EP0826199B1 - Electric air ionisation smoke detector - Google Patents

Description

The present invention relates to detectors of electrically ionized smoke.

Known smoke detectors with air ionization, such as for example that described in document FR-A-2 594 953, includes a detection chamber which contains air charged with ions and which communicates with the atmosphere, this detection chamber being the seat of a field electrostatic generated between an anode and a cathode which are connected to each other by an electrical circuit of polarization with a voltage generator creating between the cathode and anode a DC voltage less than 100 volts, so that the ions contained in the detection are drawn to each other and to the cathode others to the anode, thereby generating an electric current in the bias circuit, which circuit includes in addition to a device for measuring electrical current and alarm to measure said electric current and to trigger an alarm reaction when this electric current undergoes an abnormal variation.

In these smoke detectors, the presence of smoke in the detection chamber results in the fixing of ions present in this chamber on the constituent particles of this smoke, which weighs down these ions: this phenomenon slows down the ions, so that the intensity of the electric current flowing in the bias circuit is reduced.

When this intensity falls below a certain threshold, the detector triggers an alarm to signal the smoke in the atmosphere.

Usually in smoke detectors kind in question, the air is ionized directly in the detection chamber by means of a radiation source ionizing, generally consisting of a substance radioactive α or β.

In order to avoid using such a radioactive source, it has already been proposed to generate the ions in the electric detection chamber. Such smoke detectors are known for example FR-A-2 408 837 and FR-A-2 386 873.

But air ionization cannot be performed between the cathode and the anode of the detection chamber, as that would involve applying between these two electrodes a voltage generally greater than 1000 volts, a voltage which would attract ions to the anode and the cathode with electrostatic forces so strong that the detector would become insensitive to the presence or not of smoke particles in the detection chamber.

To remedy this drawback, it has been proposed to give the cathode of the ionization chamber the form of a grid delimiting an ionization chamber which adjoins the detection chamber and which communicates with this chamber detection through the grid holes.

The ionization chamber contains an electrode which generally has a point shape and is brought to a potential difference generally greater than 1000 volts relative to the grid, so that discharges between the grid and the electrode, ionizing the air inside the ionization chamber.

The ions generated in the ionization chamber are attracted with relatively high speeds towards each other the grid, and the others towards the electrode in the form of tip: a small proportion of the ions that are attracted towards the grid crosses the holes of this grid, and found in the detection chamber, thus allowing the creation, in this chamber of an ion current of relatively low speed between the anode and the cathode, under the effect of the voltage below 100 volts which is applied between the anode and the cathode.

These air ionization smoke detectors per channel however have the serious disadvantage that the ion concentration in the ionization chamber is very higher than the ionic concentration in the detection, because few ions manage to cross the grid holes.

Therefore, when the smoke detector is swept by a stray outside air, the ions contained in large quantities in the ionization chamber found moved in the detection chamber, which can significantly increase the current flowing in the bias circuit, for example by a factor of 10 or 100, for the duration of said external parasitic air current.

During this entire time, if particles of smoke enters the detection chamber generating a some drop in the current flowing in the circuit polarization, this decrease is not enough to compensate the considerable increase in this current caused by the stray air flow, so that no alarm is triggered.

The smoke detector no longer fills its role as soon as it is swept by a draft of outside air parasite.

Document US-A-3 028 490 discloses a smoke detector ionization using a radioactive source that includes a fan to arrive at a defined air flow in the detection chamber.

The object of the present invention is to provide a air ionization smoke detector that does not use radioactive source and which does not have the disadvantages above.

To this end, according to the invention, which is defined in claim 1, a device of the genre in question is essentially characterized in that ions are generated in at least one ionization chamber which is separate from the detection chamber and which communicates with said detection chamber, the ionization chamber also communicating with the atmosphere and comprising at least two electrodes connected together by a circuit clean power supply to generate between these two electrodes of the electric discharges ionizing the air, and that air displacement means are further provided to create a weak continuous air flow from the chamber ionization to the detection chamber, so that transfer the generated ions into said detection chamber in the ionization chamber.

Thanks to these provisions, on the one hand, it is no longer necessary to use a radioactive source to ionize air, and on the other hand, we avoid creating in any part of the detector an area where the ions would be very concentrated, so that even when the detector is swept by a parasitic outside air current, this current no significant increase in concentration ionic in the detection chamber, and therefore does not not unduly disturb the functioning of the detector.

