GB2084329A

GB2084329A - Electronic Thermometer

Info

Publication number: GB2084329A
Application number: GB8125827A
Authority: GB
Original assignee: Diatek Inc
Current assignee: Diatek Inc
Priority date: 1980-09-15
Filing date: 1981-08-25
Publication date: 1982-04-07
Also published as: SE453432B; FR2490340B1; NL8104063A; CA1177277A; AU7524181A; AU544566B2; GB2084329B; BE890332A; SE8105482L; DE3135853A1; FR2490340A1

Abstract

An electronic thermometer for the measurement of quasi static temperatures includes thermal lag compensation as a function of the rate of change of temperature of thermistor sensor 21. The temperature measurement cycle is terminated when successive compensated readings converge on a consistent value, as indicated by differences between the readings falling below a predetermined value. The compensation C to be added to a temperature T, may be of the form A (T0-T2) or of the form B log [1+A(T0-T2)] where T0 and T2 are preceding and succeeding sensor temperatures and A and B are constants. <IMAGE>

Description

SPECIFICATION Electronic Thermometer 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.

The ability to measure temperatures accurately and quickly has been desired by thermo scientists for centuries. The most common type of temperature measuring device consists of a sensing element of some sort which has a characteristic 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 temperature 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 temperature 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 prestabilization measurement is possible. This was recognized prior to the turn of the twentieth century, and many thermometers for measuring both static and dynamic temperatures have been devised over the years which take advantage of the known heat transfer characteristics of a particular thermal system to make accurate measurements, even though the sensor temperature is not yet at the object temperature.

The principle upon which these thermometers 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 1 701, discovered that the rate of change of temperature of an object (such as a sensor) which is in contact with another object is directly proportionaf 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 stabiiized.

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 disclose methods of thermal log compensation. A particularly clever apparatus was described by Botshov in a Russian Patent No.

174,398.

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 practical matter, and for a number of reasons, the goal of instantaneous reading cannot be attained. Depending on the accuracy desired, an improvement in reading time by a factor of 4 may be considered good.

Each of the above-cited prior art involves a different way of applying thermal lag compensation, and a different way of ascertaining when the reading of the thermometer 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 thermometer 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 predetermined value. Botshov discloses termination when the rate of change of sensor temperature times a fixed factor is numerically 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 invention will be described in connection with clinical thermometers, i.e., the measurement of human fever temperature. 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, a correction 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 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 present invention, there is provided an electronic thermometer comprising means for altering the electrical output signal of a temperature sensor during a measurement cycle as a function of the rate of change of temperature of said sensor, the temperature to be measured being represented by said altered electrical signal; means for monitoring changes in said altered electrical signal; and means responsive to said changes for terminating said measurement cycle.

The present invention also provides, 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 constant 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.

The present invention is further described, hereinafter, by way of example with reference to the accompanying drawings, in which: Figure 1 shows an illustrative curve of sensor temperature vs. time and of the same curve plus a correction factor; and Figure 2 is a block diagram of a presently preferred embodiment of the present invention.

Referring to Figure 2, which is a block diagram of a thermometer using the principles of the present invention, 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 use, a disposable sterile cover is normally used to prevent 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 embodiment 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 temperature 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 microprocessor computing facilities, data representing three successive temperature measurements. These may be denominated T,, T, 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 temperature 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 following formula: C=A(T0-T2) In this formula the factor (T0-T2) is very nearly numerically equal to the rate of change of sensor temperature at the time of T and A is the thermal time constant of the system.

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)J Where A and B are constants whose values depend on the characteristics 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 8=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 (by microprocessor 24 using the formula: T,=T,+C Tm is calculated each half second by microprocessor 24 and displayed on display 26.

As can be seen on illustrative curve 11, Tm typically increases rapidly at first and in some cases may even overshoot the actual value of the temperature being measured. Eventually, however, as the sensor temperature approaches the temperature being measured, and the correction factor C becomes relatively small, Tm converges on the actual temperature being measured.

The values of Tm as calculated are compared to the value of Tm calculated one half second previously, the previous value being stored in memory 25. These comparisons are made by microprocessor 24 each half second until the differences between six successive comparisons are each less than some predetermined amount, for example 0.050F. Six successive differences of iess than 0.050F. indicates that the value of Tm has converged on the temperature being measured, and at that point the temperature measuring cycle is terminated, the display locked, and the horn 27 sounded informing the user that the value of Tm then being displayed is an accurate measure of the temperature being measured.

The 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 thermometer wherein a calculation of the temperature being measured is made repeatedly during a measurement cycle, and the calculated values compared to determine whether a series of consistent 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.

Claims

1. An electronic thermometer comprising means for altering the electrical output signal of a temperature sensor during a measurement cycle as a function of the rate of change of temperature of said sensor, the temperature to be measured being represented by said altered electrical signal; means for monitoring changes in said altered electrical signal; and means responsive to said changes for terminating said measurement cycle.

2. An electronic thermometer as claimed in claim 1 wherein said measurement cycle is terminated when said altered electrical signal does not change by more than a predetermined amount during a predetermined time interval.

3. An electronic thermometer as claimed in claim 1 or 2 wherein said altered electrical signal is sampled periodically and said measurement cycle is terminated when a predetermined number of successive samples do not change by more than a predetermined amount.

4. An electronic thermometer comprising means for altering the electrical output signal of a temperature sensor during a measurement cycle as a function of the rate of change of temperature of said sensor, the temperature to be measured being represented by said altered electrical signal, wherein said function is substantially Alog (1+BT') where A and B are constants and T' is the rate of change of sensor temperature.

5. An electronic thermometer constructed and arranged to operate substantially as hereinbefore described with reference to Figure 2 of the accompanying drawings.