GB2034893A

GB2034893A - Combustible gas detector

Info

Publication number: GB2034893A
Application number: GB7844991A
Authority: GB
Original assignee: English Electric Valve Co Ltd
Current assignee: Teledyne UK Ltd
Priority date: 1978-11-17
Filing date: 1978-11-17
Publication date: 1980-06-11
Also published as: DE2942722A1

Abstract

The present invention relates to methods of manufacturing combustible gas detectors of the kind in which a heatable wire filament 2 exhibits a change in resistance occasioned by the change in its temperature which occurs due to the oxidation of a combustible gas passing over it. In the present method the heatable wire filament is enclosed in a pellet 3 comprising ceramic material which is applied to said heatable wire filament by cataphoretic coating. <IMAGE>

Description

SPECIFICATION Improvements in or relating to methods of manufacturing combustible gas detectors This invention 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 oxidation 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, or include within the mix from which the pellet is made, an oxidation catalyst which reduces the temperature at which oxidation 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.

In practice the heatable wire filament is commonly of pure platinum wire since its temperature co-efficient and specific resistance is more or less ideal for the purpose.

The present invention seeks to provide an improved combustible gas detector of the kind in which the wire filament is embedded in a pellet comprising ceramic material.

According to this invention a method of manufacturing a combustible gas detector comprises the steps of providing a heatable wire filament and enclosing said wire filament in a pellet comprising ceramic material and wherein said ceramic material is applied to said heatable wire filament by cataphoretic coating.

Said ceramic material is normally alumina.

Normally said pellet when formed has applied thereto an outer coating of catalyst material. In one example, said last mentioned catalyst material said heatable wire filament within a pellet of ceramic material is heated to a high temperature and a solution containing catalyst material is applied to the outer surface of said pellet either utilising a dropper or by repeated immersions in said catalyst solution with intermediate periods of heating.

Normally said last mentioned temperature is approximately 950 to 1,0000C.

Preferably the material chosen for said'heatable wire filament is platinum wire.

Said catalyst solution preferably comprises palladium and preferably palladium with platinum.

Other suitable catalyst solutions comprise palladium with thorium or platinum alone.

In one example of the present invention said catalyst solution is placed within an aluminium beaker and said heatable wire filament is suspended from a bench stand and lowered into said catalyst solution within said beaker whilst an electrical potential positive with respect to said beaker and negative with respect to said heatable wire filament is maintained between said beaker and said filament for a period of a few seconds (typically ten seconds) and thereafter slowly withdrawn allowed to air dry and subsequently heated within an alumina coated molybdenum boat in order cataphoretically to form an alumina pellet or coating on said heatable wire filament.

Whilst the explanation for this at present is not fully understood it has been found that a combustible gas detector manufactured in accordance with the present invention tends to exhibit improved stability in certain conditions particularly when exposed to atmospheres containing concentrations of methane for extended periods of time.

The invention is illustrated in and further described with reference to the accompanying drawing which illustrates schematically one gas detector bridge circuit including a gas detector in accordance with the present invention.

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 alumina. Surrounding, completely in this example, the pellet 3 is a layer 4 of oxidation catalyst material. In this example the oxidation catalyst material of the layer 4 is a mixture of palladium and platinum.

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 consistinq of a homogeneous mixture of alumina, and a poison, potassium hydroxide, adapted to inhibit the oxidations 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 adjustment of the slider on a potentiometer 11. Terminals 1 2 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.

In operation the gas detector 1 and compensating element 7 are exposed to normal atmosphere and the slider potentiometer 11, adjusted to give a zero reading on meter 1 0. The gas detector 1 and compensating element 7 are exposed to the atmosphere which it is required to monitor. Any combustible gases in the atmosphere oxidise on the surface of gas detector 1, but not on the surface of compensating element 7, causing the temperature of the filament 2 to rise with a consequent change in its resistance.

The reading meter 10 then provides a measure of concentration of combustible gases in the atmosphere.

In the manufacture of the gas detector 1, a wire of platinum is first wound to form filament 2 and then cleaned. The alumina suspension is made up in the following quantities Distilled water 800 ml.

Electrolyte 120 ml.

Alumina 1 560 g.

Ethyl alcohol, absolute (EYB003) 1000 ml.