• le courant d'air entre la chambre d'ionisation et la chambre de détection présente une vitesse comprise entre 1 et 20 cm par seconde ;
• le courant d'air entre la chambre d'ionisation et la chambre de détection arrive dans la chambre de détection par un dispositif de canalisation qui comprend au moins un conduit et qui offre audit courant d'air une faible section de passage totale, la chambre de détection présentant, au voisinage de ce dispositif de canalisation, une section intérieure qui, prise perpendiculairement au courant d'air, est au moins cinq fois supérieure à ladite section de passage totale, et l'air contenu dans la chambre de détection présentant un temps de renouvellement moyen de 1 à 10 minutes sous l'effet du courant d'air permanent provenant de la chambre d'ionisation ;
• les moyens de déplacement d'air comportent un dispositif de ventilation déplaçant l'air par action mécanique ;
• le dispositif de ventilation est disposé en amont de la chambre d'ionisation ;
• le dispositif de ventilation comporte des pales rotatives entraínées par un moteur électrique ;
• le dispositif de ventilation comprend une lamelle sensiblement plane qui s'étend longitudinalement entre d'une part une extrémité d'amont fixe et d'autre part une extrémité d'aval libre, cette lamelle étant actionnée par un actionneur électrique de façon que l'extrémité libre de cette lamelle se déplace perpendiculairement au plan de ladite lamelle ;
• l'actionneur électrique est choisi parmi les actionneurs électrostatiques, les actionneurs magnétiques, et les actionneurs piézoélectriques ;
• les moyens de déplacement d'air comprennent un élément chauffant pour chauffer l'air en amont de la chambre de détection, de façon à déplacer cet air par convection naturelle, le détecteur de fumée étant conformé pour que le courant d'air continu suive un chemin ascendant depuis l'élément chauffant jusqu'à la chambre de détection ;
• le dispositif comporte des moyens de commande conçus pour faire fonctionner par intermittence les moyens de déplacement d'air et la chambre d'ionisation ;
• le dispositif comporte en outre des moyens de contrôle et de commande pour mesurer la vitesse du courant d'air qui va de la chambre d'ionisation vers la chambre de détection, et pour asservir les moyens de déplacement d'air de façon à maintenir constante ladite vitesse ;
• le dispositif comporte en outre un capteur pour mesurer la vitesse du courant d'air qui va de la chambre d'ionisation vers la chambre de détection, ce capteur étant relié au dispositif de mesure d'intensité électrique et d'alarme, lequel dispositif est conçu pour prendre en compte la vitesse mesurée par le capteur et pour corriger en fonction de cette vitesse au moins certaines des données qu'il prend en compte pour déclencher une réaction d'alarme, de sorte que ce dispositif de mesure d'intensité électrique et d'alarme s'affranchit des variations de l'intensité électrique mesurée qui sont dues aux variations de la vitesse du courant d'air ;
• le courant d'air entre la chambre d'ionisation et la chambre de détection débouche au moins en partie dans un volume de réserve d'ions qui n'est pas soumis au champ électrostatique régnant entre l'anode et la cathode de la chambre de détection et qui communique avec la chambre de détection de façon à être balayé par tout courant d'air extérieur perturbateur qui balayerait également cette chambre de détection, avant le passage de ce courant d'air dans au moins une partie de ladite chambre de détection.

In preferred embodiments, use is also made of one and / or the other of the following arrangements:

• the air flow between the ionization chamber and the detection chamber has a speed of between 1 and 20 cm per second;
• the air flow between the ionization chamber and the detection chamber arrives in the detection chamber by a channeling device which comprises at least one conduit and which offers to said air stream a small total cross-section, the chamber detection having, in the vicinity of this channeling device, an internal section which, taken perpendicular to the air flow, is at least five times greater than said total passage section, and the air contained in the detection chamber having a average renewal time from 1 to 10 minutes under the effect of the permanent air current coming from the ionization chamber;
• the air displacement means comprise a ventilation device displacing the air by mechanical action;
• the ventilation device is arranged upstream of the ionization chamber;
• the ventilation device comprises rotary blades driven by an electric motor;
• the ventilation device comprises a substantially planar strip which extends longitudinally between on the one hand a fixed upstream end and on the other hand a free downstream end, this strip being actuated by an electric actuator so that the free end of this strip moves perpendicular to the plane of said strip;
• the electric actuator is chosen from electrostatic actuators, magnetic actuators, and piezoelectric actuators;
• the air displacement means comprise a heating element for heating the air upstream of the detection chamber, so as to move this air by natural convection, the smoke detector being shaped so that the continuous air flow follows a upward path from the heating element to the detection chamber;
• the device comprises control means designed to operate the air displacement means and the ionization chamber intermittently;
• the device further comprises control and command means for measuring the speed of the air current which goes from the ionization chamber to the detection chamber, and for controlling the air displacement means so as to maintain constant said speed;
• the device further comprises a sensor for measuring the speed of the air current which goes from the ionization chamber to the detection chamber, this sensor being connected to the device for measuring electrical intensity and alarm, which device is designed to take into account the speed measured by the sensor and to correct as a function of this speed at least some of the data which it takes into account to trigger an alarm reaction, so that this device for measuring electric current and alarm is freed from variations in the measured electrical intensity which are due to variations in the speed of the air current;
• the air current between the ionization chamber and the detection chamber opens out at least in part into an ion reserve volume which is not subjected to the electrostatic field prevailing between the anode and the cathode of the detection and which communicates with the detection chamber so as to be swept by any disturbing external air current which would also sweep this detection chamber, before the passage of this air current in at least part of said detection chamber.

