Low cost thermocouple apparatus and methods for fabricating the same

A thermocouple consists of a first thin layer of metal disposed on a mylar sheet. The metal layer is then covered with a varnish by employing a printing process which prints spaced apart apertures on the metal layer indicative of a hot junction and a reference junction terminal. Another top metal layer is deposited upon the varnish layer in a line grating pattern to cover one hole in each pair to thus form a hot junction by connecting the top metal layer to the first layer via the hole. The other aperture forms one terminal of the reference junction. The metal layers are extremely thin as deposited by a vapor evaporation process and hence the thermocouple exhibits a rapid response to temperature change. The method of fabrication of the devices enables one to provide mass produced units at extremely low cost.

BACKGROUND OF INVENTION 
This invention relates to thermocouples and more particularly to an 
economical thermocouple which, based on its low cost, can be employed as a 
disposable unit. 
The prior art is replete with a host of patents and devices describing 
thermocouple devices of various configurations and constructions. 
Essentially, a thermocouple is an electric circuit consisting of a pair of 
wires of different metals jointed together at one end called the sensing 
junction and terminated at the other end in such a manner that the 
terminals are both at the same and a known temperature. This is called the 
reference junction and the known temperature is called the reference 
temperature. The terminal leads of the reference junction are connected to 
a high input inpedance amplifier and due to the thermoelectric effect 
(Seebeck effect) a voltage is created across the reference junction 
whenever the sensing junction and the reference junction are at different 
temperatures. The magnitude of the voltage is an indication of the 
temperature difference. Usually, the reference junction is held at a known 
constant temperature or is electrically compensated for variations from a 
preselected temperature. 
The prior art has employed a host of different metal wires to implement the 
thermocouple structure. The materials employed have been Chromel, Alumel, 
Constanton, copper, iron, platinum, alloys of platinum as platinum and 
rhodium, tungsten, tungsten-rhenium alloys, nickel and ferrous nickel 
alloys. 
The thermoelectrical emf which causes current flow through a circuit is 
also dependent upon the junction wire materials and the temperature 
difference between the junctions. Their characteristics are well known in 
the field. See a text entitled Electronics Engineers Handbook by D. G. 
Fink and A. A. McKenzie (1975) (1st Edition) McGraw-Hill, Inc. Section 10, 
"Transducers". 
As indicated, the use of the thermocouple to measure temperature is well 
known and the applications are many. 
In any event, a major problem exists in the measurement of human body 
temperature. Essentially, each year about 1 billion temperature 
measurements are made at hospitals in the U.S.A., and the number of 
measurements made throughout the world is of a greater magnitude. Due to 
the progress made in the field of medical electronics, there has been a 
widespread use of the electronic thermometer, especially for hospital use. 
These devices employ a temperature probe which is coupled to a suitable 
electronic circuit and display to measure body temperature. The probe is 
inserted into the mouth of a user during a measurement and a display 
indicates the temperature. 
Examples of such instruments are many and many different companies supply 
and sell such devices for hospital use. It is, of course, apparent that in 
order to employ such a thermometer for hospital use, one must provide 
probe isolation from one patient to another. In this manner a plastic 
sterilized probe cover is employed. The cover is used to enclose the probe 
for each temperature measurement and is then discarded and a new cover 
inserted for the next measurement. Even though the covers are disposable 
and made from an inexpensive plastic, their cost is significant when one 
considers the number of temperature measurements made during a prolonged 
interval. 
Based upon such considerations, if a savings in this component of only a 
fraction of a penny or more could be had, this would result in a 
substantial savings over the course of a year. 
Apart from this factor is the fact that the probe section of the 
thermometer which is constantly being covered and measured is subjected to 
extensive wear and this probe has to be replaced at frequent intervals as 
well. The probe is a relatively expensive component in such devices and 
replacement of the same necessitates a recalibration of the instrument. 
A further problem with the conventional thermometer is the excessive time 
required to take a patient's temperature. Due to the above considerations 
the plastic covered probe is of a relatively large mass and hence takes an 
appreciable amount of time to indicate the temperature. In such devices, 
one must be sure that the displayed temperature is correct and hence one 
must wait for the plastic covered probe to heat up during a temperature 
measurement. Any reduction in this time results in a great savings in time 
due to the huge number of temperature measurements made as described 
above. 
