Patent Publication Number: US-2004056771-A1

Title: Apparatus and method for wireless gas monitoring

Description:
RELATED APPLICATIONS  
     [0001] This application claims the benefit of U.S. Provisional Applications No. 60/104,223 filed Oct. 14, 1998 and No. 60/122,863 filed Mar. 4, 1999. This application also claims the benefit of U.S. patent application Ser. No. 09/333,352 filed Jun. 15, 1999, now U.S. Pat. No. 6,252,510. This application also claims the benefit of U.S. patent application Ser. No. 09/854,748 filed May 14, 2001. 
    
    
     
       FIELD OF THE INVENTION  
       [0002] This invention relates to the field of gas monitoring. The invention provides a method and apparatus for wireless monitoring of gases, including toxic and combustible gases, with a device that has a radio transmitter that transmits quantitative gas levels to a master controller or multiple master controllers.  
       DESCRIPTION OF THE RELATED ART  
       [0003] Toxic gas monitoring systems are well known. Generally, gas monitors are placed around chemical producing facilities such as a chemical processing plant. These monitoring systems are configured to monitor for the presence of toxic and/or combustible gases. In addition to monitoring for the presence of these gases, typically in parts per million or lower explosive limits, these detectors could be configured to detect other important information such as wind speed and direction, temperature and other weather conditions. This information is then relayed to some sort of central reporting system. For instance, the information can be relayed back to the control center of a chemical plant and be displayed on a computer terminal or information sent directly to the plant&#39;s Distributive Control System.  
       [0004] Conventional toxic gas monitoring systems usually comprise multiple sensing units. These units are placed in and around the perimeter of a chemical processing plant, for example, to constantly monitor the targeted conditions around the plant. Upon detection of a toxic gas, usually at a predetermined level, the unit may sound an alarm in addition to relaying the information to the control center. This information can be used, for example, to determine the source of the gas so that an unexpected leak can be corrected. Alternatively, should the plant simply be operating at too high of a capacity and thus be generating too much toxic waste, its operations can be brought to within acceptable tolerances. Additionally, the wind speed, weather conditions and direction of the gas can be used to determine which people need to be warned about the presence of toxic gas and when such a warning should be issued.  
       [0005] Typically, in gas detection systems a master site provides information to a computer. U.S. Pat. Nos. 5,553,094, 5,568,121 and 5,771,004 disclose such systems. U.S. Pat. No. 5,597,534 discloses a circuit that measures a chemical sensor output. Typically, specially designed software is incorporated as well. For example, the Gastronics&#39; Event Scada Software is an unlimited tag Scada software which runs off Windows 95, 98 or 2000 or Windows NT and is designed for user friendliness along with the ability to customize and map out the geography of a plant. The Event Scada Software offers the user the flexibility to design and customize individual screens to match different applications. An assortment of tools allows the creation of trend charts, wind speed and direction, alarm settings and maintenance screens. A multilevel security feature may be included to prevent unauthorized access to customization functions.  
       [0006] Currently, the method of relaying this important information from the monitors to the control center has been through wires which physically connect each of the monitors to the output system. This is generally referred to as “hard wiring.” Hard wiring requires each monitor to be physically connected to the output system by some sort of wire or cable. Hard wiring each of the numerous monitors to the control system can be quite costly, cumbersome and require substantial and frequent maintenance. For example, should the output system ever need to be relocated, such as in a different control room or outside of the plant, the cables would need to be rerouted to this new site. Rerouting all of the cables is labor intensive and expensive.  
       [0007] To further complicate matters, the wires may need to be buried in the ground (typically below the frost line) to comply with building code requirements or simply as a precautionary measure. Burying multiple wires in the ground requires substantial excavation which is rarely inexpensive. Similarly, repairing, replacing or moving these wires also requires substantial, expensive excavation.  
       [0008] Alternatively, the wires may need to be suspended at a height substantially above ground level. Such suspension may require the installation and maintenance of some sort of suspension devices, such as telephone poles. These poles would be placed in and around the chemical plant. This, again, may be an expensive undertaking. Finally, with regard to hard wiring, the wires themselves are usually expensive and are prone to breaking, cracking or failing in some sort of way. Thus, it is apparent that a wireless toxic gas monitoring system is desirable. The present invention comprises such a wireless toxic gas monitoring system.  
       [0009] It is common to monitor gas levels around large plants. Additionally, it is not uncommon for gas monitors to be placed some distance from these large plants. Consequently, the monitors may have to transmit information a substantial distance. Moreover, because the destination of this information is often located somewhere deep within the plant, e.g., a central control room, the monitors may need to relay this information through physical objects, such s layers of concrete, steel, insulation and other building materials.  
       [0010] In addition to physical barriers, the monitors usually need to transmit the information through substantial interference as well. Electric equipment and communication systems existing in almost all plants create vast amounts of interference such as electromagnetic waves, for example. Thus, a wireless gas monitoring system that is able to transmit information over a substantial distance and through substantial amounts of interference is desirable. The current invention utilizes, but is not necessarily limited to, licensed radio frequencies that operate at higher powers and are therefore able to transmit over large distances and through substantial amounts of interference.  
       [0011] Radio telemetry has recently been used as a lower cost alternative to hard wiring the monitors to the output or control systems. A typical radio telemetry system using RTU&#39;s, while reducing significant installation costs, still requires both the high cost of the RTU as well as the installation costs to wire the gas monitors to the RTU. With the advent of the current invention, the advantages of wireless toxic gas monitoring systems are realized. This particularly true with respect to very long conduit runs, such as with perimeter monitoring applications, where the cost of the RTU and wiring the sensors to the RTU is increased by the long lengths of the conduit and installation costs.  
       [0012] Additionally, most monitors of the related art are event triggered only. By this it is meant that the monitors only relay a signal when they detect a high level of gas. The monitors merely let you know when a threshold level of gas (such as a gas denoted “alpha”) has been surpassed. For instance, if a system were set to detect 0.5 ppm of gas alpha but a dangerously high level of 20 ppm of gas alpha existed around the plant, the detector would only transmit a signal telling the controller that an amount of gas alpha above 0.5 ppm had been detected. However, the actual concentration, i.e. the dangerously high 20 ppm of gas alpha, would not be relayed back to the control room. This type of system would not provide and quantitative documentation which may be useful in any number of situations.  
       [0013] Thus, a wireless gas monitoring system with heightened sensitivity is desirable. By this it is meant that it would be desirable to have monitors that monitor and relay more detailed information. The current invention does just that. The monitors will not only relay the actual amount of gas detected, i.e. 20 ppm, but they may also relay operating parameters of the system such as the battery voltage, day, date, time, wind speed, weather conditions, etc. existing at the time the gas was detected.  
       [0014] From the foregoing it is clear that certain improvements are desired. Many of the desired improvements have been accomplished by the current invention.  
       [0015] The present invention contemplates a new and improved method and apparatus for wireless gas monitoring which is simple in design, effective in use, and overcomes the foregoing difficulties and others while providing better and more advantageous overall results.  
       SUMMARY OF THE INVENTION  
       [0016] The current invention is a system for wireless toxic gas monitoring with a monitoring device that eliminates the RTU by integrating the radio transmitter directly into the gas monitor, thus making it integral with the device. Although the current invention may utilize licensed UHF radio transmissions, the device is not limited to the type of radio, whether it be land based, cellular or satellite, the strength or radio or any safety approval classifications.  
       [0017] The transmitters feature a unique method of wireless monitoring that eliminates not only the high installation costs of hardwired systems, but also the cost of wireless Remote Terminal Units (RTU&#39;s). A typical perimeter gas monitoring system, where the monitors are completely hardwired to the master site, costing in the neighborhood of $400,000 may only cost $200,000 if the monitors are hardwired to RTUs and the RTUs transmit via radio to the master. The current invention which has the transmitters integral with the monitors would reduce the cost of this system to approximately $100,000 by eliminating the RTU&#39;s and the associated costs of installation and installation materials.  
       [0018] One advantage of the current invention is that the licensed radio frequencies enable the current invention to operate at higher powers. This allows the monitors of the current invention to transmit information over large distances and through substantial amounts of interference.  
       [0019] Another advantage of the current invention is the fact that it is wireless. This permits toxic gas monitoring and installation to be performed in an inexpensive manner not requiring substantial and frequent maintenance.  
       [0020] Yet another advantage of the current invention is the fact that remote transmitters are integrated into the monitors of the current invention. This enables equipment, maintenance, labor, manufacturing and installation costs and expenses to be reduced.  
       [0021] Still another advantage of the current invention is the fact that each of the monitoring devices and the output center may comprise a transceiver. The transceiver can both transmit and receive messages. Separate transmitters and receivers are therefore not needed and costs are thereby reduced.  
       [0022] Another advantage of the current invention is its heightened sensitivity. Upon detection of a gas, the monitors monitor and transmit a substantial amount of detailed information.  
       [0023] Still other benefits and advantages of the invention will become apparent to those skilled in the art upon a reading and understanding of the following detailed specification.  
       [0024] A plurality of gas monitor stations are provided at locations which are spaced from a potential source of a selected gas whose presence is to be detected. Each of the gas monitor stations includes a gas sensor assembly, a control assembly, a radio, and data entry apparatus. The data entry apparatus may be a keypad which is manually actuated. However, the data entry apparatus may include magnetically actuated switches and/or a remote control unit. A display is provided at each gas monitor station to provide for visual review of data entered at the gas monitor station.  
       [0025] In response to a predetermined condition, radio transmission to a master station is initiated from any one of the gas monitor stations. The predetermined condition which results in initiation of radio transmission may be one or more of a plurality of different conditions. The conditions which result in initiation of radio transmission may be varied by actuating the data entry apparatus to change data stored in the control assembly at each of the gas monitor stations.  
       [0026] The conditions which result in initiation of radio transmission from any one of the gas monitor stations may include one or more of the following conditions:  
       [0027] (a) Sensing of a predetermined concentration of the selected gas in the atmosphere at the gas monitor station.  
