Abstract:
The thermometer using differential temperature measurements utilizes a pair of adjacent temperature sensors in order to measure the temperature of a common surface over a pre-selected period of time. The thermometer includes a housing and first and second thermistors mounted adjacent one another on the housing. The first and second thermistors are positioned against the surface, which can be a body part (for oral, rectal or axial body temperature measurements) or can be any other desired surface for which a spot check temperature reading is desired. A programmable current source pre-heats the second thermistor to a pre-selected temperature, while the first thermistor is initially at room temperature. A controller inside the housing causes both the first and second thermistors to take instantaneous temperature measurements of the surface at two successive times. The controller linearizes the measurements to predict the temperature of the surface, which is then displayed to the user.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. FIELD OF THE INVENTION 
         [0002]    The present invention relates to thermometry, and particularly to a thermometer using differential temperature measurements from a pair of adjacent temperature sensors. 
         [0003]    2. DESCRIPTION OF THE RELATED ART 
         [0004]    In a conventional electronic or digital thermometer, a single temperature probe, often a thermistor, is utilized. In such a conventional thermistor probe, parallel resistors are used in the thermistor circuit to improve the linearity over the desired temperature range. Although this approach improves linearity, allowing for relatively quick temperature measurements, the sensitivity of the thermometer is dramatically compromised, leading to an inability of the circuitry to measure small temperature variations. However, for both medical purposes and also when dealing with certain chemical reactions, for example, quick temperature readings with a high degree of accuracy and sensitivity may be necessary. 
         [0005]    Thus, a thermometer using differential temperature measurements addressing the aforementioned problems is desired. 
       SUMMARY OF THE INVENTION 
       [0006]    The thermometer using differential temperature measurements utilizes a pair of adjacent temperature sensors, initially at different temperatures, in order to measure the temperature of a common surface over a pre-selected period of time. The thermometer includes a housing and first and second thermistors mounted adjacent one another on the housing. The first and second thermistors are adapted for positioning against the surface, which can be a body part (for oral, rectal or axial body temperature measurements) or can be any other desired surface for which a spot check temperature reading is desired. A programmable current source, positioned inside the housing, is provided for pre-heating the second thermistor to a pre-selected temperature, while the first thermistor is initially at room or ambient temperature. 
         [0007]    A controller is also positioned inside the housing. The controller first measures a temperature b C  with the first thermistor at a first time t 0  and a temperature b H  with the second thermistor, also at the first time t 0 . After a pre-selected time period, the controller then measures a temperature b C1  with the first thermistor at a second time t 1 , and a temperature b H1  with the second thermistor, also at the second time t 1 . Over a relatively short time duration between measurements, the temperature curves are approximately linear, thus the controller can be used to calculate a slope a C  of the linearized temperature measurement curve associated with the first thermistor as 
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         [0000]    and a slope a H  of the linearized temperature measurement curve associated with the second thermistor as 
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         [0000]    These slopes can then be used to calculate a temperature BT of the surface as 
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         [0000]    This measured temperature can then be displayed to the user on a suitable display in communication with the controller, such as a liquid crystal display or the like. 
         [0008]    These and other features of the present invention will become readily apparent upon further review of the following specification and drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is a perspective view of a thermometer using differential temperature measurements according to the present invention. 
           [0010]      FIG. 2  is a schematic diagram showing circuitry of the thermometer using differential temperature measurements according to the present invention. 
           [0011]      FIG. 3  is a schematic diagram illustrating an alternative biasing circuit for the thermometer using differential temperature measurements. 
           [0012]      FIG. 4  is a graph showing temperature measurements as a function of time for a heated thermistor and an unheated thermistor of the thermometer using differential temperature measurements according to the present invention. 
           [0013]      FIG. 5  is a graph showing response as a function of temperature for the heated thermistor and the unheated thermistor of the thermometer using differential temperature measurements. 
       
    
    
       [0014]    Unless otherwise indicated, similar reference characters denote corresponding features consistently throughout the attached drawings. 
