Gas detector

Apparatus for illuminating a gas or gas mixture and filtering the output thereof alternately with two filters. One filter has a passband at an absorption band of a gas to be detected. The other filter has a passband outside the absorption band.

BACKGROUND OF THE INVENTION 
This invention relates to arrangements for the detection of gases and/or 
vapors, and more particularly, to an arrangement for photometric analysis. 
Recreational vehicles, mobile homes and small boats frequently use bottled 
gases, e.g., propane or butane, to provide heating and lighting 
facilities. A constant hazard with such a practice is the risk of leakage 
of the gas into a confined space. As such gases are heavier than air they 
tend to form a layer on the floor of the dwelling and can cause a fire or 
an explosion, or under some circumstances, can asphyxiate the occupants. 
Conventional detectors of the semiconductor type tend to be rather 
insensitive and are also non-specific. Thus, such a detector will often 
produce a response to tobacco smoke or even to exhaled breath. 
PRIOR ART STATEMENT 
In Snowman U.S. Pat. No. 3,588,496 issued June 28, 1971, cells 13 and 14 
are alternatively exposed through chopper holes 24 and 23, respectively. 
An infrared detector 20 and magnetic detector 27 are both connected to a 
synchronous detector 21, the output of which is indicated at 22. The 
output of the synchronous detector 21 is proportional to the concentration 
of the gas to be measured. 
SUMMARY OF THE INVENTION 
According to one aspect of the invention there is provided a gas detector 
arrangement, including a light source, a light detector so arranged as to 
receive light from the source, an optical filter having a passband 
corresponding to an absorption band of the gas, and means for placing the 
filter in and out of the light path between the source and detector 
thereby causing the detector to produce an oscillatory signal the 
amplitude of which corresponds to the concentration of the gas. 
According to another aspect of the invention there is provided a gas 
detector arrangement adapted to respond to the presence of a predetermined 
gas, the arrangement including a broad band light source, a receiver 
arranged so as to receive light from the source, a first optical filter 
having a passband corresponding to an absorption band of the gas, a second 
optical filter having a passband in a region of the spectrum adjacent to 
that of the absorption band, and oscillatory means for placing the filters 
alternately in the light path between the source and the detectors thereby 
causing the detector to produce an oscillatory signal the amplitude of 
which corresponds to the concentration of the gas in the light path. 
The term "light" as used herein is hereby defined for use herein and in the 
claims to follow as to include the infrared, the visible and the 
ultra-violet regions of the spectrum.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1, a gas detector arrangement constructed in accordance 
with the present invention is mounted within a housing 11 provided with a 
relatively heavy base portion 12, the central region of which is machined 
to form spherical mirror 13. The housing 11 has openings 14 to allow the 
passage of gas therethrough and, in use, would normally be placed on the 
floor of the compartment in which an inflammable gas is used. 
A light source 15 is placed near the center of curvature of the spherical 
mirror 13, slightly displaced from the mirror axis. The resultant image is 
thereby similarly displaced to a position convenient for locating a filter 
chopper arrangement 16 and a photodetector 17. The volume of space between 
the mirror and the other optical components, which is open to the 
atmosphere, serves as the region in which light absorption takes place. 
The light from the source 15 traverses this region twice in its path to 
the photodetector 17. A collimator 18 may be provided to focus the light 
on detector 17. The upper portion 19 of the housing contains a battery, a 
detector amplifier and an alarm device. 
It should be noted that, as the gas detector arrangement is normally 
operated at infrared wavelengths, a rear-silvered glass mirror cannot be 
used because the glass would not transmit the light at such wavelengths. 
Since the required accuracy of the reflecting surface is a function of the 
working wavelength, mirrors for use in the infrared may be of 
significantly lower quality than would be required in the visible light 
region. Thus, the mirror may be a simple lathe-turned spherical recess in 
the base of the instrument, after-treatment being limited to a few moments 
polishing with a hand-held cloth and metal polish. The quality of the 
mirror need be no better than sufficient to return the major part of the 
incident light; in fact, since the objective is to achieve uniform 
illumination of the photodetector 17, it is preferable that any 
irregularities in the emission from the light source 15 should not be too 
precisely imaged upon the photodetector. In some applications the mirror 
may comprise a plastic molding flashed with a suitably reflective coating. 
