Abstract:
An apparatus and method for measuring the temperature in an oven includes a circuit and software algorithm that reads the voltage across a standard resistive temperature device (RTD) or thermistor to determine temperature measured by the device. Using an unregulated high voltage supply to increase the gain and resolution, it overcomes the problems of small changes in resistance with respect to temperature. An additional input to measure the unregulated supply voltage is used as a reference voltage input. The apparatus includes resistor dividers for both the temperature sensor and reference voltages, a microprocessor having analog inputs, and additional components for noise suppression and open sensor protection.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to U.S. Provisional Application Ser. No. 60/390,511 filed Jun. 21, 2002, having the same title and inventors as identified herein, which is incorporated by reference herein in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to temperature measurement in appliances. Specifically, the invention involves an apparatus and method for high precision temperature measurement. 
     BACKGROUND OF THE INVENTION 
     A conventional household oven allows a user to set a temperature for baking or cooking food. The oven heats an oven chamber to the desired temperature and attempts to maintain that temperature in the oven chamber for the duration of the cooking period. To heat the oven and maintain the oven temperature, the conventional household oven includes heating elements, a temperature sensor, and a controller. For the oven&#39;s basic operation, the heating elements are supplied with power to heat the oven chamber. The temperature sensor senses the temperature within the oven chamber and supplies a temperature measurement signal to the controller indicative of the temperature. Based on the temperature measurement signal, the controller compares the measured signal with the desired temperature/setpoint and sends a control signal to a heater drive. The heater drive is operatively connected to the heating elements, and is capable of varying the power to the heating elements to maintain the desired temperature setpoint within the oven chamber. 
     Typically temperature measurement using an RTD is done utilizing a regulated voltage supply along with amplifiers, and comparators that increase the gain of the voltage measured across the RTD. These measurements are usually performed using the low regulated power sources as the voltage supply. Regulation of the voltage as well as amplification of the circuit significantly increases the amount of materials required for the temperature measurement, the cost of the component, and the space required for the measurement device. Moreover, to compensate for inaccuracy based on the circuits&#39; calibration values, the offset determined at calibration is typically added to the measured temperature value during operation, which is less accurate than desired. 
     The present invention is directed to overcoming or at least reducing the effects of one or more of the problems set forth above. 
     SUMMARY OF THE INVENTION 
     The present invention relates to an apparatus for measuring the temperature in an appliance. The apparatus comprise a temperature transducer, the temperature transducer comprising a variable resistance that changes in response to the temperature. First and second resistors are coupled in series between a voltage supply and ground to form a first voltage divider. The junction of the first voltage divider is then coupled to an input of a microprocessor so as to provide the microprocessor with a signal indicative of the voltage across the first resistor. A third resistor coupled in series with the temperature transducer between the voltage supply and ground to form a second voltage divider. The junction of this voltage divider is coupled to another input of the microprocessor so as to provide a signal indicative of the voltage across the temperature transducer. The microprocessor then determines a temperature using the voltage across the temperature transducer and the second resistor to determine the resistance of the temperature transducer. 
     In another aspect, the apparatus may be constructed so that first and second resistors each comprise one or more individual resistors interconnected by one or more jumpers to provide suitable resistance values corresponding to the supply voltage. In still another aspect, the jumpers may also provides a signal to the microprocessor indicative of the supply voltage or resistance values selected. Alternatively, some other variable signal indicative of the supply voltage may be connected to the microprocessor. 
     In one aspect of the present invention, the microprocessor determines the temperature using a look-up table correlating the resistance of the temperature transducer to the temperature. In still another aspect, the temperature determined by the microprocessor is corrected by an offset value determined during a calibration routine and stored in memory of the microprocessor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other objects and advantages of the invention will become apparent upon reading the following detailed description and upon reference to the drawings. 
         FIG. 1  is a block diagram of a typical household electric oven; 
         FIG. 2  is a circuit diagram illustrating an embodiment of the present invention. 
         FIG. 3  is a circuit diagram illustrating an alternative embodiment of the present invention. 
     
