Thin film resistance thermometer detector probe assembly

A resistance thermometer probe assembly is constructed by locating a header assembly in the end of a cylindrical sleeve. The header is constructed as a stack of discs. The disc exposed to the environment is of stainless steel and the disc supporting the thin film resistance thermometer chip is of ceramic. A copper disc is brazed between the ceramic and the stainless steel to accommodate the different thermal coefficients of expansion. Nail-head pins are brazed to a thick film deposit on the exposed face of the ceramic disc and the resistance thermometer chip is connected electrically between the pins.

BACKGROUND OF THE INVENTION 
In the construction of resistance thermometer detector probes, it is 
important to provide an assembly constructed such that the resistance 
thermometer element is in good thermal contact with the object or region 
in which the temperature measurement is desired while at the same time 
being sufficiently sturdy in construction to withstand any shocks or 
vibrations to which it may be subjected and at the same time being 
resistant to other environmental conditions such as chemical attack, etc. 
When the resistance thermometer element is a thin film element, the above 
requirements are of equal importance or of more importance than they are 
with regard to the resistance thermometer assemblies of the wound-wire 
type. 
Prior art devices have not had sufficiently short response times, 
sufficiently short immersion depths, and sufficient resistance to shock 
and vibration to make them useful in many applications where accurate 
temperature measurements are desired. It is therefore an object of this 
invention to provide a detector probe assembly for thin film resistance 
elements which is resistant to shock and vibration and which has a quick 
response to temperature change while at the same time being simple and 
easy to construct, and therefore inexpensive to manufacture. 
SUMMARY OF THE INVENTION 
The resistance thermometer probe assembly of this invention utilizes a 
cylindrical sleeve with a header mounted in the end of the sleeve. The 
header includes a first button of thermally conductive material, a second 
button of soft metal having a high thermal conductivity and a high 
diffusivity along with a third button of electrically insulating material. 
Means are provided for joining these three buttons to form a sandwich such 
that the second button is between the first and third buttons with good 
thermal contact provided therebetween. A thick film is deposited on the 
exposed surface of the third button. That film has three sections, one is 
in the center region of the surface and the other two are located in 
flanking relationship to the center region. A thin film resistance 
thermometer chip with its associated thin film resistance thermometer is 
affixed to the center region with connectors being affixed to the flanking 
regions. Means are provided for connecting the contact pads of the 
resistance thermometer element to the flanking regions so that the element 
is connected between the connectors.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
In FIG. 1 there is shown a perspective view of a header which carries the 
thin film resistance thermometer and which is designed for mounting in an 
end of a cylindrical sleeve to form a resistance thermometer detector 
probe assembly in accordance with this invention. The header 10 includes a 
first button 12 of thermally conductive material which may, for example, 
be 316 stainless steel or Inconel. A second button 14 is a soft metal 
having a high thermal conductivity and a high diffusivity. Thus, the 
button 14 may be a disc of copper which may have a thickness of 
approximately the same magnitude as the first button. A third button 16 of 
insulating material provides electrical insulation between a pair of 
connectors 18 and 20. The insulating material may, for example, be 
constructed as a disc of BeO or Al.sub.2 O.sub.3 metalized on the 
underside. 
The buttons 12, 14, and 16 are stacked to form a sandwich such that the 
button 14 is between the buttons 12 and 16. The buttons are affixed to 
each other as by means of the brazes 34 and 36 so that the header 10 is an 
integral unit which may be usually welded into the end of a cylindrical 
sleeve to form a probe assembly. 
FIG. 2 provides more detail with regard to the header of FIG. 1 in that it 
shows the elements of the header in cross section. Thus, the disc 12 is 
shown as having a recessed portion 13 at the bottom part which is normally 
that part of the header which will be exposed to the region at which the 
temperature is to be measured. A soft metal disc 14 which may be of copper 
is then brazed by means of the braze 34 to the disc 12 so that there is 
good thermal contact between the two discs. An electrical insulator in the 
form of the disc 16 is then brazed by means of the braze 36 to the top of 
the disc 14. The braze is made possible by the metalized coating on the 
bottom of the disc 16 shown as 37. 
