Ionization aerosol detector
United States Patent 3913082
A completely self-contained aerosol detection device and method including a highly sensitive focused beam ionization chamber. The ionization chamber employs a radiation source located between the electrodes which emits particles having essentially a single charge into the space between the electrodes. The invention utilizes electrical or mechanical focusing of the emitted charged particles along a line passing through the two electrodes. The invention detects the presence of aerosols by electrically sensing the decrease in conductivity between the electrodes and producing a current which is proportional to the change in conductivity. The proportional current is then inverted and amplified to a level which triggers an electronic switching element that in turn activates a humanly perceptible signal.
US Patent References:
SIGNAL DETECTION AND MEASURING CIRCUIT
Amiragoff - June 1969 - 3448261
IONIZATION FIRE ALARM AND IMPROVED METHOD OF DETECTING SMOKE AND COMBUSTION AEROSOLS
Lampart et al. - July 1970 - 3521263
RESISTANCE CONTROLLED TIMED PULSE GENERATOR
Blackwell - June 1972 - 3673586
SIGNAL DETECTION AND MEASURING CIRCUIT
Amiragoff - June 1969 - 3448261
IONIZATION FIRE ALARM AND IMPROVED METHOD OF DETECTING SMOKE AND COMBUSTION AEROSOLS
Lampart et al. - July 1970 - 3521263
RESISTANCE CONTROLLED TIMED PULSE GENERATOR
Blackwell - June 1972 - 3673586
Application Number:
05/329285
Publication Date:
10/14/1975
Filing Date:
02/02/1973
View Patent Images:
Export Citation:
Assignee:
Jenson, Robert S. (North Vancouver, CA)
Primary Class:
Other Classes:
250/381, 313/54, 250/384
International Classes:
G08B17/11; G08B17/113; G08B17/10; H01J39/28; G08B17/10
Field of Search:
340/237S 250/357,381,382,384,385 313/54
Primary Examiner:
Yusko, Donald J.
Assistant Examiner:
Myer, Daniel
Attorney, Agent or Firm:
Workman, Ross H.
Claims:
What is claimed and desired to be secured by United States Letters Patent is
1. An ionization aerosol detection system comprising:
2. An ionization aerosol detection system as defined in claim 1 wherein said developing means comprises a housing for the radiation source which is charged to essentially collimate ions emitted therefrom.
3. An ionization aerosol detection system as defined in claim 1 wherein the first electrode is cylindrically configurated and wherein said radiation source is situated adjacent the cylinder upon the axis thereof, particles emitted from the radiation source having the same charge as the first electrode.
4. An ionization aerosol detection system comprising an electrical detection circuit which comprises:
5. An ionization aerosol detection system as defined in claim 4 wherein said inverting element comprises a solid state inverting operational amplifier.
6. A method of electronically detecting the presence of aerosols in ambient air comprising the steps of:
7. A method of electronically detecting the presence of aerosols in ambient air as defined in claim 6 wherein said focusing step comprises enveloping the electrodes and radiation source within an axially symmetrical cavity and electrically charging the cavity with the polarity of the emitted particles.
8. A method of electronically detecting the presence of aerosols in ambient air with a detection circuit having first and second electrodes spaced one from the other, a DC voltage source maintaining a charge difference between the electrodes, a radiation source situated between the electrodes emitting particles into the space between the electrodes to form an electrically conductive path therebetween, a detection chamber, at least a portion of which is electrically charged to the same polarity the predominant particle emitted by the radiation source, the method comprising the steps of:
1. An ionization aerosol detection system comprising:
2. An ionization aerosol detection system as defined in claim 1 wherein said developing means comprises a housing for the radiation source which is charged to essentially collimate ions emitted therefrom.
3. An ionization aerosol detection system as defined in claim 1 wherein the first electrode is cylindrically configurated and wherein said radiation source is situated adjacent the cylinder upon the axis thereof, particles emitted from the radiation source having the same charge as the first electrode.
4. An ionization aerosol detection system comprising an electrical detection circuit which comprises:
5. An ionization aerosol detection system as defined in claim 4 wherein said inverting element comprises a solid state inverting operational amplifier.
