Abstract:
A temperature sensor including a temperature sensitive device which is disposed in a flow path through which fluid flows and whose electric characteristic changes as a function of temperature of the fluid in the flow path, signal lines connected at top end sides thereof to said temperature sensitive device through electrode wires and at base end sides thereof to lead wires for connection with an external circuit, a sheath member retaining the signal lines therein, and a holding member which holds an outer circumferential surface of said sheath member directly or indirectly through another member. The resonance (primary) frequency at a top end of the temperature sensor against acceleration in a radius direction of the temperature sensor is 480 Hz or less, thereby reducing the transmission of vibration to the top end of the temperature sensor to avoid the breakage of the temperature sensitive device and the disconnection of the electrode wires 502102 even when the temperature sensor resonates.

Description:
This application is the U.S. national phase of International Application No. PCT/JP2008/057405 filed 16 Apr. 2008, which designated the U.S. and claims priority to Japan Application Nos. 2007-107517 filed 16 Apr. 2007and 2008-103995 filed 11 Apr. 2008, the entire contents of each of which are hereby incorporated by reference. 
     TECHNICAL FIELD 
     The present invention relates generally to a temperature sensor to be installed in an exhaust system of internal combustion engines. 
     BACKGROUND ART 
     There are known so-called exhaust temperature sensors which measure the temperature of exhaust gas flowing through a path such as the inside of a catalytic converter or an exhaust pipe of automobile exhaust emission control device using a thermo-sensitive device. 
     A thermistor whose electric characteristics are sensitive to temperature is disposed inside a bottomed cylindrical metallic cover. In order to enhance the thermally sensitive response of the thermistor surrounded by the metallic cover, an insulating material which is good at thermal conductivity is disposed in a space formed between an inner circumferential surface of the metallic cover and an end surface of a sheath pin so that after received by the metallic cover, the heat of exhaust gas is transmitted to the thermistor through the insulating material. An electric signal produced by the thermistor whose electric characteristics are sensitive to temperature is transmitted through electrode wires to a control device in which the temperature is to be measured. 
     Such a temperature sensor is disclosed in patent documents 1 and 2. The temperature sensor, as illustrated in  FIG. 9 , includes a thermistor  501  serving as a temperature sensitive device, a sheath pin  505  having disposed therein signal lines  503  welded to a pair of electrode wires  502  joined to the thermistor  501 , a temperature sensitive portion cover  504  that is a metallic cover disposed on a top end of the thermistor  501  to cover it, and a rib  601  retaining the sheath pin  505  at an outer circumference thereof. The installation of the temperature sensor in an exhaust pipe  800  is achieved by placing a holding member  602  and a nipple  701  on an outer circumference of the rib  601  and securing the nipple  701  to a boss  704  mounted in the exhaust pipe  800 . 
     An internal combustion engine in connection with the exhaust pipe in which the temperature sensor is typically mounted usually vibrates during running thereof. Such vibration is transmitted from the exhaust pipe  800  to the sheath pin  505  of the temperature sensor through the boss  704  and the rib  601 . Specifically, the rib  601  and the sheath pin  505  are welded together, so that the vibration is transmitted from the rib  601  directly to the sheath pin  505 . 
     Consequently, the vibration of the sheath pin  505  becomes strong (high frequency and great amplitude), which may cause the temperature sensitive portion to vibrate at high acceleration. 
     An excessive stress may, thus be exerted on the top end of the sheath pin  505 , the thermistor  505  disposed on the top of the sheath pin  505 , or a welded portion between the sheath pin  505  and the rib  601 . 
     The excessive stress on the thermistor  501  may result in breakage of the thermistor  501  or disconnection of the electrode wire  502  of the thermistor  505 . The excessive stress on the weld between the sheath pin  505  and the rib  601  may result in cracks or breakage of the sheath pin  505 . 
     Therefore, in  FIG. 9 , the length L 4  between the top end of the sheath pin  505  held by the rib  601  and the top end of the temperature sensor is shortened greatly relative to the length L 3  between the inner circumferential surface of the exhaust pipe  800  in which the temperature sensor is installed and the top end of the temperature sensor. This causes the resonance frequency of the top end of the temperature sensor to lie out of a resonance frequency band of the vibration of the exhaust pipe in which the temperature sensor is installed, thereby avoiding the disconnection of the signal lines  502 , etc. 
     Patent Document 1: Japanese Patent First Publication No. 2002-350239 
     Patent Document 2: Japanese Patent First Publication No. 2006-47273 
     The structure of Patent document 1 or 2, however, face the problem that it is impossible to bring the resonance frequency of the top end of the temperature sensor out of the resonance frequency band of the vibration of the exhaust pipe, thus resulting in possibility of the breakage of the thermistor  501  or the disconnection of electrode wires  502 . 
     When the rib  601  is prolonged toward the top end to increase the resonance (primary) frequency of the sheath pin  505 , it will cause the resonance to be about 500 times, so that the stress acting on the electrode wires  502  located on the top end side of the temperature sensor to be increased greatly. This results in a difficulty in avoiding disconnection of electrode wires  502 . 
     The present invention was made in order to solve the prior art problems. It is an object to provide a temperature sensor which reduces the transmission of vibration and is excellent in durability. 
