FIELD OF THE INVENTION

This invention relates to the field of gas sensing apparatuses, particularly the field of gas sensing apparatuses for sensing a target gas. BACKGROUND OF THE INVENTION

Air pollution monitoring systems typically contain gas sensor systems or gas sensing apparatus able to selectively sense particular target gases (for example, S0 ₂ or N0 ₂ ) or selected classes of target gases (for example, volatile organic carbon gases). It is preferable for such gas sensing apparatus to be accurate and fast-responding, thereby improving the reliability of air pollution monitoring systems.

In the interests of economy and design constraints, it is often preferable to perform such gas sensing with low-cost, small-sized gas sensors that have a long operational lifetime. It is known for a gas sensing apparatus to include one or more such sensors, for example, electrochemical gas sensors and metal-oxide semiconductor gas sensors.

Amongst the problems facing conventional gas sensor apparatus used in air pollution monitoring systems is that such gas sensor apparatus only have a limited functional lifetime, due in part to internal contamination. Furthermore, a usual known gas sensing apparatus tends to have a relatively slow response time, such that the reaction of a gas sensor to changes in air pollution is relatively slow. Typical gas sensing apparatus may thereby not be sufficiently reliable for use in air pollution monitoring systems.

SUMMARY OF THE INVENTION

It is an object of the invention to provide an improved gas sensing apparatus. The invention is defined by the independent claims. The dependent claims define

advantageous embodiments.

According to a first aspect of the invention, there is provided a gas sensing apparatus for sensing a target gas, the gas sensing apparatus comprising: a duct through which a gas may flow, the duct comprising: a first duct portion defining a first gas flow passage having a first cross-sectional area through which the gas may flow; a second duct portion defining a second gas flow passage having a second smaller, cross-sectional area through which the gas may flow such that the average flow speed of the gas flowing in a first direction from the first gas flow passage to the second gas flow passage is increased on entering the second gas flow passage from the first gas flow passage; and a third duct portion defining a third gas flow passage having a third cross-sectional area, the third cross-sectional area being greater than the second cross-sectional area such that an average flow speed of gas flowing in a second direction from the third gas passage to the second gas flow passage is increased on entering the second gas flow passage and a gas sensor adapted to sense gas passing through the second duct portion, the gas sensor being further adapted to: sense a first sensor signal corresponding to gas flowing in the first direction; sense a second sensor signal corresponding to gas flowing in the second direction; and determine a concentration of the target gas based on at least the first sensor signal and the second sensor signal, wherein the concentration of target gas in the gas flowing in either the first direction or the second direction is at least partially modified prior to the gas sensor sensing the first or second sensor signal respectively.

There is herein provided a gas sensing apparatus adapted to sense a target gas. A target gas may be understood to be a specific gas which may be selected from a range of gases and measured by the gas sensing apparatus.

There is proposed the concept of providing a duct having first, second and third duct portions, each duct portion defining a respective gas flow passage through which a gas may flow. The second duct portion (i.e. the second gas flow passage) provides a smaller cross-sectional area for gas flow such that the average speed of gas flow (measured, for example, in cm/s) from the first duct portion to the second duct portion (and/or from the third duct portion to the second duct portion) is increased or raised in or through the second duct portion. A gas sensor, positioned in, on, near or in proximity to (e.g. adjacent) the second duct portion is adapted to sense the target gas such as to provide a measure of at least the target gas.

The concept may further include sensing, at the second duct portion, a respective sensor signal corresponding to gas passing in a first direction and gas flowing in a second direction. A concentration of a target gas may then be determined based on the sensed sensor signals. The concentration of target gas may be modified in either the first direction or the second direction. Gas flowing in the first direction may flow from the first duct portion to the second duct portion, and gas flowing in the second direction may flow from the third duct portion to the second duct portion.

Put another way, the gas sensor may be adapted to detect a first and a second sensor signal associated with gases having two different concentrations of a target gas.

The modification of the target gas may, in embodiments, be performed by a target gas filter of the gas sensing apparatus, the target gas filter being adapted to filter a portion of the target gas from the gas flowing in either the first or second direction. In other embodiments, an external component or element may modify the target gas concentration. Such an external entity, component or element may, for example, be a person breathing or a gas exhaust pipe.

In some embodiments, the gas sensing apparatus may thereby be implemented in a clinical environment (e.g. a respirator) or in other target gas detection systems (such as breathalyzers). In other embodiments, they may be used to analyze output exhaust fumes (such as those from automobiles or generators) or in air pollution monitoring systems.

Such a gas sensing apparatus allows for provision of a relatively simple device which provides improved accuracy and longevity of the apparatus.

A portion of the duct may be a segment or section of the duct, such that the duct is split into at least three portions or segments so as to provide a first duct portion, a second duct portion and a third duct portion. The second duct portion is positioned such that gas may flow in a first direction from the first duct portion to the second duct portion and similarly such that gas may flow in a second direction from the third duct portion to the first duct portion. In described embodiments, the second duct portion is situated adjacent or next to the first duct portion.

A cross-sectional area of a gas flow passage should be understood to refer to the two-dimensional area (e.g. void or aperture) through which a gas flows. For example, a cross-sectional area of a gas flow passage may be the empty or void area bounded by interior surface(s) of that gas flow passage in a plane normal to the predominant or prevalent direction of gas flow in that passage. A cross-sectional area may be perpendicular to the longitudinal axis of the gas flow passage, the longitudinal axis defining the direction in which a gas may flow. Thus, a cross-sectional area should not be understood to be the total footprint area or the total interior surface area of a portion or passage, nor an area of the physical material forming the duct portions. Instead, it should be understood to be the area of the void(s), hole(s), aperture(s) or passage(s) through which the gas flow passes. To be of sufficiently smaller cross-sectional area, it may be firstly understood that the volumetric flow rate of gas (measured in, for example, m ³ /s) remains substantially constant for a gas flowing in the first duct portion and a gas flowing in the second duct portion (i.e. throughout the duct), such that gas flow in the second duct portion is made to flow at a faster speed to maintain its volumetric flow rate. As herein described, a gas flow rate or gas volumetric flow rate in a portion is a product of the flow speed in that portion and the cross-sectional area of that portion.

