Abstract:
One aspect of the present invention relates to a digitally controlled resistance standard, comprising a control system, a resistor element, hermetically sealed within an element assembly, a temperature/frequency measuring circuit for measuring the temperature of the resistor element and the frequency of an input signal, a heating/cooling assembly for raising and lowering the temperature of the resistor element a temperature controller for controlling the heating/cooling assembly, and a control system having a CPU. The control system is operable to execute computer instructions for storing the baseline characteristics of the resistor element, retrieve the temperature of the resistor element from the temperature/frequency measuring circuit, determine the actual resistance value of the resistor element, and adjust the actual resistance value of the resistor element to match a target resistance value using the temperature controller. A method for improving the accuracy of a resistance standard is also given.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to measurement standards and more particularly to a portable resistance standard capable of adjusting its actual value to a nominal value. 
     BACKGROUND 
     The unit of measurement for resistance is an ohm. The legal ohm is defined by an agreement between national laboratories on the value of the Quantum Hall Effect Resistor (QHR), a fixed reference value which cannot be adjusted. Once the value of the ohm is established, the value is disseminated to a number of working standard resistors via Cryogenic Current Comparators (CCC&#39;s). The working standard resistors are then compared against other resistance standards sent in for calibration by commercial, private, or other public laboratories. The cost, size and sensitivity of QHR&#39;s and CCC&#39;s does not permit their easy transport. Thus in practice, the legal ohm is disseminated by the transport and maintenance of passive resistance standards. 
     Prior art passive resistance standards are affected by external environmental and measurement system effects. For example, temperature, relative humidity, barometric pressure, self-heating and thermoelectric effects caused by an applied current, inductive effects at varying frequencies, and drift due to aging, among others, each have an effect on prior art resistance standards. To minimize some of these effects, prior art resistance standards are typically placed in a constant temperature oil or air bath. Constant temperature baths, however, are large, non-portable, and expensive to maintain and monitor. 
     Higher quality resistance standards are typically constructed of Manganin or Evanohm® wire, strip, or ribbon, which is heat treated to reduce the material&#39;s coefficient of temperature. The standards are manufactured to possess low thermoelectric and inductive effects and hermetically sealed to reduce the effects of humidity and barometric pressure changes. High quality resistance standards, however, require a constant temperature bath to reduce temperature effects. Additionally, high quality resistance standards are easily damaged by excessive current, exhibit drift with age, and provide no means of testing the effects of the other components in a complete measurement system. Furthermore, the accuracy of high quality resistance standards is degraded by self heating during measurement and by measurement signals other than DC or low frequency AC signals. The working uncertainty of high quality resistance standards is also laborious to calculate and maintain. 
     High quality resistance standards are only capable of realizing a single resistance value; thus multiple high quality resistance standards must be used for each resistance value. For example, two high quality resistance standards must be used to generate a 0.01 ohm value and a 1 ohm value. The use of multiple high quality resistance standards increases the expense, size, and complexity of the complete measurement system. 
     Thus, there exists a need for a resistance standard having improved accuracy, that is more immune to external environmental and measurement system effects, that does not require the use of constant temperature baths, that can realize multiple resistance values, that integrates uncertainty calculations into its value, and that overcomes other limitations inherent in prior art resistance standards. 
     SUMMARY 
     One aspect of the present invention relates to a method for improving the accuracy of a resistance standard comprising ascertaining baseline characteristics for a resistor element, determining the actual resistance value of the resistor element, selecting a target resistance value for the resistor element, and adjusting the actual resistance value of the resistor element to match the target resistance value of the resistor element. 
     Ascertaining the baseline characteristics for the resistor element may further comprise at least one of determining the resistor element&#39;s coefficients of temperature; measuring the resistor element&#39;s frequency response; and determining the resistor element&#39;s drift due to age. 
     Additionally, determining the actual resistance value of the resistor element may further comprise determining the temperature of the resistor element, determining the frequency of an applied measurement signal, determining the age of the resistance standard, and calculating the actual resistance value of the resistor element using at least one of the measured temperature, frequency, and age, and the baseline characteristics. Adjusting the actual resistance value of the resistor element may comprise altering the temperature of the resistor element to realize the target resistance value. 
