Abstract:
A method of selectively sensing the concentration of a target gas in polluted ambient air comprises the steps of: —providing a target gas sensor ( 220 ) sensitive to the target gas; —providing a first gas flow derived from the ambient air, from which first flow the target gas is substantially removed; —providing a second gas flow derived from the ambient air, substantially comprising the same target gas concentration as the ambient air; —exposing the target gas sensor to the first gas flow during a first time interval, and obtaining from the sensor a first output signal (Smf); —exposing the target gas sensor to the second gas flow during a second time interval not overlapping with the first time interval, and obtaining a second output signal (Smu); —calculating the difference (SΔ) between the first and the second output signals; calculating the concentration of the target gas from the calculated signal difference (SΔ).

Description:
CROSS-REFERENCE TO PRIOR APPLICATIONS 
     This application is the U.S. National Phase application under 35 U.S.C. §371 of International Application No. PCT/I132012/053501, filed on Jul. 4, 2012, which claims the benefit of European Patent Application No. 11173728.4, filed on Jul. 13, 2011. These applications are hereby incorporated by reference herein. 
     FIELD OF THE INVENTION 
     The present invention relates in general to the field of gas sensors. 
     BACKGROUND OF THE INVENTION 
     An important application of gas sensors lies in the domain of indoor air pollution monitoring, only for obtaining objective data, and control of air handling units and/or air cleaning units. It would be desirable that such a sensor has the following features: 
     small size; 
     low cost; 
     low power requirements; 
     minimal maintenance requirements over an operational period of at least several years; 
     a combination of high sensitivity and high selectivity with respect to a particular target gas or target class of gases. 
     Selectivity is especially important in situations where the composition of the gas to be measured is not known in advance. Lack of sufficient selectivity remains a key issue with all major sensor technologies and poses severe application problems in ordinary indoor environments where usually an unknown mixture of different gaseous pollutants is present. 
     A high selectivity towards a specific target gas or towards a specific class of target gases allows the obtained sensor signals to be interpreted unambiguously. This aspect also applies to the influence of the air humidity, the air temperature and the local air speed on the obtained sensor signals. 
     As regards the quality of ambient air, it is important to be able to unambiguously distinguish clean air from polluted air. Air may be polluted by a certain gas (for instance formaldehyde, NO x , O 3 , SO 2 ) or a certain class of gases (for instance the class of all volatile organic hydrocarbon gases, usually referred to as TVOC; or the class of acid gases, which includes HNO x , SO 2  and organic carboxylic gases). Apart from the ability to detect whether a certain pollutant is present, it is also important to be able to detect the concentration of that pollutant. Air is considered to be unacceptably polluted with a certain target gas when the concentration of that target gas is comparable to or higher than its recommended maximum concentration limit. For indoor living environments, these so-called concentration limit standards are quite low, i.e. around 50 ppb for both O 3  and NO 2 , 0.2-0.3 mg/m 3  for TVOC and 40 ppb for formaldehyde. 
     At present, no sensors or sensor technologies exist that fulfill all the above requirements to a satisfactory extent. Nevertheless, the use of metal-oxide semiconducting sensors or electrochemical sensors appears to be the most promising choice in this regard. This applies in particular to the sensing of formaldehyde, which is a recognized important air pollutant, in particular in Chinese residential environments. 
     However, an important problem with metal-oxide semiconducting sensors and electrochemical sensors is their lack of selectivity. Several attempts to overcome this problem have already been proposed. 
     The gas to be examined, for instance ambient air, may contain several pollutants, and it would be desirable to be able to measure the concentration of each one of these pollutants individually. However, pollutants tend to influence measurements directed at other pollutants. In a basic approach, it is attempted to eliminate all “other” pollutants, so that only one pollutant (i.e. the target gas) remains: a sensor output signal obtained from the thus filtered gas will be proportional to the amount of (concentration of) target gas. Such an approach to try to improve the sensing selectivity of a gas sensor is described in for instance CN101825604 and CN101776640. These documents propose to specifically remove the interfering gases from air with a “scrubbing filter”. 
