Abstract:
A temperature sensor approximates fluid temperature averaged across a location range by including an outer armour layer. Several resistance temperature detectors are spaced in an electrical circuit which is then protected in the outer armour layer. The outer armour layer is woven without any seam to enhance its longitudinal thermal conductivity. In the preferred weave, twenty-four stands of sixteen metal threads each are helically woven. The electrical circuit is sealed interior to the armour layer so any condensation or moisture within the armour layer does not affect the circuit. The armour layer is sealed on its ends to the sheathing of the underlying circuit, so the armour layer provides stress relief across the connections of the resistance temperature detectors to the circuit. The resulting sensor is robust and durable, as well as very flexible.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
   None. 
   BACKGROUND OF THE INVENTION 
   The present invention generally relates to an apparatus for electronically sensing an averaged temperature. More specifically, the present invention relates to a temperature sensor which uses multiple, spaced sensing elements such as thermistors in a circuit, such as for use in monitoring an average air temperature across an area within a large ventilation (HVAC) system. 
   Controlled, forced-air ventilation systems are known which move air within buildings. In many ventilation systems, fans draw fresh outside air into a building, and exhaust stale interior air to the outside. The ventilation systems are used with venting or ducts to provide an air flow path throughout the building, including to and from heaters and/or air conditioners. Often the ventilations systems perform heat transfer (recovery) between the interior air to be exhausted and the outside air being introduced. For proper control of these ventilation systems, parameters such as fan speeds or damper positions are set and changed based upon sensed air temperatures within the building or within the system. Particularly in systems where air of different temperatures mixes, it is important to be able to accurately determine average air temperature, such as the average air temperature across a vertical cross-section at a location within a duct. 
   As explained in U.S. Pat. Nos. 6,592,254 and 6,890,095, incorporated by reference herein, early structures for sensing average temperatures included capillary tubes and resistance temperature detectors (“RTDs”) such as platinum strand sensors, and metallic tube-enclosed thermistor-based sensors. None of these sensors were adequately easy to install and robust for use as desired in many HVAC duct sensing environments. 
   The metallic tube-enclosed thermistor-based sensors in particular had problems. Ascertaining the location of the thermistors within the tube was difficult. Particularly for long runs of measurement (typically from six to twenty four feet or more), the tube was bulky and difficult to ship. Bending the tube improperly can cause inadvertent crimping and/or kinking of the metal, which could effectively sever the electrical connections or which could lead to small holes forming in the tube. Where small holes in a metal tube are created, cycled temperature differences can result in condensation on the inside of the tube which sometimes can affect the accuracy of the temperature sensor. Condensation at the location of a thermistor could short-circuit the thermistor and lead to anomalous temperature readings. The solder connections are exposed to tension and stresses associated with adjusting and bending the wires. Over time, the solder points weaken and electrical connections break. The resulting open circuit may be difficult to locate if the wire is placed inside a tube, and may be costly to repair no matter how the sensor is situated. The metallic nature of the tube requires dielectric insulation to prevent electrical shorting between the thermistors and the wall of the tubing. The insulation/metal tube support and protection configuration thermally insulates the thermistor or platinum strand from the air, slowing the response time of the averaging temperature sensor. 
   Whether each thermistor or its overlying insulation contacted the metal tube was inconsistent and depended upon installation. If the metal tube was bent in a particular fashion during installation, a thermistor might make solid contact with the metal tube for good thermal conductivity. If the metal tube was bent in a different fashion during installation, an air gap might exist between the thermistor and the metal tube retarding heat transfer therebetween. Thus, the amount of thermal conduction from the metal tube to each thermistor varied in inconsistent and unknown ways. 
   Assembly of the electrical circuit of thermistor arrays has been problematic. An insulative card has been used, allowing solder points between the leads for the thermistor to the wires extending between thermistor locations. The soldering card further adds thermal ballast to slow response time. Response time in the control systems is fairly significant, because delays in control can lead to damage to system elements, particularly if the system manipulates outside air at a drastically different temperature than the inside air. 