The second cross-sectional area may be proportional in size to the first cross- sectional area (i.e. the cross-sectional area of the second gas flow passage may be

proportional to the cross-sectional area of the first gas flow passage). For example, the size of the second cross-sectional area may be one of the following: half the size of the first cross- sectional area, a quarter of the size of the first cross-sectional area or an eighth of the size of the first cross-sectional area. In some embodiments, the third cross-sectional area is proportional to the first and/or the second cross-sectional area. Preferably, the third cross- sectional area is substantially the same size as the first cross-sectional area,

In embodiments, the maximum allowable size for the second cross-sectional area may be no larger than half the size of the first cross-sectional area, for example no larger than a quarter of the size of the first cross-sectional area. Preferably, the size of the second cross-sectional area may be no larger than one eighth of the size of the first cross-sectional area. It will be understood that any fractional or partial value of the size of the first cross- sectional area may be realized as a maximum size for the cross-sectional area of the second gas flow passage. In other embodiments, the cross-sectional area of the second gas flow passage may be absolute, for example, less than 20 mm ² , preferably less than 10 mm ² and even more preferably less than 5 mm ² . In such embodiments, the cross-sectional area of the first gas flow passage may be generally larger than the cross-sectional area of the second gas flow passage, for example at least 50 mm ² , for example no less than 100 mm ² , for example at least 200 mm ² .

Preferably the cross-sectional area size of the second gas flow passage is a narrowed cross-sectional area of the first gas flow passage, such that the first cross-sectional area and the second cross-sectional area are of the same geometric shape or polygonal structure with different aspect ratios. Similarly, in preferable embodiments the third cross- sectional rea has the same geometric shape or polygonal structure as the first and/or second cross sectional area. Ensuring the gas sensor is only exposed to gas flowing through a narrower or smaller second gas flow passage, as herein disclosed, advantageously allows the diffusion distance of the target gas species towards the sensor to be reduced, such that an increased portion of the target gas species in the gas flow may be measured by the gas sensor. This permits, for example, the response time of the gas sensor to be reduced, as the diffusion- controlled transfer rate of the target gas species from the gas flow to the target gas sensor becomes increased. The gas flow speed in the first and/or third duct portion may be minimized (relative to the second duct portion) such that an increased longevity of the gas sensing apparatus may be realized compared to conventional apparatuses not comprising a second gas flow passage of reduced cross-sectional area.

It is particularly noted that the cross-sectional area reduction (i.e. when the second cross-sectional area is sufficiently small relative to the first and/or third cross- sectional area) can improve the contact between the gas and the gas sensor, thereby reducing the mass transfer resistance in the gas phase for target gas transport towards the gas sensor. This may reduce the sensor response time, and allow for a minimized overall airflow through the sensor system, which extends the lifetime of the gas sensing apparatus. This may, in turn, increase the accuracy and reliability of such a gas sensing apparatus.

In preferable embodiments, the gas sensing apparatus further comprises a target gas filter, preferably such that the gas flowing in either the first direction or the second direction is filtered by the target gas filter. For the purposes of explanation, the target gas filter is typically adapted to filter a target (e.g. desired or specified) gas from gas passing there-through or there-past. Various embodiments of a target gas filter will be readily understood by the person skilled in the art.

Such a gas sensing apparatus may be adapted to obtain two sensor signals: a first sensor signal corresponding to a value of gas comprising a target gas (i.e. gas which has not been filtered through the target gas filter); and a second sensor signal corresponding to a value of gas from which the target gas has been at least partially removed (i.e. gas which has to some degree been filtered by the target gas filter). It may be understood that sensed gas from which target gas has not been removed may correspond to gas flowing in one of the first direction or the second direction, and sensed gas from which target gas has been removed may correspond to gas flowing in the other one of the first direction or the second direction.

Accordingly, a measure of the concentration of target gas in the unfiltered gas (e.g. ambient air) may be calculated in an efficient and reliable manner. In embodiments, the second gas flow passage comprises: a first interior surface area, the first interior surface area being a sensing area of the gas sensor, the sensing area being adapted to aid in sensing a target gas; and a second interior surface area facing the first interior surface area, the second interior surface area positioned such that gas in the second gas flow passage passes between the first interior surface area and the second interior surface area and wherein the distance between the first interior surface area and the second interior surface area is sufficiently small such that the average flow speed of the gas flow is increased on entering the second gas flow passage from the first gas flow passage or third gas flow passage.

In other words, the interior surface of the second duct portion may be divided into at least one interior surface area. A distance between one of these interior surface area(s) of the second duct portion and a sensing area of a gas sensor may be sufficiently small such that the speed or velocity of a gas flow is increased when passing between them when compared with the speed or velocity of the gas flow in the first duct portion.

A suitably small distance between the gas sensing area and an interior surface area (e.g. an interior duct wall) improves the mass transfer rate of the target gas to the gas sensor. That is to say, a sufficiently small distance between the first interior surface area and the second interior surface area improves the contact between the gas and the gas sensor, thereby reducing the mass transfer resistance in the gas phase for target gas transport towards the gas sensor. This may reduce the sensor response time, and allow for a minimized overall airflow through the sensor system, which extends the lifetime of the gas sensing apparatus.

The gas sensing area may be, for example, a window, opening or aperture to an interior of a gas sensor. The gas sensing area may, in another exemplary embodiment, be at least one electrode of an electrochemical gas sensor or be part of a metal-oxide

semiconductor gas sensor. In other embodiments, the gas sensing area is a gas-permeable membrane of, for example, another known electrochemical gas sensor. Thus, in embodiments, the gas sensing area will contribute to the sensing by the gas sensor. Other embodiments of gas sensors will be known to a skilled person and readily utilized in the present invention.

The sensing area of the gas sensor may form an interior surface of the second gas flow passage, such that a boundary or perimeter of the second cross-sectional area comprises the sensing area. In other words, the sensing area may be formed within the walls of the second gas flow passage to define at least an interior area of the second duct portion. In some embodiments, the gas sensor is placed in a gas flow passage having the first cross- sectional area such that the space between the gas sensor and an interior surface of the gas flow passage defines the second cross-sectional area and thereby the second gas flow passage.

Preferably, the distance between the first interior surface area and the second interior surface area is no more than 5 mm, for example no more than 4 mm and preferably no more than 2 mm.

It will be readily understood to the skilled person that any sized gap or distance between the first interior surface area and the second interior surface area may be realized without departing from the scope of the present invention. For example, the distance between the first interior surface area and the second interior surface area may be no more than 4 mm, preferably no more than 2 mm, and even more preferably, no more than 1 mm. In other embodiments, this distance may be larger, for example, no more than 10 mm, no more than 20 mm, or no more than 50 mm.

