Abstract:
A smoke detection sensor ion chamber has a capacitance and a change in the permittivity of that capacitance dielectric (ionized air in the chamber) may be used to detect the presence of smoke therein. Smoke from typical fires is mainly composed of unburned carbon that has diffused in the surrounding air and rises with the heat of the fire. The permittivity of the carbon particles is about 10 to 15 times the permittivity of clean air. The addition of the carbon particles into the air in the ion chamber changes in the permittivity thereof that is large enough to measure by measuring a change in capacitance of the ion chamber.

Description:
RELATED PATENT APPLICATION 
     This application claims priority to commonly owned U.S. Provisional Patent Application Ser. No. 61/570,418; filed Dec. 14, 2011; entitled “Method and Apparatus for Detecting Smoke,” by Benjamin T. Cooke, Joseph Julicher and Keith Edwin Curtis; and is a Continuation-In-Part of U.S. patent application Ser. No. 13/633,686; filed Oct. 2, 2012; entitled “Differential Current Measurements to Determine Ion Current in the Presence of Leakage Current,” by Joseph Julicher, Keith Curtis and Paul N. Katz; both of which are hereby incorporated by reference herein for all purposes. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to smoke detection devices, and more particularly, to a smoke detection device that uses a change in permittivity that affects a capacitance value of an ion chamber when smoke is introduced therein. 
     BACKGROUND 
     A smoke detector generally uses an ionization chamber containing a radioactive ion source that is coupled to a high input impedance operational amplifier.  FIG. 1  shows a typical ionization chamber used in a smoke detector to produce a very small current (nA) that is reduced in the presence of smoke particles. Operational amplifiers are used to convert this current to a voltage that is then measured to determine the presence of smoke. Elevated temperatures cause increased leakage currents on the inputs of the operational amplifier in the smoke detector. This affects overall performance of the ionization chamber smoke detection function. Thus, such increases in leakage currents can pose a variety of problems such as inaccuracy, etc. which may require further compensation circuits when designing a smoke detector and therefore may increase the cost of the device. 
     Furthermore, the impedance of the ion chamber is extremely high, and any leakage currents, e.g., printed circuit board leakage current, masks the ion chamber current. Smoke detection ion chambers therefore require a complex manufacturing process where pins of the sensing integrated circuit operational amplifier are bent and directly welded in mid-air to the ion chamber. As mentioned above, special low leakage circuits are required to detect the small current change through the ion chamber caused by the presence of smoke therein. 
     SUMMARY 
     Therefore, a need exists for a way to detect smoke in an ion chamber of a smoke detector that does not require sensitive and expensive components nor complex manufacturing processes. 
     According to an embodiment, a method for detecting smoke may comprise the steps of: coupling an ionization chamber to a capacitive sensing module (CSM); determining a change in a capacitance of the ionization chamber using the CSM; and detecting the presence of smoke by detecting a predetermined change in the capacitance. 
     According to a further embodiment of the method, the step of determining the change in the capacitance of the ionization chamber further may comprise the steps of: determining a first change in the capacitance of the ionization chamber when the ionization chamber may be at a first polarity; determining a second change in the capacitance of the ionization chamber when the ionization chamber may be at a second polarity; determining a difference between the first change and the second change; and using the difference in determining the change in the capacitance of the ionization chamber. According to a further embodiment of the method, the predetermined change in the capacitance may be a change in the capacitance within a certain time. 
     According to a further embodiment of the method, the step of determining the change in the capacitance of the ionization chamber may comprise the steps of: charging the capacitance of the ionization chamber with a first constant current source until a charge on the capacitance may be at a first voltage, then discharging the capacitance of the ionization chamber with a second constant current source until the charge on the capacitance of the ionization chamber may be at a second voltage, and then repeating charging the capacitance; counting the number of times the charge on the capacitance of the ionization chamber may be at the first or the second voltage within a certain time period; and comparing the count numbers of subsequent time periods to determine whether the count number of any one or more of the subsequent time periods has changed by a certain number of counts. 
