Abstract:
A system and method for collecting data relating to emissions from an emissions source is disclosed. The system comprises an accumulator adapted to receive emissions from the emissions source, a sensor in flow communication with an outlet of the accumulator for generating a signal indicative of a physical property of the emissions, and a sensor interface circuit receiving the signal and generating data relating to the emissions from the emissions source. 
     A system and method for reducing emissions from an emissions source is also disclosed, including a microcontroller receiving data relating to the emissions from the emissions source and generating control signals for reducing the emissions.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of the filing date of provisional application Ser. No. 60/065,349, filed Nov. 12, 1997, entitled “Fugitive Emission Sensing System,” for all subject matter disclosed in the provisional application. 
     This application is related to copending application Ser. No. 08/968,081, filed Nov. 12, 1997, entitled “High Frequency Measuring Circuit,” copending application Ser. No. 08/968,545, filed Nov. 12, 1997, entitled “Sample Retrieval System,” and copending application Ser. No. 08/967,870, filed Nov. 12, 1997, entitled “Thermally Activated Calibration System for Chemical Sensors,” all commonly assigned with the present invention. 
    
    
     BACKGROUND OF THE INVENTION 
     A. Field of the Invention 
     The present invention relates generally to systems for monitoring environmental contaminants and, more particularly, to systems for measuring fugitive emissions from process equipment. 
     B. Description of the Related Art 
     Industrial plants that handle volatile organic compounds (VOCs) typically experience unwanted emissions of those compounds into the atmosphere from point sources such as smokestacks and non-point sources such as valves, pumps, and fittings installed in pipes and vessels containing the VOCs. Emissions from non-point sources, referred to as “fugitive” emissions, typically occur due to leakage of the VOCs from joints and seals. Fugitive emissions from control valves may occur as leakage through the packing between the valve stem and body/bonnet of the valve. Valves employed in demanding service conditions involving frequent movement of the valve stem and large temperature fluctuations typically suffer accelerated deterioration of the valve stem packing, resulting in greater fugitive emissions than valves in less demanding service. 
     While improvements in valve stem packing materials and designs have reduced fugitive emissions and lengthened the life of valve packing, the monitoring of fugitive emissions has become important as a means to identify and reduce fugitive emissions and comply with new more stringent regulation of emissions. The Environmental Protection Agency (EPA) has promulgated regulations specifying the maximum permitted leakage of certain hazardous air pollutants from control valves, and requiring periodic surveys of emissions from control valves. 
     Current methods of monitoring fugitive emissions involve manual procedures using a portable organic vapor analyzer. This manual method is time consuming and expensive to perform, and can also yield inaccurate results due to ineffective collection of the fugitive emissions from the equipment being monitored. If measurements are made on a valve exposed to wind, emissions from the valve may be dissipated before the vapor analyzer can properly measure the concentration of the emissions. Also, if the analyzer is not carefully moved around the valve to capture all the emissions from the valve, an inaccurate measurement will result. Manual measurement methods also require plant personnel to dedicate a significant amount of time to making the measurements, distracting from their other duties. 
     Automated monitoring and detection of fugitive emissions can yield significant advantages over existing manual methods. The EPA regulations require surveys of fugitive emissions at periodic intervals. The length of the survey interval may be monthly, quarterly, semi-annual, or annual; the required surveys becoming less frequent if the facility operator can document fewer than a certain percentage of control valves with excessive leakage. Thus, achieving a low percentage of leaking valves reduces the number of surveys required per year. In a large industrial facility where the total number of survey points can range from 50,000 to 200,000 points, this can result in large cost savings. By installing automated fugitive emission sensing systems onto valves subject to the most demanding service conditions and thus most likely to develop leaks, compliance with the EPA regulations can be more readily achieved for the entire facility. This results in longer intervals between surveys for all of the valves, significantly reducing the time and expense of taking measurements manually from the valves without automated sensing systems. 
     Early detection of fugitive emissions from leaking valves also enables repairs to be made on a more timely basis, reducing the quantity of hazardous material emitted and reducing the cost of lost material. Accurate sensing of fugitive emissions provides an early warning system which can alert the facility operator to a potential valve seal failure and enable preventive measures to be taken before excessive leakage occurs. 
     However, employing an automated fugitive emission sensing system in an industrial environment requires designing a sample retrieval system which can efficiently collect fugitive emissions emanating from a piece of equipment and transport the emissions to gas sensors. The sample retrieval system must be capable of delivering a sample stream at a known flow rate in order to permit the gas sensors to make accurate and consistent measurements of the concentration of fugitive emissions. 
     Furthermnore, employing gas sensors in an industrial environment requires designing sensors that perform satisfactorily in the presence of high relative humidity (up to 85%) through a broad temperature range (from −40° C. to +85° C.). The sensors must be able to discriminate between the emissions of interest and other environmental contaminants, while retaining sufficient sensitivity to detect low concentrations of the fugitive emissions. Provision also must be made to enable periodic calibration of the gas sensors. The output signals from the fugitive emission sensing system must be suitable for input into plant monitoring and control systems typically found in process plants. This will permit simple and inexpensive integration of the sensing system into existing plant process control systems. 
     The fugitive emission sensing system must be inexpensive to manufacture, and use a power source that is readily available in a typical process plant, in order to keep installation costs to a minimum. The system must be suitable for use in hazardous areas subject to a risk of explosion, requiring electrical equipment to be of intrinsically safe or explosion-proof design. It also must be able to operate in harsh environments, including areas subject to spray washing, high humidity, high and low temperatures, and vibration. The system also must be simple and reliable, in order to keep maintenance costs to a minimum. 
     Accordingly, it is an object of the present invention to provide an apparatus and method that addresses the concerns set forth above. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the present invention, a system for collecting data relating to emissions from an emissions source comprises an accumulator adapted to receive emissions from the emissions source, a sensor in flow communication with an outlet of the accumulator for generating a signal indicative of a physical property of the emissions, and a sensor interface circuit receiving the signal and generating data relating to the emissions from the emissions source. In a particular embodiment, the accumulator comprises a collecting tube, and in another embodiment, the accumulator comprises a bonnet capsule. 
     In accordance with another aspect of the invention, the system includes an ejector in flow communication with the outlet of the accumulator. The ejector draws the emissions from the accumulator to expose the sensor to the emissions. The ejector may be connected to a source of pressurized fluid so that the pressurized fluid flows through the ejector thereby creating a pressure drop to draw the emissions from the accumulator into the ejector. 
     In accordance with another aspect of the invention, the system includes a sensor calibrator in flow communication with the at least one sensor for storing a calibrant and exposing the at least one sensor to the calibrant. 
     In accordance with another aspect of the invention, the system provides that data generated by the sensor interface circuit is derived by measuring the frequency of said signal generated by the sensor. 
     In accordance with another aspect of the invention, the system includes a microcontroller adapted to receive the data from the sensor interface circuit, and a memory connected to the microcontroller for storing data from the sensor interface circuit where the data is derived from the at least one sensor&#39;s response to the calibrant. 
     In accordance with another aspect of the invention, a system for reducing emissions from an emissions source comprises an accumulator adapted to receive emissions from the emissions source, a sensor in flow communication with an outlet of the accumulator for generating a signal indicative of a physical property of the emissions, a sensor interface circuit receiving the signal for generating data relating to the emissions from the emissions source, and a microcontroller receiving the data for generating control signals for reducing emissions from the emissions source. 
     In accordance with another aspect of the invention, a method for collecting data relating to emissions from an emissions source comprises collecting at least a portion of the emissions, exposing at least one sensor to the emissions to generate a signal indicative of a physical property of the emissions, and processing the signal generated by the at least one sensor to generate data relating to the emissions from the emissions source. 
     In accordance with another aspect of the invention, a method for reducing emissions from an emissions source comprises situating an accumulator adjacent the emissions source to receive the emissions, providing at least one sensor in flow communication with the accumulator, exposing the at least one sensor to the emissions to generate a signal indicative of a physical property of the emissions, and processing the signal generated by the at least one sensor to generate control signals for controlling plant conditions to reduce the emissions from the emissions source. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features and advantages of the present invention will be best appreciated upon reference to the following detailed description and the accompanying drawings, in which: 
     FIG. 1 is a block diagram of an illustrative embodiment of the invention showing the major components of a fugitive emission sensing system. 
     FIG. 2 is a diagram of a sample retrieval system according to an embodiment of the present invention. 