    
    
     While the invention is susceptible to various modifications and alternative forms, certain specific embodiments thereof have been shown by way of example in the drawings and will be described in detail. It should be understood, however, that the intention is not to limit the invention to the particular forms described. On the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims. 
     DETAILED DESCRIPTION 
     Although the following description is in terms of a control system for an oven, it will be understood by those skilled in the art that it is applicable to all types of appliances including all types of ovens, refrigerators, freezers, washers, dryers and dishwashers. 
       FIG. 1  is a block diagram of a household electric oven  10  according to one embodiment of the present invention. The oven comprises an oven chamber  42  having at least one heating element  41  and at least one temperature sensor  40 . The oven  10  also has a user interface  18  that allows the user to control the operation of the oven  10 . The user interface  18  is a typical interface on the front of a typical household oven. The interface  18  comprises a keypad with keys and/or dials that turn the oven on and off. Additionally, the keys and/or dials present on the user interface  18  instruct the oven to operate at particular temperature set point and operational mode. For example, the user selects the appropriate set point temperature for the oven chamber  42 , such as 350° F., and selects the operating mode, such as bake mode and self-cleaning mode with the user interface  18 . 
     The user interface  18  generates signals indicating pressed keys and/or dial positions. These signals are transmitted from the user interface  18  to a control unit  20  through an analog-to-digital converter  22 . The analog-to-digital converter  22  receives the analog signals from the user interface  18  and transforms them into digital signals that are readable by the control unit  20 . Although shown as separate elements, the analog-to-digital conversion can be done internally at the control unit  20  if it is the type of microcomputer or microprocessor equipped for such a purpose. 
     The control unit  20  receives and processes the signals from the user interface  18  through the analog-to-digital converter  22 . The processing results in a series of control signals being sent from the control unit  20  to other elements of the oven to operate the oven at the desired oven temperature and in the desired oven mode. The control unit  20  sends control signals to a heater drive  24  that transmits power from a power source  26  to the heater elements  41 . The control unit  20  may also send control signals to other elements of the oven, such as a fan, depending on the oven mode. 
     The control unit  20  also receives signals representing information stored in a memory  28 . The memory  28  transmits its stored information signals over a data bus that is coupled to the control unit  20 . In an alternative embodiment, the control unit  20  includes nonvolatile memory. The memory  28  stores information representing various heat settings in the oven&#39;s modes of operation. The control unit  20  requests the information stored in memory  28  based on the signal inputs received from the user interface  18 . For example, if the user has selected the self-cleaning mode with the user interface  18 , the control unit  20  obtains information from the memory  28  relating to the self-cleaning mode. 
     The control unit  20  also receives a signal representing an oven cavity temperature from the temperature sensor  40 . The temperature sensor  40  is a standard resistive temperature device (RTD) sensor or any other temperature sensor known to those skilled in the art. The temperature signal is transmitted from the temperature sensor  40  to the control unit  20 . In an alternative embodiment, the temperature signal from the temperature sensor  40  passes through an analog-to-digital converter (not shown). The analog-to-digital converter transforms the analog signals into digital signals for reading by the control unit  20  if the control unit  20  is only equipped to read digital signals. 
     The present invention relates to a circuit and algorithm for measuring the temperature of the oven chamber using the standard RTD temperature sensor. 
       FIG. 2  illustrates one embodiment of the temperature measuring circuit of the present invention. The circuit includes an unregulated power source  11 , a microprocessor  12 , resistive temperature device (RTD)  17 , resistors R 1 , R 2 , R 3 , diode  14 , RC filter circuits  15  and  16 , and ground connection  13 . 
     The unregulated power source is a high voltage DC supply that can have nominal values of 24V, 32V, or 40V DC. Because the voltage supply is unregulated the actual voltage supply can have a variety of ranges. 
     As is known in the art, the resistance of the RTD  17  varies in proportion to the temperature being measured by the RTD  17 . As shown in  FIG. 2 , the resistor R 3  and RTD  17  are series connected to form a voltage divider for the measuring the RTD  17  resistance. The voltage divider connection  21  measures the voltage across the RTD  17  and provides the value of the measurement to input  18  of microprocessor  12 . The RC filter circuitry  15  filters any noise. Because the voltage supply is unregulated, to compensate for supply fluctuations, resistors R 2  and R 1  are connected in series to form a second voltage divider that represents the reference voltage. Voltage divider connection  22  has RC noise filter circuitry  16  and is connected to another input of the microprocessor. A diode  14  protects the microprocessor input  18  from over-voltage when the RTD  17  is disconnected. 
     Resistors R 1  and R 2  provide a signal below 5V DC for measuring the unregulated voltage supply. The values of resistors R 1 , R 2 , and R 3  depend on the range of the unregulated supply for a given application, which in turn depends on the voltage requirements of any other devices connected within the circuitry. The microprocessor  12  as an analog-to-digital-converter (ADC) that converts the analog voltage readings into corresponding digital values. 
     It should be understood that any suitable values of supply voltage and component values can be used. However for illustrative purposes, the nominal values for a typical supply voltage have been used, as well as corresponding components. The microprocessor uses the voltage input  18  across the RTD  17  and the reference voltage input  19  across resistor R 1  to compute the resistance of RTD  17 . The microprocessor  12  then converts the resistance of the RTD  17  to a temperature value. Based on the measured voltage values across the RTD  17  and across the resistor R 1  an equation is developed for a value for the resistance of the RTD  17 , which eliminates the unregulated voltage supply value. As is known in the art, the resistance of the RTD  17  can be represented by equation 1 below: 
               R   RTD     =         R   1     ·     R   2     ·     V   RTD             (       R   1     +     R   2       )     ·     V   REF       -       R   1     ·     V   RTD                   Equation   ⁢           ⁢   1             
 