       [0028] (b) Sensing of a predetermined change in the concentration of the selected gas in the atmosphere at the gas monitor station. The change in the concentration of the selected gas may be either an increase or a decrease in the concentration of the selected gas.  
       [0029] (c) The elapse of a predetermined maximum length of time since the last radio transmission was made.  
       [0030] (d) Determining that a moving average of sensed concentration of the selected gas exceeds a predetermined magnitude.  
       [0031] (e) Determining that a battery, which supplies current for the radio, has an output voltage which is less than a predetermined voltage.  
       [0032] (f) A change in battery voltage by a predetermined amount.  
       [0033] It should be understood that the data entry apparatus at each of the gas monitor stations may be utilized to select any one or more of the foregoing conditions or other conditions not set forth above, to initiate radio transmission from a gas monitor station while omitting other conditions. The data entry apparatus may also be utilized to enter data corresponding to parameters, that is, limits, utilized in association with each of the conditions which initiate radio transmission from a gas monitor station to a master station. The sensor assembly at each of the gas monitor stations may be calibrated by exposing the sensor assembly to a known concentration of the selected gas. This may be done by exposing the sensor assembly to a container of gas or gas-generating device, such as a permeation tube calibrator or gas generator. The data entry apparatus is actuated to adjust data set forth on a display at the gas monitor station to correspond to the known concentration of the selected gas.  
       [0034] Once the sensor assembly has been calibrated, the sensor assembly may be checked by applying a predetermined voltage to the sensor assembly.  
       [0035] In order to eliminate the effect of transient conditions, such as puffs of the selected gas, the sensor reading at a gas monitor station is averaged over a predetermined period of time. This period of time may be relatively short, for example, thirty seconds or less. The predetermined period of time over which the sensor readings are averaged may be entered into the control assembly by actuating the data entry apparatus at a gas monitor station.  
       [0036] It should be understood that anyone of the features of the invention may be used separately or in combination with other features. It should be understood that features which have not been mentioned herein may be used in combination with one or more of the features mentioned herein. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0037] The invention may take physical form in certain parts and arrangement of parts. A preferred embodiment of these parts will be described in detail in the specification and illustrated in the accompanying drawings, which forms a part of this disclosure and wherein:  
     [0038]FIG. 1 shows a gas monitoring system of the related art wherein the gas monitors are nard wired to the control center of a plant;  
     [0039]FIG. 2 is an alternative depiction of a gas monitoring system of the related art wherein the gas monitors are hard wired to the control center of a plant;  
     [0040]FIG. 3 shows the wireless gas monitoring system of the current invention wherein the transmitters are integral with the monitors;  
     [0041]FIG. 4 shows the current invention utilizing cellular and/or low earth orbit (LEO) satellite technology;  
     [0042]FIGS. 5 and 5 a  shows the preferred embodiment of the current invention utilizing radio and solar technology;  
     [0043]FIG. 6 shows another preferred embodiment of a monitoring device of the current invention;  
     [0044]FIG. 7 is a pictorial illustration of one gas monitor station of a plurality of gas monitor stations which are disposed at selected locations spaced from a potential source of a selected gas;  
     [0045]FIG. 8 is an exploded simplified illustration of apparatus utilized at the gas monitor station of FIG. 7 and illustrating a housing, data entry apparatus, control assembly, battery, radio, and sensor assembly which are disposed at the gas monitor station of FIG. 7;  
     [0046]FIG. 9 is an enlarged simplified schematic illustration of the control assembly utilized at the gas monitor station of FIG. 7;  
     [0047]FIG. 10 is a simplified plan view of data entry apparatus and a display provided at the gas monitor station of FIG. 7;  
     [0048]FIG. 11 is a schematic illustration depicting the relationship between manually actuated membrane switches in the data entry apparatus of FIG. 10;  
     [0049]FIG. 12 is an enlarged fragmentary schematic sectional illustration, taken along the line  12 - 12  of FIG. 10, illustrating the construction of a portion of an enclosure for the switches of FIG. 11;  
     [0050]FIG. 13 is a graph depicting the manner in which sensed gas concentration varies as a function of time;  
     [0051]FIG. 14 is a schematic illustration depicting the relationship between components of the gas monitor station;  
     [0052]FIG. 15 is a schematic illustration of circuitry associated with power for the gas monitor station;  
     [0053]FIG. 16 is a schematic illustration of a microprocessor, display circuitry, and alarm circuitry which are connected with the circuitry of FIG. 15 at the gas monitor station;  
     [0054]FIG. 17 is a schematic illustration of circuitry connected with a gas sensor at the gas monitor station and the microprocessor of FIG. 16;  
     [0055]FIG. 18 is a schematic illustration of a modem and data entry circuitry connected with the microprocessor of FIG. 16; and  
     [0056]FIG. 19 is a schematic illustration of circuitry which connects a radio at the gas monitor station with the microprocessor of FIG. 16 and the modem of FIG. 18. 
    
    
     DESCRIPTION OF ONE SPECIFIC PREFERRED EMBODIMENT OF THE INVENTION  
     [0057] Referring now to the drawings, which are for purposes of illustrating a preferred embodiment of the invention only, and not for purposes of limiting the invention, FIG. 1 shows a chemical processing plant  10 . The plant  10  is depicted as having a discharge means  12 . The discharge means  12  are a potential source of a selected gas. Multiple toxic gas monitors or monitor stations  14   a ,  14   b ,  14   c    14   d  are placed around the plant  10 . FIGS. 1 and 2 show the previous technology wherein each monitor or station  14   a ,  14   b ,  14   c ,  14   d  had to be hard wired via cables  16   a ,  16   b ,  16   c ,  16   d  to the control center or master station  18 . Should control center or master station  18  need to be relocated at a different site, such as outside of the plant  10 , the cables  16   a ,  16   b ,  16   c ,  16   d  would need to be extended to this remote site. Such a configuration and any changes to such a configuration were expensive, labor intensive and required substantial frequent maintenance.  
     [0058]FIG. 2 depicts the interconnection of some of the equipment of related art gas monitoring systems. FIG. 2 also gives some specifications on some of this equipment. Note that, in operation, the gas monitoring system  8  (FIG. 3) of the current invention may utilize much of the same equipment. However, most of the interconnection of this equipment will be by way of radio frequencies rather than wire cables  16   a ,  16   b ,  16   c ,  16   d  (FIGS. 1, 2 and  3 ). The inventive system disclosed herein has advantages over the related art because, in addition to having all of the hardware, software and other elements necessary to monitor around the plant, the transmitters are integral with the monitors  14 . Thus, no hardwiring and RTU&#39;s are necessary.  
     [0059]FIG. 4 shows a chemical plant  10  with monitors  14  around the plant. Monitors  14  employing radio telemetry are depicted comprising satellite dishes  20 . In the past, in order to have wireless connection to the control center  18 , each monitor or station  14  needed to employ an antenna more powerful than a cellular phone antenna. Additionally, these antennas needed to have a higher gain than that of a cellular phone antenna. These requirements were usually met by a remote terminal unit that sometimes included a satellite dish. Prior to the advent of low earth orbit (LEO) satellite technology, these large dishes were necessary because previous satellites were at a higher altitude and had orbits different from low earth orbit satellites  22 . Because the prior satellites were at higher altitudes and had different orbits, an antenna with a higher gain, such as a satellite dish, was required for communication between the satellite  24  and the monitor or station  14  on the ground. These remote terminal units were bulky and expensive. Should the current invention utilize low earth orbit satellite technology, the gas monitors or stations  14  need only comprise a small antenna  26  similar to a small cellular phone antenna to communicate with the low earth orbit satellites. Thus, the current invention permits an inexpensive, reliable and virtually maintenance free wireless connection to be made to the control center or master station  18 . Compared to the related art, the current invention I provides a substantially smaller and less expensive detection system.  
     [0060] When utilized in conjunction with LEO technology, the small wireless antenna  26  employed in the toxic gas monitoring devices or gas monitor stations  14  of the current invention transmit a wireless data message comprising information such as gas detected, wind speed and wind direction. The data message is transmitted to a LEO satellite  22  where it may be linked to a local gateway for validation and optimal routing to the recipient which would be the control center or master station  18 . This transmission pathway is depicted as lines  28  in FIG. 4. With this wireless technology, the control center  18  may be easily and conveniently located and relocated without the inherent difficulties of hard wiring or moving cumbersome and expensive equipment. For demonstrative purposes, FIG. 4 depicts control center  118  being located outside of plant  10 .  
     [0061] When operating with LEO technology, a monitor  14  transmits information regarding a change in toxic gas detected by way of the low earth orbiting satellites  22  to the control center  18 . This information is transmitted repeatedly as changes in reading occur. However, once the monitor  14  no longer detects toxic gas at a predetermined level, the transmitter preferably stops transmitting and waits for the next changed reading.  
     [0062] Additionally, the control center  18  may have the ability to transmit as well as receive data messages. For instance, the control center  18  may periodically poll each monitor  14  for supervisory purposes. Thus, each monitoring device  14  may also have the ability to receive as well as transmit wireless data messages, such as in the form of polling messages, for example.  
     [0063] With low earth orbit satellites, more than one monitor  14  of the wireless toxic gas monitoring system of the current invention can interactively relay data messages. Each and every one of the monitors  14  can simultaneously transmit data messages to the low earth orbit satellites and the satellites will carry through and deliver the entire data message too the control center  18 . Because of this capability, low earth orbit technology offers the advantage of not missing transmissions and information.  
