Method and apparatus for precision temperature measurement

A bandgap voltage reference and temperature sensor is used to determine accurately the temperature of the cold junction of a thermocouple for algebraic combination with the thermocouple signal to provide precision temperature measurements with thermocouple speed. In a discrete component embodiment copper slugs function as thermal inertial elements to equalize the thermal response rates of the temperature sensor and thermocouple. In an IC embodiment the cold junction is located on the chip in close proximity to the temperature sensor component in the chip thereby avoiding any thermal differential between cold junction and sensor.

BACKGROUND OF THE INVENTION 
The present invention relates to temperature measuring devices and, more 
particularly, to precision thermocouple type thermometers where fast 
reading, low cost and clinical accuracy is desired. 
While the subject invention has broad application its advantages can best 
be appreciated by considering its application to clinical measurements. 
Clinical electronic thermometers are presently known and usually employ 
either thermistors or semiconductor thermal sensors. Although these 
components along with their peripheral electronics are relatively 
inexpensive, both types of sensors have disadvantages which limit their 
use for rapid measurement with high accuracy over a wide temperature 
range. The performance of both the thermistor and semiconductor thermal 
sensor is limited to relatively slow direct or indirect measurements, 
these components having thermal rise times as long as 30 to 60 seconds as 
a consequence of their significant mass and nonhomogeneous construction. 
The voltage response with temperature of a thermistor is linear over only 
a very narrow range. Therefore, accuracy of thermistor thermometers for 
clinical use has been limited to narrow ranges, typically 96.degree. F. to 
109.degree. F. If it is desired to expand the range of such devices, it 
becomes necessary to provide range switching with each range separately 
calibrated because the rate of response in different temperature ranges 
varies markedly. To avoid this problem it is necessary to use external 
resistor networks to linearize the action of the thermistor. Thus, it is 
difficult to use this type of construction with integrated circuits. 
Both thermistor and semiconductor thermal sensors when used for clinical 
measurements are not amenable to use in the form of expendable probes both 
because of their relative expense and the significant differences in 
response from probe to probe. If the probe were to be changed from patient 
to patient, the devices would have to be recalibrated each time. The known 
thermometers of this type, therefore, use permanent probes with disposable 
envelopes or covers for the probes which covers increase the thermal mass 
and measurement time even more. 
It is well known that thermocouples generate voltage as a function of the 
metals or alloys incorporated therein, are consistent from thermocouple to 
thermocouple and are readily interchangeable. Sensitive thermocouples can 
be produced inexpensively and have very fast thermal response times, 
usually less than 1 millisecond. Nevertheless, the use of thermocouples in 
thermometers, particularly clinical thermometers, has been limited as a 
consequence of costly peripheral electronics and the need for a precise 
cold junction compensator. For example, one method which has been used is 
to maintain the cold junction at a fixed known temperature by placing the 
cold junction within an ice bath so that the temperature is known to be 
32.degree. F. However, this method is too cumbersome for practical use. In 
the alternative, expensive and complicated measuring or temperature 
stabilizing devices must be used where any finer degree of accuracy is 
required. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a very fast inexpensive 
temperature measuring device employing a thermocouple as the temperature 
sensor. It is also an object of the present invention to provide a 
thermocouple type sensing device which can be used with integrated 
circuits. It is a further object to provide an accurate clinical 
thermometer capable of reading temperatures with an accuracy of 
.+-.0.2.degree. F., capable of operating over a relatively broad range of 
temperatures and also adaptable for interchangeable, disposable probes 
requiring no calibration from probe to probe. 
In accordance with one aspect of the invention there is provided a 
temperature measuring device for providing a signal representative of a 
temperature to be measured, comprising in combination a thermocouple for 
sensing the temperature to be measured, said thermocouple having a hot and 
a cold junction for providing a first signal as a function of the 
temperature difference between said hot and cold junctions, means 
responsive to temperature arranged in a common structure with said cold 
junction to provide a second signal as a function of the actual 
temperature of said cold junction, and means coupled responsively to said 
first mentioned means and thermocouple for providing a third signal 
representative of the algebraic addition of said first and second signals, 
said third signal being representative of said temperature to be measured. 
