US3693009A

US3693009A - Ionization smoke detecting device

Info

Publication number: US3693009A
Application number: US133767A
Authority: US
Inventors: Koju Sasaki
Original assignee: Nittan Co Ltd
Current assignee: Nittan Co Ltd
Priority date: 1968-12-24
Filing date: 1971-04-13
Publication date: 1972-09-19
Anticipated expiration: 1989-09-19
Also published as: GB1275450A; DE1964669A1; NL6918515A

Abstract

An ionization smoke detector having a single ionization chamber, a pair of spaced electrodes in said chamber, a grid electrode between said spaced electrodes and a radioactive source within the chamber. A potential is applied between the spaced electrodes and means are connected between one of the spaced electrodes and the grid electrode to detect potential changes in the grid produced by the presence of smoke.

Description

@EEQ

States Patent Sasaki HONIZATION SMOKE DETECTING DEVICE Inventor: Koju Sasaki, Tokyo, Japan Nittan Company Limited, Tokyo, Japan Filed: April 13, 1971 Appl. No.: 133,767

Related US. Application Data Continuation of Ser. No. 880,023, Nov. 26, 1969.

Assignee:

Foreign Application Priority Data Dec. 24, 1968 Japan ..43/94l81 US. Cl. ..250/43.5 D, 250/44, 250/83.6 FT,

[56] References Cited UNITED STATES PATENTS 3,560,737 2/1971 Skildum ..250/83.6 FT X 3,448,261 6/1969 Amiragofi ..250/43.5 D

Primary Examiner-Archie R. Borchelt Attorney-Eugene E. Geoffrey, Jr.

[57] ABSTRACT An ionization smoke detector having a single ionization chamber, a pair of spaced electrodes in said chamber, a grid electrode between said spaced electrodes and a radioactive source within the chamber. A potential is applied between the spaced electrodes and means are connected between one of the spaced electrodes and the grid electrode to detect potential Int. Cl. "swig $2 in the grid produced by the presence of Field of Search ..250/43.5 D, 44, 83.6 FT;

340/214, 227, 228.1, 237; 313/54 1 Claim, 4 Drawing Figures VOLTAGE POWER AMPLIFIER AMPLIFIER ALARM IONIZATION SMOKE DETECTING DEVICE This application is a continuation of Ser. No. 880,023, filed Nov. 26, 1969.

This invention relates to an ionization smoke detecting device and more particularly to an ionization smoke detecting device having a single ionization chamber.

When a gas contained within a limited space is ionized by a radioactive ray from a radioactive material such as radium and a resultant ion current is measured, such ion current would vary if the composition of the gas should vary. Therefore, this change of composition of the gas can be detected by detecting the change in the ion current. This is the principle of an ionization smoke detecting device.

A prior ionization smoke detector has two ionization chambers, one of which is airtight while the other is exposed to the air. Both ionization chambers include pairs of electrodes which are mutually connected in a tivated by detecting this unbalance of the bridge circuit. Since such prior device requires two ionization chambers, it is disadvantageous in that two radioactive sources are required and the construction is complicated.

Therefore, an object of this invention is to provide an improved ionization smoke detecting device having a single ionization chamber.

According to this invention, an ionization smoke detector is provided with a smoke sensing section composed of a single ionization chamber including a pair of facing electrodes, a grid electrode located between said facing electrodes and a single radioactive source located near one of said facing electrodes; A potential drift detected at the grid electrode due to presence of smoke is amplified to activate an alarm device.

Other objects and features of this invention will be more clearly understood by reading the following description with reference to the accompanying drawings.

In the drawings:

FIG. 1 is a schematic diagram, partially in block form, of an ionization smoke detector according to the prior art;

FIG. 2 is a graph explaining operation of the device of FIG. 1;

FIG. 3 is a schematic diagram, partially in block form, of an ionization smoke detector according to this invention; and

FIG. 4 is a potential distribution diagram illustrating operation of the device of FIG. 3.