     In order to achieve the above object, is a temperature sensor including a temperature sensitive device which is disposed in a flow path through which fluid flows and whose electric characteristic changes as a function of temperature of the fluid in the flow path, signal lines connected at top end sides thereof to said temperature sensitive device through electrode wires and at base end sides thereof to lead wires for connection with an external circuit, a sheath member retaining the signal lines therein, and a holding member which holds an outer circumferential surface of said sheath member directly or indirectly through another member, characterized in that a resonance (primary) frequency at a top end of the temperature sensor against acceleration in a radius direction of the temperature sensor is 480 Hz or less. 
     There was a conventional technical idea of increasing the resonance (primary) frequency at the top end of the temperature sensor to avoid the resonance arising from external vibration. However, the present invention was based on a technical idea opposite the conventional one. Specifically, the decrease in transmission of vibration to the top end of the temperature sensor is achieved by decreasing the resonance (primary) frequency at the top end of the temperature sensor down to 480 Hz or less, thereby avoiding the breakage of the thermistor  501  or disconnection of the electrode wires  502  even when the temperature sensor resonates. 
     The resonance (primary) frequency may also be 380 Hz or less against the acceleration in the radius direction of the temperature sensor. 
     This ensures the durability further and avoids the disconnection of the electrode wires for an increased period of time. 
     If a protruding length that is a distance between an inner circumference of said flow path and a top end of the temperature sensor on an axis of the temperature sensor is defined as L 1 , and a held length that is a distance between a top end of a portion of said sheath member which is held by the holding member directly or indirectly and the top end of the temperature sensor is defined as L 2 , then a relation of L 1 &lt;L 2  is preferably satisfied. 
     The protruding length L 1  is changed frequently according to the object or intended purpose. For instance, the protruding length L 1  is changed greatly between when the temperature of a central portion of the flow path is to be measured and when the temperature of an inner edge of the flow path is to be measured. Consequently, when L 1 &lt;L 2 , the resonance (primary) frequency at the top end of the temperature sensor depends upon the protruding length L 1  as long as a condition such as the diameter of the sheath member is constant. In other words, when the protruding length L 1  is short, it will result in an increase in the held length L 2 , so that the resonance (primary) frequency at the top end of the temperature sensor will become great. 
     However, in the present exemplary embodiment, the held portion of the sheath member is designed to be located at the base end of the holding member, thereby permitting the held length L 2  to be increased sufficiently even when the protruding length L 1  is short, which results in a great decrease in resonance (primary) frequency at the top end of the temperature sensor regardless of the protruding length L 1 . 
     The breakage of the electrode wires and the temperature sensitive device located at the top end side of the temperature sensor caused by the resonance is, therefore, avoided. 
     If a diameter of a portion of the protruding length L 1  which holds said temperature sensitive device is defined as a sensor outer diameter D, the sensor outer diameter D is 3.2 mm or less, and preferably the held length L 2  is 75 mm or more. 
     This enables the resonance (primary) frequency at the top end of the temperature sensor to be decreased below 480 Hz as the sensor outer diameter D is decreased and the held length L 2  is increased, thereby avoiding the disconnection of the electrode wires. 
     Preferably, the temperature sensitive device is disposed inside a metallic cover. 
     This shields the temperature sensitive device from the atmosphere of exhaust gas to avoid the reduction-caused deterioration of the temperature sensitive device. 
     Preferably, the temperature sensitive device is implemented by a thermistor. 
     This realizes the temperature sensor which is high in measurement accuracy. 
     Preferably, temperature sensitive device is embedded in a fixing member supplied inside a top end of said metallic cover. 
     This avoids collision of the temperature sensitive device with the metallic cover so that it is broken when the temperature sensor vibrates following external vibration. Further, the temperature sensitive device is secured by the fixing member inside the metallic cover, thus reducing the vibration of the temperature sensitive device caused by the resonance. This decreases the stress acting on the electrode wires which is developed by the resonance of the temperature sensitive device. 
     Preferably, the temperature sensitive device is sealed by glass. 
     In high-temperature environments, the metallic cover is oxidized, so that the concentration of oxygen within the metallic cover drops. It is, thus, necessary to avoid the reduction-caused deterioration arising from removal of oxygen from the temperature sensitive device. The reduction-caused deterioration of the temperature sensitive device is, therefore, avoided by sealing the temperature sensitive device using the glass. This ensures the stability in the measurement accuracy of the temperature sensitive device. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross sectional view which shows an exhaust temperature sensor  1  of the invention installed in a flow path; 
         FIG. 2  is an enlarged sectional view showing a temperature sensitive portion  10  in  FIG. 1 ; 
         FIG. 3  is a schematic view which illustrates a heat/resonance durability test on an exhaust temperature sensor; 
         FIG. 4  is a graph which represents results of heat/resonance durability tests on an exhaust temperature sensor  1  of the invention; 
         FIG. 5  is an enlarged sectional view which shows a temperature sensitive portion  10  in  FIG. 4 ; 
         FIG. 6  is a cross sectional view which shows another embodiment of an exhaust temperature sensor  1  of the invention; 
         FIG. 7  is a cross sectional view which shows another embodiment of an exhaust temperature sensor  1  of the invention; 
         FIG. 8  is a cross sectional view which shows another embodiment of a temperature sensitive portion  10  of an exhaust temperature sensor  1  of the invention; and 
         FIG. 9  is a cross sectional view which shows a conventional exhaust temperature sensor  2  installed in a flow path. 
     