In preferable embodiments, the first surface area is not much larger than the cross-sectional area of the second duct portion, for example, no more than twice the size of the second cross-sectional area, and preferably less than 1.5 times larger than the size of the second cross-sectional area. In other words the size of the aperture or void through which a gas may flow in the second duct portion is optionally only slightly smaller than a first interior surface area of the second duct portion.

The gas sensing apparatus may further comprise a flow rate modifier adapted to modify the volumetric flow rate of the gas through the second gas flow passage.

A flow rate modifier is adapted to alter or change the flow rate through at least the second gas flow passage such that a reducible or restricted airflow may be provided to the gas sensor, thereby permitting the longevity of the gas sensing apparatus to be improved. Alternatively, a flow rate modifier according to at least one embodiment may increase the flow rate through the second gas flow passage such that a response time of the gas sensor may be improved (by permitting more target gas to be sensed at a gas sensor over a given time period). Such a flow rate modifier may be placed, for example, at an entrance or exit to the second gas flow passage, in the first gas flow passage itself, in the third gas flow passage itself or in the second gas flow passage itself.

A flow rate modifier may advantageously allow a flow rate to be reduced or otherwise controllable such that the flow rate does not increase above an allowable or recommended maximum flow rate, thereby increasing the longevity of the gas sensing apparatus. Alternatively such a flow rate modifier may advantageously increase the flow rate to ensure that flow speed in the second portion is sufficiently high for a gas sensor positioned therein to respond quickly, such that, for example, the size of the second cross-sectional need not be reduced beyond economically or technologically viable sizes to ensure suitable flow speed in the second duct portion.

The flow rate modifier may comprise a gas-permeable barrier, the gas- permeable barrier incurring a pressure drop when gas flows through the gas-permeable barrier. In other or further embodiments, the flow rate modifier may be an orifice plate or a plate having at least one small aperture therein.

The gas-permeable barrier may be, for example, a gas filter adapted to filter the gas flowing through the duct. A pressure drop across the gas-permeable barrier causes a reduction in the gas flow rate through the duct, and may thereby modify the gas flow rate through the second gas flow passage. The gas filter is preferably a particle filter, adapted to filter particles (for example, liquids or solids) dispersed or present in the gas, providing the additional benefit of fully or partially reducing contamination of the gas sensor through particle deposition on its surfaces.

It is additionally noted that the lifetime of such a gas or particle filter is extended relative to a conventional gas sensor by the provision of a reduced second cross- sectional the overall gas flow rate in the duct may be reduced, thereby reducing the number and frequency of particle depositions. As the flow speed in the first duct portion and/or third duct portion is reducible in comparison to conventional gas sensing apparatus (due to the reduced cross-section of the second duct portion) the effective lifetime of a gas filter positioned therein is improved.

The flow rate modifier may otherwise or additionally comprise at least one gas flow generator; for example, a fan adapted to control or otherwise modify the air flow through the second gas flow passage. Other gas flow generators will be well known to those skilled in the art, for example, a partially permeable transducer.

The gas sensing apparatus may comprise a controller adapted to control the flow rate modifier such that selection of the flow rate through the second gas flow passage is enabled.

By way of example, the flow rate modifier may be a voltage-controlled fan, wherein a controller according to an embodiment is adapted to provide a varying voltage to the fan in order to adjust the flow rate through the second gas flow passage. The controller may only be adapted to periodically control the flow rate through the second duct portion such that the gas sensor may only periodically sense at least the target gas. Such a gas sensing apparatus optionally comprises a flow rate sensor adapted to determine a flow rate in the second gas flow passage, wherein the controller is adapted to control the flow rate modifier based on at least the determined flow rate.

Such a gas sensing apparatus may be adapted wherein a desired gas flow rate is maintained within the second gas flow passage by controlling at least one flow rate modifier in response to the sensing of a present flow rate. A desired flow rate may be chosen to provide a compromise between a sufficiently fast sensor response (which may benefit from a higher flow rate) and extending the lifetime of the gas sensing apparatus (which may benefit from a lower flow rate). Controlling the flow rate in such a manner advantageously permits controlling of the flow speed. It would be particularly beneficial to effect a flow speed in the first duct portion of less than 10 cm/s, and more preferably less than 1 cm/s, particularly when the first duct portion comprises a gas filter.

It would be clear to a skilled person that a flow rate modifier may alternatively be placed in any other portion of the duct in order to control at least a flow rate in that portion, such that a flow speed may also be controlled.

In a particular embodiment, the gas sensing apparatus comprises: a first gas- permeable barrier and a second gas-permeable barrier, wherein the gas sensor is positioned between the first permeable barrier and the second permeable barrier; a first gas flow generator and a second gas flow generator, wherein the gas sensor is positioned between the first gas flow generator and the second gas flow generator; and a target gas filter adapted to at least partially filter the target gas from the gas flow.

According to another conceivable embodiment, the second duct portion and/or the third duct portion of the duct may be supplied separately to the first duct portion such that, for example, the second duct portion may be 'retrofitted' to an existing duct. In other words, an embodiment may exclude the first duct portion of the duct from the gas sensing apparatus. Thus, in such an embodiment, when a gas sensing apparatus is brought into or positioned within an existing duct, a duct having a first, second and third duct portion, thereby defining a first, second and third gas flow passage, is formed.

According to a further aspect of the inventive concept, there is provided a method of sensing a target gas, the method comprising: passing a gas in a first direction through a first duct portion defining a first gas flow passage having a first cross-sectional area; subsequently passing the gas flowing in the first direction through a second duct portion defining a second gas flow passage having a second cross-sectional area, wherein the second cross-sectional area is sufficiently smaller than the first cross-sectional area such that the average flow speed of the gas is increased on entering the second gas flow passage from the first gas flow passage; sensing a first sensor signal at a gas sensor positioned to sense gas passing through the second duct portion, the first sensor signal corresponding to gas flowing in the first direction; passing a gas in a second direction, the second direction being different from the first direction, through a third duct portion defining a third gas flow passage having a third cross-sectional area; subsequently passing the gas flowing in the second direction through the second duct portion of the duct, wherein the second cross-sectional area is also sufficiently smaller than the third cross-sectional area such that the average flow speed of the gas is increased on entering the second gas flow passage from the third gas flow passage; sensing a second sensor signal at the gas sensor, the second sensor signal corresponding to gas flowing in the second direction; and determining the concentration of the target gas based on at least the first sensor signal and the second sensor signal, wherein the concentration of target gas in the gas flowing in either the first direction or the second direction is at least partially modified prior to sensing the first or second sensor signal respectively.

The method may further comprise modifying the flow rate of the gas flow through the second duct portion using a flow rate modifier.