In accordance with another aspect of the invention there is provided a 
method for providing precision temperature measurements which comprises 
the steps of sensing the temperature to be measured with the hot junction 
of a thermocouple, determining the temperature of the cold junction of the 
thermocouple with a temperature sensor, equalizing the thermal response 
rates of said thermocouple cold junction and said temperature sensor, and 
combining the measurements from said thermocouple and said sensor to 
provide an indication of the temperature sensed by said hot junction.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIGS. 1 and 2 there is shown a disposable thermocouple probe 
designated generally by the reference numeral 10 resembling a straw and 
having a thermocouple hot junction 11 exposed at one end thereof. As best 
seen in FIG. 2, the probe consists of two concentric snug fitting plastic 
tubes 12 and 13 with the wires 14 and 15 from the thermocouple junction 11 
disposed in the interface therebetween, one on each diametral side of the 
tubular structure. For example, one of the wires such as the wire 14 may 
be of chromel while the other wire, 15, may be of constantan. The wires 
may be of the order of 3 mils in diameter and exit from the structure 10 
at the opposite end, 16, with the wire 14 being bent radially outwardly 
and folded back over the outside of the structure 10 at 17. The wire 15 
also exits at the end 16 and is folded inwardly against the inner surface 
of the structure 10 at 18. Connection to the thermocouple can be effected 
by inserting the end 16 of the probe 10 between the inner and outer 
contacts of a concentric connector, the details of which will be described 
after a consideration of the electronic measuring circuit. 
FIG. 3 shows the thermocouple hot junction 11 and its interconnecting leads 
14 and 15 joined at the broken line 19 by a connector (not shown) to 
respective chromel and constantan extension leads 20 and 21. The chromel 
extension lead 20 is connected by a solder joint at 22 to a body or slug 
of copper 23. Similarly, the constantan extension lead 21 is connected at 
a soldered junction 24 to a body or slug of copper 25. The remaining 
wiring throughout the embodiment now being described is preferably of 
copper to avoid the introduction of unwanted contact potential. Thus, a 
copper lead 26 interconnects the copper body 25 with the direct or 
positive input 27 of a chopper stabilized operational amplifier 28, 
sometimes referred to as a commutating auto zero amplifier and here 
designated by the acronym CAZ. The indirect input 29 of the CAZ amplifier 
28 is connected through a resistor 30 and a copper lead 31 to the copper 
body 23. The junction between resistor 30 and lead 31 at 32 is connected 
over a lead 33 to the output junction 34 of a voltage reference bandgap 
device 35 which is an extremely stable voltage reference source. The 
adjust input to the bandgap device 35 is connected to the adjustable 
contact 36 of the potentiometer 37 which, in turn, is connected between 
output junction 34 and ground. Voltage to the junction 34 is supplied from 
a voltage regulator 38 powered from a 9 volt battery and supplied through 
a load resistor 39 from a voltage output point 40. 
The connection points 22 and 24 between the chromel and constantan wires on 
the one hand and the copper bodies 23 and 25 on the other hand extend into 
the copper bodies and constitute the cold junction for the thermocouple. 
This will be described in greater detail below. In order to measure the 
temperature of the cold junction there is provided another bandgap device 
41 intimately associated with the bodies 23 and 25 within a central 
cavity. As shown, the bandgap temperature sensor 41 is connected from 
ground through a load resistor 42 to the output 40 of the voltage 
regulator 38. The junction 43 between the resistor 42 and device 41 is 
connected through the resistive element 44 of a potentiometer to ground. 
The adjustable contact 45 on the potentiometer 44 is connected to the 
adjust input for the bandgap device 41. The junction 43 is also connected 
through a resistor 46 in series with a resistor 47 to ground. The junction 
48 between the resistors 46 and 47 is connected to the direct input 49 of 
an operational amplifier 50. 
The CAZ amplifier 28 has an output 51 connected through an adjustable 
feedback resistor network 52 to its indirect input 29. The output 51 is 
also connected through a resistor 53 to the indirect input 54 of the 
amplifier 50. The amplifier 50, in turn, has an output 55 connected 
through a resistor 56 back to its own indirect input 54. Adjustable 
resistor network 52 is used to adjust the gain of CAZ amplifier 28. 
The output 55 from amplifier 50 is also connected over a lead 57 to the 
variable input 58 of an analog-to-digital converter 59. The converter 59 
is provided with a reference voltage at its input terminal 60 from the 
regulated output voltage point 34. In addition, the analog-to-digital 
converter 59 has an output array 61 connected to a display device 62. 
