Gas monitoring system with leak detection and flow cutoff

A gas monitoring system for a kitchen environment in a house and the like which is supplied with gas from a pressurized gas container or from a public gas supply to sense a possibly occurring gas leak. The system comprises a valve controlling the flow of gas from the container to a cooking range. Leaked gas is detected by a sensor which activates a visual and an audible alarm circuit and which operates a valve to terminate gas flow. The electronic alarm circuit is an integrated circuit, so that it can be located in a small hermetically sealed container, together with the power supply. The alarms stay on even after the gas leaked has been removed, until the system is reset by a manual switch. A manual override is provided to allow gas flow in the event of power failure or system failure in areas where service may be delayed.

The invention relates to a gas monitoring system for use in conjunction 
with a kitchen environment as in homes, mobile homes and restaurants and, 
more particularly, to a gas monitoring system to protect life and property 
in case of gas leaks from a source of combustible gas such as a 
pressurized gas container, but which has provision for use of the gas 
under emergency conditions. 
BACKGROUND OF THE INVENTION 
One known type of monitoring system for the detection of a gas leakage is 
disclosed in U.S. Pat. No. 3,955,186. The electric circuitry used in this 
safety system applies a combination of solid state devices, mechanical 
relays and switches. Such relays and switches are expensive, heavy in 
weight, take up much space and require extensive wiring. Moreover, as they 
enter into operation only rarely they are sources of faults and 
malfunctions. The system makes use of a multivibrator to supply the heater 
of the gas sensor rated at 0.5 volt with short duration current pulses 
with a mark space ratio of 50:1. This means a peak power of approximately 
24 W which constitutes a considerable loss for the power supply. 
Gas monitoring systems designed to provide protection against fires and 
explosions caused by uncontrolled gas leakage in kitchens and other places 
using gas as a source of energy must employ a magnetic valve to shut-off 
gas flow by means of a gas sensor. To obtain full security and protection 
at all times, the valve must be closed under normal conditions and 
energized to the open position by a very small current. If the system does 
not operate due to an electrical or a system failure, the valve remains 
closed, stopping gas flow from its source and creating a big problem for 
the user as he is left without gas and energy until the power is restored 
or until a qualified technician repairs the system. This problem is 
especially severe in areas where gas is often the only source of energy 
and where repair due to remoteness of places can take several days. 
SUMMARY OF THE INVENTION 
Therefore, it is an object of the invention to shut off the gas supply from 
the source in case of a leak, before the gas/air mixture forming can reach 
the lower explosion limit. 
Another object is to provide a monitoring system with an override screw to 
manually allow gas flow without interruption in the event of the system 
failure or power loss, avoiding the need for such additional equipment as 
rechargeable batteries 
A further object of the invention is to equip the monitoring system with 
electronic circuits which are light in weight, require little wiring and 
are reliable in operation over long periods of time. The circuits should 
take up so little space that they can be located together in a small 
sealed container. 
A further object of the invention is to avoid a high load on the power 
supply of the system. 
According to the invention, the gas monitoring system comprises a gas valve 
controlling the flow of gas from the combustible gas source, the flow 
being remote-controlled by a solid state switch. An integrated electronic 
alarm circuit is responsive to a sensor for the detection of the presence 
of leaked gas. The integrated alarm circuit includes a visual alarm 
circuit and an audible alarm circuit with an audio oscillator. The 
integrated alarm circuit includes circuit drivers to drive the oscillator 
and the solid state switch A voltage comparator drives, in turn, the 
circuit drivers. A direct current power supply is provided for the sensor 
and the integrated alarm circuit. The system is set and reset by a switch 
after removal of the leaked gas. A manually operable override permits the 
user to open the valve by mechanical operation to allow unprotected gas 
flow in emergency conditions.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1, a kitchen 1 includes a cooking range 2 connected to a 
pressurized combustible gas container 3 by means of a gas pipe 4, a gas 
sensor 5 is near the kitchen floor 6, where leaked gas is likely to 
accumulate first, and a control box 7 is mounted in a convenient location 
The gas pipe is provided with a pressure regulator 8 and an 
electromagnetic gas shut-off valve 9 to control the flow of gas through 
the pipe from the container to the burners 10 of the cooking range. The 
gas sensor is connected to the control box 7 by means of an electric cable 
11. 
