WO2004048959A2

WO2004048959A2 - Gas sensing assembly

Info

Publication number: WO2004048959A2
Application number: PCT/GB2003/005134
Authority: WO
Inventors: Graham Collins
Original assignee: City Technology Limited
Priority date: 2002-11-27
Filing date: 2003-11-26
Publication date: 2004-06-10
Also published as: WO2004048959A3; AU2003286267A1; AU2003286267A8

Abstract

A gas sensing assembly comprises a gas sensor having a housing (24) defining an internal cavity (40) in which is located at least one gas sensing component (20, 21), electrical contacts (4) mounted on an outer surface of the housing and coupled to the gas sensing component, and an opening (42) in the housing to allow gas to enter the cavity. A connector has a housing (28) into which the gas sensor can be removably fitted, the connector having contacts (9) which cooperate with the sensor contacts (4) when the gas sensor is fully located in the connector housing. The sensor has at least one keying feature (1, 5) mounted to the housing, the keying feature cooperating with a complementary feature (2, 11) of the connector housing when the gas sensor is fitted to the connector housing.

Description

GAS SENSING ASSEMBLY

The invention relates to gas sensing assemblies comprising a gas sensor and a connector to which the gas sensor can be removably fitted.

Electrochemical, catalytic and other sensor types may be designed in a variety of physical formats depending upon the selection of the sensing method to be employed, the availability of appropriate components and the skill of the designer. Commonly they take the form of cylinders whose height to diameter ratio is typically between 1 and 2, with typical dimensions in the region of 10 -30mm. (See Table 1.) It is widely known that smaller, more compact devices are increasingly required to address numerous applications where space and weight are at a premium, for example in portable or badge-type warning devices intended for personal protection in hazardous environments .

These are preferably mounted on printed circuit boards using a connector, providing improved noise performance and reducing the risk of sensor damage or impairment during soldering or other attachment processes . But the remaining components on the pcb generally have heights of only a few millimetres, and so there is a strong desire to produce sensors with low profiles so that more efficient mechanical designs for instrument housings can be developed. However, it is also important that reductions in the overall envelope size do not prevent easy handling by assembly and maintenance staff. Instruments in industrial applications have long service lives and normally require numerous sensor replacements during their operational duty. This is particularly true for instruments containing multiple sensors with varying replacement schedules.

Notwithstanding the desirability of such gas sensor envelope designs and the commercial advantages to be derived from their introduction, it has proved rather difficult to develop appropriate technologies at reasonable cost. For example, consumable anode electrochemical oxygen sensors can be reduced in size, but at the expense of greatly reduced life. Toxic gas sensors based on liquid electrolytes can be made smaller than current designs, but the reduced water balance capacity severely compromises performance in extremes of humidity at high temperature. Flameproof combustible sensor enclosures have proved extremely difficult to miniaturise whilst meeting the constraints of safety certification. Significant technical developments have therefore been required in a number of fields to provide a range of high performance sensors with the attributes demanded by industrial customers and size reductions giving greater design freedom without any significant increase in cost.

We have recently described electrochemical toxic gas sensors based on a novel solid polymer electrolyte system WO-A-02/11225; WO-A-02/088694 which may be produced in such low-profile formats. We have also shown how the use of a plastic moulded housing can facilitate the production of catalytic bead-based flammable gas sensors with similarly advantageous external dimensions. Work in other sensor technologies is now well advanced in order to offer similar options for the detection of further species. These sensors (the so-called "MICROceL™" range manufactured by City Technology Limited) are referred to hereafter as low profile designs, by virtue of the fact that they typically have length (or width) to height ratios in excess of 3 (see Table 1) .

Such developments require simultaneous advances in connectivity to enable maximum exploitation of the relevant ' sensor features.