     A disadvantage of this approach is that it requires knowledge of the identity of the “other” pollutants. However, it is usually not a priori known which gaseous pollutants interfere and the extent to which they interfere with the measurement of the target gas. Furthermore, gases of widely different physical properties such as H 2  and ethanol are known to be interfering gases for electrochemical formaldehyde sensors and it is far from easy to effectively remove all these gases from air at room temperature using small low-cost passive filters. It is therefore in general very difficult or even impossible to design a practical filter capable of removing all interfering gases from air while leaving everything else the same. Another approach, therefore, is to have a filter for removing the target gas from the polluted air, and to perform two measurements: one measurement on the original polluted air, which still comprises the target gas, and one measurement on the original polluted air from which the target gas has been removed. The difference between the two measurement signals obtained in these two measurements will be proportional to the amount of (concentration of) the target gas. 
     A company by the name of “Environmental Sensors” has recently proposed a portable electrochemical formaldehyde sensor equipped with a removable formaldehyde sheet filter impregnated with a chemical reactant capable of specifically removing formaldehyde from the ambient air entering the sensor interior (see http://www.environmentalsensors.com/formaldehyde-monitor-z-300.html). The formaldehyde filter furthermore serves as a diffusion barrier which limits the entry of gaseous species into the electrochemical cell. This formaldehyde filter can be manually replaced by a blank filter, which only serves as a diffusion barrier and which hence does not absorb any gases from air. By comparing the obtained sensor signal in the presence of the formaldehyde filter with the sensor signal in the presence of the blank filter, a signal difference is obtained that is directly proportional to the formaldehyde concentration, since the influence of other (interfering) gaseous pollutants is excluded. 
     A disadvantage of this approach is that the two filters can only be exchanged manually, which is inconvenient. Furthermore, the used filter is embodied as a flat fibrous sheet filter, which can be impregnated with only a very limited amount of the reactant that removes formaldehyde from air. The useful lifetime of the formaldehyde filter is therefore only short and not practical in ordinary indoor environments. It is furthermore unknown when the used formaldehyde sheet filter should be replaced. In addition, the impregnation of the fibrous sheet filter with the reactant material results in an inevitable reduction of the filter porosity, thereby changing its diffusion barrier characteristics. The latter characteristics are furthermore dependent on the ambient humidity because of the humidity-dependent moisture uptake by the reactant. The afore-mentioned circumstances result in serious interpretation difficulties with respect to the obtained signal difference in terms of the ambient formaldehyde concentration and lead to large inaccuracies. 
     Yet another approach to try to improve the sensing selectivity of a gas sensor is described in for instance CN101571506 (Huarui Scientific Instrument Shanghai). This document proposes an electrochemical formaldehyde sensor comprising a first working electrode, a compensation electrode, and a common counter electrode. The compensation electrode effectively acts as a second working electrode characterized in that it is provided with a filter capable of specifically removing formaldehyde from air. The formaldehyde filter furthermore acts as a general gas diffusion barrier. The first working electrode is provided with a dummy filter and only acts as a gas diffusion barrier. By subtracting the sensor signal obtained from the first working electrode (having contributions from both formaldehyde and interfering gases) from the signal obtained from the compensation electrode (having contributions from only the interfering gases), a differential signal is obtained that only accounts for the formaldehyde concentration in air and compensates for possible effects related to humidity and temperature changes. 
     A disadvantage of the solution offered by Huarui is that effectively two separate working electrodes are needed within a single electrochemical sensor, as illustrated in  FIG. 1 . Small physical differences between the two working electrodes can easily lead to quite different sensor responses and different signal bias, both with respect to their zero readings (in clean air) and with respect to their span (the signal difference per unit concentration of the target gas and/or of the interfering gases). It is therefore generally difficult, if not impossible, to unambiguously interpret the obtained differential sensor signal in terms of the target gas concentration. Because the filters are integrated within the electrochemical sensor, it is not feasible to remove or otherwise manipulate them, for instance for sensor calibration purposes. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to overcome or at least reduce the above problems. 
