IL44507A - Ionization-type fire sensor - Google Patents

Abstract

To permit free entry of ambient air, without clogging, dust deposits, and for easy cleaning of the sensor, the housing for the sensor is formed in at least two parts separated from each other by an essentially continuous ring-shaped slit, the parts of the housing in the vicinity of the slit being formed to provide a labyrinth path for passage of air through the slit into the chamber, in which the direction of air flow is deflected from an essentially straight path; one of the electrodes of the sensor is an air-pervious grid or mesh located to prevent mechanical access to the other electrode. [US3908957A]

Description

Ionization-type fire sensor Cerberus AG., C.42655 The present invention relates to an ionization-type fire sensor in which a housing is provided which has openings to permit the entry of ambient air to an ionization chamber, containing two electrodes and a radioactive source to provide an ion current. The ion current changes when smoke or fire · aerosols are present in the ambient air.

Ionization-type fire sensors must meet many, simultaneously applicable requirements. Their sensitivity should be high, that is, the ion current should change upon presence of even a very small smoke or fire aerosol concentration within the ionization chamber; the change in the ion current should be marked, and high. Using a low field strength result¾ in high change in ion current upon presence of smoke, or fire aerosols.

Ionization fire sensors are quite sensitive to air currents. In the Israel Patent No. 32 330 it has already been proposed to slow any air stream within the ionization chamber by means of deflecting elements or the like. The ionization fire detector described in this patent has a housing which includes an outer and inner portion having a similar geometric shape and being disposed substantially parallel to one another. Both portions are provided with apertures which are displaced such that the lateral separation of the apertures in the outer and inner .portion is greater than the separation between the two housing, portions themselves. Thereby the air entering through .the apertures of the outer housing portion is deflected and forced to flow substantially along the space between the two housing, portions and only a small amount is entering the interior of the ionization chamber. A fire detector of this design has proved to be extremely insensitive to air streams but on the other hand it has the disadvantage that with slow air circulation which may occur with smouldering fires, smoke is entering into the ionization chamber so slowly that releasing of an alarm signal is delayed to a not admissible extent. Furthermore, dust which is carried by air flowing into the chamber is apt to deposit in the space between the two portions and at the bottom of the inner portion within the ionization chamber. Therefore, certain zones of the fire sensor housing are coated with dust and dirt relatively quickly and ionization fire detectors of this kind therefore require frequent cleaning of the housing to remove deposited dust and dirt which means a good deal of labour and is rather expensive.

Other ionization type fire sensors as described in U.S. Patent 3,710,110 are provided with a housing having two slot-like annular apertures in the upper portion and on the bottom of the sensor housing. Although a direct air stream across the middle of the ionization chamber is prevented it is still possible that a certain part of the air stream is flowing in diagonal direction through the chamber so that strong air streams are still affecting the ion current. Furthermore, during the cleaning of the housing the danger is ever present that sensitive parts of the electric circuit of the fire sensor are damaged. For example low voltage ionization fire sensors which use a field effect transistor in their sensing circuit the gate electrode of which is connected to the inner electrode of the ionization chamber are particularly subject to darnag Touching the inner electrode of the sensor may lead to destruction of the field effect transistor. It has therefor 44507/2 been found necessary to protect the field effect transistor upon opening of the housing by suitable circuitry for example by protecting switches in order to avoid destruction thereof. This circuitry is additional to the regular sensor circuitry and hence adds costs.

It is an object of the present invention to provide a sensor which is so designed that it is less subject to contamination by dirt and dust, can be cleaned simply, and is so arranged that the danger of damage of components thereof by touching sensitive elements is essentially avoided, without decreasing the sensitivity of the sensor, or increasing the tendency to give false alarms based on air currents.

Subject matter of the present invention: Briefly, the housing is formed in at least two parts, so arranged that a portion thereof, at least, can be removed. The openings for the ingress of air are a ring-shaped slit, whose circumferential \ extent is interrupted as little as possible (at best by holding clips, screws or the like) . Means are provided to brake and deflect air passing through the slit; these means are, ■ preferably, a labyrinth arrangement interiorly of the slit. One of the electrodes is formed as an air-pervious grid or mesh and is so. located within the housing that it protects the other electrode which, then, can be connected to the control electrode of a field effect transistor (FET) .

