Abstract:
An electronic thermistor-based vacuum gauge and systems and methods of calibration and operation of the same that require no calibration against a known vacuum standard to obtain high accuracy through broad vacuum and ambient temperature ranges. Additional features of the invention include a construction and method of improving battery life, a construction and method of detecting faulty vacuum sensors, a method for determining the state of calibration of a vacuum sensor, a method of quantifying vacuum leak rates, and a method of automatically alerting an operator when an evacuation process has concluded.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a divisional of U.S. patent application Ser. No. 13/052,408, filed on Mar. 21, 2011 (now U.S. Pat. No. 8,504,313), which claims the priority, under 35 U.S.C. §119, of U.S. Provisional Patent Application No. 61/316,292, filed on Mar. 22, 2010, the entire disclosures of which are hereby incorporated herein by reference in their entireties. 
    
    
     FIELD OF THE INVENTION 
     The present invention lies in the field of vacuum gauges for measuring a deep vacuum in industrial, commercial, and portable applications, such as in Heating, Ventilation, and Air Conditioning (“HVAC”) systems. The present disclosure relates specifically to an electronic vacuum gauge that utilizes a temperature-sensing component, such as a thermistor, thermocouple, or a resistance temperature detector (“RTD”) (e.g., platinum wire), which requires no calibration against a known vacuum standard to obtain high accuracy through broad ranges of vacuum and ambient temperatures. 
     BACKGROUND OF THE INVENTION 
     Methods of measuring a deep vacuum have existed for some time. The Mercury McLeod Gauge, invented in 1874 by Herbert G. McLeod, is a form of manometer that utilizes a column of mercury to indicate pressure. While this type of gauge is still in use today, its relatively large size and fragility preclude it from being practical for use in most industrial, commercial, and portable applications. Consequently, electronic vacuum gauge devices have largely replaced these gauges. 
     Electronic vacuum gauges utilize vacuum sensors that are generally of the Pirani, thermocouple, or thermistor type. These gauges operate on the principle that a rate of heat transfer by conduction into a surrounding gas is dependent upon gas pressure. The Pirani gauge, invented by Marcello Pirani in 1906, utilizes a platinum wire heated by an electrical current. As the surrounding gas pressure decreases, the temperature of the wire increases due to the reduction in the heat that is being conducted away from the wire and into the surrounding gas. The resistance of the wire increases with respect to the increasing temperature of the wire. Therefore, the measured resistance of the wire is indicative of the gas pressure of the surrounding gas. 
     The thermocouple type of gauge utilizes a thermocouple thermally connected to a small wire filament to measure the temperature of the filament, which is heated via an electrical current through the filament. An output voltage from the thermocouple is indicative of the filament temperature, which increases as gas pressure decreases. 
     A thermistor-based gauge operates similarly to the Pirani gauge, but utilizes a temperature sensitive resistor (i.e., a thermistor) rather than a platinum wire. The advantage to this configuration is that thermistors generally have a much higher resistance than the platinum wires used in Pirani gauges. Accordingly, thermistors exhibit a greater resistance change versus temperature change, thereby making resistance and, therefore, temperature measurements simpler and more accurate. There are two types of thermistor-based gauges, each sensing heat by a different method. The first type relies on a heating element that is in contact with the thermistor. The second type uses an electric current to heat the thermistor, thereby directly affecting the thermistor&#39;s resistance. 
     Any of the above techniques may utilize a temperature increase to indirectly measure pressure or, alternatively, may adjust power to maintain a particular temperature (or temperature differential with the surrounding gas). In the latter case, the power required to maintain the device&#39;s temperature can be used as an estimate of vacuum pressure, as it is well known in the art that the square of a thermistor&#39;s voltage is indicative of pressure. 
     All of these vacuum sensing techniques are gas-temperature sensitive, where the amount of heat conducted away from the device and into the surrounding gas at any given gas pressure is dependent upon the difference between the temperature of the device and the temperature of the surrounding gas. Therefore, for accuracy across a broad range of ambient (gas) temperatures with these gauges, some form of temperature compensation must be employed. Generally, the sensing device is maintained at a constant differential temperature from the surrounding gas temperature using a secondary temperature-measuring device. Alternatively, the sensing device is maintained at a constant temperature, a secondary temperature-measuring device being used to compute the differential temperature between the vacuum-sensing device and the surrounding gas. The resulting value is used to adjust the vacuum-sensing device&#39;s response to changing pressure. 
     In practice, the response curve of such a vacuum-sensing device is roughly log-linear between the pressures of 1 and 25,000 microns. In this range, conduction of heat to the gas molecules dictates the response curve. When operated at constant temperature, the power dissipated by the device resembles an “S” curve on a log-linear graph. Above approximately the 25,000-microns mark, convective cooling dominates the curve and the response curve rapidly asymptotes to near the atmospheric value. Below approximately the 1-micron mark, thermal conduction through the device&#39;s metallic leads and radiative cooling dominate the response curve, thereby yielding yet another asymptote. Therefore, vacuum sensors based upon the thermal conduction of gas are generally acceptable for use only where the measurements are constrained between the two extremes—i.e., 1 and 25,000 microns. In HVAC service, for example, the approximate range of 10 to 10,000 microns is desirable. 
     Aside from temperature sensitivities, there are other disadvantageous issues with such existing vacuum sensors. First, the power required to maintain the temperature of the sensor at any given pressure not only depends on the ambient temperature, but also depends upon the construction of the sensor, its overall surface area and geometry, the materials used, the presence of any surface contamination, the diameter, length, and conductivity of the lead wire, the size and geometry of the gas cavity, and a number of other unpredictable variables. The sensor, itself, has a specified tolerance based on its manufacture, which means that the resistance of one sensor at any given temperature may be significantly different than that of another at the same temperature, especially in low-cost applications. Therefore, each sensor possesses its own unique response curve with respect to pressure and, as a result, must be individually calibrated against a vacuum reference to achieve any kind of practical accuracy. Because the response curve is only roughly log-linear, a simple two-point calibration is generally not adequate. Instead, many data points need to be calibrated throughout the specified range of the gauge in question, and over a range of temperatures. 
     Calibrating a vacuum gauge is difficult, time-consuming, and expensive. A high quality vacuum system is required, along with leak-proof gas connections. A standards-traceable master gauge must be incorporated into the system, and the pressure must be repeatedly changed and stabilized for each calibration point. Such a system can be automated to limit the amount of human interaction and decrease calibration time, but such a system still requires constant and repeated maintenance and requires a significant amount of capital resources. In addition, no field technician or end-user of the gauge will typically have this type of maintenance equipment. Therefore, such a gauge requiring calibration must be sent back to the factory for recalibration and, depending on the application, recalibration is frequently needed. Even after proper calibration, a production gauge may not operate to its published specifications in the field. This may be due to the user simply not operating the gauge at the same temperature as when it was calibrated. 
     Therefore, a need exists for an electronic vacuum gauge that requires no calibration against a vacuum reference while, at the same time, providing high accuracy across a broad range of ambient temperatures. 
     Many prior-art gauges utilize field-replaceable, per-calibrated sensors so that, in the case of a sensor failure, the sensor may be replaced without the requirement of recalibrating the gauge instrument. This is generally achieved by stamping a calibration code on the exterior of the sensor, which is input into the gauge instrument in some fashion by the operator. This process is an error prone technique and requires the attention of the operator to perform properly. Therefore, a need exists for an electronic vacuum gauge that automatically acquires calibration information from the vacuum sensor without intervention by the operator. 
     The accuracy of a vacuum-sensing device may change through time, either through component value changes or through gradual contamination of the vacuum-sensing device. There is currently no method, save utilizing a second known-to-be-good gauge, for determining that a vacuum-sensing device, or its associated gauge instrument, is operating within its specified accuracy. Therefore, a need exists for a vacuum gauge instrument that can automatically determine if it is operating within its specified accuracy and a method for automatically determining with a vacuum gauge instrument if the instrument is operating within its specified accuracy. 