In a large majority of traditional designs, pins on the sensor fit into sockets on the pcb and these can provide adequate electrical performance, although the cavities within the sockets tend to become filled with debris over the life of the instrument and their replacement is a time consuming and relatively expensive process. Furthermore, vibration and frequent changes of attitude during use may loosen the sensor (resulting in noisy/erroneous signals or complete failure) and so additional features to retain the sensor are required. Although some market standardisation of sensor envelope dimensions has occurred, in practise a range of sensor envelopes must be accommodated. This can prove awkward and mitigates against a truly flexible arrangement with interchangeable sensors, especially as gas access is also required. Bayonet fittings are used in some sensors to provide mechanical location independent of the electrical connection but these require a relatively high degree of access in order to impart the twisting necessary for location.

Table 1 shows that the pin length protruding from the sensor in order to provide a robust fit is a significant fraction of the overall height of traditional designs, and is large in comparison with the thickness of the low profile sensors considered here. This negates efforts to lower the overall profile of the system due to the need for access to and clearance- beyond the final sensor position so that pin insertion may occur correctly. Pin protrusion beyond the sensor base may also cause problems in that lateral stresses as a result of cantilever forces due to the length of the pin can weaken or loosen the pin allowing electrolyte or flame leakage paths to develop. Significant volumes within the sensor are occupied by features to ensure adequate pin retention, a particular problem when pins protrude in a direction along which the housing has a small dimension (e.g. from the base of a low profile sensor) .

For example, a typical conventional wet electrolyte toxic sensor manufactured by City Technology Ltd, (type 4CO) has a cylindrical body about 20.0mm in diameter and about 16.5mm tall. However, the connecting/locating pins protrude some 4.3mm beyond the base of the cylinder and so comprise over 20% of the total height. In addition, the pin extends 3-4mm inside the housing for the ^• urposes of attachment and sealing. Therefore, a low profile sensor housing with a height of 5mm or less, using the same pin technology, would have a protrusion which almost doubles the total height of the assembly. The pin insertion length would be virtually equal to the housing height.

A further requirement is that sensors of various types should be accommodated in similar connectors. One approach is to use "universal" connectors which are physically capable of accepting all sensor types but with the position and nature of the electrical contacts providing the customising feature (e.g. an array of receiving sockets, a subset of which mate with the pins on each sensor type) . However, where many sensor variants must be accommodated, the degrees of freedom for sensor insertion into a number of receiving sockets makes the design of a compact, foolproof and failsafe arrangement difficult. Sensor identification may also be achieved by incorporating EEPROM or other memory within the device, but this requires much more sophisticated instrument design. -Difficulties remain in cases where special features (such as specific power supply rails) are required for different sensors and hence channel signal processing reconfiguration alone is insufficient. There is also a lack of uniformity or international standards for data protocols in such applications, so economies of scale in sensor manufacture, testing and information storage can therefore be difficult to achieve.

A complementary approach to connector design is to customise every version uniquely to a single sensor type. For example, WO-A-02/088694 describes arrangements for the connection of electrochemical sensors based on solid polymer electrolytes (SPE) using receptacles having cross sectional shapes which are unique to sensors for specific species. In this case, some duplication of electrical pad positions is permissible since some sensors cannot physically engage with particular connectors. However, the number . of unique variants which can sensibly be incorporated in any given system is limited and the (potentially) wide range of connector types significantly increases the instrument design effort needed. The examples shown also provide integral gas plenums, which are necessary in pumped gas feed systems .

In accordance with a first aspect of the present invention, a gas sensing assembly comprises a gas sensor having a housing defining an internal cavity in which is located at least one gas sensing component, electrical contacts mounted on an outer surface of the housing and coupled to the gas sensing component, and an opening in the housing to allow gas to enter the cavity; and a connector having a housing into which the gas sensor can be removably fitted, the connector having contacts which cooperate with the sensor contacts when the gas sensor is fully located in the connector housing and is characterized in that the sensor has at least one keying feature mounted to the housing, the keying feature cooperating with a complementary feature of the connector housing when the gas sensor is fitted to the connector housing.

In accordance with a second aspect of the present invention, we provide a set of at least two gas sensing assemblies according to the first aspect of the present invention, wherein keying features are selected such that each gas sensor can only be fully, operatively fitted to one connector.

In accordance with further aspects of the present invention, we provide gas sensors and connectors for use in assemblies and sets according to the first and second aspects.