A satisfactory embodiment of FIG. 3 can be constructed in which the CAZ 
amplifier 28 is an Intersil type ICL 7650 chopper stablized operational 
amplifier while the amplifier 50 is a National Semiconductor Corporation 
type LM 358. Bandgap voltage regulator 35 may be a National Semiconductor 
Corporation type LM 336 that can be adjusted to provide a closely 
controlled output voltage of 2.490 volts; i.e., twice the 1.245 volt 
energy bandgap of its silicon semiconductor components. The particular 
bandgap device used for the temperature sensor 41 can be a type LM 335 
precision temperature sensor, also produced by National Semiconductor 
Corporation. The bandgap devices 35 and 41 will operate efficiently so 
long as they are provided with a reasonably stable supply voltage. In 
addition, one may use as the analog-to-digital converter 59, a National 
Semiconductor Corporation type NSB 3701 which requires a relatively stable 
voltage of between 4.6 and 5.8 volts. Therefore, the voltage regulator 38 
preferably should be designed to provide an output voltage at 40 that is 
maintained within the range of 4.6 to 5.8 volts for an input range of 5.5 
to 9 volts. 
Thermocouples produce an output voltage at a very low voltage-to-degree 
response rate that is a function of the difference in temperature between 
their hot and cold junctions. However, semiconductor devices, in 
particular the subject bandgap devices, generally operate at much higher 
voltage-to-degree response rates and provide output signals that vary as a 
function of temperature above absolute zero. In effect, therefore, 
thermocouple and bandgap devices produce signals related to different 
temperature reference points and at different voltage-to-degree response 
rates. Since the thermocouple only provides an indication of the 
difference in temperature between its hot and cold junctions it is 
necessary to know the temperature of the cold junction very accurately in 
order to determine the temperature of the hot junction. If a signal can be 
provided that represents the temperature of the cold junction and if such 
signal changes with temperature at the same rate of change as the signal 
produced by the thermocouple, it is possible to add the two signals and 
produce a third signal representative of the temperature to be measured. 
However, if the signals from the cold junction sensor and from the 
thermocouple change with temperature at different rates as observed above, 
it is necessary to modify at least one of the signals to have them both 
changing, with temperature, at the same rate. 
Assuming one wishes to indicate temperature based on a selected scale, it 
would be desirable to have a selected stable voltage level representing a 
given temperature reference point. For this, the stable voltage produced 
by bandgap device 35 can be used. Therefore, if its output voltage is 
adjusted to 2.490 V. and indications are desired in Fahrenheit degrees, 
for example, that voltage level can correspond to 0.degree. F. On the 
absolute scale this means that the 2.490 volt level corresponds to 459 
Fahrenheit degrees above absolute zero and represents 5.4248 mV./.degree. 
F. However, the response of the thermocouple based upon a 
chromel/constantan couple is approximately 34.3 .mu.V./.degree. F. Thus, 
for system compatability, it will be necessary to modify or scale up the 
output voltage of the thermocouple response to the same voltage-to-degree 
relationship, that is, to 5.4248 mV/.degree. F. 
In operation, as shown in FIG. 3, a first voltage or signal is generated by 
the thermocouple which is directly proportional to the difference in 
temperature between the thermocouple hot junction 11 and the thermocouple 
cold junction 22 and 24. In this case, using a constantan/chromel 
thermocouple, the voltage generated equals 34.3 .mu.V./.degree. F. For 
example, if the temperature at the hot junction 11 is 98.6.degree. F. and 
if the temperature at the cold junction is 70.degree. F., the voltage 
generated will be 980.98 .mu.V=0.00098098 V. A second voltage generated by 
bandgap device 41 at terminal 43 is directly proportional to the absolute 
temperature of the thermocouple cold junction. This temperature may also 
be referred to as the ambient temperature, and in this case the voltage 
generated by sensor 41 equals 5.4248 mV. per Fahrenheit degree above 
absolute zero. For an ambient temperature of 70.degree. F., the voltage 
generated between terminal 43 and ground would be 2.8697 volts, i.e., 
(459.degree.+70.degree.)5.4248.times.10.sup.-3. The voltage divider 
network consisting of resistors 46 and 47 along with potentiometer 44 
provides a voltage at junction 48 that is one half of that appearing at 
terminal 43. This is to compensate for the difference in gain of amplifier 
50 for signals furnished to its direct input 49 and indirect input 54. 