     Another aspect of the present invention relates to a self-adjusting resistance standard comprising a resistor element sealed within an element assembly, a temperature and frequency measuring circuit for measuring the temperature of the resistor element and the frequency of an applied measurement signal, a heating/cooling assembly for raising and lowering the temperature of the resistor element, a temperature controller for controlling the heating/cooling assembly, and a control system responsive to the measuring circuit to control the heating/cooling assembly to maintain the value of the resistor element. The control system may include a CPU and is operable to store the baseline characteristics of the resistor element, retrieve the temperature of the resistor element from the temperature/frequency measuring circuit, retrieve the frequency response of the resistor element, retrieve the frequency of an applied measurement signal, store uncertainty components, calculate expanded uncertainties, determine the actual resistance value of the resistor element, and adjust the actual resistance value of the resistor element to match a target resistance value using the temperature controller. The CPU also stores measurement system uncertainty components and calculates system uncertainty. 
     The present invention provides a resistance standard having improved accuracy, that is immune to external environmental and measurement system effects, that does not require the use of constant temperature baths, that contains multiple resistance values, that permits evaluation of measurement system sensitivity and accuracy, that includes uncertainty calculation, and that overcomes other limitations inherent in prior art resistance standards. Those advantages and benefits, and others, will be apparent from the Detailed Description below. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     To enable the present invention to be easily understood and readily practiced, the present invention will now be described for purposes of illustration and not limitation, in connection with the following figures wherein: 
     FIG. 1 is a perspective view of a digitally controlled resistance standard according to an embodiment of the present invention. 
     FIG. 2 is a simplified block diagram of the digitally controlled resistance standard of FIG. 1 according to an embodiment of the present invention. 
     FIG. 3 illustrates an element assembly for the digitally controlled resistance standard of FIG. 1 according to an embodiment of the present invention. 
     FIGS. 4A through 4D are detailed views of resistor elements for the element assembly of FIG. 3 according to various embodiments of the present invention. 
     FIGS. 4A-1,  4 B- 1 ,  4 C- 1  and  4 D- 1  are detailed end views of the resistor elements shown in FIGS. 4A,  4 B,  4 C, and  4 D, respectively. 
     FIG. 5A illustrates a detailed view of the housing body of the element assembly of FIG. 3 according to an embodiment of the present invention. 
     FIG. 5B illustrates a detailed view of the thermometer well assembly of the element assembly of FIG. 3 according to an embodiment of the present invention. 
     FIG. 5C illustrates a detailed view of the end cap of the element assembly of FIG. 3 according to an embodiment of the present invention. 
     FIG. 6A is a cut-away view of the heating/cooling assembly of FIG. 1 according to an embodiment of the present invention. 
     FIG. 6B is an end view of the heating/cooling assembly shown in FIG. 6A according to an embodiment of the present invention. 
     FIG. 7 illustrates a resistance/temperature curve for a typical resistor element according to an embodiment of the present invention. 
     FIG. 8 illustrates an operational process for the digitally controlled resistance standard of FIG. 1 according to an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     FIGS. 1 and 2 are a perspective view and a simplified block diagram, respectively, of a digitally controlled resistance standard  10  according to an embodiment of the present invention. The digitally controlled resistance standard  10  is comprised of a heating/cooling assembly  12 , a temperature/frequency measuring circuit  14 , control system  18 , power supply  20 , temperature controller  22 , one or more displays  24 , user terminals  25 , and memory  28 . Each component (as shown in FIG. 1) of the digitally controlled resistance standard  10  may be individually protected against noise and other environmental effects by a shield  16 . Alternatively, several components may be contained within a single shield  16 , for example, the heating/cooling assembly  12  and temperature/frequency circuit  14  may be protected by the same shield. 