We have realized that the limitations of the known sensors and connectors which have been designed to provide unique fitting combinations can be overcome by utilizing additional keying features to those which are inherent in the normal design of the components. In other words, the absence of the keying feature does not impair the performance of the gas sensor and this in turn enables keying features to be provided in a wide variety of forms and locations.

Consequently, the keying feature (s) will perform no functional role in the gas sensing process, for example it is not used to retain electrolyte in the case or an electrochemical gas sensor or as an expansion space or to define a critical potting dimension. As such, it can be removed with no distinguishable effect on sensor performance and it merely acts to constrain and retain the sensor assembly. Thus, design features key to the sensor functionality (e.g. the containment of aggressive liquid electrolytes or the provision of flameproof seals) are preferably confined to regions which are not subject to the additional demands to provide mechanical retention within the connector.

In the examples to be described, therefore, the keying features do not contain spaces which are or can be occupied either by gas (for example the expansion space of a reservoir, or as an extension of the region around the sensing elements connected to the main sensor opening) ; or electrolyte (which might be solid or liquid) ; or electrochemically active components such as electrodes.

This invention thus allows a wide variety of gas sensing assemblies to be constructed which have similar designs but which have particular keying features so that they can be correctly mounted to appropriate connectors without the need for accurate identification of the sensor assembly. The gas sensors may vary in their type, for example electrochemical, catalytic, semiconductor, optical (i.r., UN or visible wavelength), and thermal conductivity sensors and/or in the gas which they sense, for example from the group of oxygen, toxic, and/or combustible gases.

A wide variety of keying features are envisaged including the use of elongate guides such as parallel rails or slots or grooves on each side of the assembly housing, and the provision of ribs or other features on the housing. Further keying features could be added to the physical keying feature (s) such as providing unique layouts for the electrical contacts.

The longitudinal rails and guide slots or grooves need not necessarily be continuous. A pair of pegs or other protrusions on the side of the sensor housing could meet many of the demands satisfied by rails. Similarly, a perforated or discontinuous upper surface to the guide slots or grooves would work quite adequately in conjunction with rails. A further preferred feature of the invention is the provision of a connector housing having a profile in the z direction which does not extend beyond the envelope defined by the gas sensor housing. This is * particularly advantageous in the case of low profile gas sensors (as defined above) since the connector will also will present a low profile. By "low profile", we refer to sensors of the type shown in Table 1 which have a length (or width) to height ratio in excess of at least 3, preferably in excess of 4. However, it also allows different sensor heights to be mounted in the connector since there is no restriction in the z direction (in contrast to the examples in WO-A- 02/088694) .

The keying feature may also provide locating means for locating the sensor in or on the connector and/or locking means such as cooperating bumps and detents.

In a particularly preferred example, the locating means constrains movement of the gas sensor assembly in the connector in all directions apart from that of insertion/ withdrawal . It is our experience that separation of the mechanical retention and electrical connection functions within the connector is highly advantageous, so that sensor features may be optimised for their very different functions. Greater design robustness is obtained by arranging that the mechanical location and keying means is/are additional features, playing no part in the gas sensing function. Common mechanical feature (s) can therefore be incorporated into designs based on any sensing technology currently available or which becomes available in future. This approach limits the design effort required to incorporate new devices into instruments. If desired, such features can be incorporated into the housings of sensors having various profiles, or even retrofitted into earlier sensor designs. However, the benefits of the approach are greatest when using low profile devices with a standard height, and the examples below are drawn from this category.

The invention is applicable to a wide variety of gas sensors including electrochemical, catalytic etc.

