US3838283A

US3838283A - Ionization smoke detector

Info

Publication number: US3838283A
Application number: US00298750A
Authority: US
Inventors: I Andersson
Original assignee: Bevaknings AB Securitas
Current assignee: Bevaknings AB Securitas
Priority date: 1971-10-20
Filing date: 1972-10-18
Publication date: 1974-09-24
Anticipated expiration: 1991-09-24
Also published as: BE790355A; SE347377B; GB1341372A; DE2250820A1; NL7214138A; FR2175383A5; NO126977B

Abstract

An ionization smoke detector having an open and a closed chamber which are interconnected by a thin metallic membrane, constituting a common electrode. The membrane is urged against the periphery of an opening between the chambers by means provided on a removable cover defining the open chamber. A single source of radiation is provided in the closed chamber.

Description

United States Patent [191 Andersson Sept. 24, 1974 [54] IONIZATION SMOKE DETECTOR 3,666,954 5/1972 Sasaki et 1 313/54 X [75] In e tor Ing ar An ersso Ba hage 3,676,681 7/1972 Kobayashi 340/237 S Sweden [73] Assigneez Bevaknings AB Securita's Primary Examiner-Archie R. Borchelt stockholmfiammarby, Sweden Attorney, Agent, or Fzrm-Sughrue, Rothwell, Mion,

Zinn & Macpeak [22] Filed: Oct. 18, 1972 v [21] Appl. No.: 298,750

[57] ABSTRACT [30] Foreign Application Priority Data Oct 20 1971 Sweden 13 3 0 4 H 1 An ionization smoke detector having an open and a closed chamber which are interconnected by a thin metallic membrane, constituting a common electrode. 250/381 6 The membrane is urged against the periphery of an [58] Fie'ld 4 389 opening between the chambers by means provided on 31/54f340/237 a removable cover defining the open chamber. A single source of radiation is provided in the closed cham- [56] References Cited UNITED STATES PATENTS 5 Claims 8 Drawing Figu'res 3,500,368 3/1970 Abe 340/237 s IONIZATION SMOKE DETECTOR The invention relates to a smoke detector of the type, which includes a first at least substantially closed ionization chamber, within which a source of radiation is provided, and a second open ionization chamber opening into the ambient atmosphere and which chambers have a common wall. I

The open chamber, in which ionization occurs due to entering smoke gases, is subject to heavy contamination spoiling the measuring results and the function of the detector. Cleaning of the detector is expensive and can only be carried out by skilled persons acquainted with the handling of radiation sources of the types commonly used, i.e., alpha ray radiation sources.

The object of the invention is to provide a new detector comprising inexpensive, easily exchangeable parts, which when contaminated are disposed with and substituted by fresh parts and which detector can be handled without risk of radiation damages.

The above object is realized through the smoke detector defined in the claims.

An embodiment of the invention will be described with reference to the accompanying drawing, in which FIG. 1 is an axial section through a housing, containing electronic components for measuring purposes,

FIG. 2 is an axial section through an insert,

FIG. 3 is an axial section through a membrane,

FIG. 4 is an axial section through a membrane holder,

FIG. 5 is an axial section through a cup shaped cover shown, for measuring the voltages created across the closed ionization chamber and the open ionization chamber. Housing 1 is provided with a measuring electrode 2 for the closed chamber as described below. The

electrode 2 is freely located in an opening 3 in the housing and the opening 3 is surrounded by an annular flange 4, which is provided with an annular abutment surface 5.

An annular insert 6 of a good insulating material is arranged to be pressed between the inner surface of flange 4 and against the abutment surface 5 (see FIG. 7). In the embodiment shown, the insert 6 is provided in its outer wall with one or more annular grooves 7 or the like for enhancing the electric insulation. A source of radiation 8, which preferably is elongated and contains an isotope, Krypton 85 or Argon 39, for example, generating B-radiation is secured into the inner wall of the insert and within the radial boundary surfaces thereof.

FIG. 3 discloses a membrane 9 made of very thin metal sheet or another suitable material allowing B-radiation to pass through, a metal foil, for example, of the type, which is sold under the trade name HAVAR or an aluminium foil, having a thickness of 0.03 mm. The circular foil 9 is pressed onto or welded to a holder ring 10 of metal.

As shown in FIG. 7 the membrane 9 is positioned with its ring 10 engaging the engagement surface 11 of insert 6 and is held under pressure against the insert by means of a membrane holder 12 (FIG. 4) of a plastics material or similar electrically non-conducting material. The membrane holder 12 has an inner cylindrical surface 13 to be brought around an outer cylindrical surface 14 of insert 6 so that the holder is steadily held in place on the insert, the latter in turn being pressed into the housing 1. The axial position of the membrane holder 12 is determined by a stop surface 15, cooperating with the ring 10.