     FIG. 3A is a perspective view of a collecting tube in accordance with an embodiment of the invention. 
     FIG. 3B is a sectional view showing details of a bonnet capsule in accordance with another embodiment of the present invention. 
     FIG. 4 is a sectional view showing details of the ejector of the sample retrieval system of FIG.  2 . 
     FIG. 5 is a sectional view showing the arrangement of sensors in the fugitive emission sensing system of FIG.  1 . 
     FIG. 6 is a schematic of a Quartz Crystal Microbalance (QCM) oscillator for use in a fugitive emission sensing system in accordance with the present invention. 
     FIG. 7 is a diagram showing mounting details for the QCM gas sensor of FIG.  5 . 
     FIG. 8 is a schematic view, partly in section, of a remote calibrator system for use in the fugitive emission sensing system of FIG.  1 . 
     FIG. 9 is a block diagram showing the major components of a control and communications system for use in the fugitive emission sensing system of FIG.  1 . 
     FIG. 10 is a block diagram of a QCM gas sensor interface circuit for use with the QCM gas sensor of FIG.  6 . 
     FIG. 11 is a diagram of typical waveforms generated by the QCM gas sensor interface circuit of FIG.  10 . 
     FIGS. 12A-12D show a circuit diagram of a QCM gas sensor interface circuit for use with the QCM gas sensor of FIG.  6 . 
     FIG. 13A is a flowchart of a software program used by the embedded controller of FIG. 12A to implement a high frequency measuring circuit. 
     FIG. 13B is a flowchart of an interrupt service routine used by the embedded controller of FIG. 12A to implement a high frequency measuring circuit. 
    
    
     While the invention is susceptible to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and will be described in detail herein. However, it should be understood that the invention is not intended to be limited to the particular forms disclosed. Rather, the invention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the invention as defined by the appended claims. 
     DETAILED DESCRIPTION OF THE INVENTION 
     A. FUGITIVE EMISSION SENSING SYSTEM 
     Turning now to the drawings and referring initially to FIG. 1, a block diagram of an illustrative embodiment of the invention is given showing the major components of a fugitive emission sensing system  10 . An emission source  12  is shown, from which a sample stream  14  is drawn into sample retrieval system  100 . The sample retrieval system  100  includes accumulator  102 , sensor chamber  114 , and ejector  140 . A gas sensor array  200  and thermodynamic sensor array  280  are located within the sensor chamber  114 . The sample stream  14  is drawn from the accumulator  102  into the sensor chamber  114 , exposing the gas sensor array  200  and the thermodynamic sensor array  280  to the sample stream  14 . The sample stream  14  then passes into the ejector  140 . 
     A compressed air source  30  provides compressed air  32  to the ejector  140 , creating a pressure drop within the ejector  140  which draws the sample stream  14  through and sensor chamber  114  and into the ejector  140 . The compressed air  32  and sample stream  14  are mixed within the ejector  140  and exhausted to atmosphere as the mixture  36 . The sample retrieval system  100  is integrated with a remote calibration system  300 , which is arranged to inject a small quantity of the gas being measured into the sample stream to enable automated calibration of the gas sensors. 
     In addition, control and communication system  400  is provided to process the sensor outputs and perform control and communication functions for the fugitive emission sensing system  10 . The control and communication system  400  includes sensor interface circuit  402 , microcontroller  404 , memory  406 , communication interface circuit  800 , and power conversion circuit  900 . 
     The gas sensor array  200  and thermodynamic sensor array  280  are connected to sensor interface circuit  402 , which processes the signals from the sensor arrays and provides the processed signals to microcontroller  404 . The microcontroller  404  stores the data from the sensors in memory  406 , and may use the sensor data received from the fugitive emission sensing system  10  to initiate control actions to reduce or eliminate the emissions. For example, the microcontroller  404  could close a valve upstream from the emissions source  12  to stop the flow of fluid through the emissions source  12  in order to stop emissions caused by leakage of the fluid. Alternatively, the microcontroller  404  could alter the operating condition of the emissions source  12  itself to reduce or eliminate the fugitive emissions. Microcontroller  404  may use communication interface circuit  800  to provide these control signals to the upstream valve, the emissions source  12 , or any other plant equipment that may be used to reduce or eliminate the emissions. 
     Microcontroller  404  may also use communication interface circuit  800  to provide sensor data to a remote plant process control system  40 . The fugitive emission sensing system  10  may perform measurements of fugitive emissions and immediately communicate the resulting sensor data to a separate plant control system  40 . Alternatively, the fugitive emission sensing system  10  may store sensor data from each measurement for later retrieval by the plant control system  40 . 
     The communication interface circuit  800  also may receive data and control commands from the plant control system  40 . The plant control system  40  may use the sensor data received from the fugitive emission sensing system  10  to initiate control actions to reduce or eliminate the emissions. For example, the plant control system  40  could close an valve upstream or alter the operating condition of the emissions source  12  as described above to reduce or eliminate the fugitive emissions. 
     The power conversion circuit  900  receives electrical power, which may be transmitted over the communication link with the plant control system  40 , and provides power to the communication and control system  400  at a suitable voltage. 
     The fugitive emission sensing system  10  may be used to detect the presence or measure the concentration of various types of fluids emitted from the emission source  12 . The system may be used to detect hazardous, toxic, or polluting substances emitted from the source, or to detect leakage of non-hazardous substances the loss of which may be a cause of concern. The fugitive emission sensing system may be used to detect emissions from any kind of source, particularly industrial process equipment from which hazardous substances may leak. Examples include control valves, block valves, or pumps installed on lines carrying hazardous gases; agitators, screw conveyors, or other equipment installed on process vessels containing hazardous fluids, heat exchangers, reactors, etc. When emissions are detected by the fugitive emission sensing system  10 , this data may be used by the fugitive emission sensing system  10  to control the process in such a way as to reduce or eliminate the emissions. Alternatively, the data may be transmitted to a remote plant process control system  40  which may respond by controlling the process in such a way as to reduce or eliminate the emissions. 
     B. SAMPLE RETRIEVAL SYSTEM 
     Turning now to FIG. 2, a diagram is shown of the sample retrieval system  100  for use in the fugitive emission sensing system of FIG.  1 . The sample retrieval system  100  comprises an accumulator  102 , retrieval manifold  106 , sensor chamber  114 , and ejector  140 . The accumulator  102  is situated adjacent to the emission source  12  from which an emission is anticipated. The manifold  106  is connected at one end to the accumulator  102  and at the other end to the sensor chamber  114 , and permits a sample stream to flow from the emission source into the sensor chamber  114 . The manifold  106  is preferably constructed of S31600 stainless steel tubing or other suitable corrosion resistant material. 
     The sensor chamber  114  contains the gas sensor array  200 , and may also contain a thermodynamic sensor array (not shown). The outlet  116  of the sensor chamber  114  is the inlet to the ejector  140 . A pneumatic restriction is provided by a restriction orifice  118  at the inlet to the sensor chamber  114 . The restriction orifice  118  induces a pressure drop in the sensor chamber to assist in the operation of the ejector  140 . The restriction orifice  118  may be constructed from sapphire, stainless steel, or other suitable material which is inert to the emissions expected from the equipment being monitored. 
     A particulate filter  120  is located along retrieval manifold  106  to collect any particles entrained in the sample stream. Flame path restrictors  124  and  126  are provided at the inlet to the sensor chamber  114  and outlet from ejector  140 . Microvalves  130 ,  132 , and  134  are located at various positions to provide for isolation of various parts of the sample retrieval system. Microvalve  130  may be used to isolate the accumulator  102  from the sensor chamber  114 . Microvalve  132  provides the ability to draw ambient air into the sensor chamber  114 , permitting a base line calibration to be performed on the gas sensors by closing microvalve  130  and opening microvalves  132  and  134 . 
     A remote calibrator may be connected to the sample retrieval system to enable the gas sensors to be calibrated without removing them from the sensor chamber  114 . The remote calibrator analyte cell  312  containing calibrant is connected through first microvalve  332  to a dosing chamber  324 . The dosing chamber  324  is connected through second microvalve  330  to sensor chamber  114 . 
     The sensor chamber  114  is preferably constructed of cast aluminum. The interior of the chamber may be left unfinished, or coated or machined to achieve a smooth finish to reduce surface sorption of gases from the sample stream. The sensor chamber  114  may be constructed of other suitable corrosion resistant materials that are not affected by the emissions being monitored. The sensor chamber  114  is preferably constructed as a modular unit to permit replacement of the unit in the field. 