   Despite the plethora of problems noted here, the metallic tube-enclosed thermistor-based sensors became a market leading standard in the HVAC industry. In contrast to the metallic tube-enclosed thermistor-based sensors, the sensor described and claimed in U.S. Pat. Nos. 6,592,254 and 6,890,095 has begun to revolutionize averaging duct temperature sensors in the HVAC industry. Still, improvements can be made to averaging duct temperature sensors, particularly for certain environments of use. 
   BRIEF SUMMARY OF THE INVENTION 
   The present invention is a temperature sensor for approximating fluid temperature averaged across a location range. The sensor includes several RTDs spaced in an electrical circuit which is then protected in an outer armour layer. The electrical circuit is sealed so any condensation or moisture within the armour layer does not affect the circuit. The resulting sensor is robust and durable, as well as very flexible. The outer armour layer includes longitudinally extending metal strands or filaments which conduct heat longitudinally toward and away from the RTD. In one aspect, the armour layer is woven without any seam, and is sealed on its ends to the sheathing of the underlying circuit. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a plan view of a temperature sensor in accordance with the present invention. 
       FIG. 2  is an enlarged view depicting the preferred woven armour layer, showing the strands of the weave and further enlarging and showing the threads or filaments of a strand. 
       FIG. 3  is an exploded assembly view of the temperature sensor of  FIG. 1 . 
       FIG. 4  is a circuit diagram showing a square parallel/series array for a temperature sensor of the present invention. 
   

   While the above-identified drawing figures set forth a preferred embodiment, other embodiments of the present invention are also contemplated, some of which are noted in the discussion. In all cases, this disclosure presents the illustrated embodiments of the present invention by way of representation and not limitation. Numerous other minor modifications and embodiments can be devised by those skilled in the art which fall within the scope and spirit of the principles of this invention. 
   DETAILED DESCRIPTION 
   As depicted assembled in  FIG. 1  and unassembled in  FIG. 3 , an averaging temperature sensor  10  includes number of sensing elements  12  connected by cables  14 ,  16 ,  18 ,  20  in an electrical circuit  22  as generally disclosed in U.S. Pat. Nos. 6,592,254 and 6,890,095, assigned to the assignee of the present invention and incorporated herein by reference. As shown in  FIG. 3 , the electrical circuit  22  is sealed by dielectric insulators  24 . The sealed electrical circuit  22  is then disposed within an armour layer  26 . In use, the armour layer  26  is exposed for thermal transfer with the surrounding fluid (typically an air stream), and the armour layer  26  transmits heat longitudinally along the sensor  10  to or from the sensing elements  12 . As best shown in  FIG. 1 , the armour layer  26  is protective and flexible around the electrical circuit  22 , so the entire sensor  10  is very rugged and robust. 
   The electrical circuit  22  is arranged with the sensing elements  12  spaced along the length of the cables  14 ,  16 ,  18 ,  20  to form a string of sensing elements  12  each separated by a predetermined distance d. Each sensing element  12  includes a component  28  which changes electrical response between the first and second leads  30  as a function of temperature. 
   The preferred sensing elements  12  are resistance temperature detectors (“RTDs”), which change electrical resistance in a known manner responsive to changes in temperature, with the preferred RTDs being thermistors. Such thermistors  12  are commonly commercially available in various ohmic ratings, such as from BetaTHERM Corporation of Shrewsbury, Mass. For example, thermistors which have a nominal resistance at 25° C. of 0.1kΩ, 0.3kΩ, 1kΩ, 2kΩ, 2.2kΩ, 3kΩ, 5kΩ, 10kΩ, 30kΩ, 50kΩ, 100kΩ, and 1MΩ are commonly used in the heating, ventilation and air conditioning (“HVAC”) industry. Such thermistors may be formed by intimately blending high purity inorganic powders (typically transition metal oxides), which are then formed into large wafers, sintered and prepared for chip themistor production. Alternatively, the sensing element may be a platinum, nickel or balco RTD, such as rate at 0.1kΩ or 1kΩ. Each sensing element  12  has two electrical leads  30  for connection into the electrical circuit  22 . 
