Fire identification and discrimination method and apparatus

In a method and apparatus for determining the fuel of a fire, an ionization detector is positioned to detect the ionization level in the fire gases. The natural logarithm of the ratio of the ambient ionization current when there is no fire to the change in ionization current from the ambient level is multiplied times a second characteristic, which may be the optical density, the change in carbon monoxide concentration from ambient, or the change in carbon dioxide concentration from ambient. The resulting product of this multiplication will be a value indicating the fuel being consumed in the fire. A chart recorder is connected to record the product of the multiplication.

BACKGROUND OF THE INVENTION 
The present invention relates to a fire detection method and apparatus and 
more particularly to a fire detection method and apparatus designed to 
provide an indication of the fuel being consumed in the fire. 
DISCUSSION OF THE RELATED ART 
Ionization fire detectors are well-known in the art. Dual band infrared 
(IR) fire detectors and dual band infrared/ultraviolet (IR/UV) fire 
detectors are also well known in the art. In such detections dual 
radiation bands are selected so that the transmitivity of ambient air to 
radiant energy in both these bands will be significantly affected by the 
presence of combustion products from a fire, but events other than a fire 
are unlikely to significantly affect transmitivity in both these bands 
simultaneously. 
Some dual band IR and IR/UV fire detectors also screen such coincident 
changes in transmitivity for parallel or sequential confirmation of the 
coincidence, or for proportional "signature" relationships between them. 
However, the need to quickly locate the source or focus of the fire, in 
particularly to identify the chemical nature of that source of the fire, 
are important problems that these known devices do not address. 
SUMMARY OF THE INVENTION 
The present invention provides a method and apparatus for indicating the 
principal combustible material present in the fire at a given location. 
Knowledge of fuel being burned in a fire will assist fire fighting efforts 
in the selection of the optimum fire fighting tactic. It will also help 
discriminate against false alarms. Knowledge of the fuel being consumed at 
the beginning of a fire will help determine the origin of the fire. The 
preferred embodiment of the invention provides apparatus that can provide 
a record of the fuel being consumed throughout the history of the fire. In 
the case of arson, this record can be used as evidence of how the fire 
started. 
In accordance with the invention an ionization detector is located to sense 
fire gases from a fire, and the ionization current produced in the 
ionization detector is monitored. Simultaneously a second characteristic, 
local optical density, the change in local carbon monoxide concentration 
from ambient, or the change in local carbon dioxide concentration from 
ambient is also monitored. The measured values are combined in a formula 
wherein the logarithm of the ratio of the ambient ionization current 
flowing in the ionization detector to the change in ionization current 
relative to ambient is multiplied by the second measured characteristic. 
The value of the resulting product provides an indication of the primary 
fuel for the fire. 
Apparatus in accordance with the present invention comprises an ionization 
sensor producing an ionization current representing the ion concentration 
at a given location, signal processing means for determining the natural 
logarithm of the ratio of the ambient ionization current to the change in 
the ionization current from the ambient level, ln (I.sub.o /.DELTA.I), and 
an auxiliary sensor for producing a signal representing the second 
characteristic at the given location. The first and second values are 
multiplied by a multiplier to provide an index value, which when a fire 
occurs will be indicative of the primary fuel of the fire. The index value 
may be recorded by a recorder to provide a time log of the index value.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
As shown in the drawing, the preferred embodiment of the present invention 
comprises an ionization detector 20, which is positioned in a location to 
detect ionization in gases from a fire. The output signal from the 
ionization detector 20 is applied to an analog subtractor 22, which has 
stored therein a previously measured reference value representing the 
ambient ionzation current I.sub.o flowing in the ionization detector 20 
when there is no fire. The subtractor 26 subtracts the ambient ionization 
current value from the currently measured ionization current to provide an 
output signal representing the change (.DELTA.I) in the ionization current 
from ambient. This signal is amplified by a log amplifier 24 which has a 
gain characteristic such that its output signal will represent the natural 
logarithm of the input signal representing .DELTA.I. The output signal of 
the log amplifier 24 is applied to an analog subtractor 26, which has 
stored therein a previously determined reference value ln(I.sub.o) 
representing the natural log of the ambient ionization current flowing in 
the ionization detector when there is no fire. This ambient reference 
value is determined by connecting the log amplifier 24 to amplify the 
output signal of the ionization detector 20 when there is no fire and then 
storing the resulting output of the log amplifier 24 as the ambient 
reference value in the subtractor 26. The subtractor 26 subtracts the 
current value of the output of the log amplifier 24 from the ambient 
reference value stored therein to provide an output signal representing 
ln(I.sub.o /.DELTA.I). The output signal of the subtractor 26 is applied 
to a multiplier 30. 
