Thermocouple instrument
BACKGROUND OF THE INVENTION 
     The present invention relates to a thermocouple instrument and, more particularly, to a thermocouple instrument having a structure capable of enduring a vigorous vibration and impact. 
     A conventional thermocouple instrument which has been heretofore used, for example, in an aircraft engine to measure the temperature of the engine itself or of exhaust gas from the engine employed, to endure high temperature and corrosive atmosphere, a structure in which a thermocouple element formed of two dissimilar conductors joined at the ends was mounted together with an insulating powder in an inner metal protecting tube which was further mounted in an outer metal protecting tube. In order to avoid a loss responsiveness in detecting the temperature in such a structure, an air gap was normally provided in the end part of the outer metal protecting tube and a pair of small holes were formed at both side surfaces of the ends of the protecting tube, and gas to be measured at its temperature flowed through the small holes in the air gap. 
     When this thermocouple instrument was used to measure the temperature of the aircraft engine itself or of the exhaust gas of the engine, the thermocouple instrument was exposed to vigorous vibration and impact. When the thermocouple instrument thus constructed was exposed to such severe vibration and impact, the inner and outer metal protecting tubes came into violent contact with each other and were resultantly damaged. Therefore, various trials of fixedly securing both the outer and inner protecting tube have been heretofore carried out. However, the attachment has not been sufficient, additional members have been required, and the structure of the instrument has thus been complicated, resulting in a difficulty in securing both the outer and inner protecting tubes. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a thermocouple instrument which has excellent vibration resistance and impact resistance at a high temperature and in which an inner protecting metal tube for containing a thermocouple element is rigidly mounted and retained within an outer protecting metal tube. 
     According to the present invention, there is provided a thermocouple instrument comprising a thermocouple element formed of two dissimilar conductors joined at the ends thereof; an inner protecting metal tube for containing said thermocouple element together with insulating material electrically insulated from said thermocouple element via the insulating material; and an outer protecting metal tube into which said inner protecting metal tube is inserted, wherein the thermal expansion coefficient of the metal forming said inner protecting metal tube is not lower than the thermal expansion coefficient of the metal forming said outer protecting metal tube. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is best understood by reference to the accompanying drawings of which: 
     FIG. 1 is a sectional view of a thermocouple instrument according to an embodiment of the present invention; 
     FIG. 2 is an enlarged sectional view of the end part of the thermocouple instrument in FIG. 1; and 
     FIGS. 3A and 3B are respectively sectional views showing the steps of manufacturing the thermocouple instrument in FIG. 1. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An embodiment of the present invention will now be described in more detail with reference to the accompanying drawings. 
     FIG. 1 is a sectional view of a thermocouple instrument according to a preferred embodiment of the present invention. In FIG. 1, an outer protecting tube 1 is formed of a metal having excellent corrosion resistance and heat resistance such as, for example, an Hastelloy X. This outer protecting tube 1 is of a cylinder which is closed at one end and which is opened at the base. Holes 2a, 2b for passing gas, e.g., exhaust gas to be measured for its temperature are confronted and formed at the peripheral wall of the end of the outer protecting tube 1. A sheath type thermocouple 3 is inserted and mounted in the outer protecting tube 1. This sheath type thermocouple 3 is integrally pressure-molded by containing wires 5, 6 of thermocouple element and insulating powders 7, 8 in the inner protecting tube 4. The wires 5, 6 are dissimilar conductors joined at the ends. 
     The inner protecting tube 4 is formed of a metal which has a thermal expansion coefficient equal to or higher than the thermal expansion coefficient of the metal forming the outer protecting tube 1. The inner protecting tube 4 is preferably formed of a metal which has corrosion resistance and heat resistance. In this embodiment, the inner protecting tube 4 is formed of Inconel 600. The thermal expansion coefficient of Inconel 600 forming the inner protecting tube 4 is about 13.4×10 -6  /°C., and the thermal expansion coefficient of Hastelloy X forming the outer protecting tube 1 is about 7.7×10 -6  /°C. 