     According to a further embodiment of the method, the step of determining the change in the capacitance of the ionization chamber may comprise the steps of: charging the capacitance of a first ionization chamber open to smoke entrance with a first constant current source until a charge on the capacitance of the first ionization chamber may be at a first voltage, then discharging the capacitance of the first ionization chamber with a second constant current source until the charge on the capacitance of the first ionization chamber may be at a second voltage, and then repeating charging the capacitance of the first ionization chamber; counting the number of times the charge on the capacitance of the first ionization chamber may be at the first or the second voltage within a certain time period; charging the capacitance of a second ionization chamber closed to smoke entrance with the first constant current source until a charge on the capacitance of the second ionization chamber may be at a first voltage, then discharging the capacitance of the second ionization chamber with the second constant current source until the charge on the capacitance of the second ionization chamber may be at the second voltage, and then repeating charging the capacitance of the second ionization chamber; counting the number of times the charge on the capacitance of the second ionization chamber may be at the first or the second voltage within a certain time period; and subtracting a count number of the first ionization chamber from a count number of the second ionization chamber and dividing by the count number of the second ionization chamber. 
     According to a further embodiment of the method, in a first measurement, a housing of the ionization chamber may be coupled to the CSM; and in a second measurement, a collector plate of the ionization chamber may be coupled to the CSM. 
     According to a further embodiment of the method, further steps may comprise the steps of subtracting a measurement value of the first measurement from a measurement value of the second measurement then dividing by the second measurement value; and comparing the count numbers of subsequent time periods to determine whether the count number of any one or more of the subsequent time periods has changed by a certain number of counts. According to a further embodiment of the method, further steps may comprise the step of compensating for temperature change with temperature information from a temperature sensor. According to a further embodiment of the method, further steps may comprise the step of compensating for relative humidity change with relative humidity information from a relative humidity sensor. According to a further embodiment of the method, a further step may comprise the step of compensating for voltage change with voltage information from a voltage sensor. 
     According to another embodiment, an apparatus for detecting smoke may comprise: an ionization chamber coupled to a capacitive sensing module (CSM) for determining a capacitance of the ionization chamber; wherein a predetermined change in the capacitance of the ionization chamber indicates the presence of smoke in the ionization chamber. 
     According to a further embodiment, circuits may be provided for alternately coupling to the ionization chamber at a first polarity for determining a first capacitance of the ionization chamber and coupling to the ionization chamber at a second polarity for determining a second capacitance of the ionization chamber, whereby a difference between the first and second capacitances may be used in determining the presence of smoke in the ionization chamber. 
     According to a further embodiment, the CSM may be a peripheral device in a microcontroller. According to a further embodiment, a digital processor and memory may be coupled to the CSM and an alarm circuit. According to a further embodiment, a temperature sensor may be coupled to the digital processor and a temperature compensation look-up table stored in the memory coupled to the digital processor and used to compensate temperature induced changes of the capacitance of the ionization chamber. According to a further embodiment, a humidity sensor may be coupled to the digital processor and a humidity compensation look-up table stored in the memory coupled to the digital processor and used to compensate humidity induced changes of the capacitance of the ionization chamber. 
     According to a further embodiment, a voltage sensor may be coupled to the digital processor and a voltage compensation look-up table stored in the memory coupled to the digital processor and used to compensate voltage induced changes of the capacitance of the ionization chamber. According to a further embodiment, an audible alert may be actuated by the presence of smoke in the ionization chamber. According to a further embodiment, a visual alert may be actuated by the presence of smoke in the ionization chamber. 
     According to still another embodiment, an apparatus for detecting smoke may comprise: a first ionization chamber coupled to a capacitive sensing module (CSM) for determining a capacitance of the first ionization chamber, wherein the first ionization chamber may be open to smoke entrance; a second ionization chamber coupled to the CSM for determining a capacitance of the second ionization chamber, wherein the second ionization chamber may be closed to smoke entrance; wherein a predetermined difference in the capacitances of the first and second ionization chambers indicates the presence of smoke in the first ionization chamber. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete understanding of the present disclosure may be acquired by referring to the following description taken in conjunction with the accompanying drawings wherein: 
         FIG. 1  illustrates a schematic diagram of an ion chamber having a radiation source and used as a smoke detection sensor; 
         FIG. 1A  illustrates schematic diagrams of an ion chamber having a radiation source and showing current flows therethrough for different polarity voltage source connections thereto; 
         FIG. 2  illustrates a schematic elevational view of a typical ion chamber used as a smoke detection sensor; 
         FIG. 3  illustrates a schematic block diagram of a smoke detector, according to a specific example embodiment of this disclosure; 
         FIG. 4  illustrates a schematic block diagram of the capacitive sensing module shown in  FIG. 3 ; and 
         FIG. 5  illustrates a schematic block diagram of a portion of the capacitive sensing module shown in  FIG. 3  showing switching means used in rejecting common mode leakage current, according to another specific example embodiment of this disclosure. 