The multiplier 30 is also connected to an auxiliary sensor 32 located 
adjacent to the ionization sensor 20. This auxiliary sensor 32 may be an 
optical density (OD) sensor in one embodiment. Optical density is the 
natural logarithm of the inverse of the transmissivity in units of 
1/meters at a selected wave length, which in the present invention is 
selected to be 0.6328 microns. The optical density sensor may comprise a 
source of a light beam at the selected wave length and a photodetector 
positioned to be irradiated by the beam arranged so that the fire gases to 
be analyzed pass through the beam. The photodetector is connected in a 
circuit to generate a signal varying inversely with the incident light 
intensity so that the output signal from the photodetector circuit 
represents the inverse of transmissivity. This signal is amplified by 
logarithmic amplifier having a natural logarithm gain characteristic to 
provide an out signal representing the optical density OD. The output of 
the optical density sensor is applied to the multiplier 30, which 
multiplies it times the output signal of the subtractor 26 to produce an 
output signal representing [ln(I.sub.o /.DELTA.I)] OD. When a fire occurs 
and the fire gases are sensed by the ionization detector and the optical 
density detector, the product represented by the output signal of the 
multiplier 30 will provide an indication of the primary fuel being burned 
in the fire. In the preferred embodiment the signal output of the 
multiplier is boosted by an amplifier 36 and the amplified signal is then 
transmitted to a recording device located at a remote monitoring location 
such as chart recorder 40. The chart recordation provides a time log of 
the value of the multiplier output. 
When a fire starts in an area where there are installed several signal 
generator units, each comprising an ionization detector 20, a subtractor 
22, a log amplifier 24, a multiplier 30, and an auxiliary detector 32, 
connected as shown in the drawing, the change in the signal generated by 
each signal generator unit may be indicated by a pen on the recorder 40, 
each signal being recorded by a separate pen. All pens are 
zero-compensated and calibrated for gain so as to accurately record the 
value produced by each signal generator unit. With this arrangement, the 
location of the earliest deflections and the amount of the deflection of 
each pen will indicate which signal generator unit or units were affected 
when the fire began and the principal combustible material at each 
location, respectively, through time. 
In alternative embodiments, the sensor 32 may be a carbon dioxide or carbon 
monoxide sensor instead of the optical density sensor. These sensors would 
preferably be infra-red photometers adapted to measure changes in the 
concentration of these gases in parts per million (ppm), relative to the 
ambient values of these concentrations when no fire exists although 
electrochemical cells or other means could also be used, as is well known 
in the art. In these alternative embodiments, the output of the sensor 32 
representing the change in carbon dioxide concentration or the change in 
carbon monoxide concentration is multiplied in the multiplier 30 times the 
output of the subtractor 26 representing ln(I.sub.o /.DELTA.I). Thus, when 
the sensor 32 is a carbon dioxide concentration detector, the output 
signal of the multiplier 30 will represent [ln(I.sub.o 
/.DELTA.I)].DELTA.C.sub.co2 wherein .DELTA.C.sub.co2 represents the change 
in carbon dioxide concentration relative to ambient. When the sensor 32 is 
a carbon monoxide detector, the multiplier will produce an output signal 
representing [ln(I.sub.o /.DELTA.I)].DELTA.C.sub.co in which 
.DELTA.C.sub.co is the change in carbon monoxide concentration relative to 
ambient. The table below shows how the values [ln(I.sub.o /.DELTA.I)]OD, 
[ln(I.sub.o /.DELTA.I].DELTA.C.sub.co2, and [ln (I.sub.o 
/.DELTA.I)].DELTA.C.sub.co vary with different fuels being consumed in a 
fire. 
TABLE 
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Combus- 
tible [1n(I.sub.o /.DELTA.I)]OD 
[1n(I.sub.o /.DELTA.I)].DELTA.C.sub.co.sbsb.2 
[1n(I.sub.o /.DELTA.I).DELTA.C.sub.co 
______________________________________ 
Douglas 1.0 1.0 1.0 
Fir 
Heptane 2.1 0.71 0.54 
Coal 3.4 2.5 0.17 
Polyvinyl 
3.7 2.3 0.055 
Chloride 
Styrene 7.1 4.8 0.23 
Butadiene 
rubber 
Polystyrene 
8.3 7.1 0.29 
______________________________________ 
In this table, the output value of the multiplier 30 has been arbitrarily 
assigned the value of 1 when Douglas fur is the fuel being burned in the 
fire and the for the remaining fuels in the table represent the ratio of 
the output signal of the multiplier 30 to its value for Douglas Fir. 
From the table it is apparent that each of the values [ln(I.sub.o 
/.DELTA.I)]OD, [ln(I.sub.o /.DELTA.I].DELTA.C.sub.co2, and [ln(I.sub.o 
/.DELTA.I)].DELTA.C.sub.co varies widely depending upon the fuel of the 
fire and thus, will provide an accurate indication of the fuel. Thus, the 
time log of any one of these values recorded by the chart recorder 40 can 
be used as documentary evidence of the primary fuel of a fire at its 
beginning and as it progresses. 
The invention has been disclosed with particular reference to specific 
embodiments employing analog circuitry. It will be appreciated that 
instead of employing analog circuitry to cdmpute the formula represented 
by the output of the multiplier 30, a microprocessor could be employed 
programmed to carry out the arithmetic functions and provide a digital 
output representing the value of the formula. Other variations and 
modifications of the method and apparatus can be made without departing 
from the spirit and scopt of the disclosed invention which is defined in 
the appended claims.