     In the current embodiment (and as described in more detail in conjunction with FIGS.  6 A and  6 B), heating/cooling assembly  12  is comprised of a shell  90 , heat sinks  92 , thermoelectric modules  94 , fan mount  96 , fan  98 , and insulation  99  among others. In the current embodiment, the heating/cooling assembly  12  surrounds an element assembly  74 . The element assembly  74  (as described in more detail in conjunction with FIGS.  3 - 5 C), is comprised of a housing body  76 , thermometer well assembly  78 , end cap  80 , and resistor element  32 , among others. 
     In the current embodiment, control system  18  is comprised of a CPU and other circuit components (such as analog-to-digital converters, digital-to-analog converters, transistors, capacitors, and resistors, among others). The control system  18  is operable to execute a software program for achieving the resistance value&#39;s storage, display, control, and output functions of the present invention. In the current embodiment, the control system is operable to execute software functions for elapsed time measurement (which is based on comparing a current calendar reading to the resistor element&#39;s  32  date of manufacture) and uncertainty calculations (which are based on internal and external manufacturer and customer supplied data). It should be noted that internal information includes resistor element  32  data and system&#39;s  10  information. External information refers to customer supplied data. A more complete description of the software&#39;s functioning will be discussed in conjunction with FIG.  8  and operational process  800 . 
     It should further be noted that control circuit  18  is operable to calculate the expanded uncertainty of the resistor element  32 . Expanded uncertainty refers to a statistical valuation (for example, the root square sum) of all defined uncertainty components. For example: the uncertainty of a measurement at one ohm might be 1 part per million; the uncertainty of temperature measurement may be 0.01°, and 0.01° may correspond to a resistance change of 0.1 ppm. Each component may contribute a defined uncertainty. Combining the uncertainty for each components gives the expanded uncertainty. 
     Memory  28  is any combination of programmable volatile and non-volatile memory components. For example, RAM, ROM, DRAM, SDRAM, DDRAM, and FLASH memory, among others, may be used by the digitally controlled resistance standard  10  while remaining within the scope of the present invention. Memory  28  is operable to store data, for example manufacturer and user data, sent from the CPU and to return data requested by the CPU. It should be noted that other storage devices (for example, CD ROM drive, magnetic tape drive, hard disc drive, floppy disc drive, etc.) may be used while remaining within the scope of the present invention. 
     Temperature/frequency measuring circuit  14  measures the temperature of the element assembly  74  (see FIG. 3) and the frequency of a signal applied at user terminals  25 . User terminals  25  include, for example, current and potential measurement connection terminals, among others. Temperature/frequency measuring circuit  14  provides outputs, for example digital outputs, to the control system  18 . 
     In the current embodiment, the frequency measuring portion of the temperature/frequency measuring circuit  14  is comprised of an opto-isolator and an analog to digital (A/D) converter. The opto-isolator isolates the frequency measuring portion from a signal applied to user terminals  25 . The output of the A/D converter is processed by the CPU. 
     In the current embodiment, the temperature measuring portion of the temperature/frequency measuring circuit  14  is a dual element Wheatstone bridge circuit with a constant current supply. The output of the temperature measuring portion is transmitted to an A/D converter and processed by the CPU. The CPU calculates the temperature using a Steinhart-Hart equation. In the current embodiment, the accuracy of the temperature can be resolved to better than one millidegree (i.e., 0.001° C.). It should be noted that other types of circuits may be used to measure the temperature and frequency while remaining within the scope of the present invention. 
     Temperature controller  22  includes logic circuitry (such as a processor, digital-to-analog converters, analog-to-digital converters, transistors, multiplexers, and bi-power amplifiers, among others) (not shown) for executing a tuned temperature control algorithm. In the current embodiment, the tuned temperature control algorithm is a PID algorithm which generates a numerical error output signal which is converted to an analog signal by a digital-to-analog converter. The analog signal controls the power output of a bipolar power amplifier. The amplified analog signal is sent to thermoelectric modules  94  of the heating/cooling assembly  12  (see FIGS.  6 A and  6 B). 