     Even if the thermal expansion coefficient of the metal forming the inner protecting tube 4 is equal to the thermal expansion coefficient of the metal forming the outer protecting tube 1, predetermined securing effects can be obtained. More rigid securing effects can be obtained when the coefficient of the inner tube is higher than that of the outer tube, and this is accordingly preferred. It is preferable that the inner tube coefficient by higher by about 2 to 10×10 -6  /°C. It is more preferable that the inner tube coefficient be higher by about 3 to 8×10 -6  /°C. The preferable combination of both the metals will be listed in the following Table: 
     
                       TABLE                                                       
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                      Thermal expan-                                      
Metal                 sion coefficient                                    
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Outer   Cr: 18 wt %, Ni: 12 wt %,                                         
                           9.3 × 10.sup.-6 /°C.              
protecting                                                                
        Ti: ≦5 wt %, Fe: balance                                   
tube    (SUS 321)                                                         
        Cr: 17 wt %, Ni: 12 wt %,                                         
                           8.9 × 10.sup.-3 /°C.              
        Mo: 3 wt %, Fe: balance                                           
        (SUS 316)                                                         
        Cr: 21 wt %, Al: 1 wt %,                                          
                           6.2 × 10.sup.-6 /°C.              
        Ti: 2.5 wt %, Ni: balance                                         
        (NIMONIC 80A)                                                     
        Cr: 22 wt %, Co: 1.5 wt %,                                        
                           7.7 × 10.sup.-6 /°C.              
        Mo: 9 wt %, Ni: balance                                           
        (Hastelloy X)                                                     
Inner   Ni + Co: 72 wt %, 13.4 × 10.sup.-6 /°C.              
protecting                                                                
        Cr: 16 wt %, Fe: balance                                          
tube    (Inconel 600)                                                     
        Ni: 32 wt %, Cr: 21 wt %,                                         
                          14.3 × 10.sup.-6 /°C.              
        Fe: balance                                                       
        (Inconel 800)                                                     
        Cr: 19 wt %, Fe: 18 wt %,                                         
                          15 × 10.sup.-6 /°C.                
        Mo: 3 wt %, Ti: 1 wt %,                                           
        Ni: balance                                                       
        (Inconel 718)                                                     
        Ni + Co: 70 wt %, 12.6 × 10.sup.-6 /°C.              
        Cr: 15 wt %, Ti: 2.5 wt %,                                        
        Fe: balance                                                       
        (Inconel X 750)                                                   
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     The inner protecting tube 4 is formed in a cylindrical body. The end 4a of the inner protecting tube 4 is formed with a diameter smaller than the other and is hence a convergent shape, and the other end of the inner protecting tube 4 is opened. The end 4a of the inner protecting tube encloses the end joint of the wires 5, 6 forming a thermocouple element. The diameter of the end 4a is formed in consideration of the responsiveness of detecting the temperature, and its length is sufficient to enclose the joint of the wires 5, 6. The top 4b of the end 4a of the inner protecting tube 4 may be formed integrally with the inner protecting tube 4, but may also be separately formed and may be joined by welding to the end 4a of the inner protecting tube 4. 
     The wires 5, 6 are disposed along the axis of the tube 4 in the inner protecting tube 4. The ends of the wires 5, 6 are joined by welding, and are disposed within the end 4a of the inner protecting tube 4. The wires 5, 6 are formed in combination of metals of different types such as, for example, platinum-platinum rohdium, almel-chromel, etc. 
     The insulating powder 7 is filled in a space ranging from the body of the inner protecting tube 4 to a part of the end 4a of the inner protecting tube 4. Another insulating powder 8 is filled in the portion surrounding the joined end of the wires 5, 6 of the end 4a of the inner protecting tube 4, and both the insulating powders 7, 8 are respectively held to bury the entire wires 5, 6 in the inner protecting tube 4. The insulating powders 7, 8 electrically insulate the wires, 4, 5 from the inner protecting tube 4. Particularly, the insulating powder 8 rapidly transfers the temperature to be measured to the joined end of the wires 4, 5. The insulating powder 7 may be, for example, magnesia (MgO). The insulating powder 8 may be, for example, a powder containing as a main component boron nitride (BN) or beryllium oxide (BeO) which has higher heat conductivity than the general insulating material such as magnesia. 
