GB2091485A

GB2091485A - Ionization smoke detector

Info

Publication number: GB2091485A
Application number: GB8200748A
Authority: GB
Original assignee: Chloride Inc
Current assignee: Chloride Inc
Priority date: 1981-01-12
Filing date: 1982-01-12
Publication date: 1982-07-28
Also published as: CA1182592A; CH657222A5; ES508630A0; DE3200620A1; ES8307060A1; JPS57164398A; GB2091485B

Abstract

In an ionization detector comprising inner 16, outer 14, and collector 18 electrodes a test electrode 26 is provided to which a voltage may be supplied. This voltage reduces the ion current in the device and hence lowers the voltage on the collector electrode 18 thus simulating the effect of the presence of smoke in the device. Alternative circuits for energising the test electrode are provided. <IMAGE>

Description

SPECIFICATION Ionization smoke detector The invention relates to an ionization mole detector.

Ionization smoke detectors are designed to provide an output signal to acutate an alarm smoke concentration at the detector reaches a predetermined value by detecting the reduction of an iron current in a chamber caused by the presence of smoke particles. In a single chamber detector, in which electrodes within the chamber are connected in series with a resistor across a power source, the change in ion current flow changes the voltage at the junction between the detector and the resistor. In a two chamber or dual chamber detector, which has a detection chamber and a closed reference chamber connected in series across a power source, the reduction of ion current caused by smoke entering the detector chamber changes the voltage at the junction Or the chambers.In a dual chamber detector, in which the reference chamber (or reference volume) is disposed within the detector chamber with a common ion source, a collector electrode is provided in the chamber between the inner and outer electrodes (sometimes forming the separation between the two chambers), and the reduction in ion current flow changes the voltage on the collector electrode. This change in voltage is employed to actuate an alarm.

A detector of any or these kinds must be calibrated so that the alarm signal is provided at a definite predetermined smoke concentration. This smoke concentration provides a particular voltage at the detector.

It has been common practice to test such detectors by connecting the reference electrode to ground, or by providing a slowly increasing voltage across the smoke detecting chamber, and determining the voltage at which the alarm signal is produced. It has also been suggested that a test electrode may be provided in the smoke chamber, to be connected to ground or to a voltage varying device, to cause the alarm test signal.

The invention provides an ionization smoke detector comprising a chamber having first and second electrodes, means for causing ion current to flow between electrodes, including means for applying a voltage therebetween, means responsive to a decrease in ion current between the first and second electrodes to a predetermined value to provide an alarm signal, a test electrode disposed in the chamber in association with the first and second electrodes, and means for applying to said test electrode a voltage of e value that will cause the ion current to decrease to the predetermined value.

The invention also provides a dual chamber @onization smoke detector comprising a housing an inner electrode, an outer electrode, a collecter electrode between the inner and outer electrode, an ion source and a power supply for applying a voltage between the inner and outer electrodes co cause ion current therebetween, means connected to the collector electrode arranged to provide an alarm signal in response to a predetermined decrease in voltage thereon, a test electrode associated with the outer electroce ano tne collector electrode, and means for applying to the test electrode a test voltage having a value such that a voltage appears on the collector electrode that is equal to or less than the voltage value thereon that provides the alarm signal.

An ionization smoke detector in accordance with this invention is thus provided with a test electrode and means for applying to the test electrode a single predetermined voltage, having a value such that it will simulate the presence of smoke in the chamber d an amount to which the detector should respond.For example if it is intended that the detector should produce an alarm signal when the smoke concentration reaches 1% obscuration, that is, 1% of the light passing through a column of smoke 1 foot long is absorbed, the presence of this concentration of smoke causes a certain change in the voltage at the collector or at the junction, depending on the type of detector. The test circuit is consequently designed to apply to the test electrode a voltage of a value such that it causes this voltage change to occur at the collector or junction.

The test circuit can be designed to apply the test voltage in such a manner that voltage surges that might provide a false indication of satisfactory operation do not occur. Moreover means can be provided for applying the test voltage from a remote location.

