GB2332525A - Gas or vapour sensing assembly - Google Patents

Abstract

A gas or vapour sensing assembly comprising a catalytic detector 18 for coupling to a control circuit for detecting a gas or vapour incident on the surface of the detector. A removal element 10 is located upstream of the detector which, when heated, reacts and removes a gas or vapour which could poison or inhibit the detector, before the gas or vapour reaches the detector, the removal element being insensitive to the gas or vapour to be detected.

Description

1 1 GAS OR VAPOUR SENSING ASSEMBLY 2332525 The invention relates to a

sensing assembly for sensing a gas or vapour in air.

Conventional catalytic oxidation devices for sensing combustible gases or vapours incorporate an electrically heated platinum coil embedded within a detector bead. At the appropriate temperature the gas or vapour to be measured is catalytically oxidised on the detector bead.

Heat is evolved during this process thereby increasing the temperature and, consequently, the electrical resistance of the platinum coil contained within the bead. This change in resistance is a measure of the amount of combustible gas or vapour present in the atmosphere under test. In a is complete device a second, compensator bead is also employed to compensate for changes in ambient conditions, such as pressure, humidity and/or temperature, which could provide erroneous results. The matched pair of detecting and compensating beads are conveniently employed within a Wheatstone bridge measurement circuit providing a signal which is proportional to the concentration of combustible gas or vapour in the atmosphere under test. The detector and compensator beads are known as pellistors.

A problem which can arise with known pellistors is that they can be poisoned by certain gases or vapours to which the detector bead is exposed. The poison resistance of a conventional catalytic bead sensor largely depends upon the surface area of catalyst. When poisons such as silicone vapours reach the heated catalyst it is thought that the silicone is converted into a silica overlayer which progressively blocks the active catalyst sites. As this process continues the signal from the element decreases until the element is rendered inactive to combustible gases such as methane. This process is irreversible.

The effect of inhibitors such as hydrogen sulphide is also a problem although it can be reduced by raising the 2 temperature of the element the hotter the element the smaller the inhibition.

Charcoal cloth filters have been used for many years to remove poisons but suffer from the disadvantage of requiring more frequent replacement and preventing many gases which need to be detected such as heavier hydrocarbons from passing through.

In accordance with the present invention, a gas or vapour sensing assembly comprises a detector for coupling to a control circuit for detecting a gas or vapour incident on the surface of the detector; and a removal element located upstream of the detector which, when heated, reacts and removes a gas or vapour which could poison or inhibit the detector, before the gas or vapour reaches the is detector, the removal element being insensitive to the gas or vapour to be detected.

In this invention, we reduce the detector element susceptibility to poisoning, for example silicone poisoning, by positioning a heated removal element upstream of the detector so that the poison vapour or inhibiting gas can be removed before it reaches the detector. This removal element could, for example, be in the f orm of a hot ',sacrificial" element.

A possible problem in some cases with this approach is that the assembly will require additional power to that required by a conventional assembly in order to raise the temperature of the removal element to a level at which it will react the poison vapour or inhibiting gas.

Preferably, therefore, the removal element comprises a compensator element having a similar form to the detector but is insensitive to the gas or vapour to be detected, the compensator element being coupled with the control circuit in use.

As mentioned previously, a conventional pellistor arrangement includes a catalytic bead element and a corresponding compensator element, the compensator element being substantially identical to the detector bead element 3 but without a catalyst. This compensator element can be utilised as the removal element of this invention thus maintaining overall power consumption of the assembly substantially unchanged.

This use of the compensator to remove silicone and to react hydrogen sulphide before reaching the detector, offers the possibility of either a) increased silicone poison resistance on standard size elements or b) reduced power and equivalent poison resistance with smaller size elements.

This arrangement should not affect the sensor's normal response to (flammable) gases and vapours as the compensator should normally be inactive to these gases. Neither should it affect the compensator's normal operation as "extra" poison added to it should not significantly alter its performance.

In addition, by removing H2S before it reaches the detector one could operate a standard size element at lower temperature and power than at present (for example sufficiently hot to oxidise methane but not hotter so as to prevent H2S inhibition). This could in turn improve zero drift performance which is thought to be related to the temperature of the element (the cooler, the better the performance).

Another alternative could be to split the compensator into two (or more) elements to provide further poison resistance (e.g. such that three elements are in series). High surface area porous coatings of refractory materials, such as silica, alumina, and zirconia, on the compensator element could further increase poison resistance without degrading performance.

The gas sensing assembly according to the invention could be fabricated in a conventional pellistor can arrangement but is particularly suitable for implementation in an arrangement of the kind described in EP-A-0667519.

4 Although the invention is particularly suitable where the detector is a catalytic bead detector for detecting a combustible gas or vapour, it is also applicable to semiconductor sensors. A heated element that was hot enough to react with silicone vapours but didn't oxidise any of the gases reaching a heated semiconductor sensor (by virtue of not having a catalyst) could also be used to reduce poisoning effects on the semiconductor sensor whether or not this had a compensator. However, if the semiconductor sensor did have a compensator it would be advantageous to place it in the same position, i.e. upstream of the sensor.

Some examples of combustible gas sensing assemblies according to the invention will now be described with is reference to the accompanying drawings, in which:- Figure 1 is a schematic cross-section through a first example of the assembly; Figure 2 is a schematic plan of the upper layer of the assembly shown in Figure 1; Figure 3 is a schematic plan of the lower layer of the assembly shown in Figure 1; Figure 4 is a circuit diagram illustrating the circuit attached to the assembly shown in Figures 1 to 3; Figures 5 and 6 are schematic, perspective views of two further examples (with internal components shown); and, Figure 7 is a circuit for use with the Figure 6 example.

