US3521263A - Ionization fire alarm and improved method of detecting smoke and combustion aerosols - Google Patents

Description

y 1970 T. LAMPART ETAL 3,

IONIZATION FIRE ALARM AND IMPROVED METHOD OF DETECTING SMOKE AND COMBUSTION AEROSOLS 7 Filed Feb. 20, 1967 .4 Sheets-Sheet 1 INVENTORS:

The M n g LHMPQE and B Qmleen; sclneicl w-aikz %(4131 & $51 466 Cuba/Ma a y 21, 1970 T. LAMPART ETAL 3,52 63 IONIZATION FIRE ALARM AND IMPROVED METHOD OF DETECTING SMOKE AND COMBUSTION AEROSOLS Filed Feb. 20, 1967 I .4 Sheets-Sheet 2 Prior Art y 1970 T. LAMPART ETAL 3,521,263

OD 0F DETECTING SMOKE AND COMBUSTION AEROSOLS IONIZATION FIRE ALARM AND IMPROVED METH Filed Feb. 20, 1967 4 She ets-Sheet s INVENTORS! Thomas Lao mit uni BY w keae. cke'llwalea QQM'W Cdwwy y 21, 1970 T. LAMPART ETAL 3,521,263

IONIZATION FIRE ALARM AND IMPROVED METHOD OF DETECTING SMOKE AND COMBUSTION AEROSOLS Filed Feb. 20, 1967 .4 Sheets-Sheet &

United States Patent 01 Two US. Cl. 340-237 16 Claims ABSTRACT OF THE DISCLOSURE A method of detecting with increased sensitivity the presence of smoke and combustion aerosols and an ionization fire alarm operating in accordance with the inventive method and constructed to possess such increased sensitivity. There is provided at least one ionization chamber which is essentially freely accessible for the surrounding air and containing therein at least one source of radiation in order to produce ions within such ionization chamber. The method contemplates, and the ionization fire alarm is constructed such, that there is generated an electric field strength within the ionization chamber of less than 5 volts per centimeter in the region of the ionization chamber in which the greater portion of the ionization current flows.

BACKGROUND OF THE INVENTION The present invention pertains to an improved ionization fire alarm possessing increased sensitivity to smoke and combustion particles or aerosols and is of the type comprising at least one ionization chamber-also referred to as measuring chamber-which is essentially freely accessible for the ambient or surrounding air. The inventive ionization fire alarm also includes one or more radioactive sources for producing ions, and an electric circuit for producing an alarm. Additionally, the present invention also pertains to an improved method of detecting smoke and combustion aerosols by means of an apparatus structure of the aforementioned type.

It is already known that the current flowing in an ionization chamber drops upon entry of combustion gases or the like. This effect which is attributable to the deposition of gas ions as aerosol particles and the therewith associated reduction of the ion mobility has been used for quite some time for recording the presence of combustion gases, and thereby for sounding an alarm. The production of a certain ion concentration takes place by means of radioactive isotopes, generally alpha emitters, which are directly accommodated in the ionization chamber. Numerous processes or methods are known, such as taught in US. Pat. No. 2,702,898, 2,994,768, 3,233,100, among others, which in principle employ two ionization chambers connected in series, one of which is accessible by the surrounding or external air, whereas the other is not. The voltage across one of the chambers serves as a measurement value which, upon exceeding a certain value, triggers an alarm apparatus.

With the presently known ionization fire alarm of this type the voltage across the measuring chamber, that is, across the ionization chamber which is accessible to the outside air, amounts to about 100 volts. Since the spacing between the electrodes generally only amounts to several centimeters, in certain zones or regions of the ionization chamber electric field strengths appear which amount to several hundred volts per centimeter. This is especially so if there is employed a cylinder as the geo- 3,521,263 Patented July 21, 1970 metric form for the ionization chamber. The following considerations will now clearly demonstrate the influences of the electric field strength upon the smoke sensitivity of such type of ionization fire alarm.