     FIG. 3A illustrates one embodiment of the accumulator  102  shown mounted on an emission source  12 , depicted in the drawing as a control valve, in which the accumulator  102  comprises a collecting tube  160 . The collecting tube  160  facilitates mounting on various types of valve actuators and comprises a single piece of tubing. S31600 stainless steel is an example of a suitable material for the collecting tube  160 . The collecting tube  160  may be configured so as to collect gas leaking from the valve stem packing  16  located between the valve bonnet and valve stem. In the embodiment illustrated in FIG. 3A, the collecting tube  160  circumferentially encloses the valve stem packing  16 . A first end  162  of the collecting tube  160  is plugged or swagged closed, and the opposite end defines an outlet  104  that interfaces with the intake manifold  106 . 
     The collecting tube  160  defines at least one collecting orifice  164  on the side of the collecting tube  160  facing the emissions source  12 . In a particular embodiment, the collecting tube  160  defines seven collecting orifices  164 , with the diameters of the collecting orifices  164  generally increasing as the position of the orifice increases from the first end  162  of the collecting tube  160 . For instance, the collecting orifice  164  closest to the first end  162  may have a diameter of 0.156 inches, with subsequent collecting orifices  164  having diameters of 0.156, 0.0313, 0.0313, 0.0469, 0.0469, and 0.0625 inches, respectively. The decreasing fluidic resistance facilitates equal collection around the valve packing  16  circumference, carrying fugitive emissions emitted from the emission source  12  into the retrieval manifold  106  and on into the sensing chamber. 
     FIG. 3B illustrates an alternative embodiment of the accumulator  102  in accordance with the present invention, in which the accumulator  102  comprises a bonnet capsule  170 . The bonnet capsule  170  is shown mounted on an emission source  12 , depicted in the drawing as a control valve. The bonnet capsule  170  includes an outlet  104  to which the retrieval manifold  106  is connected, and may also include an opening  108  to permit installation of the bonnet capsule  170  around a valve stem  20  or other obstructing parts of the emission source. The arrangement of the bonnet capsule  170  shown in FIG. 3B is designed to collect gas leaking from the valve stem packing  16  located between the valve bonnet  18  and valve stem  20 . The opening  108  is designed to have a small clearance between the valve stem and the bonnet capsule wall to limit the entry of foreign particles into the bonnet capsule  170 . A baffle  110  is positioned inside the bonnet capsule  170  to restrict foreign particles in the bonnet capsule  170  from entering the outlet  104 , and thus, the retrieval manifold  106 . 
     The bonnet capsule  170  is mounted on the emission source so that a gap  112  remains between the bonnet capsule  170  and the emission source  12 . This creates a low impedance pneumatic restriction, which permits air to flow through gap  112 , through the bonnet capsule  170 , and into retrieval manifold  106 . This air flow carries any fugitive emissions emitted from the emission source  12  into the retrieval manifold  106  and on into the sensing chamber. This continual airflow also prevents fugitive emissions from emission source  12  from accumulating in the bonnet capsule  170 . Such an accumulation can result in a false high sensor reading due to the integration effect of an accumulation of fugitive emissions. 
     The bonnet capsule  170  may be constructed of two or more pieces to facilitate installation in situations where the bonnet capsule  170  must be installed around obstructing members. Thus, a bonnet capsule  170  as shown in FIG. 3B, comprising an enclosure split vertically into two halves, may be installed around the valve stem  20  without removing a valve actuator mounted at the top of the valve stem (not shown in FIG.  3 B). The bonnet capsule  170  is preferably constructed of S31600 stainless steel or other suitable corrosion resistant material. 
     FIG. 4 is a sectional view showing details of the ejector  140  of the sample retrieval system  100  of FIG.  2 . The ejector  140  may be integral to the sensor chamber  114  or may be constructed as a separate unit. A compressed air source  30  provides compressed air  32  to a microregulator  144  which provides regulated compressed air  34  to the ejector  140 . The compressed air is used to provide the motive power to draw the sample stream  14  from the accumulator  102 , through the sensor chamber  114 , and into the ejector  140 . The compressed air source  30  may be the instrument air supply typically used in process plants to modulate pneumatic control valves or operate pneumatic instruments, although other sources of pressurized gas or liquid may be used. The microregulator  144  is a small pressure regulator of a type commonly used in industrial applications. The microregulator  144  reduces and regulates the pressure of the compressed air to control the flow of the sample stream  14  and minimize the consumption of compressed air  32 . 
     A primary chamber  146  receives regulated compressed air  34  from the microregulator  144  and discharges air into a primary nozzle  148 . The primary nozzle  148  is tubular in shape, with an orifice  154  discharging into the throat of the secondary nozzle  152 . A secondary chamber  150  is connected to manifold  106  and to the throat of secondary nozzle  152 . The secondary nozzle  152  is tubular in shape, with a larger cross-sectional area than the primary nozzle  148 , and an orifice  156  discharges to atmosphere. 
     In operation, the regulated compressed air  34  enters the primary chamber  146  and flows into the primary nozzle  148 . The regulated compressed air  34  increases in velocity as it enters the constricted region at the outlet of the primary nozzle  148 . This high velocity stream of compressed air discharges into the secondary nozzle  152 , entraining air from the secondary chamber  150  and creating a pressure drop in the secondary chamber  150 . This pressure drop induces the flow of sample stream  14  from the accumulator  102 , through the retrieval manifold  106 , and into the secondary chamber  150 . Sample stream  14  carries any fugitive emissions from the emission source  12  through the sample retrieval system, exposing the gas sensor array  200  and thermodynamic sensor array  280  to the emissions. The regulated compressed air  34  and the sample stream  14  are mixed together in the secondary nozzle  152  and the mixture  36  is exhausted to atmosphere. 
     The ejector  140  may be made of stainless steel, or other corrosion resistant material. The primary orifice  154  and secondary orifice  156  are preferably constructed of sapphire. 
     The ejector  140  is designed to produce a sample stream  14  of known mass flow through the sample retrieval system  100 . The flow rate of the sample stream  14  is determined by the diameters of the primary orifice  154 , secondary orifice  156 , sensor chamber inlet orifice  118 , and the pressure of regulated compressed air  34 . The sample retrieval system  100  operates satisfactorily at a sample stream flow rate of about 0.425 square cubic feet per hour. This flow rate may be achieved with a primary orifice diameter of 0.011 inches, secondary orifice diameter of 0.024 inches, sensor chamber inlet orifice diameter of 0.013 inches, and regulated compressed air pressure of about 3.0 pounds per square inch gauge. However, different dimensions and operating conditions for the ejector  140  may be required to effectively collect emissions from different types of emissions sources. 
     By controlling the pressure of the regulated compressed air  34  into the ejector  140 , the pressure drop within the secondary chamber  150  can be controlled, and thus the velocity of the sample stream  14  through the retrieval manifold  106  and sensor chamber  114  can be controlled. Furthermore, the mass flow of the sample stream  14  can be calculated given the geometry of the ejector  140 , retrieval manifold  106  and sensor chamber  114 , and the pressure of the compressed air at the inlet to the primary chamber  146 . 
     The design of the sample retrieval system  100  thus eliminates the need for a mass flow sensor to measure the sample stream flow through the retrieval manifold  106 . The system described also eliminates the need for pumps or fans located near the emission source to collect the sample stream, resulting in a simple and inexpensive design. Lastly, the sample retrieval system can be designed to conform to EPA sample collection requirements. 
     C. SENSOR ARRAY 
     1. Overview 
     FIG. 5 is a sectional view showing the arrangement of sensors in the sensor chamber  114  of the fugitive emission sensing system  10  of FIG.  1 . The sensor chamber  114  is shown with an inlet from the retrieval manifold  106  and outlet  116  to the ejector  140  (not shown). An inlet orifice  118  is positioned at the inlet to sensor chamber  114 . A gas sensor array  200  and an array of thermodynamic sensors are located in sensor chamber  114 . 
     The gas sensor array  200  comprises one or more sensors responsive to the particular fugitive emission being monitored. In the embodiment shown in FIG. 5, the gas sensor array  200  comprises one or more quartz crystal microbalance (QCM) gas sensors  210  (shown in FIG.  6  and described further below). The gas sensor array  200  is incorporated into an assembly that fits within the sensor chamber  114  and can be conveniently removed and replaced in the field. 