     In the current embodiment, the thermoelectric modules  94  are Peltier-effect devices which generate heat on one surface and remove heat from the opposite surface when a current is applied. A positive polarity signal from the temperature controller  22  bi-polar amplifier causes the thermoelectric modules  94  to heat the element assembly  74 ; a negative polarity signal from the temperature controller  22  bi-polar amplifier cools the element assembly  74 . The power supplied to the temperature controller  22  bi-polar amplifier (and thus, the power supplied to the thermoelectric modules  94 ) may be limited to avoid damaging the resistor element  32 . Additionally, to extend the life of the thermoelectric modules  94 , a pulsed signal may be used. It should be noted that the CPU of the control system  12  may be used to execute the tuned temperature control algorithm while remaining within the scope of the present invention. It should further be noted that electrical shielding  16  may be used to isolate noise generated by the temperature controller  22  from the measurement terminals  25 . 
     Power supply  20  is any commercially available or custom made power unit. In the current embodiment, a 10 volt/60 hertz commercially available power supply  20 , is used. The power supply  20  is sized to supply all of the necessary power to the digitally controlled resistance standard  10 . It should be noted that multiple power supplies may be used instead of a single power supply while remaining within the scope of the present invention. Furthermore, a power supply operating at a different voltage (e.g., 220 volts) and a different frequency (e.g., 50 hertz) may be used while remaining within the scope of the present invention. 
     One or more displays  24  are used to display information from the digitally controlled resistance standard  10 . The displays  24  may be any commercially available device. For example, liquid crystal displays (LCD), light emitting diode (LED) displays, touch screen displays, etc. may be used while remaining within the scope of the present invention. Displays  24  may be controlled directly by the control system  18  or via separate display driver cards. 
     Although not shown in FIGS. 1 and 2, digitally controlled resistance standard  10  also includes one or more communication ports (for example, serial and parallel ports, etc.), input devices (for example, a keyboard, touchpad, and a mouse, etc), and measurement input devices (for example, thermometers, barometers, etc.). The communication ports are operable to connect the digitally controlled resistance standard  10  to a network, computer system, peripheral devices, and measurement systems, among others. The input devices are operable to accept user commands and data entry, among others. The measurement input devices are operable to measure and input external environmental levels (for example, temperature, pressure, humidity, etc), among others. 
     In the current embodiment, the components of the digitally controlled resistance standard  10  are contained within and supported by chassis  26 . Chassis  26  has front, back, top, bottom, and side panels (for simplicity, not all of which are shown in FIGS.  1  and  2 ). The front panel of the chassis  26  supports the displays  24 , and connection terminals  25 . The rear panel of the chassis  26  has a fused power entry module with universal select switch, redundant connection terminals  25  (for rear panel connection), communication port(s), and a filtered cooling fan exhaust, among others (not shown). Indicator lights, power switches, and element select buttons, among others may also be supported by chassis  26 . The chassis  26  may be constructed for rack mounting, bench top use, and field use, among others 
     In the current embodiment, the digitally controlled resistance standard  10  is specified to operate with no measurable degradation in accuracy between temperatures of 15° C. and 35° C., between 5% and 95% relative humidity, and between pressures of 105.15 and 69.64 kilopascals (i.e., −1000 to 10,000 feet of elevation). 
     FIG. 3 illustrates an element assembly  74  within heating cooling assembly  12  of FIG. 1 according to an embodiment of the present invention. The element assembly  74  is comprised of a housing body  76 , a thermometer well assembly  78  (comprised of a thermometer well  82  and a flange  84  and illustrated in FIG.  5 B), an end cap  80 , the resistor element  32 , and a temperature sensing device (not shown). In the preferred embodiment, electrical lead wires are supplied which connect the temperature sensing device and the resistor elements to the temperature/frequency measuring circuit  14 , among others. The element assembly  74  may include glass to metal seals through which the lead wires pass. The resistor element  32  is hermetically sealed within the housing body  76  by the thermometer well assembly  78  and the end cap  80 . The hermetically sealed housing body  76  is purged of moisture and filled with mineral oil to remove air and to provide improved thermal transfer. It should be noted that the element assembly  74  is of sufficient rigidity to eliminate measurable effects of external changes in barometric pressure changes. 
