Smoke detecting apparatus with multiple detectors connected to coincidence circuit

Abstract

Smoke detecting apparatus comprising a number of smoke detectors of the ionisation chamber type which are distributed over an area to be protected and have their outputs connected together in groups to respective AND gates which in turn feed an alarm unit through an OR gate. In this way, at least two detectors must be simultaneously operative to produce an alarm indication.

Description

BACKGROUND OF THE INVENTION

The invention relates to apparatus and systems for detecting smoke, vapor mist, and other suspensions which can be detected by the effect they produce on an atmospheric ionisation current. References hereinafter to "smoke" are to be interpreted as including reference to any suspensions which can be detected in this way.

More particularly, the invention relates to such apparatus and systems incorporating detectors in which an ionisation current is set up in the atmosphere between electrodes in a chamber to which the atmosphere in which smoke is to be detected has ingress, the presence of the smoke being indicated by a change in the ionisation current. An example of such a detector is disclosed in U.S. Pat. No. 2,408,051 to K. O. Donelian.

Ionisation smoke detectors of the general type referred to above have found quite widespread application but suffer from the disadvantage that, unless they are adjusted to have low sensitivity (with consequent risk of failure to detect a hazardous condition), they may produce spurious alarms in response to such conditions as convection currents, drafts or tobacco smoke.

Accordingly, it is an object of the invention to reduce the likelihood of such spurious alarms in smoke detectors of the ionisation chamber type.

BRIEF SUMMARY OF THE INVENTION

According to the invention, there is provided a smoke detecting system, comprising a plurality of smoke detectors of the ionisation chamber type having respective outputs at which are produced electrical warning signals in the presence of smoke, coincidence means having the same number of inputs as the number of said detectors, the said inputs being respectively connected to the outputs of the detectors, the coincidence means having an output at which is produced an output signal only when both or all its inputs simultaneously receive said warning signals, and output means connected to the output of the coincidence means to produce an alarm indication in response to the said output signal.

DESCRIPTION OF THE DRAWINGS

Smoke detecting apparatus or systems embodying the invention will now be described by way of example with reference to the accompanying diagrammatic drawings in which:

FIG. 1 shows the physical layout of one of the systems;

FIG. 2 shows the logical interconnection of the system of FIG. 1;

FIG. 3 shows the physical layout of another of the systems;

FIG. 4 shows the logical interconnection of the system of FIG. 3; and

FIG. 5 shows one form which the smoke detectors can take.

DESCRIPTION OF PREFERRED EMBODIMENTS

As shown in FIG. 1, the system comprises four smoke detectors 5, 6, 8, 10, each of the form described above in which an ionisation current is set up in the atmosphere between electrodes in a chamber to which the atmosphere in which the smoke is to be detected has access. Each detector receives a power supply by means of a line 11 and may incorporate its own means for stabilising the power supply.

Each detector incorporates circuitry which produces an output signal on a respective output line 12, 14, 16, 18 when its ionisation current is reduced below a predetermined level. As shown in FIG. 2, the outputs 12, 14 of the detectors 5, 6 are connected to the two inputs of an AND gate 20, while the outputs 16, 18 of the detectors 8, 10 are connected to the two inputs of a further AND gate 22. FIG. 2 shows the two outputs of the AND gates 20, 22 connected to control an alarm via an OR gate 26.

In operation, the alarm 24 will be actuated only when both detectors 5, 6 are producing outputs together and/or when both detectors 8, 10 are producing outputs together. If only one detector of a pair produces an output, the alarm will not be actuated. The arrangement therefore very considerably reduces the possibility of spurious alarm indications being given such as due to convection currents, drafts, tobacco smoke and the like. If each detector were able to actuate the alarm on its own, then false alarms might be quite frequently given unless the sensitivity of the detector were held at a relatively low value -- with consequent risk of non-detection of a hazardous condition.

In FIGS. 3 and 4, parts performing similar functions to parts in FIGS. 1 and 2 are similarly referenced. As shown in FIG. 4, the outputs 12, 14 of the detectors 5, 6 are connected to the two inputs of AND gate 20, while the outputs 16, 18 of the detectors 8, 10 are connected to the two inputs of AND gate 22. The two outputs of the AND gates 20, 22 are connected to control the alarm 24 via OR gate 26. However, the apparatus of FIGS. 3 and 4 differs in that an extra AND gate 30 is provided which receives the outputs from detectors 6 and 8, and feeds the alarm 24 through OR gate 26.

In operation of the apparatus of FIG. 4, the alarm 24 will be actuated when both detectors 5, 6 are producing outputs together and/or when both detectors 8, 10 are producing outputs together, and/or when both detectors 6 and 8 are producing outputs together. If only one detector of a pair produces an output, the alarm will not be actuated. As before, the arrangement therefore very considerably reduces the possibility of spurious alarm indications being given such as due to convection currents, drafts, tobacco smoke and the like and each detector can be operated at relatively high sensitivity.

In FIG. 1 the area between the two lines A.sub.1 and B.sub.1 indicates the area in which a standard fire has to be present to produce smoke which operates both detectors 5 and 6 together when they are set to a low sensitivity. Lines C.sub.1 and D.sub.1 delimit the corresponding area when the detectors have medium sensitivity, and lines E.sub.1 and F.sub.1 the area when the detectors have high sensitivity. The corresponding areas for the detectors 8 and 10 are delimited by the lines A.sub.2 and B.sub.2, C.sub.2 and D.sub.2, and E.sub.2 and F.sub.2.