     As a vacuum sensor is in direct contact with the gas being measured, any contaminants in the gas, such as oil, may contaminate the sensor. This will cause inaccurate vacuum measurements, or will cause the vacuum gauge instrument to cease functioning all together. The vacuum gauge sensor may also become faulty for any of a number of reasons, including physical failure. Therefore, a need exists for a vacuum gauge instrument that can automatically determine if a vacuum sensor is contaminated or faulty due to some other cause and a method for automatically determining with a vacuum gauge if a vacuum sensor associated therewith is contaminated or faulty due to some other cause. 
     Since a vacuum gauge sensor requires significant power to heat the vacuum-sensing device, such vacuum gauges suffer from short battery life, or require the use of mains power for long-term, continuous operation. Therefore, a need exists for a vacuum gauge instrument with a reduced power requirement for the vacuum gauge device to, thereby, increase the overall battery life or to eliminate the need for mains power and an automatic method for reducing the power requirement of a vacuum gauge device to, thereby, increase overall battery life or to eliminate the need for mains power. 
     Evacuation procedures generally require achieving a minimum predetermined pressure, and holding at least that minimum pressure for a predetermined amount of time. Generally, this is performed by an operator with a clock. This requires the continuous attention of the operator until the evacuation process is complete. Therefore, a need exists for a vacuum gauge device with an automatic method of monitoring an evacuation process and signaling an operator when the process has been successfully completed. 
     Yet another evacuation procedure may require the system under evacuation to hold a vacuum for a predetermined amount of time with any increases in pressure being indicative of leaks or moisture in the evacuated system. This is generally preformed by an operator watching the gauge for a period of time and manually computing changes in pressure during that time. Therefore, a need exists for a vacuum gauge instrument that can automatically, and instantly, compute and indicate an accurate leak rate, or the rate of pressure increase versus time. 
     SUMMARY OF THE INVENTION 
     The invention provides an electronic vacuum gauge and systems and methods of calibration and operation of vacuum gauges that overcome the hereinafore-mentioned disadvantages of the heretofore-known devices and methods of this general type and that can automatically, and instantly, compute and indicate an accurate leak rate, or the rate of pressure increase versus time. 
     With the foregoing and other objects in view, there is provided, in accordance with the invention, a method for detecting a vacuum leak rate of a vacuum system, which comprises providing a vacuum-measuring system with a vacuum sensor assembly shaped to sealingly contain a volume of gas of the vacuum system and operable to output at least one electrical signal related to the gas vacuum pressure of the contained volume of gas and an electrical circuit having a microprocessor programmed to compute a gas pressure of the volume of gas contained within the vacuum sensor assembly, an analog-to-digital converter operatively connected to the microprocessor and receiving and converting the at least one electrical signal output of the vacuum sensor assembly into a digital signal to be processed by the microprocessor, and an output device operatively connected to the microprocessor and operable to indicate an output of the microprocessor, containing a volume of gas of the vacuum system in the vacuum sensor assembly, computing a gas pressure of the contained volume of gas using the vacuum-measuring system, computing a rate-of-change of the gas pressure with the vacuum-measuring system one of periodically and continuously over a period of time, and outputting the rate-of-change of the gas pressure using the output device. 
     In accordance with another mode of the invention, the vacuum sensor assembly comprises a sensor housing shaped to form a substantially sealed cavity for containing the volume of gas of the vacuum system, the cavity having a port in fluid communication with the surrounding environment of the vacuum system to allow the volume of gas to enter the cavity from the surrounding environment. 
     In accordance with a further mode of the invention, the vacuum sensor assembly further comprises a first temperature sensing device at least partially exposed to the gas contained in the cavity and a second temperature sensing device thermally coupled to the sensor housing, a temperature of the sensor housing being representative of a temperature of the gas contained in the cavity. 
     In accordance with an added mode of the invention, the electrical circuit further comprises a constant temperature controller operatively connected to the first temperature sensing device and operable to output a signal representative of power dissipated by the first temperature sensing device and at least one circuit element operatively connected to the second temperature sensing device and operable to convert an output of the second temperature sensing device into an output signal representative of the temperature of the second temperature sensing device. 
     In accordance with an additional mode of the invention, wherein the analog-to-digital converter of the electrical circuit is further operatively connected to the constant temperature controller and the at least one circuit element and is operable to convert the output signals of the constant temperature controller and the at least one circuit element into the digital signal to be processed by the microprocessor. 
     In accordance with yet another mode of the invention, the step of containing the volume of gas comprises sealingly and fluidically connected the port to the vacuum system to cause the volume of gas from the vacuum system to enter the cavity. 
     In accordance with yet a further mode of the invention, the output device is at least one of an audible alarm operable to indicate when the rate-of-change of the gas pressure exceeds a pre-determined rate and a display visibly indicating at least one of the rate-of-change of the gas pressure and when the rate-of-change of the gas pressure exceeds a pre-determined rate. 
     With the objects of the invention in view, there is also provided a method for operating a vacuum-measuring system to indicate completion of an evacuation procedure of a vacuum system, which comprises providing a vacuum-measuring system with a vacuum sensor assembly shaped to sealingly contain a volume of gas of the vacuum system and operable to output at least one electrical signal related to the gas vacuum pressure of the contained volume of gas and an electrical circuit having a microprocessor programmed to compute a gas pressure of the volume of gas contained within the vacuum sensor assembly, an analog-to-digital converter operatively connected to the microprocessor and receiving and converting the at least one electrical signal output of the vacuum sensor assembly into a digital signal to be processed by the microprocessor, and an output device operatively connected to the microprocessor and operable to indicate an output of the microprocessor, containing a volume of gas of the vacuum system in the vacuum sensor assembly, computing a gas pressure of the contained volume of gas using the vacuum-measuring system, monitoring the computed gas pressure one of periodically and continuously over a period of time, and if the monitored gas pressure falls below a pre-determined maximum pressure during that period of time, operating a timer for a pre-determined period of time and, upon expiration of the timer, indicating a completion using the output device. 
     With the objects of the invention in view, there is also provided a method for minimizing battery consumption in a vacuum measuring system, which comprises providing a vacuum-measuring system with a vacuum sensor assembly shaped to sealingly contain a volume of gas of the vacuum system and operable to output at least one electrical signal related to the gas vacuum pressure of the contained volume of gas, an electrical circuit having a microprocessor programmed to compute a gas pressure of the volume of gas contained within the vacuum sensor assembly, an analog-to-digital converter operatively connected to the microprocessor and receiving and converting the at least one electrical signal output of the vacuum sensor assembly into a digital signal to be processed by the microprocessor, and an output device operatively connected to the microprocessor and operable to indicate an output of the microprocessor, and a power source connected to the vacuum sensor assembly and to the electrical circuit, containing a volume of gas of the vacuum system in the vacuum sensor assembly, computing a gas pressure of the contained volume of gas using the vacuum-measuring system, and conserving the amount of power dissipated from the power source by carrying out the gas pressure computing step periodically and, if the gas pressure exceeds a pre-determined pressure after a first pre-determined period of time disconnecting the power source from the vacuum sensor assembly, after a second pre-determined period of time, reconnecting and applying power from the power source to the vacuum sensor assembly, carrying out the gas pressure computing step, and if the computed pressure exceeds the pre-determined pressure, repeating the disconnecting step, the reconnecting step, and the gas pressure computing step. 
