GAS MONITOR

Abstract: An environmental gas monitor includes a gas sensing circuit (8) for detecting and generating a signal proportional to the relative quantity of gas to which the monitor is exposed. The gas sensing circuit includes a semi-conductor gas detecting device (2), and a temperature and humidity compensating circuit (4) for the gas detecting device. A visual display circuit (50) that is responsive to the signal from the gas sensing circuit is provided for displaying a visual indication of the relative quantity of gas. An audible alarm circuit (105) includes an audible alarm and activates the alarm in response to the signal from the gas sensing circuit.

Description: GAS MONITOR BACKGROUND OF THE INVENTION Field of the Invention This invention generally relates to an environmental monitor, and more specifically relates to a 5 device that detects the presence of and the amount of a gas in the environment. It is well known that many gases can accumulate in a cϊosed environment to hazardous levels. For example, carbon monoxide which normally dissipates into the atmosphere ø can reach dangerous proportions in indoor or enclosed environments due, for example, to the use of wood or coal burning stoves, kerosene heaters, thermally tight houses, inefficient or maladjusted home cooking apparatuses, heating and hot water devices and unvented gas stoves. c Carbon monoxide is a colorless, odorless gas and is undetectable by the human senses. The gas is also emitted by automobiles, trucks and other vehicles, which can lead to an accumulation of the gas in the vehicle compartment itself, or in living quarters located directed over or attached to Q garages or in proximity to highways. Carbon monoxide binds with the blood's hemoglobin displacing oxygen, resulting in discomforting headaches "at very low levels and death at higher levels and longer exposures. Carboxyhemoglobin levels of 50% cause death and carboxyhemoglobin levels as low as 2 and 3%, (only 9 ppm in - j air) are life-threatening to the 7.7 million Americans with heart disease. A level of 13 pp is hazardous to angina pectoris sufferers. Likewise, natural gas-fired devices used in 5 households produce methane as a by-product. Methane is found to exist in high concentrations in the atmosphere surrounding a number of dump sites in the United States. Also many recreational vehicles and boats rely on propane for heating, cooking and propulsion. Also, many households use propane as 1ø a primary or secondary fuel. Ammonia and hydrogen sulfide are the most threatening gaseous pollutants to livestock, such as cattle, hogs, sheep; poultry including chicken, turkey and geese; and exotic animals, such as rabbits. Only recently has the 1 effect of such gases on the general well-being of these animals, particularly pregnant specimens and in the newborn; become fully recognized. The extreme importance of low level ammonia environments has probably been most acutely realized in the domestic rabbit industry, an expanding market in the 2ø United States, and a very extensive one in yery European markets as well as in the Near and Far East,, ' particularly in China. Description of the Prior Art Gas sensing units that can detect carbon monoxide, 2ς methane, ammonia, or propane are available principally in 3 or 4 categories. Analytical tools which can detect these gases using precise and delicate infrared spectrophotometry or gas chro atography are available. These devices perform excellently in detecting the presence of gas, but they are -_ extremely expensive. They are essentially used as laboratory tools or or in large industrial applications where high accuracy recording and monitoring modes are essential. Catalytic combustion units that detect the presence of a gas in response to the temperature change due to the catalytic combustion on platinum wire sensors are widely -3- used. The sensors used in these units are expensive, and require costly amplification instrumentation. Also, only a skilled technician will be capable of operating and maintaining such devices. The catalytic combustion unit is primarily used in industrial environments and in pollution control. Detection tubes which rely on known quantitative aspects of the reaction between the gas being detected and the tube's chemical content are known. Use of a detection 0 tube, while reliable, is useful only for one detection, and is not useable for monitoring purposes. Finally, semiconductor sensors are now available which rely on the phenomenon of the reduction and resistance of certain semiconductors when combustible or reducing gasses 5 are absorbed on their surface. A number of these semiconductor sensors have been patented. One such sensor is disclosed in U.S. Patent No. 3,732,519, which issued to Noayoshi Taguchi. There are several gas sensing units on the market ø today which incorporate the semiconductor sensors described above. Most of these sensing units are designed and used as simple threshold, devices as opposed to level indicators, and lack sensitivity control. For these reasons, the gas sensing units on the market today would not be readily useable by certain populations such as the elderly, the hard of hearing or the blind. OBJECTION AND SUMMARY OF THE INVENTION It is an object of the present invention to provide ø an environmental gas monitor which is a