6. A method of electronically detecting the presence of aerosols in ambient air comprising the steps of:
7. A method of electronically detecting the presence of aerosols in ambient air as defined in claim 6 wherein said focusing step comprises enveloping the electrodes and radiation source within an axially symmetrical cavity and electrically charging the cavity with the polarity of the emitted particles.
8. A method of electronically detecting the presence of aerosols in ambient air with a detection circuit having first and second electrodes spaced one from the other, a DC voltage source maintaining a charge difference between the electrodes, a radiation source situated between the electrodes emitting particles into the space between the electrodes to form an electrically conductive path therebetween, a detection chamber, at least a portion of which is electrically charged to the same polarity the predominant particle emitted by the radiation source, the method comprising the steps of:
Description:
BACKGROUND
1. Field of Invention
The invention relates to an apparatus and method for detecting products of combustion in ambient air and more particularly to improved ionization aerosol detection devices and methods.
2. The Prior Art
For many centuries residential and commercial building fires have threatened lives and property. The increasing trends toward larger structures and more people housed and working within them has significantly increased the magnitude of the dangers due to fire. Many varied fire detection devices are known in the prior art. Among the previously known fire detection devices, photoelectric and thermostatic mechanisms have been widely employed. These fire sensing devices are only responsive to the latter stages of the combustion cycle; that is, when the fire has become well established and is producing large quantities of smoke or heat.
More recent detection devices have been centered upon the use of an ionization chamber to detect combustion aerosols while the aerosols still represent a very small percentage of the ambient air. Ionization chambers generally operate upon the principle that the conductivity between two spaced electrodes decreases as the ambient aerosols are deposited upon moving ionic particles.
Historically, it has been observed not all ionization chambers operate with the same degree of sensitivity. For example, it is well-known that high voltages across the ionization chamber (e.g. 100 volts) develops very large electric field strength which disadvantageously accelerates the velocity of ions in the chambers. High velocity ions are less easily affected by combustion aerosols and accordingly the sensitivity of the ionization chamber is reduced.
In a first effort to improve sensitivity, reduced electric field strength was maintained in a cylindrical ionization chamber as disclosed in U.S. Pat. No. 3,521,263. This approach provided a satisfactory low voltage detection device but because the electric field strength is geometry dependent, every differently configurated chamber required a new circuit design.
Some additional examples of the attempts to provide improved sensitivity are disclosed in U.S. Pat. Nos. 3,676,680; 3,233,100; and 2,994,768. Each of the above listed references seeks improved sensitivity by varying the placement of the radioactive source with relation to the position of the two electrodes. By this approach, great care has been taken to assure that the ionizing influence of particles from a radioactive source directly affects only a limited portion of the space between electrodes. Some of the prior art attempts to increase the sensitivity of an ionization chamber fire detection unit have involved the use of a reference ionization chamber in combination with a test ionization chamber to accommodate a relative electrical comparison. Still other prior art patents show attempts to increase the sensitivity of ion fire detection devices by using varied electronic circuit configurations. Examples are shown in U.S. Pat. Nos. 3,676,678; 3,673,586, and 3,559,196.
The present invention represents a significantly improved aerosol detection apparatus which advantageously overcomes many of the impediments to increased sensitivity found in the prior art and which has a surprising level of accuracy.
BRIEF SUMMARY AND OBJECTS OF THE INVENTION
The present invention comprises novel apparatus and method for detecting aerosol products of combustion resulting from early stages in the combustion process. The invention utilizes a focused beam ionization chamber for developing an ion cloud which connects two electrodes and a highly sensitive amplification system which drives a switching element to produce a humanly perceptible indication of the presence of combustion aerosols in the ion cloud. Furthermore, the present embodiment of the invention is completely self-contained and requires only very limited periodic maintenance.
It is, therefore, a primary object of the present invention to provide an improved ionization chamber fire detection apparatus.
It is another primary object of the present invention to provide a novel method for detecting and perceptively responding to the presence of combustion aerosols in ambient air.
One still further object of the invention is to provide an improved ionization chamber which develops a focused ion cloud between electrodes.
These and other objects and features of the present invention will become more fully apparent from the following description and appended claims taken in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1 and 2 schematically represent presently known types of ionization chambers useful for explaining the state of the prior art.
FIG. 3 is a schematic representation of one presently preferred ionization chamber embodiment of the present invention.
