Note: Descriptions are shown in the official language in which they were submitted.
1. ~177;~77
ELECTRONIC THERMOMETER
BAC~GROUND OF THE INVE~T~ON
Field of the Invention.
This invention relates to thermometers, and more
particularly to thermometers for providing accurate readings
of quasi static temperatures at a time prior to the time
that the sensor is stabilized at the temperature being
measured.
Prior Art.
The ability to measure temperatures accurately and
quickly has been desired by thermo scientists for centuries.
The most commGn type of temperature measuring device
consists of a sensing element of some sort which has a char-
acteristic which is a function of the sensor temperature,
and an indicator of some sort which is responsive to the
temperature dependent characteristic of the sensor. In
order to measure the temperature of an object using such a
thermometer, it is necessary that heat flow from the object
to the sensor until the sensor attains the temperature of
the object, at which time the indicator will show the temp-
erature of the object, to the accuracy of the measuring
system.
Since it takes time for heat to flow from an
object to a sensor, the rapidity with which such a temper-
ature can be measured is limited by the thermodynamic
characteristics of the system. The time delay occasioned
by the necessity of heat flow is called "thermal lag". It
is often the case that temperature stabilization of the
sensor takes longer than is desired, and in such cases it
is desirable that an accurate measurement be made before
stabilization.
The laws of heat transfer are such that a pre-
stabilization measurement is possible. This was recognizedprior to the turn of the twentieth century, and many ther-
mometers ~or measuring both static and dynamic temperatures
have been devised over the years which take advantage of the
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2.
known heat transfer characteristics of a particular thermal
system to make accurate measurements, even though the sensor
temperature is not ye~ at the object temperature. The prin-
ciple upon which these ther~ometers depend for their ability
to measure a temperature more rapidly than would otherwise
be possible is known as Newton's Law of Cooling. As applied
to a temperature measuring system, the procedure is called
thermal lag compensation. Sir Isaac Newton, in 1701, dis-
covered that the rate of change of temperature of an object
(such as a sensor) which is in contact with another object
is directly proportional to the difference in temperature
between the objects. Recognizing this, it is possible to
compute a correction factor from the rate of change of
sensor temperature, which, when added to the instantaneous
temperature of the sensor, allows one to determine the actual
temperature being sensed by the sensor even though the sensor
temperature has not stabilized.
An early definitive work discussing the above
principle was published in the Bulletin of the Bureau of
Standards Vol. 8, 1913, p. 659. This principle has been
applied many times in apparatus disclosed in U.S. patents
and elsewhere. Wormser, U.S. Patent No. 3,111,032, and
Georgi, U.S. Patent No. 3,702,076, for example, both dis-
close methods of thermal log compensation. A particularly
clever apparatus was described by Botshov in a Russian
Patent No. 174,398
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3.
SUMMARY OF THE INVENTIO~
While it might appear at first glance that using
the principle of Newton's Law of Cooling would allow an
almost instantaneous measurement of temperature, as a practi-
cal matter, and for a number of reasons, the goal of instan-
taneous reading cannot be attained. Depending on the accur-
acy desired, an improvement in reading time by a factor of
4 may be considered good.
Each of the above-cited prior art involves a dif-
ferent way of applying thermal lag compensation, and adifferent way of ascertaining when the reading of the ther-
mometer is the temperature being measured. However, none of
them permit making the measurement in the shortest possible
time.
Wormser and Georgi both disclose terminating the
measurement, (that is, adopting a reading of the thermo-
meter as being the temperature being measured) a fixed time
after the measurement is begun. Georgi discloses a second
embodiment wherein the measurement is terminated when the
rate of change of sensor temperature drops to some predeter-
mined value. Botshov discloses termination when the rate of
change of sensor temperature times a fixed factor is numer-
ically equal to the instantaneous sensor temperature.
In all of these cases, the time of termination of
the measurement has no fixed relationship to the accuracy of
the correction factor or to the accuracy of the measurement.
