GB2044987A

GB2044987A - Ion detector

Info

Publication number: GB2044987A
Application number: GB8003981A
Authority: GB
Original assignee: Institut National des Radioelements IRE
Current assignee: Institut National des Radioelements IRE
Priority date: 1979-02-14
Filing date: 1980-02-06
Publication date: 1980-10-22
Also published as: FR2449282B1; FR2449282A1; NL8000858A; GB2044987B; ES8100490A1; BE874157A; ES488603A0

Abstract

An ion detector has a reference chamber 2 and a measuring chamber 1 the latter comprising a first zone 13 wherein is generated ionization by an ???-radioactive source 9, and a second zone 14 wherein are arranged electrodes 5,8 to generate an electric field stronger than the field in said first zone, openings 6,7 being so provided and distributed in the measuring chamber that an air draft reaching the detector will move the ions formed in the first zone towards the second zone, whatever the draft direction. Dimensions are detailed. Plate 4 carries a printed circuit. Electrode 5 may be of stainless steel netting and casing 16 of plastics material. <IMAGE>

Description

SPECIFICATION lon detector This invention pertains to an ion detector with two ionization chambers, each chamber having at least one wall which is a good electricity conductor and comprises a first electrode, a second electrode and an a-radioactive source which are received within said chamber, the wall from the one chamber, the so-called measuring chamber being at leastlypartly apertured, while the wall from the other chamber, the so-called reference chamber, is closed.

The known detectors of the above-described type, which are notably used in fire-warning devices, have the drawback that they cause false alarms due to air drafts which as they spread through the measuring chamber, take along those ions which are formed therein, which results in lowering the mean number of ions being collected by the electrodes and thereby in lowering the current in the measuring chamber circuit; such current lowering being similar to the lowering that would be caused by smoke.

The invention has for object to obviate such drawback and to provide an ion detector which is particularly stable in the presence of air drafts.

For this purpose according to the invention, the measuring chamber comprises a first zone inside which is arrange the a-radioactive source and wherein is generated the ionization, and a second zone inside which are arranged the electrodes to generate an electric field which is stronger than the field inside the first zone, the openings in the measuring chamber being so provided and distributed that an air draft reaching the detector will cause the ions formed inside the first zone to move from the first zone to the second zone, whatever the air draft direction.

In an advantageous embodiment of the invention, said second zone of the measuring chambersur- round the first zone from said chamber.

In a particularly advantageous embodiment of the invention, both electrodes from the measuring chamber are so shaped that the walls thereof substantially correspond to the walls of two solids, with the same nature and common axes, the bases of which lie in substantial parallel relationship, said solids the one of which is enclosed within the other, having different heights and being so arranged relative to one another that the spacing between the solid side walls, the volume between said side walls corresponding to said second zone, is shorter than the spacing between corresponding bases of said solids, the volume between both said bases corresponding to said first zone, said openings provided in that measuring chamber wall which comprises the first electrode being distributed on the one hand to open in the volume between the side walls from both solids and on the other hand, to open in the volume between both said bases which corresponds to said first zone, the areas of those openings which open in said volumes having a ratio of about 8/1.

Other details and features of the invention will stand out from the following description given by way of non limitative example and with reference to the accompanying drawings, in which: Figure 1 is an elevation view with parts broken away, of the ion detector according to the invention.

Figure 2 is a plan view with parts broken away, corresponding to Figure 1.

In both figures, the same reference numerals pertain to similar elements.

The ion detector according to the invention as shown in the drawings comprises two ionization chambers, a so-called measuring chamber 1 and a so-called reference chamber 2, which are arranged inside a housing 3 on either side of a plate 4 that comprises a printed circuit to which the electrodes from chambers 1 and 2 as well as the voltage supply and the warning device are connected. The wiring diagram of the detector and the warning device is known and consequently it will not be described nor shown. The measuring chamber comprises a wall 5 which is a good electricity conductor that comprises a first electrode and which is provided with openings 6 and 7, a second electrode 8 and an a-radioactive source 9, for example an americium 241-source, which is associated to electrode 8.The reference chamber 2 comprises a closed wall 10 which is a good electricity conductor that comprises a first electrode, a second electrode 11 which is arranged inside the first electrode, and an a-radioactive source 12 which is fastened to electrode 10. To insure the detector stability even in the presence of air drafts and to prevent false alarms being caused thereby, the measuring chamber 1 is so arranged as to be divided into two zones 13 and 14, a first zone 13 (bounded in Figure 1 by the dot-and-dash lines 15) inside which is located the a-radioactive source 9 and inside which is generated the ionization, and a second zone 14 surrounding the first zone 13, inside which the electrodes 5 and 8 are so arranged as to generate an electric field which is stronger than the field inside said first zone 13, said openings 6 and 7 from the measuring chamber being so designed and distributed that an air draft reaching the detector under any angle, will move the ions formed inside zone 13towards zone 14. In the embodiment as shown in the drawings, the electrode 5 is comprised of a netting from stainless steel and it is covered by a casing 16 from plastic material in which said openings 6 and 7 are provided.The electrodes 5 and 8 can have various shapes as long as the spacing between the electrodes inside zone 13 is larger than the spacing between said electrodes in zone 14, said openings 6 and 7 being so designed as to open respectively in zones 14 and 13, the location of said openings 6 and 7 being arranged and the ratio between the areas thereof being so designed that thejonsformed in zone 13 move underthe action of an air draft in measuring chamber 1, caused by an air draft that reaches the detector under any angle, towards zone 14.It is thus possible to obtain a detector the measuring chamber 1 of which has on the one hand, in the absence of air movement, a weale electric field inside chamber zone 13, which insures a very good sensitivity to smoke and on the other hand, in the presence of air movement, a stronger electric field inside zone 14 than the field inside zone 13, which allows to collect those ions formed inside said zone 13 before said ions escape from zone 14, and to prevent false alarms due to a loss of ions escaping from the measuring chamber under the action of air movement, such ion loss causing as well as smoke, a lowering of the current in the measuring chamber.

