor  330 , prevents the dissipation of heat from the drilling fluid  32   b  to the rest of the drill string component  18 . 
         [0069]    Referring now to  FIGS. 6 ,  6   a , and  6   b , showing an enlarged schematic view of a second alternative ID sensor  200 ′ installed in drill string  18 . Like numbers are used to designate like parts. The second alternative ID sensor  200 ′ comprises the same components as those of first alternative ID sensor  200  shown in  FIG. 4 . However, the diameters of cavities  227 ′,  237 ′,  277 ′ in the insulator  220 ′, conductor  230 ′, and threaded plug  270 ′, respectively, and the width of passage  265   e ′ of split ring  265 ′ in sensor  200 ′ are larger than the diameters of cavities  227 ,  237 ,  277  in the insulator  220 , conductor  230 , and threaded plug  270 , respectively, and the width of passage  265   e  of split ring  265  in the first alternative ID sensor  200 . 
         [0070]    More specifically, the internal cylindrical surface  220   d ′ and through hole  220   e ′ have enlarged diameters. Further, upper external cylindrical surface  230   c ′ and upper internal cylindrical surface  230   d ′ have enlarged diameters while the diameters of lower external cylindrical surface  230   e ′ and lower internal cylindrical surface  230   g ′ remain the same as the diameters of corresponding surfaces (lower external cylindrical surface  230   e , lower internal cylindrical surface  230   g , respectively) of the first alternative ID sensor  200 . Thus, the internal cylindrical surfaces  230   d ′,  230   g ′ with internal bottom surface  230   h ′ form a larger cavity  237 ′ that is coaxial with central axis  211 ′; and upper internal cylindrical surface  230   d ′ flares outward to a greater extent from lower internal cylindrical surface  230   g ′. Internal surface  265   d ′ of split ring  265 ′ also has a wider opening to align with the larger diameter of upper internal cylindrical surface  230   d ′, and internal cylindrical surface  270   d ′ of threaded plug  270 ′ has a larger diameter forming a larger cavity  277 ′. These larger cavities  237 ′,  277 ′ are filled with air, which provide an insulating effect, helping to further prevent the dissipation of heat from the drilling fluid  32   a  to the rest of the drill string component  18 . Thus, cavities  237 ′,  277 ′ act as a thermal barrier, resisting or blocking heat transfer between the thermal conductor  230 ′ and the drill string  18 . 
         [0071]    Referring now to  FIGS. 7 and 7   a , an enlarged schematic view of a second alternative OD sensor  300 ′ installed in drill string  18  is shown. Like numbers are used to designate like parts. The second alternative OD sensor  300 ′ comprises the same components as those of first alternative OD sensor  300  shown in  FIG. 5  with insulator  320 ′ and conductor  330 ′ being the same as insulator  320  and conductor  330 , respectively. However, the diameter of cavity  315 ′, specifically the diameter of lower cylindrical portion  315   g ′ of cavity  315 ′, is larger than the diameter of corresponding cavity  315   g  of cavity  315  in the first alternative OD sensor  300 . Further, as the diameter of lower cylindrical portion  315   g ′ of cavity  315 ′ is larger while the diameter of the middle cylindrical portion  315   e ′ of cavity  315 ′ remains unchanged, the length of internal shoulder surface  315   f  is shortened and the insulator lower end  320   b ′ extends a greater amount beyond lower cylindrical portion  315   g ′ of cavity  315 ′. This larger cavity (portion  315   g ′ of cavity  315 ′) is filled with air, which provides an insulating effect, helping to further prevent the dissipation of heat from the drilling fluid  32   b  to the rest of the drill string component  18 . Thus, cavity  315 ′ acts as a thermal barrier, resisting or blocking heat transfer between the thermal conductor  330 ′ and the drill string  18 . 