The chopper 16 comprises a pair of filters disposed adjacent to each other 
on a mechanically-resonant member so that the oscillatory movement will 
interpose first one filter and then the other into the light path. An 
electronic oscillator serves to maintain the resonant member and its 
filters in continuous vibration. One filter (F1 in FIG. 4), subsequently 
to be referred to as the absorption filter, has a construction so as to 
pass light occupying a restricted bandwidth centered on the chosen optical 
wavelength at which light is absorbed by the gas to be detected. The other 
filter F2 is constructed to have a passband straddling a reference 
wavelength at which the gas does not absorb light and which is not 
disturbed by the presence of other gases of no immediate concern. 
Oscillation of the filters from FIG. 4a to FIG. 4c and back, samples first 
one wavelength, then the other, and then the one, repetitively. The 
photodetector 17 (FIG. 1) gives an output determined by the amount of 
light traversing the path at any instant, so that any unbalance between 
the light transmission at the two wavelengths generates an A.C. waveform 
having an amplitude proportional to the amount of unbalance, and a 
frequency and phase coincident with that of the filter chopper 16. 
In the absence of the gas to be detected, the light transmission at the two 
wavelengths will, for example, be equal. The A.C. component of the 
detector output may then be zero. Balancing means may be added so as to 
enhance this condition. The presence of the gas to be detected attenuates 
the light within the absorption spectra but not as the reference 
wavelength. This creates an unbalance signal which may be amplified and 
made to trigger an alarm. 
The arrangement described herein is intended to detect propane or butane 
(both heavier-than-air gases). In consequence, the optical axis has been 
disposed vertically so as to place the mirror 13 (FIG. 1) close to the 
floor, and the base of the instrument so shaped as to encourage the easy 
ingress of gas from any direction. 
In FIG. 2 one arrangement is shown for providing oscillatory motion of the 
filters F1 and F2. The filters F1 and F2 are mounted on a carrier 28 at 
one end of a spring strip 21, the other end of which is secured to a mount 
22. The other end of the carrier 28 carries a bar magnet 23, the outwardly 
facing surface of which is ground to a radius of curvature corresponding 
to or slightly less than the distance of an electromagnet 26 from the 
mount 22. Vibration of the spring 21 is restricted by limit stop springs 
24 against which the carrier 28 abuts at the extremities of its vibratory 
excursions. The arrangement is driven by electromagnet 26 which has three 
poles. Electromagnet 26 is arranged adjacent to the bar magnet 23. An 
alternating current is supplied to electromagnet 26 at a frequency equal 
to the resonant frequency resulting from the combination of the supporting 
spring 21, limit stop springs 24, the total mass of the system, the magnet 
23 and the electromagnet 26. 
The 3-pole actuating electromagnet 26 may advantageously be provided by a 
1:1 audio transformer with its core cut away in an appropriate arc. The 
electromagnet 26 is so arranged that an air gap of about 1 mm exists 
between the pole tips and the armature 23. The supporting spring 21 is 
comparatively light, providing little restoring force, so that energy 
stored in the armature 23 as it accelerates across the pole tips is 
expended mainly in depressing the limit stop springs 24. Upon recovery, 
the limit stop springs 24 return the energy to the armature 23 to launch 
it back across the pole tips, assisted by the small drive current. The Q 
of the mechanically resonant system is such that only about 20 milliwatts 
are required from the electronic driver to maintain continuous 
oscillation. The mechanically resonant system is closely coupled to the 
electronic oscillator and serves as the frequency-determining element. 
There are thus no synchronization problems. 
The absorption filter F1 may be an interference filter of 2% bandwidth 
centered on 3.35 microns--the preferred absorption wavelength for propane. 
The reference wavelength is determined by a similar filter F2 centered, 
e.g., on 3.95 microns. The selection of the absorption and reference 
wavelengths has an effect upon the choice of the remaining components of 
the optical system. Quartz cuts off at about 4.7 microns, enabling the use 
of a conventional quartz-halogen lamp. 
Some results are achieved by using a ceramic pyroelectric photodetector. 
This is not wavelength-dependent. A lead selenide photodetector is 
preferred. With it a slightly higher sensitivity at significantly lower 
cost can be obtained. 