     
    
    
     While the present disclosure is susceptible to various modifications and alternative forms, specific example embodiments thereof have been shown in the drawings and are herein described in detail. It should be understood, however, that the description herein of specific example embodiments is not intended to limit the disclosure to the particular forms disclosed herein, but on the contrary, this disclosure is to cover all modifications and equivalents as defined by the appended claims. 
     DETAILED DESCRIPTION 
     A radioactive source in an ion chamber causes some of the gas (e.g., air) in the chamber to ionize. The results is a higher than normal permittivity of the gas due to the higher than normal number of electrically polarized (ionized) gas molecules. When smoke enters the ion chamber, the smoke reacts with the ionized gas molecules thereby changing the permittivity, ∈, thereof. The ion chamber may be characterized as a leaky capacitor with the amount of leakage current determined by the ion flow between charged plates  102  and  104  ( FIG. 1 ) of the ion chamber. A capacitance, C, of a capacitor formed by plates  102  and  104  is a function of the area, A, of the conductive plates  102  and  104 ; the distance, d, between the plates  102  and  104 ; and the permittivity, ∈, of the dielectric (air) therebetween according to the formula: C=∈A/d. Thus a change in the permittivity of the gas in the ion chamber also changes the capacitance value thereof. Therefore, by using a capacitance measuring function, e.g., a capacitive sensing module (CSM) in a microcontroller, the capacitance value change caused by the permittivity change of the gas dielectric of this leaky capacitor can be detected to determine the presence of smoke therein. 
     Capacitive sensing using the period method and a capacitive sensing module (CSM), according to the teachings of this disclosure, are more fully described in Application Notes AN1101, AN1171, AN1268, AN1312, AN1334 and TB3064, available at www.microchip.com, and commonly owned U.S. Patent Application No.: US 2011/0007028 A1, entitled “Capacitive Touch System With Noise Immunity” by Keith E. Curtis, et al.; wherein all of which are hereby incorporated by reference herein for all purposes. It is also contemplated and within the scope of this disclosure that any type of capacitance measurement circuit having the necessary resolution may be used in determining the capacitance value and/or change in the capacitance value of the ion chamber, and that a person having ordinary skill in the art of electronics and having the benefit of this disclosure could implement such a capacitance measurement circuit. 
     Temperature and battery voltage variations can make significant differences in the permittivity of the gas (air) with corresponding variations in the capacitance measurements of a first ion chamber. By providing a second ion chamber that is sealed from smoke entering, a comparison of the measured capacitance values of each of the first and second ion chambers can be used to compensate for these variations and provide a sensitive way of detecting smoke particles. For example, subtracting the first ion chamber capacitance value from the second ion chamber capacitance value and then dividing by the second ion chamber capacitance value, removes the temperature and battery voltage effects, leaving a resultant value with is primarily affected by the presence of smoke in the first ion chamber. 
     Temperature, relative humidity (RH) and/or battery voltage sensors may be incorporated into a smoke detection system for determining the compensation necessary for the capacitance measurements of the ion chamber used for smoke detection. Permittivity variations due to temperature, RH and/or voltage changes generally are over a longer time period than a sudden change in the amount of contaminates (carbon particles, etc.) in the air between the plates of the ion chamber capacitor. Another less sensitive way to ignore permittivity variations due to temperature, RH and/or voltage changes, would be to use an envelope detection or averaging process to ignore the slow drift of ion chamber capacitance due to voltage and/or temperature changes but recognize a more abrupt (rapid) change of the permittivity of air due to carbon particles suddenly showing up in the ion chamber. Various techniques for measuring changes in capacitance may be used and are contemplated herein for all purposes. Those having ordinary skill in capacitor measurement circuits and the benefit of this disclosure could readily apply those capacitor measurement circuits in a smoke detection apparatus. A mixed signal (analog and digital functions) microcontroller may used for capacitance measurements, e.g., CSM, doing the calculations necessary to determine whether smoke is present in the ion chamber, and compensate for and/or average out permittivity changes due to temperature, RH and/or battery voltage changes. 