If aerosols, that is to say, combustion gases enter the measuring chamber then it will happen with a certain probability that these aerosols will deposit upon the ions which are moving in the measuring chamber under the influence of the electric field E. Since the mass of the aerosol particles is approximately ten thousand times larger than that of the ions, the speed of an ion after aerosol deposition has taken place is so small that in comparison to the gas ions one can consider such to be at standstill. Consequently, insofar as transport of charges is concerned such ion practically no longer comes under consideration. Hence, the drop in current is directly determined by the number of depositions which take place per unit of time. In this respect, the total measuring effect is additively derived from the contribution of the individual volume increments in the measuring chamber. According to the so-called deposition law discovered by Schweitler the change of the ion concentration owing to deposition at aerosols can be expressed as follows:

dt Equation 1 Here, dn/dt represents the change with time of the ion concentration n and 6 is a proportionality factor. By simple transformation of this equation it will be seen that the relative change of the ion concentration is proportional to the residence time T of the ions in the considered volume increment.

i b.E

Now, however, the ion residence time is dependent in simple manner upon the electric field strength or intensity E. Further, it is namely true that wherein dr represents the length of the volume increment in the direction of the field and b represents the ion mobility. It will thus be evident that the individual volume increments, depending upon the magnitude of the prevailing field intensity, provide different contributions to the measuring effect, with the exception of the special case of a constant field in the total measuring chamber which, however, as a practical matter can only be realized with great difficulty.

As a simple example take the case of the cylindrical ionization chamber in which the field strength increases with 1/r towards the inner electrode. In this respect, by integration of Equation 1 above it follows that the number of deposition processes in the volume increment approximately increases by the second power with the spacing from the chamber axis.

To elucidate upon this point further, it is remarked that this phenomenon is depicted in FIG. 1. There, the inner electrode 2 and the chamber housing 1 have only been schematically depicted. Field strength or intensity E is plotted along the ordinate as well as also the number of deposition processes A in the volume unit. Along the abscissa there is plotted the spacing r from the inner electrode 2. The full line curve E=a/ r shows the course of the field strength in the ionization chamber without the influence of the marginal zones. The broken line curve A k'r shows the course of the deposition process. In the above equations, the symbols a and k are proportionality factors.

As can be readily recognized, the major portion of the depositions take place in the neighborhood of the chamber housing 1, whereas the zone or region about the inner electrode 2 practically contributes nothing to the measuring effect. In this respect, there must also be considered the fact that the geometry of the chamber owing to constructive measures generally markedly deviates from the ideal cylindrical form, so that still further regions of higher field strength exist.

On the other hand, FIG. 2 shows the physical structure of a measuring chamber of a known ionization fire alarm. It incorporates a cylindrical housing 1 defining the outer electrode and provided with numerous apertures or openings 1a. The smoke or combustion gases, as a practical matter, have almost unhindered access to the cylindrical housing 1. Further, this ionization chamber is composed of an inner electrode 2 which extends from beneath into the measuring compartment, as shown. The radioactive preparations 3 and 4 are disposed at the floor of the housing 1 in the region of the inner electrode 2. At suitable non-illustrated direct-current voltage source is electrically coupled to the chamber housing 1 and the inner electrode 2 in a manner that for instance the negative pole of the source is connected to chamber housing 1 and the positive pole to the inner electrode 2. The shaded or cross-hatched region 7 represents that zone or region of the measuring chamber in which no essential amount of deposition takes place owing to the higher field strength. The dashed or broken lines 8 depict the course of the electric field lines. As can beseen, the negative ions on their way to the inner electrode 2 must pass through the region 7 of higher field intensity or strength. The flow of the positive ions is represented by reference numeral 6.

Further, FIG. 3 depicts a different embodiment of measuring chamber of a known ionization fire alarm in which likewise a larger portion of the transfer or transport of charges takes place in the region of higher field strength. The radioactive source 3 is applied to the inner electrode 2. Also in this case deposition essentially takes place only outside of the cross-hatched region 7. Further, it will be appreciated that once again the same reference numerals have been used for like or analogous elements, and accordingly, reference numeral 1 represents the outer electrode. The flow of the negative and positive ions is represented by reference numerals 4 and 5 respectively. Just as was the case with the previous prior art embodiment here also in FIG. 2 both electrodes 1 and 2 are coupled with a non-illustrated suitable directcurrent voltage source.

SUMMARY OF THE INVENTION As these examples have demonstrated the available chamber volumes of the presently known ionization fire alarms are only poorly utilized. It is for this reason. that extensive experimentations have been undertaken which have measured the smoke sensitivity of the ionization fire alarm as a function of the electric field strength. Evaluation of the measurement results have verified the theoretical considerations Which were previously explained and, in particular, demonstrated that volume regions of the measuring chamber through which current flows and in which the electric field strength considerably exceeds the value of 5 volts per centimeter, only provide a relatively small contribution to the measuring effect. The reason for this is attributable to the strongly reduced probability of deposition owing to the higher ion velocity. From this it follows that an improved utilization of the volume of the measuring chamber, and therefore, an increased smoke sensitivity can be realized it the transport of charges in the measuring chamber takes place in regions of the chamber space having smaller electric field strengths. Accordingly, the present invention is generally characterized by the fact that there is present an electric field strength of less than 5 volts per centimeter in those regions of the measuring chamber in which the greater portion of the ionization current fiows.