     Although the invention is illustrated and described herein as embodied in an electronic vacuum gauge and systems and methods of operation, it is, nevertheless, not intended to be limited to the details shown because various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims. Additionally, well-known elements of exemplary embodiments of the invention will not be described in detail or will be omitted so as not to obscure the relevant details of the invention. 
     Additional advantages and other features characteristic of the present invention will be set forth in the detailed description that follows and may be apparent from the detailed description or may be learned by practice of exemplary embodiments of the invention. Still other advantages of the invention may be realized by any of the instrumentalities, methods, or combinations particularly pointed out in the claims. 
     Other features that are considered as characteristic for the invention are set forth in the appended claims. As required, detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention, which can be embodied in various forms. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a basis for the claims and as a representative basis for teaching one of ordinary skill in the art to variously employ the present invention in virtually any appropriately detailed structure. Further, the terms and phrases used herein are not intended to be limiting; but rather, to provide an understandable description of the invention. While the specification concludes with claims defining the features of the invention that are regarded as novel, it is believed that the invention will be better understood from a consideration of the following description in conjunction with the drawing figures, in which like reference numerals are carried forward. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying figures, where like reference numerals refer to identical or functionally similar elements throughout the separate views, which are not true to scale, and which, together with the detailed description below, are incorporated in and form part of the specification, serve to further illustrate various embodiments and to explain various principles and advantages all in accordance with the present invention. Advantages of embodiments of the present invention will be apparent from the following detailed description of the exemplary embodiments thereof, which description should be considered in conjunction with the accompanying drawings in which: 
         FIG. 1  is a side elevational view of a vacuum sensor PCB assembly according to a first exemplary embodiment of the present invention; 
         FIG. 2  is an exploded perspective view of a vacuum sensor assembly according to a first exemplary embodiment of the present invention with the vacuum sensor PCB assembly of  FIG. 1 ; 
         FIG. 3  is a perspective view of the vacuum sensor assembly of  FIG. 2  from a sealed side thereof; 
         FIG. 4  is a perspective view of the vacuum sensor assembly of  FIG. 2  from a gas port side thereof; 
         FIG. 5  is a perspective view of a vacuum sensor according to an exemplary embodiment of the present invention with the vacuum sensor assembly of  FIG. 2 ; 
         FIG. 6  is a perspective view of a vacuum gauge assembly according to an exemplary embodiment of the present invention from a front side of thereof; 
         FIG. 7  is a perspective view of the vacuum gauge assembly of  FIG. 6  from a rear side thereof; 
         FIG. 8  is an exploded perspective view of the vacuum gauge assembly of  FIG. 6 ; 
         FIG. 9  is a block circuit diagram of an electronic circuit of a vacuum gauge assembly according to an exemplary embodiment of the present invention; 
         FIG. 10  is an electronic schematic circuit diagram of a vacuum-sensing thermistor driver of the vacuum gauge assembly according to an exemplary embodiment of the present invention; 
         FIG. 11  is an electronic schematic circuit diagram of a temperature-sensing thermistor driver of the vacuum gauge according to an exemplary embodiment of the present invention; 
         FIG. 12  is a graph depicting a normalized vacuum-sensing thermistor power versus gas pressure in an illustration of the curvilinear response of five vacuum sensors of the present invention; 
         FIG. 13  is a graph depicting a resulting linearization of response curves of the five vacuum sensors of  FIG. 12 ; 
         FIG. 14  is a perspective view of a vacuum sensor PCB assembly according to a second exemplary embodiment of the present invention; 
         FIG. 15  is a perspective view of the vacuum sensor PCB assembly of  FIG. 14  in an inverted position; and 
         FIG. 16  is an exploded perspective view of a vacuum sensor assembly according to a second exemplary embodiment of the present invention with the vacuum sensor PCB assembly of  FIGS. 14 and 15 . 
         FIG. 17  is a graph depicting the power dissipated by an exemplary thermistor-based vacuum-sensing device versus ambient gas temperature at atmospheric pressure according to the exemplary embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As required, detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention, which can be embodied in various forms. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a basis for any claims and as a representative basis for teaching one skilled in the art to variously employ the present invention in virtually any appropriately detailed structure. Further, the terms and phrases used herein are not intended to be limiting; but rather, to provide an understandable description of the invention. While the specification may conclude with claims defining the features of the invention that are regarded as novel, it is believed that the invention will be better understood from a consideration of the following description in conjunction with the drawing figures, in which like reference numerals are carried forward. 
     Alternate embodiments may be devised without departing from the spirit or the scope of the invention. Additionally, well-known elements of exemplary embodiments of the invention will not be described in detail or will be omitted so as not to obscure the relevant details of the invention. 
     Before the present invention is disclosed and described, it is to be understood that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting. The terms “a” or “an,” as used herein, are defined as one or more than one. The term “plurality,” as used herein, is defined as two or more than two. The term “another,” as used herein, is defined as at least a second or more. The terms “including” and/or “having,” as used herein, are defined as comprising (i.e., open language). The term “coupled,” as used herein, is defined as connected, although not necessarily directly, and not necessarily mechanically. 
     Relational terms such as first and second, top and bottom, and the like may be used solely to distinguish one entity or action from another entity or action without necessarily requiring or implying any actual such relationship or order between such entities or actions. The terms “comprises,” “comprising,” or any other variation thereof are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. An element proceeded by “comprises . . . a” does not, without more constraints, preclude the existence of additional identical elements in the process, method, article, or apparatus that comprises the element. 
     As used herein, the term “about” or “approximately” applies to all numeric values, whether or not explicitly indicated. These terms generally refer to a range of numbers that one of skill in the art would consider equivalent to the recited values (i.e., having the same function or result). In many instances these terms may include numbers that are rounded to the nearest significant figure. 
     Herein various embodiments of the present invention are described. In many of the different embodiments, features are similar. Therefore, to avoid redundancy, repetitive description of these similar features may not be made in some circumstances. It shall be understood, however, that description of a first-appearing feature applies to the later described similar feature and each respective description, therefore, is to be incorporated therein without such repetition. 
     Referring now to the figures of the drawings in detail and first, particularly to  FIG. 1  thereof, there is shown an exemplary embodiment of a vacuum sensor assembly of a vacuum gauge assembly according to the present invention. The vacuum gauge assembly is comprised of the vacuum sensor assembly and a vacuum gauge PCB. The vacuum sensor assembly may be comprised of any suitable device that enables gas vacuum pressure sensing. For example, referring now to  FIGS. 1 through 4  of the drawings, there is shown, according to first exemplary embodiment, a vacuum sensor assembly  200  that is comprised of a vacuum sensor PCB assembly  100  and a sensor housing  210 . 
     The vacuum sensor PCB assembly  100 , according to a first exemplary embodiment, is comprised of a PCB  110 , an interconnect or pin header  120 , a temperature-sensing device  140 , a vacuum-sensing temperature device  130 , and a non-volatile memory  150  (see  FIG. 2 ). The interconnect  120  has a plurality of conductive pins  125  (e.g., five in number) for electrically coupling the vacuum sensor assembly  100  to the vacuum gauge PCB as described in detail below. The interconnect  120  may be a standard pin header with appropriate lead lengths and pitch for the component side and the anterior side of the PCB  110 . Some of the pins on the anterior side may be cut to avoid mechanical interference with the vacuum-sensing temperature device  130 . 