highly sensitive device for detecting the presence of and the quantity of gas in the environment. It is another object of the present invention to provide an environmental gas monitor which is reliable " and 5 has a long useful life, and is simple in construction and easy to maintain. It is still another object of the present invention to provide an environmental gas monitor that is adaptable to be integrated into a central monitoring system. It is a still further object of the present invention to provide an environmental gas monitor that is suitable for use by persons who are deaf or blind to warn these persons of a dangerous accumulation of gas. It is yet a further object of the present invention to provide an environmental gas monitor that requires low maintenance. It is yet another object of the present invention to provide an environmental gas monitor that is calibrated for use at various concentration levels. It yet a still further object of the present invention to provide an environmental gas monitor which is a self-contained unit that monitors the presence of gas, indicates the quantity of gas tha has accumulated and sets a warning alarm or activates a system to counteract the accumulation of gas. It is yet another object of the present invention to provide an environmental gas -monitor which can accurately detect even the presence of small quantities of gas. In accordance with one embodiment of the present invention, an environmental gas monitor constructed in accordance with the present invention includes a gas sensing circuit for detecting and generating a signal proportional to the relative quantity of gas in-the environment to which the monitor is exposed. The gas sensing circuit may include a semiconductor gas detecting device, and a temperature and humidity compensating circuit for the gas detecting device. A visual indicator circuit that is responsive to the signal from the gas sensing circuit is provided for displaying a visual indication of the relative quantity of gas in the environment. The environmental gas monitor also includes an alarm circuit that is triggered in response to the signal from the gas sensing circuit. The alarm circuit includes an audible alarm. A circuit may also be included which provides an amplitude offset to the signal from the gas sensing circuit. The amount of offset provided to the signal is manually adjustable. This circuit receives the signal from the sensing circuit, and provides an amplitude offset signal to the alarm circuit. In this way, the alarm circuit can be adjustably set to trigger when the monitor checks different levels of accumulated gas. These and other embodiments, objects, features and advantages of this invention will be apparent in the following detailed description of the illustrative embodiments thereof, which is to be read in connection with the accompanying drawing. BRIEF DESCRIPTION OF THE DRAWING Figure 1 is a block diagram of a preferred form of the invention; and Figure 2 is a schematic diagram of the preferred form illustrated by Figure 1. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to Figs. 1 and 2 of the drawing, it 5 will be seen that an environmental gas monitor constructed in accordance with the present invention includes a gas detecting device 2. As mentioned previously, a suitable gas detecting device 2 that may be used in the present invention is disclosed in U.S. Patent No. 3,732,519. The gas detecting ø device disclosed in the ' 519 patent is basically a small iron oxide coil that is surrounded by a heating element. The resistance of the coil is determined by the degree of oxidation of its surface. The heating element stabilizes the state of oxidation of the oxygen measuring iron oxide.coil. The coil and heating element are incapsulated in a small cylindrical holder. The gas detecting device further includes heater leads and coil leads which protrude from one end of the holder. Positioned on the top of the device is a stainless steel mesh screen that prevents access to the sensitive element while allowing gas engress. Currently Panasonic Figaro Electronics Corporation manufactures such gas detecting devices. These devices require that they be burned in for about one week so that they become fully stabilized in their resistance characteristics. Also, the device does not stabilize sufficiently until about one minute after current is applied to the heater coil. To overcome the inherent disadvantageous characteristics of the device, the gas monitor of the present invention includes temperature and humidity compensation circuitry 4, as well as an output signal supression circuit 6 to address the problem of high signal output during initial start-up. Also, the monitor of the present invention allows the same gas detecting* device 2 to be used for detecting various types of gases and at various sensitivities. The gas detecting element 2 forms a portion of the overall gas sensing circuit 8 of the present invention. The gas sensing circuit 8 includes a wheatstone bridge-like circuit 10 which functions partly to compensate the detecting element for temperature and humidity variations. The wheatstone bridge circuit 10 includes four legs which are interconnected. The first leg 12 comprises the gas detecting elements. The second and third leg, 14 and 16 respectively each include balancing