Basically, the requirement for such type low field intensities or strengths can be generally realized in two different ways, namely:

(a) Increasing the spacing of the electrodes about tenfold while retaining the previous standard chamber voltage of approximately 100 volts; and

(b) Appropriate reduction of the chamber voltage while retaining the previous conventional spacing of the electrodes which amounts to several centimeters.

The first solution proposed above under item (a), as a practical matter, is not technically realistic owing to the large spatial dimensions which would be required of the ionization fire alarm.

Since previously the prior art did not take into consideration the influence of the electric field strength upon the smoke sensitivity of an ionization fire alarm, it was erroneously assumed that it was advantageous to operate in the region of higher chamber voltages, in other words, above 100 volts. One of the reasons for this was predicated upon striving to employ a simple indicating instrument in the case of an alarm. In this regard, a cold cathode tube provided an existing possibility which would ignite when exceeding the alarm threshold. However, the ignition voltages of these tubes require at least volts. A further argument which was advanced by those familiar with the prior art in order to support their requirement to work at higher voltages resulted from the desire of using a secondseries connected ionization chamber for producing a maximum voltage level of a saturated chamber.

Notwithstanding all of the theories of those engaged in this particular field of ionization fire alarms, and further, notwithstanding the requirements as to higher chamber voltage which were proposed by such prior art fire alarm systems, the teachings of the present invention have surprisingly found that it is possible to provide increased sensitivity to smoke and combustion aerosols for an ionization fire alarm if there is provided an electric field strength within the ionization chamber of less than 5 volts per centimeter in the region of the ionization chamber in which the greater portion of the ionic current flows. Apart from the noteworthy advantage of increased smoke sensitivity as mentioned above, the ionization fire alarm of the present invention has still further significant advantages:

(1) Owing to the much smaller field strength the extremely disturbing dust deposition at the electrodes, which can render the fire alarm system inefiectual, is much smaller than with the presently known systems and techniques;

(2) Owing to the fact that the operating voltage of the fire alarm is below 60 volts there are provided considerable advantages during installation;

(3) Climatic changes, such as pressure and temperature, have considerably less disturbing efiect upon the system; and

(4) There exists a simple possibility of transistorizing the electric circuit.

Accordingly, it is a primary object of the present invention to provide an improved method of, and apparatus for, detecting smoke and combustion aerosols with increased sensitivity.

Another, significant object of the present invention is to provide an improved ionization fire alarm which possesses increased sensitivity to smoke and combustion aerosols, thereby positively protecting the area monitored by the fire alarm, and also overcoming the drawbacks of the prior art systems mentioned heretofore.

Yet, a further important object of the present invention concerns itself with an improved method of, and ionization fire alarm for, positively and reliably detecting smoke and combustion aerosols with extreme sensitivity and wherein the physical structure of the fire alarm system and its mode of operation is such that there is minimized the danger of dust deposition at the electrodes which could adversely affect the integrity of the system.

A further noteworthy object of this invention relates to an improved ionization fire alarm which operates at such a relatively low operating voltage that the fire alarm system is much easier to install than was heretofore possible, and further, owing to this expedient it is additionally possible to transistorize the electric circuitry.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be better understood, and objects other than those set forth above, will become apparent, when consideration is given to the following detailed description thereof. Such description makes reference to the annexed drawings wherein:

FIG. 1 illustrates an ionization chamber having an inner electrode and an outer electrode and depicts the respective curves representing the field intensity and the deposition process undertaken by the combustion aerosols and the gas ions;

FIG. 2 schematically illustrates a prior art ionization chamber and is used to provide a better understanding of the principles of the invention;

FIG. 3 schematically depicts a further prior art ionization chamber, also being used for purposes of explaining and clarifying the principles of the present invention;

FIG. 4 schematically depicts the physical structure of a first embodiment of ionization chamber according to the teachings of the present invention;

FIG. 5 depicts the physical structure of a second embodiment of ionization chamber according to the present invention;

FIG. 6 is a circuit diagram depicting the cooperation of the ionization chambers of the embodiments of FIGS. 4 and 5 with the electric circuitry; and