FIG. 4 is a functional block diagram of the electronic circuitry of the present invention.
FIG. 5 is a schematic diagram representing one preferred embodiment of the electronic circuitry usable with the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
The Prior Art
Reference is now made to FIGS. 1 and 2 which schematically illustrate two prior art approaches to the formation of an ion cloud in an ionization chamber. The ionization chamber generally designated 20 is shown in FIG. 1. The ionization chamber 20 employs a generally cylindrical housing structure 22 and an electrode 24 which is disposed coaxially within the housing 22. A radioactive source 26 emitting negatively charged beta particles is affixed to the negative electrode 24. In the illustrated example, the cylindrical housing 22 is charged positively with respect to the electrode 24. As illustrated in FIG. 1, the interior of the cylindrical chamber defines a portion of an electric field, the field lines being represented by broken lines 32.
The emitted beta particles 30 follow the field lines 32 to the positively charged housing and develop within the chamber 22 an ion cloud which in turn develops a current across the electrodes. Because the beta particles 30 are found throughout the chamber 22, the amount of current traversing the chamber will be dependent upon the size and configuration of the chamber 22.
The prior art chamber of FIG. 2 illustrates positive electrode 33 and negative electrode 35 spaced one from the other. A radioactive source 35 emits alpha or beta particles adjacent and parallel to the negative electrode. The radioactive particles ionize air, the negatively charged component 37 of which migrates toward the electrode 33 to provide unipolar current in a part of the space between electrodes.
The Present Ionization Chamber
Reference is now made to FIG. 3 schematically illustrating a presently preferred ionization chamber embodiment 40 of the invention. The ionization chamber 40 is shown with a cylindrical housing 42 in order to provide a more graphic comparison with the prior art device shown in FIG. 1. As will be recognized after an explanation of the electrical characteristic of the ionization chamber 40, the housing 42 may be of any axially symmetric shape without causing a detrimental change in electrical characteristics. Electrode 44 is disposed coaxially within the housing 42 and the radioactive source 46 is affixed essentially at the center of the end 48 of the housing 42. In the illustrated embodiment of the ionization chamber 40 the housing 42 is charged positively with respect to the electrode 44.
For purposes of illustration, it will be assumed that the radiation source 46 is composed of an alpha emitter such as radium. Alpha particles will be emitted into the area surrounding the source. Unlike the wide dispersion of particles 30 within the ionization chamber 20, the ionization chamber embodiment 40 of the present invention provides a generally axial concentration of the ion cloud within the region 45 of the housing 42. The axial region 45 is created by the focusing influence of he positively charged housing upon the alpha particles emitted by the radioactive source. The air within the region 45 becomes ionized so that a current flows between the electrodes. It is observed that electric field lines 47 exist between the housing 42 and the electrode 44. However, contrary to the effect of prior art housing 22, the field lines have the effect of focusing rather than disbursing the ion cloud. Of course, the focusing effect observed in the ionization chamber 40 would be the same if the radioactive material 46 emitted negatively charged particles and the polarity of the housing 42 and the electrode 44 were reversed. Furthermore, the axial focusing of radioactive emissions may be accomplished through the use of a wide variety of housing shapes and sizes.
Ionization chamber 40 provides means for exposing the ionized particles to ambient air. When an aerosol or product of combustion enters the chamber, the electrical conductivity between the respective housings and the electrodes decreases and the decrease is electronically detectable as will hereinafter be explained. It has been discovered, however, that the axial cloud of ionized particles 45 of the present invention provides surprisingly increased sensitivity to aerosol detection over any of the previously known ionization chambers.
In summary, the ionization chamber of the present invention utilizes a beam of ionized particles which is directed along a line between the two electrodes. It should be recognized that alternate means of producing a beam of particles between the two electrodes may be employed. A deep cup surrounding the radiation source 46 would be a useful mechanical means for focusing the beam of ionized particles and may be used independently or in connection with the FIG. 3 embodiment. In addition, a magnetic field may be used to accomplish the necessary focusing. In the case where mechanical or magnetic focusing is employed, the housing 42 could be completely eliminated or replaced by a protective porous material of nearly any type and of any geometric configuration.