    
    
     DESCRIPTION OF REFERENCE NUMBERS 
     
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                  1 
                 exhaust temperature sensor (temperature sensor) 
               
               
                   
                  10  
                 temperature sensitive portion 
               
               
                   
                  20  
                 case 
               
               
                   
                 101 
                 thermistor (temperature sensitive device) 
               
               
                   
                 102 
                 electrode wire 
               
               
                   
                 103 
                 signal lines (signal lines) 
               
               
                   
                 104 
                 temperature sensitive portion cover (metallic cover) 
               
               
                   
                 105 
                 sheath pin (sheath member) 
               
               
                   
                 106 
                 fixing member 
               
               
                   
                 107 
                 glass material 
               
               
                   
                 201 
                 rib (holding member) 
               
               
                   
                 202 
                 protection tube (holding member) 
               
               
                   
                 203 
                 lead wire 
               
               
                   
                 204 
                 holder tube 
               
               
                   
                 310 
                 nipple 
               
               
                   
                 302 
                 fixing member 
               
               
                   
                 303 
                 electric furnace 
               
               
                   
                 304 
                 boss 
               
               
                   
                 400 
                 exhaust pipe (flow path) 
               
               
                   
                   
               
             
          
         
       
     
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     Embodiments of an exhaust temperature sensor  1  (temperature sensor) according to the invention will be described below based on drawings. 
     The exhaust temperature sensor  1  is applied as a sensor to measure the temperature of exhaust gas emitted from an automotive engine and to be installed in, for example, an exhaust pipe of automobiles. 
     As illustrated in  FIG. 1 , the exhaust temperature sensor  1  consists essentially of a temperature sensitive portion  10 , a case  20 , and a sheath pin  105  disposed between the temperature sensitive portion  10  and the case  20 . 
     In this specification, the lower side and the upper side of  FIG. 1  will be referred to below as a top end side and a base end side, respectively, in explanation of the structure of the exhaust temperature sensor  1 . 
     The temperature sensitive portion  10  is exposed to the exhaust gas and sensitive to the temperature of the exhaust gas. 
       FIG. 2  is an enlarged sectional view which shows the temperature sensitive portion  10  of  FIG. 1  which is formed by a sintered body made of semiconductor material whose main component is Cr-Mn and includes a thermistor  101  that is a temperature sensitive device sensitive to the temperature of exhaust gas, a pair of electrode wires  102  which transmits an electric signal, as produced by the thermistor  101  to the base end side, a pair of signal lines  103  whose top end is joined at the base end side of the electrode wires  102  through laser welding or resistor welding and other end is connected to lead wires  203 , and a temperature sensitive portion cover  104  that is a metallic cover to protect the thermistor  101 . The signal lines and the electrode wires  102  are made of platinum material. The signal lines  103  are made of stainless steel. The temperature sensitive portion cover  104  is made of Inconel material shaped by the deep drawing into a bottomed cylinder. 
     The thermistor  101  is preferably used as the temperature sensitive device to easily make the exhaust temperature sensor which has high measurement accuracy. 
     The thermistor  101  is preferably disposed inside temperature sensitive portion cover  104  to shield thermistor  101  from exhaust gas, thereby avoiding deterioration of thermistor thermistor  101 . 
     A fixing member is preferably disposed between thermistor  101  and temperature sensitive portion cover  104  to avoid collision of thermistor  101  with temperature sensitive portion cover  104  when the exhaust temperature sensor vibrates, so that thermistor  101  oscillates. This avoids damage to thermistor  101  and disconnection of electrode lines  102  of thermistor  101 . 
     As the fixing material, material which is excellent in thermal conductivity may be used to accelerate the transmission of heat outside the temperature sensitive portion cover  104  to thermistor  101 , thereby achieving the exhaust temperature sensor which is excellent in response. 
