IE56664B1 - Electric particle detector for fire detection - Google Patents

Description

The invention relates to en rleetrie particle detector for monitoring the atmosphere in a room for enawpl®^ particularly for the detection of fir®» The problem of detecting fire by the detection of 5 ions is well known» Usually^ the ionization of the air in e chamber is provoked by means of ® radioactive source or by g periodic electric sperk^ and e measurement is made, in the monitoring state,® of the Ionic current. When a fire breaks outtf heavy particles of ssoke for example are carried in the room to the detec tion chamber where they disturb the circulation of ions by picking them up or blocking them» The measured ionic current therefore reduces iund this reduction is used to trigger en al&rsu Xt should ne noted that humidity also reduces the ionic current» To prevent this humidity from triggering false 0larfeS^ It is usual to have & reference chamber that is protected from humidity^ or to have electronic compensation of the detector sensitivity® In this 1? becomes very difficult to detect a smouldering fire for example® In fact^ with these fires that do not produce large amounts of hefttf. the convection currents are not strong enough to carry the heavy partieles to tbe detector in $ reasonable time» Consequently,® the response time of detectors to β smouldering fire is very long,® which is ® disadvantage with regards to safety rul«Sn One of the purposes of the present invention is to produce a detector in which,® unlike in known detect tors,® the presence of particles emitted by the start of » fir©^ for example^ increases the eurreat passing through the Q Another purpose of the invention is to provide a remarkably simple charged particle detector which can operate with various types of power supply and with an extremely low consumption.» Accordingly, the Invention provides an electric particle detector comprising a sensor with two electrodes supplied with a voltage, and operating at a current below the zone of corona discharge, the electrodes being formed by wires bent in an alpha shape, and the operating point of the sensor being situated below the non-self ^sustaining discharge zone, that is to say at an intensity well below the self-sustaining avalanche zone corresponding to corona discharge, such that the capture of ions by said sensor causes an increase in the current passing therethrough.

