Gas detectors

A carbon monoxide gas detector comprises a housing consisting of two enclosures 10 and 12 having ports in the wall thereof to provide a tortuous path for air entering the inner enclosure 12. A gas cell transducer 16 connected to an electrical circuit 18 provides signals indicative of gas concentration. The detector is powered from a remote supply via wires 22 and 26 and provides gas concentration signals via a wire 24 to a remote central station with a fail-safe feature wherein failure of the circuit or the detector will provide signals interpreted as dangerous gas concentrations.

BACKGROUND OF THE INVENTION 
The invention relates to gas detectors. 
Gas detectors are already known including transducers which respond to the 
presence of one or more selected gases to change their electrical 
characteristics to enable, by electrically monitoring such 
characteristics, the detection of the presence and concentration of the 
selected gas in an environment. It is usual to provide an instrument 
having a transducer, an electrical monitoring circuit and a local 
electrical supply as well as a display for the instrument so that the 
presence and/or concentration of the selected gas or gases can be readily 
determined in a chosen environment. Such instruments especially when 
accuracy is required and low concentrations, a few part per million, are 
to be measured tend to be expensive and non-robust. 
The present invention is concerned with providing gas detectors which are 
comparatively cheap and are for use in detecting low concentrations of gas 
particularly carbon monoxide and methane. The gas detectors of the present 
invention have particular although not exclusive application in detecting 
the presence of fires or potentially hazardous conditions in underground 
mine workings. 
SUMMARY OF THE INVENTION 
According to one aspect of the invention there is provided a gas detector 
which comprises a housing, a gas cell transducer mounted in the housing 
and responsive to change its electrical characteristics with changes of 
concentration of selected gas in the housing, an electrical monitoring 
circuit supported in the housing and coupled to the cell which is arranged 
to receive an electrical supply from a remote control station and to 
supply monitoring signal to the control station indicative of the 
concentration of the gas. 
According to another aspect of the invention there is provided a selected 
gas concentration monitoring system for monitoring the presence of the gas 
in an environment such as the underground working of a mine, including a 
plurality of gas detectors, each detector comprising a gas cell transducer 
mounted in a housing and responsive to change its electrical 
characteristics with changes in concentration of the gas in the housing 
and an electrical monitoring circuit supported in the housing, and a 
control station electrically connected to each detector for providing an 
electrical supply thereto and for receiving monitoring signals from each 
detector indicative of concentrations of the gas in the respective 
housings. 
The gas cell transducers may be arranged to respond to a single selected 
gas, such as carbon monoxide or methane, or may be responsive to two or 
more gases and mixtures thereof. 
The housings of each detector may comprise an enclosure surrounding the 
cells having ports in the walls thereof to allow gas to enter through the 
ports. Preferably, such housings are so arranged that gas entering the 
ports is swirled slowly around the inside thereof even when the housing is 
in a position where an air flow is moving rapidly past the outside of the 
housing. 
The cell transducer is preferably arranged to be easily replaceable and 
provided for example with a plug-in mounting. This means where the 
operational life of the cell transducer is short, say only several months, 
the cell transducer can be readily exchanged as required.

DESCRIPTION OF PREFERRED EMBODIMENTS 
Referring to the drawing, in FIG. 1 the gas detector 9 has a housing 10 
which comprises an inner enclosure 12 and an outer enclosure 14, each 
enclosure having six porting slots which are non-aligned with one another. 
This means that when air is flowing relatively rapidly past the housing 10 
some of the air enters progressively into the inside of the enclosure 12 
via the slots and swirls slowly there around. A gas cell transducer 16 is 
mounted in the enclosure 12 and responds to the presence and the 
concentration of carbon monoxide to change its electrical characteristics. 
An electronic circuit 18 is mounted and encapsulated in a chamber 20 above 
the transducer 16 and is electrically coupled to the transducer 16. Three 
wires 22, 24 and 26 are connected to the circuit 18 and to a terminal 
block 27. 