         [0072]    Referring now to  FIGS. 8 and 9 ,  FIG. 8  shows an enlarged schematic view of a portion of a second embodiment of the drill string  18  of drilling system  10  shown in  FIG. 1  having sensor assembly  100 .  FIG. 9  shows an enlarged view of section  9  depicted in  FIG. 8  and includes sensor assembly  100  having an ID sensor  400  with central axis  411 . The sensor assembly  100  comprises a housing  410 , a cavity  415 , cap  430 , an RTD  450 , and epoxy  427 . RTD  450  is configured to measure the temperature of drilling fluid  32   a  flowing down the inner diameter of the drill string  18  (“ID sensor  400 ”) as shown in the present embodiment. Further, more than one sensor assembly  100  may be employed in a drilling system  10  at various locations to measure the temperature of the drilling fluid  32   a  at different locations within the drill string  18 . 
         [0073]    Central axis  411  is coaxial to the central axis  11  of the drill string  18 . Housing  410  comprises a cavity  415 , a cap  430 , and stabilizers  460  (see  FIG. 8 ). RTD  450  is adhered to the internal upper surface of cavity  415  with thermally conductive epoxy  427 . Epoxy  427  allows sensor  400  to withstand vibrations of the drill string  18  during operations; further strain relief may be added to the RTD  450  using a potting. The thermal epoxy  427  further allows the RTD  450 , via the housing  410 , to respond quickly to changes in drilling fluid  32   a  temperature. The RTD  450  comprises leads or wires (not shown), which are routed down through the bottom of housing  410  and is communicatively connected to controller  40 . 
         [0074]    Housing  410  is secured within drill string  18  via stabilizers  460 , shown in  FIG. 8  as a fin structure with a tapered outer surface  460   a . Though shown as having a fin-like structure, stabilizers  460  may follow any suitable geometry. Housing  410  may be made of any suitable material known in the art, including but not limited to metals. For example, housing  410  may be steel with a coating to prevent erosion. 
         [0075]    During operation, the drilling fluid  32   a  flowing down the inner diameter  402  of the drill string  18  flows past cap  430  and housing  410 , and subsequently, RTD  450 . The conical shape of the housing cap  430  provides an increased surface area for the drilling fluid  32   a  to contact on the RTD  450 . The increased surface area allows the RTD  450 , via the housing  410 , to respond quickly to changes in drilling fluid  32   a  temperature. 
         [0076]    Referring now to  FIGS. 10   a - 10   c , various enlarged schematic views of an alternative embodiment of the OD sensor  300  installed in drill string  18 ′ are shown. Like numbers are used to designate like parts. In this alternative embodiment, the OD sensor  300  comprises the same components as those of the first and second alternative OD sensors  300 ,  300 ′ shown in  FIGS. 5 and 6 , respectively, with insulator  320  and conductor  330  being the same as insulator  320 ,  320 ′, respectively, and conductor  330 ,  330 ′, respectively. Further, drill string  18 ′ comprises a plurality of circumferentially-spaced parallel ridges  303  separated by channels or passages  305 , the ridges  303  and corresponding channels  305  extend helically about axis  11  and axially along the drill string  18 ′. In this embodiment, drill string  18 ′ includes four uniformly circumferentially-spaced ridges  303 . However, in general, the drill string  18 ′ can include any suitable number of ridges  303 , and further, the circumferential spacing of the ridges  303  can be uniform or non-uniform. 
         [0077]    Each ridge  303  has a first side wall  303   a , a second side wall  303   b , and a radially outer generally cylindrical surface  303   c . Each passage  305  has a first side wall  305   a , a second side wall  305   b , and a bottom surface  305   c . The first ridge side wall  303   a  is coincident with first channel side wall  305   a  and the second ridge side wall  303   b  is coincident with second channel side wall  305   b . Radially outer surface  303   c  of each ridge  303  is disposed at a uniform radius R 303c , and each ridge  303  has a height H 303  measured radially from radially outer surface  303   c  to bottom surface  305   c , which has a uniform radius R 305c . The ridges  303  are spaced a distance D 303  apart measured from a first side wall  303   a  to a second side wall  303   b , and oriented at an angle θ 303  relative to a reference plane A perpendicular to axis  11  in side view (see  FIG. 10   c ). In other embodiments, the radius R 303c  of the radially outer surface  303   c  and the radius R 305c  of the bottom surface  305   c  may be non-uniform within a singular ridge  303  or channel  305 , respectively, and/or may be non-uniform between ridges  303  or channels  305 . 