     The vacuum-sensing temperature device  130  can be any temperature element that (a) can be exposed to the gas pressure, (b) can be self or externally heated to a constant temperature, and (c) can provide, either directly or indirectly, an indication (either relative or absolute) of the power required to maintain it at a constant temperature. The vacuum-sensing temperature device  130  can be, for example, a thermistor, a platinum RTD, or a thermocouple thermally connected to a heater coil. The vacuum-sensing temperature device  130  is coupled to the interconnect  120  on a reverse side of the vacuum sensor PCB  110  by, for example, soldering or welding. In the exemplary embodiment, the vacuum-sensing temperature device is a thermistor, herein referred to as the vacuum-sensing thermistor. The vacuum-sensing thermistor  130  may be any thermistor that is suitable for the application. For example, for purposes of a low power, fast warm-up, and quick thermal response operation, a thermistor with a low dissipation factor, e.g., approximately 0.1 mW/° C., may be chosen, where, in such a case, the temperature response curve and overall temperature accuracy is not critically important. 
     The temperature-sensing device  140  may be, for example, any device that is suitable for the application, including but not limited to thermistor, thermocouple, RTD, or silicon type temperature sensors. However, the best performance is achieved with a device having a high temperature accuracy (i.e., an overall accuracy of +/−0.1° C.), where, in such a case, the temperature response curve and dissipation factor are not critically important. Alternatively, the temperature sensing device  140  may be chosen such that it has high linearity (or is linearizable) and repeatability, though not necessarily high accuracy. 
     One benefit of the present invention is that, unlike existing temperature-based, vacuum measuring applications, the functions of the vacuum-sensing temperature device  130  and the temperature-sensing device  140  are separated so that the two elements do not need to be “matched,” where they cancel or track each other. Rather, the vacuum sensing element can be chosen such that the vacuum sensing characteristics are emphasized and the temperature sensing element can be chosen such that its temperature sensing characteristics are emphasized. There is no need to match the characteristics of the two. The exemplary embodiment illustrates a vacuum sensor utilizing a high accuracy thermistor as the temperature-sensing device  140 . 
     A non-volatile memory  150  may be mounted upon the vacuum sensor PCB  110 . This memory  150  may be used to store a table or coefficients representing the predetermined vacuum response curve for the particular model of vacuum-sensing thermistor  130  that is used. It may also be used to store a table or coefficients representing the predetermined temperature response curve of the particular model of the temperature-sensing thermistor  140  that is used. In this way, alternative thermistor devices or different manufactures may be used, while maintaining compatibility with existing gauges, without needing to re-program the gauge. In general, the non-volatile memory  150  may be any suitable memory that is capable of read/write access and long-term storage of data. Further, to reduce the number of header pins  125  used, a one-wire memory may be used. 
     The sensor housing  210  may be machined of brass or similar material, and can be plated for durability. A gas-pressure-measuring cavity  212  forms the gas pressure measurement volume. A hole  214  may contain the temperature-sensing thermistor  140 . A suitable fitting  205  may be disposed at the opposite side of the housing  210  to accommodate a fluid connection between the gas pressure measuring volume cavity  212  and a non-illustrated vacuum apparatus through a gas port  216 . For purposes of illustration, fitting  205  is depicted as a ¼-inch flare connection machined on the opposite side of the housing  210 . 
     To assemble the vacuum sensor assembly  200 , the vacuum sensor PCB assembly  100  is inserted into the sensor housing  210 . Epoxy-based, thermally conductive adhesive may be used to create a tight thermal connection between the temperature-sensing thermistor  140  and the hole  214  of the sensor housing  210 . An epoxy-based potting compound  220  may also be used to fill the cavity behind the vacuum sensor PCB assembly to create a tight seal and to prevent vacuum leaks. 
     In  FIGS. 14 ,  15 , and  16 , a second exemplary embodiment of a vacuum sensor PCB assembly  100   a  and vacuum sensor assembly  200   a  are illustrated. A bead support frame  135  is constructed of solid copper bus wire of suitable gauge (e.g., 24 AWG) and is mounted upon the vacuum sensor PCB  110   a . The vacuum-sensing thermistor  130  may then be stretched vertically above the vacuum sensor PCB  110   a , its leads being electrically connected to and mechanically supported by the top of the bead support frame  135  and the vacuum sensor PCB  110   a  via soldering or welding. A groove  136  formed into the bead support frame  135  may assist in proper centering of vacuum-sensing thermistor  130  during construction. Copper traces (not illustrated) etched on the vacuum sensor PCB  110   a  may then make the electrical connections between the bead support frame  135 , the opposite thermistor lead, and the interconnect  125 . This vertical construction allows for the gas pressure measuring cavity  212  and the gas port  216  of the vacuum sensor housing  210   a  to be combined into a single structure, thereby creating a significantly smaller gas measuring volume. The advantage of such a construction is that the gas confined within the smaller volume will maintain a temperature much closer to that of the vacuum sensor housing  210   a  as measured by the temperature-sensing thermistor  140 , thereby increasing the overall accuracy of the resulting vacuum gauge. 
     Referring now to  FIG. 5 , to assemble the vacuum gauge assembly  400 , the completely assembled vacuum sensor assembly  200  or  200   a  is electrically coupled to a vacuum gauge PCB  300  by way of the exposed conductive pins  125  of the interconnect  120 , which correspond to and matingly attach to a connector  310  of the vacuum gauge PCB  300 . 
     Referring to  FIGS. 6 ,  7 , and  8 , the assembly of an exemplary embodiment of the completed portable vacuum gauge instrument  900  is depicted. An input device  912  (e.g., a keypad) having controls and markings for operating the instrument, and an identifying label  914  may be applied to a front shell  910 , which, if desired, can be injection-molded. A display  940  or other suitable indicating device (e.g., an LCD) is installed along with electrically connecting polymeric connectors  945 . The vacuum gauge assembly  400  is secured in place using, for example, screws  918 . In addition, a removable hanger assembly  960  having a hanger  964  may be attached to the device using a nut  916 . A rear shell  920 , which, too, can be injection-molded, is then secured in place using, for example, screws  922  (e.g., four in number). A power source  950  is contained within a battery compartment  924  and secured in place by a battery cover  930 , which may have an identifying label  935 . Any suitable power source  950  may be employed. For example, a 9V battery could be used. It is contemplated that the vacuum gauge instrument  900  may be entirely (or partially) replaced by a computer system, as opposed to being an independent, stand-alone device. 
     For operating the vacuum gauge assembly, shown in  FIG. 9  is an electronic block circuit diagram  600  of the vacuum gauge assembly in accordance with an exemplary embodiment of the present invention. Circuit  600  may be comprised of a vacuum-sensing thermistor driver  610  (for operation of the vacuum-sensing thermistor  130 ), a temperature-sensing thermistor driver  620  (for operation of the temperature-sensing thermistor  140 ), the memory  150  (e.g., non-volatile) of the vacuum sensor PCB assembly  100  or  100   a , a reference voltage generator  640 , an A/D converter  650 , a microprocessor  660 , an input device  912  (e.g., a keypad), a display  940  (e.g., an LCD or other suitable indicating device), a second memory device  670  (e.g., non-volatile) for storing calibration values and operational data, and an indicator  680  (e.g., an audible sounding device). The electronic circuit is powered by power source  950  (see, e.g.,  FIG. 8 ). The driver circuits  610  and  620  may be incorporated into the sensor assembly  200  or  200   a , rather than within the instrument  900 . Alternatively, all the analog components up to and including the A/D converter  650 , may be incorporated inside the vacuum sensor assembly  200  or  200   a , thereby creating a field-replaceable, interchangeable, vacuum sensor assembly. 
     As shown in  FIG. 10 , the vacuum-sensing thermistor driver  610  is comprised of an operational amplifier  500 , fixed resistors R 1 , R 2 , R 3 , and R 4 , a capacitor C 1 , and a transistor Q 1 . Driver  610  is operatively connected to the vacuum-sensing thermistor  130  by, for example, the pin header or interconnect  120  and the mating connector  310  of the vacuum gauge PCB  300 . The resistance of the vacuum-sensing thermistor  130  is temperature sensitive, its value being designated as R tv . For high accuracy applications, resistors R 1 , R 2 , and R 3  may be of high tolerance (e.g., 0.1%) and the operational amplifier  500  may be of a low input offset voltage type (e.g., chopper or otherwise stabilized). 
     In operation, the operational amplifier  500  drives the base of the transistor Q 1 , which is connected as a voltage follower component. The voltage output of the transistor&#39;s emitter, designated as V ptv , is applied to the two voltage dividers, R 1  and R tv , and R 2  and R 3 . Transistor Q 1  may be eliminated if the operational amplifier&#39;s output power is capable of driving the two dividers directly. The voltage output nodes of each divider are connected to the non-inverting and inverting inputs of the operational amplifier. This creates a feedback loop that forces the voltage V ptv  to have a value such that the ratios R 1 :R tv  and R 3 :R 2  are equal, resulting in the following equation:
 