where: R 1  is the resistance of resistor R 1 ; R 2  is the resistance of resistor R 2 ; R 3  is the resistance of resistor R 3 ; V RTD  is the measured voltage across RTD  17 ; and V REF  is the measured voltage across resistor R 1 .
 
     Typical nominal values of the unregulated high voltage DC power source are 24V, 32V, or 40V DC. Because the voltage supply is unregulated the actual voltage supply can have a variety of ranges for these nominal values, as indicated in table 1 below. Moreover, based on the nominal voltage values of the power source, resistor values for the resistors R 1 , R 2 , R 3  have been chosen for illustrative purposes to maximize the analog input values and improve the resolution. These resistive values are also indicated in Table 1 below. 
     
       
         
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                 Actual 
                   
                   
                   
               
               
                 Nominal 
                 Voltage 
               
               
                 Voltage 
                 Range 
                 R1 
                 R2 
                 R3 
               
               
                   
               
             
             
               
                 24 V 
                 16-28 V 
                 41.2 kΩ ± 1% 
                 200 kΩ ± 1% 
                 14.0 kΩ ± 1% 
               
               
                 32 V 
                 22-38 V 
                 41.2 kΩ ± 1% 
                 287 kΩ ± 1% 
                 20.0 kΩ ± 1% 
               
               
                 40 V 
                 28-48 V 
                 41.2 kΩ ± 1% 
                 347 kΩ ± 1% 
                 26.1 kΩ ± 1% 
               
               
                   
               
             
          
         
       
     
     As shown in Table 1, based on these nominal voltage values  11 , the resistance of R 1  is a constant value of 41.2 kΩ. Because the typical nominal voltage can vary based on the appliance manufacturer&#39;s standards, in one embodiment of this invention, the circuit design includes three resistors for R 2  having the computed resistive values for each nominal voltage value, and three resistors for R 3  having the computed resistive values for each nominal value. In this embodiment, a jumper is installed on the printed circuit board of the device and is used to indicate the manufacturer&#39;s nominal voltage supply. The manufacturer places the jumper across the correct pins of the printed circuit board to indicate the corresponding nominal voltage. Also in this embodiment, placement of the jumper also sends an input to the microprocessor  12 , triggering the corresponding resistors R 1 , R 2 , R 3  value data stored within the memory of the microprocessor  12 . In a alternative embodiment, rather than having a hardwired input signal sent to the microprocessor  12 , based on the jumper position, the microprocessor  12  has a dial with multiple positions, indicative of the nominal voltage values. The dial is adjustable to allow the manufacture to select the desired nominal voltage value. Because the costs of the additional resistors and jumper components is minuscule, this design allows for lower manufacturing cost, by enabling the manufacture to produce one device that is end use configurable based on the end users requirements. 
     In one embodiment a data look-up table of degree Fahrenheit values an corresponding resistor values, shown in Table 2, is stored in the microprocessor&#39;s I read-only-memory (ROM). In this embodiment, based on Equation 1, the microprocessor  12  calculates the resistance of the RTD, and then using the stored ROM values indicated in Table 2, and interpolation, the microprocessor  12  calculates the temperature measured by the RTD  17 . 
     