Referring to the prior art structure shown in FIG. 1, two ionization chambers l and 2 are utilized and embody radioactive sources 3 and 4. The radioactive sources are each carried by one of the electrodes while a second electrode diagrammatically illustrated in a U- shaped configuration is positioned in spaced relationship to the electrode carrying the radioactive material. The electrodes of the chambers are connected in series and a voltage V is applied to the series connected chambers. The drain-source path of a field effect transistor 5 is connected in series with a resistor 6, and this series circuit is connected in parallel with the ionization chambers. The gate electrode of the transistor 5 is connected to the junction of the two ionization chambers. The ionization chamber 1 is airtight and will be referred to as the internal chamber. The ionization chamber 2 is exposed to air and will be referred to as the external chamber.

In FIG. 2, curves A and B represent the voltage-ion current characteristics of the internal and external chambers l and 2. When smoke is introduced into the external chamber 2, it modifies the voltage-current characteristic and produces a curve such as illustrated at B. This is caused by the reduction of the ion current resulting from absorption of the radioactive rays by the smoke and adsorption of the ions by the fine smoke particles which are heavier than ions and electrons and therefore lack mobility. Because the voltage-current characteristic of the external chamber 2 has been changed, the current changes from i to i. Furthermore, the voltage across both chambers is maintained at a constant value with the result that the voltage across the external chamber changes from v, to v',. Thus the voltage change across the external chamber 2 is Av vr-v and this voltage change appears across the resistor 6. At least a portion of the voltage across the resistor 6 is fed to an amplifier 7 and actuates an alarm device 8.

FIG. 3 illustrates a device in accordance with the invention. This device has a smoke sensing portion which comprises a pair of spaced

electrodes

11 and 12 with a grid electrode 13 disposed therebetween. A radioactive source 14 is positioned in the vicinity of the electrode 11. The electrode 11 and the grid electrode are connected to a voltage amplifier 15 which in turn is connected through a power amplifier 16 to an alarm 17. A constant voltage source 18 is connected to spaced

electrodes

11 and 12.

In operation the voltage source 18 produces a voltage E across the electrodes 11 and'12 and the potential distribution between the electrodes is a substantially straight line as illustrated bythe line H of FIG. 4. This occurs under a smoke free condition. When smoke is introduced between the

electrodes

11 and 12, the potential distribution changes markedly as illustrated by the graph K in FIG. 4. Thus the potential of the grid electrode 13 changes from e, to e and the change is Ae =e,-e

The foregoing effect is generally referred to as a space charge limiting effect, and the magnitude of the change Ae is greater with a lower voltage E and with a larger ion current. Therefore, the grid 13 is preferably located at a position which will afford the greatest voltage change. The change in the voltage is amplified by the amplifiers l5 and 16, the former having a relatively high input impedance, and the output of the amplifier 16 is fed to an alarm device 17.

As described above, the smoke detecting device in accordance with the invention comprises a single ionization chamber and can therefore be constructed simply and utilizes a single radioactive source. Moreover, the signal produced by the presence of smoke can be directly amplified by a conventional voltage amplifier and without the need for a complicated bridge circuit such as that shown in FIG. 1. The device in accordance with this invention also provides improved operation, requires simple circuitry, and affords stable operation.

While only one embodiment of the invention has been illustrated and described, it is apparent that alterations, modifications and changes may be made without departing from the true scope and spirit thereof as defined by the appended claims.

What is claimed is:

1. An ionization smoke detecting device comprising a pair of spaced electrodes having means for applying a DC potential therebetween, means supporting said electrodes to permit the movement of ambient air therebetween, a potential sensing grid disposed between said spaced electrodes, a radioactive source positioned in the vicinity of one of said electrodes, said DC potential and radioactive source producing a sub-

Claims

1. An ionization smoke detecting device comprising a pair of spaced electrodes having means for applying a DC potential therebetween, means supporting said electrodes to permit the movement of ambient air therebetween, a Potential sensing grid disposed between said spaced electrodes, a radioactive source positioned in the vicinity of one of said electrodes, said DC potential and radioactive source producing a substantially linear voltage gradient between said electrodes, a voltage amplifier having an input and output, means connecting one of said spaced electrodes and said grid to the input of said amplifier and indicating means connected to the output thereof, said amplifier being responsive to voltage changes between said one electrode and said grid produced by the production of a nonlinear voltage gradient between said electrodes upon the entry of smoke therebetween and actuating said indicating means.