 R   tv =( R   1   ·R   2 )/ R   3 .
 
     At this resistance of R tv , the vacuum-sensing thermistor  130  will be operating at a fixed, though unknown, temperature T tv  within a certain range according to its temperature curve specification. Resistor R 4  and capacitor C 1  form a low pass filter to stabilize the feedback loop and prevent oscillation. 
     From this, the value of R tv  may be chosen based on the desired thermistor operating temperature according to the manufacturer&#39;s resistance curve versus temperature specifications. For example, an R tv  of 87.5Ω of one manufacturer may result in a fixed temperature within the range of 100° C. to 135° C. It follows that R 1  is computed and chosen such that V ptv  is less than the V in,max  of the A/D converter  650  under the maximum differential temperature T D  (T tv,max −T o,min ) and maximum of the dissipation factor DF of the vacuum-sensing thermistor  130 . Further, it follows that R 2  and R 3  are chosen such that R 2 /R 3 =R tv /R 1 . Finally, R 4  and C 1  are experimentally chosen based on stability and quickness of the response. 
     When operating at a steady-state, the power dissipated P tv  by the vacuum-sensing thermistor  130  can be accurately computed by the following equation:
 
 P   tv   =V   ptv   2   /R   EQ ,
 
wherein:
 
 R   EQ   =R   1 ·( R   2   +R   3 ) 2 /( R   2   ·R   3 ).
 
As such, the tolerance of the vacuum-sensing thermistor has no bearing on the accuracy of the power computation.
 