The electromagnetic gas valve 9 is of a known type. Its armature has the 
form of a valve stem carrying a valve body which cooperates with a seat to 
control the flow of gas through the valve. The valve is connected in an 
electronic circuit such that, in normal or fresh-air operation, the 
solenoid of the valve is energized, so that the valve stem is pulled 
upwards keeping the valve body away from the seat and allowing the fuel 
gas to pass from the gas container to the burners of the cooking range. 
The gas sensor 5 is also of a known type, for example, Type TGS 813/813 C 
of Fiagaro Engineering, Inc., Japan. This sensor is a general purpose 
sensor with high sensitivity to methane, propane and butane. The sensor 
has low sensitivity to noise gases, such as gases resulting from the home 
environment, such as hair sprays and cooking fumes, thus reducing 
considerably the problem of false alarms. The gas sensing element of the 
sensor is bulk semiconductor material composed mainly of tin dioxide 
deposited on a ceramic tube. The heater of the sensor is located within 
the tube. As is known in the art, when a combustible gas such as propane 
or methane comes into contact with the sensing element, its electrical 
resistance, which is large in fresh air, decreases in accordance with the 
concentration of the gas in the air. The sensor is adjusted to a 
predetermined sensitivity by the manufacturer. 
Referring to FIG. 2, the control box 7 contains a direct current power 
supply 12, which is connected by means of an electric cable 13, is to an 
outlet 14 of the alternating current mains of the building wherein the 
kitchen is located. The monitoring system is turned on and off by means of 
a manual switch 15 of the power supply. Further, the control box contains 
circuitry to activate a visual alarm in the form of a green light emitting 
diode LED 16 which lights up when no gas leakage occurs, and a red LED 17, 
which lights up when the sensor has detected a gas leak and the gas supply 
has been shut-off by the gas valve. The gas valve is connected to the 
control box by means of a conductor 18. The control box further contains 
circuitry to operate an audible alarm in the form of a buzzer 19, at the 
same time that the red LED 17 lights up. Both LED's, the buzzer and the 
manual switch are located at the front of the control box. The control box 
is hermetically sealed so that no vapors, such as cooking fumes, dust and 
bugs can reach the electronic parts causing malfunction. 
The operation of the gas monitoring system is as follows, referring to 
FIGS. 3-5: When the system is turned on by closing the switch 15 on the 
control box, the power supply produces a regulated voltage at the input 
terminals 20 and 21 of the sensor and the audio oscillator circuit 31 
(FIG. 3), an unregulated voltage at the input terminal 22 and at the input 
terminal 23 of the alarm and gas shut-off circuit (FIG. 4) and at the 
input terminals 24 and 25 in FIG. 5. The green LED 16 is lit and the 
solenoid 26 of the gas valve is energized by the circuit including 
resistor R7 and power transistor T1, thus opening the gas valve. The 
heater 27 of the sensor starts to warm up via resistors R8 and R9, 
transistor T2 and resistor R 10, until it reaches its state of steady 
operation. At the start, the resistance of the heater being small, the 
current will be large. As a result, the potential at point B, between 
resistors R8 and R9, which is connected to the non-inverting terminal 28' 
of an integrated circuit voltage comparator 28, decreases. This in turn 
results in a decrease in the output voltage of the comparator to 
Vcs&lt;Vcn,Vcs being the voltage at the start and Vcn the voltage during 
normal operation. 