It is therefore an object of the present invention to provide a 
thermocouple device which, based on its construction, is extremely 
inexpensive and adapted to replace the conventional probe and plastic 
cover for human body temperature measurements. 
It is a further object to provide a method of manufacturing a thermocouple 
device which method enables one to provide a rugged low-cost unit 
particularly adapted for human measurements which unit can provide a rapid 
response to body temperature. 
A further object of the present invention is to provide a method of 
fabricating a thermocouple employed in measuring temperature. 
BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENT 
A thermocouple device comprising a planar plastic layer having disposed 
thereon a first thin layer of a thermocouple element metal, a layer of an 
insulator material disposed upon and covering said first layer of metal 
with said insulator layer having first and second apertures on a surface 
with said apertures positioned at opposite ends of said layer and spaced 
apart one from the other on either side of an imaginery line extending 
from one end to the other; a second thin layer of a thermocouple element 
metal disposed on top of said insulator layer and covering said first 
aperture and extending as a strip from said one end to the other, with 
said first and second metal layer contacting each other via said first 
aperture to form a hot junction and with said second aperture enabling 
contact with said first layer to form one terminal of said reference 
junction, the other terminal supplied by contact with said second metal 
layer.

DETAILED DESCRIPTION OF THE FIGURES 
Referring to FIG. 1, there is shown a top plan view of a low cost 
thermocouple 10 according to this invention. 
The thermocouple 10 has a rounded end 11 for insertion into the mouth of a 
user. 
The opposite end 12 is concave and is for insertions into a holder which 
enables the thermocouple 10 to be plugged into and removed from an 
electronic sensing circuit with a display to indicate temperature as will 
be explained. The sensing junction or hot junction 14 is in close 
proximity to the rounded end 11, while the reference junction or cold 
juntion terminals 13 and 15 are in close proximity to the concave end 12. 
As seen in FIG. 1, the upper section 16 of the thermocouple 10 consists of 
an extending strip of a first thermocouple metal, which is adjacent to a 
layer of an insulative lacquer or varnish 17. The lacquer layer 17 has a 
window 18 disposed therein which window contacts a bottom layer of metal 
19. 
The sensing junction 11, as will be explained, is formed by allowing the 
metal layer 16 to contact the metal layer 19 through an aperture in the 
lacquer layer 17 and as will be explained which is implemented during the 
construction process. 
Referring to FIG. 2, there is shown a side plan view of the thermocouple 
device 10. 
As seen from FIG. 2, the thermocouple 10 is a composite device of a 
laminated or layered construction. The device may have a bottom paper 
board layer 21 which is relatively thick and which acts as a supporting 
base plate if thickness is desired. Disposed upon the layer 21 is a thin 
plastic sheet 20 which preferably is fabricated from Mylar. Deposited on 
the plastic sheet 20 is the layer of thermocouple metal 19. Disposed on 
top of the metal layer 19 is the lacquer layer 17. The lacquer layer has 
two apertures as 13 and 22. The aperture 13 allows one to contact metal 
layer 19 to provide one terminal for the cold or reference junction, while 
the aperture 22 allows contact between the metal layer 19 and the metal 
layer 16 to form the hot or sensing junction 11. 
Referring to FIG. 3, there is shown a simple schematic of a thermocouple 
element with the reference numerals of FIGS. 1 and 2 used to designate the 
appropriate locations. 
The device 10 depicted in FIG. 1 and 2 is approximately 2 to 3 inches in 
length from end 11 to 12, with a width of about 1/4 inch. These dimensions 
can vary according to requirements and there is no intention of being 
limited by the same. Before proceeding with a description of the 
fabrication techniques, it is indicated that the metal layers 16 and 19 
are preferably elements rather than alloys. As indicated above, many 
thermocouples employ alloy composition for the elements. In this 
application element metals are employed such as tin and bismuth for the 
metal layers 16 and 19, due to the processes utilized and as will be 
further explained. 
Referring to FIG. 4A, there is shown the first step in the fabrication 
process of the thermocouple 10. A vacuum chamber 30 is shown. Such 
chambers 30 are well known and are used for potato chip bags, metallic 
wrapping paper, etc. 