The invention will be described by way of example with reference to the accompanying drawings, wherein: Fig. 1 is a longitudinal sectional view through one embodiment of a sensor which, in plan view, may be circular; and Fig. 2 is a highly schematic longitudinal view of another embodiment, in which the opening to the housing is differently arranged.

The ionization fire sensor of Fig. 1 has a socket part 1 and a sensor part 2. Socket 1 is formed with a lateral opening in which an indicator lamp 3 , for example a light-emitting diode, is located.

The sensor part 2 has a cylindrical or slightly conical housing 4 and a bottom cover 5. Housings 4 and 5 are separated by a ring-shaped slit 13 which is, preferably, no more than 1/2 cm wide. Air can pass through slit 13 into the interior of the housing. The inside of the cover 5 has an upwardly projecting baffle 12. The height of the baffle 12 is, preferably, at least 1 cm, and it extends cylindrically around the inside of the slit 13 to deflect air entering into the sensor upwardly in vertical direction.

The distance of the baffle 12 from housing 4 may be approximately in the order of the size of the width of the slit 13, somewhat greater or somewhat less. It need not be always of the same width, but can increase in size towards the top, for example if the baffle 12 is cylindrical and the housing 4 is conical (Fig. 1). The distance towards the upper opening may also decrease, for example if the baffle 12 is formed conically with a greater conical angle than the conical angle of housing 4. The bottom cover 5 and the baffle 12, which may be integral therewith, for example by being welded or soldered to the cover 5, are held or secured to the housing 4 by holding elements connecting the cover 5 and the housing 4 in such a manner that the ring-shaped labyrinth slit permitting ingress of air is interrupted as little as possible. Cover 5 may be permanently secured to the housing shield 4, for example by a few wire pins connecting the housing 4 to the cover 5; or it may be removably attached, for example by three equidis- tantly spaced circumferential screws of which one is shown at 41, similar to the holding arrangement for lamp globes in lamp sockets. Any other suitable attachment of the housing bottom 5 to the surrounding shield 4 may be used, rigidly or removably interconnecting these two parts, if care taken that the slit 13, or the inner portion of the slit is interrupted as V .5L. little as possible. iA't&^e- -t - co«. - »"-«^ iou. -irje. " Tne a^-r inlet system has particularly advantageous characteristics if the following conditions are met: 5 (aj the height H of the baffle 12 should be at least twice as high as the width B of the slit 13; (b) the maximum dimension of the distance A between the baffle 12 and the housing 4 should be at most twice the dimension of the width B of the slit 13; 10 (c) the depth T of the slit 13 should be at least half the width B of the slit 13.

In one form of the invention which has been found suitable, and which is given by way of example, the width B of the slit 13 was 3 mm; the height H of the baffle 12 15 was 9.5 mm; the maximum distance A between the baffle 12 3.5 mm; the depth T of the slit 13 was of the housing as the root of the ^e rel "tionship of the above dimensions, to each 20 other, and particularly the values of H, T and A with respect to the width B are of importance, rather than the absolute dimensions.

Within housing 4, a carrier plate 6 made of insulating material is secured. Carrier plate 6 holds all 25 other elements of the sensor. A central bore within the carrier plate has a metallic bolt 7 located therein, carrying a disk-shaped electrode 8 on which a radioactive source 9 , such as a preparation including Americium 241 is located. A wire mesh, entirely surrounding electrode 8 and formed to be cup-shaped is located at the bottomside of carrier plate 6 to electrode form counter electrode 1 1 . Wire mesh/1 1 entirely surrounds the inner electrode 8 and prevents contact with the inner electrode 8 , for example accidentally. The center electrode 8 and the counter electrode 1 1 define, between themselves, the ionization chamber 1 0. An electrical circuit, which may, for example, be an integrated circuit or otherwise an entirely encapsulated circuit 1 4 , is located above the carrier plate 6 . Electrical circuit 1 4 preferably includes an FET input transistor, and evaluates changes in ionization current within the chamber 1 0. A chamber, entirely or almost entirely separated from outer atmophere can further be provided in order to form a reference ionization chamber connected, as known, in series to the ionization chamber 1 0 . The carrier plate 6 is preferably secured to the socket part 1 , for example by holding bracket 42 molded into the carrier plate 6 or otherwise attached thereto,which brackets, in turn, may be spot-welded, or suitably connected to the socket part 1 , to securely hold the plate 6 to the socket part 1. These holding brackets 42 may, also, be removably located within the socket, so that the carrier plate 6 can be removed as a unit with the housing from the socket. In this case, suitable connectors must be provided from the electrical circuit 14 to a connecting cable 14a which, in turn, connects to outside terminals at the side of the sensor, as schematically shown.