Optionally, the method comprises a step of filtering a target gas using a target gas filter.

Such a method may be adapted wherein the first gas sensor signal is sensed at a first point in time and the second gas sensor signal is sensed at a second point in time, the method further comprising: at the first point in time, obtaining one or more characterizing values of the target gas filter; and at the second point in time, obtaining a further one or more characterizing values of the target gas filter; the determining of the concentration of the target gas is based on at least: a difference between the sensor signal sensed when the target gas is not removed from the gas flow and the sensor signal sensed when the target gas is at least partially removed from the gas flow; and one or more characterizing values of the target gas filter, wherein the one or more characterizing values comprises at least one of the following: the filter thickness, the superficial gas velocity at the face of the target gas filter, the relative humidity of the air passing through the target gas filter; and the amount of target gas absorbed in the target gas filter.

These and other aspects of the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter. BRIEF DESCRIPTION OF THE DRAWINGS

Examples of the invention will now be described in detail with reference to the accompanying drawings, in which:

Fig. 1 illustrates a first duct portion and a second duct portion of a gas sensing apparatus according to a first embodiment of the invention;

Fig. 2 illustrates a cross-sectional view of the first embodiment;

Fig. 3 illustrates a first duct portion and a second duct portion of a gas sensing apparatus according to a second embodiment of the invention;

Fig. 4 illustrates a first duct portion and a second duct portion of a gas sensing apparatus according to a third embodiment of the invention;

Fig. 5 shows a representative view of gas flow speed and volumetric gas flow rate across a first duct portion and a second duct portion of a gas sensing apparatus according to the second embodiment;

Fig. 6 illustrates a gas sensing apparatus according to a fourth embodiment of the invention;

Fig. 7a depicts a cross-section of a first duct portion according to an embodiment;

Fig. 7b depicts a cross-section of a second duct portion according to an embodiment of the invention;

Fig. 7c illustrates a cross-section of a second duct portion according to an embodiment of the invention;

Fig. 7d depicts a cross-section of a second duct portion according to an embodiment of the invention; and

Fig. 7e depicts a cross-section of a second duct portion according to an embodiment of the invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The invention provides gas sensing apparatus comprising a gas sensor positioned within a duct portion of a smaller size than a preceding, upstream, duct portion. This permits both a greater gas flow speed and a reduced target gas diffusion distance in the portion housing the gas sensor, resulting in a quick and reliable gas sensing apparatus.

A first embodiment of the invention can be readily understood with reference to Figs. 1 and 2. In Fig. 1, the gas sensing apparatus according to the first embodiment comprises a duct 1 segmented into a first duct section 101 and a second duct section 102. For the purposes of understanding, the third duct section has been omitted from Fig 1. Fig. 2 depicts the gas sensing apparatus according to the first embodiment when a viewed from the xi - x ₂ axis.

The second duct section of the first embodiment has the same polygonal structure (rectangular) as the first duct section; however the aspect ratio of the second duct section is not the same as the first duct section. As indicated in Fig. 2, each of the first duct section 101 and the second duct section 102 has the same depth di but differing widths. The first duct section has an associated first width wi and the second duct section has an associated second width w ₂ , different to the first width. It is readily apparent that there may be defined a second cross-sectional area (A ₂ ) associated with the second duct portion (i.e. w ₂ x di) that is smaller than a first cross-sectional area (Ai) associated with the first duct portion

The width, depth and cross-sectional area measurements should be taken to mean the width, depth and cross-sectional area of the void bounded by the interior walls or interior surfaces of the relevant duct portion, the void thereby defining a gas flow passage. The width and depth are one-dimensional measurements from one interior surface of a portion to another, opposing, interior surface of the portion. Typically, although not exclusively, the width is the smallest of these one-dimensional measurements.

The volumetric flow rate of gas passing through a duct portion (Φ) is dependent on both the average flow speed (or average flow velocity) (v) of gas passing through that duct portion and the cross-sectional area (A) of that duct portion. The flow rate Φ of gas in a portion is defined by:

Φ = v . A (1) The volumetric flow rate of gas Φ throughout the entirety of the duct (i.e. through the first and second duct portion) remains substantially constant, such that when the cross-sectional area of the second duct portion is less than the cross-sectional area of the first duct portion the flow speed or flow velocity of gas flowing in the second duct portion (v ₂ ) is greater than the flow speed of gas flowing in the first duct portion (vi).

From equation 1, it can be calculated that for a constant volumetric flow rate of gas Φ through the duct, the relationship between the flow speed through the first duct portion (vi) and the flow speed through the second duct portion (v ₂ ) is: (2)

Thus, as the cross-sectional area of the second duct portion (A ₂ ) is less than the cross- sectional area of the first duct portion (Ai), such that A ₂ < A _{l s} the flow speed in the second duct portion is proportionally greater.

It would be readily apparent to a skilled person that the cross-sectional area of a duct is not limited to only rectangular geometric shapes, but may rather be any shape, either regular (for example, a circle or triangular) or irregular (for example an L-shape). The cross- sectional area of the first duct portion need not be the same geometric shape or polygonal structure as the second duct portion (e.g. a rectangular first duct portion but a circular second duct portion).

A gas sensor 105 is positioned within the second duct portion 102, the gas sensor 105 being adapted to sense at least a target gas flowing through the second duct portion 102. The increased flow speed of gas flowing through the second duct portion (i.e. v ₂ > vi) reduces the diffusion distance of the target gas to the gas sensor (when compared, for example, to a gas sensor positioned in the first duct portion 101) by reducing the thickness of the diffusional boundary layer adjacent to the gas sensor. Such a reduction in diffusion distance advantageously allows a larger portion of the target gas species to be detected by the gas sensor such that the response time of the gas sensor is reduced (when compared, for example, to a gas sensor positioned in the first duct portion).

In the present embodiment, wherein each portion has a rectangular cross- section, the width w ₂ of the second duct section is preferably less than 4 mm, for example, less than 2 mm. This will advantageously allow the diffusion distance of the target gas species towards the sensor to be optimally reduced, such that an increased proportion of the target gas species may be measured by the gas sensor. Furthermore, such a distance allows the average flow speed of the gas flowing through the second duct portion to be optimally greater than an average speed of gas flowing through the first duct portion, to realize the greatest advantage.

Preferably the cross-sectional area of the first duct portion is sufficiently large to cause the first flow speed vi to be less than 10 cm/s, and more preferably less than 1 cm/s, particularly for known or controlled flow rates. Purely by way of example, if a flow rate is known to be 100 cm ³ /s, to realize a flow speed of less than 10 cm/s, the first cross-sectional area must be at least 10 cm ² large, whereas to realize a flow speed of less than 1 cm/s, the first cross-sectional area must be at least 100 cm ² .