The thermocouple voltage is fed between the direct input to CAZ amplifier 
28 and terminal 32. However, terminal 32 is maintained at a fixed offset 
reference voltage by bandgap device 35. In the example shown, the 
reference voltage is adjusted by adjustable arm 36 of potentiometer 37 so 
that it is maintained very accurately at 2.490 volts. As previously 
stated, this voltage establishes the level for 0.degree. F. based on a 
voltage-to-temperature scale of 5.4248 mV./.degree. F. from absolute zero. 
In order to scale up the thermocouple voltage so that its rate of voltage 
change per .degree. F. matches that of the bandgap device 41, it is 
necessary for the CAZ amplifier to have a gain of 5.4248 mV. divided by 
34.3 .mu.V., i.e., a gain of 158.157. This gain is adjusted and settable 
by the resistance network 52. With the CAZ amplifier 28 connected as shown 
its output voltage at output 51 will be equal to the offset or reference 
voltage at 32 minus the CAZ amplifier gain multiplied by the thermocouple 
voltage. For the selected example the voltage at 51 will be 
[2.490-158.157(0.00098098)]=2.33485 V. 
The amplified or scaled up thermocouple voltage combined with the offset 
voltage is then fed through resistor 53 to the indirect input 54 of 
operational amplifier 50. Amplifier 50 is connected to provide an output 
signal corresponding to the algebraic addition of the voltage signals 
appearing at 49 and 54. The output 55 of amplifier 50 for unity gain would 
then be a voltage which is proportional to the temperature being measured 
at the thermocouple hot junction in .degree. F. which voltage would vary 
at the rate of 5.4248 mV./.degree. F., the signal from terminal 43 
corresponding to the absolute scale having been corrected to Fahrenheit by 
subtracting the offset voltage, i.e., 2.490 V.=459.degree.. The output 
voltage at 55 in this form could be used for any desired purpose as a 
signal representative of the hot junction temperature. 
If is is desired to display the temperature in digital form, for example, 
the signal at terminal 55 is fed into the input 58 of an analog-to-digital 
converter 59. The reference voltage, which corresponds to 0.degree. F. 
(here 2.490 volts), is fed into terminal 60 of the analog-to-digital 
converter 59. Since the voltage at 58 corresponds to the temperature at 
the hot junction, the display 62, in conventional manner, can be made to 
read the precise digital temperature at the hot junction. This display 
will respond extremely fast and will be very accurate. With the equipment 
shown, temperatures can be read within 3 seconds and will be accurate to 
.+-.0.2.degree. F. over a range of 80.degree. F. to 140.degree. F. In 
addition, the device shown in FIG. 3 is very compact, lightweight and easy 
to carry. It has the added advantage of being usable over a wide range of 
temperatures and is relatively simple and inexpensive. 
As previously mentioned, it is preferred at present to use an NSB 3701 
analog-to-digital converter. This device has a count capacity of 4000 and 
if it is desired to display temperature readings to the nearest tenth it 
provides a capacity of zero to 400 degrees, providing an output display of 
400.0 when the voltage at its input terminal 58 equals the voltage at its 
reference terminal 60, i.e., 2.490 V. But 2.490 V. has been equated in the 
system to a range of 459.degree. rather than 400.degree.. To compensate 
for this it is necessary to increase the gain of operational amplifier 50 
from unity to the ratio of 459 to 400 or to 1.1475. 
For the selected example of a hot junction temperature of 98.6.degree. F. 
and a cold junction temperature of 70.degree. F. the voltage at point 43 
will be equal to 5.4248 mV. (70.degree.+459.degree.)=2.86972 V. The 
voltage at terminal 51 was previously noted as being 2.33485 V. Therefore, 
the voltage at terminal 55 will be (2.86972-2.33485)1.1475=0.6137633 V. 
Applying this signal through input 58 to the A/D device 59 yields an 
output equal to 0.6137633 divided by 2.490 multiplied by 400 which equals 
98.597.degree. F., i.e., 98.6.degree. F., the desired temperature reading. 
