GB2606375A

GB2606375A - Air quality sensor device

Info

Publication number: GB2606375A
Application number: GB2106430.8A
Authority: GB
Inventors: Edward Thomas White Matthew
Original assignee: Elta Group Ltd
Current assignee: Elta Group Ltd
Priority date: 2021-05-05
Filing date: 2021-05-05
Publication date: 2022-11-09
Also published as: WO2022234260A1; GB202106430D0

Abstract

An air quality sensor device 10 comprises an inner cover element 52 mounted on a base 12 defining a first housing chamber 56, the inner cover element comprising a channel recess (68, fig 4) received within a housing cap 14 defining a second housing chamber 70 and an air flow vent 62 interconnecting the first and second housing chambers. An air inlet 16 on a first side of the housing cap communicates with the first chamber and an air outlet 20 on an opposite second side 18b of the housing cap communicates with the second chamber. A fan 36 located in the first chamber communicates air between the inlet and outlet via the chambers, and a least one sensor (46a – 46c, fig 3) is mounted in the first chamber. The at least one sensor may be mounted to a circuit substrate 28a, and along with a second circuit substrate 28b defines a baffle directing air flow within the first chamber. The vent may be positioned at the top of the inner cover element defining a vertical air flow path from an exhaust 40 of the fan and the at least one sensor provided in the vertical flow path.

Description

Air Quality Sensor Device The present invention relates to an air quality sensor device which is adapted for determining the composition of at least one component of air in a room, and preferably which is wirelessly communicable with other ventilation and filtration systems for the 5 room.

For ventilation and filtration systems in buildings, it is useful to know what the status of the air being treated is. Typically, the system owner will be interested in the gas composition, particularly in relation to carbon dioxide and pollutants, as well as an indication of the level of particulate matter in the air.

An air quality sensor can be used to provide this information. However, there are various challenges associated with air quality monitoring.

Firstly, where lots of different air quality characteristics are to be measured, an equivalent number of sensors is required. This can result in a bulky device, which is not aesthetically pleasing, and/or is cumbersome to move.

Where bulky components are present, this can necessitate a large internal volume of the device, which can lead to recirculation of the air flowing through the device. This means that the sensors may be monitoring recirculated air, rather than fresh air from outside of the device. This can skew the measurements being made.

Furthermore, the largely sealed nature of such air quality sensor devices is required to 20 prevent damage or contamination of the sensor components. However, this can lead to clogging of the air inlet and outlet of the device with dust, since the user cannot readily clean the device.

It is an object of the present invention to provide an air quality sensor device which overcomes or obviates the above-referenced problems.

According to a first aspect of the invention, there is provided an air quality sensor device comprising: a base, an inner cover element mountable to the base to form a first housing chamber, and a housing cap which is receivably engagable over the inner cover element to form a second housing chamber, the inner cover element having an air flow vent to interconnect the first and second housing chambers; an air inlet in communication with the first housing chamber; an air outlet in communication with the second housing chamber; an electrically-energisable fan located in the first housing chamber for driving air between the air inlet and the air outlet via the first and second housing chambers, the outlet being spaced apart from the air flow vent; and at least one sensor element mounted in the first housing chamber.

In the air quality sensor device of the present invention, there are two separate chambers within; a first chamber which houses and protects the electrical components, and in particular the sensors, and a second chamber which forms an exhaust channel for the device. Dust cannot thus ingress into the first chamber, but the housing cap can be removed for ease of cleaning the second chamber, where dust may still accumulate. This cleaning process can thus be achieved without the user needing to access the electronics.

Preferably, the at least one sensor element may be mounted to a circuit substrate which is vertically mounted in the first housing chamber.

Vertical insertion of the circuitry into the housing of the device has the advantage of 15 reducing the horizontal footprint of the device, as well as allowing the formation of vertical channels within the device via which the air can be channelled.

Two said circuit substrates may be provided, the circuit substrates acting as baffles to direct airflow within the first housing chamber.

A pair of circuit substrates, typically a sensor board and a communications board will be 20 able to form a passage through the device for air channelling without the need to provide additional ducting.

Preferably, an exhaust of the electrically-energisable fan may be located at or adjacent to a floor of the base.