     FIGS. 4A through 4D are detailed side views of resistor elements  32  for the element assembly of FIG. 3 according to various embodiments of the present invention. FIGS. 4A-1,  4 B- 1 ,  4 C- 1  and  4 D- 1  are detailed end views of the resistor elements  32  shown in FIGS. 4A,  4 B,  4 C, and  4 D, respectively. As shown, each of the resistor elements  32  is mounted on a fixture  52 ,  54 ,  56 ,  58 , respectively, which supports the element  32  without mechanical strain and which provides thermal uniformity. As best seen in FIGS. 4A-1 through  4 D- 1 , each fixture  52 ,  54 ,  56 ,  58  is constructed with a hollow core. An additional support may be inserted into the hollow core and used to secure the resistor element  32  within housing body  76 . For example, as shown in FIG. 3, the thermometer well  82  is inserted into the hollow core for additional support. A temperature sensor (such as a glass encapsulated thermistor selected for high stability, among others) (not shown) is attached to the resistor element  32 . 
     FIGS.  4 A and  4 A- 1  illustrate a resistor element  32  comprised of an Evanohm® strip  38  placed over an insulating sleeve fixture  52 . In the current embodiment, the Evanohm® strip  38  is connected to ends of copper lead wires  50  at a nickel-alloy interface  48 . The other ends of the copper lead wires  50  are connected to the measurement terminals  25  (not shown in FIGS.  4 A and  4 A- 1 ). In the current embodiment, the resistor element  32 , having the Evanohm® strip  38 , is constructed to have a resistance value between 0.001 and 1 ohms. 
     FIGS.  4 B and  4 B- 1  illustrate a resistor element  32  comprised of an Evanohm® ribbon  40  over a mica strip fixture  54 . The Evanohm® ribbon  40  is connected to ends of copper lead wires  50  at a nickel-alloy interface  48 . The other ends of the copper lead wires  50  are connected to the measurement terminals  25  (not shown in FIGS.  4 B and  4 B- 1 ). In the current embodiment, the resistor element  32 , having the Evanohm® ribbon  40 , is constructed to have a resistance value between 1 and 10 ohms. 
     FIGS.  4 C and  4 C- 1  illustrate a resistor element  32  comprised of wire bifilar  42  wound on an insulating hollow cylinder fixture  56  cut with a double-helical groove. The wire bifilar  42  is connected to ends of copper lead wires  50  at a nickel-alloy interface  48 . The other ends of the copper lead wires  50  are connected to the measurement terminals  25  (not shown in FIGS.  4 C and  4 C- 1 ). In the current embodiment, the resistor element  32 , having the bifilar wire  42 , is constructed to have a resistance value between 10 and 100 ohms. 
     FIGS.  4 D and  4 D- 1  illustrate a resistor element  32  comprised of insulated wire  44  wound on an ceramic bobbin fixture  58 . The insulated wire  44  is connected to ends of copper lead wires  50  using nickel-alloy interface solder tabs  46 . The other ends of the copper lead wires  50  are connected to the measurement terminals  25  (not shown in FIGS.  4 D and  4 D- 1 ). In the current embodiment, the resistor element  32 , having the insulated wire  44 , is constructed to have a resistance value between 100 ohms and 10 megohms. 
     During manufacture, the baseline characteristics for each resistor element  32  are measured. For example, a resistor element&#39;s  32  actual resistance value, coefficients of temperature, drift due to age, and frequency response, as well as other factors which contribute to the resistor element&#39;s  32  uncertainty, are measured. These factors are then stored in memory  28 , along with corresponding equations for temperature and frequency response. The digitally controlled resistance standard  10  uses this information to determine a resistance vs. temperature curve (among others) for each resistor element  32 . The digitally controlled resistance standard  10  uses the resistance vs. temperature curve to adjust a resistor element&#39;s  32  actual resistance value to the nominal resistance value. For example, a user may select a nominal value of 1 ohm. The actual value of the resistor element  32 , however, may be 0.95 ohms (i.e., a deviation 0.05 ohms) at a room temperature of 68° F. By raising the resistor element&#39;s  32  temperature to 70° F., the digitally controlled resistance standard  10  changes the resistor element&#39;s  32  resistance value to the nominal resistance (i.e., 1 ohm) and reduces the deviation from 0.05 ohms to zero ohms. It should be noted that additional information can be added to memory  28  by the measurement system or by the user (among others) during testing. 