     To meet this objective, the present invention proposes a method and a sensing arrangement for determining the concentration of a target gas in ambient air. The sensing arrangement comprises a target gas sensor, a target gas filter, means for air displacement through the sensing arrangement, a controller capable of controlling the means for air displacement, and an evaluation unit capable of receiving and interpreting output signals from the target gas sensor. The target sensor is exposed to a first gas flow of displaced air, wherein the target gas concentration is substantially the same as in the polluted ambient air. The target sensor is exposed to a second gas flow of displaced air which comprises substantially the same pollution as the first gas flow of displaced air except that the target gas has substantially been removed from the second gas flow of displaced air through selective filtration by the target gas filter. The difference between the correspondingly obtained sensor output signals is then proportional to the concentration of the target pollutant only, thus enabling selectivity. An advantage of this method and apparatus is that it requires only one gas sensor. 
     Further advantageous embodiments and elaborations are mentioned in the dependent claims. 
     The present invention also proposes an embodiment of a sensing arrangement in which two separate gas sensors are used for obtaining a differential signal that is characteristic of the concentration of the target gas, and in which means are provided to equalize the respective sensor responses when the two sensors are exposed to the same gaseous environment. The latter possibility at least partly compensates for possible differences in the measured sensor responses of the two sensors as a function of the target gas concentration, the concentrations of the respective interfering gases, the temperature and the relative humidity. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other aspects, features and advantages of the present invention will be further explained by means of the following description of one or more preferred embodiments with reference to the drawings, in which same reference numerals indicate same or similar parts, and in which: 
         FIG. 1  schematically shows a prior art electrochemical sensor; 
         FIG. 2  shows the sensor exposure to two possible flows of displaced air, wherein one flow first passes the target gas filter before reaching the sensor; 
         FIGS. 3A-3C  schematically illustrate a first embodiment of a gas sensing apparatus according to the present invention; 
         FIGS. 4-9  schematically illustrate several variations of embodiments of a gas sensing apparatus according to the present invention, requiring only a single target gas sensor; 
         FIGS. 10-12  schematically illustrate several variations of embodiments of a gas sensing apparatus according to the present invention comprising at least two target gas sensors. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  schematically shows a prior art electrochemical sensor  100  comprising a first working electrode  101 , a second working electrode  102 , a reference electrode  103 , and a common counter electrode  104 . Both working electrodes  101  and  102  share the same counter electrode  104  and the same reference electrode  103  and are exposed to the same electrolyte solution  111  from an electrolyte reservoir  110 . 
     Over the first working electrode  101 , a filter  121  for a specific target gas is arranged. This filter  121  effectively removes the target gas from an airflow. An air gap between the filter  121  and the first working electrode  101  is indicated at  123 . Over the second working electrode  102 , a dummy filter structure  122  is arranged. This structure  122  has mechanical properties similar to filter  121 , but does not filter out any of the air pollutants. An air gap between the dummy filter structure  122  and the second working electrode  102  is indicated at  124 . Insulators adjacent the working electrodes are indicated by reference numerals  140 . An air-permeable sensor cover  130  covers the filter  121  and the dummy filter structure  122  and holds them in place. 
     The gas to be monitored, for instance ambient air  150 , slowly passes the cover  130  and the filter  121  or the dummy filter structure  122 , respectively, to reach the first working electrode  101  or the second working electrode  102 , respectively. Thus, the first working electrode  101  is passively exposed to air from which the target pollutant has been removed, while the second working electrode  102  is passively exposed to unfiltered ambient air. If the two working electrodes  101 ,  102  have mutually equal characteristics, the difference between their output signals is proportional to the amount (or concentration) of target gas in the air. 
       FIG. 2  is a drawing schematically illustrating the basic principles behind the present invention. A gas sensing apparatus according to the present invention is generally indicated by reference numeral  1 , and comprises a gas sensor  2  and a target gas filter  4 . As long as it is sensitive to at least the target gas to be sensed, the gas sensor  2  may be any known sensor; therefore a more detailed description of the gas sensor  2  is omitted here. It is noted that the gas sensor  2  as such does not need to have selectivity for the target gas. In fact, the gas sensor may be sensitive to various different target gases or classes of target gases. 