The housing 10 has a lid 28 provided with a support eye 30 so that the 
detector 9 can be readily suspended at a chosen location. 
In use, the detector 9 is positioned in a chosen location in underground 
workings and is associated with a number of like detectors at various 
suitable positions in a district of underground workings. All the 
detectors are electrically supplied from a central control station to 
which monitoring signals are supplied from the detectors indicative of the 
concentration of carbon monoxide in the region of respective detectors. 
In FIG. 2, the detector is the same as the detector of FIG. 1 except that a 
small fan 32 is fitted inside the lid 28. The fan 32 is supplied from the 
station via the wire 26. The fan 32 draws air into the chamber 12 via the 
slots in the walls of the enclosures 12 and 14. The provision of the fan 
32 enables the detector 9 to respond efficiently and rapidly to changes of 
carbon monoxide concentration even where the detector is mounted in a 
region where there is little or no air flow. Normally in mine workings, 
conventional forced air ventilation provides adequate circulation past the 
detector when it is located in or near a main airway. However, it is 
possible that the detector 9 is positioned in a region of relatively poor 
air circulation or the air ventilation may be turned off for maintenance 
or be inoperative due to a fault. In such cases, the fan 32 ensures that 
there is adequate movement of air so that air enters into the enclosure 12 
to enable the transducer 16 to respond to and provide detection of carbon 
monoxide. 
In FIG. 3, the circuit includes a supply circuit which receives a -15 volt 
direct current supply from a central control station between lines 22 and 
26. A stabilising network 40 reduces the voltage to -12 volts and supplies 
a divider circuit, including an amplifier 42, to provide a common zero, 
together with a +6 volts and a -6 volts output for the remainder of the 
circuit. 
As seen in FIG. 4, gas cell transducer 16 is connected to a matching 
amplifier 44, with a resistor 46 in its feedback circuit, the output of 
which is connected via an amplifier 48 to a further amplifier 50 
incorporating temperature correction feedback. Further amplifiers 52, 54 
and 56 are connected in series to a current driver 58, the output of which 
is connected to the wire 24. 
The inputs of the amplifiers 52 and 54 are adjustable to alter the zero 
setting of the circuit and the range span respectively. The input of the 
amplifier 56 is also adjustable to provide an off-set of the overall 
output of the circuit. 
In the described circuit, an output for supply along the wire 24 is chosen, 
indicative of carbon monoxide concentration, so that when the 
concentration of carbon monoxide is zero, the output of the current driver 
58 is one milliamp. This corresponds to a zero output at the amplifier 54 
so that the off-set input of the amplifier 56 must be 1 volt. 
The zero setting for the circuit is adjusted by altering an input to the 
amplifier 52. This adjustable input is required to balance the small 
current which flows through the transducer 16 when no carbon monoxide is 
present. 
The span of detection selected is controlled or adjusted by an input to the 
amplifier 54 and is chosen to be zero to 100 parts per million of carbon 
monoxide; this span can also be adjusted if required by altering the 
resistance value of the resistor 46. After initial setting up of the 
circuit, the detector is then tested by subjecting the transducer 16 to a 
known concentration of carbon monoxide and the span adjusted to suit the 
chosen range. 
Normally each of the adjustments mentioned can be carried out by the 
provision of suitable variable resistors which can be accessed, using a 
screwdriver for example, without damaging or interfering with the 
encapsulation of the circuit in the chamber 20. 
In use, the transducer 16 which per se forms no part of the invention, 
responds to the presence of the carbon monoxide to change its electrical 
characteristics so that the inputs to the amplifier 48 become unbalanced 
and the output current of the current driver 58 reduces appropriately. For 
monitoring mine workings, the range chosen as explained is 100 parts per 
million so that if the concentration rises to this level the output 
current of the current driver 58 reduces from 1 milliamp to zero. 