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0015]    Referring now to  FIG. 1 , there is shown a thermometer  10  using differential temperature measurements which, as will be described in greater detail below, makes use of a pair of temperature sensors, such as thermistors  12 ,  14 . Housing  16  can be formed from any suitable material having a relatively high thermal conductivity, such as stainless steel or the like. As shown in  FIG. 1 , housing  16  can have an overall contouring similar to that of a conventional tongue depressor, though desirably thinner, allowing the thermometer  10  to be comfortably used in a patient&#39;s mouth, rectum or armpit. Exemplary dimensions for the exemplary housing  16  of  FIG. 1  are a length of approximately 7 cm, a width of approximately 5 mm, and a thickness of approximately 2 mm. It should be understood that the overall contouring and relative dimensions of housing  16  can be varied, dependent upon the particular intended function of thermometer  10 . Although the interior of housing  16  can be filled with thermally conductive materials, at a sensor end or sensor area  15  of the housing  16  the temperature sensors, such as the thermistors  12 ,  14 , are positioned in an adjacent, spaced apart relation to each other and the space or gap between the temperature sensors, such as the thermistors  12 ,  14 , is desirably empty or filled with a thermally insulating material to prevent or substantially prevent heat transfer between the temperature sensors, such as between the thermistors  12 ,  14 . 
         [0016]    Each thermistor  12 ,  14  is desirably of the negative temperature coefficient (NTC) type, although a positive temperature coefficient (PTC) type thermistor can be also used for the thermistors with some modification, for example, in the thermometer  10 . As shown in  FIG. 2 , the thermistor  12  is energized by a power source, such as a constant current source  18 , whereas thermistor  14  is energized by a power source, such as a programmable current source  20 , so as to pre-heat thermistor  14  to a temperature T p . As shown, each of thermistors  12 ,  14  is buffered by a respective one of amplifiers  22 ,  24  before feeding measurement signals, via analog-to-digital channels ADC 1  and ADC 2 , respectively, of a controller  30 . The controller  30  is in communication with the components described herein, of the thermometer  10  to control, send or receive information or data in relation to temperature measurement by the thermometer  10 . Also, a power source, such as can include a battery, as can be in conjunction with the constant current source  18  and the programmable current source  20 , is typically provided to power operation of the thermometer  10  for temperature measurement, for example. 
         [0017]    Controller  30  can be or be included in any suitable type of computer implemented device, such as a microprocessor, programmable controller, programmable logic controller (PLC), microcontroller, system on chip (SOC) processor, application specific integrated circuit (ASIC), or the like, for example. Calculations and implementation of temperature measurement, such as implementing a program or programs to carry out the steps or methods for temperature measurement by the thermometer  10 , are performed or controlled by the controller  30 . A program or programs or instructions to carry out the steps or methods for measuring temperature using the thermometer  10  can be stored in a memory  31 , which can be any suitable type of computer readable and programmable memory. Memory  31  is desirably a non-transitory, computer readable storage medium, such as a semiconductor memory (for example, RAM, ROM, etc.), and the memory  31  can be separate from or integrated with the controller  30 . Data or instructions can be entered into the thermometer  10  via a suitable type of interface  32 , and such data or instructions can be stored in the memory  31 . 
         [0018]    Switch S 1  of controller  30  can be any suitable type of manual switch, microswitch or the like, which desirably actuates the thermometer  10  when the thermometer  10  is removed from a handset, casing or the like. Upon actuation of switch S 1 , controller  30  causes the programmable current source  20  to deliver a current of approximately 50 mA to pre-heat thermistor  14  for a relatively short period of time (on the order of 500 ms). This current is used to raise the temperature T p  of thermistor  14  to a pre-heated, pre-selected temperature up to approximately 42° C. The warming process is monitored by continuous measurement of the voltage drop on pre-heated thermistor  14  (i.e., a function of the resistance of thermistor  14  which is, in turn, a function of the temperature T p ). 