FIG. 7 is a variant form of ionization fire alarm system employing two series connected ionization chambers.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Describing now the drawings and, in particular, directing attention to the exemplary embodiment of ionization chamber 20, also referred to as measuring chamber, depicted in FIG. 4 it will be seen that such comprises a perforated or apertured cylindrical housing 1 providing the outer electrode as well as an inner electrode 2 arranged within the outer electrode 1. Further, a source of radioactive material 3 is applied to the inner electrode 2, as shown. The arrows 9 represent the ionized chamber region brought about by the sphere of influence of the radiation. The direction of the radioactive rays is essentially parallel to the direction of the field lines 8. Naturally, the sphere of influence of the radioactive material 3 can be also selected such that ions are produced almost in the entire measuring or ionization chamber 20. Continuing, and as clearly apparent from the course of the field lines 8, transport of the charges essentially takes place in the chamber region or space which is subjected to a small field strength or intensity. Those locations 7 of the measuring or ionization chamber 20 in which higher field strengths occur make only a very small contribution to the total current since such regions 7 are essentially located outside of the ionization zone.

A variant embodiment of measuring or ionization chamber 20 is depicted in FIG. 5, where once again the same reference numerals have conveniently been employed to designate like or analogous components. Here, in comparison to FIG. 4, there is employed a different method of ionization. The radioactive sources 3, 4 emit their rays, not as with the previous embodiment in the direction of the electric field lines 8, but rather transverse to such field. By appropriate scattering of the rays there approximately appears an ionized zone as represented by the depicted arrows 9. Also in this case the 6 ionic current 5 essentially flows in the region of lower field strength.

Keeping the foregoing in mind, attention is invited to FIG. 6 which schematically depicts a simple embodiment of ionization fire alarm designed according to the teachings of the present invention. Here, the ionization or measuring chamber 10, which may be of the type previously considered with regard to FIGS. 4 and 5, is electrically coupled at its inner electrode 2 with the negative pole of the operating voltage U via a resistor 11 and at its chamber housing 1 with the positive pole of said operating voltage U By way of example, at the inner electrode 2 there is provided a radioactive source 3 similar to the arrangement of FIG. 4. Naturally, there also could be provided a plurality of radioactive sources, such as shown for instance in the embodiment of FIG. 5. In any case, the voltage U across the measuring or ionization chamber 10 is, at the same time, the gate voltage for a suitable semi-conductor element such as the field-effect transistor 12 providing a sensing or indicating means which senses voltage changes across the electrodes 1, 2 of the ionization chamber 10. This voltage U is chosen such that the field effect transistor 12 in its quiescent state blocks current flow, that is to say, at the working resistor 13 there is no voltage drop. Moreover, it should be appreciated that in the absence of smoke and com bustion aerosols the voltage across the ionization chamber is advantageously of a magnitude smaller than 20 volts. The gate-controlled rectifier 14 is therefore likewise blocked and the relay 15 is not energized. Now, if smoke or combustion gases penetrate into the measuring or ionization chamber 10 then the chamber voltage U increases and upon exceeding a predetermined threshold triggers ignition of the controlled rectifier 14, whereby the relay 15 triggers a suitable alarm. Triggering of the alarm can be carried out in any appropriate manner, as for instance shown and described in the commonly assigned US. Pat. 3,233,100 of Thomas Lampart, one of the co-inventors herein, and entitled Determining Presence of Aerosols in Gases, and granted Feb. 1, 1966. In this instance, the field-effect transistor 12 operates both as a threshold device as well as also as an amplifier element.

FIG. 7 depicts a different embodiment of fire alarm system designed according to the teachings of the present invention, and wherein there are employed two symmetrical ionization chambers 10' and 16 which are connected in series. Here, the chamber 10 is the measuring chamber whereas the other chamber 16 serves as a reference or comparison chamber. Both of these chambers 10, 16 are constructed in a manner similar to that previously described herein.

In this case, the field effect transistor 12 functions as an impedance transformer which transforms the high-ohm intrenal chamber resistance of about 10 ohms into the range of several kiloohms. The voltage U serves to regulate the alarm threshold. When smoke and combustion gases enter into the measuring chamber 10, also in this instance, the voltage at the gate of the field effect transistor 12 increases, and therefore, also at its cathode or emitter If this value exceeds the regulated threshold U then the controlled rectifier 14 ignites and the relay 15 is energized, for the reasons previously considered. On the other hand, in a very simple manner it is possible to check the func tionality or operability of the fire alarm system by caus ing the controlled rectifier 14 to fire by dropping the volt age U Finally, by way of completeness it is once again mentioned that in all of the ionization fire alarm systems of the invention herein disclosed there may advantageously be employed an operating power voltage supply which is less than 60 volts, bringing about the advantages previously noted herein.