Other characteristics and advantages of the invention will appear during the detailed description following of one of its embodiments, given as non-limiting example, with reference to the accompanying drawings.

• la figure 1 est une vue schématique d'un détecteur de fumée selon une forme de réalisation de l'invention,
• les figures 2 à 4 sont des vues de détail schématiques illustrant des variantes de dispositifs de ventilation utilisables dans le détecteur de la figure 1,
• et la figure 5 représente schématiquement une autre variante du détecteur de fumée de la figure 1.

In the drawings:

• FIG. 1 is a schematic view of a smoke detector according to an embodiment of the invention,
• FIGS. 2 to 4 are detailed schematic views illustrating variants of ventilation devices which can be used in the detector of FIG. 1,
• and FIG. 5 schematically represents another variant of the smoke detector of FIG. 1.

In the different figures, the same references denote identical or similar elements.

As shown in Figure 1, the smoke according to the invention conventionally comprises a chamber detection 1 which is delimited between an anode 2 and a cathode 3 forming the outer wall of said chamber detection. The detection chamber communicates with the atmosphere exterior via openings 4, 5 in cathode 3.

The cathode and the anode are connected to each other by an electrical bias circuit 16 which includes a voltage generator 17 generating between the anode and the cathode a potential difference V for example of the order of 20 volts and generally less than 100 volts, so that the detection chamber 1 is the seat of a field electrostatic.

In addition, the air contained in the detection 1 is charged with ions which are generated in a ionization chamber 6 separate from the detection chamber and which are then transferred to said chamber detection via one or more transfer 7, 8, by means of a weak permanent air flow going from the ionization chamber 6 to the detection 1.

Ions are generated in the ionization chamber 6 by electrically ionizing the air between two electrodes 9, 10 of which at least one preferably presents one or more points directed towards the other electrode, these two electrodes being connected to a voltage generator 11.

This voltage generator applies between the two electrodes 9, 10 an electrical voltage, alternating or continuous, which is sufficient to cause the appearance of electric discharges in the air between these two electrodes, which ionizes the air: the voltage in question can be example of the order of 6000 volts, and it is generally at least a few hundred volts.

Furthermore, the permanent air current which goes from the ionization chamber 6 to the detection chamber 1 is obtained, in the example considered, by a small fan 12 consisting of an electric motor 13 causing a propeller 14, this fan being arranged in a duct air inlet 15 upstream of the ionization chamber 6.

The ions thus transferred into the detection 1 are subject to the prevailing electrostatic field in this room: this field attracts negative ions or the electrons contained in the chamber 1 towards the anode 2 and the positive ions contained in said chamber towards the cathode 3.

These ions discharge respectively on the cathode and on the anode, so that an electric current i is generated in the bias circuit 16.

The intensity of this electric current is measured by an electronic device 18 for measuring intensity and alarm, known per se. When the electric current i drops below a certain threshold, the device 18 triggers an alarm reaction on an output 19 connected by example to an alarm center.

So that the permanent air flow coming from the ionization chamber does not disturb the operation of the detection chamber, this air flow preferably has a speed of the order of 1 to 20 centimeters per second in the conduits 7, 8, this speed being by example of the order of 1 to 5 centimeters per second and especially about 2 centimeters per second.

For the same purpose, advantageously, the conduits 7, 8 together provide a section of very low total passage (for example 10 times lower) to the interior section of the ionization chamber taken in the vicinity of these conduits perpendicular to the direction of arrival of the ion-carrying air stream.