The output Vcs of the comparator is applied to the input 29 (FIG. 4) of 
transistor amplifier TDC 5. Its loads, transistor amplifier TDC 6 and TDC7 
in Darlington connection (FIG. 5), and now being in the semi-off state, 
constitute large resistance to transistor amplifier TDC 5. The output of 
TDC 5 as applied to the inputs of transistor amplifier TDC 6 and TDC 7 
will be low, so that these turn off completely. With transistor amplifier 
TDC 6 being off, current flows from the unregulated terminal 23 (FIG. 4) 
through resistor R4 and the red LED 17, which lights up for a short 
duration. Transistor amplifier TDC 7 also being off, it switches off the 
power transistor T1, so that no current passes through the solenoid 26 of 
the gas valve and the valve will prevent gas from leaving the container 3. 
When the sensor heater warms up further, thereby increasing its resistance, 
the output of the comparator 28 increases from Vcs and therefore the red 
LED 17 is extinguished. Power transistor T1 is now activated, so that 
solenoid 26 is energized, opening the gas valve and permitting gas to flow 
through the gas pipe from the container to the cooking range. The green 
LED 16 remains lit. 
In case of a gas leak, the sensor output voltage increases as its 
resistance decreases. The higher voltage is applied to the non-inverting 
terminal 28' of the comparator 28, resulting in a decrease in output of 
the comparator to a value Vcd&lt;Vcs&lt;Vcn. The transistor amplifiers TDC 6 and 
TDC 7 will now be in the semi-off state and constitute large load 
resistances to the transistor TDC 5. Further, transistor amplifier TDC 6 
being off, the red LED 17 will conduct current from the unregulated 
terminal 23 via resistor R4 and light up. Transistor amplifier TDC 7 also 
being off, power transistor T1 will also be off, so that the solenoid 26 
of the gas valve is deenergized, closing the gas valve and shutting off 
the flow of gas through the gas pipe 4. 
At the same time, the output Vcd of the comparator 28 being small, this 
decrease is transmitted to the input 30 of the audio oscillator 31 via 
resistor R13, zener diode Z1 and capacitor C1 (FIG. 3), so that the 
oscillator is activated. The output of the oscillator is applied to the 
buzzer 19 on the control box via conductor 32, transistor amplifiers TDC1, 
TDC2 and TDC 4 (FIG. 4. It is seen that both visual alarm, red LED 17, and 
an audible alarm, buzzer 19, are activated when the gas supply from the 
gas container is shut off by reason of a gas leak. It must be noticed that 
even if the gas has been removed from the kitchen, both alarms continue to 
operate until the main switch 15 on the control box is manually turned off 
and then turned on again. Also, the green LED 16 remains extinguished by 
the signal from the audio oscillator 31 via transistor amplifier TDC 1 and 
resistor R1 (FIG. 4). To reiterate, even after removal of the leaked gas, 
the green LED 16 is extinguished, the red LED 17 is lit and the buzzer 19 
keeps as long as the monitoring system has not been reset by means of the 
main switch 15. 
Further, it is to be noticed that even if no gas leakage occurs, but the 
electric mains supply is interrupted for some reason, the power transistor 
T1 is off as there is no input at regulated terminal 23, so that the 
solenoid 26 remains deenergized and the gas valve remains closed thus 
preventing gas to leave the gas container. This is an important feature as 
in this way no gas leak can occur during interruption of the mains, which 
would not be detected by the system. The gas supply could be restored by 
connecting the power supply 12 of the control box 7 to a battery. 
All electronic circuits described are integrated circuits which take up 
little space, so that they can be easily located in a control box of small 
dimensions. 
As indicated above, when the power fails, the gas flow is terminated and 
the user may be without energy for an extended interval. In remote 
locations where obtaining restoration of power could be delayed, this can 
be a serious hardship. It would, of course, be possible to arrange a 
system of backup batteries and a recharger but these are often expensive 
and may not be reliable. Accordingly, the invention includes provision for 
a manual override of the safety system. 