     Referring now to the drawing, the details of specific example embodiments are schematically illustrated. Like elements in the drawings will be represented by like numbers, and similar elements will be represented by like numbers with a different lower case letter suffix. 
     Referring to  FIG. 1 , depicted is a schematic diagram of an ion chamber having a radiation source and used as a smoke detection sensor. The ion chamber  102  may be characterized as a capacitor with some ionized gas molecules between the capacitor plates  104  and  106 . The gas molecules are ionized by the radiation source and when a voltage is applied between the two capacitor plates  104  and  106  a current will flow through the ionized gas and a resistor  108  connected in series with the capacitor plates  104  and  106 . This current produces a voltage across the resistor  108 . By measuring the voltage across the resistor  108 , the permittivity, ∈, of the gas may be determined. Smoke in the ion chamber will cause an abrupt change in the permittivity, ∈, causing an abrupt change in the current flow and voltage across the resistor  108 . This voltage is measured by a very high impedance operational amplifier (not shown) which requires complex circuitry and manufacturing processes. A better way, according to the teachings of this disclosure, is to measure the capacitance values of the ion chamber before and after smoke entry therein. As the ionized gas permittivity, ∈, changes so does the capacitance value of the ion chamber. By using a capacitive measurement module having high enough capacitance value measurement resolution, the change in capacitance caused by smoke entry into the ion chamber may be detected and used to generate a smoke detection alarm. 
     Referring to  FIG. 1A , depicted are schematic diagrams of an ion chamber having a radiation source and showing current flows therethrough for different polarity voltage source connections thereto. The ion chamber  102  may be characterized as three electrodes, e.g., electrodes  104 ,  106  and  210 , having some ionized gas (e.g., air) molecules therebetween. The gas molecules are ionized by a radiation source  108 . When a voltage potential  112  is applied between the two electrodes  104  and  106  at a first polarity (positive to electrode  106  and negative to electrode  104 ), a positively biased ionization electron current  116 , I chamber , will flow through the ionized gas. When the voltage potential  112  is applied between the two electrodes  104  and  106  at a second polarity (positive to electrode  104  and negative to electrode  106 ), substantially no negatively biased ionization electron current  116   a  will flow through the ionized gas since now the electrode  104  will repel the ionized gas electrons. However, leakage current  114 , I leakage , e.g., printed circuit board contaminates, grease, dust, etc., will flow irrespective of the connected polarity of the voltage potential  112 . 
     Thus when the voltage potential  112  is connected at the first polarity across chamber  102  electrodes  104  and  106 , the total current flow through the current meter  110  is the ionized electron current  116 , I chamber , plus the leakage current  114 , I leakage . And when the voltage potential  112  is connected at the second polarity across chamber  102  electrodes  104  and  106 , the total current flow through the current meter  110  is substantially no ionized electron current  116   a  plus the leakage current  114 , I leakage , which results in substantially only the leakage current  114 , I leakage . Therefore, by subtracting the leakage current  114 , I leakage , from the total current flow, the actual ionized electron current  116 , I chamber , may be determined. This allows more sensitive measurements of any change in the ionized electron current  116 , I chamber , without these changes being masked by the undesired leakage current  114 , I leakage . It is contemplated and within the scope of this disclosure that any fluid, e.g., gas or liquid, that can be ionized by the ion source  108  will function as described hereinabove. 
     Referring to  FIG. 2 , depicted is a schematic elevational view of a typical two chamber smoke detection sensor having a radiation source. The ion chamber  102  is comprised of two chambers  102   a  and  102   b . The top chamber  102   a  is open to ingress of smoke therein, and the bottom chamber  102   b  is closed to smoke ingress. A conductive screen  210  is located between the two chambers  102   a  and  102   b . The radiation source  108  proximate to or in the ion chamber  102  causes some of the gas in the chambers  102   a  and  102   b  to ionize. This ionization of the gas within the chambers  102   a  and  102   b  causes an ionization current  116 , I chamber , through both chambers  102   a  and  102   b  to increase between the electrodes  104  and  106  of the ion chamber  102 . 