     The base of the inner insulating tube 4 is sealed with a sealing member 9 which is, formed of glass, silicone, heat resistant resin, etc., and the wires 4, 5 externally extend through the sealing member 9. In general, the base of the sheath type thermocouple is not necessarily sealed, but may be sealed depending on the environments. In other words, the thermocouple instrument, by the use of the sealing member 9, can be used in an aggressive worse environment where oil mist is scattered in the interior of an aircraft engine. 
     The sheath type thermocouple 3 which is formed of the inner protecting tube 4, wires 5, 6 and insulating powders 7, 8 is inserted into the outer protecting tube 1, and is held and secured to the outer protecting tube 1 by welding the bases of the inner and outer protecting tubes 4 and 1 or by reducing the diameter of the base of the outer protecting tube 1. 
     The thermocouple instrument thus constructed can be, for example, installed in the exhaust gas passage of an aircraft engine and can be used to measure the temperature of the exhaust gas. In this case, the exhaust gas to be measured is introduced from the holes 2a, 2b into the air gap at the end of the outer protecting tube 1 to fill the periphery of the end 4a of the inner protecting tube 4 for containing the joined portion of the wires 5, 6. The temperature of the exhaust gas is measured in such a manner that the temperature of the gas is transferred through the insulating powder 8 to the joined end of the wires 5, 6, causing a thermoelectromotive force to be produced between the wires 5 and 6, and the thermoelectromotive force is then detected by an instrument connected to the wires 5 and 6. 
     As described above, when the thermocouple instrument of the present invention is arranged to measure at the high temperature of exhaust gas of an aircraft engine, e.g., 700° to 1,400° C., the air gap between the inner protecting tube 4 and the outer protecting tube 1 disappears due to the thermal expansion of the inner protecting tube 4, and the sheath type thermocouple 3 is rigidly engaged to the outer protecting tube 1 to be mounted and held therewith. Therefore, even if severe vibration and impact are applied to the thermocouple instrument of the present invention when it is disposed in the exhaust gas stream flowing at a superhigh speed, the outer surface of the inner protecting tube 4 will not come into violent contact with inner surface of the outer protecting tube thus causing damage. When vibration and impact are applied to the conventional thermocouple instrument at a high temperature, there have been problems such as disconnection or deformation of wires, and improper insulation between the wires and the inner protecting tube due to the deformation of the wires. However, according to the thermocouple instrument of the present invention, such problems cannot take place at all. Since the sheath type thermocouple 3 can be held in the outer protecting tube 1, without using a supporting member, the above-described problems can be eliminated while maintaining a simple structure in the thermocouple instrument of the present invention. 
     In the embodiment described above, the diameter of the end 4a of the inner protecting tube 4 may be made smaller than the other portion. Since the distance between the outer surface of the end 4a of the inner protecting tube 4 and the joined end of the wires 4, 5 can be shortened in this case, the temperature of the exhaust gas can be transferred to the joined end of the wires 5, 6 in an extremely short time, with the result that the responsiveness of measuring the temperature can be improved. In the embodiment described above, boron nitride or beryllium oxide which has high thermal conductivity is employed as the insulating powder 8 to enclose the joined end of the wires 5, 6, and therefore, further improving the responsiveness of measuring the temperature. The boron nitride is not limited in use into the end 4a of the inner protecting tube 4, but may also be filled in the other portion. 
     The sheath type thermocouple 3 of the thermocouple instrument of the present invention can be, for example, manufactured as below. As shown in FIG. 3A, the wires 5, 6 are first disposed in the inner protecting tube 4 before being joined with the end 4b of the end 4a, and the insulating powder 7 is filled therein. Subsequently, drawing of the end 4a is performed. Swagging is employed as the drawing. In this state, the ends of the wires 5, 6 are not yet joined, but are disposed at the opening of the end 4a of the inner protecting tube 4. In this state, the wires 5, 6 are joined by welding at the ends. Since the wires 5, 6 are buried and held stably in the insulating powder 7, the welding work can be facilitated. Subsequently, as shown in FIG. 3B, the insulating powder 8 is filled from the opening of the end 4a of the inner protecting tube 4, and the top 4b of the end of the inner protecting tube 4 is welded to the opening of the end 4a of the inner protecting tube 4. Thus, the insulating powder 8 can be readily filled therein.