     In the current embodiment, the digitally controlled resistance standard  10  has multiple resistance ranges that can be selected by the user during testing. A single resistor element  32  with a number of measurement points tapped off of a single piece resistor element  32  and a number of discrete resistor elements  32  having different resistance ranges, among others, may be used in the digitally controlled resistance standard  10  while remaining within the scope of the present invention. 
     In the current embodiment, resistor elements  32  are fabricated to a value close to a desired nominal value (e.g., 1 ohm, 10 ohms, 100 ohms, etc.). The value of the resistor elements  32  are measured at temperatures above and below a reference temperature to determine the resistor elements&#39;  32  unique alpha and beta temperature coefficients. Measurements are also taken at various frequencies to determining the resistor elements&#39;  32  frequency responses. The resistor elements  32  are then processed for stability (e.g., stress relieved) and final adjustments are made. The resistor elements&#39;  32  construction allows the manufacturer to readjust their value at a future time should, for example, a resistor element&#39;s  32  value change beyond desired limits due to age or damage. A series of measurements are then taken over a given time period to establish the drift rate of each resistor element  32 . Uncertainties to all measurements are calculated and recorded during manufacture for each resistor element  32 , for use in the digitally controlled resistance standard&#39;s  10  internal uncertainty calculator. In the current embodiment, user data fields for entering external measurement system uncertainties allow the user to maintain complete system uncertainty on the digitally controlled resistance standard  10 . 
     It should be noted that other materials, construction methods, and configurations may be used to construct a resistor element  32  while remaining within the scope of the present invention. For example, manufacturing steps that are well know in the art, such as heat treating and stress relieving, among others, may be employed while remaining within the scope of the present invention. Additionally, the resistance value of the resistance element  32  may be varied while remaining within the scope of the present invention. 
     FIGS. 5A through 5C illustrate detailed views of the component parts of the element assembly  74  of FIG. 3 according to an embodiment of the present invention. Specifically, FIG. 5A illustrates a detailed view of the housing body  76 , FIG. 5B illustrates a detailed view of the thermometer well assembly  78 , and FIG. 5C illustrates a detailed view of the end cap  80 . 
     Referring now to FIG. 5A, in the current embodiment, the housing body  76  is machined from a single block of nickel plated copper. The first end of the housing body  76  is machined to accept the thermometer well assembly  78  and the second end of the housing body  76  is formed to accept the end cap  80 . It should be noted that the shape of the element assembly  74  may be varied while remaining within the scope of the present invention. 
     In the current embodiment, the thermometer well assembly  78  is comprised of a hollow nickel plated copper well  82  with a proximal end soldered to a nickel plated copper flange  84 . One or more holes are drilled through flange  84  to allow access to the interior of the thermometer well  82  and the interior of the housing body  76  (e.g., for lead wires, etc.). Once the element assembly  74  is assembled, the holes may be hermetically sealed to prevent external environmental factors from interfering with the operation of the resistor element  32 . The well  82  provides access to the interior of the element assembly  74  for temperature sensing device, such as a thermometer (not shown), and may provide support for the resistor element  32  (as shown in FIG. 2) that is contained with the housing  76 . 
     In the current embodiment, end cap  80  is comprised of a threaded bolt having a chamfered opening. The chamfered opening is sized to accept and support the distal end of well  82  when the element assembly  74  is assembled. The threads of end cap  80  are operable to engage a set of threads on the end of housing body  76 . It should be noted that other configurations may be used for end cap  80  (for example, a flange, etc.) while remaining within the scope of the present invention. 