         [0019]    In use, the approximately 500 ms warming period is desirably completed before introducing the thermometer  10  into the patient&#39;s mouth, rectum or armpit. As shown in  FIG. 2 , a display  34  is desirably in communication with controller  30 . Display  34  can be any suitable type of display, such as a liquid crystal display (LCD) or the like, and can provide the user with a visual indication of when the thermometer  10  is ready for use or can provide a readout of the measured temperature. In addition to a visual display, such as the display  34 , any suitable type of audio output, tactile output or data output can also be provided, such as to an alert/information device  35 , such as can include a speaker, a buzzer, vibration member or other type of peripheral device, for example. Additionally, any suitable type of interface  32  can be provided, allowing the user to program the controller  30 , such as control buttons, a keypad, a touchscreen or the like. It should be understood that display  34  can be used to provide the user with any desired information, such as readiness, battery level, etc. and, similarly, interface  32  can be used to program controller  30  with any desired data or instructions. 
         [0020]    Once the warming of pre-heated thermistor  14  to a temperature of T p  has been completed, the controller  30  controls both current sources  18 ,  20  deliver an excitation current of approximately 500 μA to thermistors  12 ,  14 , respectively. In order to eliminate possible variation between current sources  18 ,  20 , the alternative circuit of  FIG. 3  can be utilized to bias both thermistors  12 ,  14  with only programmable current source  20 . In use, the switch S 2  is normally in the “off” or open state. As in the previous embodiment, the controller  30  sets the output of programmable current source  20  to approximately 500 μA for a pre-set period of time. Once the pre-set temperature of thermistor  14  is reached, the switch S 2  is closed (or set to the “on” position) by the controller  30 , biasing both thermistors  12 ,  14  with only current source  20 , which is producing a current of approximately 500 μA for both thermistors. 
         [0021]    In order to detect the placement of the thermometer  10  in the patient&#39;s mouth, for example, the controller  30  monitors the temperature of each thermistor  12 ,  14  to detect a sudden decrease in temperature Tp of pre-heated thermistor  14  and a sudden increase in temperature T of non-heated thermistor  12 . As an alternative, a capacitance sensor can be used to detect the contact of the housing  16  with the human body. Such an arrangement can be implemented by utilizing the metal housing  16  and/or a thermometer cover. Any suitable type of capacitance sensor can be utilized. 
         [0022]    Immediately following the detection stage, the temperature is measured by each thermistor  12 ,  14 . The temperature T measured by non-heated thermistor  12  should be greater than or equal to approximately 32° C. and the temperature T p  measured by pre-heated thermistor  14  should be less than or equal to approximately 42° C., otherwise the controller  30  will trigger an alert signal, such as to provide an alert by one or more of the display  34  or the alert/information device  35 , indicating that the patient whose temperature is being measured by the thermometer  10  is likely suffering from hypothermia or hyperthermia, dependent upon the temperature reading. 
         [0023]    If the measured temperatures are within the expected ranges, the temperature measurements, as a function of time, should follow the curves shown in  FIG. 4 . The decreasing line in  FIG. 4  (i.e., temperature linearly decreasing as a function of time) indicates the temperature T p  measured by pre-heated thermistor  14 , and the increasing line in  FIG. 4  (i.e., temperature linearly increasing as a function of time) indicates the temperature T measured by non-heated thermistor  12 . Given the relatively small temperature range and expected linear behavior of the temperature increases and decreases, a straight line equation can be used to predict the patient&#39;s body temperature or a temperature of a surface (BT). It should be understood that thermistors  12 ,  14  do not take direct temperature measurements. As is well known, thermistors have a varying resistance which is dependent upon temperature. It is this variation in resistance which is being measured (in terms of the corresponding variation in current and/or voltage) and the controller  30  (via channels ADC 1  and ADC 2 ) converts the received signals into corresponding temperature measurements. As opposed to a conventional single-thermistor temperature probe, no linearization is typically required at this point, as the voltage drop across the thermistors can be used against a lookup table, for example, (desirably stored in the memory  31  associated with the controller  30 ) to calculate the temperature in a very small segment of the thermistor resistance curve, which is typically very linear. 