       
         
               
               
               
             
               
               
               
             
           
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 ° F. 
                 Ω 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 0 
                 932.060 
               
               
                   
                 10 
                 953.340 
               
               
                   
                 20 
                 974.572 
               
               
                   
                 30 
                 995.766 
               
               
                   
                 40 
                 1016.922 
               
               
                   
                 50 
                 1038.042 
               
               
                   
                 60 
                 1050.124 
               
               
                   
                 70 
                 1080.169 
               
               
                   
                 80 
                 1101.177 
               
               
                   
                 90 
                 1122.148 
               
               
                   
                 100 
                 1143.081 
               
               
                   
                 110 
                 1163.978 
               
               
                   
                 120 
                 1184.837 
               
               
                   
                 130 
                 1205.659 
               
               
                   
                 140 
                 1226.445 
               
               
                   
                 150 
                 1247.192 
               
               
                   
                 160 
                 1267.903 
               
               
                   
                 170 
                 1288.577 
               
               
                   
                 180 
                 1309.213 
               
               
                   
                 190 
                 1329.812 
               
               
                   
                 200 
                 1350.374 
               
               
                   
                 210 
                 1370.899 
               
               
                   
                 220 
                 1391.387 
               
               
                   
                 230 
                 1411.838 
               
               
                   
                 240 
                 1432.251 
               
               
                   
                 250 
                 1452.628 
               
               
                   
                 260 
                 1472.967 
               
               
                   
                 270 
                 1493.269 
               
               
                   
                 280 
                 1513.534 
               
               
                   
                 290 
                 1533.762 
               
               
                   
                 300 
                 1553.952 
               
               
                   
                 310 
                 1574.106 
               
               
                   
                 320 
                 1594.222 
               
               
                   
                 330 
                 1614.301 
               
               
                   
                 340 
                 1634.348 
               
               
                   
                 350 
                 1654.343 
               
               
                   
                 360 
                 1674.316 
               
               
                   
                 370 
                 1694.246 
               
               
                   
                 380 
                 1714.140 
               
               
                   
                 390 
                 1733.996 
               
               
                   
                 400 
                 1753.815 
               
               
                   
                 410 
                 1773.597 
               
               
                   
                 420 
                 1793.341 
               
               
                   
                 430 
                 1813.049 
               
               
                   
                 440 
                 1832.720 
               
               
                   
                 450 
                 1852.353 
               
               
                   
                 460 
                 1871.949 
               
               
                   
                 470 
                 1891.508 
               
               
                   
                 480 
                 1911.030 
               
               
                   
                 490 
                 1930.514 
               
               
                   
                 500 
                 1949.962 
               
               
                   
                 510 
                 1969.372 
               
               
                   
                 520 
                 1988.746 
               
               
                   
                 530 
                 2008.082 
               
               
                   
                 540 
                 2027.381 
               
               
                   
                 550 
                 2046.642 
               
               
                   
                 560 
                 2065.867 
               
               
                   
                 570 
                 2085.054 
               
               
                   
                 580 
                 2104.205 
               
               
                   
                 590 
                 2123.318 
               
               
                   
                 600 
                 2142.392 
               
               
                   
                 610 
                 2161.433 
               
               
                   
                 620 
                 2180.435 
               
               
                   
                 630 
                 2199.399 
               
               
                   
                 640 
                 2218.326 
               
               
                   
                 650 
                 2237.217 
               
               
                   
                 660 
                 2256.070 
               
               
                   
                 670 
                 2274.886 
               
               
                   
                 680 
                 2293.665 
               
               
                   
                 690 
                 2312.406 
               
               
                   
                 700 
                 2331.111 
               
               
                   
                 710 
                 2349.778 
               
               
                   
                 720 
                 2368.408 
               
               
                   
                 730 
                 2387.001 
               
               
                   
                 740 
                 2405.557 
               
               
                   
                 750 
                 2424.076 
               
               
                   
                 760 
                 2442.557 
               
               
                   
                 770 
                 2461.002 
               
               
                   