     Referring to FIG. 5A, in the current embodiment length L1 is equal to 6 inches (15.24 cm), L2 is equal to 5.75 inches (14.605 cm), and L3 is equal to 1.25 inches (3.175 cm). Diameter D1 is equal to 0.968 inches (2.45872 cm), D2 is equal to 0.875 inches (2.2225 cm), and D3 is equal to 0.375 inches (0.9525 cm). Referring to FIG. 5B, length L4 is equal to 5.0 inches (12.7 cm) and L5 and L6 are both equal to 0.500 inches (1.27 cm). Diameter D4 is equal to 0.967 inches (2.45618 cm) and D5 is equal to 0.125 inches (0.3175 cm). Referring to FIG. 5C, length L7 is equal to 1.0 inches (2.54 cm), L8 is equal to 0.500 inches (1.27 cm), and diameter D6 is equal to 0.375 inches (0.9525 cm). It should be noted that all dimensions provided for FIGS. 5A through 5C are exemplary and that other dimensions may be used while remaining within the scope of the present invention. 
     FIGS. 6A and 6B illustrate a cut-away view and an end view, respectively, of heating/cooling assembly  12  according to an embodiment of the present invention. In operation, it is desirable to maintain the resistor element  32  at a constant temperature. The heating/cooling assembly  12  is used to achieve and maintain the desired temperature. 
     Heating/cooling assembly  12  is comprised of a shell  90 , heat sinks  92 , thermoelectric modules  94 , fan mount  96 , fan  98 , and insulation  99  among others. In the current embodiment, one or more thermoelectric modules  94  are carried by the element assembly  74 , which is then surrounded by insulation  99 . The thermoelectric modules  94 , the element assembly  74 , and the insulation  99  are surrounded by one or more heat sinks  92 . The heat sinks  92 , thermoelectric modules  94 , the element assembly  74 , and insulation  99  are placed within shell  90 . A fan mount  96  is used to secure a fan  98  to one end of the shell  90 . The fan  98  is operable to push or pull air across the heat sinks  92 . It should be noted that a filter may be used to prevent contaminants from depositing on the heat sinks  92 . 
     In normal operation, the temperature of the resistor element  32  is sensed by a temperature sensing device (such as a glass encapsulated thermistor). In the current embodiment, the thermoelectric modules  94  are Peltier-effect devices which generate heat on one surface and remove heat from the opposite surface when a current is applied (note: reversing the current&#39;s polarity causes the first side to remove heat and the opposite side to generate heat). Thus, if the temperature of the resistor element  32  is too low, current is applied to the thermoelectric modules  94  such that the element assembly  74  is heated, and thus, the temperature of the resistor element  32  increases. If the temperature of the resistor element  32  is too high, current (with the opposite polarity) is applied to the thermoelectric modules  94  such that the element assembly  74  is cooled, and thus, the temperature of the resistor element  32  decreases. Additionally, fan  96  may be activated to help regulate the element assembly  74  temperature, and thus, the resistor element  32  temperature. 
     It should be noted that multiple heating/cooling assemblies  12 , each containing an element assembly  74  having a different magnitude resistance range (for example, 0.01-1 ohm, 1-10 ohm, 10-100 ohm, and 100-1M ohm, etc.), may be used within the digitally controlled resistance standard  10 . Additionally, a single heating/cooling assembly  12  containing a multitude of different magnitude element assemblies  74  or a single element  32  with multiple tap points may be used while remaining within the scope of the present invention. Thus, the user is able to select the desired resistance range needed for testing. 
     FIG. 7 illustrates a resistance/temperature curve  60  for a resistor element  32  according to an embodiment of the present invention. As previous discussed, the actual value of the resistor element  32  is measured during manufacture. The resistor element&#39;s  32  coefficients of temperature, drift due to age, and frequency response are determined, as well as other factors which contribute to the resistor element&#39;s  32  uncertainty. These factors are then stored in the memory  28 , along with corresponding equations for temperature and frequency response. These and other measurements are used to determine a resistor element&#39;s  32  resistance/temperature curve  60 . 
     In the current embodiment, the digitally controlled resistance standard  10  uses the resistance/temperature curve  60  to accurately adjust the resistor element&#39;s  32  actual resistance to a nominal resistance. Referring to FIG. 7 for example, when the resistor element  32  is at 68° F., the resistor element  32  has an actual resistance of 0.95 ohms (i.e., a deviation 0.05 ohms). By raising the resistor element&#39;s  32  temperature to 70° F., the digitally controlled resistance standard  10  changes the resistor element&#39;s  32  resistance value to the nominal resistance (i.e., 1 ohm) and reduces the deviation from 0.05 ohms to zero ohms. 