Some examples of gas sensing assemblies according to the invention will now be described with reference to the accompanying drawings, in which: -

Figures 1A-1E are a bottom plan, section on the line B-B, end view, side view, and section on the line C-C respectively of a first example of a gas sensor;

Figures 2A-2D are a bottom plan, end view, plan and section on the line A-A respectively of a connector for use with the gas sensor in Figure 1;

Figures 3A-3C are a plan, side view and end view of the gas sensor and connector of Figures 1 and 2 wh'en assembled; Figures 4A-4D are views similar to Figures 1A-1D respectively but of a second example of a gas sensor according to the invention;

Figures 5A-5D are views similar to Figures 2A-2D but of a connector for use with gas sensor of Figure 4; Figures 6A-6C are views similar to Figures 3A-3C respectively but showing the gas sensor and connector of Figures 4 and 5 when assembled;

Figures 7A-7C are a bottom plan, end view and side view respectively of a third example of a gas sensor according to the invention; Figures 8A-8E are a bottom plan, end view, plan, section on B-B and section A-A respectively of a connector for use with the gas sensor shown in Figure 7; and,

Figures 9A-9C are views similar to Figures 6A-6C respectively but showing the gas sensor and connector of Figures 7 and 8 assembled.

In the following descriptions, dimensions are defined as follows; x is the width of the sensor, y is the depth of the sensor in the connector insertion (sliding) direction and z is the height of the sensor. Also, those components which are substantially the same in each example have been given the same reference numeral .

Example 1 - Pellistor Sensor and Connector

Sensor Description

A low profile combustible gas sensor employing catalytic sensors (pellistors) is shown in Figures 1A-1E. This has overall dimensions of 17 (x) by 17 (y) by 4 (z) mm and is fabricated by injection moulding a plastic housing 24 for example of PEI, (polyetherimide) , PPS (polyphenylsulphide) , PTFE (polytetrafluoroethylene) around a metal lead frame 25 thereby enclosing a cavity 40 which does not extend into either of a pair of longitudinal rail keying features 1 extending in parallel along opposite sides of the gas sensor. The cavity 40 contains a conventional pair of catalytic bead sensors (compensator 20 and detector 21) , together with glass wool material 22 for shock absorbance and filters 23 for the removal of inhibiting or poisoning species. The cavity 40 is closed by joining a metal mesh flame arrestor 26 to the top of the moulded housing 24 and a further filter 29 is held in place on top of the mesh and under an opening 42 by a bezel 27 which clips into place over the housing 24. Although it is possible to conceive sensor/connector assemblies based on sliding fits where the sensor pins protrude beyond the sensor envelope, in practise it is greatly preferable for the sensor pads 4 to be flush or sub flush in relation to the housing 24. In the example shown here, the pads 4 are sub flush and are formed by moulding the sensor housing around the pads which are made from copper alloy 0.3mm thick, additionally plated with nickel and gold to provide good electrical contact. The overall body thickness in this region is therefore 0.6mm which compares very favourably with the much larger pin insertion and retention dimensions discussed earlier. Figure 1 shows the mechanical keying and locating features ie a pair of rails 1 on the external edges of the sensor. As noted previously, these features are not integral to the gas sensing functionality of the device and in the example shown they comprise moulded parts of the sensor housing 24 fabricated simultaneously with the remainder of the sensor housing but which do not form part of the sensing gas chamber or cavity 40 within the device. If desired, similar features may be added to the sensor body in a subsequent step by other attachment means (e.g. welding, gluing etc.), but simultaneous moulding is a preferable approach.

It will be seen in Figure IE that parts 25 of the lead frame extend into the rails 1. However, this is purely for convenience and in other examples _.the lead frame 1 does not extend into the rails because this does not affect the functionality of the sensor.

Connector Description

Figure 2 shows the general format of a connector for use with the sensor of Figure 1. Spring contacts 9 are housed within a plastic enclosure or housing 28 and are positioned to locate in the recess containing the sub flush pads 4 on the sensor during the final stages of insertion. The system is designed such that the insertion requires a sliding motion in the y direction only, i.e. perpendicular to the low profile dimension of the sensor. Thus, the clearance between the top of the sensor rest position and the external wall of the instrument can be kept to an absolute minimum. No access perpendicular to the direction of sliding is required for the purposes of sensor insertion or removal, thereby allowing highly effective use of the available space.

