GB2088559A - Gas sensing equipment - Google Patents

Abstract

A detector head defines a gas sensing chamber 4 and is provided with a detector pellistor 9 and a compensating pellistor 10 housed within two protective shields or cans 28 and 29, respectively. An insulating component 12 of relatively high thermal resistance provides thermal insulation to reduce heat losses from the gas sensing chamber and an insert component 21 of relatively low thermal resistance effectively thermally connects the two protective shields. <IMAGE>

Description

SPECIFICATION Gas sensing equipment This invention relates to gas sensing equipment.

In particular, although not exclusively, the present invention relates to gas sensing equipment, for example, methane sensing equipment having a detector head comprising a sensitive detector pellistor and a compensator pellistor housed within protective shields or casings and connected in a Wheatstone bridge circuit provided with an amplifier for amplifying out of balance voltage due to the detector pellistor sensing the presence of methane.

With known methane sensing equipment the performance of the equipment can deteriorate rapidly when subjected to concentrations of methane above 0.8% by volume in conditions of high relative humidity as frequently exist in underground coal mine atmospheres. At such underground sites it is normally adopted practise to regularly change the detector head, typically the head is changed at weekly intervals. However, in mine atmospheres where the methane concentration is above 1% and the relative humidity is greater than 95% detector heads have to be changed after a few days, due to less of sensitivity and zero drift.

Investigations have revealed that the rapid failure of the pellistors in conditions of high methane concentrations and high relative humidity is due to water formation inside the detector peilistor shield, the water being formed as a result of condensation of the products of combustion inside the detector shield. A secondary effect of the water was rusting of the pellistorsupport pillars and base and of the protective shield housing the pellistor. Loose flakes of rust could settle on the catalytic bead and cause unacceptably high zero shift.

An object of the present invention is to provide methane sensing equipment which tends to overcome or reduce the above mentioned problem.

According to the present invention gas sensing equipment comprises a detector head defining a chamber for the gas to be sensed, the head having a detector element and a compensator element housed within two protective shields, respectively, and connectable such that an out of balance condition of the pellistors can be detected, an insulating component of relatively high thermal resistance tending to provide thermal insulation to reduce heat losses from the protective shields, and an insert component of relatively low thermal resistance tending to effectively thermally connect the two protective shields.

Preferably, the insert component is mounted between the two protective shields.

Preferably, the insert component comprises a block having two recesses for accommodating the two protective shields, respectively.

Preferably, the insulating component comprises a block having a recess for accommodating the insert component.

Advantageously, the two elements are connected in a Wheatstone bridge arranged to derive a signal indicative of the out of balance condition of the two pellistors.

Conveniently, the two elements comprise pellistors.

Preferably, each protective shield comprises a generally cylindrical can having an aperture providing a passage for gas flow.

By way of example only, one embodiment of the present invention will be described with reference to the accompanying drawings, in which: Figure lisa longitudinal section taken through a detector head of methane sensing equipment constructed in accordance with the present invention; Figure 2 is a section, partly exploded view of a detail of Figure 1 drawn on an enlarged scale; Figure 3 is an end view of a further detail of Figure 1; Figure 4 is a section taken along line IV - IV of Figure 3; and Figure 5 is an end view of the further detail of Figure 3, but viewed from the opposite end to that shown in Figure 3.

Figure 1 shows the detector head of methane sensing equipment suitable for use in an underground coal mine atmosphere. The detector head comprises an outer casing 1 provided with a threaded portion 2 for removably securing the head in an operational position mounted on a supporting portion of the sensing equipment (not shown). The end of the casing remote from the threaded portion is provided with an open window 3 permitting diffusion of mine atmosphere into gas sensing chambers 4 defined by the head.

Within the casing 1 is mounted an electrical circuit block 5 for receiving input and output cables 6 leading from the supporting portion of the sensing equipment and cables 7 and 8 connecting the block 5 to a detector element for detecting the presence of methane in the sampled mine air and a compensating element which is insensitive to the presence of methane constituted by a detector pellistor 9 and a compensator pellistor 10, respectively. The block 5 includes a Wheatstone bridge circuit with the two pellistors 9 and 10 arranged in the bridge such that an out of balance condition existing between the two pellistors is detected by the bridge, a derived signal indicative of the out of balance condition is amplified and fed along cables 6 to monitor means provided elsewhere in the sensing equipment.

The two pellistors 9 and 10 are mounted in protective shields constituted by perforated generally cylindrical cans 28 and 29, respectively, fixedly mounted on a backing plate 11 located on support pillars (not shown), the backing plate abuting a generally cylindrical insulating component 12 (details of which are shown in Figures 1 to 5) which is made of relatively high thermal resistance material tending to thermally insulate the protective shields to thereby reduce the amount of heat loss from the shields. For example, the insulating component 12 is made from plastic.

The radially outer margin of the insulating component 12 is provided with a groove 13 accommodating an 'O' ring seal 14which as seen in Figure 1 forms an air tight seal engagement with the innermost surface of the casing 1. The end margin of the insulating component 12 remote from the backing plate 11 is provided with a thread portion 15 onto which a retaining cap 16 is reieasably fixedly engaged, the cap having an open window 17 permitting diffusion of gas into the sensing chamber 4 (in Figure 2 the retained cap is shown disengaged from the insulating component).