     [0064]FIG. 3 shows the preferred embodiment of the current invention that does not use satellite technology. A chemical plant  10  is shown with gas monitor stations  14   a ,  14   b ,  14   c ,  14   d  of the current invention around the plant. In this embodiment, the transmitters are integral with the monitor stations  14 . A wireless data message comprising information such as the actual amount of gas detected, battery voltage, wind speed, wind direction, etc.  36  is transmitted from the gas monitor stations  14   a ,  14   b ,  14   c ,  14   d  to the control center  18 . It is contemplated that the gas monitor stations  14   a ,  14   b ,  14   c ,  14   d  may be constructed without including apparatus to monitor wind speed and/or wind direction. With this wireless technology, the control center  18  may be easily and conveniently located and relocated without the hassle of hard wiring or moving cumbersome and expensive equipment. For demonstrative purposes FIG. 3 depicts control center or master station  18  being located outside of plant  10 . A mobile control center of master station  18  may be provided if desired.  
     [0065] So that the system may be further wireless, applicant envisions that the monitors or gas monitor stations  14   a ,  14   b ,  14   c ,  14   d  may be solar  7  or battery  9  powered (FIGS. 5 and 5 a ) or powered by any other source of power chosen with sound engineering judgment. In the preferred embodiment, each monitor comprises a 4 amp lead acid battery capable of supporting the system a nominal operation for up to five days without power as well as a 20 watt solar panel. Additional battery power is optional. Preferably, the monitoring devices operate properly at temperature range between −40 degrees C. to +50 degrees C. It is also preferred that the monitors are shielded against lightning strikes through a lightning arrestor  100  combined with a copper ground rod  21  (FIG. 5). It is further preferred that the solar panel be 5 inches by 13 inches and that each monitoring device be comprised with a 6 inch by 6 inch housing.  
     [0066] With continuing reference to FIG. 3, the gas monitoring system  8  disclosed herein may operate at any frequency, but preferably utilizes licensed radio frequencies. Licensed radio frequencies provide better and more advantageous overall results than the radio frequencies used in the related art. This is because nonlicensed radio frequencies operate at lower power than licensed radio frequencies. The lower powered nonlicensed radio frequencies are unable to transmit data from the monitors  14   a ,  14   b ,  14   c ,  14   d  to the control room  18  typically located inside the plant  10 . Additionally, the nonlicensed radio frequencies are unable to transmit over substantial distances or through substantial interference. The monitors  14   a ,  14   b ,  14   c ,  14   d  disclosed herein can transmit detailed information over substantial distances and through substantial amounts of interference.  
     [0067] The monitors  14   a ,  14   b ,  14   c ,  14   d  disclosed herein can accurately transmit information through walls  11  as well as interference in the form of electromagnetic waves, for example. These particular types of interferences and impedances are encountered in almost all gas monitoring applications. This is because the destination of the information transmitted by the monitors  14   a ,  14   b ,  14   c ,  14   d  is usually located somewhere within the plant  10 . Typically, the control room  18  is centrally located somewhere deep within the plant  10 . Consequently, the monitors  14   a ,  14   b ,  14   c ,  14   d  must transmit the data through physical barriers of the plant  10  such as concrete and steel walls  11 . Additionally, this information must successfully traverse interference created by electric equipment and communication systems. Such interference typically presents itself in the form of electromagnetic waves which exist within virtually all plants  10 .  
     [0068] The licensed radio frequencies disclosed herein may be obtained by way of application to the federal Communications Committee “FCC”. Applicant notes that a particularly useful bandwidth of licensed radio frequencies would be between 450-470 megahertz “mHz”. Preferably, the current invention operates within this bandwidth. Preferably, the transmitters  42  that are integral with the monitoring devices  14  comprise an up to a 5 Watt 450-470 mHz (UHF) radio transmitter. This eliminates the need for wiring to a Remote Radio Terminal Unit (RTU), thereby significantly reducing costs.  
     [0069] Regardless of the frequency used, the current invention, even without the use of satellite technology, will not miss transmissions. This is because each monitoring device  14  will very rapidly transmit its readings to an output center. Because these transmissions occur so often, there is insufficient time for data readings to accumulate between transmissions.  
     [0070] The gas monitor stations  14  may comprise more than one sensor  38  to sense various gasses. In the preferred embodiment, the gas sensor  38  may be an electrochemical, infrared or catalytic gas sensor. Some of the sensors may operate in the range of 4 mA to 20 mA. Preferably, the gas detection system  8  disclosed herein comprises means  5  (FIG. 5) to interface with each monitor  14   a ,  14   b ,  14   c ,  14   d  (FIG. 3). Preferably, this is an easily accessible keypad  5   a ,  5   b ,  5   c ,  5   d  on the face of each monitor  14   a ,  14   b ,  14   c ,  14   d . This keypad  5   a ,  5   b ,  5   c ,  5   d  allows the monitoring devices to be manipulated. For instance, they may enable the monitoring devices  14   a ,  14   b ,  14   c ,  14   d  to be programmed for the particular type of gas to be monitored for as well as the level of this gas which causes the monitors to start transmitting information.  
     [0071] Preferably, each monitoring device  14  comprises a display  80  (FIG. 5). Each device can be configured so that any number of readings taken by the device are displayed on the display  80 . With reference to FIG. 6, each monitoring device also comprises a microprocessor  34  for driving the display  80  and causing the appropriate reading to be displayed on the display  80 . The microprocessor  34  also enables a user to interface with the device via the interface means  5 . Preferably, the microprocessor links the display  80  and the interface means  5  so that the display  80  is useful in assisting a user to interface with the monitoring device  14 . It is further preferred; that the microprocessor  34  monitors sensor readings and initiates and controls transmissions. Optionally, the microprocessor may cause the monitoring device to periodically transmit polled data readings.  
     [0072] Additionally, it is preferred that the monitoring devices  14   a ,  14   b ,  14   c ,  14   d  are reprogrammable. By this it is meant that the monitors  14   a ,  14   b ,  14   c ,  14   d  may be used time and again for the detection of different gases. It is further preferred that via the interface means  5  selective monitoring may be accomplished. By this it is meant that the particular condition(s) to be monitored for may be selected and the monitoring devices  14   a ,  14   b ,  14   c ,  14   d  calibrated and configured accordingly. It is preferred that the monitoring devices can at least monitor for chlorine, ammonia, hydrogen fluoride, hydrogen cyanide, phosphine, fluorine, chlorine dioxide, phosgene, carbon monoxide, ozone, diborane, methyl mercaptan, hydrogen sulfide, sulfur dioxide, hydrazine, silane and germane at a plurality of concentrations. An alternative embodiment within the scope of the invention has a receiver  44  connected to the microprocessor  34 . The receiver  44  allows remote transmissions to be received by the monitoring device  14 . This may allow, for example, the operating parameters of the monitoring device to be reprogrammed from a remote location.  
     [0073] For optimum performance, simplicity and efficiency, it is preferred that the transmitter  42  and receiver  44  are integrated into a single component known as a transceiver  43  (FIG. 6). In this fashion, each monitoring device  14  is very compact and is literally a remote transmitting unit as well as a remote receiving unit. The monitoring devices  14  then do not require remote terminal units RTU&#39;s to transmit or receive data messages. The cost of toxic gas monitoring is thereby significantly reduced.  
     [0074] In operation, preferably the monitors “sleep” unless a change causes them to transmit. By this it is meant that the monitors do not transmit data unless a preprogrammed level of a particular gas is detected or upon a battery voltage change or in reply to a transmission from the master site(s). Once this event occurs, the monitors begin to transmit data. The monitors  14   a ,  14   b ,  14   c ,  14   d  transmit information regarding changes in gas detected. This information is transmitted repeatedly as changes in readings continue. However, once the monitors  14   a ,  14   b ,  14   c ,  14   d  no longer detect gas at a predetermined level, the monitors  14   a ,  14   b ,  14   c ,  14   d  again rest and wait for the next changed reading. Upon a subsequent changed reading that exceeds the predetermined threshold level, the monitors once again begin to transmit information. According to another embodiment of the invention, the control center  18  may periodically poll each monitor  14   a ,  14   b ,  14   c ,  14   d  for supervisory purposes.  
     [0075] The monitors of the current invention allow the transmission of information which may be quantified. By this it is meant that the monitors transmit detailed information. The monitors transmit not only the actual amount of gas detected but also the status of important surrounding circumstances. For instance, this information may include the parts per million (ppm) of gas detected, explosion limit levels, battery voltage (voltmeter  32 ), alarm statuses, date, time, wind speed, wind direction (weather sensing means  36 ), etc. existing at the time the gas was detected.  
     [0076] According to another embodiment of the invention, upon detection of a toxic gas, usually at a predetermined level, the system may sound an alarm box  19  (FIG. 3). The alarm may be sounded alone or in addition to relaying the aforementioned information to the output center  18 . Alternatively, the master site may only be the alarm boxes rather than an output center  18 . Additionally, alarms  19  could also be on the monitors  14  and/or in the control room  18 . Preferably, the alarm box is a 5 Watt UHF radio receiver device providing 3 amp to 10 amp rated relays that are synchronized to the alarm settings of the transmitters.  
     [0077] Preferably, the output center or master station  18  has a receiver  74  for receiving transmissions from a monitoring device or gas monitor station  14 . The output center  18  also has a housing  60 , microprocessor  64 , and power means  62  for powering the output center. Additionally, the output center  18  has a display  70  and interface means  65 . Preferably, the microprocessor  64  connects the receiver  74 , display  70 , interface means  65  and signaling means  66  for producing a signal. The signal may be an audible or visual signal that signal the detection of a certain gas.  
     [0078] As mentioned above, the output center or master station  18  may periodically poll each monitoring device or gas monitor station  14 , such as for supervisory purposes, for example. Thus, as with each monitoring device  14 , the output center  18  preferably can transmit as well as receive data messages. To effect this the output center  18  may have a transmitter  75  operatively connected to the microprocessor  64 . However, for optimum compactness and operating efficiency, again as with each monitoring  14 , the output center  18  has a single transceiver  77  that both transmits and receives data messages (FIG. 6). This eliminates the need for the output center  18  to have both a transmitter  75  and a receiver  74 .  