The Detector Circuit
Referring now to FIG. 4, a four-stage circuit is employed in combination with the ionization chamber 40 of FIG. 3 to produce a humanly perceptible indication when aerosol products of combustion enter the ionization chamber 40. The stage A of the circuit is a detection stage which produces a voltage the level of which corresponds to the conductivity of the ionization chamber and therefore to the presence or the absence of aerosol products of combustion in the ambient air. The second stage of the circuit is the isolation-amplification stage. Stage B receives the voltage signal of stage A and produces a corresponding voltage which is an amplified and isolated image of the input signal. Stage C is an inversion-amplification stage. Stage C receives the voltage output signal of stage B and inverts and amplifies the voltage to a level which will drive a switching element. The output signal from stage C is characterized by a trigger voltage only when the input signal to stage C is at smaller than a predetermined reference voltage. The output of of stage c is characterized by a reduced or zero output when the input to stage C is equal to or greater than the predetermined reference voltage. Stage D is an alarm stage wherein the output voltage signal of stage C activates a humanly perceptible alarm device. When the output of stage C reaches trigger voltage, the alarm signal of stage D is activated. It should be recognized that appropriate sensitivity adjusting resistance may be incorporated into any one or more of the four stages. In the circuit embodiment example of FIG. 5, the sensitivity regulation is accomplished through the use of two separate potentiometers, one in stage C and the other as the interstage coupling between stages B and C.
For illustration purposes, a circuit schematic of FIG. 4 is shown in FIG. 5. The stages within the circuit embodiment are indicated by dotted lines and designated to correspond with the stages illustrated in the block diagram of FIG. 4. With continued reference to the circuit embodiment of FIG. 5, a DC voltage source 51 is employed as the power supply for the alarm circuit. The voltage of the DC source is determined by the characteristics of the solid state devices used in the circuit and is also limited by the required voltage across the ionization chamber 40. A power supply having 12.6 volts has been found adequate for this circuit embodiment and miniature batteries manufactured by Eveready and Ray-O-Vac are readily available and adequate. The ionization chamber 40 is connected to the positive power supply bus 49 and is in series with a resistor R51 which is in turn connected to the negative power supply bus 53. R51 is a fixed resistor of a value which preferably will place approximately 6 volts across the ionization chamber 40. A value of 5 × 10 10 ohms has been found appropriate.
An insulated gate field effect transistor MOSFET Q52 is employed to isolate and amplify the voltage differential which appears at node 54. The drain of the MOSFET is connected to the positive bus 49 and a trickle of current i 1 is passed drain to source by the MOSFET Q52. The current i 1 is proportional to the voltage at node 54. A series of biasing resistors R56, P58 and R60 are connected between the source of Q52 and the negative power supply bus 53. Potentiometer P58 serves as a sensitivity control by transferring a selective portion of the voltage drop across the potentiometer and resistor R60 to stage C.
Stage C is primarily composed of an integrated circuit operational amplifier 62 and biasing and stabilizing circuitry. Operational amplifier 62 is an integrated circuit device which drives the gate of a switching device such as SCR 73. Potentiometer P64 is included in the biasing circuitry of the operational amplifier 62 in order to provide additional sensitivity regulation. The operational amplifier 62 is connected in an inverting configuration which yields a significantly positive voltage at terminal 66 when the voltage at terminal 68 goes less positive. R70 is an interstage coupling resistor resistor which feeds the output voltage of the operational amplifier 62 to stage D. The resistor R74 between the positive bus 49 and the non-inverting input 72 of the amplifier 62 in cooperation with resistors P64 and R76 maintain a reference voltage of predetermined magnitude at input 72.
In the steady state, no aerosol products of combustion are present within the ionization chamber 40 and the ionization chamber 40 conducts a predetermined amount of current which yields a comparatively high voltage at node 54. The MOSFET is an N-type channel device which conducts a given current, i 1 , corresponding to a given positive gate to source voltage and therefore is responsive to changes in voltage at node 54.
Since the voltage at node 54 is a comparatively large positive voltage, the MOSFET will conduct a current in the range of 30 to 40 microamps. Accordingly, the voltage at node 70 is high depending upon the setting of potentiometer P58.
