FIRE DETECTORS

DETECTEURS D'INCENDIE

English Abstract

ABSTRACT OF DISCLOSURE.
A fire detector of the kind comprosing a housing defining
an ionization chamber, an electrode supported in the
ionization chamber by means of a support member made of an
electrically insulating material, and an electronic ciruit
to apply a voltage between the housing and the electrode
and to detect variations in the resulting electrical current
flowing through the ionization chamber. In order to prevent
leakage currents flowing between the electrode and the housing
via the support member a heat generating component of the
electronic circuit associated with the detector is mounted in
a cavity in the support member to heat that part of the support
member thereby to prevent the formation of moisture on that
part.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH
AN EXCLUSIVE PROPERTY OR PRIVILEGE IS CLAIMEDS ARE DEFINED AS FOLLOWS:

1.
A fire detector of the kind comprising a housing defining
an ionization chamber, an electrode supported in the
ionization chamber by means of a support member made of an
electrically insulating material, and an electronic circuit
to apply a voltage between the housing and the electrode and
to detect variations in the resulting electrical current
flowing through the ionization chamber characterised in that
a heat generating component of the electronic circuit is
mounted in a suitable cavity formed in the support member
between the electrode and the housing to heat that part of
the support member surrounding the component in order to
prevent the formation of moisture on that part thereby to
prevent leakage currents flowing between the electrode and
the housing via the support member.

2.
A fire detector as claimed in claim 1 characterised in that
the heat generating component is an operational amplifier.

Description

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THIS invention relates to fire detectors of the type used
in mines and similar installations for detecting the out-
` break of a fire.

Such detectors generally comprise a housing defining a cavity
known as an ionization chamber, an electrode supported in
, ~ the ionization chamber by means of a support member made of
an insulating material, and an electronic curcuit to apply
y a voltage between the housing and the electrode and to
-~ - detect variations in the resulting electrical current flowing
-, 10 through the ionization chamber. Usually, the electrical
~ current flowing through the ionization chamber is substantially
-i~ constant when no combustion particles are present. On the ~-
outbreak of a fire, however, the resulting combustion
;~ particles cause a decrease in the magnitude of the current.
This decrease is detected and an alarm or the like is
, energised.

Under certain conditions, such as those encountered in mines,
moisture and dust can accumulate on the surface of the support
member. This can cause small leakage currents to flow
~ between the electrode and the housing which, in turn, can
i seriously affect the operation of the detector. In this regard
it should be pointed out that the current flowing through
$ the ionization chamber is usually very small e.g. of the order
~ of 500 pico-amps.
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It is an nbject of the present invention to provide a fire
:~ detector in which the problem of leakage currents caused by
~ the accurnulation of moisture on the support member is overcome.
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In a fire detector according to the invention a heat
generating component of the electronic circuit is mounted
in a suitable cavity formed in the support member between
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the electrode and the housing to heat that part of the support
member surroundlng the component in order to prevent the
formation of moisture on that part thereby to prevent leakage
currents flowing between the electrode and the housing via
the support member.

Further according to the invention the heat generating component ~-
is an operational amplifier.

Thus, in accordance with the present teachings, a fire detector

is provided of the kind which comprises a housing defining an
ionization chamber, an electrode supported in the ionization
chamber by means of a support member made of an electrically
insulating material, and an electronic circuit to apply a
voltage between the housing and the electrode and to detect

variations in the resulting electrical current flowing through
the ionization chamber. A heat generating component of the
electronic circuit is mounted in a suitable cavity formed in
the support member between the electrode and the housing to
: .
~ heat that part of the support member surrounding the component

; in order to prevent the formation of moisture on that part
--, 20
thereby to prevent leakage currents flowing between the
electrode and the housing by way of the support member.

To illustrate the invention an embodiment thereof is described
hereunder with reference to the accompanying drawing which

is a sectional side elevation of the fire detector of the
invention.




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- With reference to the drawing the fire detector comprises,
in essence, a housing 10 defining an ionization chamber 12
having suitable openings therein through which smoke can ;~
pass. An electrical grid 14 is supported in the ionization
chamber by means of a supporting structure 16 which is
secured to one inner wall of the housing 10 at 18.

The usual electronic circuit for detecting variations in the
currents flowing through the ionization chamber is located in

a container 20 which is mounted in a cavity 22 in the
supporting structure 16.


The supporting structure 16 includes a vertical limb 24, the ~ ~ `
electrical grid 14 being attached to the lower end of the ~ -
: limb.
... .
A cavity is formed in the limb 24, and one of the heat trans~
mitting components of the electronic circuit is located in
this cavity. The component in question is preferably an opera-

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tional amplifier 28 which is of course connected into the circuit



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- located in the container 20. The heat generated by the
-- operational amplifier will prevent the formation of moisture
on that part of the limb 24 surrounding the operational
amplifier and this, in turn, will minimize the possibility
of leakage currents flowing between the grid and the housing
via the supporting structure 16.
.
Experiments have shown that approximately 50 milliwatts of
heat power applied to the inner surface of the limb 24 over
: a length of approximately 5mm will cause a temperature rise
on the outer surface of that part of the limb of about
5 degrees C, and this seems to be sufficient to keep that
:~ part of the leakage current path dry under extreme humidity
~, conditions. By suitable choice of an operational amplifier
`.j it is possible to obtain one with a quiescent power con-
' 15 sumption (in the form of heat) of 45 milliwatts.

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