IE41452B1 - Improvements in or relating to fire detectors - Google Patents

Abstract

1492193 Ionization detectors ANGLO AMERICAN CORP OF SOUTH AFRICA Ltd 19 Aug 1975 [26 Aug 1974] 34390/75 Heading H1D In a fire detector having an ionization chamber 12 in which a grid electrode 14 is mounted from the housing 10 by an insulating support 16, a heat generating component 28 of the electronic circuitry associated with the detector is mounted within a cavity 26 formed in the support 16 to heat the surrounding part of the support 16 and thus prevent leakage currents between the electrode and housing due to the formation of moisture on the support 16. As shown, the component 28, e.g. an operational amplifier is mounted within a vertical limb 24 of support 16 to which is attached the grid 14. The remainder of the electronic circuitry is mounted in a box 20 in a cavity 22 in the support 16.

Description

THIS invention relates to fire detectors of the type used in mines and similar installations for detecting the outbreak 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 circuit to apply a voltage between the housing and the electrode and to detect variations in the resulting electrical current flowing lo through the ionization chamber. Usually, the electrical current flowing through the ionization chamber is substantially 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 in the magnitude of the current.

This decrease is dected 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 electrodes and the housing whioh, in turn, can 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.

It is an object of the present invention to provide a fire detector in which the problem of leakage currents caused by the accumulation of moisture on the support member is overcome.

In a fire detector according to the invention a heat generating Component of the electronic circuit it mounted in a suitable cavity formed in the support member between - 2 41463 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.

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

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.

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 26 is formed in the limb 24, and one of the heat transmitting components of the elctronic circuit is located in this cavity. The component in question is preferably an operational amplifier 28 which is of course connected into the circuit 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 45 milliwatts of heat power applied to the inner surface of the limb 24 over a length o.f approximately 5mm will cause a temperature rise on the outer surface of that part of the limb of about 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 it is possible to obtain one with a quiescent power con15 sumption (in the form of heat) of 45 milliwatts.

Claims (3)

1. CLAIMS:1A 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 5 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 10 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 15 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.

3. A fire detector substantially as herein described with reference to the accompanying drawing.