         [0024]    The initial temperature T of non-heated thermistor  12  is expected to be room or ambient temperature. For the linear curves of  FIG. 4 , after approximately 250 ms, for example, the initial temperature reading for non-heated thermistor  12  is given as b C  and the initial temperature reading for pre-heated thermistor  14  is given as b H  (both taken at a time denoted t 0 ). Using this convention, after another 250 ms, for example, the temperature readings are respectively given by b C1  and b H1  (measured at a time denoted as t 1 ). Given the linear nature of the curves in  FIG. 4 , only these readings typically are needed to predict the overall straight line curves. The predicted temperature T C  for non-heated thermistor  12  and the predicted temperature T H  for pre-heated thermistor  14  can be described by the following linear equations: 
         [0000]        T   C   =a   C   t+b   C ; and   (1)
 
         [0000]        T   H   =a   H   t+b   H ,   (2)
 
         [0000]    where t represents time and a C  and a H  are the slopes of the T and T p  curves of  FIG. 4 , respectively; i.e., 
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         [0000]    After a certain point of time, both thermistors  12 ,  14  will ultimately read the same temperature (i.e., the point in  FIG. 4  where curves T and T p  cross). From equations (1) and (2), this time t BP  is given as: 
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         [0000]    and the temperature at time t BP  (i.e., the body temperature or surface temperature BT) is given by: 
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         [0025]    The above calculations are performed by controller  30 . The controller  30  in conjunction with the thermistors  12 ,  14  can therefore provide a means for measuring and calculating a temperature BT of a body or a surface, for example. Controller  30  desirably also includes timing circuitry for performing the successive measurements over the pre-set time duration, although it should be understood that a separate timer can be in communication with controller  30 . The thermometer  10  uses one non-heated thermistor  12  and one pre-heated thermistor  14  to guarantee that the two temperature outputs from the thermistors  12 ,  14  must level off or cross at a specific time with a specific temperature reading BT, as indicated by the dashed line in  FIG. 4 . Once each of thermistors  12 ,  14  generate the same temperature reading, the body or surface temperature BT has been found, and any further output should be in the form of the flat, single curve BT. It should be understood that, alternatively, rather than using pre-heating, the slopes a c  and a H  could be found by controlling the heat transfer, controlling the sensitivity, or by any other suitable technique that would result into different slopes. As a further alternative, this could also be accomplished by choosing one of the thermistors to have a positive temperature coefficient while the other would be selected with a negative temperature coefficient. 
         [0026]    The accuracy of the dual-thermistor technique described above is relatively higher than that of a conventional single temperature sensor due to the fact that the pair of thermistors  12 ,  14  allow for a very small part of the curve to be used for evaluating the temperature measurement. In general, the smaller the temperature range measured, the greater the linearity of the curve which, consequently, increases the accuracy in prediction.  FIG. 5  illustrates measurement within a relatively small temperature range TR. The vertical axis in  FIG. 5  measures the typical resistance variation (Rt) relative to thermistor resistance at 25° C. (R 25 ). Over the entire range between 31° C. and 63° C., both the T and T p  curves show non-linear behavior. However, in the limited range TR, each curve is approximately linear. The linearized approximations are shown by the dashed lines in  FIG. 5 . The accuracy (in terms of error percentage) for non-heated thermistor  12  is given by 
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         [0000]    and the accuracy for pre-heated thermistor  14  is given by 
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         [0027]    The accumulated error (E) of both sensors is given by E=√{square root over (E T   2 +E T     p     2 )}. When the linearity of the small temperature range is taken into account, the error percentage approaches zero. Additionally, due to the linearity over this small range, only a single point is required for calibration of the thermometer  10 . 
         [0028]    It should be understood that thermometer  10  can be used in the place of a conventional handheld thermometer, as a part of a larger vital sign monitoring system, or as part of any other medical device where an oral/rectal/axillary temperature spot check can be needed. In addition to medical uses, it should be understood that thermometer  10  can be used for any application where high performance and sensitivity over a relatively small temperature range are needed. 
         [0029]    It is to be understood that the present invention is not limited to the embodiments described above, but encompasses any and all embodiments within the scope of the following claims.