In the embodiment as shown in the drawings, the electrodes 5 and 8 are so shaped that the walls thereof correspond substantially to the walls from two solids in the shape of co-axial truncated straight revolution cones the great bases of which lie substantially in the same plane, the apex angle 19 from that truncated cone which comprises electrode 5 being larger than the apex angle 20 from that truncated cone which comprises electrode 8, the diameter of the small base 17 from said latter truncated cone being larger than the diameter of the small base 18 from that truncated cone which comprises electrode 5.Moreover the spacing 21 between the small bases 17 and 18 of said truncated cones is larger, in the range of four times larger than the spacing 22 between the side walls 23 and 24 form said truncated cones, the radioactive source 9 being arranged in the center of said small base 17, the openings 6 being uniformly distributed in casing 16 to let same open on the one hand, on the circumference of that truncated cone comprising electrode 5 adjacent the great base thereof, and on the other hand adjacent the small base 18 from said truncated cone, the total surface area of said openings 6 being about 280 mm2 for a total surface area of said openings 7 of about 35 mm2, with spacings 21 and 22 being respectively about 25 mm and 6 mm.

It must be understood that the invention is in no way limited to the above embodiments and that many changes can be brought therein without from the scope of the invention as defined in the appended claims.

Claims

1. Ion detector with two ionization chambers, each chamber having at least one wall which is a good electricity conductor and comprises a first electrode, a second electrode and an a-radioactive source which are received within said chamber, the wall from the one chamber, the so-called measuring chamber being at leastly partly apertured, while the wall from the other chamber, the so-called reference chamber, is closed, in which the measuring chamber comprises a first zone inside which is arranged the a-radioactive source and wherein is generated the ionization, and a second zone inside which are arranged the electrodes to generate an electric field which is stronger than the field inside the first zone, the openings in the measuring chamber being so provided and distributed that an air draft reaching the detector will cause the ions formed inside the first zone to move from the first zone to the second zone, whatever the air draft direction.

2. Ion detector as defined in claim 1, in which said second zone of the measuring chambersur- rounds the first zone from said chamber.

3. lon detector as defined in claim 2, in which both electrodes form the measuring chamber are so shaped that the walls thereof substantially correspond to the walls of two solids, with the same nature and common axes, the bases of which lie in substantial parallel relationship, said solids the one of which is enclosed within the other, having different heights and being so arranged relative one another that the spacing between the solid side walls, the volume between said side walls corresponding to said second zone, is shorter than the spacing between corresponding bases of said solids, the volume between both said bases corresponding to said first zone, said openings provided in that measuring chamber wall which comprises the first electrode being distributed on the one hand to open in the volume between the side walls from both solids and on the other hand, to open in the volume between both said bases which corresponds to said first zone.

4. lon detector as defined in claim 3, in which the spacing between said side walls is about four times shorter than the spacing between said bases.

5. lon detector as defined in either one of claims 3 and 4, in which the total surface area of those openings which open between the side walls from both solides is about thirty times larger than the total surface area of those openings which open in the space between both said bases.

6. Ion detector as defined in either one of claims 3 and 4, in which the total surface area of those openings which open between the side walls from both solids is about 8 times larger than the total surface area of those openings which open in the space between both said bases.

7. lon detector as defined in any one of claims 3 to 6, in which both said solids are in the shape of coaxial truncated straight revolution cones, the great bases thereof lying in the same plane.

8. Ion detector as defined in claim 7, in which the apex angle of that truncated cone comprising the first electrode is larger than the apex angle of that truncated cone comprising the second electrode, the diameter of the small base from said latter truncated cone being at least equal to the diameter of the small base from that truncated cone comprising the first electrode.

9. lon detector as defined in claim 8, in which the a-radioactive source is arranged in the center of the small base from that truncated cone comprising the second electrode, the openings provided in the first electrode being uniformly distributed on the one hand on the circumference of that truncated cone comprising the first electrode adjacent the great base thereof, and on the other hand adjacent the center of the small base from said truncated cone.

10. Ion detector as described above with refer ence to the accompanying drawings.