In order to keep dust and dirt from the interior of the ionization chamber 10, the housing portion, or shield 4 and the bottom cap 5 are made of metal and placed at a voltage which is different from that of the voltage applied to the mesh or grid electrode 11. A connection, therefore, is brought out from electrode 11 through the insulating plate 6 to terminal 111 which, preferably, is connected to a source of positive voltage. Alternatively, the entire housing may be made of non-conductive material, so that dust particles will precipitate on the housing, due to static charge of the material, rather than being carried into the ionization chamber.

In a preferred form, the cover 5 of the housing is removable from- the shield part 4 in order to facilitate cleaning.

The shield part 4 is retained on the plate 6 by suitable means, for example by slight depressions formed circumferentially around the plate 6, in which punched-in resilient projections, formed in shield part 4, can engage. Thus, shield part 4 can be removed from the plate 6, as a ' unit with the bottom cover 5, if desired; for assembly, shield part 4 is merely pushed on the plate 6 until the snap openings engage. Alternatively, the plate 6 may be formed with a ring groove, in which spring fingers engage, or projections engaging bayonet-holes in the shield 4 which are then covered internally by the plate 6 , or by other suitable holding and separable engagement arrangements.

The construction of the ionization-type sensor provides a slit-ring-shaped opening 1 3 in which the air is braked and dammed immediately behind the opening by the baffle 1 2 , and air flow is deflected in vertical direction. The deflection is so arranged that the air flow is tangential to the grid or mesh 1 1. The relatively heavy dust particles will largely precipitate in the damming region; the much lighter smoke particles will, hardly, precipitate however and will be carried into the chamber 1 0. Dust particles, therefore, will precipitate on the housing, or at least in the immediate vicinity of the entry slit 1 3 and will not penetrate into the ionization chamber 1 0. Ingress of smoke is, however, hardly impeded. By removing housing 4 from the carrier plate 6 , or even by merely removing the bottom cap 5 from the housing 4 , dust which has precipitated can be easily removed, thus permitting easy cleaning of the sensor. Rather than screws 41 , resilient snaps or bayonet-slits in the housing 4 , engaging bent-over lugs projecting from bottom cap 5 or the baffle 1 2 may be used for simple disassembly. The mesh or grid 1 1 protects the center electrode 8 connected to the control - electrode of an FET in the circuit 14 when the housing 4, or the cover 5 has been removed from the sensor. Thus, upon cleaning, damage to the sensitive electrical circuit and particularly to the FET is prevented.

In the embodiment of Fig. 2, the housing 21 is separated by a ring-shaped slit 22 from an attachment surface 27. The measuring ionization chamber 20 has a central electrode 23 and an outer electrode 24 which is formed as a fine mesh grid. The housing 21 is removable from the base plate 26 which, preferably, is of insulating material, such as a plastic block. The attachment of the housing 21 to the by means of pins molded outwardly, passing through holes and slits formed in the housing 21, in form of a bayonet connection. The air which passes into the chamber 20 is dammed and deflected upon entry through the slit 22 by the side surfaces 25 of the insulating block 26. This construction, also, provides for damming and deflection of air passing into the sensor. Dust will usually mostly precipitate at the inner wall of the housing 21. Mesh electrode 24 rejects dust partides,without preventing passage of air, carrying smoke or fire aerosols along. The mesh or grid electrode 24 further protects the field effect transistor connected to center electrode 23. The electrical circuit is preferably contained within a hollowed portion of the block 3 , and then sealed therein by means of a casting compound. The electrical connections have been omitted from Fig. 2.

