Isothermal panel assembly for terminating a plurality of thermocouple leads

A panel assembly for terminating a plurality of thermocouple leads is disclosed. The panel assembly has sufficient mass that it can stabilize at the temperature of the surrounding air. All thermocouple termination posts are of a common metal such as copper so that all subsequent leads can be copper and thereby reduce termination errors due to subsequent thermoelectric effects. A temperature measuring device is embedded in the mass of the panel assembly for measuring the temperature of the termination points of the thermocouple leads. Recorder jacks are provided so that an external recorder may record the outputs of the thermocouple.

TECHNICAL FIELD 
This invention relates to temperature measurements using thermocouples 
consisting of wires of dissimilar metals joined together which produce an 
electromotive signal when heated and particularly to an apparatus for 
terminating a plurality of these thermocouples. 
BACKGROUND ART 
When two dissimilar metals are connected and that junction exposed to an 
elevated temperature, a thermal electromotive force is generated. This 
phenomenon is known as the Seebeck effect and is the basis of temperature 
measurements using thermocouples. When two similar metals are joined, no 
thermoelectric effect takes place. Theremorcouple leads are, therefore, 
made from the same material from which the thermocouples are made. 
Eventually, in the circuit, however, connections must be made to copper, 
such as at the binding post of a potentiometer or other measuring device. 
Unfortunately, these connections form two new junctions which are 
themselves thermocouples. The error produced by these new thermocouple 
junctions can be eliminated by placing the copper to thermocouple lead 
junction in a bath of melting ice. This keeps the two junctions at the 
same temperature and keeps that temperature constant. This cancels the 
thermoelectric effect of the new junction. If all additional terminations 
are copper to copper, no new thermal electromotive error is introduced. 
The electromotive force generated by the thermocouple is then measured 
with a potentiometer. The electromotive force generated by the 
thermocouple when an ice bath is used with the reference junction is 
represented by the equation: 
##EQU1## 
where E equals electromotive force generated by the thermocouple; t equals 
temperature in degrees Celsius; A, B, C are constants dependent upon the 
materials used to fabricate the thermocouples. If the thermocouple to 
copper lead junctions are at some temperature other than the melting point 
of ice, then the above equation must be corrected by adding a term 
dependent upon the temperature of the junction. 
EQU E=E.sub.c +E.sub.r 
where E equals electromotive force generated by thermocouple if an ice 
junction were used; E.sub.c equals correction factor; E.sub.r equals 
electromotive force actually measured. Once the electromotive force is 
determined, it is a simple matter to look up the temperature in a chart of 
electromotive force versus temperature which have been developed for 
various thermocouples and published in books such as the CRC Handbook of 
Chemistry and Physics. 
Electronic devices are available to replace the ice bath for reference 
junction compensation. One such device is described by Ihlenfeldt et al. 
In U.S. Pat. No. Re. 30,735. The problem encountered when using a 
plurality of thermocouples is that a separate ice bath junction or 
electronic junction compensation device is needed for each thermocouple. 
The present invention overcomes this problem by terminating all 
thermocouples at a common point to an isothermal block such that all 
terminations are at the same temperature. The temperature of the 
isothermal block which is used for termination is then measured. A method 
then can be used to simultaneously compensate all thermocouple signals as 
is disclosed in the Applicants' co-pending application, Ser. No. 398,491. 
DISCLOSURE OF THE INVENTION 
This invention is an apparatus for terminating a plurality of individual 
thermocouple leads to a common panel assembly. All termination points are 
held at the same temperature by a mass of metal. The mass of metal forms a 
faceplate which structurally holds the thermocouple termination points and 
maintains them at a common temperature. The faceplate is electrically 
insulated from the individual termination points to prevent disturbing the 
thermocouple signals. The faceplate also serves to connect the panel 
assembly to a standard equipment rack not shown. An integrated circuit 
temperature transducer is embedded in the mass of metal forming the 
faceplate to determine the temperature of the faceplate. Vertical pairs of 
holes are spaced longitudinally down the faceplate to accept jacks which 
form the thermocouple terminations. 
The thermocouple terminations are formed from jacks constructed of hollow 
copper tubes which have an electrically insulating plastic tip molded on 
one end. The plastic tip is bigger in diameter than the outside of the 
copper tube, and has a hole in the middle concentric with and the same 
diameter as the inside of the copper tube. The hole diameter is chosen 
such that a standard equipment jack may fit through the plastic tip and 
make electrical contact with the inside diameter of the copper tube. The 
other end of the copper tube, opposite the end with the plastic tip, has 
internal threads formed in the interior of the tube. This thread receives 
a copper screw which forms the binding post for the thermocouple lead. The 
exterior of the copper tube also has threads formed on it for mechanical 
attachment to other components in the assembly. 