Terminal 68 of the operational amplifier 62 is the inverting input terminal and terminal 72 is the non-inverting terminal. Balancing resistors R74, R76 and P64 are provided to produce a reference voltage at input terminal 72 which is sufficiently small to keep the output at terminal 66 of the operational amplifier 62 below trigger voltage when the high positive voltage appears at terminal 68. Feedback resistors R80 and R78 provide the appropriate gain to improve the slow rate of the amplifier output.
In the steady state, node 70 has been assumed at a comparatively high voltage. Terminal 72 of the operational amplifier 62 is maintained at a reference voltage which is low compared to the normal voltage at node 20. Since terminal 68 is an inverting terminal, the output terminal 66 is near zero volts or at least lower than the trigger voltage of SCR73
When aerosols enter the ionization chamber 40, the resistance increases thereby increasing the voltage drop across the chamber and decreasing the voltage at node 54. The MOSFET drain to source current, i 1 , decreases in response to the drop in voltage at node 54. The decrease in i 1 causes the voltage at node 70 to decrease to a level below the reference voltage on non-inverting terminal 72 of the operational amplifier 62. The reference voltage at the non-inverting terminal 72 then dominates therby causing the output terminal 66 of the operational amplifier to rise to a significant positive value.
Stage D of the circuit embodiment of FIG. 4 is primarily composed of a silicon-controlled rectifier (SCR) 73 which drives a suitable alarm such as a buzzer 83. While the illustrated embodiment depicts an audible signal, clearly any suitable humanly perceptible response could be developed from the output signal at terminal 66. When a triggering voltage appears at node 84, SCR 73 switches on and activates the alarm buzzer 83. The power supply bus terminals 88 and 90 may be connected, if desired, to an appropriate battery level indicator.
The SCR 73 will not switch on until a significant positive voltage appears at node 84. When terminal 66 is low or near zero volts (i.e. when aerosols are not present in chamber 40) R86 holds the gate of the SCR at a minimum potential. The SCR, therefore, remains off and the alarm 84 is not activated.
The four-stage alarm circuit of FIGS. 4 and 5 incorporating the ionization chamber of FIG. 1 presents a surprisingly sensitive fire alarm device. The fire alarm may be a self-contained unit which can be placed in an area where fire monitoring is deemed necessary. The extreme sensitivity of the fire alarm makes it possible for one such alarm to monitor a very large area. Of course, the alarm could be connected in combination with other similar fire alarms to a central notification station and thereby sound a general fire alarm signal.
The invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive and the scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.
1. Field of Invention
The invention relates to an apparatus and method for detecting products of combustion in ambient air and more particularly to improved ionization aerosol detection devices and methods.
2. The Prior Art
For many centuries residential and commercial building fires have threatened lives and property. The increasing trends toward larger structures and more people housed and working within them has significantly increased the magnitude of the dangers due to fire. Many varied fire detection devices are known in the prior art. Among the previously known fire detection devices, photoelectric and thermostatic mechanisms have been widely employed. These fire sensing devices are only responsive to the latter stages of the combustion cycle; that is, when the fire has become well established and is producing large quantities of smoke or heat.
More recent detection devices have been centered upon the use of an ionization chamber to detect combustion aerosols while the aerosols still represent a very small percentage of the ambient air. Ionization chambers generally operate upon the principle that the conductivity between two spaced electrodes decreases as the ambient aerosols are deposited upon moving ionic particles.
Historically, it has been observed not all ionization chambers operate with the same degree of sensitivity. For example, it is well-known that high voltages across the ionization chamber (e.g. 100 volts) develops very large electric field strength which disadvantageously accelerates the velocity of ions in the chambers. High velocity ions are less easily affected by combustion aerosols and accordingly the sensitivity of the ionization chamber is reduced.
In a first effort to improve sensitivity, reduced electric field strength was maintained in a cylindrical ionization chamber as disclosed in U.S. Pat. No. 3,521,263. This approach provided a satisfactory low voltage detection device but because the electric field strength is geometry dependent, every differently configurated chamber required a new circuit design.