                 780 
                 2479.409 
               
               
                   
                 790 
                 2497.779 
               
               
                   
                 800 
                 2516.112 
               
               
                   
                 810 
                 2534.408 
               
               
                   
                 820 
                 2552.666 
               
               
                   
                 830 
                 2570.888 
               
               
                   
                 840 
                 2589.072 
               
               
                   
                 850 
                 2607.219 
               
               
                   
                 860 
                 2625.330 
               
               
                   
                 870 
                 2643.402 
               
               
                   
                 880 
                 2661.438 
               
               
                   
                 890 
                 2679.437 
               
               
                   
                 900 
                 2697.398 
               
               
                   
                 910 
                 2715.322 
               
               
                   
                 920 
                 2733.210 
               
               
                   
                 930 
                 2751.059 
               
               
                   
                 940 
                 2768.872 
               
               
                   
                 950 
                 2786.648 
               
               
                   
                 960 
                 2804.386 
               
               
                   
                 970 
                 2822.088 
               
               
                   
                 980 
                 2839.752 
               
               
                   
                 990 
                 2857.379 
               
               
                   
                   
               
             
          
         
       
     
     It is known to those skilled in the art that deviations in the circuit&#39;s components value compromise the accuracy of the RTD  17  temperature measurement and creates an offset in measured value. To compensate for this offset, calibration of the circuit is required. Calibration is performed by replacing the RTD  17  with a known resistance, representative of an ideal temperature. The microprocessor  12  is put in calibration mode and prompts the programmer to input the known resistance value. Based on the known resistance value, the microprocessor  12  chooses a temperature value corresponding to the known resistance, referred to as an ideal temperature. The microprocessor  12  then using Eq. 1 calculates the actual measured resistance and the corresponding temperature value. The microprocessor  12  then subtracts the measured temperature from the ideal temperature; the resulting value is the circuit&#39;s offset. This offset is stored in the ROM of the microprocessor  12 . 
     In one embodiment of this invention, during normal operation of the circuit, the offset value is added to the measured temperature value, to provide a more accurate representation of the actual measured temperature. In a further aspect of this embodiment, for increased accuracy, the offset value is multiplied by the resistance of the ideal temperature used for calibration purposes and this value is then divided by the actual measured temperature, resulting in a percentage offset value. In this embodiment, rather than add the entire offset amount to the measured temperature value, the percentage offset value is added to the measured temperature value, providing a more accurate representation of the actual measured temperature. 
     In a further embodiment of this invention, efficiency in calculating the measured temperature and simplification of the software is achieved by manipulating Equation 1 to include a constant K and developing a value termed ‘internal value’, which can be used to determine the measured temperature. Equation 1 can be manipulated to include the constant K resulting in Equation 2: 
                 (       R   1     ·     R   3       )       K   ·     R   RTD         =           (       R   1     +     R   2       )     ·     V   REF       -       R   1     ·     V   RTD           K   ·     V   RTD                 Equation   ⁢           ⁢   2             
 
     Equation 2 above represents the ‘internal value’ as well as Equation 7 below. The ‘internal value’ is inversely proportional to sensor resistance. In Equation 2, the voltage units cancel each other out. This allows the raw 10-bit analog input values to be used directly for V REF  and V RTD  without actually converting them to volts. 
             InternalValue   =             K   R     ·     V   REF       -       K   S     ·     V   RTD           V   RTD       =         R   1     ·     R   3         K   ·     R   RTD                   Equation   ⁢           ⁢   7             
 
where: K R =(R 1 +R 2 )/K; K S =R 1 /K; and K=(R 1 ·R 3 )/(InternalValue·R RTD ).
 
Equation 3 represents the value of the constant K. Because the value of K depends upon the values of R 1  and R 3 , a value for K has to be determined for each of the unregulated nominal voltage supply values. Because the internal value is inversely proportional to the resistance of the RTD, and the resistance of the RTD increases as the temperature increases, the value of 12288 decimal or 3000 hex is selected as the maximum to indicate a temperature of 0 deg F., which has an ideal resistance value of 963.63 Ω, based on Table 2. In binary form, this 3000 hex maximum value is much lower than the maximum 16-bit value. Based on this, the maximum ‘internal value’ can be determined using Equation 3. 
             K   =       (       R   1     ·     R   3       )       InternalValue   ·     R   RTD                 Equation   ⁢           ⁢   3             
 
     Using these values, a value for K at each nominal voltage value can be determined as shown in Table 3 below. Also values for K R  and K S  can be computed as shown above. Therefore using these equations, values for K, K R  and K S  are computed and shown in Table 3 below. 
     