The vacuum chamber contains two rotatable spools 31 and 32 having disposed 
between them a sheet of mylar 33. Essentially, spool 32 is a takeup spool 
while spool 31 is a feed spool. The mylar sheet 33 may be a few feet in 
width for large production capability and is between 0.001-0.01 inches 
thick. Mylar sheets of such size are conventional and are employed for 
many purposes. The sheet 33 is caused to move from spool 31 to spool 32. 
Disposed in the vacuum chamber 30 is a vapor deposition apparatus 34. The 
apparatus 34 evaporates or sputters a thin layer of metal 19 on the Mylar 
sheet 33. The mylar sheet 33 as is ascertained functions as the sheet or 
layer 20 shown in FIG. 2. 
The vapor evaporation apparatus 34 is also well known and one can coat 
mylar or plastic sheets with thin layers of metal by vapor evaporation or 
other processes. Essentially, the metal layer 19 may be tin or bismuth or 
some other thermocouple metal element. It is important to note that by 
utilizing vapor evaporation or vacuum sputtering, which are reliable 
processes, one cannot conveniently employ an alloy due to the fact that 
the evaporation processes will alter the alloy and change its 
characteristics. 
The vacuum chamber 30 is held at a high vacuum condition (10.sup.-5 to 
10.sup.-6 torr). The metal material to be evaporated is heated by an 
electron beam or otherwise until it vaporizes. The vaporized material 
radiates (dashed lines) where it is deposited upon the mylar film 33. The 
film may also be preheated to maintain good adhesion. This is a 
conventional technique used for coating plastic film with metals. One 
could also employ a sputtering technique which can be implemented in a low 
pressure gas atmosphere. The layer of metal 19 is about 2,000 to 5,000 
Angstroms thick and hence is an extremely thin layer which serves to coat 
the mylar sheet. See cross sectional view of FIG. 5A to the right of FIG. 
4A. 
Referring to FIG. 4B, the metal coated Mylar sheet as gathered on spool 32 
is now removed from the chamber 30 and positioned as the feed spool 36 on 
a printing press structure. The printing process is also a well known 
technque and a varnish is used to coat the metal with a varnish layer 36. 
The printing cylinder 37 contains a die surface wherein the varnish is 
coated over the metal layer 19 leaving holes as 38 and 39 in the lacquer 
layer 36, which holes are used to provide the hot junction as well as the 
terminal of the cold junction as apertures 13 and 14 of FIG. 2. The 
printing of varnish-like lacquers on any surface is well known. During the 
process depicted in FIG. 4B, the entire metal coating is printed with the 
varnish or lacquer layer 33 leaving the spaced apart apertures. Hence the 
processed sheet appears from a top view as shown in FIG. 5B. The hole 
spacing as between 38 and 39 is about 2 inches with the offset being about 
1/8 of an inch. Each aperture 38 and 39 (FIG. 5C) may be a square of 1/16 
inch or a rectangle with a 1/16 inch width and 1/8 inch or more in length. 
As seen from FIG. 5C, each aperture as 38 and 39 are printed on either side 
of an imaginery center line 70. The apertures are successively staggered 
along the length of the coated sheet as shown in FIG. 5C. 
FIG. 5D shows a representative cross section depicting apertures 38 and 39 
and showing the openings coacting with the metal layer 19. 
Referring to FIG. 4C, the varnished printed sheet is now returned to the 
vacuum chamber 30 where the second metal layer 40 is evaporated in a line 
pattern as the sheet 33 is moved from spool to spool. A grid mark 41 is 
disposed in the chamber to force the radiated metal from the evaporation 
apparatus 34 to deposit on the sheet in a line pattern as shown in FIG. 
5E. 
The lines of metal 40 cover each of the holes 39 to form a contact Junction 
or hot junction with the metal layer 19 via the aperture 39. The other 
apertures as 38 remain exposed to allow contact to the metal layer 19. The 
metal layer 40 evaporated during this step 4C would be bismuth or tin as 
opposed to the metal used for layer 19. The cross sectional view of a 
portion of the structure is depicted in FIG. 5F. The layer 40 is also 
between 2,000 to 5,000 Angstroms thick and hence is also an extremely thin 
layer. 
