GB2278719A

GB2278719A - Ionisation smoke detector

Info

Publication number: GB2278719A
Application number: GB9408719A
Authority: GB
Inventors: Jose Luis Gallardo Lamas
Original assignee: Kilsen SA
Current assignee: Kilsen SA
Priority date: 1993-05-17
Filing date: 1994-05-03
Publication date: 1994-12-07
Anticipated expiration: 2014-05-03
Also published as: ES2070748R; ES2070748B1; GB2278719B; ES2070748A2; GB9408719D0

Abstract

An ionisation smoke detector has a radioactive source carried by a support 12 which can be assembled and disassembled independently of other detector parts inside a housing. The support may be a snap fit inside the detector, or, as shown, be slid into place and retained by a latch 16. <IMAGE>

Description

IONIC SMOKE DETECTOR The present invention relates to an ionic smoke detector.

Such ionic smoke detectors form a fundamental part of fire detection and alarm systems.

Fire detectors constitute main elements in fire detection and alarm systems. There exist a great many types of detectors, among which it is worth mentioning ionic, thermovelocimetric and optical detectors.

In the field of detection, the most perfected and efficient type is, in most cases, the ionic detector. When combustion begins, gases and smoke are given off and may also be accompanied by an increase in temperature or by luminous radiation. The function of the smoke detector is to activate the alarm in those cases where the people in the affected area are unable to detect the smoke given off, .e.g., because it cannot be seen or because it is inaccessible.

Among the known varieties of ionic detector it is worth mentioning the following types according to the kind of ionization chamber: Single chamber ionic detector: consisting of two separated, parallel conducting plates with a radioactive source (americium 241 or the like) whose emission sweeps and ionizes the air in the space which separates them. The assembly is enclosed by a housing, thereby forming what is known as ionization chamber, which may be one of the two plates described and which is provided with openings to allow the smoke to enter.

Once installed, the control and operation of said ionic detector is carried out by means of an electronic circuit which applies a voltage across the conducting plates, causing an electric field to be set up between them.

The particles ionized by the radioactive source are attracted or repelled correspondingly by the plates as a result of this electric field, giving rise to a current of ions which the electronic circuit interprets as a weak electric current.

When smoke particles enter the ionization chamber the ions produced attach themselves to their surfaces and cause the ionic current between the plates to decrease, because the smoke particles are much heavier and therefore slower than the ions. This gives rise to a reduction in the electric current in the electronic circuit which is detected by suitable devices which activate the corresponding fire alarm.

Double chamber ionic detector, consisting of two ionization chambers as described above except in that the housing of one of them, known as the reference chamber, has no openings to allow the smoke to enter1 unlike the other, which is the measuring ionization chamber.

The purpose of this arrangement is to use the reference chamber to compensate for variations due to atmospheric agents in the measuring chamber. Therefore, the variations in current in the measuring chamber are compared to the current of the reference chamber.

One simplification of this design consists of joining one of the plates of each chamber, i.e. using three plates instead of four, where one face of one of the plates belongs to one chamber and the other face of said plate belongs to the other chamber. In this case a radioactive source which is active on both faces is normally used, one face for each chamber.

Pseudo-double ionic chamber1 in which the reference chamber is reduced in size and included inside the measuring chamber in order to reduce costs, at the cost of a loss in the reliability of compensation. Three parallel conducting plates are used, the middle plate being perforated to allow the radioactive source, which is normally fitted inside the inner reference chamber, to ionize the air of the measuring chamber, which is larger and has openings, in the housing which encloses the whole assembly and which may be the un-shared plate of said measuring chamber, that are oriented towards it as much as possible. The measurements for the smoke detection are made by comparison, like in the previous case.

The radiation f-rom americium 241 is in the form of alpha particles, soft gamma rays and X-rays with relative energies of 5500-60-18 respectively. This, together with the fact that the number of alpha particles emitted is much higher than that of gamma rays which is in turn much higher than that of X-rays, means that the radiation from americium 241 is taken to be predominantly alpha particles. The average range of the alpha particles, which is the average distance travelled before losing all of their energy, is of the order of 4 cm, taking into account that any surface stops this travel.All of this, together with the fact that the activity of the pellets of radioactive material used is less than 1 uci, results in an absorbed dose due to an ionic smoke detector of the order of 1 pSv/year at 5 cm, much less than the 50 mSv/year allowed by law.

The use of ionic smoke detectors has increased considerably as a result of their superior reliability and efficiency, leading to an increase in the amount of handling of radioactive material. This increase in handling does not give rise directly to a increase in the presence of radioactive material, since said material is periodically recycled and recovered.

One of the drawbacks of ionic smoke detectors arises during their manufacture. The use of radioactive material means that certain minimum routine precautions have to be observed, in particular checks, during the production process. These precautions are taken at all stages involving the radioactive source, which usually means a major part of the process since the source is normally situated at a fairly inner part of the device, among other reasons to ensure that it is well isolated from the outside.

Furthermore, the position of the radioactive source means that maintenance and repair are also difficult during the useful life of the detector. The cleaning of the detector, which is critical due to its importance regarding any other maintenance operation, has to take into account the presence of the radioactive source, which means that not just any method of cleaning can be used. The cleanness of the source is crucial since, due to its small size, it can easily become affected by any dirt which may come into contact with it, and cleaning it is difficult due to its inaccessibility.

Finally, when the detector reaches the end of its useful life, it has to be handled again in order that the radioactive source can be extracted and subsequently recovered and reprocessed by specialized companies before the detector is recycled.