     Further, temperature sensitive portion cover  104  preferably uses alloy such as Inconel that is excellent in oxidation resistance, thereby avoiding oxidation of temperature sensitive portion cover  104  and avoiding a change in characteristic of thermistor  101  arising from a decrease in concentration of oxygen in temperature sensitive portion cover  104 . The oxidation of temperature sensitive cover  104  usually results in a drop in concentration of oxygen in the cover. This may cause the oxygen to be removed from the thermistor  101  to compensate for the drop in concentration of oxygen, thereby changing the characteristics of thermistor  101 . This is avoided by making temperature sensitive portion cover  101  by the anti-oxidation metal. 
     As the anti-oxidation metal, there is, for example, stainless steel or Inconel 
     In the thus constructed temperature sensitive portion  10 , the sheath pin  105  which corresponds to a sheath member is inserted and disposed at an end thereof. 
     The sheath pin  105  is cylindrical and made of Inconel. The sheath pin  105  is secured to the temperature sensitive portion cover  104  by crimping or laser-welding. The sheath pin  105  may be press-fitted or resistor-welded into the temperature sensitive portion cover  104 . 
     Thermistor  101  is preferably sealed by a glass material. This reduces the deterioration of the temperature sensitive device and makes it excellent in durability. 
     The sheath pin  105  has signal lines  103  disposed therein and insulates and protects them. The sheath pin  105  includes the two signal lines  103  made of stainless steel, an insulating portion made of insulating powder such as magnesia which is disposed around signal lines  103 , and an outer tube portion made of stainless steel covering the outer circumference of the insulating portion. 
     Next, the case  20 , as illustrated in  FIG. 1 , will be described below. The case  20  serves to install the exhaust temperature sensor  1  to the exhaust pipe and is joined to the temperature sensitive portion  10  through the sheath pin  105  corresponding to the sheath member. 
     The case  20  includes a rib  201  coupled to the outer periphery of the sheath pin  105 , a protection tube  202  welded to the outer periphery of the rib  201 , and the lead wires  203  connected electrically to the base end of the sheath pin  105 . 
     In  FIG. 1 , a substantially integrated member formed by securing the rib  201  and the protection tube  202  together through laser-welding corresponds to the holding member. 
     As illustrated in  FIG. 1 , a boss  301  is fixed in exhaust pipe  400  which corresponds to the flow path. The boss  301  has an internal thread formed on an inner circumferential surface thereof. The installation of the exhaust temperature sensor  1  in exhaust pipe  400  is achieved by pressing it to the top end side in contact of nipple  202  with the base end surface of rib  201  and, at the same time, engaging an external thread of nipple  202  with the internal thread of boss  301 . 
     The top end surface of the rib  201  is seated firmly on the inner peripheral surface of the boss  301  to hermetically seal the exhaust gas flowing inside the exhaust pipe  400 . 
     The thus constructed exhaust temperature sensor  1  outputs an exhaust gas temperature signal, as produced by the thermistor  101 , to an external circuit (e.g., an ECU) not shown through the lead wires  203  to detect the temperature of the exhaust gas. 
     The fixing of the rib  201  and the protection tube  202  is achieved by placing a portion of the outer circumferential surface of the rib  201  in abutment with the inner circumferential surface of the protection tube  202  and welding the outer circumferential surface of the protection tube  202 . 
     The sheath pin  105  is fit in a central hole of the rib  201 . The sheath pin  105  and the rib  201  are welded together at a contact between the inner circumferential surface of the rib  201  and the outer circumferential surface of the sheath pin  105 . 
     The holding member, as referred to in this specification, is the substantially integrated member made by laser-welding the rib  201  and the protection tube  202 . A held portion of the sheath member is a contact between the sheath member and the rib  201  disposed around the outer periphery of the sheath pin  105 . 
     The features of the exhaust temperature sensor  1  according to the embodiment of the invention will be described below. 
     The fixing of the rib  201  and the protection tube  202  is, as illustrated in  FIG. 1 , achieved by placing the portion of the outer circumferential surface of the rib  201  in contacting abutment with the inner circumferential surface of the protection tube  202  and welding the outer circumferential surface of the protection tube  202 . 
     The sheath pin  105  is fit in the central hole of the rib  201 . The sheath pin  105  and the rib  201  are welded together at the contact between the inner circumferential surface of the rib  201  and the outer circumferential surface of the sheath pin  105 . 