It is advantageous to direct the incoming air through the device towards a floor of the 25 device, since this provides an opportunity for creating a linear and vertical subsequent air pathway through the device. This significantly reduces the risk of turbulence within the device which would lead to recirculation over the sensor elements.

The air flow vent may be positioned at a top of the inner cover element to thereby define a vertical air flow path from the exhaust of the electrically-energisable fan in the first 30 chamber.

The provision of a vertical and linear air flow path ensures that a continuous airflow passes through the centre of the device between the air inlet and the air outlet.

Optionally, the at least one sensor may be provided on the vertical air flow path.

The vertical flow path has a continuous air flow across it in use due to the channelling 5 effects described. Positioning of the sensors on this air flow path reduces the risk of sensing of recirculated air which would otherwise skew the measurements of the sensor.

Preferably, the air inlet may be positioned on a first side of the housing, and the air outlet on a second side of the housing which is opposite the first side of the housing.

Keeping the air inlet and air outlet opposite one another encourages a continuous flow 10 through the device in spite of the vertical turns in the air flow path.

The housing cap may be releasably engageable with the base to permit removal for cleaning. In a preferred embodiment, the housing cap may be magnetically engagable with the base.

Releasable engagement of the housing cap allows for the user to easily access the inner 15 cover element to dust and sanitise.

A said sensor element may be a particulate sensor, and the electrically-energisable fan is a fan of the particulate sensor.

In order to minimise the power drain and bulk of the device, one option is to utilise the existing fan provided with a particulate sensor, which can be sufficient to drive air though 20 a compact device. This obviate the need to provide a specific additional fan or motor for the device.

The inner cover element may comprise a channel recess on an outer surface thereof which forms the second housing chamber with the housing cap.

If a channel recess is provided on the inner cover element, then not only can the volume 25 of the second housing chamber be dimensioned for best flow capabilities, but the housing cap be have an internal volume which snugly fits the inner cover element therein. This allows the device to remain compact.

Optionally, the air flow vent may be elevated with respect to the air outlet.

If there is a fall between the air flow vent and air outlet, the risk of dust ingress via the air outlet into the air flow vent is significantly reduced.

In one embodiment, there may be a vertical turn in a flow path between the air inlet and the electrically-energisable fan.

A vertical turn on the inflow into the air inlet reduces the risk of dust ingress into the inlet, since there is formed a vertical barrier to entry into the electrically-energisable fan.

Optionally, the device may further comprise a hairpin duct which forms the vertical turn.

The duct having a hairpin bend formed therein forms a near vertical wall which inhibits the ingress of larger dust particles into the first housing chamber, which can then be 10 cleaned out once the housing cap is removed.

Preferably, a flow path between the air inlet and the electrically-energisable fan may be at least in part formed by a channel on the outside of the inner cover element.

As with the channel recess for the air outlet, the channel for the air inlet reduces space requirements for the air quality sensor device as a whole.

The inner cover element may be or substantially be light-transmissible, and further comprising at least one illuminable status-indicator element positioned in the first chamber.

Status indicators, typically coloured lights, are useful for outputting information to the user. Under normal circumstances, however, the lights would need to extend to an outer surface of the device. If the inner cover element is light transmissible, however, illumination signals can be emitted without needing to provide electrical contacts outside of the first housing chamber.

According to a second aspect of the invention, there is provided an air quality sensor device comprising: a housing having an air inlet and an air outlet; an electrically-energisable fan located in the housing for driving air between the air inlet and the air outlet; first and second circuit substrates arranged in the housing to form a baffle for air between the air inlet and the air outlet; and at least one sensor element mounted to the first and/or second circuit substrate.

The use of vertical circuit substrates within the housing to form an air channel, with the substrates acting as baffles, provides a mechanism for channelling air into the correct regions of the device, and in particular, towards the sensor elements so that a continuous non-recirculated airflow is achieved.

According to a third aspect of the invention, there is provided an air quality sensor device comprising: a housing having an air inlet and an air outlet; and a particulate sensor at or adjacent to the air inlet, the particulate sensor comprising an electrically-energisable fan for driving air between the air inlet and the air outlet.

The use of a particulate sensor for driving the air flow through the device reduces the 10 overall size and cost to manufacture the device, allowing for the creation of a more portable air quality sensor device to be created.