In FIG. 2, photodetector 17 is shown, the output of which is connected to 
an amplifier 18. The output of amplifier 18 is synchronously detected in a 
detector 40 having an input connected from a drive oscillator 41. 
The output of detector 40 is impressed upon an integrator 42, the output of 
which, in turn, is impressed upon a conventional alarm device 43. 
FIG. 3 shows an alternative filter vibrator arrangement. In this 
arrangement a loop of high-tensile wire 31, e.g., steel, is rigidly 
attached at one end at 29. It then passes around a freely-rotatable pulley 
32 before returning to another point of attachment adjacent to the first, 
at which point a suitable tensioning means 33 is provided so as to enable 
adjustment of the resonant frequency of the system. The arrangement thus 
provides a pair of parallel wires of equal length and, by the intervention 
of the pulley, equal tension, thereby endowing them both with the same 
resonant frequency. The two points of attachment are electrically 
insulated from each other, and serve also as input terminals for a 
suitable driving waveform. The pulley 32 is also insulated so as to avoid 
short-circuiting the drive waveform. The filter chopper assembly 16 is 
attached so as to bridge the pair of wires, as shown, at their centers 
where the amplitude of vibration will be at a maximum. The bridge, also, 
is insulated so as not to short-circuit the drive waveform. 
The resonant wires are embraced by a magnet system 34 which generates lines 
of magnetic force disposed at right angles to the direction of the wires. 
Polarities are so arranged that when drive current is passed around the 
loop, the forces on the two wires are such as to displace them both in the 
same direction as well as assembly 16. The two-wire support provides 
accurate control of the filter angle and position while allowing freedom 
for the desired sideways movement. 
In the absence of a gas to be detected it is preferable that the amount of 
light passing through the filters F1 and F2 should be equalized. One 
method of effecting balancing is achieved by adopting the filter 
configuration shown in FIGS. 4a to 4i, in which the light path is taken to 
be emerging normal to the paper surface. 
In the normal chopping cycle, the filters F1 and F2 will swing from side to 
side in relation to the optical aperture, exposing first the reference 
filter F2, (FIGS. 4a, 4d and 4g) and then the absorption filter F1 (FIGS. 
4c, 4f and 4i). Between these extremes (FIGS. 4b, 4e and 4h) the carrier 
or chopper 28 assumes a transition mode in which one filter progressively 
takes over from the other, passing through a neutral position where the 
optical aperture is momentarily shared equally between the two filters. 
This sequence will be seen to prevail in FIGS. 4d to 4f, where the optical 
aperture is fully exposed essentially throughout the entire chopping 
cycle. 
A balancing adjustment can be achieved by moving the whole vibrator unit, 
including the drive electromagnet upwards or downwards in relation to the 
detector 17. 
If the vibrator unit is displaced downward the aperture may be partially 
obscured during the passage of filter F2 as at FIGS. 4a and 4b. If the 
unit is displaced upward, as in FIGS. 4g to 4i the converse will occur. By 
displacement of the vibrator unit, this masking effect may be made to 
compensate for static differences in the light transmitted at the two 
wavelengths, the setting-up adjustment being made such as to achieve 
minimum photodetector output under no-gas conditions. 
The drive circuit for the vibrator, shown in FIG. 5, is a simple 
tuned-base, tuned-collector transistor oscillator. Feedback is effected 
via 1:1 audio transformer adapted to serve also as the drive electromagnet 
in the manner described earlier. In operation, the magnetically polarized 
vibrator armature generates an EMF which reacts on the circuit so as to 
lock its oscillation rigidly to the mechanical resonance. This ensures 
that temperature or supply voltage changes cannot cause the electronic 
oscillator to lose synchronism with the mechanical vibrator. The coupling 
is so close that, if the vibrator is physically restrained, the circuit 
also stops oscillating. Nevertheless the oscillation is self-starting at 
switch-on. The mechanically resonant mode of operation ensures that power 
losses are minimal. 
The photodetector is followed by a two-stage ac-coupled amplifier. 