This override in constructed as part of the solenoid and valve structure 
and is illustrated in FIGS. 6-9. As seen in FIGS. 6 and 7, the solenoid 
includes a coil 26 which, when energized, causes a plunger 35 to move 
axially within a generally tubular housing 37. Plunger 35 is a tubular 
body of magnetic material which carries at its lower end a body of 
elastomeric material 39 which acts as a valve member. Housing 37 is 
threadedly attached to an inlet-outlet body 41, the connection 
therebetween being sealed by an O-ring 43. 
Body 41 has inlet means 45 to receive an inlet fitting connected to the 
tank or other source of gas under pressure and threaded outlet means 47 to 
receive an outlet fitting which is connected to the appliance using the 
gas. A passage 48 extends into body 41 coaxially with plunger 35 and 
terminates at a valve seat 50 which is covered by valve member 39 when the 
plunger is in the position shown in FIG. 6. A passage from inlet means 45 
also reaches the region around the valve seat but there is no 
communication between the inlet means and the interior of passage 48 when 
the valve member 39 rests on the valve seat. Plunger 35 is urged to the 
position shown by a compression coil spring 52 and is moved upwardly 
against the force of the spring when solenoid coil 26 is electrically 
energized as described above. 
The manual override includes a rotatable screw member which includes a 
cylindrical plug 54 rotatably received in a generally cylindrical recess 
56 in body 41. The outer surface of plug 54 is penetrated by a 
non-circular recess 58 which is preferably hexagonal to receive an end of 
a hexagonal wrench. The inner end of plug 54 is formed with operator means 
comprising a circular cylindrical eccentric 60 the central axis of which 
is perpendicular to the central axis of plunger 35 and parallel with the 
central axis of plug 54 but offset therefrom. As best seen in FIG. 7, the 
periphery of eccentric 60 contacts a bottom surface of plunger 35 which 
extends below valve member 39. 
To operate the manual override, an end of a hexagonal wrench is inserted in 
recess 58 and rotated clockwise (with reference to FIG. 7) about 
90.degree., thereby rotating the plug and the eccentric 60 through the 
same angle. The plug can be provided with a scribe 62 adjacent recess 58 
as an indicator of its rotational position and the outer surface of body 
41 is preferably marked with reference letters or numbers such as the 
numbers 0 and 1 shown in FIGS. 7 and 9. The eccentric is oriented relative 
to the plug, as shown in FIGS. 7 and 9, so that its lobe of greatest 
eccentricity is opposite the scribe. Thus in the position of FIGS. 6 and 
7, the eccentric may be in contact with the plunger but exerts no 
significant force thereon. However, as the plug is rotated clockwise so 
that the scribe approaches the 0 indicator, the eccentric pushes plunger 
35 upwardly against the force of spring 52, lifting valve member 39 away 
from valve seat 50 as shown in FIGS. 8 and 9 and opening a flow path 
between inlet means 45 and outlet means 47 through passage 48. Gas will 
then flow between the tank and appliance without regard to the existence 
of power or leakage conditions. The manual override must, of course, be 
returned to the inactive position of FIGS. 6 and 7 when power is restored 
in order for the sensing circuits to perform their unctions. 
As seen in FIG. 10, a pin 64 is inserted into a hole in body 41 to prevent 
plug 54 from being forced outwardly. Also, the periphery of plug 54 is 
sealed against leakage by an O-ring 65. 
Various changes may be made in the monitoring system described. For 
example, the system can be adapted to be operated by the car battery of a 
mobile home by providing a suitable transformer between the battery and 
the direct current power supply of the control box. Also, the system can 
be adapted to operate a ventilator to remove leaked gas after the gas 
supply has been shut off by the system. An additional alarm of some sort 
and located in a suitable place may be connected to the system. The same 
applies to an additional visual alarm. Further, the control box may be 
located in a place different from the sensor.