According to a fourth aspect of the invention, there is provided an air quality sensor device comprising: a housing having an air inlet and an air outlet; an electricallyenergisable fan located in the housing for driving air between the air inlet and the air outlet; a circuit substrate arranged vertically or substantially vertically in the housing; and at least one sensor element mounted to the circuit substrate.

The invention will now be more particularly described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 shows a front perspective representation of one embodiment of an air 20 quality sensor device in accordance with the first aspect of the invention; Figure 2 shows a rear perspective representation of the air quality sensor device of Figure 1; Figure 3 shows an exploded front perspective representation of the air quality sensor device of Figure 1; Figure 4 shows an exploded rear perspective representation of the air quality sensor device of Figure 1; Figure 5 shows a vertical cross-section through the air quality sensor device of Figure 1; and Figure 6 shows the air quality sensor device of Figure 5 indicating an air flow pathway through the device using the block arrows.

Referring to Figures 1 and 2, there is shown an air quality sensor device, indicated globally at 10. The air quality sensor device 10 is intended to be a freestanding unit, 5 drawing in air, determining various particulate and gas quantities in the air, before communicating with other filtration and ventilation devices accordingly.

The air quality sensor device 10 has a base 12 which is preferably weighted so that the air quality sensor device 10 can stand in an upright or vertical condition without toppling.

A housing cap 14 is provided which is receivably engagable with the base 12 to form the outer surface of the air quality sensor device 10. The housing cap 14 may have a tapered design which has a pleasing smooth aesthetic appearance. Magnets may be provided at an interface between the housing cap 14 and the base 12, to permit magnetic releasable engagement.

An air inlet 16 is provided on the front side 18a of the air quality sensor device 10, since this is most likely to be the room-facing direction of the air quality sensor device 10. The air inlet 16 is here formed as a notch positioned at a bottom edge of the housing cap 14, which then forms an opening of the air inlet 16 once the housing cap 14 is in position on the base 12. The air inlet 16 is thus positioned at a top edge of the base 12. This air inlet 16 position assists with the direction of air flow through the air quality sensor device 10, as will be described in more detail below, but it will be apparent that ducting or channels inside the air quality sensor device 10 could create the same effect for an air inlet 16 positioned elsewhere on the air quality sensor device 10.

An air outlet 20 is provided on the rear side 18b of the air quality sensor device 10. As with the air inlet 16, the air outlet 20 may be formed as a notch positioned at a bottom edge of the housing cap 14, which then forms an opening of the air outlet 20 once the housing cap 14 is in position on the base 12. The air outlet 20 is thus positioned at an upper edge 22 of the base 12. Again, it will be apparent that ducting or channels inside the air quality sensor device 10 could create the same effect for an air outlet 20 positioned elsewhere on the air quality sensor device 10.

The housing cap 14 preferably is rotationally symmetric, so that it can be positioned with the notches forming either of the air inlet 16 and air outlet 20.

A power inlet 24 is provided preferably on the rear side 18b of the air quality sensor device 10. This allows the air quality sensor device 10 to be powered without having a detrimental effect on the appearance of the front side 18a. The power inlet 24 is positioned at a lower portion of the base 12, since this is likely where the electronic components will be located.

Figures 3 and 4 shows the structure of the air quality sensor device 10 in more detail.

The base 12 forms a receiver for the electronic components of the air quality sensor device 10. At the upper edge 22 of the base 12, there is an upstanding lip 26 which helps to locate additional components thereon. The upstanding lip 26 terminates at a position 10 inline with the air inlet 16 and air outlet 20 so as to not occlude airflow thereacross.

In this instance, the electronic components are mounted to first and second circuit substrates 28a, 28b which are interconnectable. These circuit substrates 28a, 28b are intended to be mounted into the base 12 in a vertical or substantially vertical condition, with the circuit substrates 28a, 28b being thus being perpendicular or substantially perpendicular to a support surface onto which the air quality sensor device 10 is mounted. The base 12 may have a grip element 30 on its lower surface to prevent slippage on said support surface. The base 12 may include one or more locators 32 integrally or otherwise formed therewith which are configured to support the first and second circuit substrates 28a, 28b in the vertical condition. These locators 32 could be part of the floor of the base 12 and/or part of the upstanding walls of the base 12.