     2. Quartz Crystal Microbalance Gas Sensors 
     FIG. 6 shows a quartz crystal microbalance (QCM) circuit comprising a QCM gas sensor  210 , which may be included in the gas sensor array  200 , and oscillator circuit  240 . The QCM gas sensor  210  comprises a quartz crystal substrate  212 , polymer coatings  214  and  216 , and electrodes  218  and  220  located between the substrate and the coatings. The oscillator circuit  240  comprises NAND gates  222  and  224 , and AND gate  226 , connected in series. Resistor  228  is connected between the output of NAND gate  222  and circuit power supply voltage +V, and resistor  230  is connected between the output of NAND gate  224  and circuit power supply voltage +V. Resistor  232  is connected across NAND gate  222 , connecting a first input to the output. A select signal  234  is connected to the second input of NAND gate  222 , and the same select signal is also connected to an input of AND gate  226 . An enable signal  236  is connected to an input of NAND gate  224 . 
     When the select signal  234  and enable signal  236  are both high, NAND gate  222  and  224  act as high-gain inverting amplifiers and cause oscillator output  244  to oscillate between high and low voltage, producing an oscillating square wave output. The oscillating voltage from the oscillator output  244  is transferred through AND gate  226  and applied across the crystal substrate  212 , exerting a physical stress on the crystal due to the piezoelectric effect and causing the QCM gas sensor  210  to physically resonate. The resonating crystal interacts with the oscillating circuit causing the oscillating circuit to oscillate at the resonant frequency of the QCM gas sensor  210 . Thus, the frequency of oscillator output  244  will vary as the resonant frequency of the QCM gas sensor  210  varies. 
     AND gate  226  provides isolation between the oscillator circuit  240  and the QCM gas sensor  210  when the sensor is not selected. The output from NAND gate  224  is connected to a first input of AND gate  226 , the second input being connected to select signal  234 . When the QCM gas sensor  210  is selected for measurement, select signal  234  is high and the output from AND gate  226  follows any change of state present at its first input. Thus, the oscillating output from NAND gate  224  will be transferred to terminal  220  of quartz crystal substrate  212  and the QCM gas sensor  210  will be connected into the oscillator circuit  240 . When the QCM gas sensor  210  is not selected for measurement, select signal  234  is low and the output from AND gate  226  will be low regardless of the signal at the first input of AND gate  226 . This will result in the QCM gas sensor  210  being isolated from oscillator circuit  240 . 
     The resonant frequency of the QCM gas sensor  210  is a function of the size, shape, and cut of the quartz crystal substrate  212 . Quartz crystal exhibits a natural resonant frequency that is a function of the mass and structure of the crystal. The precise size, type of cut, and thickness of the quartz crystal substrate  212  are selected to result in a particular resonant frequency. An AT-cut crystal with a nominal resonant frequency of 9 MHz is suitable for gas sensor applications. Suitable quartz crystal substrates may be obtained from Standard Crystal Corporation of California. Other types of piezoelectric acoustic wave devices may also be used in place of the QCM gas sensor, including surface acoustic wave (SAW) devices, acoustic plate mode (APM) devices, or flexural plate wave (FPW) devices. However, these alternative devices may have higher operating frequencies of over 100 MHz, and alternative operating modes, necessitating the use of circuitry capable of measuring such high frequencies. The electrodes  218  and  220  may be constructed of gold-on-chromium, although other suitable corrosion resistant conductors may be used. 
     The resonant frequency of the QCM gas sensor  210  is a function of the total mass of the device. The mass of the polymer coating  214  and  216  affects the total mass of the device, and thereby affects the resonant frequency of the QCM gas sensor  210 . When gas molecules are sorbed into or deposited onto the polymer coating  214  and  216 , the mass of the polymer coatings is slightly increased, and the resonant frequency of the QCM gas sensor  210  changes. The resonant frequency of QCM gas sensor  210  is also a function of the viscoelectric properties of the coatings, and mechanical stresses caused by temperature effects and the QCM mounting arrangement. However, these effects are either negligible or can be compensated for, allowing the QCM gas sensor  210  of the present invention to function principally as a mass sensor. Thus, a very sensitive gas detector may be constructed by selecting a polymer coating that has a chemical affinity with a particular gas or class of gases of interest. 
     When the gas of interest comes in contact with the QCM gas sensor  210 , gas molecules are absorbed and deposited onto the polymer coating  214  and  216  through various sorption processes. The sorption of gas molecules increases the mass of the QCM gas sensor  210 , thereby altering its resonant frequency and causing a corresponding change in the operating frequency of oscillator  230 . The quantity of gas molecules absorbed and deposited, and the resulting change in the operating frequency of oscillator  230 , is a function of the concentration of the gas being measured in the environment surrounding the QCM gas sensor  210 . The frequency changes linearly with change in gas concentration, within certain limits. Some variation in the resonant frequency of the quartz crystal substrate  212  also will occur due to aging of the crystal and temperature effects. 
     Thus, a change in concentration of the gas of interest may be measured by measuring the change in frequency of oscillator output  244 . The gas sensor may be calibrated by exposing the QCM gas sensor  210  to known concentrations of gas and recording the resulting frequency of oscillator output  244 . The gas sensor may then be used to measure the absolute concentration of a gas. The gas sensor of FIG. 1 may be designed to detect very low concentrations of gas. However, in order to measure low gas concentrations, a means of measuring small variations in frequency of the oscillator output  244  is required. A QCM gas sensor interface circuit in the communication and control system  400  is described below to make these measurements. 
     The QCM gas sensor  210  is sensitive to vibration and to the flow characteristics of the gas sample stream  14 . Such vibration may be caused by the operation of pumps, motors, or other equipment which is connected to the valve on which the fugitive emission sensing system  10  is mounted. The mounting arrangement for the QCM gas sensor  210 , illustrated in FIG. 7, is designed to isolate the sensor from these vibrations. 
     A base  250  supports two rigid support members  252  and  254 , each having a slit opening ( 256  and  258 ). The QCM gas sensor  210  is formed in the shape of a flat disk, and is positioned between the rigid support members  252  and  254  so that the periphery of the sensor disk protrudes through the slits  256  and  258  in the support members. Electrode  218  of the QCM gas sensor  210  has a circular portion in the center of the sensor disk and an elongated portion extending outwards to the support member  252  and through the slit  256 , where electrode  218  and support member  252  make electrical contact. The support member  252  is electrically connected to electrical terminal  262 , thus completing an electrical path between electrode  218  and electrical terminal  262 . Electrode  220  (not shown) is located on the opposite side of the sensor disk and shaped similarly to electrode  218 . However, the elongated portion of electrode  220  extends towards support member  254  and through slit  258 , completing an electrical path from electrode  220 , support member  254 , and electrical terminal  260 . Electrical terminals  260  and  262  connect the QCM gas sensor  210  into the oscillator circuit  240  shown in FIG.  6 . 
     The QCM gas sensors are preferably mounted in a removable module to facilitate replacement and maintenance of the sensor array. The QCM gas sensors are densely packed to reduce the effect of any gradient in the concentration of the fugitive emission within the sensor chamber. Multiple QCM gas sensors  210  may be used with each sensor having a different polymer coating, permitting discrimination between a variety of different fugitive emissions. 
     3. Thermodynamic Sensors 
     The thermodynamic sensor array comprises one or more sensors responsive to the thermodynamic conditions in the sensor chamber  114 . In the embodiment shown in FIG. 5, the thermodynamic sensor array comprises a temperature sensor  282 , a relative humidity sensor  284 , and a differential pressure sensor  286 . 
     The QCM gas sensors are sensitive to variations in temperature. Measurement of the temperature in the sensor chamber  114  may be used to compensate for gas sensor measurements affected by temperature variation. Temperature sensor  282  is located within the sensor chamber  114 , and may optionally be located in the same removable assembly as the gas sensor array  200 . A QCM sensor without any polymer coating may be used as the temperature sensor  282 . The uncoated QCM sensor is constructed similarly to the QCM gas sensor  210  described above, having a quartz crystal substrate and being connected to an oscillator circuit, but lacking any polymer coating. The QCM temperature sensor  282  is hermetically sealed to prevent absorption of fluid from the sample stream  14  or ambient air. Any variation in the temperature of the quartz crystal substrate of the sensor will result in a corresponding change in the resonant frequency of the uncoated QCM temperature sensor  282 . As with the QCM gas sensor  210 , some variation in the resonant frequency of the quartz crystal substrate also will occur with aging of the device. As an alternative to the use of a QCM device, a resistance temperature detector or other common type of temperature sensor also may be used. 