The connector is preferably formed by overmoulding a high temperature thermoplastic polymer, for example one of the liquid crystal polymers available from Ticona or other manufacturers around the sprung contacts 9. Alternatively, the connector could be in two parts which are snapped together around the contacts. Different forms of sprung contact may be utilised; there is considerable freedom in this respect by virtue of the mechanical retention being achieved by other means. However, the selection should provide adequate repeatability and longevity given the anticipated number of insertion operations and the demand for high integrity contacts in challenging operational environments (including wide temperature ranges, humidity etc . ) . The connector is provided with features to allow attachment to a pcb or other base plate. The device is physically located on to a pcb (not shown) using corner pegs 7 which are designed to mate with preformed holes in the board and provide the required xy location. Adhesive may then be used to complete the location process in common with standard pcb assembly techniques. Alternatively, the connector pins may be made from solderable material, or pcb locks could be used to secure the connector in a preformed aperture in the board. These and other methods which are compatible with known surface mount techniques for circuit assembly are appropriate. Electrical connection in this example is via solderable tabs 10 (conventional reflow or hot iron methods may be used as preferred) , but other methods for connection (e.g. conducting epoxy) could be employed. The electrical connections may exit the connector at other points if this is more convenient to the overall design. The sensor rails 1 are a slide fit into a pair of parallel connector guides or grooves 2 formed in the housing 28 and would typically be manufactured with 0.1 to 0.15mm clearance in both the x and z dimensions. Any movement within this interface in the x direction is taken up by ensuring that the width of each sensor pad 4 is significantly greater than the area of the corresponding spring contact 9, thus allowing lateral sliding to occur without any loss of electrical continuity. In the z direction, movement is absorbed by the spring overtravel potential of the contact 9. This is designed as a cantilever such that substantial pressure is maintained on the surface of the pad 4 thereby pressing the sensor rail 1 against the surface of the connector guide 2. The contact 9 is provided with a radius hook at its end which is able to "roll" across the surface of the sensor pad and thereby maintain good electrical contact in a variety of attitudes and positions.

It is feasible that the location of the rail and guide features could be reversed (i.e. by providing indentations on the sensor mating with protruding rails on the connector base) . However, this is a less favourable approach, especially in small low profile sensors where space is at a premium. Locking of the sensor into the connector is achieved via detent features incorporated in both the connector and the sensor. These features are designed to engage in the final few mm of travel in the y direction during insertion and to provide no interference to the sliding fit until the very end of the insertion travel. As shown in Figures 1 and 2, such features may be located on the sensor rail (a male bump or "pip" 6) with a mating female receptacle 12 on the connector guide. Alternatively (or in addition, as shown) , features may be incorporated on the sensor base (female receptacle 3) and the connector base (male "pip" 8) . In all cases, the detent features rely on the compliance of the polymer materials used to fabricate the sensor and/or connector to allow one or both parts to move within their elastic limit (s) and return to their original^' position (s) .

Figure 3 shows the sensor fully inserted in the housing and the relationship between the rail 1 and guide 2 on each side of the assembly.

Apart from the use of visual aids such as colour coding the sensor and matching connector mouldings, a range of mechanical features are employed in order to provide the required keying of the assembly;

(1) the thickness and profile of the sensor rail 1 is matched to the connector guide 2 ;

(2) the number and position of the contacts 9 is matched to the layout of the sensor pads 4; (3) the sensor housing 24 (and rail 1) have chamfered corners 5 which mate with corresponding features 11 in the connector base .

Other keying features providing further selectivity are described in the examples below. Of course, these features can be provided individually or in any combination.

It will be noted in Figure 3 that the sensor housing 24 protrudes through an opening 50 in the upper surface of the connector housing 28 so that the Z dimension of the sensor is not constrained.

This example should be contrasted with conventional pin and socket arrangements in which it is usual to have multiple spring contacts per connection arranged around the intended pin position. This is not only in an attempt to maintain electrical contact in a variety of orientations, but also to provide some guidance of the pin into the centre of the socket during insertion. However, wrongly angled insertion of the pin can obviously force the springs to move outside of their elastic limits and thereby damage the assembly. In this example, we choose to use a single spring contact which allows travel in the z direction only. Therefore, additional guiding features must be provided if correct location is to be achieved. Our guide design ensures that the sensor is properly aligned before significant movement of the spring occurs, thereby reducing the risk of damage and increasing the likelihood of a good connection.

