GB1567736A - Combustible gas detectors - Google Patents

Description

(54) IMPROVEMENTS IN OR RELATING TO COMBUSTIBLE GAS DETECTORS (71) We, ENGLISH ELECTRIC VALVE COMPANY LIM1TED, a British Company, of 106, Waterhouse Lane, Chelmsford, Essex CMl 2QU, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to combustible gas detector arrangements and more particularly to combustible gas detector arrangements of the kind which are responsive to a change in the resistance of a heatable wire filament occasioned by the change in its temperature which occurs due to the oxidation of a combustible gas passing over it.Such arrangements usually comprise a bridge circuit in which the heatable wire filament is included, the change in the balance of which bridge circuit, 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 resist anode is more or less ideal for the purpose.

However, whether the wire filament is used as a naked "hot wire" detector or is enclosed in a pellet, crystal growth deformation and movement of the wire during processing or subsequent service can, under certain conditions, give rise to premature failure.

Platinum alloys are available which are of improved mechanical strength, but in general their use has not been favoured since the electrical characteristics of the filament which results are not as appropriate as those of pure platinum wire and less efficient detectors tend to be the result.

The present invention seeks to provide improved combustible gas detectors in which the above disadvantages are reduced.

According to this invention a combustible gas detector arrangement of the kind referred to is provided in which said heatable wire filament is formed of an alloy of platinum with zirconia as a dispersant strengthener.

The alloy itslef from which the filaments of the detector arrangements in accordance with the present invention are made may be known per se and as described, for example, in an article entitled "Dispersion Strengthened Platinum" by G. L. Selman, J. G. Day and A. A. Bourne published in "Platinum Metals Review" 1974 Volume 18, No. 2.

Preferably said heatable wire filament is embedded in a pellet comprising ceramic material.

Preferably, and in accordance with our copending application number 25464/76 (Serial No. 1516039) said heatable wire filament is embedded in a pellet consisting of a homogeneous mixture of an oxidation catalyst material and a substantially non catalytic carrier material and a layer of oxidation catalyst material is provided on the outside of said homogeneous pellet. Preferably said layer completely surrounds said pellet.

Preferably said catalyst material of said homogeneous mixture forming said pellet and said catalyst material of said layer are both palladium and/or platinum.

Preferably again said carrier material is alumina.

As is well known, it is usual in use to include the heatable wire filament in one arm of a balanced bridge circuit in another arm of which is included a compensating element which is similar to the gas detecting element.

Where said heatable gas detecting element and said compensating element are naked said gas detecting element will be arranged to be exposed to gas, but not said compensating element. Where said filament of said gas detecting element is naked, said compensating efementmay be constituted by a suitable compensating resistor.

Where 'aid - filament of said-gas dtecting element is enclosed in a pellet including oxidation catalyst material, a compensating- element is utilised which is similar to the gas detecting element save for the fact that no catalyst material is provided and the refrac tory. material -- is poisoned to suppress the oxidation of combustible gases. The functionof the compensating element is to compensate for changes in ambeint conditions, its value of resistance being sensitive only to changes in ambient conditions.

LE one gas detecting arrangement in eluding a balanced bridge circuit arrangement as described above, the wire filament of the compensating element is similar to that of the gas detecting element itself and is - em- bedded in a pellet consisting of a homogeneous mixture of a substantially non catalytic carrier material and a poison acting to suppress the oxidation of combustible gases. Preferably said poison is potassium hydroxide.

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

Referring to the drawing, 1 is a gas detecting element consisting of a heatable wire filament 2 of ZGS platinum embedded in a pellet 3 consisting of a homogeneous mixture of alumina, and an oxidation catalyst material, palladium and platinum. 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 also a mixture of palladium and platinum formed by applying relatively high concentration solutions to the outside of the pellet 3.

The gas detecting element 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 ZGS platinum embedded in a pellet 9 consisting 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. Themeter 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.