     [0079] Preferably, each monitoring device  14  has its own housing  30  and site address with respect to the rest of the system. This address distinguishes one monitoring device from another. Preferably, each monitor can be tied into other detection systems by providing an ASCII formatted RS232 signal to a DCS. Since many DCS systems require Modbus for their driver, the Master RTU is often directly compatible. Analog outputs for each transmitter can be provided through the use of an analog output expansion card.  
     [0080] Reliable and appropriate sensor technology will be incorporated into the system. A couple of sensor manufacturers, with whose products the current invention performs optimally, are Sensoric and GmbH. Some benefits of the sensors used in the current invention are: no temperature effects, no humidity effects, no periodic zeroing required, no background current, greater chemical selectivity, no drying out, no taking on moisture, no costly recharges or refilling of electrolyte.  
     [0081] Specifications regarding the one specific embodiment of the current invention are as follows:  
                                      Housing   Nema-4X (Optional Explosion Proof Version       Temperature Range   −40° C. to +50° C.       Humidity Range   0-100% RH       Power Options   115/220 VAC or 12 VDC Solar Powered       Power Consumption   40 mA Nominal, 1000 mA during transmission       Internal Battery   12 VDC, 4 Amps       Radio Power   2 watts or 5 watts       Antenna Gain   3 db       Radio Frequency   UHF Licensed, Provided by Gastronics       Frequency Adjust   Crystals       Microprocessor   32 bit       Analog Resolution   16 bit       Keypad Settings   Trigger on Change, Drop Out, Analog Filtering,           STEL/Hi/HIHi Alarms, Remote Site Address,           Master Site(s) Address, Alarm Box Site Address       Jumper Settings   Display Resolution, Sensor Type, Sensor Range                  
 
     [0082] A gas monitor station  14  is illustrated in FIG. 7 of the application drawings. The gas monitor station  14  is one of a plurality of identical gas monitor stations which are disposed at selected locations spaced from a potential source  12  of a selected gas. The gas monitor stations  14  may be arrayed around a potential source  12  of a selected gas in the manner illustrated in FIG. 3. Alternatively, one or more of the gas monitor stations  14  may be located inside the plant  10 . The master station  18  may be located outside the plant  10 , as shown in FIG. 3, or located inside the plant, as shown in FIG. 4.  
     [0083] The environment around the source  12  of the selected gas is monitored for the presence of the selected gas by the gas monitor stations  14  (FIGS. 3 and 4). The gas monitor stations  14  (FIGS. 3 and 4) may all have the same construction and mode of operation as the gas monitor station  14  of FIG. 7. However, the gas monitor station  14  of FIG. 7 is not equipped to cooperate with satellites  22  and  24  (FIG. 4). If desired, the gas monitor station  14  of FIG. 7 could be equipped to cooperate with satellites.  
     [0084] Upon the occurrence of a predetermined condition, for example, the sensing of a predetermined concentration of the selected gas in the atmosphere adjacent to a gas monitor station  14  (FIG. 7), a radio in the gas monitor station transmits to a master station or control center  18  (FIG. 3). Although it is believed that it may be desired to have the master station or control center  18  in a building or plant  10  and the gas monitor stations  14  disposed in an array about the building, one or more of the gas monitor stations could be provided within the building.  
     [0085] The master station or control center  18  may be a stationary control center disposed within the building or plant  10  which provides the source of the gas which is to be detected by the gas monitor stations  14 . However, the master or control station  18  could be disposed at a location which is remote from the source of the gas which is to be detected. The potential source of gas could be outside of any building. Although only a single master station or control center  18  has been illustrated schematically in FIG. 3, it should be understood that a plurality of master or control stations could be provided if desired.  
     [0086] It is contemplated that a stationary master station  18  could be provided within the building  10  and a mobile or secondary master station (not shown) could be provided in a vehicle which is capable of traveling to the gas monitor stations  14 . Data from the gas monitor stations  14  could be utilized to plot, on a suitable display, the probable configuration of a cloud of gas from the potential source  12  of gas. This would enable the mobile master station to approach one of the gas monitor stations  14  from a direction which would minimize exposure of occupants of the vehicle containing the movable master station or control center to a gas from the potential source  12  of gas. It is contemplated that radio communications would be maintained between the mobile master station  18  and the stationary master station which may be disposed within the building  10 .  
     [0087] It should be understood that the gas monitor stations  14  could all be provided within the building  10  if desired. A stationary master station  18  could be provided in the building  10  along with the gas monitor stations  14 . Alternatively, the master station  18  could be disposed at a location remote from the building  10  and the potential source  12  of gas. A mobile master station could be provided within the building  18  in a movable housing which may be carried by an individual, such as a technician, to any one of the gas monitor stations in the building.  
     [0088] The gas monitor station  14  illustrated in FIG. 7 is intended to be located in the environment around and spaced from the source of the gas to be monitored. Thus, the gas monitor station  14  would be mounted at a location remote from the building  10  and the master station  18  in the manner illustrated schematically in FIG. 3. The gas monitor station  14  is constructed in such a manner that being outdoors in the environment around a source of gas will not result in degradation of the operating characteristics of the gas monitor station. However, the gas monitor station  14  could be disposed in the building  10  if desired.  
     [0089] The gas monitor station  14  includes a housing  30  (FIG. 7) which is connected with a stationary mounting post or rod  200 . A solar panel  7  is mounted on the post  200  above the housing  30 . The solar panel  7  is positioned to face toward the sun. The sun&#39;s rays activate the solar panel  7  to provide energy for the gas monitor station  14 . The solar panel  7  is connected with the housing  30  by a cable  202 .  
     [0090] The gas monitor station  14  includes a gas sensor  38  (FIG. 7) which is fixedly connected to the housing  30  and is exposed to the atmosphere around the housing. In the embodiment of the gas monitor station  14  illustrated in FIG. 7, the gas sensor  38  is disposed outside of and is connected to the housing  30 . If desired, the gas sensor  38  could be enclosed in the housing and exposed to the atmosphere adjacent to the housing by suitable openings within the housing. If desired, a pump could be provided to induce air adjacent to the housing  30  to flow into the housing. Alternatively, the gas sensor  38  could be spaced from the housing  30  and connected with the housing by a suitable cable, in much the same manner as in which the solar panel  7  is connected with the housing  30  by the cable  202 .  
     [0091] Data entry apparatus  5  (FIGS. 7 and 14) is connected with the housing  30  (FIG. 7) and is exposed to the environment around the housing. Therefore, the data entry apparatus  5  is constructed in such a manner that exposure of the data entry apparatus to the environment around the gas monitor station  14  does not result in malfunctioning of the data entry apparatus. In the embodiment of the invention illustrated in FIG. 7, the data entry apparatus  5  includes a keypad  206  which is manually actuated from outside of the housing  30  to enter data at the gas monitor station  14 .  
     [0092] The keypad  206  is exposed to the environment around the gas monitor station  14 . Therefore, depending upon the specific location where the gas monitor station  14  is positioned, the keypad  206  may be exposed to rain, sleet and/or snow. In addition, the data entry apparatus  5  will be exposed to hot summer sun and cold winter winds. Therefore, it is important that the data entry apparatus  5  be capable of withstanding a wide range of adverse environmental conditions.  
     [0093] In the embodiment of FIG. 7, the data entry apparatus  5  includes the keypad  206  which is manually actuated to enter data. However, it is contemplated that the data entry apparatus could have any one of many other known constructions. For example, the data entry apparatus  5  could include a plurality of switches which are contained within the housing  30  and are magnetically actuated from outside of the housing. Alternatively, a remote control unit, similar to the remote control units commonly utilized in association with television sets, could be utilized to actuate data entry apparatus contained within the housing  30 .  
     [0094] A display  80  (FIGS. 7, 14 and  16 ) is provided in the housing  30 . When the data entry apparatus  5  (FIGS. 7 and 14) is actuated to enter data at the gas monitor station  14  (FIG. 7), indicia at the display  80  changes to indicate the data entered. This enables an individual entering the data by manually actuating the keypad  206 , to view the display  80  and determine whether or not the data was correctly entered. Of course, if the data was not correctly entered, the individual entering the data would actuate the keypad to revise the data. The display  80  also sets forth indicia which prompts an individual actuating the keypad  206  to enter the required data. The display  80  is effective to increase the user friendliness of the data entry apparatus  5  at the gas monitor station  14 .  
     [0095] The keypad  206  is accessible from outside of the housing  30 . The display  80  is visible from outside of the housing  30 . This enables an individual desiring to enter data at the gas monitor station  14  to enter the data without opening the housing  30 . However, if desired, the data entry apparatus  5  and/or display  80  could be enclosed within the housing. Of course, this would require an individual desiring to enter data at the gas monitor station  14  to open a cover or other part of the housing to obtain access to the data entry apparatus  5  and/or make the display  80  visible. Alternatively, the display  80  could be visible from outside of the housing and the data entry apparatus  5  enclosed within the housing. If this was done, a remote control apparatus, which may be similar to that utilized in association with a television set, may be used to effect the entry of data at the gas monitor station  14 . Alternatively, while the housing  30  remains closed, one or more magnets outside of the housing could be utilized to actuate switches within the housing. By having the display  80  visible from outside the housing, an individual entering the data would be able to review the data. In addition, the individual would be able to follow instructions provided at the display  80  as to the next data to be entered.  
     [0096] A radio or transceiver  248  (FIGS. 8 and 14), corresponding to the transmitter  42  of FIG. 6, is enclosed within the housing  30 . An antenna  26  is mounted on the outside of the housing  30 . When a radio  248  (FIG. 8), disposed within the housing  30  is activated, radio signals are transmitted from the antenna  26  to the master station  18 . These radio signals transmit data from the gas monitor station  40  to the master station  18 .  