The jacks have an electrically insulating sleeve whose length is equal to 
the thickness of the faceplate placed over the exterior of the tubular 
member of the jack. This insulating sleeve has an interior diameter larger 
than the outside diameter of the copper tubular member of the jack and an 
outside diameter smaller than the interior diameter of the hole in the 
faceplace. The jack with the insulating sleeve fits through the hole in 
the faceplate such that the plastic tip of the jack is on the exterior of 
the faceplate. An insulating washer is then placed over the exterior 
threaded portion of the copper sleeve, and a copper nut is threadably 
engaged with the copper sleeve of the jack. In this manner, the jack is 
firmly attached mechanically to the faceplate but is electrically isolated 
from the faceplate by the plastic tip on the front side, the sleeve as it 
passes through the faceplate, and the insulating washer on the back side. 
A printed circuit board is then placed over the jack assemblies which 
protrude through the faceplate. The body of the printed circuit board is 
composed of a flat sheet of non-conductive material. The printed circuit 
board has pairs of vertical holes spaced longitudinally along its length 
which correspond to the diameter and location of the holes in the 
faceplate. A conductive copper surface is imprinted on the board around 
each of these holes and leads to an adjacent position where copper male 
terminals are soldered in position. After the printed circuit board is 
placed over the jacks, a copper washer and copper nut secure the printed 
circuit board to each of the jack assemblies. The board is then 
electrically and mechanically joined to each jack assembly. The signal 
from the jack is led by the copper circuitry of the printed circuit board 
to the copper terminal. A female, non-conductive insulator block connector 
is placed over pairs of terminals corresponding to the positive and 
negative inputs of each of the thermocouples. This insulator block is held 
in place mechanically by barbs on the terminals which have been soldered 
to the printed circuit board. The insulator block and terminals provide a 
convenient and quick way for connecting the thermocouple signals to 
external devices. 
The integrated circuit temperature transducer which has been embedded in 
the metal mass of the faceplate has its leads soldered to the printed 
circuit board. Copper exit wires are then soldered to the printed circuit 
board such that the integrated circuit temperature transducer signal can 
be led to external devices. 
In operation, copper screws are threaded into the internal threads of the 
jack assemblies. A thermocouple lead, for instance the lead composed of 
platinum plus 10% rhodium, is wrapped around the body of the screw and the 
screw is turned until it makes a tight joint with the jack. The other lead 
of the thermocouple, for instance a lead composed of pure platinum, is 
similarly attached to the jack immediately below the jack above. In this 
manner, the positive signal is led from the jack across a printed circuit 
board by means of the copper circuitry to the output terminal. The other 
terminal, which is inside the insulation block, leads back to the negative 
signal of that particular thermocouple. This entire panel assembly is 
designed to fit into a standard equipment rack. An operator standing in 
front of the rack may use an external recording device by plugging the 
recorder terminals through the plastic tips of the jack and thus receiving 
the signal from the thermocouple.

BEST MODE FOR CARRYING OUT THE INVENTION 
The main purpose of this invention is to conveniently terminate a plurality 
of thermocouple leads at a common panel such that all the terminations are 
held at the same temperature. FIG. 3 shows a preferred embodiment of an 
isothermal panel assembly for accomplishing this end. 
A metallic faceplate or first layer 1, preferably aluminum, has vertical 
pairs of holes spaced longitudinally down its length as shown in FIG. 1. 
Each pair of holes corresponds to the positive and negative leads of a 
thermocouple. In the preferred embodiment, the isothermal panel assembly 
is used to terminate 18 thermocouples. There would be, therefore, 18 
pairs, or 36 individual holes spaced in faceplate 1. The faceplate 1 is 
machined to fit into a standard equipment rack (not shown) with 
appropriate holes for fastening as shown in FIGS. 1 and 2. The mass of the 
faceplate 1 is such that it will come to thermal equilibrium with the 
ambient air and hold all termination points at the same temperature as 
will be seen from the following discussion. 
The holes in the faceplate 1 receive jack assemblies 20 which consist of a 
copper tubular member 2 with a plastic tip 3 molded on one end. The 
plastic tip 3 has a flat backside designed to fit flush with faceplate 1 
and has a hole molded into the front which is concentric with and the same 
diameter as the inside of copper tubular member 2. The inside diameter of 
the copper tubular member 2 is sized to accept an equipment or recorder 
plug (not shown). The end of the copper tubular member 2 opposite the 
plastic tip 3 has external and internal threads formed thereon. 