Thus, the termination time must be set conservatively so that
under all expected operating conditions the accuracy of the
measurement will be acceptable.
Termination, according to the present invention,
is related to the accuracy of the measurement, and therefore
it is possible, using the principles disclosed herein, to make
measurements in the shortest possible time, given the thermal
system and operating parameters.
While useful in other contexts, the present inven-
tion will be described in connection with clinical thermo-
meters, i.e., the measurement of hu~an ever temperature.
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4.
The sensor used in thermometers for human fever temperature
measurement is typically a thermistor on the end of a probe.
A disposable sterile cover is usually used to prevent cross
infection. Electronic circuitry including an A/D converter
and a digital display allows the temperature of the sensor,
plus any correction, to be displayed.
When the probe and cover combination is initially
inserted in the mouth the sensor starts to heat rapidly.
As time goes on the temperature of the sensor gets closer
to the mouth temperature and, in accordance with Newton's
Law of Cooling, the rate of change of sensor temperature
drops. A representative curve of sensor temperature vs.
time is shown as curve 10 in FIG. 1. It can be seen that
as time passes after insertion of the probe, curve 10
approaches the temperature being measured asymptotically
and after some relatively long time it is close enough to
be called the temperature being measured.
As referred to above f a ccrrection factor can be
added to any instantaneous value of sensor temperature along
curve 10 to obtain the value of the temperature being
measured. This correction factor is the product of a factor
of proportionality ~known as the thermal time constant)
times the rate of change of sensor temperature at that time.
Unfortunately, the thermal time constant of most thermal
systems is not precisely known and, in fact, it is not
usually a constant at all but a value that varies somewhat
from measurement to measurement, depending on the initial
conditions as well as other factors, and also as a function
of time. Consequently, a reliable reading of the temperature
being measured cannot be obtained early in the measurement
cycle. Curve 11 in FIG. 1 shows an illustrative plot of
readings vs. time of curve 10 plus a correction based on an
assumed thermal time constant of the system times the rate
of change of sensor temperature. As can be seen, an accurate
reading of temperature cannot be achieved until some time
after insertion of the probe.
As mentioned above, this problem was handled in
the past by either waiting for some fixed period of time
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5.
or by waiting for the rate of change of sensor temperature
to drop to a low enough value to assure that the correction
added to the sensor temperature is low enough that any
inaccuracy in the correction is small. Neither of these
methods results in a temperature reading in the shortest
possible time.
In accordance with the principles of the present
invention, a reading can be had at the earliest possible
time. This is accompl-ished by monitoring the corrected
sensor temperature and terminating the measurement when
the corrected temperature reading arrives at a substantially
stable value, that is, when the reading does not vary by
more than some predetermined amount during a predetermined
time interval.
As mentioned previously, the "thermal time
constant" of thermal systems is not actually constant, but
varies during the measurement cycle. This is particularly
true in clinical thermometry since the thermal system is
complicated by the presence of a cover over the sensor.
It is possible to improve the speed of making temperature
readings by assuming a varying value of thermal time con-
stant in calculating the thermal lag correction factor,
rather than a fixed value, as has been disclosed in the
prior art. In fact, it has been found that a particular
form of correction factor equation, as will be disclosed
below, fits the characteristics of the usual thermistor
and disposable sterile cover of a clinical thermometer
very well.
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6.
DESCRIPTION OF THE DRAWINGS
.
FIGURE 1 shows an illustrative curve of sensor
temperatures ~s. time and of the same curve plus a correc-
tion factor.
FIGURE 2 iS a block diagram of a presently
preferred embodiment of the present invention.
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7.
DETAILED DESCRIPTION OF THE IN~ENTION
Referring to FIGURE 2, which is a block diagram
of a thermometer using the principles of the present inven-
tion, a sensor 21 is shown coupled to an amplifier 22. In
clinical thermometry the sensor is typically a thermistor
mounted on the end of a probe and in ~se, a disposable
sterile cover is normally used to pervent cross-infection.