     The vibration applied externally to the exhaust temperature sensor  1  is transmitted to the contact (i.e., the held portion) between the inner circumferential surface of the rib  201  and the outer circumferential surface of the sheath pin  105  to induce the vibration and the resonance to which the sheath pin  105  has the contact as a fixed end is subjected. The resonance, as referred to herein, is the characteristic vibration of each member such as the sheath pin  105  having energy which arises from vibration applied thereto. 
     In this embodiment, the resonance (primary) frequency at the top end of the exhaust temperature sensor  1  is specified to be 480 Hz relative to acceleration of exhaust temperature sensor  1  in a radius direction thereof. 
     The reduction in transmission of vibration to the top end of exhaust temperature sensor  1  may be achieved by bringing the above resonance frequency below 480 Hz. Even when the resonance occurs at exhaust temperature sensor  1 , it avoids breakage of thermistor  501  or disconnection of electrode wires  502 . The resonance frequency is preferably set to 380 Hz or less, thereby enabling exhaust temperature sensor  1  which has a high degree of durability and vibration resistance. The vibration resistance is ensured, especially when exhaust temperature sensor  1  is used in an exhaust system which is designed to vibrate greatly or required to be prolonged. 
     Note that the resonance (primary) frequency at the top end of the exhaust temperature sensor  1  may be measured using a laser Doppler oscillometer. 
     The adjustment of the resonance (primary) frequency at the top end of the exhaust temperature sensor  1  may be achieved by selecting a relation between a protruding length L 1  and a held length L 2  where the protruding length L 1  is, as can be seen in  FIG. 1 , a distance between the inner circumferential surface of the exhaust pipe  400  on the axis of the exhaust temperature sensor  1  and the top end of the exhaust temperature sensor  1 , and the held length L 2  is a distance between the top end of the held portion where the sheath pin  105  is held or retained directly or indirectly by the rib  201  that is the holding member and the top end of the exhaust temperature sensor  1 . 
     The inner circumference surface of exhaust pipe  400  on the axis of exhaust temperature sensor  1 , as referred to above, is an intersection a between an imaginary line (a broken line in  FIG. 1 ) extending through opposed ends of exhaust pipe  400  in which exhaust temperature sensor  1  is installed and the axis of exhaust temperature sensor  400 . The protruding length L 1  is a distance between the intersection a and the top end of exhaust temperature sensor  1 . 
     The top end of the held portion, as referred to above, is an end A of the held portion in which the rib  201  and the sheath pin  105  are welded, that is, portions of the rib  201  and the sheath pin  105  which are placed in constant contacting abutment with each other. In this embodiment, the protruding length L 1  is also a distance between an end of a tapered portion of the rib  201  o the base end side and the top end of the temperature sensitive portion  10  (i.e., a length of a portion of the exhaust temperature sensor  1  protruding into the exhaust pipe  400 ). 
     In this embodiment, the protruding length L 1  and the held length L 2  preferably have a relation of L 1 &lt;L 2 . 
     The resonance frequency at the top end of the exhaust temperature sensor  1  may be decreased by prolonging the held length L 2  as much as possible relative to the protruding length L 1 , thereby avoiding the disconnection of the electrode wires  102 . 
     The relation among the resonance frequency of exhaust temperature sensor  1 , protruding length L 1 , and held length L 2  was checked by heat/resonance durability tests, as illustrated in  FIG. 3 . 
     As illustrated in  FIG. 3 , the heat/resonance durability tests are to apply acceleration to the exhaust temperature sensor  1  in a radius direction thereof while the top end of the exhaust temperature sensor  1  is being heated in an electric furnace  303 . Test conditions are to place the top end of the exhaust temperature sensor  1  in the furnace  303  to bring the temperature of the top end up to 850° C., apply an acceleration of 20G to a mount  302  in which the exhaust temperature sensor  1  is installed through the nipple  301 , and at the same time sweep a frequency band near the (primary) resonance frequency of each sample (i.e., the top end of the sheath pin  105 ) to observe the presence of breakage of the electrode wires  102 . 
     The tests were performed for different values, as listed in table 1, of a sensor outer diameter D that is a diameter of the sheath pin  105  which is the holding member holding the thermister  101  within the protruding length L 1  and the held length L 2 . Results of the tests are shown in table 1 and  FIG. 4 . 
     Note that the resonance was measured by the above described laser Doppler oscillometer. In the tests, the protruding length L 1  was smaller than the held length L 1  by 20 mm (L 1 =L 2 −20). 
     
