IL32330A

IL32330A - Ionisation fire alarm

Info

Publication number: IL32330A
Application number: IL32330A
Authority: IL
Original assignee: Cerberus Ag
Priority date: 1968-06-18
Filing date: 1969-06-02
Publication date: 1972-04-27
Also published as: FR2011126A1; NL164407B; HK20378A; NL164407C; NL6909253A; GB1217792A; US3731093A; CH475614A; DE1928874B2; BE734060A; SE340971B; DE1928874A1; IL32330A0; DK125938B

Description

32330/2 lonisation fire alarm CEHB!RUS AG 0:-30638 This invention relates to a fire alarm of the ionisation type, comprising at least one ionisation chamber including at least a source of ionising radiation and two electrodes, one of which may be constructed as a wall of the chamber, of which the interior is in communication with the ambient air* Ionisation fire alarms which serve to detect the presence of combustio aerosols in the ambient air, comprise at least one ionisation chamber which inoludes a radioactive substance for ionising the air within the chamber. A voltage is applied between two electrodes within the chamber, or between a single electrode and the chamber wall, so that a flow of electric current arising from the flow of ions occurs between the electrodes. Upon the entrance of aerosols, or other particles, e.g. smoke or dust, within the chamber, the ionisation current alters.

Swiss Patent Specifications Uos. 297,463 and 355,380 describe an arrangement in which such an ionisation chamber is connected in series with a resistor, while the change of current in the ionisation chamber upon the appearance of aerosols or other particles in the chamber leads to a change in potential of an electrode which is appropriately amplified and employed to give an alarm.

In order to produce sufficiently high sensitivity in such apparatus, care must be taken that the air has, so far as possible, free and unhindered entry to the space between the electrodes. As a rule this is achieved by making the outer wall of the ionisation chamber permeable to air, for example, it is partly open or consists in part of a grid.

In more recent times, transistorised ionisation fire alarms which operate at low voltages have been developed. Swiss chamber voltage of 20V, corres onding to a field strength of less than 5 V/cm. In ionisation chambers of this kind, however, the velocity of the ions moving between the electrodes is so greatly reduced that even a low wind speed or a slight circulation of the air is sufficient to affect the ion current.

Constructions have therefore been proposed in which it is sought to prevent this sensitivity of the ionisation chamber to air currents by means of wind shields of plate or tube form* It has proved, however, that even in these constructions a certain flow of air penetrates the ionisation chamber, which affects the ionisation current.

The object of the invention is to provide a construction of ionisation fire alarm which is protected by screening devices so that the ionisation current within the chamber is not affected by air flowing through it.

According to the present invention there is provided an ionisation fire alarm comprising at least one ionisation chamber which includes at least a radioactive substance and two electrodes, to which chamber the external air has access, wherein the ionisation space of said chamber is surrounded by spaced outer and ; inner geometric walls, the inner surface of the outer wall having a/form similar to that of the outer surface of the inner wall and the two surfaces being disposed substantially parallel with one another, the inner and the outer walls being each pierced by at least one aperture and the aperture or apertures in the outer wall being displaced with respect to the aperture or apertures in the inner wall so that the lateral separation between the margins of the or each aperture in the outer wall and the or each aperture in the inner wall is grea ter than the separation between the two walls.

Preferred features and advantages of the invention will become apparent from the following description taken in conjunction with the accompanying drawings, of which: Figure 1 shows an axial cross-section through an ionisa-tion chamber of cylindrical form; Figure 2 shows a partly sectional side elevation of an ionisation chamber of the kind shown in Figure lj Figure 3 shows a cross-section through an ionisation chamber of more complex form forming a rough approximation to the hemi-spherical; Figure 4 is a partly cut-away perspective view of the ionisationchamber of Figure 3» Figure 5 shows an ionisationchamber built into a surround.

Figures 1 and 2 show an ionisationchamber with a cylindrical sensitive space 1. An insulating baseplate 2 carries an inner electrode* 3 and an outer electrode 4 of mesh form. The sensitive ionisation space 1 is surrounded by two spaced walls, an inner wall 5 which i turn is surrounded by an outer wall 6. The two walls surround the measuring chamber up to the fixing plane 7. In the interior of the chamber there are disposed portions 8, 9 of radioactive material, the ionising radiation emitted by which ionises the air in the space between the two electrodes 3 and 4 and gives rise to an electric current between the electrodes.

The outer wall 6 has a geometrical form similar to that of the inner wall 5# so that the separation between the two walls is approximately the same at all places. The air to be investigated may enter through apertures 10 into the interspace between the two walls and thence through further apertures 11 and 12 in the inner wall into the interior of the chamber. Since the separation between the two since the apertures in the outer wall are displaced with respect to-the apertures in the inner wall so that the separation of the margins of the access apertures is greater than' the separation between the two walls, the flow of air into the ionisation fire alarm is so greatly retarded that it has only a low velocity as it enters into the measuring space.

By the arrangement of access apertures which is employed it is arranged that the additional mobility of the ions which results from the speed of flow of the air is small in comparison with the mobility due to the electric field, that is, that the ionisation current is independent of the speed of flow of the air.