In FIG. 3, the lines A.sub.1, B.sub.1, C.sub.1, D.sub.1, E.sub.1, F.sub.1, A.sub.2, B.sub.2, C.sub.2, D.sub.2, E.sub.2, and F.sub.2 correspond to those in FIG. 1 while the lines A.sub.3 and B.sub.3 delimit the areas for the detectors 6 and 8 when they have low sensitivity.

FIG. 5 shows, by way of example only, one form which each detector can take. As shown the detector comprises a chamber 50 having at least some of its sides in the form of gauze to allow ingress of the atmosphere in which smoke is to be detected. The chamber is made of metal and contains a radioactive source 52 on one internal wall face which ionises the atmosphere in the chamber. An electrode 54 is mounted in the chamber on an electrically insulating support 56. An electrical power supply (not shown) applies a voltage across a potential divider comprising a resistor 58 in series with the gap between the electrode 54 and the metal wall of the chamber, and a field effect transistor 60 monitors the voltage at the junction between the chamber and the resistor 58. When smoke enters the chamber 50, the ionisation current between the electrode 54 and the chamber wall changes, thus altering the voltage sensed by the FET 60. The resultant output activates an amplifier and trigger unit 62 to produce a warning signal. The resistor 58 may be replaced by a second chamber which is similar to chamber 50 in that it contains an electrode and radioactive source but which is sealed off to prevent ingress of the atmosphere. Instead of the field effect transistor, another high impedance detector can be used.

As will be apparent to those skilled in the art, many modifications can be made without departing from the spirit and scope of the invention as defined in the appended claims.

Claims

What is claimed is:

1. A smoke detecting system, comprising

a plurality of smoke detectors of the ionisation chamber type having respective outputs at which are produced electrical warning signals in the presence of smoke,

coincidence means having the same number of inputs as the number of said detectors, the said inputs being respectively connected to the outputs of the detectors, the coincidence means having an output at which is produced an output signal only when the plurality of its inputs simultaneously receive said warning signals, and

output means connected to the output of the coincidence means to produce an alarm indication in response to the said output signal.

2. A system according to claim 1, including

a second plurality of smoke detectors of the ionisation chamber type having respective outputs at which are produced electrical warning signals in the presence of smoke,

second coincidence means having the same number of inputs as the number of detectors in the said second plurality, the inputs of the second coincidence means being respectively connected to the outputs of the detectors of the second plurality, the second coincidence means having an output at which is produced an output signal only when the plurality of its inputs simultaneously receive said warning signals, and

output signal connecting means connecting the output of the second coincidence means to the said output means in parallel with the output of the other said coincidence means to produce a said alarm indication in response to either said output signal.

3. A system according to claim 2, in which the said output signal connecting means comprises an OR gate.

4. In a smoke detecting system, comprising

a plurality of ionisation smoke detectors physically distributed about an area to be protected, each detector of which plurality includes a chamber to which the atmosphere in which smoke is to be detected has ingress and having two polarised electrode means, means for ionising the atmosphere in the chamber, and means responsive to a change in the ionisation current between the electrode means to produce at a detector output a warning signal indicative of the presence of smoke in the chamber, the improvement comprising

at least two coincidence means each having a plurality of inputs and a single output,

means connecting the said outputs of a first group of the said detectors to the inputs of one said coincidence means whereby the coincidence means produces an output signal at its said output only when all its said inputs simultaneously receive said warning signals,

means connecting the said outputs of a second group of the said detectors to the inputs of the other said coincidence means, the second group of detectors including at least one detector from the first group thereof, whereby the said other coincidence means produces an output signal at its said output only when all of its said inputs simultaneously receive said warning signals, and

output means connected to the outputs of both the coincidence means to produce an alarm indication in response to either said output signal.

5. A smoke detecting system, comprising

first and second pluralities of smoke detectors of the ionization chamber type having respective outputs at which are produced respective electrical warning signals in the presence of smoke,

first and second coincidence means each having the same number of inputs as the number of said detectors in the first and second pluralities of detectors respectively, the inputs of the first coincidence means being respectively connected to the outputs of the detectors of the first plurality and the inputs of the second coincidence means being respectively connected to the outputs of the detectors of the second plurality, each coincidence means having an output at which is produced an output signal only when the plurality of its inputs simultaneously receive said warning signals,

third coincidence means having two inputs and an output,

means connecting the said output of one of the detectors of the first plurality thereof to one of the inputs of the third coincidence means in parallel with the input of the first coincidence means to which that output is also connected,

means connecting the said output of one of the detectors of the second plurality thereof to the other input of the third coincidence means in parallel with the input of the second coincidence means to which that output is also connected,

the third coincidence means producing an output signal at its said output only when both of its said inputs simultaneously receive said warning signals,

output means for producing an alarm indication in response to the said output signal, and

output signal connecting means connecting the outputs of the first, second and third coincidence means to the said output means in parallel so that the said output means produces an alarm indication in response to any one of the said output signals.

6. A system according to claim 5, in which the said output signal connecting means comprises an OR gate.