GB2310726A

GB2310726A - Fire detection sensor

Info

Publication number: GB2310726A
Application number: GB9704137A
Authority: GB
Inventors: Edward Watkin Williams
Original assignee: KES Ltd
Current assignee: KES Ltd
Priority date: 1996-03-01
Filing date: 1997-02-28
Publication date: 1997-09-03
Also published as: GB9704137D0

Abstract

A sensing device for detecting and giving early warning of fires includes a sensor 10 comprising a substrate 12 having a layer of a composition comprising a semiconducting metallic oxide, a first pair of electrodes 13,14 connected to the oxide layer and connectable to a circuit for monitoring the resistance between the electrodes which varies in the presence of carbon monoxide gas in the vicinity of the sensor and a second pair of electrodes 14,15 connected to the oxide layer and connectable to a circuit arranged to pass a current between the electrodes and monitor the value of the current, an optical fibre 17 being attached at one end 16 to the oxide layer between the electrodes of the second pair and its other end terminating in a smoke chamber 19, means 20 being provided to shine light through the smoke chamber and optical fibre to the oxide layer.

Description

DESCRIPTION SENSOR This invention relates to a sensor for gases and vapours, and is particularly intended to provide a multi-purpose sensor having at least two simultaneously operable sensing modes and which is particularly useflil for the early warning of fire.

Sensors to detect gases such as carbon monoxide and to detect smoke from a fire are well known.

A sensor for the detection of carbon monoxide and water vapour, for example, is disclosed in WO 94/23289 which teaches the use of a sensor based on a composition containing a semi-conductor metallic oxide, a catalyst and a rheological agent to induce porosity into the layer.

The present invention aims to provide an improved sensor that can detect and give early wanting of most types of fires, i.e. including fires that generate smoke but little or no carbon monoxide and fires that give off carbon monoxide but little or no smoke.

Accordingly the invention provides a sensing device including a sensor comprising a substrate having a layer of a composition comprising a semi-conductor metallic oxide, a first pair of electrodes attached to the metallic oxide layer and connectable into an electrical circuit measuring and monitoring the resistance between the electrodes, a second pair of electrodes attached to the metallic oxide layer and connectable to an electrical circuit through which a current can be passed and monitored, an optical fibre connected at one end to the metallic oxide layer between the second pair of electrodes and its other end terminating in a smoke chamber and means to shine light into the smoke chamber and thereby through the optical fibre to the sensor.

The first and second pairs of electrodes may conveniently share one electrode, i.e. only three electrodes may be required rather than four.

The resistance of the semi-conductor metallic oxide layer should be sufficiently high to prevent interference between the two electrical circuits operating via the two pairs of electrodes. Thus the resistance of that layer should preferably be between 10 megohms and 100 gigaohms, i.e. 10" ohms.

The sensor of the invention enables monitoring for a gas such as carbon monoxide by virtue of the monitoring circuit between the first pair of electrodes and monitoring for smoke by virtue of the monitoring circuit between the second pair of electrodes.

Thus, if carbon monoxide impinges on the sensor between the first pair of electrodes, its reaction with the semi-conductor metallic oxide layer causes a change in the resistance of that layer, as is well known, and that change will be detected by the resistance monitoring circuit.

Alternatively, or additionally, if smoke enters the chamber of a smoke detector connected to the sensor, the light passing through the optical fibre from the chamber will decrease due to masking by the smoke.

Light impinging on the sensor causes electron excitation in the semiconductor from the valence band to the conduction band and results in a flow of current. When the light is obscured or partially obscured by smoke, the amount of current generated will fall and this fall will be detected by the current monitoring circuit.

Thus the detector can provide early warning of fires that generate carbon monoxide rather than smoke, of fires that generate smoke rather than carbon monoxide and of fires that generate both.

In a preferred embodiment of the invention a third sensing and monitoring circuit is provided. The third circuit is based on the use of another optical fibre connected at one end to the sensor and extending around the area to be protected. Thus this fibre may extend around a building into particular areas of fire risk, e.g. a boiler room, or furnace, a laboratory and the like, and be provided with a window in its sheathing to admit light at each sensitive position desired to be protected. Light entering each window passes along the fibre to the sensor where it excites electrons, as previously described, and generates a current. This current is monitored in an electrical circuit in a similar manner to that for the smoke detector circuit.

Conveniently this optical fibre may be connected to the sensor between the same pair of electrodes as the optical fibre connected to the smoke chamber. The same monitoring circuit may be used, if desired.

Beam splitters, as conventionally known, may be positioned at each window of the optical fibre to increase the effect of the light transmission.

In this embodiment, if a fire starts near to one of the windows of the optical fibre, additional light from the fire will pass via the window along the fibre to the sensor. This will cause increased electron excitation and hence increased current in the monitoring system above a set threshold to give an early warning of the fire.