Figures 3 and 4 show an ionisation fire alarm of a somewhat more complex form, which has a rough approximation to the hemispherical. The measuring volume 13 is again surrounded by an inner ¾rall 14 which once more is surrounded at least as far as the fixing plane 16 by an outer wall 15 of similar geometrical form, so that the separation between the internal and external walls is approximately constant.

Through the insulating base plate 17 passes an inner electrode 18 which carries a radioactive substance 19. The inner surface of the inner wall 14 acts as the second electrode.

The air to be investigated can enter into the measurement space through access apertures 20 in the outer wall and access apertures 21 and 22 in the inner wall. The arrangement of the access apertures in relation to one another is once again such that even with high flow velocities of the ambient air the velocity of air flow within the chamber is small.

In this embodiment the access apertures are displaced in relation to one another so that it is not possible to draw a straight line through an aperture in the outer sheathing portion and through any aperture in the inner wall. This is arranged by placing the apertures in the inner wall and in the outer wall on different surfaces which are not parallel with one another but include an angle between them. In this embodiment the apertures 22 are constructed as a wire grid.

The access apertures 20 in the outer sheath are inclined to the axis of the alarm, here the vertical, by an angle which lies between 30° and 60° and is here shown as 45°. It is thus ensured that the ionisationchamber has sufficient sensitivity, remaining as uniform as possible, for both vertical and horizontal air flows.

Many variations are naturally possible as regards the arrangement of the access apertures in the two walls. An adequately low speed of air-flow in the interior of the chamber can always be attained if the inner and outer access apertures are displaced so far from one another that no direct flow can take place through the measurement chamber, and if on the othv.r hand, owing to an approximately constant separation between the two walls, without obstructions, a uniform and so far as possible turbulence-free air-flow occurs in the interspace between the two walls to the inner- access apertures.

In addition the walls may also consist of a plurality of appropriately assembled parts. Advantageously the inner and outer walls are mechanically separate from one another and the outer wall is removable. The apertures in the inner wall advantageously consist wholly or in part of grids.

The embodiments illustrated show ionisation fire alarms which are intended for building into a surface. In this case there may be situated on the other side of the base plate 2 or 17 a further ionisationchamber operating as a reference chamber, which is In loniaation fire alarms which are intended for surface mounting or suspended operation, the other parts of the circuit of the ionisation fire alarm are combined with the measuring chamber into a compact unit. In this case the mounting plane for the two walls lies approximately in the plane of the insulating base plate 2 or 17. One of the two walls for the measuring chamber can in this case serve also ao a casing for the circuit components.

Figure 5 shows a further embodiment in which the outer wallof the measuring chamber, which consists of the base plate 24» the electrode 25 which the radioactive sub-stance 26 and the inner wall 27 acting as the second electrode, is formed ae a part of a trim member 28 for the fire alarm. In thio caee the access aperture in the outer wall is considered ac being formed by the circular aperture 29 in thetrici member 28, which, i order further to reduce the affect of air currents, may be provided with a cover, here not shown, pierced by small apertures* Here a^ in the inner apertures 30 are displaced from the aperture in the trim member by more than the separation between the inner surface of the trim member and the inner wall.

Claims

1. An Ionisation fire alarm comprising at least one ionisation chamber which includes at least a radioactive substance and two electrodes, to which chamber the external air has access, wherein the ionisation space of said chamber is surrounded by spaced outer :nd inner walls, the inner surface of the outer wall having a geometric form similar to that of the outer surface of the inner wall and the two surfaces being disposed substantially parallel with one another, the inner and the outer walls each being pierced by at least one aperture and the aperture or apertures in the outer wall being displaced with respect to the aperture or apertures in the inner wall so that the lateral separation between the margins of the or each aperture in the outer wall and the or each aperture in the inner wall is greater than the separation between the two walls.

2. An ionisation fire alarm in accordance with Claim 1, wherein the separation between the inner surface of the outer wall and the outer surface of the inner wall is generally uniform and is smaller than the dimensions of the ionisation chamber.

3. An ionisation fire alarm in accordance with Claim 1 or 2 in which the inner and outer walls of the ionisation chamber are mechanically separate from one another and the outer wall is removable.

4. An ionisation fire alarm in accordance with any of the preceding claims wherein the spertures in the two walls are ormed in surfaces which are inclined to one another.

5. An ionisation fire alarm in accordance with any of the preceding claims wherein the apertures in the outer wall are so disposed in relation to the apertures in the inner wall that no r - 9 - 23220/2 - ' rectilinear path exists from a point in an outer aperture to any point of an inner aperture,

6. · An lonisation fire alarm in accordance with any of the preceding claims which is axially symmetrical and wherein the apertured surface of the outer wall forms an angle of between 30° and 60° with the axis of symmetry of the fire alarm.

7. Ah ionisation fire alarm in accordance with any of the preceding claims wherein the access apertures in the inner wall consist wholly or in part of grids.

8. · An ionisation fire alarm in accordance with any of the preceding claims wherein one electrode of the ionisation chamber is formed by the inner wall thereof.

9. An ionisation fire alarm substantially as described with reference to Figures 1 and 2, Figures 3 and 4 or Figure 5 of the accompanying drawings. For the Applicants RTNjifiS mz