resistors, 18 and 20 respectively. The fourth leg 22 includes a resistor 24 in series with the parallel combination of a thermistor 25 and a shunt resistor 27. The first and second leg 12, 14 , second and third leg 14, 16, third and fourth leg 16, 22, and fourth and first leg 22, 12 of the wheatstone bridge 10 are connected at first, second, third and fourth nodes 26, 28, 30, 32, respectively. The fourth node 32 is connected to a common return, such as ground, while the second node 28 is connected to a highly regulated D.C. voltage source 34. The wheatstone bridge circuit 10 was selected for use in the present invention because it provides for high signal discrimination and the ability to detect small changes in the resistance of the gas detecting element 2. It generates a signal due to the imbalance in the values of the 0 products of opposite resistance values of the components incorporated into the four legs of the wheatstone bridge. The wheatstone bridge circuit 10 also allows the environmental gas monitor of the present invention to easily adapt to operation for various gases and with various 5 detecting elements. Also, the bridge circuit allows quick adjustment for different concentration ranges for any one particular gas, by a single variation or adjustment of at least one resistor, in particular, the resistor 20 that is opposite the detecting element. ø Furthermore, the wheatstone bridge circuit 10 compensates for the gas detecting element's sensitivity to temperature and humidity by placing such compensating circuitry (i.e., the thermistor 25) in a bridge leg which is part of the product of resistances which comprise the -bridge circuit. The choice of the resistors and the thermistor 25 used in the bridge circuit will determine the actual-.signal level generated by the bridge and the level of current through the bridge components. The level of current-is very 2 Q important with reference to leakage levels as well as potential "self-heating" of various resistances, in particular the thermistor 25, and is thus important to the reliability and accuracy of the gas monitor. Preferably, precision resistors are used in ' the __ wheatstone bridge circuit 10, the resistors having a tolerance of 1% or better. 1 The first and third nodes 26, 30 of the wheatstone bridge circuit 10 are connected respectively to the noninverting inputs of first and second operational amplifiers 36, 38 respectively. The first and second operational amplifiers 36, 38 are configured to provide unity gain, and act as buffers with a high input impedance to isolate the wheatstone bridge circuit 10 from the rest of the circuitry of the monitor. Thus, the precision of the wheat¬ stone bridge 10 remains unaffected by the additional 10 circuitry of the monitor. The output signals from the first and second operational amplifiers 36, 38 are directed to the inverting and non-inverting inputs of a third operational amplifier 40, respectively through a first and second input resistor 42, 15 44, respectively. A feedback resistor 46 is connected from the output of the third operational amplifier 40 to the inverting input thereof, and a shunt resistor 48 is connected between the non-inverting input of the third operational amplifier 40'.and ground. 0 The third operational amplifier 40 acts to amplify the differential signal between the first and third nodes 26, 30 of the wheatstone bridge circuit 10. The use of three operational amplifiers in the mode as described herein makes the gas monitor of the present 5 invention extremely sensitive, very reliable and especially suitable for .low concentration operations. The output signal from the third operational amplifier 4Q, which is a positive going signal, is provided to a visual display circuit 50. The visual display circuit 2 Q 50 includes a dot/bar display driver 52 which receives the output signal of the third operational amplifier 40 on its signal input terminal 54. The display driver includes a negative voltage supply terminal 56, which is grounded; a positive voltage supply terminal 58, which is connected to a _ positive voltage supply ; a R, Q terminal 60 which is preferably grounded through a shottky diode 62; a mode terminal 64, which is connected to the positive voltage supply; a reference adjust terminal 66 which is connected between a first and second resistor 68, 70, respectively of a voltage divider network 72; a reference high input 74 and reference output 76, the two of which are interconnected and joined to the first resistor 68 of the voltage divider network 72. The other end of the second resistor 70 of the voltage divider network 72 is grounded. A capacitor may be 0 positioned near the mode terminal 64 to provide additional filtering. With the mode input 64 at a logic high level, that is, connected to the positive voltage supply, the display driver 52 will function as a bar graph driver. 