     Relative humidity affects the measurements made by gas sensor array  200  because the water molecules within the sample stream  14  compete with the molecules of the fugitive emission being measured for sorption by the polymer surfaces of the QCM gas sensor  210 . Relative humidity sensor  284  is located in the sensor chamber  114 . A QCM sensor similar to the QCM gas sensor  210  may also be used for the relative humidity sensor  284 . When used as the relative humidity sensor  284 , the polymer coating applied to the quartz crystal substrate of the QCM sensor is selected to be hydrophilic. The resonant frequency of the QCM relative humidity sensor  284  varies with the amount of water deposited on the polymer coating on the surface of the sensor. 
     The differential pressure sensor  286  measures the flow of the sample stream  14  through the sensor chamber  114 . Pressure taps  288   a  and  288   b  measure the pressure in the retrieval manifold  106  and sensor chamber  114  respectively, thus measuring the pressure drop across orifice  118  at the inlet to the sensor chamber  114 . The flow of gas into the sensor chamber  114  can be calculated from the differential pressure measurement using well known techniques. 
     D. REMOTE CALIBRATOR SYSTEM 
     QCM gas sensors typically degrade due to the effects of aging, temperature, humidity, poisoning, and oxidation on the polymer coating. Periodic calibration of the gas sensors permits the fugitive emission sensing system to compensate for these effects. To permit efficient and consistent calibration of the gas sensors, the fugitive emissions sensing system includes a remote calibrator. FIG. 8 is a sectional view of an embodiment of a remote calibrator system for use in the fugitive emission sensing system of FIG.  1 . 
     The calibration technique selected for use with the fugitive emissions sensing system provides for exposing the gas sensor array  200  to the same type of emissions that the system is designed to measure. By exposing the sensors to known quantities of the emissions, the analysis of the resulting data from the sensors is reduced to a regression problem. The gas sensor array  200  is exposed to the process plant atmosphere containing three increasingly greater concentrations of the emission of interest. The three calibration points are chosen to encompass the entire operational range of the sensor (from the lowest concentration of the emission of interest to the highest concentration) and define the sensor&#39;s performance for a specific measurement interval. The frequency of measurement may be as often as daily with measurement times not to exceed 10 minutes. Power consumption is a critical parameter in all aspects of the system and drives many aspects of the design. 
     FIG. 8 shows a remote calibrator  300  for performing automatic calibration of the gas sensors use with the fugitive emission sensing system  10 . The remote calibrator  300  is mounted in the field adjacent to the gas sensors. Remote calibrator  300  includes a reservoir  312  which contains a quantity of liquid analyte calibrant  314 , which is preferably the same material as is running through the valve to the monitored. 
     Remote calibrator  300  includes a conduit  316  which extends between the reservoir  312  and an outlet nozzle  318 . Conduit  316  includes a bore  320  extending therethrough, and further includes an intermediate or central portion  322 , a portion of which defines a dosing chamber  324 . Dosing chamber  324  is preferably of predetermined volume, which for purposes of the preferred embodiment is in the range of 2 microliters (2×10 −6  cubic centimeters). Conduit  316  is preferably constructed of stainless steel tubing having an inside diameter of 0.008 inches and an outside diameter of 0.50 inches, or any other suitable thickwall small diameter tubing. A thermal activator  326 , which is preferably a resistive coil or a radio frequency heating unit, surrounds the conduit  316  adjacent the dosing chamber  324 , enabling the activator  326  to heat a measured quantity  328  of calibrant  314  contained within the dosing chamber  324 . The thermal activator  326  is preferably capable of bringing the measured quantity  328  contained within the dosing chamber  324  to its boiling point very quickly, as in the range of about 10 milliseconds. 
     An outlet valve  330  having a magnetically coupled actuator  331  is located at outlet nozzle  318 , and is movable between an open position in which the bore  320  and dosing chamber  324  are in flow communication with the surrounding atmosphere, and a closed position in which the bore  320  and dosing chamber  324  are isolated from the surrounding atmosphere. A second valve  332  having a magnetically coupled actuator  333  is disposed along conduit  316  between dosing chamber  324  and reservoir  312 . Valve  332  is movable between an open position in which dosing chamber  324  is in flow communication with reservoir  312 , and a closed position in which the dosing chamber  324  is isolated from the reservoir  312 . Preferably, each of valves  330 ,  332  are remotely operable from a remote calibrator control circuit  750 . Remote calibrator control circuit  750  is also used to energize the thermal activator  326  as will be discussed in greater detail below. Further, the pneumatic impedance through valve  330  is preferably about fifty (50) times greater than the pneumatic impedance through valve  332 , the importance of which will be discussed in greater detail below. Valve  330  preferably includes a chemically resistant soft seat, such as VITON or TEFLON. These fluorinated materials prevent calibrant absorption into the seat, thus preventing “off-gassing.” The closure force of valve  330  may be relatively low, such as in the range of 25 pounds per square inch of closure force on nozzle  318 . 
     In operation, when the remote calibrator  300  is inactive, valve  330  is closed, valve  332  is open, and the calibrant  314  in reservoir  312  is free to flow into the dosing chamber  324 . When it is desired to activate the remote calibrator  300 , the remote calibrator control circuit  750  closes valve  332 , thus seriously impeding or preventing flow between dosing chamber  324  and reservoir  312 , and thermal activator  326  is energized. Simultaneously, or shortly thereafter, valve  330  is opened. The now vaporized calibrant  314  contained within dosing chamber  324  is at boiling point, and is ejected through the open nozzle  318  into the sensor chamber  114  (not shown). At that point, the exhausted calibrant can be mixed with a known quantity of ambient air drawn from around the emissions source  12 , for measuring or predicting the leak emissions. The gas sensor array  200  can be calibrated by comparing the obtained sensor reading to empirical data, or by using other known methods. 
     Alternatively, the impedance between the dosing chamber  324  and the reservoir  312  may be achieved using a mechanical restriction rather than a closeable valve. Also, in less severe environments or in environments where inertial dispersion of calibrant is not expected, it is conceivable that surface tension and pneumatic impedance may be sufficient to prevent evaporation as well as backward flow of the calibrant, thus making it possible to dispense with one or both of the valves. 
     E. CONTROL AND COMMUNICATIONS SYSTEM 
     1. Overview 
     FIG. 9 is a block diagram showing the major components of a control and communications system for use in the fugitive emission sensing system of FIG.  1 . The control and communications system  400  includes circuits to interface to the sensors (QCM interface circuit  500  and thermodynamic sensor interface circuit  700 ) and to control the remote calibrator (remote calibrator control circuit  750 ). A microcontroller  404  communicates with each of these and sends data to the communication interface circuit  800  for transfer to a plant control system  40 . A power conversion circuit  900  provides power to the communication and control system  400 . 
     2. Microcontroller and Memory 
     The microcontroller  404  controls the operation of the fugitive emission sensing system  10 . The microcontroller  404  manages communications between the components of the fugitive emission sensing system  10 , and communication with a plant control system  40 . The microcontroller  404  also provides storage of measurement data from the gas sensor array  200  and thermodynamic sensor array  280 , as well as data derived from calibration of the gas sensors, in memory  406 . 
     The microcontroller  404  may be programmed to perform fugitive emission measurements upon request from the plant control system  40 . The data may be stored in memory  406  temporarily and uploaded to the plant control system  40  after each measurement cycle. Alternatively, the microcontroller  404  may be programmed to perform fugitive emission measurements on a set schedule. The measurement data may be stored in non-volatile memory  406  and uploaded only upon request for the data from the plant control system  40 . 
     3. QCM Gas Sensor Interface 
     Several techniques can be used to determine the resonant frequency of QCM gas sensor  210 . One method involves resonant frequency determination based upon impedance measurements. This technique is an analog-digital hybrid circuit that is prone to noise, is complex, and expensive to implement. However, the use of a frequency counter provides a low cost fully digital circuit that has high noise immunity, and simple integration of commercially available components make this technique novel and robust. 