     The power dissipated P tv  by the vacuum-sensing thermistor  130  is comprised of three components: (1) power conducted away from the thermistor by the surrounding gas molecules via convection and/or conduction; (2) power conducted away from the thermistor by the thermistor&#39;s own electrical connections; and (3) power radiated away from the thermistor&#39;s surface. The power dissipated by the surrounding gas molecules is the dominant source of power dissipation and is the quantity used to measure the gas pressure. The remaining sources of power dissipation are parasitic, but can be normalized away according to the present invention. 
     In addition, an enable pin EN of the operational amplifier provides a way for facilitating disconnection of power to the driver circuit, thus allowing implementation of a method for conserving battery power under some conditions according to an exemplary embodiment of the invention. 
     As shown in  FIG. 11 , an exemplary embodiment of the temperature-sensing thermistor driver  620  is comprised of a voltage divider having, for example, a fixed resistor R 5  and the temperature-sensing thermistor  140 . R 5  may be chosen such that V tt  provides the most accurate temperature derivation within the operating temperature range. This may be determined, for example, through a spreadsheet that maps V tt  versus temperature using R tt  as specified by the manufacturer. Driver  620  is operatively connected to the temperature-sensing thermistor  140  by, for example, the pin header  120  and the mating connector  310  on the vacuum gauge PCB  300 . The resistance of the temperature-sensing thermistor  140  is temperature sensitive, its value being designated as R tt . A reference voltage powers the voltage divider, and the voltage output of the divider, V tt , is easily converted into absolute temperature using a curvilinear equation or lookup table as provided by the manufacturer of the temperature-sensing thermistor. The reference voltage generator  640  provides the reference voltage. For high accuracy applications, resistor R 5  may be of high tolerance (e.g., 0.1%) and the reference voltage  650  may be of high quality (i.e., low drift, low noise). 
     The voltage outputs V ptv  and V tt  of the driver circuits  610  and  620  and the reference voltage generated by the reference voltage generator  640  are operatively connected to the A/D converter  650 . The A/D converter  650 , the non-volatile memory device  150 , the keypad  912 , the LCD  940  (or suitable indicating display), the non-volatile memory device  670 , and the audible sounding device  680  are each operatively connected to the CPU  660 . The A/D converter  650  may be any suitable analog-to-digital converter that is capable of converting analog voltages, V ptv  and V tt , into corresponding digital signals that can be processed by the CPU  660 . A high-resolution device will facilitate greater overall accuracy and ultimate computational resolution, though lower resolution A/D converters may be used to reduce the overall cost at the expense of diminished accuracy and/or resolution. 
     The keypad  912  may, for example, be a tactile laminated keypad, a set of pushbuttons, a capacitive touch pad, or any suitable device for receiving input from an operator. Signals from the keypad  912  may be operatively connected to and processed by the CPU  660  or processed by a dedicated keypad driver operatively connected to the CPU  660 . 
     The LCD  940  or suitable indicating device may be any device capable of displaying vacuum pressure data to an operator. The LCD  940  or suitable indicating device may be driven directly from the CPU  660  or through a dedicated discrete display driver operatively connected to the CPU  660 . 
     The audible sounding device  680  may be a loud speaker or any device capable of alerting an operator to particular conditions relating to, for example, leakage rate, as explained in detail below. The audible sounding device  680  may be directly driven by the CPU  660  or driven by an audio controller operatively connected to the CPU  660 . 
     The second memory device  670  may be any memory device (e.g., non-volatile) that is capable of storing calibration values and operational data. The memory  670  may be a discrete device or integrated into the CPU  660 . 
     The CPU  660  may be any suitable microprocessor, microcontroller, digital signal processor (DSP), distributed computing system, or other computer capable of capturing and processing the data from the A/D converter  650 , receiving input from the keypad  912 , and displaying data on the LCD  940 . 
     For any given vacuum-sensing thermistor  130 , there is a great deal of variance in its properties, even among devices of the same part number from the same manufacturer and within the same lot. In addition, variations during manufacturing of the vacuum sensor assembly  200  and the materials used will affect the thermal response of the vacuum-sensing thermistor  130  in unpredictable ways. Finally, component tolerances of the vacuum-sensing thermistor driver  610 , the voltage reference generator  640 , and the A/D converter  650  will add yet additional errors with respect to the computation of actual power dissipated by the thermistor  130 . A method of normalizing these sources of variations is important to achieve a vacuum gauge instrument of high accuracy, an example of which is provided by the invention as described below. 
     The power dissipated P tva  by a thermistor having a dissipation factor DF in free air of ambient temperature T A  at atmospheric pressure is approximated by the following equation:
 
 P   tva   =DF ·( T   tv   −T   A )
 
where, as mentioned earlier, T tv  is the operating temperature of the thermistor. However, this approximation is not satisfactory for high-accuracy, high-resolution, vacuum-measurement applications. It has been discovered that the response curve of the thermistor is more accurately reflected by the following second order equation (1):
 
 P   tva   =DF ′·( T   tv   −T   A ) 2   +DF ·( T   tv   −T   A ).  (1)
 
Equation (1) governs the power dissipated within a few hundredths of a percent over an ambient temperature range of less than 0° C. to near T tv , which is quite satisfactory for vacuum measurement applications. Equations of additional orders may be used, but there are rapidly diminishing improvements in accuracy and rapidly increasing computation complexities for each additional order.  FIG. 17  illustrates a power vs. temperature response curve of an actual vacuum sensing thermistor operating at atmospheric pressure in a varying ambient gas temperature from 0° C. through its constant operating temperature, i.e., T tv , of approximately 130° C.
 
     As there is a great deal of variation from part-to-part in the constant operating temperature T tv  of the thermistor (its temperature at constant R tv ) and in its dissipation factors DF and DF′, these are unknown values and must be determined to fully quantify the thermistor&#39;s characteristics. As the unknown T tv  is held constant by the vacuum-sensing thermistor driver  610  (by holding R tv  known and constant), the second order equation (1) above can be restated as the following second order polynomial equation (2):
 
 P   tva   =A·T   A   2   +B·T   A   +C   (2)
 
wherein:
 
 DF′=A;  
 
 DF =√( B   2 −4 ·A·C ); and
 
 T   tv =( B +√( B   2 −4 ·A·C ))/((−2)· A ).
 