In addition, the average air renewal time included in the detection chamber is preferably between 1 and 10 minutes: in other words, the report between the interior volume of the detection chamber and the volume flow of the permanent ion-charged air stream is preferably between 1 and 10 minutes.

In addition, preferably, the smoke detector has an ion reserve volume 20 which is supplied with ions through at least one conduit 8 through which at least passes part of the permanent air stream charged with ions.

As taught in document FR-A-2,594,953, this ion reserve volume 20 is not subject to the field electrostatic which prevails in the detection chamber 1, and it communicates with this detection chamber so as to be swept away by any disturbing outside air flow which would sweep the detection chamber, before the passage of this air flow in at least part of said chamber detection.

This prevents such a disturbing air flow does not cause too great a drop in the current i flowing in the bias circuit 16.

Advantageously, the volume of ion reserve 20 is entirely delimited between on the one hand a metal plate 21 and on the other hand an openwork cage 22 in electrical contact with the plate 21, this plate and this cage constituting anode 2.

In addition, the smoke detector can optionally further include a speed sensor 41 for measure the speed of the continuous air current generated by the fan 12. This speed sensor can be arranged in particular in one of the conduits 7, 8, or possibly in the air inlet duct 15, or the like.

The air speed sensor 41 can be constituted for example by a small propeller which is driven in rotation by the air stream charged with ions and whose rotational speed is detected by an electrooptical system or electric.

This air speed sensor 41 is connected to a electronic control circuit 42 which is designed to control the speed of rotation of fan 12 in function of the measurement carried out by said sensor 41, of so as to keep the speed of the continuous air flow from ionization chamber 6 to the detection chamber 1.

Furthermore, the electronic circuit 42 can if necessary also order the voltage generator 11 and be designed to operate this voltage generator as well as the fan 12 intermittently, to limit the energy consumption of the smoke detector.

For example, the control circuit 42 can be designed to operate the voltage generator 11 and the fan 12 by generating the continuous current of air charged with ions for periods during each a few seconds, for example about 5 seconds, these periods of being separated from each other by stop periods lasting for example around 20 seconds.

In addition, the circuit 42 is connected to the device 18 to stop the measurement of current i during fan off periods 12.

Optionally, circuit 42 could have only function to control intermittent operation smoke detector, in which case this detector would not contain no air speed sensor 41.

Note also that the fan 12 could be replaced by any other ventilation system, mechanical or not.

For example, fan 12 can be replaced by a substantially planar strip 23 which extends longitudinally between on the one hand an upstream end 24 integral of the inlet duct 15 and on the other hand a downstream end 25 free, this strip being actuated by an actuator electric so that the free end of this slat moves perpendicular to the plane of said strip in thereby generating the aforementioned permanent air flow.

In this case, as in the example in Figure 1, the smoke detector could possibly include a electronic control circuit 42 associated if applicable to an air speed sensor 41, to control the operation intermittent smoke detector and / or for control the beat frequency as a function of speed of air measured by the sensor 41, of the strip 23 so to maintain constant the speed of the continuous air stream charged with ions.

In the example shown in Figure 2, the actuation of the strip 23 is piezoelectric.

To this end, the strip 23 is formed by two layers 26, 27 of piezoelectric material separated by a conductive layer 28 which is connected to a voltage source alternative 29.

In addition, the two layers 26, 27 of material piezoelectric are covered to the outside of the strip 23 respectively by two conductive layers 30, 31 connected to ground (zero potential).

The layers 26, 27 of piezoelectric material are oriented to deform longitudinally (elongation or shortening) in phase opposition one with respect to the other under the effect of the alternating voltage generated by the source 29, and the fixed end 24 of the strip is embedded in a support 32 integral with the inlet duct 15.

So under the effect of alternating voltage generated by the source 29, the longitudinal deformations opposites undergone by the layers 26, 27 of piezoelectric material deform the lamella 23 alternately in a direction and in the other by bimetallic effect, which creates alternate beats of the free end of the lamella 25 by generating the permanent air current which brings the ions in the detection chamber.

In the example shown in Figure 3, the actuation of the strip 23 is obtained electrostatically.

For this purpose, the strip 23 is metallic and is connected to ground (zero potential), the fixed end 24 of this strip being pivotally mounted on a support 33 integral with the inlet duct 15.