The first circuit substrate 28a includes a power supply 34 which is in communication with an electrically energisable fan 36. The electrically energisable fan 36 has an enclosed housing with a fan inlet 38 at or adjacent to the air inlet 16, and an exhaust 40 positioned at or adjacent to a floor of the base 12. The electrically energisable fan 36 and power supply 34 therefore provide much of the weighting of the base 12, being comparatively heavy to the base 12 itself.

To connect the air inlet 16 to the fan inlet 38, there may be provided a ducting component 42, which ensures that there is an airflow channel which extends from the air inlet 16 into the electrically energisable fan 36 without needing to provide significant moulding internally to the base 12. The ducting component 42 is formed as a hairpin duct, which thereby creates a vertical turn prior in the air flow path between the air inlet 16 and the fan inlet 38. This helps to prevent large dust particles falling into the electrically-energisable fan 36, with the hairpin duct acting as a wall or similar barrier to physical entry into the fan inlet 38. The hairpin duct has a substantially pyramidal profile with a lower air inlet and an internal triangular baffle which urges air up and then down into the fan inlet 38.

The power supply 34 may have a connector which engages with an adaptor piece 44 of the base 12. This adaptor piece 44 will be dimensioned for the power coupling to be used with the device, such as a USB power adaptor, and thus may be manufactured as a separate piece to the base 12, though it will be apparent that it could be unitarily formed with the base 12.

The first circuit substrate 28a may also include the sensor elements 46a, 46b, 46c of the air quality sensor device 10. The depicted embodiment illustrates a carbon dioxide sensor 46a, such as a low-power infra-red sensor, a particulate sensor 46b, and additional sensing packages 46c. The circuit substrate 28a itself can act as a baffle to attempt to direct airflow across the sensor elements 46a, 46b, 46c.

The sensor elements described here are indicative only. The skilled person will be aware of the desired sensing capabilities of their air quality sensor device, and utilise appropriate sensors accordingly.

In the present embodiment of the invention, the electrically energisable fan 36 is the fan of the particulate sensor 46b, and therefore, unusually, no separate dedicated fan unit is 20 required.

The first circuit substrate 28a also here includes a through-board stacking connector 48 for engaging with the second circuit substrate 28b, which acts as a communications board, having one or more communications elements 50 thereon. Typically, these would be W-Fi (RTM) communicators designed for communication with air filtration or ventilation units as part of an air conditioning system. It will be appreciated that a non-connected version of the air quality sensor device 10 could be provided, and the second circuit substrate 28b and/or components thereon could be removed.

An inner cover element 52 is provided which engages with the upstanding lip 26 of the base 12 to seat in position. One or more fasteners 54, such as screws, can be provided 30 for securing the inner cover element 52 to the base 12. It is not intended that the user access the first housing chamber 56 that is formed by the base 12 and inner cover element 52.

The inner cover element 52 may have a seal 58 at its lower perimeter which provides an airtight seal with the base 12. This ensures that the only air access to the first housing chamber 56 is via the air inlet 16, which accesses the first housing chamber 56 through the ducting component 42, and an air flow vent 62 which is provided at an top surface 64 of the inner cover element 52. The ducting component 42 could, of course, be provided as part of the inner cover element 52.

The inner cover element 52 may have one or more locator portions 66 thereon which 10 assist with alignment to the housing cap 14. These are formed as linear channels on the side of the inner cover element 52 which matingly engage with corresponding structures on the housing cover 14.

The air flow vent 62 is preferably positioned at least in partial contact with the top surface 64, but may additionally or alternatively extend at least in part down a rear side of the 15 inner cover element 52. The air flow vent 62 is designed to direct air towards the air outlet 20.

To this end, there is a channel recess 68 which is positioned on an outside of the inner cover element 52 which forms a second housing chamber 70 with the housing cap 14. This second housing chamber 70 forms an exhaust duct for the air quality sensor device 10 towards the air outlet 20. A corresponding channel 72 may be provided on a front side of the inner cover element 52 which at least in part forms an air flow path between the air inlet 16 and the electrically-energisable fan 36. The ducting component 42 may be receivably engagable in this channel 72.