Typically, although not exclusively, gas is made to flow from the first duct portion to the second duct portion, such that the second duct portion may be considered downstream from the first duct portion. There may, in embodiments, be further duct portions positioned between the first duct portion and the second duct portion.

A gas sensing apparatus 2 according to a second embodiment of the invention is shown in Fig. 3. The gas sensing apparatus 2 according to the second embodiment also comprises a duct 2 segmented into first 201 and second 202 duct sections. For the purposes of understanding, the third duct section has been omitted from Fig 3. The dimensions of the first 201 and second 202 duct sections according to the second embodiment are identical to those provided in the first embodiment.

In this second embodiment the gas sensor 205 is provided adjacent to and in contact with the second duct portion, such that a sensing area 221 of the gas sensor defines a surface area of the second duct portion 202. Thus, there may be considered to be an interior surface area 222 of the second duct portion 202 opposite to or facing the sensing area 221 of the gas sensor 205 to thereby define the second duct portion 202. A distance between the sensing area 221 of the gas sensor and an opposing interior surface 222 of the second duct portion 202 may be considered to be a width w ₄ , less than a width w ₃ of the first duct portion.

Preferably this width w ₄ is no greater than 5 mm, for example no greater than

4 mm, for example less than 2 mm and preferably less than 1 mm.

A third embodiment of the invention may be described with reference to Fig. 4.

The third embodiment comprises all the features of the second embodiment and the additional feature of a gas filter 310 positioned in the first duct section 301. The gas filter 310 may be either a target gas filter or a particle filter. A target gas filter is adapted to at least partially filter a target gas (i.e. one which the gas sensor is designed or adapted to measure).

In other words, the target gas filter is adapted to modify a concentration of the target gas in the gas.

A gas filter 310 may cause a modification of the flow rate through the duct; and will typically reduce the flow rate of the gas. Such a gas filter 310 may be associated with a pressure drop, such that the volumetric flow rate of a gas crossing the gas filter 310 is reduced. Such a filter may be selectively chosen to result in a desired flow rate drop across the filter. The choice of the size, thickness or other characteristics of such gas filters may be determined based on the cross-sectional area of the second duct portion 202 or vice versa. Typical (i.e. known in the prior art) gas sensing apparatus might conceivably comprise a gas filter and a gas sensor in a duct portion of the same width, such that the flow speed or velocity of gas is the same through the filter and past the gas sensor. Thus, in the prior art a compromise is typically made between improving the longevity of the gas filter (by providing a lower flow speed) and improving the response time of the gas sensor (by providing a higher flow speed). It has been advantageously recognized that embodiments according to the invention allow for a first flow speed in or associated with the gas filter and a second, different, flow speed at the gas sensor, such that improved longevity of the gas filter and improved response time at the gas sensor can be simultaneously realized.

The operation of a gas sensing apparatus according to the second embodiment may be more easily elucidated with reference to Fig. 5. For the described operation, gas flows from the first duct portion to the second duct portion (i.e. the second duct portion is downstream from the first duct portion)

Fig. 5 depicts an indication of the average gas flow speed 502 and the volumetric gas flow rate 501 of a gas flow along the length of the duct (e.g. along the horizontal axis 512) according to the second embodiment. A first vertical axis 510 is indicative of the value of the gas flow speed, and a second vertical axis 513 is indicative of the value of the volumetric gas flow rate.

Initially, the gas flow passes through the first duct portion at a first value v ₅ .i of gas flow speed 502. The gas flow also has an associated gas flow rate 501, having an initial value of Φ ₅ . When gas flow reaches the boundary to the second duct portion 550 the gas flow speed increases from the first value v ₅ .i to a second value v ₅ . ₂ . This is due to the volumetric flow rate 501 through the duct remaining at a substantially constant value Φ ₅ , such that the change in cross-sectional area from the first duct portion to the second duct portion induces a respective and proportional inverse change in the gas flow speed. As the second value V5.2 is five times greater than the first value V5.1 , it follows that the cross- sectional area of the second duct portion 202 is five times smaller than the cross-sectional area of the first duct portion 201. It will be readily understood that any proportional size of the first cross-sectional area to the second cross-sectional area may be realized without parting from the scope of the invention, such that a second duct portion may be any number of times smaller in cross-sectional area.

A fourth embodiment of the inventive concept can be described with reference to Fig. 6A. There is shown a gas sensing apparatus 6 having a duct comprising a first duct portion 601, a second duct portion 602, and a third duct portion 603. Gas is permitted to flow in a first direction 641 from the first duct portion 601 to the second duct portion 602 and subsequently to the third duct portion 603. Alternatively or additionally, gas is also permitted to flow in a second direction 642 from the third duct portion 603 to the second duct portion 602 and subsequently to the first duct portion 601.

Positioned in the first 601 and third 603 duct portion is a respective first 611 and second 612 gas filter. In the present embodiment, the gas filter is a particle filter adapted to filter particles from the gas. Also positioned in the first 601 and third 603 duct filter is a respective first 631 and second 632 gas flow generator, embodied as a voltage-controlled fan.

In the described embodiment, the cross sectional area of the second duct portion 602 is smaller than both the cross-sectional area of the first duct portion 601 and the cross-sectional area of the third duct portion 603. The cross-sectional areas of the first and third duct portions are not necessarily the same, although this may advantageously provide a more consistent or predictable change in gas flow speed of a gas flow in the gas sensing apparatus.

A sensor signal associated with the gas flowing in the first 641 or second 642 direction is obtained in the second duct portion by a gas sensor 650. The gas sensor is adapted to detect at least a target gas (e.g. S0 ₂ or C0 ₂ ). Gas flowing in the first direction is at least partially filtered by a target gas filter adapted to filter only the target gas (i.e. the target gas filter filters a selected gas) before being detected at the gas sensor. In other words, the target gas filter modifies a concentration of the target gas in the gas flowing in the first direction. Thus, gas detected or measured at the gas sensor when flowing in the first direction does not comprise the target gas or comprises a reduced target gas concentration when compared with the target gas concentration in the gas upstream from the target gas filter. Gas detected at the gas sensor when flowing in the second direction does comprise at least the target gas, as no targeted filtering occurs.