     FIG. 10 is a block diagram of the main functional components of a digital QCM gas sensor interface circuit for use in the control and communications system of FIG.  9 . The QCM gas sensor  210  and oscillator  240  are shown, and the oscillator output is connected to counter  504  and a first input of digital mixer  506 . The counter  504  is connected to subtract circuit  516 , which is used to generate “coarse” measurement  518 , as described below. Coarse measurement  518  is an input to digital frequency synthesizer  520 , which generates reference frequency  522 . Reference frequency  522  is a second input to digital mixer  506 . The output of digital mixer  506  is connected to low pass filter  526 , whose output is connected to a logic gate  530 . The logic gate  530  may be a buffer or inverter, or a Schmitt trigger to provide noise immunity. The logic gate output is connected to timer  534 , which is used to generate “fine” measurement  536 , as described below. Coarse measurement  518  and fine measurement  536  are inputs to add circuit  538 , which generates final measurement  540 . Clock circuit  542  generates gate signal  544  which is an input to counter  504  and internal clock frequency  546  which is an input to timer  534 . 
     Initially the output of oscillator  240  is the QCM frequency  502 , which has the same frequency as the resonant frequency of QCM gas sensor  210 , typically 9 MHz. As mentioned earlier, this frequency will vary as a result of the sorption of gas molecules into and onto the polymer coatings  214  and  216  of the QCM gas sensor  210 . The counter  504  counts the number of cycles (measured by the rising edges of low to high transitions) of QCM frequency  502 . This count is initial frequency measurement  514 . Counter  504  is a 16-bit device so the maximum count possible for the 16-bit initial frequency measurement  514  is 2 16  or 65,536. To prevent an overflow in the 16-bit count, the counter  504  must be enabled for a sufficiently short time such that the total expected count is less than 65,536. To prevent such an overflow, the clock circuit  542  generates a periodic gate signal  544  to enable the counter  504  for a short period. The counter  504  counts the number of cycles of QCM frequency  502  that occur between each gate signal. 
     The gate period selected is dependent on the frequency of the signal being measured. A longer gate period will provide greater resolution, while a shorter gate period will provide for greater variation in the frequency being measured without causing an overflow. For example, a 9 MHz signal will provide 54,000 counts in a 6 ms gate period. The resolution of the 16-bit count for a 9 MHz signal and a 6 ms gate period is 9 MHz/54,000 counts, or approximately 167 Hz (i.e. each count represents approximately 167 Hz). The actual error is not symmetrical due to truncation of the digital values that occurs during count accumulation. However, to precisely calculate the mass of gas molecules sorbed into the polymer coating of QCM gas sensor  210 , greater accuracy is required. 
     Higher resolution is achieved by digitally mixing the QCM frequency  502  with a reference frequency and measuring the difference frequency between the two signals. The reference frequency is derived from the initial frequency measurement  514  produced by the counter  504 . One count is subtracted from the initial measurement  514  by subtract circuit  516 , and the resulting “coarse” measurement  518  is an input to the digital frequency synthesizer  520 . The digital frequency synthesizer  520  generates a reference signal  522  which has a frequency corresponding to the value of coarse measurement  518 . The subtraction of one count to give coarse measurement  518  ensures that the frequency of the reference signal  522  is always less than the frequency of QCM frequency  502 . This simplifies reconstruction of the final measurement  540  by eliminating the need to determine whether the output from the digital mixer  506  represents a positive or negative difference in frequency (i.e. whether fine measurement  536  should be added or subtracted from the coarse measurement  518 ). 
     Reference signal  522  and QCM frequency  502  are both inputs to digital mixer  506 . Digital mixing may be accomplished by performing a Boolean Exclusive OR operation on the two inputs. The digital mixing of the two high frequency signals produces a sinusoidally varying pulse width modulated signal  524 . The pulse width modulated signal  524  varies sinusoidally due to the periodic phase variations between the frequencies of the reference signal  522  and QCM frequency  502 . The pulses are integrated by a first order low-pass filter  526  to remove the high frequency carrier and passed through a logic gate  530  to provide a square wave difference frequency signal  532 . The difference frequency signal  532  is an input to timer  534 . 
     The difference frequency signal  532  has a much lower frequency than the QCM frequency  502 , and can be measured very precisely. The timer  534  is configured to count the number of cycles of internal clock signal  546  (measured by the rising edges of low to high transitions) during each cycle of difference frequency signal  532 . For an internal clock signal  546  with a frequency of 5 MHz, the internal clock cycle time is 200 nanoseconds. Thus, timer  534  increments its count every 200 nanoseconds during one cycle of difference frequency signal  532 . 
     Coarse measurement  518  has the same resolution as initial measurement  514 , approximately 167 Hz. The frequency of reference signal  522  is nominally 167 Hz less than QCM frequency  502 , because reference signal  522  is generated from coarse measurement  518  which is one count less than initial frequency measurement  514 . Thus, the difference in frequency between reference signal  522  and QCM frequency  502  may theoretically vary from approximately 167 Hz to 333 Hz (the actual difference in frequency will be greater due to truncation errors), and the difference frequency signal  532  will thus vary between 167 Hz and 333 Hz. The timer  534  measures this low frequency difference frequency signal  532  with a resolution of at least 0.1 Hz, to produce “fine” measurement  536 . 
     Finally, the reconstruction circuit  540  adds fine measurement  536  to coarse measurement  518  to produce final measurement  540 . Thus, a vernier frequency counter has been developed to accurately determine the operating frequency of the QCM gas sensor  210 . 
     FIG. 11 is a diagram of typical waveforms of various signals generated by the high resolution frequency measurement circuit of FIG.  10 . Waveform  560  represents the QCM frequency  502 . This is a square wave oscillating at the resonant frequency of the QCM gas sensor  210 . The frequency of waveform  560  is a function of the mass of QCM gas sensor  210 , which is a function of gas concentration. 
     Waveform  562  represents reference signal  522 . This signal is generated by digital frequency synthesizer  520 , and has a frequency determined by the value of coarse measurement  518 . Waveform  562  has a lower frequency than waveform  560 , because coarse measurement  518  is always less than QCM frequency  502 . 
     Waveform  564  represents the output from digital mixer  506 . This waveform is a pulse-width modulated signal created by the phase variance between waveform  560  (QCM frequency  502 ) and waveform  562  (reference signal  522 ). The pulse width of waveform  564  varies sinusoidally, and the period of the sinusoidal variation is a function of the difference in frequency between waveform  560  and waveform  562 . 
     Waveform  566  represents the output from low-pass filter  528 . The pulses of waveform  566  are integrated by the low-pass filter  526 , removing the high frequency carrier and converting the sinusoidal variation of pulse width of waveform  564  into low frequency sinusoidal waveform  566 . The frequency of waveform  566  equals the difference in frequency between waveform  560  and waveform  562 . 
     Waveform  568  represents the difference frequency signal  532 . Waveform  568  is generated by passing the sinusoidal waveform  566  through logic gate  530  to produce a square wave having the same frequency as waveform  566 . Thus, waveform  568  is a square wave having a frequency equal to the difference in frequency between waveform  560  (QCM frequency  502 ) and waveform  562  (reference signal  522 ). 
     Turning now to FIGS. 12A-12D, a circuit to implement a high resolution frequency measurement circuit is shown. The circuit has three main components: a PIC embedded controller  602 , a direct digital synthesis (DDS) integrated circuit  604 , and a digital mixer  606 . The embedded controller  602  contains two 8-bit counter-timers and a 16-bit counter-timer. The embedded controller  602  also contains program and variable memory to provide for control of the counter-timers and analysis of their outputs, and includes a communications port, either serial or parallel, and external address and data bus. The embedded controller  602  also should be capable of executing floating point math algorithms. A suitable controller is the PIC16C62 controller made by Microchip Technology Inc. of Arizona, although other controllers having the required functionality may also be used. 
     The DDS circuit  604  must be capable of creating periodic waveforms (square or sinusoidal) at frequencies equal to the resonating frequency of a QCM gas sensor. A monolithic DDS integrated circuit model AD9850, made by Analog Devices, Inc. of Massachusetts, is suitable for this application. The AD9850 generates the desired signal with 32-bit resolution. The digital mixer  606  is a single Boolean Exclusive OR gate, of a commonly available type. 