Some additional examples of the attempts to provide improved sensitivity are disclosed in U.S. Pat. Nos. 3,676,680; 3,233,100; and 2,994,768. Each of the above listed references seeks improved sensitivity by varying the placement of the radioactive source with relation to the position of the two electrodes. By this approach, great care has been taken to assure that the ionizing influence of particles from a radioactive source directly affects only a limited portion of the space between electrodes. Some of the prior art attempts to increase the sensitivity of an ionization chamber fire detection unit have involved the use of a reference ionization chamber in combination with a test ionization chamber to accommodate a relative electrical comparison. Still other prior art patents show attempts to increase the sensitivity of ion fire detection devices by using varied electronic circuit configurations. Examples are shown in U.S. Pat. Nos. 3,676,678; 3,673,586, and 3,559,196.
The present invention represents a significantly improved aerosol detection apparatus which advantageously overcomes many of the impediments to increased sensitivity found in the prior art and which has a surprising level of accuracy.
BRIEF SUMMARY AND OBJECTS OF THE INVENTION
The present invention comprises novel apparatus and method for detecting aerosol products of combustion resulting from early stages in the combustion process. The invention utilizes a focused beam ionization chamber for developing an ion cloud which connects two electrodes and a highly sensitive amplification system which drives a switching element to produce a humanly perceptible indication of the presence of combustion aerosols in the ion cloud. Furthermore, the present embodiment of the invention is completely self-contained and requires only very limited periodic maintenance.
It is, therefore, a primary object of the present invention to provide an improved ionization chamber fire detection apparatus.
It is another primary object of the present invention to provide a novel method for detecting and perceptively responding to the presence of combustion aerosols in ambient air.
One still further object of the invention is to provide an improved ionization chamber which develops a focused ion cloud between electrodes.
These and other objects and features of the present invention will become more fully apparent from the following description and appended claims taken in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1 and 2 schematically represent presently known types of ionization chambers useful for explaining the state of the prior art.
FIG. 3 is a schematic representation of one presently preferred ionization chamber embodiment of the present invention.
FIG. 4 is a functional block diagram of the electronic circuitry of the present invention.
FIG. 5 is a schematic diagram representing one preferred embodiment of the electronic circuitry usable with the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
The Prior Art
Reference is now made to FIGS. 1 and 2 which schematically illustrate two prior art approaches to the formation of an ion cloud in an ionization chamber. The ionization chamber generally designated 20 is shown in FIG. 1. The ionization chamber 20 employs a generally cylindrical housing structure 22 and an electrode 24 which is disposed coaxially within the housing 22. A radioactive source 26 emitting negatively charged beta particles is affixed to the negative electrode 24. In the illustrated example, the cylindrical housing 22 is charged positively with respect to the electrode 24. As illustrated in FIG. 1, the interior of the cylindrical chamber defines a portion of an electric field, the field lines being represented by broken lines 32.
The emitted beta particles 30 follow the field lines 32 to the positively charged housing and develop within the chamber 22 an ion cloud which in turn develops a current across the electrodes. Because the beta particles 30 are found throughout the chamber 22, the amount of current traversing the chamber will be dependent upon the size and configuration of the chamber 22.
The prior art chamber of FIG. 2 illustrates positive electrode 33 and negative electrode 35 spaced one from the other. A radioactive source 35 emits alpha or beta particles adjacent and parallel to the negative electrode. The radioactive particles ionize air, the negatively charged component 37 of which migrates toward the electrode 33 to provide unipolar current in a part of the space between electrodes.
The Present Ionization Chamber
Reference is now made to FIG. 3 schematically illustrating a presently preferred ionization chamber embodiment 40 of the invention. The ionization chamber 40 is shown with a cylindrical housing 42 in order to provide a more graphic comparison with the prior art device shown in FIG. 1. As will be recognized after an explanation of the electrical characteristic of the ionization chamber 40, the housing 42 may be of any axially symmetric shape without causing a detrimental change in electrical characteristics. Electrode 44 is disposed coaxially within the housing 42 and the radioactive source 46 is affixed essentially at the center of the end 48 of the housing 42. In the illustrated embodiment of the ionization chamber 40 the housing 42 is charged positively with respect to the electrode 44.