The gas sensor 650 may have a definable gas sensing surface 651 adapted to sense at least the target gas. Such a gas sensing surface may, for example, be the gas- permeable membrane of an electrochemical gas sensor or of a metal-oxide semiconductor gas sensor. Preferably, a gas sensing distance 655 between the gas sensing surface 651 and an interior surface 660 of the second duct portion facing the gas sensing surface 651 is very small, for example, less than 10 mm, preferably less than 4 mm and even more preferably less than 2 mm. The gas sensing distance 655 may be selected based on at least the desired target gas to be detected and/or the characteristics of the gas sensor (e.g. sensitivity or longevity). An exemplary operation of the gas sensor apparatus 6 shall now be elucidated in the following description. Firstly, the first 631 and/or second 632 gas flow generators cause a gas (e.g. ambient air received from outside the gas sensing apparatus) to be moved through the duct in a first direction 641 , such that the gas is passed through the first particle filter 61 1 and the target gas filter 620 before a first value of gas concentration is sensed at the gas sensor. The value of gas concentration may be alternatively considered to be a sensor measurement S, and the sensor measurement of gas flowing in the first direction a filtered sensor measurement (S _F ). Subsequently, the first 631 and/or second 632 gas flow generators may cause a gas to be moved through the duct in a second direction 642, such that the gas is passed through the second particle filter 612 before being detected at the gas sensor. In other words, gas flowing in the second direction is not subject to a target gas filter before being detected at the gas sensor. Thus, the sensor measurement of gas flowing in the second direction is an unfiltered sensor measurement (Su).

It will be understood that the operation of the gas sensor apparatus may be reversed, such that gas in the second direction 642 (i.e. unfiltered gas) is sensed at the gas sensor 650 before gas in the first direction 641 (i.e. filtered gas).

Having obtained a filtered sensor measurement (S _F ) and an unfiltered sensor measurement (Su), a target gas concentration (c _gas ) may be determined. Typically, the target gas concentration is proportional to the difference between the unfiltered sensor measurement and the filtered sensor measurement. In other words: cgas ^{= K} (.Su ^~ (3) where κ is a proportionality factor, obtainable through calibration of the gas sensing apparatus. The above equation is particularly accurate over the lifetime of the target gas filter when the target gas filter consistently filters the same proportion of the target gas from successive iterations of gas flowing in the first direction. Thus, if the target gas filter always filters a consistent proportion (for example, all or 90%) of the target gas from the gas flowing in the first direction 641 , the target gas concentration may be reliably calculated using the calibrated proportionality factor.

There may be a controller (not shown) connected to the gas flow generator so as to control the gas flow in the first and second direction by, for example, controlling the gas flow generators. Such a controller may also receive an input from a gas flow speed detector (not shown) or a volumetric gas flow rate detector (not shown) positioned in at least one of the first 601, second 602 or third 603 duct portions. Such a controller connected to a gas flow speed detector or volumetric gas flow rate detector may thereby provide and control a desired gas flow speed or gas flow rate to flow in the first, second or third duct portion. A gas flow speed or volumetric flow rate may be selected so as to improve the lifetime of the gas filters. For example, a gas flow speed at a gas filter may advantageously be maintained at no more than lOcm/s, preferably no more than lcm/s.

The gas sensing apparatus 6 may further comprise a display unit (not shown), for example an LED screen or an LED light, adapted to indicate when the determined target gas concentration (c _gas ) is above an allowable or suitably safe value (c _gas , max).

In embodiments, the controller may be adapted to determine the released exit concentration of target gas from the gas filter (c _ex it), Cexit being a concentration of target gas remaining in the filtered gas flow. Such a controller may be adapted to determine this exit concentration c _ex it based on at least the determined target gas concentration and one or more characterizing values of the target gas filter. The one or more characterizing values may, for example, comprise at least one of the following: the filter thickness, the superficial air velocity at the target gas filter face, the relative humidity of the gas passing through the target gas filter; and the amount of target gas absorbed in the target gas filter.

In at least one preferable embodiment, there may be provided an algorithm for controlling the gas sensing apparatus, said algorithm incorporating several pre-defined values.

As previously identified, a measure of the concentration of target gas detected by the gas sensor may be named c _gas . There may be an assumed initial concentration of gas (Cgas,start) when the sensor system is switched on. It is preferred to set c _gas , _sta rt at a relatively low value below a clean air guideline concentration of the target gas.

There may be a set maximum allowable or permissible value for c _gas named c _gas , _max stored. When c _gas ^ c _gas , _max is measured, a waiting time tid _le is applied, during which no air is passed through the gas filter, before proceeding with subsequent c _gas measurements. Preferably c _gas , _max is a value that is at least several times the clean air guideline concentration of the target gas. The waiting time (tidie) is therefore the waiting time before initiating the next determination of c _gas when c _gas ^ c _gas , _max . When an exiting gas concentration c _ex it from the target gas filter 620 is greater than a predetermined saturation gas concentration c _gas,sat (i.e. when Cexit > c _gas , _sat ), a user should be informed that the gas filter is to be replaced, possibly together with the gas sensor.

There may be a maximum permissible rate (Ψ) at which the measured concentration c _gas is allowed to change between two successive determinations of c _gas . For example, when a maximum change of 20% is allowed, Ψ = 1.2. There may be a time interval (At) during which a sensor signal S (e.g. Su or S _F ) is measured and during which a volumetric flow rate φ ₀ passes through the target gas filter.

The algorithm makes use of successive sensor measurements S(t _n ), where n is any positive integer (including 0). In a typical operation, at even values for "n", the sensor is exposed to an unfiltered gas flow (e.g. unfiltered ambient air) i.e. S = Su, whereas at odd values, the sensor is exposed to a filtered gas flow (e.g. filtered ambient air), i.e. S = S _F . It will be understood that this may reverse, such that at even values the sensor is exposed to a filtered gas flow.

It is herein recognized that any target gas filter with a defined filter structure and known absorption characteristics with respect to the target gas at concentration c _gas can be characterized with a response function Z(L, Amter, v _s , RH, Γ, c _gas ). Here, L is the filter thickness, Amter is the filter face area, v _s is the superficial air velocity at the filter face, RH is the relative humidity of the air passing through the target gas filter, Γ is the amount of target gas absorbed in the filter, and c _gas is the target gas concentration in the gas flow entering the target gas filter. There is defined an airflow rate through the target gas filter (j) _c , being the product of the filter face area and the superficial air velocity at the filter face (i.e. φο = Amter ^x v _s ). The significance of this response function Z(L, Amter, v _s , RH, Γ, c _gas ) is that at any arbitrary point in time t _n , the exit gas concentration c _ex it(t _n ) from the target gas filter (i.e. the concentration of target gas in a gas flow emerging from and immediately downstream from the target gas filter) that is targeted with an airflow rate φο comprising a target gas concentration c _2as can be obtained from: cexit (^n) ^— Z(L, A^i _ter , V _s , RH(t _n r(t _n ), Cg _as (t _n ) ^{^} ) (4) At t = to, the gas filter is assumed to have a determined or determinable target gas loading r(t ₀ ) obtained from previous exposures to target gas-polluted air.