The invention is further described below, by way of example, with reference to the accompanying drawings, in which: Figure 5 is a schematic view or an ionization detector embodying the present invention, in which the detector is provided with a test electrode and means for applying a predetermined voltage to the test electrode to simulate a predetermined amount of smoke in the chamber;; Figure 2 is a graph illustrating the relationship between the voltage on the est electrode and the percentage smoke which the test voltage simulates; and Figures 3 and 4 illustrate respective modified forms of test circuit which allow the detector of the invention to be tested from a remote location.

Figure 1 of the drawings shows an ionization smoke detector 10 which comprises a housing 12 having an inner electrode 1 6, an outer electrode 14, and a collector electrode 18. T The physical structure of the detector provides an inner or reference chamber 20 between the inner electrode 16 and the collector electrode 18, and an outer or smoke detecting chamber 22 between the reference electrode and the outer electrode 14. The physical structure of such a detector is illustrated, for example, in U.Q. Patent 3 935 466 and U.K. published patent Application 2 013 393A. An ionization source 24 is provided in the housing to provide ion current flow when the detector is is connected to a power source P. The outer electrode 14 and the inner electrode 16 are connected across the power source so that ion current flow of a definite magnitude flows between the inner and outer electrodes 16, 14 when clean air is in the detector.

This ion current flow establishes a voltage on the collector electrode of predetermined value, which voltage is applied to a terminal T1 of a comparator C for comparison with a reference voltage established at the comparator terminal T2 from the junction J1 of a voltage divider consisting of resistors R1 and R2 connected in series across the power source P.

When smoke enters the chamber, the smoke particles reduce the ion current flow, which reduces the voltage on the collector electrode 1 8.

For example, with a supply voltage of 9 volts, the voltage on the collector electrode during clean air conditions may be 5.5 volts. The resistors R1 and R2 have values such that the voltage on terminal T2 of the comparator C is 4.5 volts. Therefore when the smoke density in the chamber becomes great enough to reduce the collector voltage to 4.5 volts, the comparator produces an output to an alarm actuating device A.

In accordance with this invention, a test electrode 26 is provided in the housing between the collector and the outer electrodes in the smoke detecting chamber 22. To apply a predetermined voltage to the test electrode 26, a voltage divider is provided, which consists of resistors R3 and R4 in series through a junction J2 and in series with a switch S1 across the power source P, the test electrode being connected to the junction J2.

The test electrode 26 is normally maintained at the same potential as the outer electrode, because it is connected to the outer electrode and the power source through resistor R3 through which no substantial current flows when the switch S1 is open.

However when switch S1 is closed, current through R3 and R4 establishes a predetermined voltage on the test electrode 26 that is less than the voltage on the outer electrode 14. This causes a change in the electric field distribution in the chamber between the collector and the outer electrodes 1 8, 14 which causes a decrease in the voltage on the collector electrode 1 8. If the voltage applied to the test electrode 26 is of the proper value, the collector voltage is reduced to a value just equal to that of the comparator reference voltage. Applying the proper voltage to the test electrode thus simulates the amount of smoke in the housing at which the alarm is to be actuated.

The graph of Figure 2 illustrates the relation between the voltage on the test electrode 26 and the percent smoke that the voltage simulates, for a particular detector.

If the detector is intended to provide an alarm signal when the concentration of smoke in the chamber reaches 1%, established by the ratio of the values of the resistors R1 and R2, then it is seen from Figure 2 that a test voltage of about 5.8 volts will be required to produce a test alarm under clean air conditions. However in the manufacture of the detectors in large quantities, manufacturing tolerances make it impractical to test the detectors to the exact alarm point.

Therefore if a regulation issued by government or by an industry regulatory body requires that the detector respond to 6% smoke, for example, the detector can be manufactured to 1% smoke, with the test circuit being designed to simulate a smoke percentage intermediate the required alarm point of 6% smoke and the designed alarm point of 1% smoke. The resistors R3 and R4 will therefore have values that will produce a test voltage when the switch S1 is closed that will simulate 3% smoke, for example.

In the detector of Figure 1, the switch S1 may be a magnetically operated switch so positioned that the test may be conducted by applying a magnet to the exterior of the detector housing.