For purposes of illustration, it will be assumed that the radiation source 46 is composed of an alpha emitter such as radium. Alpha particles will be emitted into the area surrounding the source. Unlike the wide dispersion of particles 30 within the ionization chamber 20, the ionization chamber embodiment 40 of the present invention provides a generally axial concentration of the ion cloud within the region 45 of the housing 42. The axial region 45 is created by the focusing influence of he positively charged housing upon the alpha particles emitted by the radioactive source. The air within the region 45 becomes ionized so that a current flows between the electrodes. It is observed that electric field lines 47 exist between the housing 42 and the electrode 44. However, contrary to the effect of prior art housing 22, the field lines have the effect of focusing rather than disbursing the ion cloud. Of course, the focusing effect observed in the ionization chamber 40 would be the same if the radioactive material 46 emitted negatively charged particles and the polarity of the housing 42 and the electrode 44 were reversed. Furthermore, the axial focusing of radioactive emissions may be accomplished through the use of a wide variety of housing shapes and sizes.
Ionization chamber 40 provides means for exposing the ionized particles to ambient air. When an aerosol or product of combustion enters the chamber, the electrical conductivity between the respective housings and the electrodes decreases and the decrease is electronically detectable as will hereinafter be explained. It has been discovered, however, that the axial cloud of ionized particles 45 of the present invention provides surprisingly increased sensitivity to aerosol detection over any of the previously known ionization chambers.
In summary, the ionization chamber of the present invention utilizes a beam of ionized particles which is directed along a line between the two electrodes. It should be recognized that alternate means of producing a beam of particles between the two electrodes may be employed. A deep cup surrounding the radiation source 46 would be a useful mechanical means for focusing the beam of ionized particles and may be used independently or in connection with the FIG. 3 embodiment. In addition, a magnetic field may be used to accomplish the necessary focusing. In the case where mechanical or magnetic focusing is employed, the housing 42 could be completely eliminated or replaced by a protective porous material of nearly any type and of any geometric configuration.
The Detector Circuit
Referring now to FIG. 4, a four-stage circuit is employed in combination with the ionization chamber 40 of FIG. 3 to produce a humanly perceptible indication when aerosol products of combustion enter the ionization chamber 40. The stage A of the circuit is a detection stage which produces a voltage the level of which corresponds to the conductivity of the ionization chamber and therefore to the presence or the absence of aerosol products of combustion in the ambient air. The second stage of the circuit is the isolation-amplification stage. Stage B receives the voltage signal of stage A and produces a corresponding voltage which is an amplified and isolated image of the input signal. Stage C is an inversion-amplification stage. Stage C receives the voltage output signal of stage B and inverts and amplifies the voltage to a level which will drive a switching element. The output signal from stage C is characterized by a trigger voltage only when the input signal to stage C is at smaller than a predetermined reference voltage. The output of of stage c is characterized by a reduced or zero output when the input to stage C is equal to or greater than the predetermined reference voltage. Stage D is an alarm stage wherein the output voltage signal of stage C activates a humanly perceptible alarm device. When the output of stage C reaches trigger voltage, the alarm signal of stage D is activated. It should be recognized that appropriate sensitivity adjusting resistance may be incorporated into any one or more of the four stages. In the circuit embodiment example of FIG. 5, the sensitivity regulation is accomplished through the use of two separate potentiometers, one in stage C and the other as the interstage coupling between stages B and C.
For illustration purposes, a circuit schematic of FIG. 4 is shown in FIG. 5. The stages within the circuit embodiment are indicated by dotted lines and designated to correspond with the stages illustrated in the block diagram of FIG. 4. With continued reference to the circuit embodiment of FIG. 5, a DC voltage source 51 is employed as the power supply for the alarm circuit. The voltage of the DC source is determined by the characteristics of the solid state devices used in the circuit and is also limited by the required voltage across the ionization chamber 40. A power supply having 12.6 volts has been found adequate for this circuit embodiment and miniature batteries manufactured by Eveready and Ray-O-Vac are readily available and adequate. The ionization chamber 40 is connected to the positive power supply bus 49 and is in series with a resistor R51 which is in turn connected to the negative power supply bus 53. R51 is a fixed resistor of a value which preferably will place approximately 6 volts across the ionization chamber 40. A value of 5 × 10 10 ohms has been found appropriate.