In order to solve, among others, the drawbacks described above, the ionic smoke detector which forms the object of the present invention has been devised.

The ionic smoke detector is provided with a protective housing with a plurality of openings and comprising therein at least two substantially parallel electrically conducting elements. Said elements are arranged in such a way that they form at least one chamber, known as the ionization chamber, in which is provided at least one support with at least one radioactive source arranged in such a way that it emits radiation towards the interior of said ionization chamber.

The ionic detector is characterized in that the assembly and disassembly of the support of the radioactive source is independent of the assembly and disassembly of the other elements within the housing.

This makes the operations of assembling and disassemblina the detector easier.

Preferably, the detector is characterized in that the support of the radioactive source is removably mounted inside the ionization chamber.

Said embodiment allows the support of the radioactive source to be easily arranged inside the ionization chamber.

According to a preferred embodiment the support with the radioactive source can be snap fitted inside the ionization chamber.

This makes it easy to place the radioactive source within the detector or remove it therefrom.

Another preferred embodiment of the detector is characterized in that the support with the radioactive source comprises means of fixing it inside the ionization chamber.

Said means enable the support with the radioactive source to be secured inside the ionization chamber.

Advantageously, the assembly and disassembly of the support of the radioactive source is carried out by sliding it through the ionization chamber.

This enables the support of the radioactive source to be easily inserted into the ionization chamber or extracted therefrom without disassembling the ionization chamber.

In order that the characteristics of the present invention be better understood, the accompanying drawings show by way of non-limiting examples a number of practical embodiments of ionic detectors according to the present invention.

In said drawings, figure 1 is an elevation view in cross section of an installed ionic smoke detector where the support of the radioactive source is slidable, according to the object of the invention.

Figure 2 is an elevation view in cross section of an installed ionic smoke detector where the support of the radioactive source is snap fitted, according to another embodiment of the object of the invention.

Figure 3 is an elevation view in cross section of the sliding support with the radioactive source, according to figure 1.

Figure 4 is a plan view of the support with the radioactive source of figure 3.

Figure 5 is an elevation view in section of the snap fitted support with the radioactive source, according to figure 2.

Figure 6 is a plan view of the support of figure 5.

Figure 7 is an exploded perspective view of the interior of the housing of the ionic smoke detector of figure 1.

As can be seen in figures 1 and 2, the ionic smoke detector of an embodiment of the present invention comprises a protective housing 1 consisting of a base 2 and a lid 3, said lid 3 being provided with a number of holes 4 to allow air to circulate.

Inside the housing 1 is situated a support 5 for the electronic circuit and two casings which form the ionization chamber 6, one of them external 7, close to the lid 3 of the housing, and the other internal 8. The external casing 7 comprises openings 9 and a conducting plate 10.

Inside, said ionization chamber 6 comprises an electrode 11 and a support 12,120 which contains a radioactive source 13 therein. The support 12,120 comprises a conducting element, which can be an interior plate 14 or the support 120 itself depending on the embodiment. The radioactive source 13 is connected to the outside via an opening 15 in the support 12,120.

As can be seen figures 3 and 4, one embodiment of the support 12 of the radioactive source 13 comprises a shape having two parallel faces for sliding said support 12 into its final position. The surface in which the opening 15 is situated is provided with a moveable element 16 for fixing said support 12 inside the ionization chamber.

According to figures 5 and 6, another embodiment of said support 120 comprises a cylindrical element in the centre of which is situated the radioactive source 13 with its corresponding opening 15. The peripheral face is provided with a rib 17 for its snap fitting and the face opposite the face provided with the opening 15 is provided with a grip element 18.

Figure 7 shows a detail of the assembly inside the ionic smoke detector described in figure 1, in which the support 12 with its radioactive source 13 slides, through the opening provided for the purpose, inside the casing 8, and the electrode 11 is arranged inside the ionization chamber which is formed by the casing 7, provided with openings 9 and conducting plate 10, and the casing 8.

Various practical embodiments of ionic smoke detectors, according to the invention, have been described by way of non-limiting examples, although said smoke detectors may be subject to several modifications and variations, obvious to an expert in the field, which are therefore included in the scope of the invention and the corresponding claims.

Claims

1. An ionic smoke detector consisting of a protective housing (1,2,3) with a plurality of openings (4), said housing (1,2,3) comprising inside at least two substantially parallel electrically conducting elements (10,11,14,120), said elements (10,11,14,120) being arranged in such a way that they form at least one chamber (6), known as the ionization chamber (6), in which is provided at least one support (12,120) with at least one radioactive source (13) arranged in such a way that it emits radiation towards the interior of said ionization chamber (6), characterized in that the assembly and disassembly of the support (12,120) of the radioactive source is independent of the assembly and disassembly of the other elements within the housing (1,2,3).

2. A detector according to claim 1 characterized in that the support (12,120) of the radioactive source (13) is removably mounted inside the ionization chamber (6).

3. A detector according to claim 1 characterized in that the support (12,120) with the radioactive source (13) can be snap fitted inside the ionization chamber (6).

4. A detector according to the previous claims characterized in that the support (12,120) with the radioactive source (13) comprises means (16,17) of fixing it inside the ionization chamber.

5. A detector according to any of the previous claims characterized in that the assembly and disassembly of the support (12) of the radioactive source (13) is carried out by sliding it through the ionization chamber (6).

6. A detector substantially as herein described with reference to the accompanying drawings.