     FIG. 8 illustrates an operational process  800  for the digitally controlled resistance standard  10  of FIG. 1 according to an embodiment of the present invention. Operational process  800  is initiated by operation  801 . Operation  801  powers up and initiates a self-test sequence for the digitally controlled resistance standard  10 . The self-test sequence may include, for example, testing of CPU functions, memory integrity, and I/O status, as well as other tests typical to microcomputers. After the power is turned on and self-test sequence is completed, operational control is passed to operation  802 . 
     Operation  802  retrieves relevant data stored within the memory  28  of the digitally controlled resistance standard  10 . Data stored in the memory  28  may include manufacturer&#39;s data, secured user data, and last user data, among others. Manufacturer&#39;s data may include, for example, information related to the resistor element  32  (such as the serial number, date of manufacture, uncertainty components, temperature coefficients, frequency response, upper and lower deviation limits, and drift rate of the resistor element  32 , among others) and equations used to control the resistance standard  10  (such as a three-mode PID control equation and temperature coefficient equation, among others). Secured user data may include calibration data, user ID information, nominal value, nominal temperature, and uncertainty data, among others. Last user data may include element selection and deviation selection, among others. 
     Operation  803  assumes control after operation  802  retrieves the relevant data from memory  28 . Operation  803  uses, for example, the results of the self-test sequence completed by operation  801  and the relevant data retrieved by operation  802  and updates the displays  24 . Operation  803  may also calculate and display uncertainty information related to each component and the expanded uncertainty for the system. Operation  803  may also output relevant data to the user via a printer, chart recorder, and communications link, among others. After operation  803  updates the displays  24 , operational control is the passed to operation  804 . 
     Operation  804  alters the temperature of the resistor element  32  until it reaches a desired point. For example in the current embodiment, operation  804  utilizes the temperature/frequency measurement circuit  14  to determine the actual temperature of the resistor element  32 . If the resistor element&#39;s  32  actual temperature deviates from the desired temperature, operation  804  activates the thermoelectric modules  94  to heat or cool the resistor element to the desired level. In addition to controlling the resistor element&#39;s  32  temperature, operation  804  continuously updates the values shown on the displays  24 . 
     After the temperature of the resistor element  32  reaches the desired level, operational process  805  determines whether any user commands have been entered. If user commands are present, operational control branches YES and operation  806  assumes control. Operation  806  retrieves the user commands (for example, the nominal resistance value, element selection, etc.). Operational  807  then updates the status of the user commands in memory  28 . After the status of the user commands are updated, operational control is returned to operation  804 . 
     If user commands are not present, operational control branches NO and operation  808  assumes control. Operation  808  updates the device settings. Device settings include the temperature settings used to achieve a desired resistance value, the calculation of displayed and data output resistance values corrected for the frequency of an applied signal, the age of the resistance element and application of correction to compensate for drift, the application of uncertainty calculations, and the update of display/output of a calculated expanded uncertainty, among others. 
     Operation  809  stores the updated device settings in the memory  28 . After the updated device settings are stored, operation  810  determines whether the actual temperature of the resistor element  32  needs to be adjusted. Control branches YES and operation  804  assumes control if the actual resistance value of the selected resistance element  32  needs to be adjusted to match the desired nominal resistance value. Control branches NO and operation  811  assumes control if the actual resistance value of the selected resistance element  32  does not need to be adjusted. 
     Operation  811  determines whether the testing is complete. If testing is not complete, operational control branches NO and operation  804  assumes control. If testing is complete, operational control branches YES and operation  812  assumes control. Operation  812  powers down the digitally controlled resistance standard  10 . 
     It should be recognized that the above-described embodiments of the invention are intended to be illustrative only. Numerous alternative embodiments may be devised by those skilled in the art without departing from the scope of the following claims. For example, a non-digital resistance standard (e.g., a resistance standard implemented using analog circuitry) may be employed while remaining within the scope of the present invention.