Throughout the above discussion it has been assumed that the temperature 
sensor 41 is at the same temperature as the thermocouple cold junction 
such that the sensor output is truly indicative of the cold junction 
temperature. FIGS. 4 and 5 show a detailed description of the bandgap 
device 41 and the copper bodies 23 and 25. The purpose of this structure 
is to assure that the bandgap device 41 produces a signal voltage that is 
truly representative of the temperature of the cold junction. This is 
accomplished by locating the bandgap device and the cold junction of the 
thermocouple within a common structure that serves to equalize the thermal 
response of the cold junction and the bandgap device to temperature 
changes. Thus, copper body 23 may be in the form of a slug having a 
central aperture 65. It is stacked above another copper slug 25 having a 
central aperture 68 and is separated from slug 25 by a thin layer of a 
quantity of thermally conductive but electrically insulating material 66 
such as silicon grease. The layer 66 of silicon grease extends from the ID 
of the respective slugs 23 and 25 radially outwardly for a short distance 
to a point where it is surrounded and contained by an encircling body 67 
of an epoxy potting compound. The bandgap temperature sensor device 41 is 
disposed with a snug fit within the cavity formed by apertures 65 and 68 
in slugs 23 and 25 at the direct center of the composite assembly. A 
quantity of heat transmitting silicon grease 69 is disposed about the 
device 41 and contained by a sealing quantity of epoxy potting material 70 
at the top and 71 at the bottom. In a working embodiment, the bodies 23 
and 25 are each about 0.250" high with an outside diameter of about 0.5" 
and with a separation between bodies 23 and 25 of about 1 to 2 mils. The 
apertures 65 and 68 have a diameter of about 0.2". 
As seen in FIG. 5, the copper leads 26 and 31 have their respective ends 72 
and 172 secured by solder 73 and 74 within respective bores 75 and 76 
passing radially part way through the respective slugs 25 and 23. The 
chromel and constantan wires 20 and 21 are connected, respectively, to the 
bodies 23 and 25 in a slightly different manner. The ends of the wires 20 
and 21 at 77 and 78 have been stripped of insulation and inserted in the 
reduced diameter bores 79 and 80 so as to be in contact with both the 
bandgap device 41 and the respective copper bodies 23 and 25. The ends of 
the wires are held in place by solder 22 and 24, as shown. The insulation 
on the respective wires enters the enlarged diameter bore sections 81 and 
82 with and is encased in solder as shown. Thus, the cold junction of the 
thermocouple is actually embedded within the copper bodies 23 and 25 in 
direct physical and thermal contact with the envelope of the bandgap 
temperature sensor 41. By proper dimensioning of the bodies 23 and 25 they 
function as thermionic inertia elements to equalize the thermal response 
rates of the cold junction of the thermocouple and of the bandgap 
temperature sensor 41. 
Also as shown in FIG. 5 the chromel and constantan wires 20 and 21 connect, 
respectively, to the outer contact or shell 83 and to the coaxial inner 
contact 84 of a connector 85 for receiving the end 16 of the thermocouple 
probe 10 illustrated in FIGS. 1 and 2. It should be understood that the 
probe 10 telescopes over the contact 84 within the cylindrical outer 
contact 83 such that the thermocouple wire ends 17 and 18 make electrical 
contact with, respectively, contacts 83 and 84. The outer contact element 
83 should be fabricated of chromel while the inner contact 84 should be of 
constantan so as to avoid contact potentials that would introduce an 
unwanted cold junction into the circuit. While not shown it is preferred 
to provide connector 85 with an insulating handle and to enclose wires 20 
and 21 within a coiled elastically extendable sheath. 
It is important to observe that the thermal inertia introduced by the 
copper bodies 23 and 25 insures that the temperature measured by the 
bandgap temperature sensor 41 is the same as that affecting the cold 
junction of the thermocouple. By using a commom voltage reference for the 
operational amplifiers and the analog-to-digital converter, any 
fluctuation in voltage as a result of temperature changes will be 
automatically compensated. However, the bandgap device 35 is extremely 
constant in its operation and substantially independent of temperature. 
Bandgap device 41 is similar to bandgap device 35 but with slightly 
different biasing it provides an accurate output voltage which is a 
function of the ambient temperature. With the circuit shown, temperature 
indications are obtained with reasonable accuracy from 0.degree. F. to 
400.degree. F. and with an accuracy within .+-.0.2.degree. F. over a range 
from 80.degree. F. to 140.degree. F. 
The potentiometer 44 is used to adjust the output at junction 43 to the 
desired correspondence to ambient temperature. In the example just 
described it would be adjusted so that the output at junction 43 varies at 
the rate of 5.4248 mV./.degree. F. 