Synchronous rectification of the unbalanced waveform is performed by the 
switch S1. This may be provided by one section of a COS/MOS quad bilateral 
switch, but may also be a suitable FET device. The switching square wave 
is derived from the vibrator drive oscillator by an adjustable 
phase-shifting network followed by a squaring amplifier. The phase 
adjustment is provided to permit the substitution of photodetectors, 
because whereas the lead selenide cell produces an output proportional to 
illumination, the ceramic pyroelectric cell has an output proportional to 
the rate of change of illumination, that is, in quadrature with the phase 
of vibration. In some applications this adjustment could be eliminated, 
and the phase preset by initial choice of component values. 
The output from a synchronous switch S1 is fed to a simple RC integrator 
and thence to the non-inverting input of the threshold-detecting and 
squaring amplifier. This input is normally held at a dc potential of about 
+6 volts, via S1, by the pair of resistors immediately preceding S1. 
Similarly, the inverting input is raised in potential by the resistor 
chain incorporating the "adjust sensitivity threshold" potentiometer. On 
setting up, this potentiometer is adjusted to place the inverting input of 
an amplifier OA3 slightly positive of its non-inverting input. In this 
state, the output of amplifier OA3 lies close to the negative supply rail 
and a transistor T2 is cut off. 
In the presence of the gas to be detected, a resulting unbalance waveform 
appears at the output of the photodetector amplifier, the "on" phase of 
the switch S1 being made such that successive positive-going half-cycles 
of the waveform pass so as to raise the state of charge of the integrator 
capacitor. In the manner familiar to the art, the potential on this 
capacitor will rise to the mean value of the successive samples taken over 
the integration time, thus truly reproducing the unbalanced amplitude 
despite the presence of random photodetector noise or other 
non-synchronous disturbances. Should the unbalanced amplitude become 
sufficient to carry the non-inverting input of amplifier OA3 positive of 
its inverting input, amplifier OA3 output will switch immediately to the 
positive supply potential, turning-on transistor T2 and activating the 
alarm device. 
In FIG. 5, a conventional photodetector is illustrated at 44. Photodetector 
44 may be an infrared detector model PPC522 of the pyroelectric type, if 
desired. 
Detector 44 includes a field effect transistor (FET) 45 having a drain 46, 
a source 47 and a gate 48. 
Gate 48 is connected to ground through a capacitor 49. A resistor 50 is 
connected in parallel with capacitor 49. 
A resistor 51 is connected from source 47 to ground. A capacitor 52 and a 
resistor 53 are connected in that order from source 47 to the inverting 
input of an amplifier OA1. 
A capacitor 54 is connected from drain 46 to ground. 
Junctions are provided at 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 
67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81 and 82. 
A resistor 83 is connected between junctions 55 and 56. A capacitor 84 is 
connected from junction 56 to ground. Junctions 56, 57 and 58 are 
connected together. 
A resistor 85 is connected between junctions 57 and 63. A capacitor 86 is 
connected from junction 58 to ground. A resistor 87 is connected between 
junctions 58 and 59. A resistor 88 is connected between junctions 59 and 
66. A resistor 89 is connected between junctions 60 and 68. A resistor 90, 
a potentiometer 91 and a resistor 92 are connected in that order from 
junction 61 to junction 82. 
Potentiometer 91 has a winding 93 and a wiper 94 connected to the inverting 
input of an amplifier OA3. 
Junctions 63, 69 and 70 are connected together. 
Resistor 53 is connected to a junction 95 which, in turn, is connected to 
the inverting input of amplifier OA1. A resistor 96 is connected between 
junctions 64 and 95. 
A resistor 97 and capacitors 98 and 99 are connected in parallel from 
junction 70 to ground. 
A capacitor 100 and a resistor 101 are connected in that order from 
junction 64 to junction 65. 
Junction 64 is connected from the output of amplifier OA1. Junction 65 is 
connected to the inverting input of an amplifier OA2. A resistor 102 is 
connected between junctions 65 and 67. An output of amplifier OA2 is 
connected to junction 67. Junctions 66 and 71 are connected together. A 
resistor 103 is connected in parallel with a capacitor 104. Resistor 103 
and capacitor 104 are connected from junction 71 to ground. 