       
         
               
               
               
               
               
             
           
               
                   
                 TABLE 3 
               
               
                   
                   
               
               
                   
                 Nominal Voltage Supply 
                 K 
                 K R   
                 K S   
               
               
                   
                   
               
             
             
               
                   
                 24 V 
                 50.36119 
                 4789 
                 818 
               
               
                   
                 32 V 
                 74.94456 
                 4562 
                 573 
               
               
                   
                 40 V 
                 93.88765 
                 4422 
                 439 
               
               
                   
                   
               
             
          
         
       
     
     The values of K, K R  and K S  shown in Table 3 are programmed into the microprocessor&#39;s 12 ROM. Based on the circuit&#39;s nominal voltage supply  11 , determinable by a jumper connection and/or a dial setting on the microprocessor  12 , the microprocessor selects the correct values to calculate the RTD  17  temperature measurement. 
     Based on the ‘internal values’ for various resistance and equivalent temperature values, a look-up table, illustrated in Table 4, is generated and stored in the microprocessor&#39;s 12 ROM. Based on a 256 value decimal decrement being subtracted initially from the maximum ‘internal value’ 12288, and each iteration thereafter, Table 4 has 34 reference points termed i, that range consecutively from 0 to 33. Each i value corresponds to a specific ‘internal value’ and corresponding temperature value in degrees Fahrenheit, as illustrated in Table 4. 
     
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 4 
               
             
             
               
                   
               
               
                 ROM Table for Degrees Fahrenheit Conversion 
               
             
          
           
               
                   
                 i 
                   
                 Internal Value 
                 Table 
               
               
                   
                   
               
             
          
           
               
                   
                 0 
                 3840 
                 0F00 hex 
                 1056 
               
               
                   
                 1 
                 4096 
                 1000 hex 
                 955 
               
               
                   
                 2 
                 4352 
                 1100 hex 
                 864 
               
               
                   
                 3 
                 4608 
                 1200 hex 
                 783 
               
               
                   
                 4 
                 4864 
                 1300 hex 
                 713 
               
               
                   
                 5 
                 5120 
                 1400 hex 
                 650 
               
               
                   
                 6 
                 5376 
                 1500 hex 
                 594 
               
               
                   
                 7 
                 5632 
                 1600 hex 
                 543 
               
               
                   
                 8 
                 5888 
                 1700 hex 
                 498 
               
               
                   
                 9 
                 6144 
                 1800 hex 
                 456 
               
               
                   
                 10 
                 6400 
                 1900 hex 
                 418 
               
               
                   
                 11 
                 6656 
                 1A00 hex 
                 383 
               
               
                   
                 12 
                 6912 
                 1B00 hex 
                 351 
               
               
                   
                 13 
                 7168 
                 1C00 hex 
                 322 
               
               
                   
                 14 
                 7424 
                 1D00 hex 
                 294 
               
               
                   
                 15 
                 7680 
                 1E00 hex 
                 269 
               
               
                   
                 16 
                 7936 
                 1F00 hex 
                 245 
               
               
                   
                 17 
                 8192 
                 2000 hex 
                 223 
               
               
                   
                 18 
                 8448 
                 2100 hex 
                 203 
               
               
                   
                 19 
                 8704 
                 2200 hex 
                 183 
               
               
                   
                 20 
                 8960 
                 2300 hex 
                 165 
               
               
                   
                 21 
                 9216 
                 2400 hex 
                 148 
               
               
                   
                 22 
                 9472 
                 2500 hex 
                 132 
               
               
                   
                 23 
                 9728 
                 2600 hex 
                 116 
               
               
                   
                 24 
                 9984 
                 2700 hex 
                 102 
               
               
                   
                 25 
                 10240 
                 2800 hex 
                 88 
               
               
                   
                 26 
                 10496 
                 2900 hex 
                 75 
               
               
                   