     [0097] The housing  30  (FIG. 8) includes a main section  212  and a cover section  214 . When the housing  30  is in the closed condition illustrated in FIG.  7 , the cover section  214  is fixedly connected to the main section  212  (FIG. 8) by suitable fasteners  216 . Although the illustrated cover section  214  and main section  212  of the housing  30  are formed of aluminum, it is contemplated that the housing could be formed of other materials which would protect the contents of the housing from the environment around the gas monitor station  14 . Of course, if the gas monitor station  14  is to be located within a building, rather than outside a building, the housing  30  would not have to be as rugged.  
     [0098] The housing  30  enclosed a control apparatus  220  (FIG. 8). In the illustrated embodiment, the control apparatus  220  includes a single printed circuit board  222  on which components of the control apparatus  220  are mounted. By mounting the components of the control apparatus  220  on a single circuit board, construction and/or maintenance of the control apparatus  220  is facilitated. The control apparatus  220  may include a plurality of circuit boards if desired.  
     [0099] The printed circuit board  222  (FIGS. 8, 9,  14  and  16 ) forms part of display  80  and is mounted on the inside of the cover section  214 . A liquid crystal display  226  is mounted on the printed circuit board  222  (FIGS. 8 and 9) and is disposed in alignment with an opening  228  formed in the cover section  214  of the housing  30 . A clear window  232  is disposed in a cover plate  234 . It should be understood that the display  80  could have a construction other than the liquid crystal display  226 .  
     [0100] The cover plate  234  has a multi-layered construction (FIG. 12) and includes a plurality of membrane switches (FIG. 11) disposed between layers of the cover plate. A suitable adhesive is provided on the back side of the cover plate  234  to fixedly secure the cover plate to the cover section  214  (FIG. 8). The adhesive on the back of the cover plate  234  secures the cover plate to the cover section  214  with the window  232  aligned with the opening  228  in the cover section  214 . The cover plate  234  seals the opening  339  to prevent moisture from entering the housing  30 .  
     [0101] A flexible conductor  240  (FIG. 8) extends from the cover plate  234  through a slot  241  in the cover section  214  to connector pins  243  (FIG. 18) on the printed circuit board  222  (FIG. 8) in the control apparatus  220 . The flexible conductor  240  includes a plurality of flexible conductive ribbons which are connected with the membrane switches  236  (FIG. 11). The conductive ribbons extend from the membrane switches  236  to a connector  242  (FIG. 8) at one end of the conductor  240 . The connector  242  is connected with the connector pins  243  (FIGS. 9 and 18) on printed circuit board  222  (FIGS. 8 and 9) of the control apparatus  220 .  
     [0102] A radio cable  246  (FIG. 8) extends between a radio  248  (FIGS. 8 and 14) and the printed circuit board  222  of the control apparatus  220 . The radio cable  246  is connected to the printed circuit board  222  at connectors  249  (FIG. 19). An antenna cable  250  (FIG. 8) extends between the radio  248  and the antenna  26  (FIGS. 7, 8 and  14 ). The radio or transceiver  248  (FIG. 8) can send signals to the master control station  18  and receive signals from the master control station. The radio or transceiver  248  performs the functions of the transmitter  42 , transceiver  43 , and receiver  44  of FIG. 6. The radio  248  is connected with a mounting bracket  254  (FIG. 8). The mounting bracket  254  is secured in the main section  212  of the housing  30  by suitable fasteners.  
     [0103] The control apparatus  220  is effective to control the operation of the radio or transceiver  248 . Thus, when predetermined conditions, corresponding to data entered at the switches  236  of the keypad  206 , have been met, the control apparatus  220  initiates transmission from the radio  248  to the master station  18 . The master station  18  can initiate transmission to the radio  248 .  
     [0104] A power supply  250  (FIG. 14) includes a battery  260  (FIG. 8), corresponding to the battery  9  of FIG. 6, is secured to the mounting bracket  254  by a battery clamp  262 . The battery  260  is connected with the printed circuit board  222  in the control apparatus  220  by a pair of conductors  266  and  268 . The conductors  266  and  268  are connected with the printed circuit board  222  at terminals  274  (FIG. 15). The battery  260  provides power for the radio  248 .  
     [0105] The solar panel  7  (FIG. 7) is connected with the printed circuit board  222  and the control apparatus  220  by the cable  202  (FIG. 8). The solar panel  202  is connected with the printed circuit board  222  at terminals  271  (FIG. 15). Power from the solar panel  7  charges the battery  260  in the power supply  250  (FIG. 14) to supplement the power provided by the battery. It is contemplated that other sources of power may be provided to supplement the battery  260  (FIG. 8) if desired. For example, a 110 volt, AC, power may be connected with a transformer  294  (FIG. 9) in the power supply  250 .  
     [0106] The sensor  38  (FIGS. 7 and 14) is mounted on the outside of the main section  212  (FIG. 8) of the housing  30 . The sensor  38  is connected with the control apparatus  220  by a sensor cable  274  (FIG. 8). The sensor cable  274  is connected with the printed circuit board  222  at terminals  276  (FIG. 17). The sensor  38  is an electrochemical sensor of the type which is commercially available from Sensoric, Inc. The sensor  38  may be of the aqueous or the organic non-aqueous type.  
     [0107] The sensor  38  may include a sensing electrode which is covered by a membrane of a suitable material, a counterelectrode, and a reference electrode. The selected gas to which the sensor  38  responds seeps through the membrane and reacts at the sensing electrode and/or electrolyte. Although the sensor  38  could have many different constructions, it is contemplated that the sensor could be constructed in a manner similar to that disclosed in U.S. Pat. Nos. 5,958,214; 5,538,620 or 6,129,825. It should be understood that the foregoing are merely examples of known sensors having operating principles which may be utilized in the sensor  38 . It is contemplated that the sensor  38  could have any one of many different constructions.  
     [0108] The control apparatus  220  includes a microprocessor  280  (FIGS. 9, 14 and  16 ) which is mounted on the printed circuit board  222 . The microprocessor  280  is connected with an EEPROM  284  (FIGS. 9 and 17) which functions as a storage bank for data transmitted to the microprocessor when the microprocessor is shut down. The flexible conductor  240  (FIG. 8) for the switches  236  in the keypad  206  is connected with the printed circuit board  222  at a connector  243  (FIGS. 9 and 18). This enables data input at the keypad  206  to be transmitted to the microprocessor  280 .  
     [0109] A modem  288  (FIGS. 9, 14 and  18 ) is built onto the printed circuit board  222  and is connected with the microprocessor  280  (FIGS. 9, 14 and  16 ) and with the radio  248  (FIGS. 8 and 14) through the radio cable  246 . When the microprocessor  280  makes a determination that predetermined conditions corresponding to data entered at the keypad  206  are present, the microprocessor initiates transmission with the radio  248  which is connected with the modem  288 . The microprocessor  280  controls operation of the radio  248  in accordance with the data which is input at the keypad  206 .  
     [0110] The sensor  38  (FIG. 8) is connected with the printed circuit board  222  by the sensor cable  274  (FIG. 9) at terminals  276  (FIG. 17). The sensor cable  274  is connected with the microprocessor  280  through an analog-to-digital converter  290  (FIGS. 9, 14 and  17 ). The sensor  38  may be specifically constructed to detect a selected gas.  
     [0111] The sensor  38  is effective to provide an analog output signal corresponding to the concentration of the selected gas in the atmosphere adjacent to the sensor. The output from the sensor  38  corresponding to the concentration of the selected gas in the atmosphere to which the sensor is exposed is transmitted through the cable  274  to the control apparatus  220 . The cable  274  is connected with the analog-to-digital converter  290 . The analog-to-digital converter  290  converts the analog output signal from the sensor  38  to a digital signal. The digital signal, corresponding to the analog output signal from the sensor  38 , is transmitted to the microprocessor  280 .  
     [0112] When the microprocessor  280  detects that a predetermined concentration of the selected gas is present in the atmosphere, the microprocessor initiates transmission by the radio  248  (FIGS. 8 and 14) to the master station  280 . The specific control apparatus  220  illustrated in FIG. 9 is effective to transmit signals to the master station in response to detection of either one of two concentrations of the selected gas.  
     [0113] When the concentration of the selected gas reaches a first, relatively low, concentration, the microprocessor  280  initiates radio transmission of a HI signal to the master station  18 . The HI signal indicates that the concentration of the selected gas has increased to a level which is of interest. When the concentration of the gas increases to a second level, the microprocessor  280  initiates transmission of a HIHI alarm signal with the radio  248 . This HIHI alarm signal indicates to personnel at the master station  18  that the concentration of the selected gas in the atmosphere at the gas monitor station  14  has reached a level of concern and that suitable action should be taken.  
     [0114] In order to promote understanding of the situation by personnel at the master station  18 , the radio  248  transmits data which is indicative of the actual concentration of the selected gas in the atmosphere adjacent to the gas monitor station  14 . The data transmitted by the radio  248  to the master station  18  is displayed at the master station and indicates the actual concentration of the selected gas sensed by the sensor  38 . Visual and/or audible alarms may be activated at the master station  18  when the data transmitted by the radio  248  corresponds to either a HI alarm or a HIHI alarm. Of course, visual and/or audible alarms may be provided when the data transmitted by the radio  248  corresponds to other predetermined conditions. In addition, visual and/or audible alarms may be provided at the gas monitor station  14 . The alarms at the gas monitor station  14  may be connected with the printed circuit board  222  (FIGS. 8 and 9) at terminals  291  (FIG. 16).  
     [0115] It is contemplated that personnel at the master station  18  will want to know when there is a predetermined variation in the sensed quantity of the selected gas in the atmosphere at the gas monitor station  14 . Therefore, the microprocessor  280  determines when the sensed concentration of the selected gas in the atmosphere adjacent to the gas monitor station  14  has either increased or decreased by a predetermined amount.  
     [0116] When the microprocessor  280  determines that the predetermined variation in the sensed concentration of the selected gas has occurred, the microprocessor initiates any alarm provided at the gas monitor station  14  and transmission to the master station with the radio  248 . The microprocessor initiates transmission of a signal indicating the magnitude of the change in the concentration of the selected gas. The predetermined variation in concentration of the selected gas may occur when the concentration of the selected gas either increases or decreases by the predetermined amount.  