A capacitor 105 is connected between junctions 67 and 68. Junctions 68 and 
72 are connected together. A resistor 106 is connected between junctions 
72 and 80. FET S1 is provided to act as a switch. The FET S1 has a drain 
107, a source 108, and a gate 109 connected respectively to junction 72, a 
resistor 110 and a junction 111. Resistor 110 is connected from source 108 
to junction 73. A capacitor 112 is connected between junctions 73 and 81. 
Junction 73 is connected to the non-inverting input of amplifier OA3. 
Transistor T2 has a collector 113 connected to junction 62 through alarm 
114. Junction 62 is connected to +12 volts. 
Transistor T2 has a base 115 which is connected from the output of 
amplifier OA3 through a resistor 116. Transistor T2 has an emitter 117 
which is connected to junction 82. 
Also shown in FIG. 5, for connection with junction 111, is a transformer 
including windings 118 and 119 connected between junctions 120, 121 and 
122 and 123, respectively. A capacitor 124 is connected between junctions 
120 and 121. Junction 120 is connected to a junction 125. 
Junction 122 is connected to a junction 126. Junction 126 is maintained at 
a potential of +12 volts. A junction is provided at 127. A capacitor 128 
is connected between junctions 122 and 127. Junctions 123 and 127 are 
connected together. A resistor 129 is connected between junctions 125 and 
126. 
A transistor T1 is provided having a base 130 connected from junction 121, 
a collector 131 connected to junction 123 and an emitter 132 connected to 
a junction 133. 
Junctions 134, 135, 136, 137, 138, 139 and 140 are also provided. 
A resistor 141 is connected between junctions 133 and 137. A capacitor 142 
is connected between junctions 133 and 138. A resistor 143 is connected 
between junctions 136 and 139. A capacitor 144 is connected between 
junctions 136 and 140. Junctions 125, 135 and 136 are connected together. 
An amplifier is provided at OA4 having an output connected to junction 111. 
A resistor 145 is connected from junction 111 to 0 volts. Junctions 137, 
138, 139 and 140 are all connected to 0 volts and to ground. A resistor 
146 and a capacitor 147 are connected in that order from junction 127 and 
junction 134. Junction 134 is connected to one input of amplifier OA4. A 
variable resistor 148 is connected from junction 121 to junction 134. 
A fixed resistor 149 is connected with junction 135 to another input to 
amplifier OA4. 
As explained previously, the input to FET S1 over gate 109 synchronously 
detects the optical signal received by photodetector 44. 
Values of components of FIG. 5 may be as follows: 
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Capacitor 54 100 microfarads 
Capacitor 84 100 microfarads 
Capacitor 86 1 nanofarad 
Capacitor 98 1 nanofarad 
Capacitor 99 100 microfarads 
Capacitor 100 4.7 microfarads 
Capacitor 104 1 nanofarad 
Capacitor 105 4.7 microfarads 
Capacitor 112 10 microfarads 
Capacitor 124 100 microfarads 
Capacitor 128 300 microfarads 
Capacitor 142 1000 microfarads 
Capacitor 144 1000 microfarads 
Capacitor 147 0.1 microfarad 
FET S1 CD 4016 
Photodetector 44 Pyroelectric infrared 
detector PPC522 
Potentiometer 91 100,000 ohms 
Resistor 83 2,200 ohms 
Resistor 85 1.8 megohms 
Resistor 87 330 ohms 
Resistor 88 1.8 megohms 
Resistor 89 100,000 ohms 
Resistor 90 47,000 ohms 
Resistor 92 47,000 ohms 
Resistor 96 5.6 megohms 
Resistor 97 1.8 megohms 
Resistor 101 56,000 ohms 
Resistor 102 5.6 meg 
Resistor 103 1.8 megohms 
Resistor 106 100,000 ohms 
Resistor 116 10,000 ohms 
Resistor 129 1,600 ohms 
Resistor 141 270 ohms 
Resistor 143 820 ohms 
Resistor 145 100,000 ohms 
Resistor 146 100,000 ohms 
Resistor 148 470,000 ohms 
Resistor 149 27,000 ohms 
Transistor T1 2N3053 
Transistor T2 2N3053 
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Synchronous detector 40 (FIG. 2) is entirely conventional as are detector 
17, amplifier 18, integrator 42, alarm 43 and oscillator 41 shown in FIG. 
2