The resultant treated sheet after the process depicted in FIG. 4C may be 
then placed on one feed spool of a liminator 45 as shown in FIG. 4D if 
required. The treated Mylar sheet 33 is positioned on a feed spool 45, 
with the other feed spool containing the cardboard support layer as layer 
21 of FIG. 1. The treated sheet 33 is then bonded to the cardboard sheet 
48 within the laminator 45. Lamination of plastic to paper, cardboard and 
so on is also a well known technique and various inert adhesives can be 
employed as hot or cold melt adhesives which are completely non-toxic as 
those used for stamps, envelopes and so on. The cardboard sheet may be 
approximately 0.02 inches thick to act as a base or support. 
The takeup spool 48 of the laminator collects the processed sheet. The 
material from spool 48 is then fed into a die cutter apparatus 50 as 
depicted in FIG. 4E where it is cut to form a plurality of thermocouples 
as shown in FIGS. 1 and 2. 
The above described process enables one to mass produce extremely large 
quantities of thermocouples for use in measuring temperature. Due to the 
fact that the netal layers and the substrate are extremely thin, the 
device has a rapid response time to temperature. The thermocouple is 
sterilized and packaged separately and is used once for a temperature 
measurement and then discarded. The resultant structure is less expensive 
than the plastic tube covering devices of the prior art and is a complete 
thermocouple. Thus the apparatus eliminates the expensive temperature 
probe as well as the plastic covering for the probe. Due to the structure, 
the device responds rapidly to temperature because of the thickness of the 
thermocouple metal elements and hence provides a rapid readout with 
associated electronic circuitry. 
Referring to FIG. 6, the thermocouple device 10 is shown inserted into a 
socket 60 associated with an insulated housing 61. 
The thermocouple as shown in FIG. 6A has two output leads via the cold 
junction which are coupled to the input of a high impedance operational 
amplifier as 70. As can be seen from FIG. 6, there is an internal hollow 
62 within the housing. The hollow contains a metal block 63 which may be 
formed of copper. The block 63 has an aperture on the top surface into 
which is inserted a heating element 64. Underlying the heating element 64 
is a RTD temperature sensor or temperature sensitive resistor 65. The 
heating element as well as the sensor has output leads which are coupled 
through appropriate slots in the housing. 
Also shown in FIG. 6 is a contact 66. Contact 66 is on of two contacts 
which are located within the housing and are spaced apart to provide the 
required contact with the thermocouple terminals as shown in FIG. 3 as 15 
and 13. Essentially, as depicted in FIG. 6A, the output of the amplifier 
70 is directed to the input of a temperature detection circuit and 
display. 
There are a plethora of circuits as 71 for responding to a change in emf as 
desired form a thermocouple to provide an output display via an LCD 
display or other to indicate the temperature as manifested by the change 
in voltage. In regard to FIG. 6A the heater element as 64 of FIG. 6 is a 
metal resistor 72. The sensor as 65 operates to provide the variable 
resistance in response to temperature. 
As shown in FIG. 6A, a comparator 80 has one input coupled to the sensor 65 
and another input coupled to a source of reference voltage indicated as 
reference temperature. The compartor 80 may be a digital comparator and 
serves to compare the sensor temperature 65 with the reference temperature 
as determined by circuit 81. The output of the comparator controls the 
application of potential to the heater 72 which then serves to heat or 
cool the copper block as desired. In this manner, the temperature of the 
thermocoupled junction can be held very close to the desired measuring 
temperature such as the temperature of the human body. Hence the circuit 
need only respond to a small temperature differential to obtain accurate 
temperature reading by using the thermocouple of the invention. 
It is, of course, a major factor that the above described application 
discloses a simple and inexpensive thermocouple which can be mass-produced 
in large quantities and which device is simple to use and disposable to 
allow an increased savings in time and money when implementing body 
temperature measurements. 
This allows wide variations in EMF to be acceptable which allows higher 
yield and lower manufacturing costs. For example, if a 25.degree. F. 
temperature differential exists without a heater and a 25.degree. F. 
variation exists with a heater, the acceptable tolerance of variations in 
EMF is increased by a factor of ten.