The monitoring circuits may be connected to any suitable alarm systems, visual and audible, to trigger an alarm when the set thresholds of current or resistance are passed.

In the mode of the sensor employing an optical fibre connected to a smoke chamber, the light shone into the smoke chamber should be of a sufficiently constant brightness and should have sufficient energy to excite electrons in the sensor layer. LED light is preferred and should be at most of wavelength in the visible green range, blue wavelength being preferred.

Thus the light should be of wavelength at most about 577 to 492 nanometers and of frequency 520 to 610 THz (i.e. green), preferably from about 492 to 455 nanometers of frequency 610 to 659 Thz (i.e. blue) and may be of lower wavelength, about 455 downwards and of frequency from 659 Thz upwards (i.e. violet).

In the embodiment employing one optical fibre connected to a smoke chamber and another optical fibre with windows to detect firelight, the two fibres may, if desired, be attached jointly to the surface of the sensor, i.e. they may be glued at their ends to a single fibre which is glued at its other end to the sensor.

Stannic oxide is the preferred semi-conductor metallic oxide but, alternatively for example, indium oxide or aluminium oxide may be used.

In addition to the semi-conductor metallic oxide, the sensor composition should contain a catalyst, e.g. platinum, preferably in the form of a platinum black, or palladium, rhodium, ruthenium, osmium, or iridium.

Preferably, the composition is formulated according to the teaching of WO 94/23289. Thus it may contain a rheological agent, e.g. kieselguhr or sepiolite. Preferred proportions of the composition are as follows: Weight ratios Stannic oxide (or other semi-conductor oxide) 70-92% Catalyst 3-30% Rheological agent 5 -20% Other ingredients may be added as desired. For example, additives, well known per se, to change the electrical conductivity of the layer may be added, e.g. in an amount of from 0.5 to 5% by weight.

The substrate, which should be a good electrical insulator, may be, for example, a sheet of glass or ceramic material. The sensor layer may be applied as a film of the sensor composition from 100 nanometers to lmm thick, i.e. both so-called thin and thick sensors may be used, and may be formed on the substrate by applying a paste of the composition in water to the substrate and annealing at a temperature of, e.g. from 500 to 10000C to form a hardened layer. The coating composition may alternatively be applied to the substrate, for example, by r.f. sputter deposition or by screen printing a paste onto the substrate.

To prepare a usable sensor after the oxide layer has been applied to the substrate and annealed, any suitable means may be used to provide the required electrodes. Thus, for example, silver, aluminium or tin electrodes may be formed on the surface of the semi-conductor oxide after masking desired portions of its surface. The electrodes may be applied by evaporation from a filament or a boat using a conventionally known vacuum system.

The sensor, or at least its sensitive surface, should be housed to avoid light - except of course the light transmitted down the optical fibres this being conventional practice. For example, light blockers, e.g. tape or black paint, can be applied to the otherwise exposed sides of the sensor.

Embodiments of the invention will now be described by way of example only with reference to the accompanying drawings in which: Figure 1 is a diagrammatic representation of a sensor according to a preferred embodiment of the invention; Figure 2 is a graph of current v. time for the smoke detection portion of the sensor; Figure 3 is a graph of current v. time for the firelight detection portion of the sensor; Figure 4 is a graph of resistance v. time for the carbon monoxide detection portion of the sensor; Figure 5 is an electrical circuit to monitor for carbon monoxide detection in the sensor of Figure 1; and Figure 6 is an electrical circuit to monitor for smoke and light detection in the sensor of Figure 1.

In Figure 1 a sensor 10 has a layer 11 of annealed stannic oxide composition applied to a glass substrate layer 12. Three electrodes 13, 14 and 15 have been applied to the surface of layer 11 to form two pairs of electrodes 13, 14 and 14, 15. A short optical fibre 16 is attached at one of its ends by gluing to the surface of layer 11 between electrodes 14 and 15.

The other end of fibre 16 is attached to two separate optical fibres 17 and 18. The other end of fibre 17 terminates in a smoke chamber 19 into which blue LED light is constantly fed from source 20. Smoke chamber 19 is positioned in a sensitive area to receive smoke should a fire start in that area.

Fibre 18 is extended around an area, e.g. a building, it is desired to protect. It has a multiplicity of windows 21, only one of which is shown for convenience.

The surface of the sensor between electrodes 13 and 14 is arranged to receive any carbon monoxide generated from a fire starting in the adjacent area.

Electrodes 13 and 14 are connected to an electrical circuit (see Figure 5) which is a standard microprocessor (inc1) circuit incorporating a thermistor TH1 and an alarm SP1.

Electrodes 14 and 15 are connected to an electrical circuit (see Figure 6) including an operational amplifier 24 to measure the signal currents. A photon 23 is diagrammatically illustrated impinging on the sensor between electrodes 14 and 15.

The sides ofthe sensor are masked by tape or paint 22 (Figure 1).