     When smoke is present in the top chamber  102   a , it combines with the ionized gas, neutralizing some of the ionized gas from the current path of the ionization current  116 , I chamber . As a result the permittivity of the top chamber  102   a  is smaller than it is in the lower chamber  102   b . The ionization current  116 , I chamber , flows in series through chambers  102   a  and  102   b  and therefore will be lower when smoke is in the chamber  102   a . When the voltage across the chambers  102   a  and  102   b  is reversed substantially no reverse ionization current  116   a  will flow and the only current flow between the electrodes  104  and  106  will be the leakage current  114 . The presence of the leakage current  114  reduces the sensitivity in measuring changes in the ionization current  116 . By removing this common mode leakage current  114  from the determination of smoke in the chamber  102   a , a more sensitive smoke detector results. 
     Referring to  FIG. 3 , depicted is a schematic block diagram of a smoke detector, according to a specific example embodiment of this disclosure. A smoke detector, generally represented by the numeral  300 , may comprise a capacitive sensing module  208 , a smoke detection sensor ion chamber  102   a , a digital processor and memory  314 , an alarm driver  316 , and an audible/visual alert  318 . The capacitive sensing module  208 , digital processor and memory  314 , and alarm driver  316  may be provided in an integrated circuit microcontroller  330 . The smoke detection sensor ion chamber  102   a  is coupled to the capacitive sensing module  208  wherein representations of capacitance values thereof are measured and then each representative capacitance value is read by and processed in the digital processor and memory  314 . When there is a change in the capacitance value representations within a certain time, the digital processor  314  will enable the alarm driver  316  which turns on the audible/visual alert  318  to indicate the presence of smoke in the location of the smoke detector  300 . 
     The smoke detector  300  may further comprise a second ion chamber  102   b  that is closed to outside air that may contain smoke. The first and second ion chambers  102   a  and  102   b  may be used for making a comparison of the measured capacitance values of each of the first and second ion chambers  102   a  and  102   b , and compensate for these variations, thereby providing for a more sensitive way of detecting smoke particles, as more fully described hereinabove. 
     The smoke detector  300  may further comprise a temperature sensor  320 , a relative humidity sensor  322 , and/or a voltage sensor  324  coupled to a power supply, e.g., battery (not shown). Wherein the digital processor  314  may compensate for capacitance measurements that may change under different temperature, humidity and/or voltage conditions, e.g., using look-up tables that contain calibration and compensation data for the smoke sensor ion chamber  102 . In addition, the digital processor  314  may perform smoothing, time averaging, noise suppression, over sampling, and/or digital signal processing to enhance the capacitance change detection sensitivity and/or reduce noise pick-up. 
     Referring to  FIG. 4 , depicted is a schematic block diagram of the capacitive sensing module shown in  FIG. 3 . The Capacitive Sensing Module (CSM)  208  measures capacitance based upon a relaxation oscillator methodology. The CSM  208  produces an oscillating voltage signal for measurement by a frequency determining circuit, at a frequency dependent upon the capacitance of the ion chamber  102 . The frequency at which the CSM  208  generates the oscillating voltage signal will change when smoke is introduced into the ion chamber  102   a . This change in frequency indicates smoke being present in the ion chamber  102   a . Also a further enhancement to more reliable smoke detection is to require that the change in frequency occurs in less than or equal to a certain time period so as to reject slow frequency change due to changes in temperature, relative humidity and/or supply voltage (e.g., battery not shown). 
     The Capacitive Sensing Module (CSM)  208  may comprise an ion chamber selection switch  440 , a current source selection switch  442 , an internal or external capacitor  444 , a first constant current source  446 , a second constant current source  448 , a first voltage comparator  450 , a second voltage comparator  452 , an RS flip-flop  454 , waveform counter(s)  456 , latch(es)  458 , and a period timer  460 . The selection switch  440  may be controlled by the digital processor  314  to select between ion chambers  102   a  and  102   b . The capacitor  444  may be added in parallel with the capacitance of the ion chamber  102  for lowering the oscillation frequency. The current source selection switch  442  couples either the first constant current source  446  or the second constant current source  448  to the capacitance of the ion chamber  102  (and capacitor  444  if used). Comparators  450  and  452  monitor the charging/discharging voltage on the capacitance of the ion chamber  102  (and capacitor  444  if used). 