5 The display driver 52 includes a number of output terminals which are internally open collector circuits capable of sinking current from a LED, LCD or vacuum fluorescent display. When the driver is in the bar graph mode, as in the present case, one or more of th'e outputs of o the driver will be activated in response to a particular amplitude level of the output signal of the third operational amplifier 40. The visual display circuit 50 of the present invention further includes a visual indicator which is 5 preferably an LED bar graph display 80. There are eight segments which comprise the bar graph display 80. Each of the eight segments include first and second LED's 82 and 84 respectively, that-are connected in series. The anodes of the first LEDs 82 of the second 0 through eight segments 86-98 respectively, are connected to a positive voltage supply; the cathodes of the second LEDs 84 are connected to respective outputs of the bar graph driver 52. The first segment 100 of the bar graph has the anode of its first LED 82 connected to a positive voltage supply through a current limiting resistor 102; the cathode of the second LED 84 of the first bar segment 100 is grounded. In operation, the resistance of the gas detector element 2 will vary in accordance with the concentration of gas in the environment sampled by the gas monitor. This resistance will cause the wheatstone bridge 10 to provide a differential signal to the third operational amplifier 40 through the first and second buffer operational amplifiers 36, 38. Thus, the output signal of the third operational amplifier 40 will decrease or increase in proportion to the concentration of gas in the atmosphere. The proportionate increase in amplitude of the output signal provided to the driver circuit 52 will correspondingly cause the first and second LEDs 82, 84 of the second through eighth bar elements 86-98 to conduct sequentially. Thus, the visual display will provide a quantitative, as opposed to a merely qualitative, fume concentration indication. This simultaneously provides for a monitoring mode indicating an increase or decrease in fume concentration with reference to an earlier level indicated by the bar graph display, as opposed to merely an alarm signal being generated. It was preferable to connect the first and second LEDs 82, 84 of each bar segment in series rather than in parallel. This configuration provides a longer life for the LEDs and, when the first LEDs 82 are connected to a 5 volt supply, it eliminates the need for seven current limiting resistors in series with the LED's. The first segment 100 of the bar graph display is chosen to be part of the fume concentration monitoring circuit. It acts as a "normally on" indicating element, and is lit when the unit is operative. Its brightness is matched to that of the other elements by the choice of an appropriate value for the current limiting resistor 102. This configuration eliminates the need of an extra on/off switch and pilot light. The gas monitor of the present invention also includes an adjustable threshold circuit 104 and an alarm 1 circuit 105. The adjustable threshold circuit 104 includes a fourth operational amplifier 106. The output signal of the third operational amplifier 40 is provided to the non-inverting input of the fourth operational amplifier 106 5 through an input resistor 108. The threshold circuit also includes a voltage divider network 110. The network 110 includes a first resistor 112 connected between the inverting input of the fourth operational amplifier 106 and a voltage source. A 10 second resistor 114 is adjustable, such as a potentiometer with its wiper connected to either end of the potentiometer, and is connected between the inverting input of the fourth operational amplifier 106 and ground. The threshold circuit 104 provides an adjustable 5 offset to the output signal from the third operational amplifier 40. The offset output signal from the fourth operational amplifier 106 is provided to the base of an NPN transistor 116 through a base resistor 118. The alarm circuit 105 of the present invention is 2Q triggered in response to the output signal from the threshold circuit 104 or, in the specific embodiment illustrated in Figure 2, the output signal from the fourth operational amplifier 106. The preferred alarm circuit includes an audible alarm 120, and a driving circuit 122 for the audible alarm 120 and for causing the audible alarm 120 to emit a pulsating sound. The alarm circuit 105 includes the NPN transistor 116 previously described. As mentioned, the base of the NPN transistor 116 is connected through a base resistor 118 to _ the fourth operational amplifier 106. The collector of the NPN transistor 116 is connected to one terminal of the audible alarm 120. A high resistance emitter resistor 124 interconnects the emitter of the NPN transistor 116 to ground. 35 To protect the NPN transistor 116 so that it is not overdriven by the fourth operational amplifier 106, a diode 126 that is normally reversed biased is shunted across the base/emitter junction of the NPN transistor 116, with its anode and cathode respectively connected to the emitter and base of the transistor 116. The driving circuit further includes a PNP transistor 128. The collector of the PNP transistor 128 is connected to the second terminal of the audible alarm 120, and its emitter is connected to a source of positive voltage. A resistor 121 is connected across the base-emitter junction of the PNP transistor 128. The audible alarm 120 is preferably a piezo- electronic alarm. The circuit for causing the audible alarm 120 to emit a pulsating sound includes an astable multivibrator 130, which can be one half of a 556 dual timer integrated circuit 132 manufactured by Signetics or National Semiconductor. The first