The metal portions of the housing - if metal, is used -for example socket 1 , are preferably grounded. If the shield 4 (Fig. 1) is metal, a metallic connection is made between the socket 1 and the shield 4, for example by engagement of the socket^'with a depressed resilient projection 1b formed on a cross plate 1a of the socket 1 .

The block or plate 6 likewise is attached to cross plate' la, which can also support such releasable plug-socket connections for the electrical circuit as may be needed. The luminescent diode 3 is shown connected in circuit with the sensor.

For a circuit for use with a sensor, reference is made to copending application Serial 374,795 , filed June 28, 1973, and assigned to the assignee of the present application.

Various changes and modifications may be made within the scope of the inventive concept.

Claims (1)

1. CLAIMS 1. Ionizatlon-type ire sensor having a housing defining an ionization chamber^ an ion source and two electrodes in the chamber, the ion current between the electrodes changing upon presence of fire or smoke aerosols within the chamber* wherein the housing comprises at least two parts, said parts bein separated from each other by an essentially continuous ring-shaped slit, the parts of the housing, in the vicinity of the slit, formin a labyrinth path for passage of air through the slit into the chamber, in which the direction of air flow is deflected from an essentially straight path; at least one of aid parts bein removable} ■ and one of said electrodes comprises an air-pervious grid or mesh located to prevent mechanical access to the other electrode and hence accidental contact with the other electrode* 2. Sensor according to claim 1, wherein an evaluation circuit including a field effect transistor ( ET) is provided., the other electrode being connected to an input electrode of the FET. 3. Sensor according to claim 1, wherein th housing comprises a sleeve-like shield and a cover, the slit and hence the entry opening for air to the ionization chamber being formed between the end of the sleeve and the cover. 4. Sensor according to claim 3, wherein the cover is removable and separable from the feeve. 5. Sensor according to claim 5, wherein the slit S between the sleeve and the cover has a width of about 1/2 cm or less, 6. Sensor according to claim 3» wherein the inner side of the cover is formed with a projecting baffle to deflect air passing into the slit. 7. Sensor according to claim 6 , wherein the projecting baffle is an essentially cylindrical, o slightly conical projecting haffle ring. 8. Sensor accordin to claim 6 , wherein the projecting baffle has a height of a least 1 cm. 9. Sensor according to claim 6, wherein the height (H) of the projecting baffle is at least twice the width (jS) of the slit. 10. Sensor according to claim 9» wherein the maximum distance (A) of the baffle from the interior of the housing sleeve is at most twice the width (B) of the alit and the depth (T) of the slit is at least half the width (B) o the slit. according to claim 1» wherein the housing is formed as a ca^ the senso is secured! to an attachmen plate, the slit bein formed b a; ring-shaped ga between the cap and said.attachment plafte. 12. Sensor according to claim 11, wherein the senso includes a block of insulating material located inside said cap and ormed with surrounding lateral surfaces located behind the gap and forming the deflecting surfaces for air passing through said path. 13. Sensor according to claim 12 j, wherei the mesh or grid electrode is secured to the block of insulating material. 14. Sensoa?r according to c&aim 1, , wherein the housin isf of metal, and the grid or mesh electrode and the housing are connected across a source of voltage. 15. Sensor accordin to claim 1, wherein the housing is ormed of electrically non-conductive material. 16. Sensor according to claim 1, wherein the housing parts defining said labyrinth path, and the grid electrode, are 17. Sensor according to claim 1, wherein an essentially 1 horizontal plate-like support block of insulating material is provided, said electrodes being supported from said support block, and said grid or mesh-shaped electrode being shaped i form of an inverted cup depending from said plate-like block to define within said oup the ionization chamber, the other electrode being located within the chamber and enclosed by said cup-shaped grid or mesh electrode. 18· Sensor according to claims3 and 17, wherein said sleeve is removably attached to the block of insulating material. For the Ap iQnt*