According Co other characteristics of the invention: - the two electrodes of the sensor are connected to the power supply via two resistances, one of which is of the order of a gigaohm and the other of which provides a measurable voltage drop for determining the current passing through the sensor; - the sensor is powered via at least one diode operating at the start of the characteristic and 2o behaving like a resistance; - said other resistance has its two ends connected to earth via capacitors; - the wires forming the alpha shaped electrodes have barbs® By way of example^ In the appended drawing 1s represented^ figure a simplified electric circuit diagram of an embodiment of the charged particle detector according to the invention^ Figures 2 to various embodiments of the elec•erodes of the detector of Figure Figure 12^ a variant of embodiment ofthe electric circuit diagram of Figure Figure 13, an enlarged view of an alpha shaped 35 electrode according to Figure 5ω Referring to the drawing one can see a sensor 1 of charged particles, which forms the sensitive part of the defectorA The case of this sensor is not showne This case must be open to the gases likely to pass through It and preferably Includes a grid allowing the passage of gases and charged particles emitted by the start of a fire. The electrodes 2 and 3 of the sensor will be described later» The sensor 1 Is connected to ground via a resistor 4 the value of which is for example in the order of 10 Megaohms · A first measuring point 5 is provided between p the sensor 1 and the resistor 4a The sensor 1 is connected to a power supply source via a scries resistor 6 of very high value, in the order of 1 Gigaohm. The power supply source is symbolised by the point 7» Xt is decoupled to ground through a capacitor SQ The power supply of the circuit is shown diagrammatically by a voltage step-up transformer of which the primary 12 is supplied aeross terminals 13 and 14 and of which the secondary 15 is connected between ground on the one hand, and point 7 on the other hand via a diode 10^ Xt should be specified that the power supply of the circuit can be Pt, at point 7? with a voltage in the order of 1000 V for example, that It can be rectified^ according to the circuit of Figure 1, the primary being powerd with and finally that it can can be pulsed, again in the case of Figure 1, the primary then being powered by pulses coming for example from the discharge of a capacitor» Taking account of the value of the resistor 6 and of the value of the supply voltage, the current likely to pass through the sensor 1 is in the order of 10**^ to 10~^2 A. The operating point of the sensor must be below the non-meintained discharge sone, i.e» well below, in current, the self-maintaining 9 avalanche sone corresponding to the corona effect» Xn Figure 12 we can see that the resistor 6 of t «1 Figure 1 has been replaced by diodes 17 which operate at the start of characteristic and behave like a high value resistor^ This arrangement has a currant regulat® Ine effect In order to Increase the voltage on the sensor on aging.. The capacitor 3 of Figure 1 has also been deleted» On the other hand, the measuring point 1s removed to after the high value resistor the ends of which are connected to ground through two capacitors 10 and 19 of very low value» We thus obtain an Integrated current output 1n the order of 10*^ Λ· The shape of the electrodes 2 and 3 Is provided to limit the electrical field between them so as to prevent discharges» That Is why the electrodes are not formed from points facing each other, but rather by fingers (Figure 2), balls (figure 3), wires bent In serai-circles (Figure or 1n alpha shape (Figure 5), or even of straight wires arranged In parallel planes (Figure 6) the projection of one of the wires in the plane of the other wire cutting that Hire, by parallel wires (Figure 7), by coiled wires arranged In pacalw lei (Figure 8) or in parsLleL planes (Figure 9)» Me can also have one of the electrodes in the forts of a winding spread over a cylinder and she other electrode arranged along th© axis of the cylinder*, either straight (Figure 10) or colled (Figure 11)» Those embodiments of the electrodes are only described by way of Illustration» The electrodes are of wire folded back In alpha shape according to the diagram in Figure Sv In Figure 13, the alph&”shaped electrode 1s shown In the form of a folded wire 20. This wire Is not perfect and It has barbs symbolised in 21, as the micro® geometry of the wire 1s useful for questions of electric field» By way of example, a wire of diameter 100/ira and folded with a radius of curvature of Less than or equal to 1 rara« This electrode shape appears fo be particularly effective for picking up the charged and not completely burnt particles omitted by a fire and particulary by a smouldering fire» When those particles are picked up by the electrodes, they are discharged there and this β creates a current, but they flare up there again and this causes an emission of secondary ions that are also picked up, which results in a net Increase in current, This secondary combustion is improved by making or covering the electrodes with special metals or alloys» Xn certain cases, a nonecharged particle, such as a speck of dust, is deposited on an electrode.» Xf an ion positions Itself on top of this speck of dust, a discharge is produced between this ion and the 2o electrode, which corresponds with an avalanche mult” iplication of electrical charges, and a larger current is seen to appear^ ALso, this discharge is accompanied by a reaction equivalent to a chemical reaction of oxidation of the dust, the result of which is combustion of the dust, be® th® cleaning of the electrode» This results in an Increase in the sensitivity of the sensor and a seIf-cLeaning of the electrodess the sensor can therefore work with a lower supply vol·3 tag®» 2q The basic difference with respect to traditional Ionic sensors is that in the sensor according to the invention, the ions picked up and the secondary ions, also picked up, increase the current in the sensor,On the other hand, variations in ambient temperature, sir currents end th® presense of non^charged particles have no significant effect on the current in th® sensor end this prevents false alarms» Xf w® return to th® diagram in Figure 1, we can analyse th® functioning of th® detector in a simplified 3q way ss follows: in th® absence of ions, th® measuring point 5 is at a stable potential; In th® presence of picked up ions, the current in th® sensor increases and th® potential et this point 5 varies» Th® power supply to th® sensor can be bC, rectified AC or pulses with sampling pulses in th® order of 10 to 20 ms per second for example» Th® invention is particularly well suited to dot ecting the start of a fire, particularly smouldering ii fires, by Its sensor which picks up the sms 11 charged particles emitted at the start of a fire® Xt is also suited to the detection of smokos, gases, vapors (of sodium for example), dust, aerosols or ionse

Claims (6)

CLAIMS:

1. An electric particle detector comprising & sensor with two electrodes supplied with a voltage, and operating at a current below the zone of corona discharge, the electrodes being formed by wires bent in an alpha shape, and the operating point of the sensor being situated below the non-self-sustaining discharge zone, that is to say at an intensity well below the self-sustaining avalanche zone corresponding to corona discharge, such that the capture of ions by said sensor causes an increase in the current passing therethrough.

2. A detector according to Claim 1, wherein the two electrodes of the sensor are connected to the power supply via two resistances, one of which is of the order of a gigaohm, and the other of which provides a measurable voltage drop for determining the current passing through the sensor.

3. A defector according to Claim 1, wherein the sensor is supplied via af least one diode which operates af the beginning of its characteristic and behaves like a resistance.

4. A defector according to Claim 2, wherein said other resistance has its two ends connected to earth via capacitors.

5. A detector according to Claim 1, wherein the wires forming the electrodes ln an alpha shape have barbs. A

6. An electric particle detector according to Claim 1, substantially as hereinbefore described with particular reference to and as illustrated in the accompanying drawings.