An insulating sleeve 4 having an inside diameter equal to the outside 
diameter of the copper tubular member 2 of jack assembly 20 and an outside 
diameter equal to the inside diameter of the holes in the faceplate 1 has 
a flange molded integral with one end. The insulating sleeve 4 is inserted 
into the hole in the faceplate 1 from the backside. The tubular member 2 
of jack assembly 20 is placed through the insulating sleeve 4 from the 
front side of faceplate 1. A copper nut 5 is placed over the external 
threads of copper tubular member 2 and tightened to hold the jack assembly 
20 firmly to the faceplate 1. The jack assembly 20 is thus completely 
electrically isolated from the faceplate 1 by the plastic tip 3 of jack 
assembly 20 and the flange and body of insulating sleeve 4. 
A printed circuit board or second layer 30 is fabricated of a 
non-electrically conductive substrate 10 coated with conductive copper 
circuitry 11. Pairs of holes are spaced longitudinally through the 
non-conductive substrate 10 corresponding to the hole spacing in faceplate 
1. The diameter of the holes are slightly greater than the outside 
diameter of the copper tubular member 2 of jack assembly 20. Smaller holes 
slightly larger than the copper terminals 12 are spaced inboard toward the 
center of the substrate 20 to accept the terminals 12. The copper 
conductive elements 11 are printed through the holes and on both sides as 
substrate 10 to electrically connect the hole which will receive the jack 
assembly 20 to the hole which will accept the terminal 12. In this manner, 
any signal impressed on one jack assembly 20 will be impressed on only one 
output terminal 12. The terminal 12 is pressed into its proper hole and is 
soldered to the copper conductive element 11 of printed circuit board 30. 
A female insulator block 13 is positioned over a pair of terminals 20 and 
is held in place by barbs on the body of terminal 12 which engage a molded 
rib on the insulator block 13. The terminal 12 and insulator block 13 are 
designed to accept a male insulator block 16 holding female connector 17 
to which are soldered leads for ultimate connection to external devices. 
To complete the assembly, insulating washers 6 are placed over the tubular 
member 2 of the jack assembly 20 to act as insulators and spacers. A 
copper washer 7 is then placed over the tubular member 2. The printed 
circuit board assembly 30 is placed over the jack assemblies 20 which are 
held in place on faceplate 1. Copper washers 7 are then placed over the 
tubular members 2 of jack assembly 20 and a copper nut 5 is threadably 
engaged with the external threads on the tubular member 2. The copper 
washers 7 placed on each side of the printed circuit board assembly 30 
insure good electrical contact between the jack assembly 20 and a printed 
circuit element 11 of the printed circuit board 30. 
An integrated circuit temperature transducer 14, such as a Model AD590 
Temperature Transducer manufactured by Analog Devices, which had 
previously been imbedded in faceplate 1 is now soldered to copper pads 
printed on circuit board assembly 30. Provisions are incorporated into 
printed circuit board assembly 30 to solder external leads to these 
connections such that the signals from the temperature transducer 14 can 
be led to external devices. Copper screws 15 are threadably engaged to the 
internal threads formed in the tubular member 2 of jack assembly 20. The 
thermocouple input leads 18 are wrapped around the copper screw 15 and the 
screws are tightened into the threaded opening of tubular member 2. The 
thermocouple signal, which is carried by thermocouple lead 18, is 
electrically transmitted to jack assembly 20 by the combination of copper 
screw 15 and the copper tubular member 2 of jack assembly 20. The signal 
is transmitted from jack assembly 20 by the copper nut 5 and copper washer 
7 to the copper printed element 11 on the printed circuit board 30. 
Printed circuit element 11 transmits the output signal to the copper 
connectors 12. When mating insulator block 16 and female connector 17 are 
inserted into the insulator block 13 and connectors 12 on printed circuit 
board 30, the signal can be led to external devices. All connections, 
terminations and leads in this invention, except for the input 
thermocouple leads, are copper. No thermoelectric effect takes place 
between common metals so that the only thermolelectric effect other than 
at the thermocouple junction occurs at the termination to the isothermal 
blocks. Since temperature tranducer 14 measures this temperature, 
corrections can be made. 
The faceplate 1 is of sufficient mass that it stabilizes to the temperature 
of the surrounding air. The faceplate 1 acts as a heat sink for the jack 
assembly 20 by keeping the jack assembly at this common temperature. 
Temperature transducer 14 continuously monitors the temperature of 
faceplate 1. 
INDUSTRIAL APPLICABILITY 
This invention provides a quick and inexpensive method of terminating a 
plurality of thermocouple leads to an isothermal panel assembly. All 
thermocouple leads are terminated at the same area and, due to the mass of 
the metal in the faceplate, are kept at the same temperature which 
minimizes the error due to thermoelectric effects of the terminations.