The fact that the switch S1 is in series with the resistors R3 and R4 prevents a surge of current when the switch is closed which might erroneously actuate the alarm circuitry.

To enable the test to be conducted from a remote location, a modified form of test circuit may be provided, as shown in Figure 3. In this circuit, the resistors R3 and R4 are connected in series with the collector-emitter path of a transistor T. The base of transistor T is connected to the junction J3 of resistors R5 and R6, which resistors are connected to the power source P through a switch S2, which may be at a remote location, for example, at a central control panel.

Closure of the switch S2 causes a predetermined voltage to be applied to the base of the transistor T sufficient to cause conduction in its emittercollector path so that the test voltage appears at junction J2. If the line from the switch S2 to the detector is iong enough for voltage transients to be a problem, a noise suppression capacitor F may be provided between the base of the transistor T and ground.

Another modified test circuit that allows a remote testing is shown in Figure 4, in which the resistors R3 and R4 are connected in series with a photo-responsive device PT, such as phototransistor of an optically coupled isolator. The light source L of the isolator may be connected to the power source P through a resistor R7 of suitable value and a switch S3. As in the circuit of Figure 3, the switch S3 may be located at a location remote from the detector. When the switch S3 is closed, the resulting illumination of light source L allows conduction through the photo-responsive device PT to cause the test voltage to appear at the junction J2.

Although in the illustrated embodiments of the invention, the test electrode and circuit are shown as being incorporated into a dual chamber detector, the invention can also be incorporated into a single chamber detector or into the detector chamber of a two chamber detector.

Claims

1. An ionization smoke detector comprising a chamber having first and second electrodes, means for causing ion current to flow between electrodes including means for applying a voltage therebetween, means responsive to a decrease in ion current between the first and second electrodes to a predetermined value to provide an alarm signal, a test electrode disposed in the chamber in association with the first and second electrodes, and means for applying to said test electrode a voltage of a value that will cause the ion current to decrease to the predetermined value.

2. An ionization detector as claimed in claim 1 wherein the test electrode is normally maintained at the same potential as one of the first and second electrodes, and wherein the voltage applied to the test electrode is less than the voltage on one electrode and greater than the voltage on the other electrode.

3. A dual chamber ionization smoke detector comprising a housing, an inner electrode, 'an outer electrode, a collector electrode between the inner and outer electrodes, an ion source and a power supply for applying a voltage between the inner and outer electrodes to cause ion current therbetween, means connected to the collector electrode arranged to provide an alarm signal in response to a predetermined decrease in voltage thereon, a test electrode associated with the outer electrode and the collector electrode, and means for applying to the test electrode a test voltage having a value such that a voltage appears on the collector electrode that is equal to or less than the voltage value thereon that provides the alarm signal.

4. An ionization detector as claimed in claim 1, 2 or 3 wherein the voltage applied to the test electrode is obtained from the junction of two resistors connected across a power source for the first and second electrodes in series with a switch, whereby the voltage at the junction of the two resistors is applied to the test electrode when the switch is closed.

5. An ionization detector as claimed in claim 1, 2 or 3 wherein the means for applying the voltage to the test electrode comprises a resistor connected between the outer electrode and the test electrode and a second resistor connected in series with a switch between the test electrode and the inner electrode, whereby the test electrode is maintained at the same potential as the outer electrode when the switch is open, and the test voltage is applied to the test electrode when the switch is closed.

6. An ionization detector as claimed in claim 4 or 5 wherein the switch comprises a transistor with the collector-emitter path thereof connected in series with the two resistors, and means for applying a voltage to the base of the transistor to cause current flow in the collector-emitter path to cause the voltage to be applied to the test electrode.

7. An ionization detector as claimed in claim 4 or 5 wherein the switch comprises 7n optically coupled isolator comprising a photo-responsive device connected in series with the second resistor between the test electrode and ground, and a light source positioned to illuminate the photo-responsive device when energized, and means for energizing the photoresponsive device.

8. An ionization smoke detector substantially as herein described with reference to Figure 1, Figure C3 or Figure C4 of the accompanying drawings.