     [0117] It is contemplated that transient conditions may result in an instantaneous increase and/or decrease in the concentration of the selected gas in the environment around the gas monitor station  14 . Thus, a relatively small puff of the selected gas may be blown past the gas sensor  38  (FIG. 7) at the gas monitor station  14 . In order to prevent the transmission of data from the gas monitor station  14  to the master station  18  in response to these transient conditions or puffs of the selected gas, the microprocessor  280  is effective to average the input received from the sensor  38  over a predetermined period of time.  
     [0118] The period of time over which the microprocessor averages the input from the sensor  38  is relatively short to enable the control apparatus  220  to quickly respond to conditions which are not transient. Thus, the microprocessor  280  may average the input from the sensor  38  over a period of time of thirty seconds or less. For example, the microprocessor  280  may average the data received from the gas sensor  38  over a period of approximately ten seconds.  
     [0119] Before the microprocessor  280  initiates transmission with the radio  248  of an alarm signal, whether it is a HI signal or a HIHI signal to the master station, the level of concentration of the selected gas in the atmosphere adjacent to the gas monitor station will have been present for a short period of time, for example, ten seconds. By averaging the output from the gas sensor  38  over a short period of time, false or spurious alarms in response to transient conditions are avoided.  
     [0120] It is contemplated that it may be desired to have a short-term exposure limit (STEL) alarm. A short-term exposure limit alarm averages exposure level over a predetermined length of time. When a multiple of the average sensed concentration of the selected gas and the elapsed time over which the average sensed concentration is determined exceeds a predetermined magnitude, the microprocessor  280  initiates transmission with the radio  248  to inform the master station  18  that short term exposure limit has been exceeded.  
     [0121] For example, the average short-term exposure limit could be set for 0.3 parts per million (ppm) over a period of time, for example, fifteen minutes. The multiple of the average concentration of the selected gas (0.3 ppm) over the period of the selected time (15 minutes) is 4.5. Therefore, if there is an average exposure to 0.3 ppm of the selected gas for a period of fifteen minutes, the microprocessor  280  initiates transmission to the master station  18  with the radio  248 .  
     [0122] It is contemplated that it will be desired to have some check at the master station  18  to determine whether or not the gas monitor station  14  is functioning. This is particularly true when the circumstances are such that the microprocessor  280  does not initiate transmission with the radio  248  in response to a change in sensed concentrations of the selected gas or a change in status for a long period of time. Therefore, when the microprocessor  280  determines that a predetermined maximum length of time has elapsed since the last transmission was made with the radio  248 , the microprocessor initiates transmission with the radio  248  to the master station  18 .  
     [0123] For example, if a time period of thirty minutes passes after a transmission is made by the radio  248  to the master station, the microprocessor  280  initiates transmission with the radio to the master station. This informs the master station  18  that the gas monitor station  14  is still functioning. The master station then resets a timer for the maximum length of time between communications from the gas monitor station  14 .  
     [0124] If more than the predetermined time period, for example, sixty-five minutes, passes between communications from the gas monitor station  14  to the master station  18 , an alarm is provided at the master station. This alarm indicates to personnel at the master station that the gas monitor station  14  has not transmitted to the master station for more than the predetermined period of time. Personnel at the master station  18  can then initiate an inspection of the gas monitor station  14  to determine why the gas monitor station  14  had not transmitted to the master station for more than the predetermined period of time.  
     [0125] The microprocessor  280  is also effective to determine when the power level, that is the output voltage, from the battery  260  is below a predetermined level. Thus, the output voltage of the battery  260  is transmitted to the control apparatus  220  through the conductors  266  and  268  (FIGS. 8 and 9). The microprocessor  280  (FIG. 9) receives an input indicative of the output voltage of the battery  260 . When this output voltage falls below a predetermined level, for example, 12 volts (direct current), the microprocessor  280  initiates a transmission with the radio  248  to the master station  18  to indicate that the output of the battery is below a desired level. When they are not being used, the microprocessor  280  and radio  248  are shut down to a de-powered or standby condition to minimize the load on the battery  260 .  
     [0126] The microprocessor  280  also initiated a transmission with the radio  248  to the master station  18  if the output from the battery changes by more than a predetermined amount. For example, if the battery voltage should increase or decrease by more than 0.5 volts within the predetermined period of time, the microprocessor  280  would initiate transmission with the radio  248  to indicate to the master station that there has been a change in battery voltage.  
     [0127] It should be understood that when the microprocessor  280  initiates transmission with the radio  248  to the master station  18 , the radio is effective to transmit data indicative of the condition which is present. For example, when the sensed concentration of the selected gas in the atmosphere at the gas monitor station  14  exceeds a concentration necessary to trigger the HI alarm, the microprocessor  280  initiates transmission with the radio  248  to transmit data indicative of the actual sensed concentration of the gas in the environment adjacent to the gas monitor station. Similarly, when the concentration of the selected gas in the environment adjacent to the gas monitor station  14  reaches a level sufficient to trigger a HIHI alarm, the radio  248  transmits data indicative of the actual concentration of the selected gas in the atmosphere. When the microprocessor  280  initiates operation of the radio  248  in response to a predetermined variation in the concentration of the selected gas in the atmosphere at the gas monitor station  14 , data indicative of the actual concentration of the selected gas and the actual variation in the concentration of the selected gas is transmitted from the gas monitor station to the master station by the radio  248 .  
     [0128] In addition to the microprocessor  280 , the control apparatus  220  includes other components including a transformer  294  (FIGS. 9 and 15). The transformer  294  forms part of the power supply  250  (FIG. 14). The transformer  294  may be connected with a source of alternating current, such as a 110 volt power line. The alternating current is connected with the printed circuit board  222  at connection locations indicated by the numeral  296  (FIGS. 9 and 19) on a terminal block  298  (FIG. 9). The transformer  294  is connected with the connection locations  296  through a suitable fuse  300  (FIGS. 9 and 15). The transformer  294  transforms either 115 volt or 230 volt line current to a relatively low level (approximately 13 or 14 volts) required to charge the battery  260 .  
     [0129] In addition to the transformer  294  (FIG. 9), the control apparatus  220  includes a radio power regulator  304  (FIGS. 9 and 19) which is mounted on the printed circuit board  222  and connected with the radio  248 . A solar panel regulator or voltage converter  306  (FIGS. 9 and 15) is also provided on the circuit board  222  in the control apparatus  220 . A radio squelch setting potentiometer  310  (FIGS. 9 and 18) is provided on the circuit board  222  in the control apparatus  220 .  
     [0130] The construction of the cover plate  234  is illustrated in FIGS. 10 and 12. As was previously mentioned, the cover plate  234  has a multi-layered construction. A front layer  320  (FIG. 12) of the cover plate  234  is connected with a rear layer  322  at a sealed connection  324 . The sealed connection  324  is formed by heat and a polyester base dry film adhesive. The connection  324  is free of pressure sensitive adhesive which tends to lose its adhesive properties when exposed to heat provided by the sun.  
     [0131] The sealed connection  324  extends completely around the periphery of the cover plate  234  and securely seals membrane switches  236  disposed within the cover plate  234  from the environment around the cover plate. The cover plate  324  has a known construction and is commercially available from Berquist Company. However, it should be understood that the cover plate  234  could have different construction if desired.  
     [0132] Membrane switches  236  (FIG. 11) are provided between the front and rear layers  320  and  322  of the cover plate  234 . The front layer  320  forms a continuous layer which extends across the entire front of the cover plate  234 , with the exception of the window  232 . The window  232  is formed of a clear plastic or other transparent material. The front and rear layers  320  and  322  are sealed around the periphery of the window so that liquid, cannot seep into the space between the front layer  320  and rear layer  322  of the cover plate  234 .  
     [0133] The front layer  320  of the cover plate is provided with indicia  328  (FIG. 10) which overlies the membrane switches  236  (FIG. 11). By manually pressing against the indicia  328  on the front layer  320  of the cover plate  234 , the corresponding membrane switch is actuated. Thus, by pressing on indicia indicative of the numeral “4”, a membrane switch  334  (FIG. 11) corresponding to the numeral  4  and disposed between the rear layer  322  and front layer  320  is actuated.  
     [0134] The relationship between the various switches which form the keypad  206  is illustrated schematically in FIG. 11. For example, if the indicia  328  indicated by the numeral “4” in FIG. 10 is manually depressed, the switch indicated at  334  is actuated. Actuation of the switch  334  completes a circuit between a terminal pin  336  and a terminal pin  337 . Completing the circuit between the terminal pins  336  and  337  indicates to the microprocessor  280  that the indicia  328  for the numeral “4” on the keypad  206  was manually actuated. The microprocessor  280  then effects the transmission to the liquid crystal display  226  (FIG. 9) to have the numeral “4” appear in the display at the window  232  (FIG. 10).  
     [0135] Similarly, if the indicia  328  corresponding to the numeral “9” is manually actuated on the keypad  206 , a switch  338  (FIG. 11) is closed to complete a circuit between a terminal pin  340  and a terminal pin  342 . The microprocessor  280  will then cause the numeral “9” to appear on the display  226 . By having the display  226  set forth the indicia which is actuated on the keypad  206 , a person actuating the keypad can manually view the display through the window  232  and check the data which has been entered.  
     [0136] By depressing right arrow indicia  344  on the keypad  206 , a switch  346  (FIG. 11) is closed. The resulting completion of a circuit between the terminal pin  342  and a terminal pin  350  indicates to the microprocessor  280  that the menu which prompts the individual entering data should be advanced. Similarly, depressing left arrow indicia  352  on the keypad  206  results in actuation of a switch  354  (FIG. 11). Depressing of the switch  354  completes a circuit between the terminal pin  350  and the terminal pin  337 . This indicates to the microprocessor  280  that the menu provided on the liquid crystal display  226  should be scrolled backward to enable the operator to enter data which had previously been missed or to revise data which had previously been entered.  