It will be appreciated that other circuit arrangements can be provided and 
also other settings used in the circuit. The use of a reducing current 
output as gas concentration increases is however preferred because, as 
would be evident to those having skill in the art, this introduces a 
fail-safe feature since, for example, loss of supply current to the unit 
would also result in the indicating device reading otherwise than 
full-scale deflection, which would indicate a fault. Also, where recording 
is made, especially by chart recorders, full-scale or maximum deflection 
at zero gas concentration is advantageous. 
The transducer 16 is plugged into position in the housing 10. This enables 
the transducer 16 to be changed easily. Suitable transducers presently 
available have a working life of 6 to 18 months in mining environments and 
must therefore be replaced normally every 6 to 12 months. As will be 
appreciated, the described circuit enables each replaced transducer to be 
calibrated and the circuit adjusted when a replacement transducer is first 
plugged into the housing 10 and also at other times if required. 
The fan 32 when fitted is connected to the wire 26 as shown dotted in FIG. 
3. 
As mentioned earlier the detectors are suitably positioned in chosen 
different locations throughout a district of underground workings; the 
central control station is normally and conveniently situated at the 
surface of the mine. At the control station it is preferable to have a 
number of chart recorders respectively coupled to each of the detectors to 
monitor the changes of concentration of carbon monoxide. The chart 
recorder can each be associated with an alarm circuit and arranged to 
provide an audible and/or visable alarm whenever the concentration rises 
to a predetermined level. The alarm circuit, or an additional circuit 
incorporated with the described electrical circuit in the encapsulation in 
the chamber 20 in each detector, can be provided to produce such an alarm 
signal. Further, the detectors or the alarm circuit at the surface may be 
provided with an additional circuit arrangment which responds to rates of 
chanqe of carbon monoxide concentration and produces a further alarm 
signal whenever this rate exceeds a predetermined rate. 
A telemetry system can be used to communicate the magnitude of the output 
current of the current driver 58 to the central control station. In such a 
case the telemetering link is used to power the electrical circuits of the 
detectors as well as telemetering circuitry provided in each detector. 
The monitoring of carbon monoxide is especially advantageous for detecting 
the on-set of an underground fire or even the presence or occurrence of 
fire hazardous conditions as they develop particularly in a coal mine. 
Although the presence of other gases, for example carbon dioxide may also 
be used. However occasional overall comparatively high background 
concentration of carbon dioxide in a mine even when there is no fire or 
fire hazard, means that changes indicative of a fire are not so easily 
distinguishable from normal safe conditions. By monitoring the presence of 
carbon monoxide, methane, or mixture thereof, the on-set and increase of 
small concentrations of such gases provides a generally much more reliable 
and more easily distinguishable condition on which to detect rapidly the 
outbreak of a fire or other hazardous conditions. 
As each described detector in a system for monitoring the concentration of 
gas in an underground district does not require a local power source and 
has remote indicators, chart recorders for example, each detector can be 
made relatively cheaply. Further, the remote station not only readily 
provides an early indication of a fire but the central monitoring of the 
concentration of gas has the added advantage that faulty or inoperative 
detectors can be identified. This can be done either by observing 
uncharacteristic changes in their output monitoring signals or by 
comparing those signals with the signals supplied from detectors 
positioned nearby in the same district. 
The system may also be used to monitor the presence or the on-set of 
dangerous gas concentrations either in mine or even in long roadway 
tunnels which need not be associated with fires or fire hazards but may be 
related to health hazardous conditions. 
Preferably, the central monitoring station is of the type already used with 
ionisation type smoke detectors. Further, same housings for the detectors 
that are used and currently available for such smoke detectors can be 
advantageously used. This enables the gas detectors to be produced in bulk 
quantities using the same machinery as for producing the housings of the 
smoke detectors. Additionally, the gas detectors can be incorporated if 
desired into a system which monitors a number of smoke detectors and a 
number of carbon monoxide detectors, distributed in the same district, at 
the same time as may be desired.