Another way of viewing the operation of the circuit of FIG. 3 is that the 
thermocouple produces a first signal which is a function of the difference 
between the temperatures at the hot and cold junctions. Bandgap 
temperature sensor 41 provides a second signal as a function of the 
temperature of the thermocouple cold junction. The CAZ amplifier 28 
modifies the first signal, provided thereto by the leads 26 and 31 from 
the thermocouple, to change its voltage-to-degree relationship and 
combines therewith an offset voltage. The operational amplifier 50 is 
coupled to the bandgap device 41 and the CAZ amplifier 28 to provide a 
third signal representative of the algebraic addition of the first and 
second signals as modified. Finally, the analog-to-digital converter, 59, 
and display, 62, provide an output indicative of the temperature to be 
measured. 
It is of particular significance that the principles underlying the circuit 
of FIG. 3 can be adapted to the production of an integrated circuit 
version. Referring to FIG. 6, the essential details are shown somewhat 
schematically. One corner of an IC chip 90 is shown provided with two gold 
lead connections 91 and 92 from conventional connecting pins 93 and 94. 
The chromel and constantan leads 20 and 21 from the probe connector 85 
shown in FIG. 5 are connected as shown in FIG. 6 to the pins 93 and 94, 
respectively. Since gold when connected to either chromel or constantan 
produces junctions of relatively low contact potential the principle cold 
junction for the thermocouple assembly will be located where the gold 
leads 91 and 92 make contact with the aluminum bonding points 95 and 96 on 
the chip proper. If the bandgap device, since it is semiconductor based, 
is formed directly in the chip in close proximity to the entry points 95 
and 96 such as shown by the phantom lines 97 there will be essentially no 
thermal gradient between the temperature at junctions 95 and 96 and at the 
location of the bandgap circuit component 97. Thus, the component 97, if 
connected in a circuit similar to that shown in FIG. 3, will provide a 
signal representative of the temperature of the cold junction of the 
thermocouple. Here, rather than using a relatively large mass as shown in 
FIGS. 4 and 5, which tends to slow down temperature changes at the cold 
junction so that the bandgap sensor will be at the same temperature as the 
cold junction, the same desired result is accomplished by using a 
structure of relatively little mass and by having the cold junction and 
semiconductor sensor so close together that, for all practical purposes, 
there can be no significant temperature gradient between the cold junction 
and the bandgap sensor. 
The difference between a bandgap temperature sensor and a bandgap voltage 
reference resides in the manner in which the connections are made thereto. 
Consequently, it is also possible to combine the separate discrete 
assemblies shown in FIG. 3 into a unitary assembly with different outputs 
depending upon whether a temperature responsive sensor or voltage 
reference is desired. This is contemplated in the integrated circuit shown 
in FIG. 6. It may be assumed that in all other respects the circuit 
incorporated in the integrated circuit of FIG. 6 is the same as that shown 
in discrete form in FIG. 3. However, IC technology permits all of the 
structure incorporated in the chip, i.e., bonding points 95 and 96 and 
associated circuitry including bandgap device 97, to occupy an area 
approximately 1/16" in diameter. Consequently, it can be assumed that 
there is no temperature difference between any of the parts. Therefore, an 
important advantage of the integrated circuit embodiment is that it is not 
necessary to provide thermal masses such as the copper bodies 23 and 25 
used in FIG. 3. 
Where bandgap devices are mentioned throughout the specification it is to 
be understood that in order to obtain a reference voltage of 2.490 V. it 
is presently preferred to employ a device selected from the silicon family 
of semiconductors. Other semiconductor materials can be used to obtain 
different voltage levels and different temperature sensor characteristics 
if desired. 
Little mention has been made of the display. Nevertheless, it should be 
understood that the temperature indicative signal furnished to the 
analog-to-digital converter can be used in various ways. The display can 
be interrupted and fixed after any desired time interval or it can be 
fixed after the rate of change of the temperature indicating signal has 
decreased below a preset level. Since the present apparatus is capable of 
producing a valid reading within about 3 seconds, a 5 second reading 
interval can be used with an adequate margin for reliability. 
Alternatively, the display can be permitted to operate continually with 
the operator noting the indication whenever it holds a reading long enough 
to permit reading. 
Although the specific example described above provides temperature readings 
in terms of the Fahrenheit scale, it should be apparent that the system 
can be adapted to provide readings based on the Centigrade scale or in 
terms of any other desired scale. The various amplifier and digital 
components of the circuit of FIG. 3 can be altered as will appear to those 
skilled in the art. 
Having described the presently preferred embodiments of the subject 
invention it should be understood that various changes in details of 
construction can be incorporated without departing from the true spirit of 
the invention as defined in the appended claims.