A typical thermistor probe and cover for use in clinical
thermometry is described in U.S. Patent No. 4,054,057,
Kluge. The thermistor sensor is usually connected into a
bridge-type circuit, the output voltage of which is a
function of the temperature of the sensor. Other types of
temperature sensors and other circuitry can be used in the
present invention, suitable sensors and circuitry being
well known in the art.
Amplifier 22 amplifies the output of the sensor
circuitry and provides a voltage for operating analog to
digital converter (A/D) 23. In a presently preferred em-
bodiment of the invention, the output of A/D 23 is in
parallel binary form. Microprocessor 24 accepts the output
of A/D 23 and performs certain mathematical operations on
the data as will now be described.
Digital information representing the sensor tem-
perature appearing at the output of A/D 23 is sampled twice
per second by the microprocessor 24 and temporarily stored
in memory 25. Sufficient memory capacity is provided to
store data representing the three immediately preceding
samples. There is therefore available to the micro-
processor computing facilities, data representing three
successive temperature measurements. These ma~ be denomi-
nated To~ Tl and T2. The most recent temperature is To
and the oldest ~two seconds older) is T2.
If the sensor in combination with the object
whose temperat~re is being measured is what is known as a
single time constant system (that is, if the time constant
is truly constant), a correction factor C may be calculated
each half second using the ~ollowing formula:
C = A(TG ~ T2)
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In this formula the factor tTo - T2) is very nearly numeri-
cally equal to the rate of change of sensor temperature at
the time of T and A is the thermal time constant of the sys-
tem.
It has been found that in a practical clinical
thermometer using a sensor and cover of the type described
in U.S. Patent No. 4,054,057 referred to above, the
thermal system is much more complicated than the single time
constant system discussed in the previous paragraph and a
more accurate value for the correction factor can be obtained
by using the following formula:
C = B log[l~A(T0-T2)]
Where A and B are constants whose values depend on the char-
acteristics of the particular sensor and cover being used.For purposes of example, the constants A and B which result
in a correction factor C which closely matches one particular
practical cover and sensor combination in wide use are A=40
and B=0.8.
After the correction factor C is calculated by the
microprocessor, whether it be by one or the other of the
formulas listed above, or by using some other formula which
fits the particular sensor/thermal system being used, the
temperature being measured, Tm is calculated tby micropro-
cessor 24 using the formula:
Tm = Tl + C
Tm is calculated each half second by microprocessor
24 and displayed on display 26. As can be seen on illustra-
tive curve 11, Tm typically increases rapidly at first and
in some cases may even overshoot the actual value of the tem-
perature being measuredO Eventually, however, as the sensor
temperature approaches the temperature being measured, and
the correction factor C becomes relatively small, Tm con-
verges on the actual temperature being measured.
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The values of Tm as calculated are compared to the
value of Tm calculated one half second previously, the pre-
vious value being stored in memory 2~. These comparisons are
made by microprocessor 24 each half second until the differ-
ences between six successive comparisons are each less thansome predetermined amount, for example 0.05F. Six succes-
sive differences of less than 0.05F. indicates that the
value of Tm has converged on the temperature being measured,
and at that point the temperature measuring cycle is termin-
ated, the display locked, and the horn 27 sounded informingthe user that the value of Tm then being displayed is an
accurate measure of the temperature being measured.
m e calculation and control functions described
above are well known operations in the microprocessor art
and are therefore not described in detail. Persons skilled
in the art of programming microprocessors will readily
mechanize the functions described without further details.
What has been described is an electronic themo- ~
meter wherein a calculation of the temperature being measured
is made repeatedly during a measurement cycle, and the calcu-
lated values compared to determine whether a series of consis-
tent values has been calculated. A series of consistent
values of calculated temperature is indicative of an accurate
temperature reading, and when such a series is found, the
measurement is terminated and the last calculated value
adopted as the measurement of the temperature being measured.