     [0137] When the gas monitor station  14  (FIG. 7) is to be utilized to sense a selected gas, a gas sensor  38  which is capable of sensing the selected gas is connected with the sensor cable  274  (FIG. 8). This connects the gas sensor  38  with the control apparatus  220 . The sensor for the selected gas is connected to the housing  30  in the manner illustrated in FIG. 7. The selected gas may be any one of the gases previously mentioned herein.  
     [0138] The keypad  206  (FIGS. 8 and 10) is then actuated to transmit data to the microprocessor  280  (FIG. 8) indicative of the selected gas. As was previously mentioned, the gas sensor  38  may be constructed to sense any one of many different gases. Of course, the keypad  206  would be manually actuated to transmit data to the microprocessor  280  indicative of the selected gas.  
     [0139] The gas sensor  38  may then be calibrated. The sensor  38  is calibrated by exposing the sensor to a known concentration of the selected gas. This may be done by exposing the sensor  38  to the interior of a container containing a known concentration of the selected gas. Rather than being exposed to a container containing a known concentration of the selected gas, the sensor  38  could be exposed to a gas-generating device, such as a permeation tube calibrator or gas generator. The output of the sensor  38  in response to exposure to the known concentration of the selected gas is transmitted to the analog-to-digital converter  290  (FIGS. 9 and 17) in the control apparatus  220 . The analog-to-digital converter  290  converts the analog output of the gas sensor  38  to a digital signal which is transmitted to the microprocessor  280 .  
     [0140] A number corresponding to the output of the gas sensor  38  is then displayed on the liquid crystal display  226 . The number corresponding to the output of the gas sensor  38  can be viewed through the window  232  (FIG. 10) by the individual operating the keypad  206  (FIG. 7). The individual operating the keypad  206  then actuates the indicia  328  (FIG. 10) to close switches  236  (FIG. 11) corresponding to the known concentration of the selected gas in the container.  
     [0141] This results in the transmission of data to the microprocessor  280  (FIG. 9) indicating a gas concentration corresponding to the known concentration of the selected gas to which the sensor  38  is exposed. When this has been done, the microprocessor  280  is calibrated so that it will effect actuation of the display  226  to indicate the known gas concentration to which the sensor  38  is exposed. In addition, when the microprocessor  280  initiates operation of the radio  248  to transmit to the master station, the microprocessor will cause the radio to transmit data corresponding to the sensed gas concentration of the selected gas to the master station  18 . Calibration data stored in the microprocessor  280  (FIGS. 9 and 16) is transmitted to the EEPROM  284  (FIGS. 9 and 17) when the microprocessor is shut down, that is, de-powered.  
     [0142] The illustrated gas sensor  38  has a hose barb  360  (FIG. 7) which connects a hose extending from a container of known concentration of the selected gas, to the sensor  38 . Once the control apparatus  220  has been calibrated, the hose and container of known concentration of the selected gas are disconnected from the hose barb  360  on the gas sensor  38 . Of course, the container of a known concentration of the selected gas could be connected with the gas sensor  38  in a different manner if desired. It should be understood that the gas sensor  38  could be exposed to a known concentration of the selected gas in any one of many different ways.  
     [0143] When the gas monitor station  14  is to be configured to initiate radio transmission in response to the occurrence of predetermined conditions, an individual at the gas monitor station  14  manually depresses “ENT” indicia  366  on the keypad  206  (FIG. 10). Manually depressing the indicia  366  actuates an “ENT” switch  368  (FIG. 11). Actuation of the “ENT” switch  368  connects terminal pin  340  with a terminal pin  370 . This causes the microprocessor  280  to change the liquid crystal display  226  (FIG. 9) to indicate either option “1 SUPERVISOR” or option “2 USER”. The Supervisor can change all of the parameters of the gas monitor station  14  while the User can only zero and span the sensor.  
     [0144] Since the gas monitor station  14  is being configured to set the conditions which result in initiation of radio transmission, the individual actuating the keypad  206  will be a Supervisor and will select option “1 SUPERVISOR”. This will be accomplished by depressing the indicia  374  on the keypad  206  (FIG. 10). Actuation of the indicia  374  results in the switch  376  (FIG. 11) being closed to complete a circuit between the terminal pin  336  and a terminal pin  378 .  
     [0145] This indicates to the microprocessor  280  that a Supervisor&#39;s security code is to be entered next. Therefore, the microprocessor  280  changes the display  226  to request entry of the Supervisor&#39;s security code. The Supervisor actuates the indicia  328  on the keypad  206  to enter the security code. The security code may be a four-digit number, such as 1234. The “ENT” (enter) indicia  366  is then actuated to close the switch  368  to input the security code to the microprocessor  280 .  
     [0146] By pressing a right arrow indicia  344 , the right arrow switch  346  is closed and request for the settings for the battery alarm and a battery dead band will appear at the display window  232 . The keypad  206  will then be actuated to enter the battery output voltage at which the microprocessor  280  will initiate operation of the radio  248  to transmit a battery alarm to the master station. Usually, the battery alarm voltage is set at 12 VDC. However, it should be understood that a different voltage could be utilized if desired.  
     [0147] The battery dead band voltage is the amount by which the output of the battery  260  must change to cause the microprocessor  280  to initiate transmission by the radio  248  to the master station informing the master station of the change in battery output voltage. The battery dead band setting is entered into the microprocessor  280  by actuating the keypad  206 .  
     [0148] It is contemplated that the battery dead band may be set at 0.5 volts. This will be accomplished by manually actuating indicia  386  (FIG. 10) for the numeral zero (0). This will result in an actuation of a switch  388  to complete a circuit between a terminal pin  390  and the terminal pin  370 . The decimal point (.) is entered by actuating indicia  392  (FIG. 10) on the keypad  206 . This results in actuation of a switch  394  to complete a circuit between terminal pins  350  and  370 . Indicia  396  (FIG. 10) on the keypad  206  for the numeral five (5) is then manually actuated to close a switch  398 . Closing of the switch  398  completes a circuit between the terminal pins  337  and  390 .  
     [0149] Once the battery alarm and battery dead band settings have been entered into the microprocessor  280  (FIG. 9), the right arrow indicia  344  (FIG. 10) is again manually depressed. This results in closing of the switch  346  (FIG. 11) and a change in the display at the window  232 . The display at the window  232  will then request settings for the sensor type and analog filter. Indicia  328  corresponding to a numerical code for the selected gas sensor  38  is then actuated. This results in the inputting of data to the microprocessor  280  indicating the type of sensor  38  which is being used and the selected gas which is to be sensed by the sensor. The data indicating the type of sensor  38  which is being used is stored in the EEPROM  284  when the microprocessor  280  is shut down.  
     [0150] An analog filter setting is then entered into the microprocessor  280 . The analog filter setting corresponds to a period of time over which readings by the sensor  38  are to be averaged. The period of time over which readings by the sensor are averaged is relatively short, for example, thirty seconds or less.  
     [0151] By averaging the output of the sensor  38  over a period of time, the effect of transient conditions, such as puffs of the selected gas, are eliminated. If the output from the sensor  38  was not averaged over a short period of time in order to eliminate the effect of transient conditions, it is possible that numerous nuisance or false alarms could be provided as a result of short duration variations in the amount of the selected gas which is immediately adjacent to the sensor  38  at any given instance. In one specific configuration of the gas monitor station  14 , the keypad  206  was actuated to indicate that the analog filter or averaging time was to be ten seconds. Data corresponding to the analog filter averaging time is stored in the EEPROM  284  when the microprocessor  280  is shut down.  
     [0152] Once the sensor type and analog filter time has been entered by actuating the keypad  206 , the right arrow indicia  344  is again actuated. This results in the microprocessor  280  (FIG. 9) changing the display  226  to request a STEL (short term exposure limit) setting and a Gas Dead Band setting. The short-term exposure limit (STEL) setting is a number which corresponds to the maximum permissible moving average concentration of gas over a predetermined length of time.  
     [0153] The manner in which sensed gas concentration in the atmosphere at the gas monitor station  14  may vary is illustrated by a curve  400  in FIG. 13. A predetermined length of time over which the moving average gas concentration is determined is represented by a line  402  in FIG. 13. The line  402  extends from the present time back for a predetermined amount of time, for example fifteen minutes. The moving average gas concentration is represented by the dashed line  404  in FIG. 13.  
     [0154] The line  402  representing the predetermined length of time over which the average gas concentration is determined continuously moves to the right, as viewed in FIG. 13, with the passage of time. This rightward movement of the time line  402  is indicated by an arrow  405  in FIG. 3. Therefore, the moving average gas concentration represented by the line  404  may be referred to as a sliding or rolling average.  
     [0155] The moving average gas concentration  404  may be multiplied by the predetermined length of time  402  over which the moving average is determined. This results in a number having a magnitude which corresponds to an area  406  under the curve  400  in the predetermined length of time  402 . The area  406  is the moving integral of gas concentration. Since the predetermined length of time  402  is a constant, the multiple of the moving average gas concentration  404  times the predetermined length of time  402  is equal to a constant times the moving average gas concentration.  
     [0156] By setting a short-term exposure limit alarm which is a function of both the duration of exposure to the selected gas and the concentration of the selected gas, an alarm is provided when the short-term exposure is relatively high. The microprocessor  280  provides a continuous series of sensing periods for which the multiple of elapsed time in the sensing period and sensed concentration of the selected gas is continuously determined by the microprocessor. The microprocessor  280  continuously determines the moving average gas concentration  404  over the predetermined length of time  402 . If the moving average gas concentration exceeds a predetermined number, the microprocessor  280  initiates transmission to the master station  18  with the radio  248 .  
     [0157] In one specific embodiment of the invention, a permissible short-term exposure number equal to the area  406  (FIG. 13) was entered into the microprocessor  280 . The actual short-term exposure number determined by the microprocessor  280  was continuously compared to a permissible short-term exposure limit number. The permissible short-term exposure limit number for one selected gas was, for example, determined by operating personnel to be 4.5. When the microprocessor  280  is shut down, the short-term exposure number is stored in the EEPROM  284 .  
