GB2182198A

GB2182198A - Ionisation detection chamber

Info

Publication number: GB2182198A
Application number: GB08526892A
Authority: GB
Inventors: Stephen Henry Ellwood; David Appleby; Stephen Brown
Original assignee: Gent Ltd
Current assignee: Novar Systems Ltd
Priority date: 1985-10-31
Filing date: 1985-10-31
Publication date: 1987-05-07
Anticipated expiration: 2005-10-31
Also published as: EP0221770A2; GB2182198B; EP0221770A3; DE3683827D1; EP0221770B1; GB8526892D0

Abstract

An ionisation detection chamber for detecting the presence of small particulate solids, such as smoke particles, carried in a gaseous stream comprises an electrically insulating base 1, a first, measuring, electrode 2 carried by the base, means for ionising gas associated with a central region of the measuring electrode, eg. radioactive source 14 in central recess 13, a second, negative, electrode 4 comprising a metal grid disposed over the base and together with the base defining the chamber and a third, positive, electrode 3 located in register with the ionisation means 14 and nearer to the measuring electrode 2 than to the negative electrode 4, wherein the third electrode 3 comprises at least one rod-like element 25 extending generally parallel with the measuring electrode and carried by an electrically insulating member 22 supported by the base 1. <IMAGE>

Description

1 1 50 GB 2 182 198 A 1

SPECIFICATION

Ionisation detection chamber This invention relatesto an ionisation detection 70 chamber for detecting the presence of small particulate solids,such as smoke particles, carried in a gaseous stream.

It is now commonplace to construct such chambers with three electrodes; an ionisation 75 source being attached to one electrode and a second electrode being disposed nearer the ionisation source than the third electrode. Depending upon the detailed construction of the chamberthe ionisation source may be attached to the measuring electrode or one of the other electrodes. In all such arrangementsthe chamber is effectively divided into two intercommunicating sub-chambers one of which provides much greater sensitivity of detection than the other and the likelihood of a spurious alarm is reduced as compensation for changes in temperature pressure and other non smoke conditions is provided. At regular intervals it is necessaryto clean the electrodes and insulation spacers to ensurethat a predetermined sensitivity is maintained. This sensitivity can be seriously affected by dirt and the bodies of insects.

In one prior construction a flat negative electrode is mounted on an insulated basewith a wafer-like ionisation source disposed centrally thereon. A positive electrode in the form of a dome-like metal partwhich may be a grid or perforated sheet is disposed overthe base to define the chamber and a measuring electrode in theform of an apertured disc is disposed above and nearerto the negative electrode than the grid with the aperture in the disc in register with ionisation source. Such an arrangement is operationally satisfactory butwhen cleaning orservicing is required both the grid and the measuring electrode must be removed. Reassembly 105 frequency results in a change of electrode spacing which affects sensitivity and reduces long term reliability.

In another arrangement typified in U.K. patent specifications 1581236 and 1603702, granted to predecessors of the present applicants, the positive electrode is in the form of a rod extending through an insulated mount carried bythe 9 rid which in this case is the negative electrode. Aflat electrode, in this case the measuring electrode, is mounted on the base and carries the wafer-like ionisation source in register with the inner end of the positive electrode. Again this arrangement is operationally satisfactory and it has an advantage over the construction described above. Should the creep path between the flat electrode and the grid become spuriously conductive then because the flat electrode is the measuring electrode and the grid is negative it provides a decrease in signal level which makes it possible for an auxiliary alarm circuitto be activated to indicate thatthe particular chamber is faulty.

However, the arrangements of the U.K.

specif ications 1581236 and 1603702 are expensiveto manufacture since the grid needs to be a structural element carrying the amount and the positive 130 electrode and there is also as requirementfor carrying an electrical connection around the outside of the grid. Also during cleaning and maintenance, removal and replacement of the grid can modifythe position of the inner end of the positive electrode relative to the ionisation source so altering the sensitivity of the chamber.

It is an object of the present invention to provide an improved ionisation detection chamberwhich is comparatively easy and inexpensiveto manufacture; may readily be cleaned and maintained without alteration of electrode spacing and will fail safe asthe shortest path is nowtothe positive electrode causing an increase in signal level if the chamber is contaminated.