   The first length of cable  14  includes a positive voltage wire  32  and a ground wire  34  within sheathing  36 . The sheathing  36  terminates to provide assembly access to the positive voltage wire  32  and the ground wire  34 . The insulators  38  for positive voltage wire  32  and ground wire  34  are stripped to provide ends  40  for electrical connection. The positive voltage wire  32  is electrically connected to one of leads  30  of the thermistor  12 . In the preferred series/parallel circuit  22 , a parallel positive voltage wire  42  is also connected to the positive voltage wire  32 , and an intermediate connection wire  43  runs between RTDs  12 . The RTDs  12  are preferably arranged in a parallel/series square array (additional thermistors shown in  FIG. 4 ). All the wires  32 ,  34 ,  42 ,  43  are common electrical wires for carrying the specified current and voltage within a dielectric insulator  38 . For instance, RTDs commonly use relatively low current, typically less that 100 μA, so the wires  32 ,  34 ,  42 ,  43  can be about 30 A.W.G. or thicker, of a common conductor such as tin or copper, within a common insulator such as polyimide. 
   The sheathing  36  is electrically insulative and flexible. Each single piece of sheathing  36  defines a sheathing lumen which preferably contains all circuit wires  32 ,  34 ,  42 ,  43  running between adjacent, electrically connected RTDs  12 . The sheathing  36  may provide low flammability. For example, the sheathing  36  may be plenum-rated cable sheathing (such as UL - 94 VO). Many other types of materials for the sheathing  36  could be used. If desired, the circuit wires  32 ,  34 ,  42 ,  43  may alternatively be used within the armour layer  26  without an outer sheathing. 
   As shown in  FIG. 3 , once the electrical connections are made, an insulator  24  is positioned over each thermistor  12 . Insulators  24  are electrically insulative or dielectric, preventing electrical conduction between the armour layer  26  and the thermistors  12 . The preferred insulator  24  is formed of a flexible, elastomeric material, and may be plenum-rated like the sheathing  36 . For example, the insulator  24  may be formed of a cross-linked modified polyolefin tubing having an adhesive coated interior as disclosed in application Ser. No. 10/436,451, assigned to the assignee of the present invention and incorporated by reference. The insulator material should be as thin as possible without being so thin as to allow holes or to rupture during assembly or use. The thinner the insulator material, the less it thermally insulates its underlying thermistor  12 , and the faster the thermal response of the sensor  10 . Other types of RTDs may not require the insulator  24  to be distinct from the thermistor  12 . 
   In the preferred embodiment, the insulator  24  extends from the sheathing  36  on one side of the thermistor connection, over the thermistor  12 , to the sheathing  36  on the other side of the thermistor connection. The length of the insulator  24  thus covers the exposed wires  30 ,  40 , the thermistor  12 , the various connections, and the ends of the wire insulation  38  up to the sheaths  36  on both sides, so that the entire connection area is covered. By positioning the insulator  24  so that it overlaps the sheathing  36  on both sides of the thermistor connection, the connections are protected against tension placed on the cables  14 ,  16 ,  18 ,  20 . The insulator  24  helps transfer both the tensile stress and the bending stress away from the electrical connections and instead to the sheathing  36 , thereby reducing wear and stress on the electrical connections and improving the durability of the sensor  10 . 
   The preferred conductor connections are made by splicing. Spliced connections can be more quickly made during manufacturing than soldering or other types of connections. Because the present invention provides ample stress relief for the electrical connections, pulling out of the spliced connections is not a problem. Alternatively, the electrical connections may be made using a solder bead, adhesive, taping or through other means. 
   The insulator  24  helps to maintain the electrical connections by tightening around the existing connections. Whether the connections are made by splicing, using a solder bead, adhesive, taping or through other means, the insulator  24  helps secure the connections. The insulator  24  also effectively seals the electrical connections and leads  30  of the thermistor  12  from airflow, and avoids the condensation problems associated with prior art tubing. 