     An oscillating triangular voltage waveform is generated by the first constant current source  446  charging the capacitance of the ion chamber  102  (and capacitor  444  if used) and the second constant current source  448  discharging the capacitance of the ion chamber  102  (and capacitor  444  if used). The RS flip-flop  454  controls the current source selection switch  442  as follows: When the voltage on the capacitance of the ion chamber  102  (and capacitor  444  if used) (charging) reaches voltage VH, the output of the first voltage comparator  450  goes to a logic 1 and “sets” the Q-output of the RS flip-flop  454  to a logic 1, thereby causing the current source selection switch  442  to select the second constant current source  448 , whereby the capacitance of the ion chamber  102  (and capacitor  444  if used) start to discharge through the second constant current source  448 . When the voltage on the capacitance of the ion chamber  102  (and capacitor  444  if used) (discharging) reaches voltage VL, the output of the second voltage comparator  452  goes to a logic 1 and “resets” the Q-output of the RS flip-flop  454  to a logic 0, thereby causing the current source selection switch  442  to select the first constant current source  446 , whereby the capacitance of the ion chamber  102  (and capacitor  444  if used) start to charge through the first constant current source  446 . Switches  440  and  442  may be solid state field effect transistor (FET) switches. 
     This switching oscillation between the two charging and discharging constant current sources  450  and  452 , respectively, have an oscillation frequency dependent upon the capacitance value of the ion chamber  102  (and capacitor  444  if used). If the ion chamber  102  capacitance value increases then the oscillation frequency decreases and if the capacitance value decreases then the oscillation frequency increases. By accurately measuring this oscillation frequency, detection of a capacitance change (e.g., smoke) may be determined. 
     A waveform counter  456  may be used to count the number of cycles of the oscillation waveform (e.g., positive going logic levels from Q output of the RS flip-flop  454 ) within a certain time period. If the time period of counting and number of cycles within the time period are known, the frequency of the waveform may be determined. However since only a change in the capacitance value is of interest, just comparing the number of cycles within different time periods is all that is necessary in determining whether there is smoke in the ion chamber  102   a . Waveform counters  456   a  and  456   b  may be concatenated and/or a high resolution counter (timer) may be, for example but not limited to, 24 or 32 bits. A high resolution period timer  460  provides accurate time periods for determining the number of cycles of the oscillation waveform in each of the time periods. Latch(es)  458  capture the number of cycles counted in the waveform counter(s)  456  for each time period and the digital processor may then read the cycle count per time period of the oscillation waveform from the latch(es)  458 . 
     According to various embodiments, in one measurement the housing  106  of the ion chamber  102   a  ( FIG. 2 ) may be coupled in parallel with the internal capacitor  444  and the respective resulting frequency may be measured. In another measurement the internal collector plate  104  of the ion chamber  102   a  may be connected in parallel with the internal capacitor  444 . Subtracting the ion chamber  102   a  capacitance value from the ion chamber  102   b  capacitance value and dividing by the ion chamber  102   b  capacitance value, removes temperature and battery voltage effects, leaving a capacitance value which is primarily affected by the presence of smoke in the ion chamber  102   a.    
     Referring to  FIG. 5 , depicted is a schematic block diagram of a portion of the capacitive sensing module shown in  FIG. 3  showing switching means used in rejecting common mode leakage current, according to another specific example embodiment of this disclosure. Switches  550  and  552 , and  554  and  556  change the polarity connections of the chambers  102   a  and  102   b , respectively. Two sample counts of each of the chambers  102   a  and  102   b  are taken, one sample count at a first polarity and a second sample count at a second polarity opposite the first polarity. These sample count values are stored in the memory of the digital processor  314  for further computational processing, e.g., subtracting the lower sample count value from the higher sample count value of each chamber  102   a  and  102   b , thereby canceling out what is caused by the leakage current  114 , with a result of only a representation of the chamber ionization current  116 . Since each chamber  102   a  and  102   b  is independently measured, any difference in the ionization currents  116  of the two chambers will indicate influence of smoke on the ionization of the gas in the chamber  102   a . Determining a count value representing the ionization current  116  of the closed to the count value representing the smoke ionization chamber  102   b  thereby allows a base value that can be used to track or “float” a base count reference value for chamber  102   a  so that a small change thereof can be more easily recognized as indicating detection of smoke therein. 
     While embodiments of this disclosure have been depicted, described, and are defined by reference to example embodiments of the disclosure, such references do not imply a limitation on the disclosure, and no such limitation is to be inferred. The subject matter disclosed is capable of considerable modification, alteration, and equivalents in form and function, as will occur to those ordinarily skilled in the pertinent art and having the benefit of this disclosure. The depicted and described embodiments of this disclosure are examples only, and are not exhaustive of the scope of the disclosure.