The disclosed algorithm proceeds as follows:

At t = to

Measure Su(to) and RH(t ₀ );

Assume c _gas ^— c _gas ,start

At t tl

Measure S _F (ti) and RH(ti); c = κ(5υ(ί ₀ ) -5 _F (ti)) + (L, 4 ii _ter » r(t ₀ ),c) Solve this equation for c.

if C > f Cgas,start then Cgas(tl)— Ψ X Cgas,start

1 1

if ^c ψ ^c gas, start then C _gas (ti)—— ^c gas, start

else c _gas (ti) = c if (z,^ _/ter ,v _s ,RH( ,r( ,^( )≥

then display message:- "filter replacement needed!"

i ^l f ⁱ c gas (t)>c gas,max

then apply a standby time tid _le during which φ ₀ = 0 before proceeding with the measurement of Su(t ₂ );

else proceed with the measurement of Su(t ₂ ).

At t = t ₂

Measure Su(t ₂ ) and RH(t ₂ );

c = κ(5 _ϋ (ί ₂ ) - S _P t )) + Z& RHfaXrfaXc

Solve this equation for c.

if c> Ψ x c _gas (ti) then c _gas (t ₂ ) = Ψ x c _gas (¾)

1 1

if c < - x c _gas (ti) then c _gas (t ₂ ) = - c _gas (¾)

else c _gas (t ₂ ) = c

then display message:- "filter replacement needed!"

then apply a standby time tid _le during which φ ₀ = 0 before proceeding with the measurement of Su(t ₃ );

else proceed with the measurement of Su(t ₃ ).

This algorithm can be generalized such that

At t = t _n Measure S _F (t _n ) and RH(t _n );

C = - S _P t _n )) + Z Afuter.Vs.RHit _r J.rit _n - .c

Solve this equation for c.

if if c<-xc (t^) then c {t _n ) =— xc {t _n _ _x ) ψ ψ

then display message:- "filter replacement needed!" i ^L f ⁱ c gas (Vtn )J>r gas,max

then apply a standby time ti _dle during which φ ₀ = 0 before proceeding with the measurement of Su(t _n + 1 );

else proceed with the measurement of Su(t _n + 1 ) .

At t = t _n+ i

Measure Su(t _n+ i) and RH(t _n+ i);

c = K(Su t _n+1 ) - S _F (tj) + Z(L > A fiiter> ^v s> RH{t _n+1 ),r{t _n ),c)

Solve this equation for c.

if then c _gas (t _n+i ) = xc _gas (t _n ) if c <— x c (t ) then c (t _^ ) =— x c (t )

ψ ψ

else c _g t _n+1 ) = c

Apply

if z(L,A _filter , Y _s ,RH(t _n+l ),T(t _n+l ),c _gas (t _n+l ))≥ c _ga ^ _sat then display message:- "filter replacement needed!"

then apply a standby time tid _le during which φ ₀ = 0 before proceeding with the measurement of Sp(t _n +2);

else proceed with the measurement of SF(t _n + ₂ ).

With the above algorithm, target gas concentrations c _gas (t _n ) are determined in the course of time from a series of serial measurements S(t _n ). The rate of concentration change over time is bound to an imposed maximum while the influence from the existence of a non-zero c _ex it on the sensor signals is compensated for. The functionality of the gas filter is extended by minimizing the rate of filter loading with target gas. This is done both by minimizing v _s in the first duct portion and by changing the sensor system operation to intermittent-mode when a high ambient target gas concentration c _gas > c _ga s,max is encountered.

Because the values c _gas (t _n ) are obtained after adjustments related to the filter loading with target gas, it must be ensured that these adjustments remain relatively minor otherwise the inaccuracy can rise unacceptably. If the adjustments become dominating, the filter should be replaced for a fresh one. This is taken care of by the system issuing a warning message when c _exi t(t _n ) ^ c _gas ,sat.

For the purposes of further explanation, and to allow the skilled person to readily identify a concept of the present invention, a further embodiment may be described with reference to Fig. 6B.

The concept hereafter describes relates to one of sensing a first sensor signal associated with a first flow of gas, and a second sensor signal associated with a second flow of gas. A target gas concentration (e.g. associated with the first or second flow) may be determined based on a comparison between the two sensed sensor signals.

There is provided a gas sensing apparatus 6 having a first duct portion 601, a second duct portion 602 and a third duct portion 603. The second duct portion 602 has a smaller cross section than the first duct portion 601 and the third duct portion 603. Gas may flow or be passed from the first duct portion 601, to the second duct portion 602, to the third duct portion 603 in a first direction 641. Similarly, gas may flow or be passed from the third duct portion 603, to the second duct portion 602, to the first duct portion 601 in a second direction 642.

The gas flowing in the first direction 641 (first flow) may have a first target gas concentration, being a concentration of a target gas in the gas of the first flow. The gas flowing in the second direction 642 (second flow) may have a second, modified target gas concentration. Such modification may be a result of a target gas filter (which filters the target gas) or via another modifying element, which may be external to the gas sensing apparatus, such as a patient, a user or a machine.

It will be readily understood that, in some embodiments, the gas flowing in the first direction may be associated with the modified target gas concentration. In this way, the modified (second) target gas concentration may be greater or less than the first target gas concentration.

It will be apparent that there is typically a difference in the concentration of target gas in the gas flowing in the first direction 641 and the concentration of target gas in the gas flowing in the second direction 642. In this way, the concentration of target gas flowing in the first or second direction may be considered to be modified. This difference in concentration of target gas may, in some embodiments, be caused by a target filter or an element external to the gas sensing apparatus 6 (e.g. a user breathing or a machine processing the gas).

A concentration of the target gas may be determined by comparing or otherwise algorithmically processing (such as previously described) signals associated with the gas flowing in the first direction to gas flowing in the second direction. In this way, a measure of the target gas concentration may be calibrated, or accuracy of determining the target gas concentration may be more accurately attained.