The membrane 9 will accordingly be held under pressure against the insert establishing a substantially air tight sealing. The spacing defined by the membrane 9, the insert 6 and the portion of the housing 1 containing the electrode 2 constitutes the closed ionization chamber and the membrane 9 constitutes together with the electrode 2 the two measuring electrodes of the closed ionization chamber.

The open chamber of the smoke detector comprises a cup shaped cover 16 of a metal or another electrically conducting material, a metallized plastics, for example. The cover 16 has an inner cylindrical surface 17, which is pressed around the membrane holder 12 so as to be positioned as shown in FIG. 7. The cover 16 has through flow openings 18 around its periphery through which air is allowed to flow to the inner of the chamber. The membrane 9 forms together with the cover 16 the two measuring electrodes of the open chamber. The membrane holder 12 has been designed such that an electrical insulation as good as possible will be obtained between the outer electrode 16 and the intermediate electrode 9.

The electrical wiring from membrane 9 and cover 16 to the electronic measuring circuits is not shown in the drawing, but it is obvious to those skilled in the art how the wiring is to be arranged After a longer period of operation of the detector causing contamination of the membrane, the membrane holder and the cover, the cover and the membrane holder with the membrane are removed and substituted by clean components. To facilitate the change of components the membrane ring 10 is preferably bound by an adhesive to the membrane holder, the latter being in turn bound to the cover, enabling these components to be removed as an integral unit. The risk of radiation damages has been minimized, because the new detector allows a low radiation source to be used. On the substitution of contaminated parts, the cover is preferably handled by a pole provided with a suction member or the like, so that a close contact with the radiation source is avoided.

It is obvious that the embodiment shown can be modified in different ways. Instead of a press fit connection between the cover and the membrane holder, a thread connection could be used. It is further not necessary to use a separate insert 6, because the radiation source can be mounted in the housing 1 in any other convenient way.

The invention enables the production of very small detectors, which in turn enables the use of a small strength B-radiation source, which can be lower than I millicurie. Detectors including B-radiation sources hitherto used have a source strength of the order 20 millicurie. A preferred embodiment, which enables an exact determination of the amount of radiating material, the radiation source comprises a thin walled, air tight tube of aluminium, having a predetermined diameter and length, which is filled with a gaseous isotope and which has ends sealed by welding. The axis of the tube is parallel with the plane of the membrane and the radiation source is close to the membrane.

Another modification consists therein that the membrane 9 is joined to another membrane 19 (FIG. 8), having its periphery sealingly connected to the membrane 9. The radiation emitting substance, Krypton, for example, is confined in the spacing between the two

membranes

9 and 19. The membrane 19 could be smaller than the membrane 9 and positioned centrally on the latter.

I claim:

1. An ionization smoke detector comprising,

a. A housing having a first ionization chamber closed to ambient air and an opening therein,

b. a single source of radiation located in said first ionization chamber,

0. an air permeable cover having a second ionization chamber therein releasably attached to said housmg,

d. a thin metallic membrane covering'said opening in said housing, said membrane being a radiation per meable wall and a measuring electrode common to said first and second ionization chambers, and

e. means affixed to said air permeable cover to urge said membrane against the peripheral edge of said opening to provide air tight sealing of said first ionization chamber.

2. The smoke detector, of claim 1 wherein a substantially air tight insert member (6) is connected to the housing and in which insert said opening is provided.

3. The smoke detector according to claim 1 wherein the source of radiation includes the isotope Krypton or Argon 39.

4. The smoke detector according to claim 1 wherein the source of radiation comprisesa closed tube, filled with a gaseous isotope emitting B-radiation, positioned adjacent the membrane and having its longitudinal axis substantially parallel with the plane of the membrane.

5. The smoke detector according to claim 1 wherein said membrane (9) is gas tight secured to a second membrane (19) and that the radiation emitting substance is provided between said two membranes.

Claims

1. An ionization smoke detector comprising, a. A housing having a first ionization chamber closed to ambient air and an opening therein, b. a single source of radiation located in said first ionization chamber, c. an air permeable cover having a second ionization chamber therein releasably attached to said housing, d. a thin metallic membrane covering said opening in said housing, said membrane being a radiation permeable wall and a measuring electrode common to said first and second ionization chambers, and e. means affixed to said air permeable cover to urge said membrane against the peripheral edge of said opening to provide air tight sealing of said first ionization chamber.

3. The smoke detector according to claim 1 wherein the source of radiation includes the isotope Krypton 85 or Argon 39.

4. The smoke detector according to claim 1 wherein the source of radiation comprises a closed tube, filled with a gaseous isotope emitting Beta -radiation, positioned adjacent the membrane and having its longitudinal axis substantially parallel with the plane of the membrane.