The assembly shown in Figures 1 to 3 comprises a pair of printed circuit boards (PCBs) 1,2 secured together one above the other with a layer of glass wool 12 between them. Each PCB 1,2 has a respective aperture 3,4 which are aligned. The lower surface of the PCB 1 carries conductive tracks 5,6 connected at respective pins 7,8. The track 5 is connected via a lead 9 to a compensator bead 10 while the track 6 is connected to the compensator bead 10 via a lead 11. The compensator bead 10 is suspended in the aperture 3 via the leads 9,11 and is supported against 1-, 1 is vibratory movement and the like by the layer of glass wool 12 which extends into the aperture 3, and by a f urther layer of glass wool 13.

The lower surface of the PCB 2 carries tracks 14,15 which are coupled with pins 16,8 respectively. The track 15 is connected to a lead 17 which is coupled with a detector bead 18 while the track 14 is connected with the detector bead 18 via a lead 19. The detector bead 18 is supported in the aperture 4 by the leads 17,19 and is again protected against vibratory movement and the like by the glass wool layer 12. The layer 12 also thermally isolates the beads 10,18 while allowing gas to travel through. This allows the beads to be located close together and thereby reduce response times.

The PCBs 1,2 are mounted in a stainless steel outer housing 30 having a gas inlet aperture 31 through which gas to be sensed passes. The gas then passes through a conventional sinter layer 32 before entering the aperture 3. The housing 30 is closed by a layer of epoxy 33. The aperture 4 is shielded from the gas inlet so that the gas can only reach the aperture 4 after passing through the aperture 3.

The pins 7,8,16 are connected in use into a control circuit forming a Wheatstone bridge (Figure 4). The pin 8 forms one output point 41 directly while the pins 7,16 are coupled to respective resistors R,,,Rl at 43,42 respectively. The resistors R1,R2 are connected at 44 to a zero set variable resistor 45 which can be adjusted between 0 and 1 kohm. The point 44 constitutes the other output pole. A trimming resistor 71 is shown connected between the pins 7,8 and is provided to compensate for differences in performance of the elements 10, 18 with temperature. DC power is supplied from a source 46 to the two points 42,43. The resistors R,,R2 would each typically be fixed at 27 ohm although in some cases these could be varied. operation of the circuit is the same as in a conventional device. Thus, the supply 46 causes current to flow through the elements 6 10,18 thus raising their temperature until a balance condition is reached. Thereafter, any combustible gas to which the detector 18 is sensitive will combust in the region of the detector 18, thus raising its temperature but will not combust on the compensator element 10. Gas combustion causes a change in resistance of the element 18 thereby causing a signal to be generated between the outputs 41,44 which can be measured.

Any poisoning vapours or inhibiting gas will react at the heated compensating element 10 and will therefore not reach the detector element 18.

The detector and compensator elements 18,10 will have conventional construction, typically comprising a bead of ceramic material. In the case of the detector element is 18, the ceramic material will be additionally coated with a layer of a catalyst, such as platinum and/or palladium, which may be chosen according to the gas which is to be detected. In an alternative arrangement, the ceramic material could be impregnated with the catalyst.

It is important that in use, the temperature of the compensator element 13 is not raised to a level at which the combustible gad to be detected reacts.

Figure 5 illustrates a second example in which the compensator element 101 and detector element 181 are separately mounted in a stainless steel can 80 having an inlet aperture 81. The elements 101,181 are separated by a layer of glass wool 82. In this case, each element 101 181 is connected to a respective pair of pins 83,84 for connection to a detection circuit.

In further examples, the compensator element 13 could be sub-divided into two or more elements electrically connected in series but located one above the other in the gas flow path. This will increase the surface area exposed to poisons and the like thus increasing the surface area exposed to the poisoning and inhibiting gases and vapours.

An example is illustrated in Figure 6 where three compensating elements lOA-10C are located one above the 1 I_ 7 other in a stainless steel tube 85, the compensator elements being separated by respective layers of glass wool 86 and from a detector element 87 similar to the detector element 18. The components are housed in an outer can 88 having an gas inlet aperture 89.

Figure 7 illustrates a Wheatstone bridge circuit for use with the assembly shown in Figure 6.

8

Claims (7)

1. A gas or vapour sensing assembly comprising a detector for coupling to a control circuit for detecting a gas or vapour incident on the surface of the detector; and a removal element located upstream of the detector which, when heated, reacts and removes a gas or vapour which could poison or inhibit the detector, before the gas or vapour reaches the detector, the removal element being insensitive to the gas or vapour to be detected.

2. An assembly according to claim 1, wherein the removal element comprises a bead of material with which a poisoning or inhibiting gas or vapour will react.

3. An assembly according to claim 1 or claim 2, wherein the removal element comprises a compensator element having a similar form to the detector but is insensitive to the gas or vapour to be detected, the compensator element being coupled with the control circuit in use.

4. An assembly according to claim 3, wherein the detector and compensator element are connected in a Wheatstone bridge.

5. An assembly according to any of the preceding claims, further comprising a housing defining an aperture into which gas or vapour diffuses, the detector and the removal element being mounted in the aperture.

6. An assembly according to any of the preceding claims, wherein the detector is a catalytic bead detector for detecting a combustible gas or vapour.

7. A gas or vapour sensing assembly substantially as 30 hereinbefore described with reference to any of the examples shown in the accompanying drawings.