timer of the 556 circuit may be connected to function in the astable mode, with its ' output connected to .the base of the PNP transistor 128 through a base resistor 134. So that the first timer of the 556 package 132 functions astably, the reset terminal 138 should be connected to a positive voltage supply; the discharge terminal 140 should be connected through an appropriate discharge resistor 142 to a positive voltage supply; and the threshold terminal 144 should be joined to the trigger terminal 146 and connected through a timing resistor 148 to a positive voltage supply, and through a timing capacitor 150 to ground. The free-running first timer of the 556 unit 132 will cause the PNP transistor 128 to ultimately conduct current to the audible alarm 120, in accordance with the duty cycle of the signal on the output of the first timer. Of course, it is possible to eliminate the astable multivibrator 130 so that the audible alarm 120 will be continuous. However, a pulsed audible alarm is far more annoying to the human ear, and thus more effective, than a continuous alarm. It is preferred if the timing resistor 148 and timing capacitor 150 are selected so that the alarm will pulsate at an approximately 25% duty cycle. An alarm signal suppression circuit 152 is also included in the design of the present invention. The purpose of the alarm suppression circuit 152 is to prevent an inadvertent signal when the gas monitor is first energized or accidentally deenergized and reactivated, or as a result of a momentary power failure. Also, this give sufficient time for the gas detecting element 2 to heat to its operating temperature so that the " alarm circuit 105 will disregard any erroneous signals generated by the element 2 during the initial warm-up period. .. The alarm signal suppression circuit 152 preferably- includes a monostable multivibrator 154, which can conveniently be the second timing unit of the 556 dual timer 132. The second timing unit has its terminals connected so that it operates in a monostable mode. To do this, the discharge terminal 156 and threshold terminal 158 are joined together and to a positive voltage source through an appropriate timing resistor 160 and to ground through an appropriate timing capacitor 162; the reset terminal 164 is connected to a positive voltage source; and the trigger terminal 166 is connected to a positive voltage source through a trigger resistor 168, and to ground through a trigger capacitor 170. The output of the second timer of the 556 circuit 132 is connected to the emitter of the NPN transistor 116. When the gas monitor is first energized, a positive going pulse will be generated as an output signal from the second timer. This output signal will back bias the base-emitter junction of the NPN transistor 116, which will prevent current flow through the audible alarm 120. When the positive going pulse returns to its normal low state, the NPN transistor 116 will be forward biased and conduct current through the alarm 120, depending upon the output signal level of the fourth operational amplifier 106 and the state of the 0 NPN transistor 116. The length of time that the alarm signal is suppressed is determined by the choice of values of the timing resistor 160 and timing capacitor 162. A 50 second delay is preferably chosen, and provided by a pulse width of 5 the same duration for the output signal of the second timer. It will be noted from the circuit of Figure 2 that, should the output signal from the fourth operational amplifier 106 be sufficiently high to trigger the NPN transistor 116, the audible alarm 120 will emit a pulse ø signal when the gas concentration is high enough to surpass the preset, signal level, controllable by the potentiometer 114. Furthermore, because the pulsed output signal from the first timer is not directly provided to the alarm input, c but rather is routed through the PNP transistor 128 to the terminal- of the piezo-electronic alarm 120, the alarm will operate at a constant voltage each time the PNP transistor 128 is triggered by the pulsed output of the first timer. The alarm thus will emit a pulsed alarm signal that is 0 constant in amplitude. The gas monitor of the present invention further includes a circuit 164 which will activate an external device or circuit. The activation circuit 164 may include a relay 166 that can control the line current to a device external to the gas monitor in the event a high accumulation of gas is detected. Thus, the gas monitor may be used to automatically control a discharge fan 168, for example, to remove the high concentration of gas from the environment. If desired, a diode 170 may be connected across the coil 172 of the relay 166 to provide a path for arc suppression when the magnetic field of the relay coil collapses. Also, a capacitor 174 may be included from one end of the relay 166 to ground. The same end of the relay 166 is connected to the collector of the NPN transistor 116, while the other end is connected to a positive voltage source. In this way, when the NPN transistor 116 conducts current through the audible alarm 120, it will also energize the relay 166 and control any device connected thereto. The environmental gas monitor of the present invention can be powered in at least three different ways. First, the gas monitor may include a transformer