       
         
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 * L1 = L2 − 20 
               
             
          
           
               
                 Sample 
                 Sheath pin outer 
                 L1 
                 L2 
                 Resonance 
                   
               
               
                 No. 
                 dia. D (mm) 
                 (mm) 
                 (mm) 
                 frequency (Hz) 
                 Judgment 
               
               
                   
               
             
          
           
               
                 1 
                 2.3 
                 30 
                 50 
                 808 
                 X 
               
               
                 2 
                   
                 40 
                 60 
                 561 
                 X 
               
               
                 3 
                   
                 45 
                 65 
                 478 
                 ◯ 
               
               
                 4 
                   
                 50 
                 70 
                 412 
                 ◯ 
               
               
                 5 
                   
                 55 
                 75 
                 359 
                 ⊚ 
               
               
                 6 
                   
                 60 
                 80 
                 316 
                 ⊚ 
               
               
                 7 
                   
                 65 
                 85 
                 280 
                 ⊚ 
               
               
                 8 
                   
                 70 
                 90 
                 249 
                 ⊚ 
               
               
                 9 
                 2.9 
                 30 
                 50 
                 980 
                 X 
               
               
                 10 
                   
                 40 
                 60 
                 680 
                 X 
               
               
                 11 
                   
                 45 
                 65 
                 580 
                 X 
               
               
                 12 
                   
                 50 
                 70 
                 500 
                 X 
               
               
                 13 
                   
                 55 
                 75 
                 435 
                 ◯ 
               
               
                 14 
                   
                 60 
                 80 
                 383 
                 ⊚ 
               
               
                 15 
                   
                 65 
                 85 
                 339 
                 ⊚ 
               
               
                 16 
                   
                 70 
                 90 
                 302 
                 ⊚ 
               
               
                 17 
                 3.2 
                 30 
                 50 
                 1066 
                 X 
               
               
                 18 
                   
                 40 
                 60 
                 740 
                 X 
               
               
                 19 
                   
                 45 
                 65 
                 631 
                 X 
               
               
                 20 
                   
                 50 
                 70 
                 544 
                 X 
               
               
                 21 
                   
                 55 
                 75 
                 474 
                 ◯ 
               
               
                 22 
                   
                 60 
                 80 
                 416 
                 ◯ 
               
               
                 23 
                   
                 65 
                 85 
                 370 
                 ⊚ 
               
               
                 24 
                   
                 70 
                 90 
                 329 
                 ⊚ 
               
               
                 25 
                 3.4 
                 55 
                 75 
                 495 
                 X 
               
               
                 26 
                 3.5 
                 55 
                 75 
                 506 
                 X 
               
               
                   
               
               
                 X: breakage of electrode wires 102 within target time (equivalent to 300,000 km) 
               
               
                 ◯: breakage of electrode wires 102 after lapse of one or two times the target time 
               
               
                 ⊚: no breakage of electrode wires 102 even after elapse of two times the target time or more. 
               