As should now be apparent, the objects set forth at the outset of the specification have been successfully achieved. Accordingly, what is claimed is:

1. An ionization fire alarm having increased sensitivity to smoke and combustion aerosols, comprising: means for forming at least one ionization chamber which is essentially freely accessible for the ambient air, said ionization chamber providing a measuring chamber; at least one source of radioactive means for producing ions within said ionization chamber; electric circuit means electrically coupled with said ionization chamber for producing an alarm; means for generating an electric field strength within said ionization chamber of less than volts per centimeter in the region of said ionization chamber in which the greater portion of the ionization current flows.

2. An ionization fire alarm as defined in claim 1, wherein said generating means includes means for applying a voltage across said ionization chamber of a magnitude smaller than volts in the absence of smoke and combustion aerosols.

3. An ionization fire alarm as defined in claim 1, wherein said generating means includes an operating power voltage supply which is less than 60 volts.

-4. An ionization fire alarm as defined in claim 1, wherein said at least one source of radioactive means produces both negative and positive ions.

5. An ionization fire alarm as defined in claim 1, wherein said source of radioactive means only produces ions throughout a relatively small portion of the ionization chamber.

6. An ionization fire alarm as defined in claim 1, further including: resistance means in series with said ionization chamber; sensing means in circuit with said electric circuit means for detecting changes in voltage across said ionization chamber and controlling said electric circuit means for producing an alarm.

7. An ionization fire alarm as defined in claim 6, wherein said ionization chamber includes spaced electrode means, and wherein changes in voltage across said electrode means are employed to control operation of said sensing means.

8. An ionization fire alarm as defined in claim 6, wherein said resistance means in series with said ionization chamber includes means providing a second ionization chamber.

9. An ionization fire alarm as defined in claim 8, wherein said second ionization chamber essentially possesses the same current-voltage characteristics as the other ionization chamber defining said measuring chamber.

10. An ionization fire alarm as defined in claim 1, further including sensing means in circuit with said electric circuit means for detecting changes in voltage across said ionization chamber and controlling said electric circuit means for producing an alarm, said sensing means including a field-effect transistor means.

11. An ionization fire alarm as defined in claim 10, wherein said electric circuit means includes a controlled 8 rectifier means, said field-effect transistor means controlling said controlled rectifier means.

12. A method of detecting the presence of smoke and combustion aerosols in a gaseous atmosphere resulting from combustion comprising the steps of:

introducing a gaseous atmosphere into a chamber;

ionizing at least a portion of said gaseous atmosphere;

providing an electric field within said chamber to cause movement of the ions and a production of ionic current, the strength of said electric field being not greater than 5 volts per centimeter throughout an area within said chamber through which the greater portion of said ionic current flows;

introducing smoke and combustion aerosols resulting from combustion into said chamber, said aerosols attaching to at least some ions to increase the mass of said at least some ions and thus reduce said ionic current flow; and,

utilizing said reduction of said ionic current flow to trigger an alarm.

13. A method as defined in claim 12, wherein the step of ionizing at least a'portion of said gaseous atmosphere eflfects the production of both positive and negative ions.

14. A method as defined in claim 12, wherein the step of ionizing at least a portion of said gaseous atmosphere further includes the steps of:

providing a source of radiation; and

placing said source of radiation within said chamber in a manner ,such that ions are only produced throughout a relatively small portion of said chamber.

15. A method as defined in claim 12, wherein the step of ionizing at least a portion of said gaseous atmosphere further includes the step of: providing a source of radiation which has a sphere of influence within said chamber such that ions are only produced throughout a relatively small portion of said chamber.

16. A method as defined in claim 12 wherein said step of providing an electric field within said chamber includes the step of applying a voltage across said chamber having a magnitude less than 20 volts.

References Cited UNITED STATES PATENTS 2,145,866 2/ 1939 Failla. 3,233,100 2/1966 Lampart.

FOREIGN PATENTS 398,722 9/ 1933 Great Britain. 1,009,271 11/1965 Great Britain.

JOHN W. CALDWELL, Primary Examiner D. MYER, Assistant Examiner US. Cl. X.R.

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