which will convert 100 volt, 60 cycle line current, or 220 volt, 50 cycle line current to appropriate voltages for powering the circuit components. Second, an external dedicated transformer may be employed which delivers voltages to the gas monitor that are appropriate to power the circuit components- Third, the circuit of the gas -monitor may be directly powered from a battery source for applications where power line voltage sources are not always available. The gas monitor preferably provides a twelve volt rectified and regulated DC voltage to all the controlling integrated circuits and the on-board relay, if such is present, the alarm and the measuring wheatstone bridge circuit. Also, the power supply circuit provides a five volt regulated voltage for the sensor heating coil 176 and other components, and a five volt unregulated voltage to power the segments of the bar graph indicator. A power supply circuit 180 of the monitor includes a line current primary, center tapped secondary transformer 182. The center tap of the transformer's secondary winding provides the five volt unregulated supply for the visual indicator 80. The center tap is connected to two filtering capacitors 184, 186 whose other ends are grounded, and to the input of a five volt voltage regulator 188. The output of the five volt voltage regulator is connected to a filtering capacitor 190, and to one end of the heater coil 176 of the gas detecting element 2. The other end of the heater coil 176 of the element is connected to ground. A twelve volt regulated voltage source is provided in the following manner. The ends of the secondary winding of the transformer 182 are connected to first and second terminals 192 and 194, respectively, of a full wave rectifying bridge circuit 196. A third terminal 198 of the rectifying circuit 196 is connected to a reverse capacitor-diode circuit 200, which includes a capacitor 202 connected to ground, and a back biased diode 204 also connected to ground. The fourth terminal 206 of the rectifying bridge 196 is connected to two filtering capacitors 208, 210 and- to the input of a 12 volt voltage regulating circuit 212. The output of the 12 volt regulating circuit 212 is connected to a filtering capacitor 214 and provides a regulated positive voltage for many of the components of the gas monitor. The advantages of providing a power supply configured in the manner disclosed above are numerous. The 12 volt and 5 volt circuits are fully filtered due to the filtering capacitors, two before each of the 5 and 12 volt regulators and one after each of the 5 and 12 volt regulators. Thus, only one rectifying bridge 196 is required, resulting in a considerable saving in both direct component cost as well as board space and assembly labor. Furthermore, the third terminal 198 of the full wave rectifier bridge circuit 196 is connected to the capacitor 202 and diode 204. As such, a small negative., voltage (which is preferably -.67 volts) is provided on the 12 volt rectifying bridge. More conventional power supply circuits directly ground the third terminal of the rectifying bridge circuit. The negative voltage level becomes the "ground" connection for the first through fourth operational amplifiers in the circuit and substantially enhances the sensitivity of the gas monitor, in particular, the signal output from the wheatstone bridge 10, at low gas concentrations or in operation closer to the more conventional ground level. It is in these situations that conventional units do not provide with sufficient resolution a true quantitative measurement of gas concentration. 0 In addition, because the 5 volt regulated supply is "center tapped" from the secondary winding of the transformer 182 and directly provides power to the sensor heater coil 176 without the need for a full wave recitifier circuit, the number of components required have been reduced, and the 5 stability of the gas detecting element is maintained. The 5 volt unregulated supply from the center tap of the secondary winding is provided to the LEDs 84 of the indicator 80 from the input side of the 5 volt regulator 188; such prevents loading and voltage variation in the sensor ^. heater, which is important for maintaining the stability of the gas detecting element. The gas monitor of the present invention includes a number of additional features. For example, the visual indicator 80 is an 8 element, bar graph display. The long r axis of the bar graph display is vertically disposed. Each incremental increase of gas concentration (PPM) in the monitored ambient atmosphere will cause an additional element of the array to be activated and lighted. The vertically oriented LED array will thus be a quantitative indicator and Q simultaneously an indicator of the state or relative condition of the environment. This allows the operator to get a quantitative indication of the gas concentration by merely glancing at the display. Also, by designing the first and lowest segment 100 of the bar graph display as indicating whether the unit is powered and operational, rather than J including a separate on/off indicator lamp, the chances that the operator will mistakenly think that gas concentration levels are satisfactory, had no elements of the bar graph display been illuminated, are significantly reduced. Also, the audible alarm 120 of the gas