The insulating component 12 is provided with an axially extending slot or recess 20 which accommodates an insert component 21 constituted by a block of relatively low thermal resistance material, for example, brass. The insert component 21 has two through bores or recesses 22 and 23 accommodating the detector and compensator pellistor cans 28 and 29 respectively. The bores 22 and 23 are counterbored to accept flanges 25 provided on the pellistor cans and '0' ring seals 26. The insert component 21 tends to thermally connect the two peilistor shields such that in use when the pellistor shields become heated during the operation both the shields are maintained at substantially the same temperature.Thus, when the sensing equipment is subjected to changes in ambient temperature both shields tend to be maintained at an equal temperature and zero drift tends to be eliminated or maintained to an acceptably low level.

The detector head also comprises a gas filter arrangement which comprises a carbon cloth filter 30 sandwiched between two bronze sinter disc fiiters 31 and 32 and which is provided to remove poisons and inhibitors contained in the gas to be sampled.

An anti-skid shim 33 is provided to ensure the engaged retained cap 16 retains the filter arrangement in position. The shim 33 is generally an annular washer having two projections 34 engageable in recesses 35 provided in the insulating component such that the shim is prevented from rotational movement with the retainer cap.

In use when the methane sensing equipment is installed to detect the methane concentration in mine atmosphere, the detector head is mounted such that a continuous sample of mine air to be monitored is allowed to diffuse through the sinters and carbon cloth filter into the gas sensing chambers 4. As mentioned above the filter arrangement removes from the gas to be sampled all traces of substances likely to poison or inhibit the action of the detector pellistor. The air to be sampled passes through holes 40 provided in the pellistor cans 28 and 29 to reach the pellistors 9 and 10. The detector pellistor 9 typically comprises a helical coil type resistance filament of platinum alloy wire mounted within a bead of refractory material typically, alumina, the bead being coated with a catalyst,for example, palladium and thoria.In the sensing operation, the detector pellistor is mounted to sense the mine atmosphere within the can 28. A sufficient potential is applied across the Wheatstone bridge arrangement until the outer surface of the bead reaches a temperature of approximately 500"C when combustible gases, ie methane start to burn on the bead. Thus, the temperature of the detector pellistor is increased by the burning methane and, in consequence the resistance of the filament coil increases. Any increase in temperature and, thereby, increase in filament coil resistance being proportion al to the methane concentration in the sampled mine atmosphere.

The compensating pellistor 10 also includes a bead of refractory material. However, the compensating bead is treated to prevent combustible gases burning on the surface. As seen in Figure 1 the compensating pellistor is mounted along side the detector pellistor to compensate for changes in ambient temperature, bridge voltage, or in the water vapour concentration level, or in the atmospheric pressure, both the detector and the compensating elements being substantially equally affected by these parameters. The other two arms of the Wheatstone bridge circuit arrangement include two fixed resistances.

During operation of the Wheatstone bridge both pellistor shields become warm (for example 35"C) and during when the presence of methane is sensed the small increase in the operational temperature of the detector bead has negligible effect on the shield temperature. The effect of the insulating component 12 is to provide thermal insulation for the detector shield which thereby during the sensing operation tends to be maintained at a sufficiently high temperature to substantially prevent or reduce to an acceptable level the amount of condensation of the products of combustion taking place in the detector can. Thus, the amount of rusting taking place tends to be reduced to an acceptable level and the sensing equipment tends to have a relatively long operational life.

The affect of the brass insert component 21 is to effectively thermally connect the detector and compensating cans such that both shields are maintained at substantially the same temperature therefore ambient temperature variations do not cause zero drift problems which could occur if the insert was not provided and only an insulating material component was used. Thus, upon the sensing equipment being checked in air both shields substantially are at the same temperature and zero shift tends to be eliminated or reduced to an acceptable level.

From the above description it will be seen that the present invention tends to overcome or reduce the aforementioned problems associated with the prior equipment.

Claims (8)

1. Gas sensing equipment comprising a detector head defining a chamberforthe gas to be sensed, the head having a detector element and a compensator element housed within two protective shields, respectively, and connectable such that an out of balance condition of the pellistors can be detected, an insulating component of relatively high thermal resistance tending to provide thermal insulation to reduce heat losses from the protective shields, and an insert component of relatively low thermal resist ance tending to effectively thermally connect the two protective shields.

2. Equipment as claimed in claim 1, in which the insert component is mounted between the two protective shields.

3. Equipment as claimed in claim 1 or 2, in wh the insert component comprises a block having tv recesses for accommodating the two protective shields, respectively.

4. Equipment as claimed in claim 1,2 or 3, in which the insulating component comprises a bloc having a recess for accommodating the insert con ponent.

5. Equipment as claimed in any one of the preceding claims, in which the two elements are connected in a Wheatstone bridge arranged to derive a signal indicative of the out of balance condition ofthetwo pellistors.

6. Equipment as claimed in any one of the preceding claims, in which the two elements com prise pellistors.

7. Equipment as claimed in any one of the preceding claims, in which each protective shield comprises a generally cylindrical can having an aperture providing a passage for gas flow.

8. Gas sensing equipment substantially as described herein and substantially as shown in the accompanying drawings.