An insulated gate field effect transistor MOSFET Q52 is employed to isolate and amplify the voltage differential which appears at node 54. The drain of the MOSFET is connected to the positive bus 49 and a trickle of current i 1 is passed drain to source by the MOSFET Q52. The current i 1 is proportional to the voltage at node 54. A series of biasing resistors R56, P58 and R60 are connected between the source of Q52 and the negative power supply bus 53. Potentiometer P58 serves as a sensitivity control by transferring a selective portion of the voltage drop across the potentiometer and resistor R60 to stage C.
Stage C is primarily composed of an integrated circuit operational amplifier 62 and biasing and stabilizing circuitry. Operational amplifier 62 is an integrated circuit device which drives the gate of a switching device such as SCR 73. Potentiometer P64 is included in the biasing circuitry of the operational amplifier 62 in order to provide additional sensitivity regulation. The operational amplifier 62 is connected in an inverting configuration which yields a significantly positive voltage at terminal 66 when the voltage at terminal 68 goes less positive. R70 is an interstage coupling resistor resistor which feeds the output voltage of the operational amplifier 62 to stage D. The resistor R74 between the positive bus 49 and the non-inverting input 72 of the amplifier 62 in cooperation with resistors P64 and R76 maintain a reference voltage of predetermined magnitude at input 72.
In the steady state, no aerosol products of combustion are present within the ionization chamber 40 and the ionization chamber 40 conducts a predetermined amount of current which yields a comparatively high voltage at node 54. The MOSFET is an N-type channel device which conducts a given current, i 1 , corresponding to a given positive gate to source voltage and therefore is responsive to changes in voltage at node 54.
Since the voltage at node 54 is a comparatively large positive voltage, the MOSFET will conduct a current in the range of 30 to 40 microamps. Accordingly, the voltage at node 70 is high depending upon the setting of potentiometer P58.
Terminal 68 of the operational amplifier 62 is the inverting input terminal and terminal 72 is the non-inverting terminal. Balancing resistors R74, R76 and P64 are provided to produce a reference voltage at input terminal 72 which is sufficiently small to keep the output at terminal 66 of the operational amplifier 62 below trigger voltage when the high positive voltage appears at terminal 68. Feedback resistors R80 and R78 provide the appropriate gain to improve the slow rate of the amplifier output.
In the steady state, node 70 has been assumed at a comparatively high voltage. Terminal 72 of the operational amplifier 62 is maintained at a reference voltage which is low compared to the normal voltage at node 20. Since terminal 68 is an inverting terminal, the output terminal 66 is near zero volts or at least lower than the trigger voltage of SCR73
When aerosols enter the ionization chamber 40, the resistance increases thereby increasing the voltage drop across the chamber and decreasing the voltage at node 54. The MOSFET drain to source current, i 1 , decreases in response to the drop in voltage at node 54. The decrease in i 1 causes the voltage at node 70 to decrease to a level below the reference voltage on non-inverting terminal 72 of the operational amplifier 62. The reference voltage at the non-inverting terminal 72 then dominates therby causing the output terminal 66 of the operational amplifier to rise to a significant positive value.
Stage D of the circuit embodiment of FIG. 4 is primarily composed of a silicon-controlled rectifier (SCR) 73 which drives a suitable alarm such as a buzzer 83. While the illustrated embodiment depicts an audible signal, clearly any suitable humanly perceptible response could be developed from the output signal at terminal 66. When a triggering voltage appears at node 84, SCR 73 switches on and activates the alarm buzzer 83. The power supply bus terminals 88 and 90 may be connected, if desired, to an appropriate battery level indicator.
The SCR 73 will not switch on until a significant positive voltage appears at node 84. When terminal 66 is low or near zero volts (i.e. when aerosols are not present in chamber 40) R86 holds the gate of the SCR at a minimum potential. The SCR, therefore, remains off and the alarm 84 is not activated.
The four-stage alarm circuit of FIGS. 4 and 5 incorporating the ionization chamber of FIG. 1 presents a surprisingly sensitive fire alarm device. The fire alarm may be a self-contained unit which can be placed in an area where fire monitoring is deemed necessary. The extreme sensitivity of the fire alarm makes it possible for one such alarm to monitor a very large area. Of course, the alarm could be connected in combination with other similar fire alarms to a central notification station and thereby sound a general fire alarm signal.
The invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive and the scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.