According to the present invention there is provided an ionisation detection chamberfor detecting the presence of small particulate solids, such as smoke particles, carried in a gaseous stream comprising an electrically insulating base, a first, measuring, electrode carried bythe base, meansfor ionising gas associated with a central region of the measuring electrode, a second, negative, electrode comprising a metal grid disposed overthe base and togetherwith the base defining the chamber and a third, positive, electrode located in registerwith the ionisation means and nearertothe measuring electrodethan to the negative electrode characterised in thatthe third electrode comprises at least one rod-like element extending generally parallel with the measuring electrode and carried by an electrically insulating membersupported bythe base.

One embodimentof the invention will now be described byway of examplewith referencetothe accompanying drawings in which:- Figure 1 shows a vertical section through an ionisation detection chamber of a smoke detector and Figure2 is a plan view of the chamberwith the grid removed.

Referring to the drawings an electrically insulated base 1 carries a measuring electrode indicated generally at 2, a positive electrode indicated generally at3 and a negative electrode 4 in theform of a metal grid constituting an insect screen and, togetherwith the base 1 defining an ionisation chamber. Itwill be understood thatthe chamberwill be mounted in a casing (not shown) incorporating suitable aperturesforthe passage of smoke intothe chamberand extra wind baffles. Usuallythe chamberwill be mounted on a ceiling so as to be upside down relative to the disposition shown in Figure 1.

The base 1 is a high density polyethylene moulding onthe lowersurface 5of which is a sheet metal cover which isatthe sameelectrical potential asthegric14 to act as an electrical screen.

The measuring electrode 2 comprises a stainless steel disc 7 fixed at 8 and 9 to the base 1 and formed with a central dished part 10 extending into a recess 11 in the base. A stain less steel cover disc 12 formed with a central aperture 13 is secured to the disc 7 in good electrical contact therewith. The aperture 13 is smaller than the dished part 10 and a wafer-1 ike 2 GB 2 182 198 A 2 ionisation source comprising an Americium 241 ot particle emitter 14 is trapped within the recess. The disc 7 is formed with diametral apertures 15 and 16 in registerwith slots 17 and 18 formed in the edges of cover disc 12. A connector 19 extends through an aperture 20 in the base 1 and is soldered at 21 to the disc 12.

The positive electrode 3 comprises an electrically insulated mount22 of inverted -L- shape with a moulded in electrode 23 the lower part 24 of which constitutes a connection to the circuit and the upper part 25 of which is the active positive electrode. This upper part 25 extends parallel with the electrode 2 diametrally in registerwith the emitter 14 and is nearerthereto than the nearest part of the negative electrode 4. The mount 22 has a cylindrical lower part 26 extending through an aperture 27 in the base. Above the part 26 the mount has an eccentric formation 28 to ensure precise orientation relative to the base.

The mount is fitted to the base whilst still hot and the two parts are effectively welded together.

It has been found thatthe above arrangement operates in the same way as the construction described in U.K. specification 1581236 but provides greaterflexibility in the arrangement of the positive electrode. Although this has been shown as asingle rod extending diametrally of the emitter 14 itcould be provided with other configurations and could also bewholly or partly offsetwith respectto the emitter. More particularly, however, for cleaning and maintanance, upon removal of the negative electrode 4, access can be gained to the remaining electrodes without physical disturbance thereof.

Furthermore the grid 4, constituting the negative electrode can be of lightweight construction.

Claims

1. An ionisation detection chamber for detecting the presence of small particulate solids, such as smoke particles, carried in a gaseous stream comprising an electrically insulating base, a first, measuring, electrode carried by the base, means for ionising gas associated with a central region of the measuring electrode, a second, negative, electrode comprising a metal grid disposed overthe base and togetherwith the base defining the chamber and a third, positive, electrode located in registerwith the ionisation means and nearerto the measuring electrode than to the negative electrode characterised in thatthe third electrode comprises at least one rod-like element extending generally parallel with the measuring electrode and carried by an electrically insulating member supported bythe base.

2. An ionisation detection chamber substantially as herein described with reference to Figure land Figure 2 of the accompanying drawing.

Printed for Her Majesty's Stationery Office by Croydon Printing Company (UK) Ltd,3187, D8991685. Published byThe Patent Office, 25 Southampton Buildings, London, WC2A lAY, from which copies maybe obtained.

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