Figure 5 shows the internals of the fully assembled air quality sensor device 10. The first housing chamber 56 fills most of the internal volume of the air quality sensor device 10, and encloses all of the electronic components. Access to these components is therefore very restricted. The second housing chamber 70 is comparatively small, but allows for the air flow vent 62 to be physically dislocated from the air outlet 20. This significantly reduces the risk of dust ingress into the first housing chamber 56. This is enhanced by not only the air outlet 56 being downstream of the air flow vent 58, but also being physically lower than the air flow vent 58, and therefore there is a gravitational effect which inhibits dust ingress.

The air flow path can be seen from Figure 6. Air is drawn in through the air inlet 16 by the electrically energisable fan 36, up through the ducting component 42 and around the hairpin turn. Preferably, the hairpin turn may be a 1800 or substantially 1800 turn, or some form of U-turn. The 180° turn may maximise a compaction of the flow path. Given that this is the particulate sensor 46b in the present embodiment, it is preferred that a particulate filter is positioned downstream of the electrically energisable fan 36 so as to protect the downstream sensors from particulate matter.

The shape of the particulate sensor 46b used is such that the exhaust 40 is positioned at right angles to the fan inlet 38. This is very useful in the present context, as it means that air can be vented into the first housing chamber 56 at the floor of the base 12. As such, air travel within the first housing chamber 56 follows a largely linear path in an upward vertical direction.

The positioning of the first and second circuit substrates 28a, 28b serve to form a channel up through the centre of the first housing chamber 56, creating a baffle. The second circuit substrate 28b is dimensioned so as to abut the top of the electrically energisable fan 36, thereby discouraging airflow away from the said channel. The air flow path between the first and second circuit substrates 28a, 28b is here created by the distance enforced by the stacking connector 48, but alternative connection means between the first and second circuit substrates 28a, 28b could be considered, for example, one or more spacers.

If only one circuit substrate is provided, then one or more internal walls of the inner cover element 52 could be used to generate airflow direction capabilities, or indeed a separate dedicated baffle unit. Furthermore, it may be possible to provide sealing between the circuit substrates 28a, 28b and the base 12 and/or the inner cover element 14 which inhibits air flow around the circuit substrates 28a, 28b. This could be in the form of adhesive, or a potting compound, for example.

The baffle has the effect of urging air past the sensor packages 46c without creating too much turbulence. It is desirable to prevent air lingering inside the first housing chamber 56, as this will result in inaccurate measurements at the sensors.

The air flow is directed up and out of the first housing chamber 56 and through the air flow vent 62 at the top of the first housing chamber 56. The provision of several apertures at the air flow vent 62 limits the risk of rebound of airflow within the first housing chamber 56, instead, forcing air into the exhaust channel that is the second housing chamber 70. Air is thus urged out of the air outlet 20.

The air flow path through the air quality sensor device 10 is therefore well-defined to ensure that freshly obtained air is drawn through the inlet and passed across the various 5 sensors in a sequential manner. There is no opportunity for recirculation of the air.

The air flow path is also constructed to as to directly inhibit the accumulation of dust in the air inlet 16 and air outlet 20, due to the gravitational barriers respectively positioned at either end of the device 10. This has the effect of creating a tortuous or serpentine air flow path in the vertical direction, as the air flows through the air quality sensor device 10.

The issue of dust accumulation is also further improved by the removability of the housing cap 14. Since the housing cap 14 can be readily lifted off the base 12. The user can then dust the outer surfaces of the inner cover element 52 and thus remove larger dust particles which would not otherwise be suspended in the air, and therefore should not be sensed. The gravitational barriers provided ensure that any dust accumulated settles away from access points to the first housing chamber 56, and thus cleaning is straightforward.

A further advantage of the inner cover element 52 is that, if it is provided so as to be at least in part light-transmissible, then one or more illumination elements, typically in the form of light-emitting diodes (LEDs), can be provided as part of the electrical componentry. The illumination elements can be linked to a processor of the air quality sensor device 10, which allow for a visual output of a status of the air quality sensor device 10.

For example, a green LED could be used to indicate a correct activation status of the air 25 quality device 10, whereas amber or red LEDs could be used to indicate either a device problem or an issue with the air quality. If the housing cap 14 is at least in part translucent, then this will create a pleasing aesthetic appearance.