In another scenario, such gas sensing apparatus could be used in a clinical or training environment, with, for example, gas flowing in the first direction 641 being associated with a breathing gas or gas/air inhaled by a user, and gas flowing in the second direction 642 being associated with gas/air exhaled by a used. In some embodiments, this allows a measure of the target gas concentration in a patient's or user's exhaled breath to be calibrated based on a measure of the target gas concentration in a patent/user's inhaled breath.

By way of example, if in one scenario a gas sensor 650 is adapted to detect 0 ₂ (oxygen) as a target gas; such gas sensing apparatus 600 may be used to determine an efficiency of the patient's or user's breathing. Assuming that the gas flowing in the second direction 642 is representative of exhaled breath (and gas flowing in the first direction 641 is representative of inhaled breath), a calibrated determination of the amount of target gas (e.g. 0 ₂ ) used by the patent or user may be determined. Purely by way of example, such apparatus may advantageously be used to test a user's response or breathing efficiency to a low input oxygen level (e.g. simulating high altitude air), in the event that the gas flowing in the first direction is associated with air to be breathed having a low oxygen concentration.

The gas sensing apparatus 600 may, in embodiments, thereby be a respirator, inhaler, or other breathing apparatus suitable for use in a clinical environment.

In yet another example, the gas sensing apparatus may be a breathalyzer adapted to detect a target gas (e.g. ethanol in a vapor phase). Such a breathalyzer may have a higher degree of accuracy, as it may allow for calibration of exhaled breath to an

environment or ambient air or inhaled air.

Gas may be caused in flow in the first 641 and/or second 642 direction due to a flow rate modifier (e.g. a gas flow generator 631 such as a fan). In other embodiments, gas may be caused to flow in the first and/or second direction due to external elements (such as a patient or user breathing in and out), such that the gas sensing apparatus may be considered 'passive'. In some such embodiments, there may be a default setting in which gas flows in a particular direction (e.g. a first direction), which may be overcome by a user exhaling air in an opposing direction (e.g. the second direction). In yet other conceivable embodiments, movement of the device may cause gas to flow through the device (e.g. a user may move the device through a gas to cause movement of the gas through the device in a particular direction).

The gas sensing apparatus allows for provision of a relatively simple device using a single gas sensor capable of detecting a target gas of gas moving in a first and/or second direction, whilst also providing improved accuracy and longevity of the apparatus.

Optionally, such a gas sensing apparatus 600 may comprise at least one target gas filer adapted to filter at least a portion of the target gas from the first and/or second flow prior to the gas of the respective flow being sensed at the gas sensor 650. Such a filter may be particularly advantageous if the gas sensor is only reliably operable in a particular (e.g. low) range of percentages. Knowledge about the target gas filters characteristics may allow a method to account or calibrate for such filtered gas, if required.

In some embodiments, the gas flowing in the first direction and the gas flowing in the second direction, prior to any filtering, is substantially the same (e.g. ambient air). Such an embodiment was previously described with reference to Figure 6 A. In this way, the gas sensor 650 may sense two sensor signals, a first sensor signal associated with a first gas, and a second sensor signal associated with a second gas, wherein the first and second gas are substantially the same, excepting one of the first or second gas has a target gas at least partially removed therefrom. Removal of the target gas in this manner permits an improved accuracy of sensing a target gas concentration, as described in previous embodiments.

Various possible cross-sectional areas of an exemplary first duct portion and second duct portion may be described with reference to Figs. 7a-7e. Fig. 7a shows a cross- section of a first duct portion 700 according to an embodiment of the invention. The first duct portion comprises a hollow structure 701, embodied as a hollow cylinder, i.e. a circular pipe. A void or aperture 702 of the first duct portion is defined by the interior wall 703 of the hollow structure. The void or aperture 702 defines a gas flow passage through which a gas may flow in the first duct portion, as well as defining the cross-sectional area of the first duct portion 700. A width w _7a of the cross-sectional area 702 of the first duct portion is defined as the distance between a first surface area of the interior wall 703 and a second surface area of the interior wall 703, the second surface area facing the first. In the present embodiment, this is realized as the diameter of the cross sectional area 702.

A cross-section of an embodiment of a second duct portion 710 is shown in Fig. 7b. It can be clearly seen that the void or aperture 712 of the second duct portion 710 is much smaller than the void or aperture 702 of the first duct portion 700, such that the width w ₇ b of the void or aperture 712 (in this case the diameter) of the second duct portion 710 is much smaller than the width w _7a of the void 702 of the first duct portion 700. In the present embodiment, the overall size of the hollow structure 711 that makes up the second duct portion 710 is the same as the hollow structure 701 that makes up the structure of the first duct portion. However, it will be readily understood that the size of the hollow structure is inconsequential, as long as the cross-sectional area of the void or aperture (that defines that gas flow passage) is smaller in the second duct portion than the first duct portion.

The cross-sectional area of the void or aperture need not be the same geometric shape in the second duct portion as in the first. For example, as shown in a further embodiment of a second duct portion 720 in Fig. 7c, the cross-sectional area 722 may be a different geometric shape (e.g. a square). Furthermore, the interior walls 723 of a hollow structure 721 that makes up a duct portion (e.g. the second duct portion 720) need not match the shape of the exterior walls 724. However, the restriction that the size of the cross- sectional area of the second portion is smaller than the size of the cross-sectional area of the first portion still holds true. Furthermore, the width w ₇ c between two opposing surfaces of the interior walls 723 of the second duct portion 720 is less than the width w _7a between two opposing surfaces of the interior wall 703 of the first duct portion 700. As shown in Fig. 7d, in an alternative embodiment of a second duct portion 730, the exterior walls of the hollow structure 731 need not be circular, but rather may be any shape, regular or irregular, as long as the cross-sectional area 732 through which gas may flow in the second portion is smaller than the cross-sectional area of the first portion 702.

The cross-sectional area of either the first or second portion need not be a continuous or unbroken shape, but may rather comprise a plurality of cross-sectional area to define a plurality of gas flow passages through which a gas may flow. For example, as shown in Fig. 7e, a second duct portion 740 may comprise a plurality of voids or apertures 742 within a hollow structure 741, each defining a respective gas flow passage. The overall cross- sectional area of the second duct portion (i.e. the total cross-sectional area of the plurality of gas flow passages 742) is less than a cross-sectional area 702 of the first duct portion 700. Alternatively, a second duct portion may be considered to be any number (e.g. a single one, or a pair) of these gas flow passages 742, such that the cross-sectional area of such a second duct portion is the cross-sectional area of a selection of the gas flow passages 742.

Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other features or steps, and the indefinite article "a" or "an" does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measured cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.