Example 2 - Oxygen Sensor and Connector

Figure 4 shows an oxygen sensor 13 based on consumable anode technology and employing a solid membrane diffusion barrier 43 in the opening 42 of housing 2 ' . The sensor has parallel, laterally extending rail keying features 14 which are analogous to those in Example 1. The sensor housing has square corners 15 on the insertion edge, which prevent the device mating fully with connectors having chamfered features (such as that shown in Figure 2) .

In this case, the sensor requires only 2 electrical connections which are facilitated by circular sub flush pads 16 in the base of the sensor housing. These have the same separation as the outermost pads on the combustible gas sensor shown in Example 1 but the differing nature of the circuit connections in each case provides an additional keying feature which ensures that no harmful effects ensue even if the other keying features within the design are circumvented and sensors from Examples 1 and 2 are used in the wrong connectors .

Figure 5 illustrates the connector designed for use with this sensor, showing a housing 28' supporting two sprung contacts 17 which are designed and positioned to mate with the sensor pads 16. Both the sensor rails 1 and bottom face 45 of the sensor housing are provided with detent locking features 3 , 8 analogous to those described in Example 1. The connector guides are provided with mating features, again similar to those described in the previous example. Figure 6 shows the sensor located within the connector and clearly illustrates the relationship between the sensor rail 14 and the connector guide 18. Example 3 - SPE Sensor and Connector

Figure 7 shows an electrochemical toxic gas sensor (for example for the detection of CO or H₂S) based on a solid polymer electrolyte. As for the oxygen sensor in Example 2, only 2 electrical connections are required and these are provided by the sub flush pads 32, whose separation is the same as that between adjacent pads in the combustible gas sensor shown in Figure 1. However, pads 32 are symmetrically positioned along the centre line of the sensor, so providing a third different connection format.

The sensor housing 24' ' has a pair of parallel, laterally extending keying rails 1 and in this example also includes an additional rib on its upper surface at 31. This engages with a flexible housing extension 30 (shown in Figure 8) the corresponding connector housing 28 ' ' so locking the sensor in place at the end of its insertion travel. Extension 30 also acts as a further keying feature, since the sensor thickness in the region of rib 31 is significantly less than for either of the sensors in the earlier examples. It therefore prevents full insertion of oxygen or combustible gas sensors into the connector shown in Figure 8.

A further keying feature of the pair is illustrated in Figure 9 which shows the sensor inserted into the connector. The corners of the sensor base on the insertion edge are provided with triangular cut outs 33 on the upper surface, so reducing the thickness of the base in these regions. The corresponding corners of the connector 34 have a matching triangular profile on their upper surface but are undercut to provide full access for the square corner sections at the bottom.

These examples, whilst not exhaustive, illustrate a number of the keying and locking concepts which may be incorporated into a range of sensors and matching connectors meeting the desirable criteria outlined earlier.

Many further keying concepts may also be envisaged and incorporated within this advantageous overall design philosophy. For example, ribs on the connector base extending in the insertion direction may be provided which mate with receiving apertures in particular sensor designs . The use of multiple rib/aperture positions could, if required, be used to provide a further level of selectivity and uniqueness in sensor/connector paring.

All or some of the ideas described may readily be combined in the design of low profile gas sensors and matching connectors in order to provide a range offering the required degree of security against incorrect connection.

TABLE 1

Typical Sensor Dimensions of Electrochemical CO Sensors Manufactured By City Technology Ltd

Claims

1. A gas sensing assembly comprising a gas sensor having a housing defining an internal cavity in which is located at least one gas sensing component, electrical contacts mounted on an outer surface of the housing and coupled to the gas sensing component, and an opening in the housing to allow gas to enter the cavity; and a connector having a housing into which the gas sensor can be removably fitted, the connector having contacts which cooperate with the sensor contacts when the gas sensor is fully located in the connector housing characterized in that the sensor has at least one keying feature mounted to the housing, the keying feature cooperating with a complementary feature of the connector housing when the gas sensor is fitted to the connector housing.

2. An assembly according to claim 1, wherein the keying features comprise one or more pairs of complementary rails and grooves .