     [0158] The permissible short-term exposure limit number of the foregoing example, that is, 4.5, was transmitted to the microprocessor  280  (FIG. 9) by actuating the indicia  328  (FIG. 10) for the numeral  4  with a resulting closing of the switch  334  (FIG. 11). This completes the circuit between the terminal pins  336  and  337 . The indicia for the decimal point, that is, the indicia  392  (FIG. 10), on the keypad  206  is then manually actuated. This results in closing of the switch  394  (FIG. 11) to complete a circuit between the terminal pins  350  and  370 . The indicia  396  for the numeral 5 would then be manually depressed. This would result in closing of the switch  398  and a completion of the circuit between the terminal pins  390  and  337 . This results in the inputting to the microprocessor  280  of a short-term exposure limit number of 4.5.  
     [0159] The duration of the sensing periods over which the moving average gas concentration for the short-term exposure limit is measured may also be entered into the microprocessor  280  from the keypad  206 . Thus, in one specific instance, the measuring period  402  for the short-term exposure limit was fifteen minutes. The measuring or sensing period  402  could have a duration either longer or shorter than fifteen minutes if desired. This data is also stored in the EEPROM  284  when the microprocessor  280  is shut down.  
     [0160] Once the data entry has been completed and the predetermined permissible short term exposure limit has been entered into the microprocessor  280 , the microprocessor will initiate transmission of an alarm to the master station  18  whenever the predetermined multiple of the time  402  and the moving average gas concentration  404  is reached. In the previous example, a multiple of 4.5 was input to the microprocessor as the permissible short-term exposure limit number. Thus, if there is a moving average sensed gas concentration  404  of 0.3 parts per million (ppm) for the measuring period  402  of fifteen minutes, the short exposure limit multiple would be 4.5. This would be equal to the permissible short-term exposure limit number and would result in the microprocessor  280  initiating radio transmission of an alarm to the master station  18  with the radio  248 .  
     [0161] Rather than entering a permissible short term exposure limit number corresponding to the area  406 , the permissible short term exposure limit number could correspond to the moving average gas concentration represented by the dashed line  404  in FIG. 13. In the foregoing example, the permissible moving average gas concentration was 0.3 parts per million over a predetermined length of time  402  of fifteen minutes. Rather than entering the selected short-term exposure number of 4.5, the moving average gas concentration number of 0.3 parts per million over the predetermined length of time  402  could be entered.  
     [0162] During determination of the moving average gas concentration  404 , the analog filter time is in effect. This means that readings by the gas sensor  38  are averaged over a short period of time to determine the curve  400  and the moving average gas concentration  404 . As was previously mentioned, the analog filter time is relatively short, thirty seconds or less. In the previous example, the analog filer time was selected to be ten seconds.  
     [0163] The previous examples of analog filter time of ten seconds and a length of time  402  (FIG. 13) over which the moving average gas concentration  404  is determined of fifteen minutes are assumed to have been entered into the microprocessor  280  at the keypad  206 . The moving average gas concentration  404  would then be determined based on ninety gas concentration values. This is because there would be six average analog filter gas concentration values determined over ten second time averaging periods in one minute. In the fifteen minute length of time  402 , over which the moving average gas concentration  404  is determined, there would be a determination of six times fifteen or ninety gas concentration values.  
     [0164] It should be understood that the analog filter time for averaging readings by the sensor  38  could be different than the foregoing example of ten seconds. Similarly, the length of time  402  over which the moving average gas concentration is determined could be different than the foregoing example of fifteen minutes.  
     [0165] If desired, the short-term exposure limit could be determined without the analog filter to average output readings from the sensor  38 . Alternatively, the analog filter could be used to average output readings from the sensor  38  without utilization of the short-term exposure limit setting. It is contemplated that the short-term exposure limit could be used without the analog filter while the analog filter is used for other purposes. For example, the analog filter could be used for averaging of reading of the sensor  38  over a short length of time, that is, thirty seconds or less, to determine whether or not a predetermined level of concentration of the selected gas is present in the atmosphere at the gas monitor station. At the same time, the short-term exposure limit could be determined by obtaining the moving average  404  of the unfiltered output of the sensor  38  over the predetermined period of time  402 .  
     [0166] The magnitude of a gas dead band is then transmitted from the keypad  206  to the microprocessor  280 . The gas dead band is the amount of change in the output of the sensor  38  which is required to cause the microprocessor  280  to initiate radio transmission of data to the master station. For example, the gas dead band could be set to be a variation of 0.1 parts per million (ppm) in the concentration of the selected gas. If this was done, the microprocessor  280  would initiate radio transmission of data to the master station whenever the amount of the selected gas in the atmosphere changed by more than 0.1 parts per million. The required variation in the amount of the selected gas in the atmosphere to initiate transmission by the radio  240  may be either an increase or a decrease in the concentration of the selected gas.  
     [0167] The settings for the HI alarm and the HIHI alarm are then input from the keypad  206  to the microprocessor  280 . The HI alarm is a relatively low setting, for example, 1.0 parts per million of the selected gas, while the HIHI alarm is a higher concentration of the selected gas, for example, 2.0 parts per million. The HI alarm is set at a gas concentration level which initiates investigative action on a non-urgent basis. However, the HIHI alarm would initiate investigation on an urgent basis.  
     [0168] It should be understood that the analog filter setting applies to both the HI and the HIHI alarms. Therefore, the HI alarm or the HIHI alarm is transmitted by the radio  248  when the average concentration of the selected gas over the predetermined time represented by the analog filter setting exceeds either the HI alarm setting or the HIHI alarm setting. The analog filter setting would also apply to the gas dead band determination.  
     [0169] By again pressing the right arrow indicia  392 , the Site Address and Master Address appear at the window  232 . Each gas monitor station has a unique Site Address which is coordinated with the controller at the master station  18 .  
     [0170] By again pressing the right arrow indicia  382 , an Alarm Address indication and a Set Pole Timer indicia appear at the window  232 . The Alarm Address data is input to the microprocessor to indicate the location of Alarm Sites where transmission from the radio  248  at the gas monitor station  14  is to result in an alarm.  
     [0171] The Set Pole Timer data is entered into the microprocessor  280  by actuating the keyboard  206 . The Set Pole Timer data corresponds to a maximum predetermined length of time which may elapse between radio transmissions. Thus, when a radio transmission is made, the Set Pole Timer data indicates the maximum length of time which will pass before a next radio transmission.  
     [0172] For example, if the Set Pole Timer data results in the microprocessor  280  being set to have a transmission every thirty minutes, the microprocessor will initiate a transmission from the radio  248  to the master station  18  after thirty minutes has elapsed from the last previous communication with the master station. This enables the master station to check to be certain that the gas monitor station  14  is functioning normally. The master station is set to report a communication failure alarm if there is no communication from a gas monitor station after a time period which is longer than the Set Pole Timer period has elapsed. In the previous example, the Set Pole Timer period was set for thirty minutes. The master station may initiate a communication failure alarm if forty-five minutes elapses between communications from a particular gas monitor station.  
     [0173] By again pressing the right arrow key  382 , the setting for RF Diagnostics and Line Rejection appear at the window  232 . The RF Diagnostics allow an individual to transmit a three second radio signal which is long enough to be displayed on a watt meter for radio/antenna integrity. The RF Diagnostics also allows an individual to send a message to the master station to verify a radio link with the master station. The Line Rejection is set to either 50 or 60 hertz power.  
     [0174] The CLR indicia on the keypad  206  is manually depressed to exit from the keypad after all of the desired data has been input to the microprocessor  280 . All of the data which has been entered into the microprocessor by actuation of the keypad  206  is stored in the EEPROM  284  when the microprocessor  280  is shut down.  
     [0175] When the microprocessor  280  initiates operation of the radio  248  to transmit to the master station, the power requirements for the radio increase substantially. When the radio  248  is in a standby mode, that is, when the radio is not transmitting, the radio  248  requires a relatively small amount of current. Thus, when the radio is in a standby mode it uses less than 50 milliamps. When the radio  248  changes from the standby mode to the transmit mode, the radio uses more than 100 milliamps of current. When the radio  248  is in the transmit mode, it may use 1,000 milliamps of current. Since the radio  248  will be in a standby mode for a large majority of the time, the radio will draw a relatively small amount of current from the battery  260  and thereby tend to promote the operating life of the battery. Current drain on battery is also reduced by shutting down (de-powering) the microprocessor  280  when it is not in use.  
     [0176] After the gas monitor station  14  has been used for a substantial period of time, it is contemplated that it may be desired to check the sensor  38  to determine whether or not the sensor needs to be replaced. To check the sensor  38 , the microprocessor  280  effects the application of a predetermined voltage to leads in the sensor cable  274 . Application of this voltage to the sensor  38  results in the transmission of a different current output from the sensor back to the microprocessor  280 . If the sensor  38  has not degraded and does not require replacement, the current transmitted from the sensor  38  back to the microprocessor will be a first function of the voltage which is transmitted from the microprocessor to the sensor. However, if the sensor  38  has degraded to an extent that it requires replacement, the current transmitted from the sensor  38  to the microprocessor will be a different function of the initial voltage applied to the sensor by the microprocessor.  
     [0177] There are a substantial number of features associated with the gas monitor station  14 . It is contemplated that each of these features could be used separately or together with other features. It is contemplated that various combinations of the features disclosed herein will be used in association with other known features which are not described herein.  
     [0178] The invention has been described with reference to the preferred embodiment. Obviously, modifications and alterations will occur to others upon a reading and understanding of the specification. For instance, the current invention may be used to monitor for other than toxic gases and at other than chemical processing plants. And, applicant anticipates that the system will comprise other than four monitors and/or four configuration means. It is intended by applicant to include all such modifications and alterations.