Although the air quality sensor device is defined as portable, it will be appreciated that the device may not necessarily be portable. Additionally, although described as portable 30 this would not necessitate a portable power supply and would not preclude the device from being energised by a mains or utility power supply. The device may be considered to be portable due to its portable size It is therefore possible to provide an air quality sensor which has improved resistance to dust ingress therein, as well as being easier to clean or sanitise, which also ensuring that the airflow characteristics through the device are well-suited towards accurate air quality sensing. This can be achieved by the provision of vertical circuit substrates within the freestanding device, which then sit on a well-defined air flow pathway.

The words 'comprises/comprising' and the words 'having/including' when used herein with reference to the present invention are used to specify the presence of stated 10 features, integers, steps or components, but do not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof.

It is appreciated that certain features of the invention, which are, for clarity, described in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the invention which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable sub-combination.

The embodiments described above are provided by way of examples only, and various other modifications will be apparent to persons skilled in the field without departing from the scope of the invention as defined herein.

Claims

Claims 1. An air quality sensor device comprising: a base, an inner cover element mountable to the base to form a first housing chamber, and a housing cap which is receivably engagable over the inner cover element to form a second housing chamber, the inner cover element having an air flow vent to interconnect the first and second housing chambers; an air inlet in communication with the first housing chamber; an air outlet in communication with the second housing chamber; an electrically-energisable fan located in the first housing chamber for driving air between the air inlet and the air outlet via the first and second housing chambers, the outlet being spaced apart from the air flow vent; and at least one sensor element mounted in the first housing chamber.
2. An air quality sensor device as claimed in claim 1, wherein the at least one sensor 15 element is mounted to a circuit substrate which is vertically mounted in the first housing chamber.
3. An air quality sensor device as claimed in claim 2, wherein two said circuit substrates are provided, the circuit substrates acting as baffles to direct airflow within the 20 first housing chamber.
4. An air quality sensor device as claimed in any one of the preceding claims, wherein an exhaust of the electrically-energisable fan is located at or adjacent to a floor of the base.
5. An air quality sensor device as claimed in claim 4, wherein the air flow vent is positioned at a top of the inner cover element to thereby define a vertical air flow path from the exhaust of the electrically-energisable fan in the first chamber.
6. An air quality sensor device as claimed in any one of the preceding claims, wherein the at least one sensor is provided on the vertical air flow path.
7. An air quality sensor device as claimed in any one of the preceding claims, wherein the air inlet is positioned on a first side of the housing, and the air outlet on a 35 second side of the housing which is opposite the first side of the housing.
8. An air quality sensor device as claimed in any one of the preceding claims, wherein the housing cap is releasably engageable with the base to permit removal for cleaning.
9. An air quality sensor device as claimed in claim 8, wherein the housing cap is magnetically engagable with the base.
10. An air quality sensor device as claimed in any one of the preceding claims, wherein a said sensor element is a particulate sensor, and the electrically-energisable 10 fan is a fan of the particulate sensor.
11. An air quality sensor device as claimed in any one of the preceding claims, wherein the inner cover element comprises a channel recess on an outer surface thereof which forms the second housing chamber with the housing cap.
12. An air quality sensor device as claimed in any one of the preceding claims, wherein the air flow vent is elevated with respect to the air outlet.
13. An air quality sensor device as claimed in any one of the preceding claims, wherein there is a vertical turn in a flow path between the air inlet and the electricallyenergisable fan.
14. An air quality sensor device as claimed in claim 13, further comprising a hairpin duct which forms the vertical turn.
15. An air quality sensor device as claimed in any one of the preceding claims, wherein a flow path between the air inlet and the electrically-energisable fan is at least in part formed by a channel on the outside of the inner cover element.
16. An air quality sensor device as claimed in any one of the preceding claims, wherein the inner cover element is or is substantially light-transmissible, and further comprising at least one illuminable status-indicator element positioned in the first chamber.
17. An air quality sensor device comprising: a housing having an air inlet and an air outlet; an electrically-energisable fan located in the housing for driving air between the air inlet and the air outlet; first and second circuit substrates arranged in the housing to form a baffle for air between the air inlet and the air outlet; and at least one sensor element mounted to the first and/or second circuit substrate.
18. An air quality sensor device comprising: a housing having an air inlet and an air outlet; and a particulate sensor at or adjacent to the air inlet, the particulate sensor comprising an electrically-energisable fan for driving air between the air inlet and the air outlet.