The top of the disc 16 has three areas which have a thick film deposit, 
namely the deposits 28, 24, and 26. These conductive deposits are utilized 
to provide electrical connections between the connectors 18 and 20 which 
are shown as nail-head pins. The nail-head pins 18 and 20 are brazed to 
the areas 26 and 24 respectively by the brazes 40 and 42. The centrally 
located thick film deposit 28 is brazed by means of braze 44 to thin film 
resistance thermometer detector chip 22, which contains upon its surface 
the thin film deposit 48 which forms the resistance thermometer. In FIG. 2 
one of the contact pads of the resistance thermometer is shown connecting 
the resistance thermometer detector to the thick film area 24 by way of 
the connector strap 30. 
In a typical example of a useful resistance thermometer detector probe 
header, the disc 12 may be made of stainless steel or Inconel as 
previously mentioned, and the disc 14 can be of copper with the disc 16 
being of BeO or Al.sub.2 O.sub.3 with the metalized facing on the bottom, 
namely the facing 37 being Mo-Mn thick film corresponding with the Mo-Mn 
thick film areas 28, 24, and 26 which are deposited on the top of the disc 
16. Typically, the copper disc 14 can be brazed between the stainless 
steel disc 12 and the ceramic disc 16 at the same time as the braze of the 
nail-head pins 18 and 20 to the areas 26 and 24 is accomplished using a 
gold/copper braze melting at 1010.degree. C. The resistance thermometer 
detector chip 22, which has a metalized back side, is brazed in a 
subsequent brazing operation at a lower melting point with an alloy of 
Cu-Au or Pt-Au-Ag which melts at 800.degree. C. The platinum ribbon straps 
30 can be resistance welded between the contact pad on the resistance 
thermometer chip and the thick film 24, for example. 
With the structure as shown, the copper disc 14 provides a stress relief 
between the disc 12 and the disc 16 which may be, respectively, steel and 
ceramic. In the absence of the copper disc 14, the ceramic disc would 
fracture when cycled between the brazing temperature and the room 
temperature during the manufacturing process because of the thermal 
expansion mismatch between the discs 12 and 16. There is thus provided by 
the structure of resistance thermometer detector header 10 of FIGS. 1 and 
2 a structure for an element which can be finally welded into the end of a 
protecting cylindrical sleeve so as to form a probe assembly, the 
structure being such that it is both small and easy to manufacture, and 
therefore inexpensive. 
The header 10 can be easily mounted into a probe assembly as shown in FIG. 
3 where the header 10 is inserted into the end of a cylindrical sleeve 50 
to which it is welded by the peripheral weld 52. 
The probe body may be typically a swaged, mineral (MgO) insulated structure 
wherein the insulator 56 provides means for carrying the leads 58 and 60 
to the pins 18 and 20 to which the leads are welded. The insulator 56 
forms the core of the sheath 61 which may be of the same material as the 
sleeve 50 and to which the sleeve 50 can be welded at a peripheral weld 
62. 
It has been found that a probe with a header assembly of the type shown and 
described in connection with FIGS. 1, 2, and 3 can, with the materials 
given as an example, have a response time of less than one second compared 
with the typical response times of prior art probes in the order of 5 to 8 
seconds while having great rigidity since the structure has no loose parts 
and since the lead pins are short and are welded to heavier probe leads 
with the header welded to become an integral part of the assembly. 
It will be evident to those skilled in the art that the material used for 
the various discs which make up the header assembly, namely 12, 14, and 16 
can be of material other than that specifically mentioned above so long as 
the material of disc 14 is of a sufficiently soft material so that it can 
accommodate the differences in the thermal expansion coefficients of the 
material of discs 12 and 16. It is, of course, necessary that the 
combination of the discs 12, 14, and 16 be such that the good thermal 
conductivity is provided between the disc 12 and the resistance detector 
chip 22 to maintain the desired fast response time and it is likewise 
important that the soft metal used as disc 14 and the insulator used as 
disc 16 have a high diffusivity (ratio of thermal conductivity to specific 
heat) also for the purpose of maintaining a short response time.