   The insulator  24  also tightens around its RTD  12  for intimate contact and good thermal conductivity between the insulator  24  and its RTD  12 . If desired, the material of the insulator  24  may be specially fabricated to increase its thermal conductivity. Minimizing or eliminating any airgap between the insulator  24  and its RTD  12  helps make the temperature sensor  12  more consistent and accurate in sensing temperature of the flow. 
   Once the connections of the electrical circuit  22  are sealed and insulated, the electrical circuit  22  is positioned within the armour layer  26 . As best shown in  FIG. 2 , the armour layer  26  is formed of at least four strands  44  extending longitudinally. The armour layer  26  protects the electrical circuit  22  and conducts heat along the strands  44  in the longitudinal direction. By conducting heat longitudinally, the armour layer  26  allows the RTDs  12  to acquire a reading which is more representative of an average temperature of the sensor  10  rather than merely at point locations. The longitudinal strands  44  provide a metal conduction path which continuously extends at least half of the distance d between neighboring RTDs  12 , so the multiple point RTDs  12  measure a temperature which is truly an average of the local temperatures witnessed all along the length of the sensor  10 . 
   The preferred armour layer  26  is braided or woven without a seam. The preferred weaving pattern is provided by twelve strands  44  each helically oriented in a parallel orientation (e.g., clockwise when viewed axially from the distal end) interwoven with twelve strands  44  each helically oriented in an opposite parallel orientation (e.g., counterclockwise when viewed axially from the distal end). The preferred weave is an over-two, under-two pattern relative to the opposite helix arrangement. Strands  44  in both directions (clockwise and counterclockwise) are tightly placed next to neighboring strands  44  so the weave provides minimal gaps in its armouring protection. 
   To further increase the flexibility of the armour layer  26 , each strand  44  is made up of a plurality of longitudinally extending threads or filaments  46 . Each metal filament  46  is significantly thinner than the temperature sensor  10  as a whole, and the large number of thin metal filaments  46  results in an armour layer  26  which is strong but still very flexible. For instance, each filament  46  member should have a thickness which is less than 10% of the overall thickness of the temperature sensor  10 . 
   In the preferred embodiment, sixteen metal filaments  46  are used in each strand  44  of the weave. If a four strand weave were used, this would result in sixty-four different continuous metal filament members  46 . The preferred armour layer  26  contains 16 filaments/strand×24 strands=384 continuous filaments  46  extending longitudinally and helically about the electrical circuit  22 . Each filament  46  is preferably provided by a tin plated copper thread having a diameter of about 0.003 inches. The sensor  10  has a diameter or thickness of about 1/10 th  of an inch, so each filament  46  has a diameter which is about 3% or less than the overall thickness of the sensor  10 . 
   The preferred weave provides several distinct advantages over the metal tubing of the prior art. With a woven configuration, the armour layer  26  is much more flexible than prior art metal tubing. The sensor  10  can be wrapped, bent, flexed, even tied into a knot much like cord or rope. No tubing bender is needed either for wrapping the sensor  10  or straightening the sensor  10 . The sensor  10  can be easily shipped in a small box (not shown). 
   The helical wrapping permits the armour layer  26  to change diameter, particularly during assembly of the sensor  10  as shown in  FIG. 3 . To place the electrical circuit  22  into the armour layer  26 , a compressive force C is placed on the armour layer  26  to increase its helix angle and increase the inside diameter of the armour layer  26 . Once the electrical circuit  22  is positioned as desired within the armour layer  26 , a tensile force T is placed on the armour layer  26  to decrease its helix angle and decrease the inside diameter of the armour layer  26 . The armour layer  26  is pulled until it snugly fits around both the sheathing  36  and the insulator protected thermistors  12 . With this snug fit, the armour layer  26  makes intimate and consistent contact around each RTD  12  for consistent thermal conductivity from the armour layer  26  through the insulator  24  to each RTD  12 . Minimizing or eliminating any airgap between the armour layer  26  and each RTD  12  helps make the temperature sensor  12  more consistent and accurate in sensing temperature of the flow. 