monitor is preferably arranged to have its face and sonic cavity exposed through the front cover of the gas monitor's housing to make maximum use of the signal generated decibel level. Exposure of the vibrating face of the audible alarm 120 to the free 0 air space provides an undampened sonic pressure wave, just ahead of the vibrating disk of the piezo-electronic device, which acts as a self-cleaning and dust rejecting feature. This feature is significant in dusty applications such as in grain elevators, domestic animal confinements and certain 5 agricultural and industrial environments. As mentioned previously, the audible alarm 120 may be adjusted to activate at different levels of gas concentration. This adjustment is regardless of the specific sensitivity range of the gas detecting element 2. For a gas o detecting element with a full range of sensitivity set for 0 to 1,000 PPM, that is, the eighth element 98 of the bar graph indicator is set at around 1,000 PPM, the alarm could be set to sound when any one of the elements of the bar display is activated. 5 Furthermore, the pulsating ceramic piezo-electric element is selected as the audible alarm. The piezo-electric element causes vibrations in a freely suspended nomferrous (copper) disk. The alarm circuit is not subject to .- mechanical wear that would normally be associated with the o opening or closing of relay contacts. As mentioned previously, the relay 166 may be used to activate a further protective device such as lights, sirens, exhaust fans, telephone message hook-ups, quality control devices or process monitoring equipment. 5 1 The gas detecting element 2 of the gas monitor is removably mounted in a socket on the gas monitor's housing. Thus, the gas detecting element 2 is externally accessible. This allows the operator to exchange or replace one detecting 5 element for another. Also, by mounting the detecting element on the outside of the housing, gas fumes have direct access to the element. This also allows the unit to be used as a hand held device which, by pointing the detecting element in a particular direction, can locate undesirable fume 10 concentrations. The gas monitor of the present invention further includes a hand held wand-type device. The wand device includes a socket into which the gas detecting element may be inserted, and an extension cord which is insertable into the ι c . socket of the gas monitor that held the detecting element. The wand and gas monitor may be used as a proximity probe, a "sniffer", or leak detector. Furthermore, it allows the gas monitor to be operated with the gas sensing element in a location remote from the rest of the gas monitor. The remote , operation of the gas monitor may be desirable in environments containing corrosive chemicals or fumes, in deep tank operations or for computer monitored readouts. Also, the remote operation of the gas monitor may be useful when multiple units, positioned in various locations, need -,-r monitoring from a central location. Although illustrative embodiments of the present invention have been described herein with reference to the accompanying drawing, it is to be understood that the invention is not limited to those precise embodiments and f , that various other changes and modifications may be effected therein by one skilled in the art without departing from the scope or spirit of the invention. 35

Claims: Clai s What is claimed is: 1. An environmental gas monitor, which comprises: gas sensing means for detecting and generating a signal proportional to the relative quantity of gas to the the monitor is exposed; visual indicator means for displaying a visual indication of the relative quantity of gas, the visual indicator means being responsive to the signal from the gas sensing means; and audible alarm means for providing an audible alarm in response to the signal from the gas sensing means. - 2. An environmental gas monitor as defined by claim 1, wherein the gas sensing means includes a gas detecting device. 3. An environmental gas monitor as defined by claim 2, wherein the gas sensing means further includes means for compensating the gas sensing device ^ .for temperature and humidity variations. 4. An environmental gas monitor as defined by claim 1, which further includes means for surpressing the audible alarm for a predetermined period of time. 5. An environmental gas monitor as defined by claim 4, wherein the audible alarm surpression means includes a timer, the timer providing a control signal to deactivate the audible alarm for a predetermined period of time. 6. An environmental gas monitor as defined by claim 1, wherein the audible alarm means, . further includes means for pulsating the audible alarm at a preselected frequency. 7. An environmental gas .monitor as defined by claim 6, wherein the alarm pulsating means includes an astable multi-vibrator. 8. An environmental gas monitor as defined by claim 1, wherein the audible alarm means responds to the amplitude of the signal from the gas sensing means, and wherein the gas monitor further includes means for controlling the amplitude of the signal from the gas sensing means.

Inventors: GIELISSE PETER J, GIELISSE, PETER, J.
Applicants: CAPITAL TECHN INT, CAPITAL TECHNOLOGIES INTERNATIONAL
Application year: 1986
Main classification: G08B17
IPC codes: G01N27, G01N33, G01R19