     Therefore, once A, B, and C are determined, the thermistor characteristics of DF′, DF, and T tv  can be fully quantified. As equation (2) is a standard second-order polynomial equation, a solution for the coefficients A, B, and C can be realized through well-known statistical techniques. Specifically, by varying T A  (an independent variable) in increments from high-to-low or low-to-high and over a broad range, and by measuring P tva  (a dependent variable) at various points within that range, a polynomial regression computation can quickly and efficiently deduce the values of A, B, and C. When generating the data points for the statistical derivation of A, B, and C, it does not matter whether the temperature is increasing, decreasing, or fluctuating randomly as long as enough data points are acquired over time and representing a sufficient portion of the desired operating temperature range. For example, applying power vs. temperature data from a temperature of 0° C. to 20° C. with 0.1° C. increments acceptably computes a satisfactory A, B and C for an operating temperature range of −15° C. to 50° C. 
     Once the coefficients A, B, and C are computed to within an acceptable percentage of error, the thermistor&#39;s power at any gas pressure and any ambient temperature T A  can be normalized, for example, in the following manner. First, the ambient temperature T A  is measured from the temperature-sensing thermistor driver  620 . Second, the power dissipated P tv  by the vacuum-sensing thermistor is computed from the vacuum-sensing thermistor driver  610 . Third, the expected power dissipation P E  at atmospheric pressure and temperature T A  is computed using equations (1) or (2), where P E  is substituted for P tva . Lastly, the normalized power dissipation P N  is computed by the following equation:
 
 P   N   =P   tv   /P   E .  (3)
 
     At atmospheric pressure, the value of P N  will be close to 1.000. In other words, the resulting value P N  should be within a few hundredths of a percent of 1.000 at any given ambient temperature within the specified range, regardless of the style, construction, or variations of the vacuum-sensing thermistor, the variations in the vacuum sensor construction, or the component tolerances of the electronic circuits involved in the measurements. Accordingly, all sources of error are normalized away at atmospheric pressure. 
     As the gas pressure inside the vacuum sensor is reduced, fewer gas molecules are available to pull heat from the vacuum-sensing thermistor, thereby causing a reduction in P tv  and a corresponding reduction in the calculated value of P N . Therefore, under vacuum conditions, P N  will always be a value less than 1.000. 
     Under vacuum conditions between 1 and 25,000 microns, the curve of P N  versus gas pressure is roughly log-linear. Deduction of the shape of the curve is important for accurately computing gas pressure at any given P N . At the higher range, near 25,000 microns, the curve asymptotes near a P N  value of 1.000 due to convective conduction effects of the gas. At the lower range, near 1 micron, conductive effects of the heat through the vacuum-sensing thermistor&#39;s electrical connections and radiative effects of heat from the thermistor&#39;s surface cause yet another asymptote at a P N  value of above 0.000 (typically, about 0.120). It has been discovered that these parasitic effects are also largely normalized out by the normalization process of equation (3), resulting in an accurately quantifiable curve for a given manufacturer and vacuum thermistor part number. 
     Depicted in  FIG. 12  is the normalized vacuum-sensing thermistor power P N  plotted against the base-2 logarithm of gas pressure, log 2 (P), in an illustration of the curvilinear response of five vacuum sensors of the present invention. A curve mapping P N  to log b (P) can be expressed as a sixth-order polynomial equation (4) as follows:
 
 P   lc =log b ( P )= C   6   ·P   N   6   +C   5   ·P   N   5   +C   4   ·P   N   4   +C   3   ·P   N   3   +C   2   ·P   N   2   +C   1   ·P   N   +C   0   (4)
 
where P is the actual gas pressure, P lc  is the logarithm base b of the computed gas pressure, P N  is the computed normalized power from equation (3), and the coefficients, C 0 -C 6 , are pre-determined constants for a particular manufacturer and thermistor part number. The base of the logarithm, b, can be any suitable value to facilitate computation, such as “10,” “e,” or “2,” with the corresponding coefficients adjusted for the particular base value used. The coefficients are generated using statistical analysis of a statistically significant sample of vacuum sensor assemblies  200  and  200   a  constructed with a particular vacuum-sensing thermistor model. The coefficient values may be stored permanently in a non-volatile memory device  150  of the vacuum sensor PCB assembly  100 . As a result, alternative thermistors with different sets of coefficients may be used in production while still maintaining the accuracy of the pressure calculations for that particular sensor. Alternatively, these values may be stored permanently in the non-volatile memory device  670  of the vacuum gauge instrument  900 .
 
     From equation (4), the computed pressure P c  is computed as follows:
 
 P   c   =b   Plc   (5)
 
The resulting computed pressure P c  may be constrained from about 0 to about 25,000 microns for the computation to be valid.  FIG. 13  illustrates the calculated pressure versus the actual pressure of the five vacuum sensors of  FIG. 12  using the pre-determined coefficients C 0 -C 6  of the sensors. In all cases, each pressure computation is within +/−6% of the actual measured pressure, with the exception of a few outliers due to experimental error. In no case does the error exceed 10%. It has been discovered that, through the application of rigorous manufacturing processes, the overall accuracy can be maintained better than ±5%±5 microns from 0 to 25,000 microns on a mass-production basis.
 
     It is contemplated that, within the scope of the present invention, yet another function, log b (P)=f(P N ), or even another function P=f(P N ) (eliminating the exponential calculation), may be deduced. Also, another function, a 5th order polynomial of the form:
 
log b ( P+O )= C   5   ·P   N   5   +C   4   +P   N   4   +C   3   +P   N   3   +C   2   ·P   N   2   +C   1   ·P   N   +C   0 ,  (6)
 
actually provides superior low-pressure accuracy while maintaining high-pressure accuracy and requires significantly fewer CPU operations to perform. In this case, the pressure offset O is additionally computed and stored with the C 0 -C 5  coefficients.
 
     Accordingly, a beneficial consequence of the normalization method described above is a method of accurately calibrating the vacuum gauge without the need to calibrate under vacuum conditions against a known vacuum standard. In accordance with an exemplary embodiment of the present invention, this calibration method is comprised of the following steps:
         (a.i) exposing the vacuum sensor to atmospheric pressure;   (a.ii) bringing the vacuum sensor to a predetermined low temperature;   (a.iii) raising the temperature of the vacuum sensor while periodically computing the vacuum-sensing thermistor&#39;s power using the output signal of the constant temperature controller and the temperature-sensing thermistor&#39;s temperature using the output signal of the voltage divider until a predetermined high temperature is achieved;   (a.iv) applying each power/temperature data set from step (iii) to a polynomial regression algorithm to compute the coefficients A, B, and C of the polynomial equation (2) such that the power computed by the equation versus temperature is predictive of the power dissipated by the first thermistor at atmospheric pressure at any temperature within a predetermined operative temperature range of the gauge as reported by the second thermistor; and   (a.v) storing the computed polynomial coefficients A, B, and C and/or the derived coefficients DF′, DF, and T tv  in non-volatile memory for future use in computing the actual pressure of the gas contained in the sensor housing.       