         [0078]    Drill string  18 ′ further comprises a bore or cavity  315 ″ that extends from the bottom groove surface  305   c  toward the ID  202  of drill string  18 ′, where cavity  315 ″ has a central axis coaxial with the central axis  311  of sensor  300 . In this alternative embodiment, the characteristics of the cavity  315 ″ are similar to those of the cavity  315 ,  315 ′ in other embodiments described herein and configured similarly to house and engage the components of the OD sensor  300 . The quantity of ridges  303  and corresponding channels  305  as well as the distance D 303  between ridges  303  is configured such that the cavity  315 ″ is disposed within groove bottom surface  305   c  between the first and second ridge sides  303   a ,  303   b , respectively. As in prior embodiments, when OD sensor  300  having a uniform radius R 300  is disposed in cavity  315 ″, an upper end  330   a  of conductor  330  protrudes radially beyond the bottom surface  305   c  of groove  305  having radius R 305c  of drill string  18 ′. However, the upper end  330   a  of conductor  330  does not extend radially beyond radially outer ridge surface  303   c  having radius R 303c . Thus, the radius R 303c  of the ridge  303   c  is greater than the radius R 300  of the OD sensor  300 , which is greater than the radius R 305c  of the bottom channel surface  305   c . In other embodiments, upper conductor end  330   a  may be flush with or below the bottom surface  305   c  of drill string  18 ′. In such embodiments, the radius R 303c  of the ridge  303   c  is greater than the radius R 305c  of the bottom channel surface  305   c , which is either approximately equal to or greater than the radius R 300  of the OD sensor  300 . 
         [0079]    During operation, drilling fluid  32   b  flowing up the annulus  28  or outer diameter of the borehole  26  up the OD  202  of the drill string  18 ′ flows over conductor upper end  330   a , into channel  318  (see  FIG. 5 ), and around upper external cylindrical surface  330   c  of conductor  330 . By locating the OD sensor  300  in the bottom surface  305   c  of the groove, while the drilling fluid  32   b  flows up the annulus  28 , a portion of the drilling fluid  32   b  enters and flows upward within channels  305 . The drilling fluid  32   b  then flows over and around the OD sensor  300  and because channels  305  are generally oriented along the same direction as the flow of the drilling fluid  32   b , the fluid  32   b  can continue to flow past OD sensor  300  through channel  305  and not become packed around the conductor  330 . The channels  305  provide a gap or space that allows the drilling fluid  32   b  and cuttings to flow past the cavity  315  with OD sensor  300  while protecting the OD sensor  300  from coming in direct contact with the wall of the borehole  26 . The passage  305  acts as a self-cleaning mechanism for the OD sensor  300  by creating a path for the drilling fluids  32   b  to pass through. Specifically, the channels  305  allow the OD sensor  300  (with a radius R 300  less than the radius R 303c  of the ridge  303 ) to protrude into the drilling fluid  32   b  flowing up the annulus  28  while remaining within the gage diameter of drill string  18 ′ based on the radius R 303c  of the ridge  303 , which is larger than the radius R 300  of OD sensor  300 . The drilling fluid  32   b  can flow across the OD sensor  300  without becoming packed around OD sensor  300  to provide realistic temperature measurements of the drilling fluid  32   b.    
         [0080]    Exemplary embodiments are described herein, though one having ordinary skill in the art will recognize that the scope of this disclosure is not limited to the embodiments described, but instead by the full scope of the following claims. The claims listed below are supported by the principles described herein, and by the various features illustrated which may be used in desired combinations.