Combustible gas detectors

#This invention, like that which is the subject of our United Kingdom Pat. No. 1,387,412, relates to combustible gas detectors and more particularly to combustible gas detectors of the kind in which a heatable wire filament exhibits a change in resistance occasioned by the change in its temperature which occurs due to the oxidisation of a combustible gas passing over it. Such gas detectors are usually included in a bridge circuit, the change in the balance of which as the resistance of the wire filament changes being utilised to provide an indication of the concentration of the combustible gas.

#Whilst it is possible to use a naked wire filament it is also common to use a wire filament which is embedded in a pellet of ceramic material to provide a more rugged structure. It is also common to coat such a pellet with an oxidisation catalyst which reduces the temperature at which oxidisation of the combustible gas takes place, in order to reduce or prevent evaporation of the wire filament so as to reduce any tendency for the characteristics of the gas detector to change in service.

#A gas detector as described above suffers from the disadvantage that the manufacturing process is a relatively involved one. In order to provide the pellet of ceramic material it is usual to immerse the wire filament in a solution of material which, when heated, decomposes to ceramic material, remove the liquid, heat the filament in order to dry out the solution adhering to the filament and repeat the process for as many times as is necessary to obtain the desired thickness of ceramic material forming the pellet. A similar process is then required to be repeated to form the layer of catalyst material on the pellet.

#The invention is illustrated in and further described with reference to the accompanying drawing which illustrates a balanced bridge arrangement including a gas detector in accordance with the present invention for use in the detection of combustible gases.

#Referring to the drawing, 1 is a gas detector consisting of a heatable wire filament 2 of platinum embedded in a pellet 3 consisting of a homogeneous mixture of alumina, and an oxidisation catalyst material, palladium and platinum. Surrounding, completely in this example, the pellet 3 is a layer 4 of oxidisation catalyst material. In this example the oxidisation catalyst material of the layer 4 is also a mixture of palladium and platinum formed by applying relatively high concentration solutions to the outside of the pellet 3.

#The gas detector 1 is included in one arm of a balanced bridge arrangement consisting of resistors 5 and 6 of equal value and a compensating element 7. The compensating element 7 consists of a wire filament 8 of platinum embedded in a pellet 9 consisting of a homogeneous mixture of alumina, and a poison, potassium hydroxide, adapted to inhibit the oxidisation of combustible gases. Across the bridge is connected a voltmeter 10, calibrated to indicate combustible gas concentrations. The meter 10 is arranged to be set to zero by the adjustment of the slider on a potentiometer 11. Terminals 12 are provided to be connected to a source of power providing both the heating current for the filaments 2 and 8 and voltage for the bridge.

#Except for the nature of the gas detector 1, the arrangement is, in fact, as known per se.