3. An assembly according to claim 2, wherein at least one of the rails and/or grooves is continuous.

4. An assembly according to claim 2 or claim 3, wherein the or each rail is mounted to the sensor housing.

5. An assembly according to any of the preceding claims, wherein the keying feature on the gas sensor housing comprises a portion of the perimeter of a lateral flange mounted to the sensor housing.

6. An assembly according to claim 5, wherein the portion of the perimeter defines a chamfered corner.

7. An assembly according to any of the preceding claims, wherein the keying feature (s) of the gas sensor is integrally formed with the sensor housing.

8. An assembly according to any of the preceding claims, wherein the gas sensor is a slide fit in the connector housing.

9. An assembly according to claim 8, wherein the slide fit is achieved using the keying features.

10. An assembly according to any of claims 1 to 7, wherein a further keying feature is provided defined by the physical location of the contacts on one or both of the sensor and connector housings.

11. An assembly according to any of the preceding claims, further comprising complementary locking means on the sensor housing and connector housing respectively which engage when the sensor is fully located in the connector.

12. An assembly according to claim 11, wherein the complementary locking means are defined by a bump and cavity.

13. An assembly according to any of the preceding claims, wherein the sensor housing opening comprises a gas diffusion barrier.

14. An assembly according to claim 13, wherein the opening in the sensor housing comprises a solid phase or gas phase diffusion barrier.

15. An assembly according to any of the preceding claims, wherein the sensor contacts are flush or sub-flush to the sensor housing.

16. An assembly according to any of the preceding claims, wherein the gas sensor housing has a low profile.

17. An assembly according to claim 16, wherein the height of the gas sensor housing is no more than 5mm.

18. An assembly according to claim 16 or claim 17, wherein the ratio of the height of the gas sensor housing to its maximum lateral dimension is no more than 1:3, preferably no more than 1:4.

19. An assembly according to any of the preceding claims, wherein the gas sensor comprises one of an electrochemical, catalytic, semiconductor, optical (i.r., UN or visible wavelength), and thermal conductivity sensor.

20. A set of at least two gas sensing assemblies according to any of the preceding claims, wherein keying features are selected such that each gas sensor can only be fully, operatively fitted to one connector.

21. A set according to claim 20, wherein the gas sensors are of different types and/or for sensing different gases.

22. A set according to claim 21, wherein the gas sensors are selected from the group comprising electrochemical, catalytic semiconductor, optical (i.r., UN or visible wavelength), and thermal conductivity sensor.

23. A set according to claim 21 or claim 22, wherein the gas sensors are designed to detect gases from the group of oxygen, toxic, and/or combustible gases.

24. A set according to any of claims 20 to 23, when dependent on any of claims 2 to 4 , wherein the keying features are defined by complementary rails and grooves which differ in thickness between different gas sensing assemblies .

25. A set according to any of claims 20 to 24, when dependent on claim 5 or claim 6, wherein the keying features are defined by portions of the perimeter of a lateral flange mounted to each sensor housing, the perimeters differing between gas sensors .

26. A. set according to claim 25, wherein the perimeter of one gas sensor has at least one chamfered corner while the perimeter of another gas sensor has a 90° corner in the same relative position.

27. A set according to any of claims 20 to 26, when dependent on claim 10, wherein an additional keying feature is defined by the physical location of the contacts on the sensor and/or connector housings, the location being different for different gas sensing assemblies.

28. A gas sensor for use in a gas sensing assembly according to any of the preceding claims, the gas sensor comprising a housing defining an internal cavity in which is located at least one gas sensing component, electrical contacts mounted on an outer surface of the housing and coupled to the gas sensing component, an opening in the housing to allow gas to enter the cavity, and at least one keying feature mounted to the housing, the keying feature cooperating in use with a complementary feature of a connector to which the gas sensor -is fitted.

29. A connector for use in an assembly according to any of claims 1 to 27, the connector comprising a housing into which a gas sensor can be removably fitted, the connector having contacts which cooperate with contacts on the gas sensor, and at least one keying feature which cooperates with a complementary keying feature of the gas sensor housing.