Furthermore, in the inlet duct 15 are arranged two metal plates 34, 35 which are arranged so as to form a divergent corner downstream from the fixed end 24, without however being in contact with said fixed end 24.

These two plates 34, 35 are preferably pierced of holes 36 and they are respectively connected to a voltage generator device 37 which is intended for alternately apply to either of the two plates 34, 35 a positive voltage, the other metal plate then being brought to zero potential.

Thus, the strip 23 is alternately applied against one or the other of the plates 34, 35 by effect electrostatic, so that its free end 25 undergoes an alternating beat which creates the permanent air current mentioned above, in particular through holes 36.

In the example of Figure 4, the actuation of the coverslip 23 is obtained by magnetic means.

For this, the strip 23 is made of a material magnetic such as iron, and the fixed end 24 of this lamella can for example be rigidly secured to a support 38 fixed inside the inlet duct 15.

An electromagnet 39 is disposed facing the strip 23: this electromagnet is alternately supplied then not supplied by a circuit 40, so that it attracts towards him the strip 23 during each feeding period, in thus forcing this lamella to deform elastically, then the coverslip returns to its initial position by elasticity during the next period of non-feeding of the electromagnet.

The free end 25 of the lamella thus moves following an alternating beat, which generates the current above-mentioned permanent air.

Furthermore, as shown in FIG. 5, the fan 12 could be replaced by a resistor electric heater 43 traversed by an electric current generated by a power supply 44.

In this case, the smoke detector is arranged to so that the continuous air flow follows an upward path from the resistor 43 to the detection chamber 1, so this continuous stream of air is generated by a natural correction phenomenon due to overheating of the air produced by the resistor 43.

In particular, the smoke detector is preferably arranged so that the detection chamber 1, the ionization chamber 6 and the inlet duct 13 are vertically aligned.

In the example shown in Figure 5, the smoke detector has an air speed sensor 41 similar to that already mentioned with regard to FIG. 1, but this sensor is here connected to the electronic circuit 18 of intensity measurement and alarm instead of being connected to a control circuit 42.

The electronic circuit 18 is designed to correct the measurement of the electric current i or to correct the threshold alarm detection based on speed measurement of air carried out by the sensor 41, so that the detection smoke is made substantially independent of variations of the electric current i due to variations in the speed of the continuous air stream charged with ions.

For example, the electronic device 18 can have in memory a correspondence table giving the corrected values of the electric current i or the threshold of alarm detection according to the measured air speed by sensor 41.

• soit un coefficient de correction à appliquer à la mesure réelle du courant i pour obtenir une mesure corrigée insensiblement indépendante des variations de la vitesse du courant d'air continu chargé d'ions,
• soit une valeur du seuil de détection d'alarme qui peut être égale à un certain pourcentage (par exemple environ 80 %) de la valeur normale du courant électrique i en l'absence de fumée.

These correspondence tables can be determined experimentally for a given type of smoke detector, for example by measuring the variations in the electric current i as a function of the variations in the measurement carried out by the sensor 41 in the absence of smoke. The correspondence table can then give, depending on the measurement of the sensor 41:

• either a correction coefficient to be applied to the actual measurement of the current i in order to obtain a corrected measurement imperceptibly independent of the variations in the speed of the flow of continuous air charged with ions,
• or a value of the alarm detection threshold which may be equal to a certain percentage (for example around 80%) of the normal value of the electric current i in the absence of smoke.

dans ce cas, il n'est plus nécessaire d'asservir le fonctionnement du ventilateur 12 ou de la lamelle 23 à la mesure effectuée par le capteur 41.

It will be noted that in the embodiments of FIGS. 1 to 4, the sensor 41 could possibly be connected to the electronic device 18 so as to correct the operation of this device as described above:

in this case, it is no longer necessary to control the operation of the fan 12 or the lamella 23 to the measurement made by the sensor 41.

Furthermore, it will also be noted that in the form of Figure 5, it would be possible to provide an electronic control circuit 42 connected to the sensor 41 and to the power supply 44, to control the operation of electrical resistance 43 to the measurement made by the sensor 41, so as to maintain substantially constant the speed of the continuous flow of air charged with ions: in this case, the sensor 41 would generally no longer be connected to the electronic device 18.

As it goes without saying, and as it results from elsewhere of the above, the invention is in no way limited to those of its embodiments which have been specially envisaged: on the contrary, it embraces all variants, the invention being limited only by the claims.