In operation the gas detecting element 1 and compensating element 7 are exposed to normal atmosphere and the slider potentio meter 11, adjusted to give a zero reading on meter 10. The gas detecting element and com pensating element 7 are then exposed to the atmosphere which it is required to monitor.

-Any combustible gases in the atmosphere oxidise on the surface-of detecting element 1, but not on the surface of compensating element 7, causing. the temperature of the filament 2 to rise lviffi å CQnseqUent change in its - resistance. - The readi-ug- meter 10 then provides a measure- - of -edneentration of combustible gases in the atmosphere.

In the .manufacture of the gas detecting element 1, a wire of ZGS platinum (that is to say an alloy of platinum with zirconia as a dispersant strengthener) is first wound to form filament-2 and tin cleaned A solution of palladium chloride, platinum chloride, concentrated hydrochloric acid, distilled water and an aluminium nitrate solution is then made up in the following quantities: 1 gram of palladium chloride 1 gram of platinum chloride 12-5 ml of concentrated hydrochloric acid 12-5 ml of distilled water 100 ml of saturated aluminium nitrate solution at 200 C.

A pellet is then built up upon the filament 2 by an evaporation technique as known per se.

A solution of palladium chloride, platinum chloride, concentrated hydrochloric acid, distilled water and an aluminium nitrate solution is then made up on the following quantities: 1 gram of palladium chloride 1 gram of platinum chloride 25 ml 2-5N concentrated hydrochloric acid 3 ml saturated aluminium nitrate solution at 20"C.

Using this solution a layer is then formed around the pellet previously formed, again by an evaporation technique. The compensat ing element 6 is also formed byan evaporation technique in a manner similar to that in which the basic pellet of the gas detecting element I is formed except that potassium hydroxide replaces the chlorides in the solution.

As with known gas detector elements, prior to operation the element is "activated" by operating the element at elevated tem perature in a reducing atmosphere.

With the constituents above mentioned used for the gas detecting element and com pensatory element, the former naturally is dark in colour whilst the latter is light in colour. Thus the former has a thermal emissivity which is different from that of the latter. This leads to a change in the balance of the bridge upon a change of voltage applied to terminals 12. In order to minimise this, a compensating resistor (not shown) may be connected across the compensating element 7.

Claims (11)

WHAT WE CLAIM IS:

1. A combustible gas detector arrangement of the kind referred to in which said heatable wire filament is formed of an alloy of platinum with zirconia as a dispersant strengthener.

2. An arrangement as claimed in claim 1 and wherein said heatable wire filament is embedded in a pellet comprising ceramic material.

3. An arrangement as claimed in claim I or 2 and wherein said heatable wire filament is embedded in a pellet consisting of a homogeneous mixture of an oxidation catalyst material and a substantially non catalytic carrier material and a layer of oxidation catalyst material is provided on the outside of said homogenous pellet.

4. An arrangement as claimed in claim 3 and wherein said layer completely surrounds said pellet.

5. An arrangement as claimed in claim 3 or 4 and wherein said catalyst material of said homogeneous mixture forming said pellet and said catalyst material of said layer are both palladium and/or platinum.

6. An arrangement as claimed in any of the above claims 3 to 5 and wherein said carrier material is alumina.

7. An arrangement as claimed in any of the above claims and comprising a balanced bridge circuit including said heatable wire filament as a gas detecting element in one arm and a compensating element in another arm.

8. An arrangement as claimed in claim 7 as dependent upon any of claims 3 to 6 and wherein said compensating element is similar to said gas detecting element save for the fact that no catalyst material is provided and the refractory material is poisoned to suppress the oxidation of combustible gases.

9. An arrangement as claimed in claim 7 as dependent upon any of the above claims 3 to 6 and wherein said compensating element comprises a wire element similar to that of the gas detecting element and embedded in a pellet consisting of a homogeneous mixture of a substantially non catalytic carrier material and a poison acting to suppress the oxidation of combustible gases.

10. An arrangement as claimed in claim 9 and wherein said poison is potassium hydroxide.

11. A gas detector arrangement substantially as herein described with reference to the accompanying drawing.