     Embedded controller  602  is connected to address decoder  608  which is connected to the sensor select gates  610 ,  612 ,  614 ,  616 ,  618 , and  620 , and to sensor isolation gates  611 ,  613 ,  615 ,  617 ,  619 , and  621 . The sensor select gates and sensor isolation gates each connect to a terminal of a QCM gas sensor and operate to connect the sensors into or isolate the sensors from the high resolution frequency measurement circuit. The following describes the detailed connections and operation of only one of the QCM gas sensors and its sensor select gate and sensor isolation gate, although it can be readily appreciated that additional sensors may be connected similarly and operated in the same manner, and that the high resolution frequency measurement circuit is designed to operate with multiple sensors. 
     QCM gas sensor  210  has a first terminal  218  (shown in FIG. 6) connected to a first input of sensor select gate  610 , and a second terminal  220  (shown in FIG. 6) connected to the output of sensor isolation gate  611 . The second input to the sensor select gate  610  and one input from sensor isolation gate  611  are both connected to sensor select line  622  from address decoder  608 . Address decoder  608  is connected to controller  602  via sensor select lines  624  and  626 . To select a particular sensor to measure, controller  602  generates a select signal on line  624  and a sensor address on lines  626  which is decoded by address decoder  608 . Address decoder  608  outputs a high signal on the sensor select line corresponding to the selected sensor (and a low signal on all the other sensor enable lines), causing the corresponding sensor select gate and sensor isolation gate to connect the selected sensor to the oscillator circuit. Thus, to select QCM gas sensor  210 , a select signal is generated by embedded controller  602  which causes address decoder  608  to output a high signal on sensor enable line  622 . This high signal causes sensor select gate  610  and sensor isolation gate  611  to pass logic signals from QCM sensor  610  to oscillator NAND gate  646 , completing the oscillator circuit through the QCM gas sensor  610  and providing feedback from the QCM gas sensor  610  to permit sustained oscillation at the resonant frequency of the sensor. In this way, multiple QCM gas sensors may be connected in parallel across the oscillator circuit, with the sensors being selected one at a time for measurement by embedded controller  602 . Alternatively, other common digital techniques may be employed to individually select the sensors. 
     Upon selection of one of the QCM gas sensors, embedded controller  602  generates a QCM enable signal to enable operation of oscillator NAND gate  646 . The oscillator output  648  (this is equivalent to the oscillator output  244  shown in FIG. 6) is connected to a timer-counter input of embedded controller  602 . Because the particular model of embedded controller used in this embodiment does not have a 16-bit timer-counter that can be used to generate a coarse measurement of oscillator output  648  (i.e. the function performed by counter  504  in the circuit of FIG.  10 ), two 8-bit timer-counters are used. The first 8-bit timer-counter (the “8-bit prescaler”) counts every cycle of oscillator output  648 . The second 8-bit timer-counter increments only after a preset number of cycles (the “8-bit counter”). In this application, the 8-bit counter increments only once every 256 cycles of the oscillator output  648 . Together, the 8-bit counter and 8-bit prescaler provide a 16-bit count of oscillator output  648 ; the 8-bit prescaler providing the least significant 8 bits and the 8-bit counter providing the most significant 8 bits of the 16-bit count. The outputs from the 8-bit counter and 8-bit prescaler are concatenated by the embedded controller  602  to yield a 16 bit count. This count is the initial frequency measurement  514 , described above in the discussion of FIG.  10 . 
     The following example illustrates the method of deriving a full 16 bit count from outputs of the 8-bit counter and 8-bit prescaler. If the oscillator output  648  is 9 MHz and the gate time is 6 ms, then the number of counts recorded by the 8-bit counter is (9×10 6  Hz)×(6×10 −3  s)/256=210.9375 counts. The 8-bit counter increments every 256 cycles of the oscillator output  648 , yielding a counter value of 210 or D2 [base 16]. This value is the upper 8 bits of the total 16-bit count of initial frequency measurement  514 . The 8-bit prescaler increments on every cycle of the oscillator output  648 . The 8-bit prescaler rolls over at 256 counts, so the number of counts recorded is the fractional count (the count remaining in the counter at the end of the 6 ms gate period), equal to 0.9375×256, which equals 240 [base 10] or F0 [base 16]. This value is the lower 8 bits of the 16-bit initial frequency measurement  514 . The full 16 bit value is thus D2F0 [base 16]. 
     However, the embedded controller  602  can only access the count accumulated by the 8-bit counter. To derive the full 16-bit count, embedded controller  602  performs the following steps. First, embedded controller  602  sends a QCM enable signal (i.e. a high logic voltage) to oscillator NAND gate  646  for a 6 millisecond gate period. During this period, the 8-bit counter and 8-bit prescaler count the pulses appearing at oscillator output  648 . At the end of the gate period, the QCM enable signal is removed which disables the oscillation of oscillator output  648 , and embedded controller  602  stores the count accumulated by the 8-bit prescaler. To determine the count accumulated by the 8-bit prescaler, embedded controller  602  then toggles the input to NAND gate  644  from high to low, causing oscillator output  648  to toggle from low to high, which causes the 8-bit prescaler to accumulate additional counts. Embedded controller  602  continues to toggle the input to NAND gate  644  until the 8-bit prescaler overflows, causing the 8-bit counter count to increase by one count. Embedded controller  602  then subtracts the number of toggles required to cause this overflow from 256 to calculate the count accumulated by the 8-bit prescaler during the 6 millisecond gate period. Lastly, embedded controller concatenates this derived count with the stored count from the 8-bit counter to result in the 16 bit initial frequency measurement  514 . 
     The frequency of the oscillator output  648  will be the frequency at which the QCM gas sensor is resonating, typically 9 MHz, and the model of embedded controller  602  used in this embodiment cannot measure such a high frequency directly. The internal clock of the embedded controller  602  is limited to one fourth the rate of the master clock frequency, resulting in an internal clock frequency of 5 MHz for a typical master clock frequency of 20 MHz. To permit the embedded controller  602  to measure the 9 MHz frequency, the oscillator output  648  is used as the clock input to the 8-bit counter and 8-bit prescaler, and a fixed frequency signal having a 6 ms period is generated from the internal clock and is used as the other input. In this configuration, the 8-bit counter and 8-bit prescaler count the number of cycles of oscillator output  648  occurring during a 6 ms gate period. 
     The 16 bit count of cycles occurring during the gate period is the initial measurement  514  of the frequency of oscillator output  648 . Embedded controller  602  subtracts one count to from initial measurement  514  to produce coarse count. The embedded controller  602  then performs a floating point calculation to convert the integer coarse count to coarse measurement  518  in engineering units. The coarse count is divided by the gate period to convert the integer count value into a frequency value. For example, a QCM frequency of 9.12345 MHz and a 6 millisecond gate period will result in a initial measurement  514  of: 9.12345 MHz×6×10 −3  s=54740 [base 10] or D5D4 [base 16]. Subtracting one count yields a coarse count of D5D3 [base 16]. Thus, the coarse measurement  518  in engineering units is: D5D3 [base 16]×6×10 −3  s=9.123166667 MHz. 
     However, the DDS  604  requires an integer input scaled to its clock frequency. To produce the DDS input, the embedded controller  602  converts the engineering unit coarse measurement  518  into an integer control word for input to the DDS  604 . The DDS control word is calculated by multiplying the coarse measurement  518  by the full-scale count value of the 32-bit DDS  604 , and dividing by the DDS clock frequency. For example, using the data given above and assuming the DDS  604  has a clock frequency of 50 MHz, the DDS control word would be: 9.123166667 MHz×2 32 /50 MHz=783,674,049 [base 10] or 2EB5EAC1 [base 16]. 
     The embedded control transmits the DDS control word and control signals on data lines  628  to DDS  604 . DDS  604  generates reference frequency  522  (shown in FIG. 2) having a frequency equal to the frequency represented by the control word (which is the same frequency as that represented by coarse measurement  518 ) from embedded controller  602 , transmitting the result on DDS output  632 . Digital mixer  606  receives DDS output  632  (the reference frequency  522 ) and oscillator output  648  (the QCM frequency  502 ). The digital mixer  606  performs an Exclusive OR operation on the two inputs to produce a pulse width modulated output. This output passes through a simple single-pole filter comprising resistor  634  and capacitor  636 . The output  638  from the low pass filter  526  is fed to buffer  640 , comprising an open collector NAND gate, to provide a square wave at output  642  to the 16 bit counter-timer of embedded controller  602 . 