Claims (13)

1. Air ionization smoke detector, comprising a detection chamber (1) which contains air charged with ions and which communicates with the atmosphere, this detection chamber being the seat of an electrostatic field generated between an anode (2) and a cathode (3) that are connected to each other by a polarizing electrical circuit (16) provided with a voltage generator (17) creating a continuous voltage (V) between the cathode and the anode of less than 100 volts, so that the ions contained in the detection chamber (1) are attracted, some to the cathode (3) and others to the anode (2) , generating in this way an electric current (i) in the polarizing circuit (16), which circuit additionally includes an electrical intensity measuring and alarm (18) device for measuring the said electric current and for triggering an alarm reaction when this electric current undergoes an abnormal variation, characterized in that the ions are generated in at least one ionization chamber (6) which is distinct from the detection chamber (1) and which communicates with the said detection chamber, the ionization chamber also communicating with the atmosphere and having at least two electrodes (9, 10) connected together by an electrical supply circuit capable of generating electrical discharges between these two electrodes ionizing the air, and in that air displacement means (12, 23, 43) are additionally provided in order to create a slight continuous air flow from the ionization chamber (6) to the detection chamber (1) so as to transfer the ions generated in the ionization chamber to the said detection chamber.
2. Smoke detector according to claim 1, wherein the air flow between the ionization chamber (6) and the detection chamber (1) has a velocity of between 1 and 20 cm per second.
3. Smoke detector according to either of claims 1 or 2, wherein the air flow between the ionization chamber (6) and the detection chamber (1) arrives in the detection chamber via a channelling device which comprises at least one conduit (7, 8) and which presents the said air flow with a small total cross section of passage, the detection chamber (1) having, in the vicinity of this channelling device, an internal cross section which, taken perpendicular to the air flow, is at least five times greater than the said total cross section of passage, and the air contained in the detection chamber (1) having an average renewal time of 1 to 10 minutes under the effect of the permanent air flow coming from the ionization chamber.
4. Smoke detector according to any one of the preceding claims, wherein the air displacement means includes a ventilating device (12, 23) displacing air by mechanical action.
5. Smoke detector according to claim 4, wherein the ventilating device (12, 23) is positioned upstream to the ionization chamber.
6. Smoke detector according to either of claims 4 or 5, wherein the ventilating device (12) has rotating blades (14) driven by an electric motor (13).
7. Smoke detector according to either of claims 4 or 5, wherein the ventilation device (12) has a substantially flat blade (23) which extends longitudinally between on the one hand a fixed upstream end (24) and on the other hand a free downstream end (25), this blade being actuated by an electric actuator (26, 27; 34, 35; 39) so that the free end (25) of this blade moves perpendicularly to the plane of the said blade.
8. Smoke detector according to claim 7, wherein the electric actuator (26, 27; 34, 35; 39) is chosen from electrostatic actuators, magnetic actuators and piezo-electric actuators.
9. Smoke detector according to any one of claims 1 to 3, wherein the air displacement means includes a heating element (43) for heating the air upstream to the detection chamber (1), so as to displace this air by natural convection, the smoke detector being formed so that the continuous air flow follows an ascending path from the heating element (43) to the detection chamber (1).
10. Smoke detector according to any one of the preceding claims, having control means (42) designed to cause the air displacement means (12, 23, 43) and the ionization chamber (6) to operate intermittently.
11. Smoke detector according to any one of the preceding claims, additionally having monitoring and control means (41, 42) for measuring the velocity of the air flow that passes from the ionization chamber to the detection chamber, and for controlling the air displacement means (12, 23, 43) so as to keep the said velocity constant.
12. Smoke detector according to any one of claims 1 to 10, additionally including a sensor (41) for measuring the velocity of the air flow passing from the ionization chamber to the detection chamber, this sensor being connected to the electrical intensity measuring and alarm device (18), which device is designed to take account of the velocity measured by the sensor (41) and to correct as a function of this velocity at least some of the data that it takes into account for triggering an alarm reaction, so that this electrical intensity measuring and alarm device (18) is free from variations in the measured electrical intensity (i) that are due to variations in the velocity of the air flow.
13. Smoke detector according to any one of the preceding'claims, wherein the air flow between the ionization chamber (6) and the detection chamber (1) emerges at least partly into an ion reserve volume (20) that is not subjected to the electrostatic field existing between the anode (2) and the cathode (3) of the detection chamber and which communicates with the detection chamber (1) so as to be swept by any external disturbing air flow that would also sweep this detection chamber, before this air flow passes into at least part of the said detection chamber.

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