             
          
         
       
     
       FIG. 4  shows that the resonance (primary) frequency at the top end of the exhaust temperature sensor  1  drops with a decrease in the sensor outer diameter D and an increase in the held length L 2 . 
       FIG. 4  and Table 1 show that samples Nos. 3-8, 13-16, and 21-24 in which the resonance frequency is 480 Hz or less have improved durability against the disconnection of the electrode wires  102  which exceeds one time of the target time and is shorter than two times of the target time. 
     Particularly, it is advisable that the sensor outer diameter D be 3.2 mm or less, and the held length L 2  be 75 mm or more. This, as can be seen from  FIG. 4 , causes the resonance (primary) frequency to be 480 Hz or less. 
     It is also found that samples Nos. 25 and 26 in which the held length L 2  is as long as 75 mm, but sensor outer diameter D is more than 3.2 mm exceed 480 Hz in the resonance frequency and do not satisfy the target time. 
     It is also found that samples Nos. 5-8, 14-16, 23, and 24 in which the resonance frequency is 380 Hz or less have improved durability against the disconnection of the electrode wires  102  which is two times longer than the target time or more. 
     When the sensor outer diameter D is 3.2 mm or less, and the held length L 2  is 85 mm or more, the resonance (primary) frequency will be 380 Hz or less. 
     In the above described embodiment, the rib  201  and the sheath pin  105  are designed so that they placed in direct contact with each other and welded together at the contact therebetween, but however, they may be, as illustrated in  FIGS. 5(   a ) and ( b ), secured together through an additional member. 
     For instance, the temperature sensitive portion cover  104  is so fixed as to cover a portion of the outer circumferential surface of the top end of the sheath pin  105 , but the longer temperature sensitive portion cover  104 , as illustrated in  FIGS. 5(   a ) and  5 ( b ), may alternatively be used to dispose the sheath pin  105  therewithin. Specifically, the temperature sensitive portion cover  104  is welded at the base end side thereof to the base end side of the rib  201  to achieve the same effects as in the above embodiment. The fixing member  106  may be disposed between the inner circumferential surface of the temperature sensitive portion cover  104  and the outer circumferential surface of the sheath pin  105 . 
     In the structure of  FIG. 5 , the outer diameter of a portion of the temperature sensitive portion cover  104  except a relatively smaller diameter portion thereof. The distance between the top end of the contact between the inner circumferential surface of the rib  201  and the outer circumferential surface of the temperature sensitive portion cover  104  and the top end of the temperature sensitive portion cover  10  is defined as L 2 . It is also important that L 1 &lt;L 2  is satisfied. 
     The protection tube  204  may be, as illustrated in  FIG. 6 , disposed between the inner circumferential surface of the rib  201  and the outer circumferential surface of the sheath pin  105 . Additionally, although not illustrated, the top end of the protection tube  204  may be decreased in diameter because it results in interference with the sheath pin  105  to reduce the resonance of the sheath pin  105 . In this case, the protection tube  204  may be placed in direct contact with the sheath pin  105 , but such contact only serves to suppress the resonance and never as a fixed end of vibration of the sheath pin  105 . 
     The protection tube  202  may also be, as illustrated in  FIG. 7 , crimped at the outer periphery thereof to decrease the diameter so as to make a contact with the sheath pin  105  located inside. The retaining of the sheath pin  105  may be achieved by welding the contact. 
     The glass material  107  which has the heat resistance may also be, as illustrated in  FIG. 8 , disposed to cover the thermister  101  to block the thermister  101  from the atmosphere in the temperature sensitive portion cover  104 . This avoids the reduction-caused deterioration of the thermister  101  arising from oxidation of the temperature sensitive portion cover  104 . 
     The structures may be modified in various ways without departing from the principle of the invention.