     At any time after performing this calibration method, the vacuum gauge instrument  900  may be re-calibrated in the field by repeating steps (i) through (v) above. Indeed, in an exemplary embodiment, the CPU  660  of the vacuum gauge instrument  900  may be preprogrammed to automate the above sequence. 
     As indicated previously, equation (4) with its associated coefficients C 0 -C 6  normalize out nearly all sources of error, providing an accurate method for determining actual gas pressure across a broad range of temperatures within about +/−5%. For even greater accuracy, it is possible to generate additional sets of coefficients C 0 -C 6  at different temperatures, resulting in multiple curves that can be used to interpolate (or extrapolate) even more accurate pressure computations. 
     After completing the calibration steps of the vacuum gauge instrument  900  as described above, the vacuum gauge instrument  900  may be operated to compute the actual pressure of the gas contained in the sensor housing by the following steps according to an exemplary embodiment of the present invention:
         (a.i) exposing the vacuum sensor to the gas pressure to be computed by connecting the gauge to the gas of the vacuum system through, for example, the fitting  205 ;   (a.ii) computing the vacuum-sensing thermistor&#39;s power using the output signal of the constant temperature controller and the temperature-sensing thermistor&#39;s temperature using the output signal of the voltage divider;   (a.iii) applying the second thermistor&#39;s temperature to the polynomial equation (2) with the coefficients A, B, and C, or to the equation (3) with the derived coefficients DF′, DF, and T tv , generated during the calibration step to compute the expected first thermistor&#39;s power at atmospheric pressure and at the second thermistor&#39;s temperature;   (a.iv) normalizing the computed first thermistor&#39;s power at the current gas pressure by dividing by the computed first thermistor&#39;s power at atmospheric pressure and at the second thermistor&#39;s temperature; and   (a.v) computing the gas pressure by applying the normalized first thermistor&#39;s power to a predetermined curvilinear equation or lookup table, i.e. equations (4) and (5).       

     Alternatively, a set of correction factors versus T D  (for each set of coefficients, T D  is a corresponding temperature differential that is equal to the difference between T tv  and T A ) for a specific vacuum-sensing thermistor  610  part number may be determined and stored in non-volatile memory devices  150  or  670  that can be used to correct the computations of equations (3), (4), and/or (5) for varying values of T D . Where correction factors are taken into consideration, the operation of the vacuum gauge would involve the following additional steps in accordance with an exemplary embodiment of the present invention:
         (i) applying the coefficients computed in the calibration step to a polynomial transformation to compute the actual operating temperature of the vacuum-sensing thermistor;   (ii) computing the temperature difference between the vacuum-sensing thermistor and temperature-sensing thermistor by subtracting the temperature-sensing thermistor&#39;s temperature from the computed actual operating temperature of the first thermistor;   (iii) computing a correction factor by applying the temperature difference to a predetermined curvilinear equation or lookup table; and   (iv) applying the correction factor to any one of:
           the computed normalized first thermistor&#39;s power; the value proportional to the logarithm of the computed gas pressure; or   the computed gas pressure.   
               

     In accordance with another exemplary embodiment of the present invention, the enable (EN) control of the operational amplifier of the vacuum-sensing thermistor circuit  610  provides a method for elongating the battery life of the vacuum gauge  900 . While operating at pressures above 25,000 microns, the vacuum sensor provides no useful information to the operator (the normalized power is close to 1.000), and the power dissipation by the vacuum-sensing thermistor  130  is at a maximum under these conditions. The EN control of the amplifier may be operatively connected to the CPU  660 . When disabled, the output of the amplifier is set to high impedance, turning off the base of transistor Q 1 , thereby disconnecting current flow into the vacuum-sensing thermistor  130 . A power-saving feature, therefore, may be implemented where the CPU operates utilizing two modes, i.e., “active” and “sleep” modes. The periods of “sleep” between “active” modes may be 30 seconds or more, significantly extending the life of the battery by up to a factor of 10 at a pressure of 25,000 microns and above. 
     Because it has been determined that the value of P N  will always be 1.000 or less for all gas pressures from deep vacuum to atmosphere, the present invention also provides a method for detecting a faulty vacuum sensor based on the knowledge that values of P N &gt;1.000 indicate a faulty vacuum sensor. Generally, the causes for a fault are disconnection or destruction of the vacuum-sensing thermistor  130 , by contamination with oil or other fluids that greatly increase the power required to maintain constant temperature T tv , and/or a failure to maintain the constant temperature T tv  due to excessive contamination. As the vacuum-sensing thermistor  130  and the temperature-sensing thermistor  140  are electrically connected to the vacuum gauge PCB assembly  400  by interconnect  120  and mating connector  310  in the exemplary embodiment, an analysis of the computed value P N , the voltage V tt  of the temperature thermistor driving circuit  620 , and the voltage V ptv  of the vacuum-sensing thermistor driver  610  yields at least the following possible conclusions:
         1. P N ≦1.000 and V tt  is not saturated (max value): vacuum gauge sensor is operating normally;   2. P N &gt;1.000 and V tt  is not saturated: oil or contamination in sensor or vacuum-sensing thermistor  130  is faulty;   3. P N &gt;1.000 and V ptv  is saturated: heavy oil or contamination in sensor; or   4. V tt  is saturated: sensor disconnected or temperature-sensing thermistor  140  is faulty.
 
The state of the sensor may then be reported to the operator through the LCD  940  or suitable indicating display and/or through an audible alarm by the audible sounding device  680 .
       

     Additionally, the value P N  may be analyzed during periods when the gas pressure within the vacuum sensor is known to be at atmospheric pressure, thereby determining whether or not a calibration sequence is required. If P N  is not sufficiently close to a value of 1.000, i.e. different by 0.05% to 0.1% or greater, under the condition of atmospheric pressure, the vacuum gauge instrument may alert the operator that such a calibration is required, for example, through an appropriate indication on the LCD  940  and/or an alarm from the audible sounding device  680 . 
     In an additional aspect of this invention, it is useful to provide information to an operator regarding the possible presence and size of a vacuum leak. The CPU  660  may make successive gas vacuum measurements and compute a vacuum leak rate in, for example, units of microns per second. This leak rate may be indicated, for example, on the LCD  940  or suitable indicating display, and an audible alarm may sound from the audible sounding device  680  if the leak rate exceeds a specified maximum, as could be pre-programmed and/or selectively programmed by the operator. 
     In yet another exemplary aspect of this invention, an evacuation procedure may specify the need to reduce the gas pressure to a particular level and hold the pressure at that level or less for a minimum period of time. In this example, the CPU  660  may continuously monitor the gas pressure and start a timer when a certain pressure is achieved. At the expiration of a proper amount of time, evacuation success may be indicated, for example, on the LCD  940  or suitable indicating display and/or an audible alarm may sound from the audible sounding device  680 . The pressure threshold and time period may be pre-programmed and/or selectively programmed by the operator. 
     The foregoing description and accompanying drawings illustrate the principles, preferred embodiments and modes of operation of the invention. However, the invention should not be construed as being limited to the particular embodiments discussed above. Additional variations of the embodiments discussed above will be appreciated by those skilled in the art and the above-described embodiments should be regarded as illustrative rather than restrictive. Accordingly, it should be appreciated that variations to those embodiments can be made by those skilled in the art without departing from the scope of the invention as defined by the following claims.