The suspension is then placed in an aluminium beaker and the platinum wire is suspended over it by means of a bench stand. A source of d.c.

potential is connected with its positive terminal connected to the beaker and its negative terminal connected to the platinum wire (via the bench stand). The voltage of the potential source is approximately 40 volts. The platinum wire is then lowered into the beaker until it is completely immersed and it is maintained so for approximately ten seconds whereupon the platinum wire is withdrawn slowly and uniformly. The wire thus becomes cataphoretically coated with alumina.

The coated platinum wire is then air dried for five to ten minutes. The coated platinum wire is then heated in air at a temperature of 950 to 1,00qOC by passing current through the coil and a catalyst solution as known per sue is applied to the surface of the alumina bead formed on the platinum wire either by a dropper or by repeated immersions in the catalyst solution with intermediate heating periods.

In this particular example the catalyst solution is one containing palladium and platinum, this solution being made up as follows:- 1 gram of palladium chloride 1 gram of platinum chloride 25 ml of 2.5N concentrated hydrochloric acid 3 ml of saturated aluminium nitrate solution at 200 C.

Claims

1. A method of manufacturing a combustible gas detector comprising the steps of providing a heatable wire filament and enclosing said wire filament in a pellet comprising ceramic material and wherein said ceramic material is applied to said heatable wire filament by cataphoretic coating.

2. A method as claimed in claim 1 and wherein said ceramic material is alumina.

3. A method as claimed in claim 1 or 2 and wherein said pellet when formed has applied thereto an outer coating of catalyst material.

4. A method as claimed in claim 3 and wherein said last mentioned catalyst material said heatable wire filament within a pellet of ceramic material is heated to a high temperature and a solution containing catalyst material is applied to the outer surface of said pellet either utilising a dropper or by repeated immersions in said catalyst solution with intermediate periods of heating.

5. A method as claimed in claim 4 and wherein said last mentioned temperature is approximately 950to 1,0000C.

6. A method as claimed in any of the above claims and wherein the material chosen for said heatable wire filament is platinum wire.

7. A method as claimed in any of the above claims 4 to 6 and wherein said catalyst solution comprises palladium.

8. A method as claimed in claim 7, in which the final heating is to a temperature of from 9500 to 1 ,0000C.

9,A method of manufacturing a combustiblegas detector element as claimed in any of the preceding claims and substantially as hereinbefore described.

8. A method as claimed in any of the above claims 4 to 7 and wherein said catalyst solution comprises palladium with platinum.

9. A method as claimed in any of the above claims 4 to 7 and wherein said catalyst solution comprises palladium with thorium.

10. A method as claimed in any of the above claims 4 to 7 and wherein said catalyst solution comprises platinum alone.

11. A method as claimed in any of the above claims 4 to 10 and wherein said catalyst solution is placed within an aluminium beaker and said heatable wire filament is suspended from a bench stand and lowered into said catalyst solution within said beaker whilst an electrical potential positive with respect to said beaker and negative with respect to said heatable wire filament is maintained between said beaker and said filament for a period of a few seconds and thereafter slowly withdrawn allowed to air dry and subsequently heated within an alumina coated molybdenum boat in order cataphoretically to form an alumina pellet or coating on said heatable wire filament.

12. A method as claimed in claim 11 and wherein said period of a few seconds is of the order of ten seconds.

13. A method of manufacturing a combustible gas detector substantially as herein described with reference to the accompanying drawing.

New claims or amendments to claims filed on DATE OF ISSUE OF SEARCH REPORT Superseded claims - 1-13 New or amended claims:

1. A method of manufacturing a detector element of a combustible-gas detector, in which method a heatable wire filament is enclosed in a pellet of ceramic material, the ceramic material being applied to the heatable wire filament by cataphoretic coating.

2. A method as claimed in claim 1, in which the cataphoretic coating is effected by making the filament the cathode of a cell containing an aqueous alcoholic suspension of the ceramic base material.

3. A method as claimed in claim 2, in which the ceramic base material is alumina.

4. A method as claimed in any of the preceding claims, in which the material chosen for the heatable wire filament is platinum.

5. A method as claimed in any of the preceding claims, in which the pellet when formed has applied thereto an outer coating of catalyst material.

6. A method as claimed in claim 5, in which the catalyst is platinum, palladium with platinum, or palladium with thorium.

7. A method as claimed in either of claims 5 and 6, in which the catalyst material is applied to the pelletised heatable wire by heating the pellet to a high temperature and then applying to the pellet's outer surface a solution containing catalyst material, either utilising a dropper or by repeated immersions in the catalyst solution with intermediate periods of heating.