   Preferably the distal end  48  of the electrical circuit  22  extends slightly, such as about ¼-½ inch, beyond the distal end  50  of the armour layer  26 . Proximal leads  52  for the electrical circuit  22  extend out of the proximal end  54  of the armour layer  26  a sufficient distance for attachment into an HVAC controller (not shown). As an alternative to inserting the electrical circuit  22  into an already formed armour layer  26 , the armour layer can be woven around the electrical circuit  22 . 
   After the armour layer  26  has been pulled about the electrical circuit  22 , both the proximal and distal ends  50 ,  54  of the armour layer  26  are attached to the sheathing  36  of the electrical circuit  22 . By securing the armour layer  26  to the sheathing  36  both proximally and distally of the RTDs  12 , the armour layer  26  provides the stress relief bridging clips of U.S. Pat. Nos. 6,592,254 and 6,890,095. The preferred method of attachment involves end cap attachments  56  to secure the armour layer  26  to the sheathing  36 . The preferred end cap attachments  56  are provided by a cross-linked modified polyolefin tubing having an adhesive coated interior as disclosed in application Ser. No. 10/436,451. If desired and as depicted in  FIG. 1 , the end caps  56  may be transparent or translucent. A proximal end cap tube  56  is placed around an external surface of the proximal end  54  of the armour layer  26  so it also extends over a proximal section of the sheathing  36 . A distal end cap tube  56  is placed around an external surface of the distal end  50  of the armour layer  26  so it extends further around the distal end  48  of the electrical circuit  22  and even further to completely seal the distal end of the sensor device  10 . The proximal and distal end caps  56  also prevent any fraying of the armour layer  26 . 
   The woven armour layer  26  forms a seamless or circumferentially continuous lumen about the electrical circuit  22 . With no seam, there is no position of weakness or likely fraying of the armour layer  26 , and the sensor  10  retains an attractive, “looks-like-new” appearance over an extended period of time. 
   The temperature sensor  10  of the present invention is particularly contemplated for use in an HVAC system within a ventilation duct (not shown), to be read by an HVAC control unit (not shown). The air flow may have different temperatures at different locations in the duct, and the different temperatures may change differently as a function of time. The sensor  12 , when placed in the duct, will provide a single equivalent or average temperature reading. The armour layer  26  conducts heat along the length of the sensor  12 , so each RTD  12  will be heated or cooled via the armour layer  26  in accordance with the distance that heat needs to travel through the armour layer  26  (i.e., how far away a local hot or cool spot in the flow is). The sensor  10  thus provides two separate, complementary forms of temperature averaging; one due to having multiple RTDs  12  in a single circuit, a second because the temperature at each RTD  12  is affected by heat conduction along the armour layer  26 . The RTDs  12  are spaced along the length of the temperature sensor  10  as desired for positioning and support of the RTDs  12 . In a preferred embodiment, the spacing d between RTDs  12  is selected to be equal, such as about three feet. 
   With the armour layer  26 , the RTDs  12  and insulators  24  are no longer visible on the outside of the sensor  10 . The insulators  24  are preferably slightly thicker and/or slightly less flexible than the sheathing  36 , so an installer can still readily determine the locations of the RTDs  12  by the slight increase in thickness of the sensor  10  or slight decrease in flexibility at the RTD locations. 
   The armour layer  26  is very durable and protects the electrical circuit  22  from damage. In contrast to the point averaging provided by the primary embodiment of U.S. Pat. Nos. 6,592,254 and 6,890,095, the temperature sensing of the armour sensor  10  is more representative of the entire length of the sensor  10  rather than just the locations of the RTDs  12 . Openings in the armour layer  26  permit a limited amount of air flow therethrough, beneficial both for a quicker response time and so any condensation within the armour layer  26  can be dissipated via evaporation. The electrical circuit  22  is sealed separately from the armour layer  26 , so any condensation within the armour layer  26  does not short out the circuit  22 . 
   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.