                 27 
                 10752 
                 2A00 hex 
                 63 
               
               
                   
                 28 
                 11008 
                 2B00 hex 
                 51 
               
               
                   
                 29 
                 11264 
                 2C00 hex 
                 40 
               
               
                   
                 30 
                 11520 
                 2D00 hex 
                 29 
               
               
                   
                 31 
                 11776 
                 2E00 hex 
                 19 
               
               
                   
                 32 
                 12032 
                 2F00 hex 
                 9 
               
               
                   
                 33 
                 12288 
                 3000 hex 
                 0 
               
               
                   
                   
               
             
          
         
       
     
     An example of operation of the circuit is illustrated below, using the following values and referring to FIG.  2 . As illustrated by this example, the RTD  17  actual temperature is 350° F. 
     Nominal Voltage Supply=32 V 
     Actual Voltage Supply=31 V 
     R RTD =1654.3 
     V REF =3.852 V=788 after ADC 
     V RTF =2.379 V=487 after ADC 
     Using these values as well as the values for K R  and K S  shown in Table 3, the internal value is calculated using Equation 7 to be 6809 (rounded). 
     As shown in Table 4, the smallest ‘internal value’ is 3840, which represents the largest temperature value of 1056, indicated as Table 1 . Hence calculation of i is computed as follows: i=(InternalValue=8340)/256 (truncated)=11. Once i has been calculated, the Degree Measurement (Deg.Meas.) is computed using the equation below, and referring back to Table 4. The microprocessor. 12 interpolates to determine the Deg. Meas. value. 
         Deg   .   Meas   .     =         Table   i     -       (       Table   i     -     Table     i   +   1         )     ·   InternalValue     -   3840   -     256   ·   i       256         
 
     Using the above equation and the example above, the Deg.Meas. value is 364° F., whereas the actual temperature of the RTD  17  is 350° F. As previously mentioned, a further embodiment of this invention includes adjustment of the temperature for the offset determined by the initial calibration. In this embodiment of the invention, for increased accuracy, the calibration adjustment to the temperature measurement is proportional to the actual resistance measured. To further increase the accuracy, the calibration is performed at a resistance that corresponds to a relatively high temperature, so that the adjustment can be proportionally reduced for lower temperatures. The internal value is inversely proportional to the resistance, therefore the internal value is in the denominator of the calibration adjustment equation. 
     An example of the calibration, to determine to offset is illustrated below. In this example, the RTD  17  is replaced with a resistance that has a value of 2199.4 Ω that represents an ideal temperature of 630° F. During calibration the following is an example of the actual supply voltage and measured RTD  17  and reference voltages used: 
     Nominal Voltage Supply=32 V 
     Actual Voltage Supply=34V 
     R RTD =2199.4 
     V REF =4.224 V=864 after ADC 
     V RTF =3.384 V=692 after ADC 
     The internal value, i, and Deg. Meas. are calculated using the same method used for calculating the example actual operational measurement. Thus the following values are computed by the microprocessor  12 : 
     Calibrated InternalValue=5123 
     Calibrated i=4 
     Calibrated Deg.Meas=649° F. 
     As indicated by the calibration, the offset of the circuit is −19° F., determined by subtracting the Cal. Deg. Meas. from the actual value (equivalent temperature value based on the resistance of the calibrating resistor). 
     Using the offset value of −19° F., the Compensated Deg. Meas. is more accurately determined using the Deg. Meas. of 364° F. computed earlier and the offset. Using the offset calculation, which compensates for the error determined at calibration, the Compensated Deg. Meas. of 349° F. is much closer to the Actual Deg. Meas. of 350° F. 
     Although the embodiments have discussed the use of only one RTD  17 , in a further embodiment of the present invention, multiple RTDs  17 ,  17 ′ are used along with multiple series resistors R 3 , R 3 ′, as illustrated in  FIG. 3   
     The foregoing description of preferred and other embodiments is not intended to limit or restrict the scope or applicability of the inventive concepts conceived of by the Applicants or defined in the appended claims. In exchange for disclosing the inventive concepts contained herein, the Applicants desire all patent rights afforded by the appended claims. It is intended that the inventive concepts defined by the appended claims include all modifications and alterations to the full extent that such modifications or alterations come within the scope of the appended claims or the equivalents thereof.