As shown in Figure 2, the current v. time plot is a straight line being the set and monitored current in the circuit of Figure 6 when no smoke is detected. When smoke enters chamber 19, it obscures the light from source 20 causing a drop in the amount of light travelling along fibre 17 and hence a drop in current generated between electrodes 14 and 15 - see dotted line on graph. This fall triggers the alarm.

As shown in Figure 3, the current v. time plot is the same straight line as for Figure 2, being the same set and monitored current in the circuit of Figure 6. When light enters a window 21 of optical fibre 18, it causes an increase in light travelling along fibre 18 and hence an increase in current generated between electrodes 14 and 15 - again see dotted line on graph. This rise triggers the alarm.

Clearly, it will be necessary to ensure that this sensor mode is not activated by daylight or lights being switched on so that its use may be restricted to dark areas and night time and means should be provided to deactivate it, for example, if it is necessary to switch on a light to enter the particular area to be monitored. Circuitry can be provided to isolate individual windows for this purpose rather than the whole of the extent of the fibre.

As shown in Figure 4, the resistance v. time plot is a straight line being the set monitored resistance in the circuit of Figure 5. When carbon monoxide impinges on the sensor between electrodes 13 and 14, the chemical reaction with the semi-conductor oxide causes a change in the measured resistance - again see dotted line of the graph. This change triggers the alarm.

In Figure 5, the circuit details are as follows: R5 = 27 K ohms (kilo ohms) R6 = 27 K ohms R7 = 330 ohms R8 = 330 ohms R9 = 330 ohms R10 = 10 K ohms C3 = 0.1 microfarad C4 = 0.1 microfarad C5 = 18 PF (picofarad) C6 = 18PF C7 = 0.1 microfarad C8 = 0.1 microfarad S1 = test switch S2 = reset switch CB = connector board D1 = light emitting diode D2 = light emitting diode D3 = light emitting diode TH 1 = thermistor SP1 = alarm IC1 = microprocessor

Claims

CLAIMS 1. A sensing device including a sensor comprising a substrate having a layer of a composition comprising a semi-conductor metallic oxide, a first pair of electrodes attached to the metallic oxide layer and connectable into an electrical circuit measuring and monitoring the resistance between the electrodes, a second pair of electrodes attached to the metallic oxide layer and connectable to an electrical circuit through which a current can be passed and monitored, an optical fibre connected at one end to the metallic oxide layer between the second pair of electrodes and its other end terminating in a smoke chamber and means to shine light into the smoke chamber and thereby through the optical fibre to the sensor.
2. A sensing device according to Claim 1, in which the first and second pairs of electrodes share one electrode.
3. A sensing device according to Claim 1 or 2, in which the resistance of the semi-conductor metallic oxide layer is between 10 megohms and 100 gigaohms.
4. A sensing device according to Claim 1, 2 or 3, which includes a third sensing and monitoring circuit comprising a second optical fibre connected at one end to the sensor and extending around an area to be protected.
5. A sensing device according to Claim 4, in which the second optical fibre is provided with a window in its sheathing at each desired position to be monitored, whereby light entering each window passes along the fibre to the sensor where it excites electrons and generates a current, and monitoring means are provided to monitor that current.
6. A sensing device according to Claim 4 or 5, in which the second optical fibre is connected to the sensor between the same pair of electrodes as the optical fibre connected to the smoke chamber.
7. A sensing device according to Claim 6, in which the same monitoring circuit is used to monitor current generated via either optical fibre.
8. A sensing device according to Claim 5, 6 or 7, in which a beam splitter is positioned at each window of the optical fibre.
9. A sensing device according to any preceding claim, in which the monitoring circuits are connected to a visual and/or audible alarm.
10. A sensing device according to any preceding claim, in which the light shone into the smoke chamber is LED light.
11. A sensing device according to Claim 10, in which the LED light is of wavelength from about 577 to 492 nanometers and of frequency 520 to 610 THz.
12. A sensing device according to Claim 10, in which the light is of wavelength from about 492 to 455 nanometers and of frequency from 610 to 659 THz.
13. A sensing device according to Claim 10, in which the light is of wavelength from about 455 nanometers or lower and of frequency 659 THz or higher.
14. A sensing device according to any one of Claims 4 to 13, in which the two optical fibres are attached jointly to the surface of the sensor.
15. A sensing device according to any preceding claim, in which the semi-conductor metallic oxide is stannic oxide, indium oxide or aluminium oxide.
16. A sensing device according to any preceding claim, in which the sensor composition contains platinum, palladium, rhodium, ruthenium, osmium or iridium as catalyst.
17. A sensing device according to any preceding claim, in which the sensor composition contains kieselguhr or sepiolite as a rheological agent.
18. A sensing device according to any preceding claim, in which the sensor composition is applied to the substrate as a layer from 100 nanometers to 1 mm thick.
19. A sensing device according to Claim 1, substantially as hereinbefore described with reference to and as shown in the accompanying drawings.