Abstract:
A temperature measuring probe utilizing an RTD element and an improved mode of assembling the element in a reliable manner which is proof against shock and vibration. Specifically, the invention has its most important applications in connection with miniaturized RTD&#39;s.

Description:
BACKGROUND AND SUMMARY OF THE INVENTION 
     RTD elements are frequently mounted in a metallic sheath of very small diameter. In the prior art, a typical mode is to locate the device within a sheath, say stainless steel, and to obtain rigidity by surrounding the device with a highly compacted mass of some particulate refractory, say, MgO. One common type of RTD capsule comprises a resistance in the form of a conductor helically wound on an insulating mandrel and the embedment of the winding in a glass or ceramic insulant which, when viscous, fuses with the mandrel. Since numerous applications prescribe the utmost possible accuracy in conversion of temperature to voltage, every precaution is taken to avoid the effects of vibration and shock by immobilizing the element. Prior practice frequently overlooked the possibility of damage resulting from compacting the MgO and careless handling. 
     The present invention relates to an improved RTD probe and the method of fabricating the same. The improved method disclosed herein decreases the cost and increases reliability, while eliminating problems encountered with prior methods, e.g. wherein an RTD element is joined to two or more conductors carried in the sheath while these latter are exposed for access; in such methods the element and joints are subsequently enclosed by a metal sleeve which is then joined to the metallic sheath of the probe, and a powdered refractory is then packed between the sheath and elements therewithin. The present invention eliminates the problems of these prior practices. 
    
    
     DESCRIPTION OF THE DRAWING 
     FIG. 1 is a side elevational view of a typical RTD, approximately full size; 
     FIG. 2 is a vertical, medial, magnified, partial cross section of a probe prepared to receive the element; 
     FIG. 3 is a cross section taken on the line 3--3 of FIG. 2; 
     FIG. 4 is a cross section taken on the line 4--4 of FIG. 5; 
     FIG. 5 is a cross section similar to that of FIG. 2 but showing the completed assembly. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In a preferred form of the invention we employ a commercial RTD element indicated at 10, comprising a solid, insulating mandrel upon which a helical winding (not shown) of suitable resistance wire is wound. The winding is encased in glass fused in place or fired ceramic and the terminal ends 14 of each winding are brought out for connection to a suitable circuit. A representative probe 11 incorporating the principles of the invention will comprise a metallic sheath 16, e.g. stainless steel, within which the element 10 is supported in a manner to be described. 
     The conductors 18 (in the example, three in number) by which the terminal leads 14 of the element are connected to the circuitry (not shown) in which the element plays its part are, in accordance with common practice, supported and insulated with respect to the sheath 16 and each other, by means of a firmly compacted mass 19 of a pulverized refractory, e.g. MgO. Conductors 18 may number two or more. In acccordance with the invention the leads 18 and refractory are leveled adjacent one end of the sheath, as by counterboring, for example, to the plane X--X, to provide a chamber 21 to house the connections between the terminal leads of the RTD element and its conductors 18. It is to be noted that this chamber 21 has an interior portion of the sheath as its lateral wall 22. 
     A bore 31 is drilled on the axis of the sheath of such length and diameter as to receive the RTD element with a comfortable fit yet loose enough to allow placement of an adequate layer of cement. Prior to insertion of the element, it is coated with a ceramic cement, such as Sauereisen No. 8 or equivalent. At this point it is important not to impose strain on the element in order to avoid fracture. The cement referred to is available from Sauereisen Cement Co., Pittsburgh, Pennsylvania. 
     After insertion of the RTD element into the bore and curing of the ceramic cement, the three exemplificative conductors 18 of the probe are welded to the terminal leads of the respective RTD&#39;s. 
     In the example, a jumper (FIG. 4) is shown connected between two of the three cable wires. However, since the electrical circuitry, per se, forms no part of the invention, elaboration is deemed unnecessary. The chamber 21 is then filled with Sauereisen No. 8 cement which is then cured to integrate the electrical connections with the insulation. The cement is trimmed to a level calculated to avoid interference with a closure cap 41, in the form of a simple disc or plug which is welded to the end of the sheath 16 (FIG. 5). 
     It will be understood that the total length of the probe will be as required an that its connection to the temperature indicating or controlling means will be conventional. 
     In order to provide better understanding of the magnitude of the probe and its milieu, a typical RTD element may be essentially the shape of a medicinal capsule, having a diameter of about 3 mm and a length of about 30 mm.