     The 16 bit counter-timer circuit produces a fine count. Embedded controller  602  converts the integer fine count into fine measurement  536  in engineering units by dividing the embedded controller  602  internal clock frequency by the fine count. For example, if the embedded controller clock frequency is 5 MHz, a fine count of 17647 [base 50] or 44EF [base 16] would yield a fine measurement  536  of: 5 MHz/44EF=283.334 Hz. Because the fine count (representing the difference frequency  524  of FIG. 10) is much less than internal clock frequency of the embedded controller  602 , the resulting fine measurement  536  has a very high resolution. 
     To calculate final measurement  540 , embedded controller  602  performs a floating point add of coarse measurement  518  and fine measurement  536 . Lastly, embedded controller  602  converts the floating point final measurement  540  into a format suitable for transmission over a serial communication link to a central monitoring system. 
     A typical reading profile involves enabling each of the individual QCM gas sensors one at a time taking a measurement for each one. A final measurement  540  is calculated for each QCM gas sensor and transmitted to the central monitoring system with appropriate information identifying which sensor generated the data. 
     The above described functions of the embedded controller  602  may be implemented according to the software program flowchart depicted in FIG.  13 A and the interrupt service routine flowchart depicted in FIG.  13 B. FIG. 13A shows the main program which executes cyclically to implement the high frequency measuring circuit. Upon initial startup, the PIC embedded controller  602  is initialized and the variables stored within the PIC embedded controller  602  are reset. The program then enters a loop, beginning with a reset of the DDS  604  and clearing of the embedded controller&#39;s 16 bit timer, 8-bit counter, and 8-bit prescaler values. A QCM gas sensor is then selected for measurement and an enable signal sent to the oscillator NAND gate  646  to enable the QCM oscillator circuit. 
     At the end of a 6 millisecond delay, a disable signal is sent to the oscillator NAND gate  646 , the accumulated count value of the 8-bit counter is read and temporarily stored by the embedded controller  602 . The embedded controller  602  then sends signals to toggle the input to NAND gate  644  to cause the 8-bit prescaler to accumulate additional counts. The toggle signals are sent until the 8-bit counter increments by one count. The embedded controller  602  keeps an accumulated count of the number of toggle signals sent and subtracts this count from 256. One count is then subtracted from the resulting value, and it is concatenated with the previously stored 8-bit counter value to give a 16 bit coarse measurement. This coarse measurement is then converted to floating point format and scaled to produce a word suitable for input to the DDS  604 . The 16 bit timer of the embedded controller  602  is cleared, an enable signal sent to the oscillator NAND gate  646 , and the 16 bit timer enabled. 
     The program then waits for the interrupt service routine, shown in FIG.  13 B and described below, to complete. The output from digital mixer  606  drives the input to the interrupt circuit, and completion of the interrupt service routine indicates that one complete cycle of the output of digital mixer  606  has occurred and the 16-bit timer has accumulated a fine measurement count. The embedded controller  602  then performs the reconstruction algorithm to derive the final measurement from the coarse measurement and fine measurement values previously obtained. The embedded controller  602  stores the final measurement value and outputs the value onto the embedded controller&#39;s data bus. The program execution then returns to the beginning of its loop, resets the DDS  604 , and continues execution as described above. 
     The interrupt service routine shown in FIG. 13B starts when a leading (or rising) edge of the output from digital mixer  606  is detected by the embedded controller  602  and an interrupt signal generated. The first execution of the interrupt service routine will proceed down the right leg of the flowchart. The 16-bit timer is cleared and turned on, the interrupt register cleared and enabled to permit detection of a second leading edge of the output from digital mixer  606 . The second execution of the interrupt service routine, triggered by detection of a second leading edge of the output of digital mixer  606 , begins execution of the left leg of the flowchart. A disable signal is first sent to oscillator NAND gate  646  to disable the QCM oscillator circuit. The embedded controller  602  then stores the accumulated value from the 16-bit timer as the fine measurement. The 16-bit timer is reset, the interrupt circuitry is reset, and a flag is set to indicate that the interrupt service routine has completed. 
     The flowcharts illustrate one method of programming the embedded controller  602  to implement the high frequency measuring circuit of the present invention, although many other methods may be used that will be apparent to one of ordinary skill in the art. 
     4. Thermodynamic Sensor Interface 
     The thermodynamic sensor interface circuit  700  receives signals from the thermodynamic sensor array  280 , which may comprise temperature sensor  282 , relative humidity sensor  284 , and differential pressure sensor  286 . The thermodynamic sensor interface circuit  700  processes the sensor signals to generate digital signals representing the measured variables. The temperature sensor  282  and relative humidity sensor  284  are preferably QCM devices, and the interface circuits for these sensors operate similarly to the QCM gas sensor interface circuit shown in FIG. 10,  11 , and  12 D— 12 D and described above. The interface circuit for the differential pressure sensor  286  uses components and techniques known to one of skill in the art. 
     5. The Remote Calibrator Control Circuit 
     The remote calibrator control circuit  750  controls operation of the remote to calibration system  300 . The remote calibrator control circuit  750  may receive commands from the microcontroller  404 , or directly from the plant control system  40 . When it receives a command to initiate a calibration cycle of the gas sensor array  200 , the remote calibrator control circuit  750  activates the thermal activator  326 , the actuator  331  of outlet valve  330 , and actuator  333  of second valve  332  or remote calibration system  300  (shown in FIG. 8) in a timed sequence in order to inject calibrant in the sensor chamber  114 . 
     6. Communication Interface Circuit 
     The communication interface circuit  800  provides a means to send data from the fugitive emission sensing system  10  to a remote plant process control system  40 , and to receive data and control signals from the plant process control system  40 . The data sent to the process control system  40  may include measurement data from the gas sensor array  200  and thermodynamic sensor array  280 , and calibration data for the sensor arrays. The data and control signals received from the process control system  40  may include commands to take emission measurements, commands to perform a calibration of the sensors, and commands to download stored measurement and calibration data. 
     The fugitive emission sensing system  10  may also be integrated with the valve it is monitoring so that the communication interface circuit  800  may also send valve stem position data and other valve related data to the process control system  40 , and may receive valve position control signals from the process control system  40 . This data exchange between the fugitive emission sensing system  10  and the plant control system  40  may include any operational or maintenance data appropriate to the equipment integrated with fugitive emission sensing system  10 . 
     The preferred method of communicating data between the fugitive emission sensing system  10  and the plant process control system  40  is by means of a single two-conductor communication link, although other communication links, including fiber optic cabling, may be used. The communication interface circuit  800  may use the communication link to send and receive both analog and digital signals. For example, an analog 4-20 milliamp signal may be used to send a valve position output from the plant control system  40  to a control valve integrated with the fugitive emission sensing system  10 , where the 4-20 milliamp signal is used to modulate a compressed air supply to control the valve stem position. The same two-wire cable may also used to exchange data in digital format between the fugitive emission sensing system  10  and the process control system  40 . A suitable communication interface circuit for use with the fugitive emission sensing system  10  is described in U.S. Pat. No. 5,451,923, the disclosure of which is hereby incorporated by reference in its entirety. Another communication interface circuit is described in U.S. Pat. No. 5,434,774, the disclosure of which is hereby incorporated by reference in its entirety. 
     The fugitive emission sensing system  10  may use gas sensor measurement data to take control actions designed to reduce or eliminate emissions from the plant. This may include shutting off the stream of fluid passing through an emissions source from which emissions have been detected, or changing the operational state of the emissions source itself to reduce the possibility of continuing emissions. The plant process control system  40  also may use gas sensor measurement data received from the fugitive emissions sensing system  10  to take control actions designed to reduce or eliminate emissions from the plant. 
     7. Power Conversion Circuit 
     The power conversion circuit  900  provides power to the fugitive emission sensing system  10 . The power conversion circuit  900  performs voltage conversion and regulation of incoming power to provide a regulated and continuous power to the fugitive emission sensing system  10 . The power conversion circuit  900  may receive power from an auxiliary power supply line or a battery integrated into the fugitive emission sensing system  10 , or may use the signal generated by the plant control system  40  to provide power. A suitable circuit for utilizing the voltage on the communication link to the plant control system  40  is described in U.S. Pat. No. 5,451,923, the disclosure of which is hereby incorporated by reference. Other techniques and circuits that may be used for the power conversion circuit  900  are well known to those of skill in the art. 
     Many modifications and variations may be made in the techniques and structures described and illustrated herein without departing from the spirit and scope of the present invention. Accordingly, it should be understood that the methods and apparatus described herein are illustrative only and are not limiting upon the scope of the present invention.