     The apparatus  1  furthermore comprises controllable means capable of causing air  3 , derived from polluted ambient air, to follow either one of at least two different airflow paths, as illustrated by two flow arrows  5  and  6 . The concentration of the target gas in air  3  is substantially the same as in the ambient air from which air  3  is derived. In one airflow  5 , the air  3  passes the target gas filter  4 , so that the target gas is substantially removed from the air  3  before the air reaches the sensor  2 . In another airflow  6 , the air  3  does not pass the target gas filter  4 , so that the airflow  6  reaching the sensor  2  has substantially the same concentration of target gas as the ambient air. The air  3  may be totally unfiltered ambient air, so that the composition of air  6  is substantially equal to the composition of ambient air. It is also possible that air  3  is derived from ambient air by passing the ambient air through a filter that removes one or more gas components but does not affect the concentration of the target gas. This means that both gas flows  5  and  6 , when reaching the sensor  2 , have the same composition as far as all other components are concerned, except for the target gas which is substantially absent in the first gas flow  5  and which is substantially present in the second gas flow  6  to the same extent as in the original ambient air. Thus, the sensor is exposed either to air WITH or air WITHOUT the target gas, all other components being the same, and any difference in the sensor output signal in these two situations is representative of the amount of target gas. 
     Several implementations are possible, as will be explained in the following. For allowing unfiltered ambient air  6  to reach the gas sensor  2 , it may be sufficient to use (natural) convection as the driving force for air displacement, but it is also possible to use airflow generating means, for instance a ventilator or a pump. For making air pass the filter  4 , convection will usually be insufficient to generate the airflow  5 , so that the apparatus preferably comprises airflow generating means, for instance a ventilator, but it is also possible to connect the apparatus to a source of pressure difference. Airflow selection in the gas sensing apparatus  1  can for instance be done by using controllable valves and/or controllable ventilators. 
     A first embodiment of a gas sensing apparatus according to the present invention is generally indicated by reference numeral  200  and is schematically illustrated in  FIGS. 3A and 3B . The gas sensing apparatus  200  comprises a measuring chamber  210  and a gas sensor  220  arranged in the measuring chamber  210 . 
     The measuring chamber  210  has at least one passageway  211  allowing direct entry of ambient air and allowing this ambient air to reach the measuring chamber  210  unfiltered. In the embodiment shown, there are two different passageways  211 ,  212  connecting the measuring chamber  210  to the ambient environment such as to allow for convection. 
     The gas sensing apparatus  200  further comprises an air duct  230  having an entrance  231  communicating with the ambient environment and an output  232  communicating with the measuring chamber  210 . The gas sensing apparatus  200  further comprises a gas filter  234  and a controllable airflow generating means  233 , for instance a ventilator, arranged in the air duct  230 . The figure shows the gas filter  234  arranged between the entrance  231  and the ventilator  233 , but this order may also be reversed. The filter  234  is selected for substantially removing the target gas to be sensed from any air passing the filter, and will also be indicated as target gas filter. 
     The gas sensing apparatus  200  further comprises a control device  240  for controlling the ventilator  233 , the control device  240  having a control output  243  coupled to a control input of the ventilator  233 . The control device  240  may for instance be implemented as a suitably programmed microprocessor, microcontroller, or the like. The control device  240  in this embodiment is integrated with means for sensor signal evaluation, for which purpose the control device  240  is provided with a measuring input  241  connected to receive an output signal from the sensor  220 . It is noted that the functionality of sensor signal evaluation may be implemented in a different unit, which then would have an output communicating with an input of the control device, so that control of the ventilator can take place on the basis of the outcome of the sensor signal evaluation. 
     The gas sensing apparatus  200  is capable of operating in two different operational modes. In a first operational mode, the gas sensor  220  is exposed to unfiltered ambient air: this mode will hereinafter be indicated as “unfiltered mode”, and the measuring output signal of the sensor  220  in this mode will be indicated as Smu. In a second operational mode, the gas sensor  220  is exposed to a flow of ambient air filtered by the filter  234 : this mode will hereinafter be indicated as “filtered mode”, and the measuring output signal of the sensor  220  in this mode will be indicated as Smf. The control device  240  is capable of calculating a differential signal SΔ=Smu−Smf, which is proportional to the target gas concentration in the unfiltered ambient air. This differential signal SΔ can be considered as constituting the measuring output signal of the sensing apparatus  200 . It is noted that the tasks of receiving and processing the sensor output signals on the one hand and controlling the ventilator on the other hand may alternatively be performed by separate calculating/evaluating and control units. 
       FIG. 3A  illustrates the gas sensing apparatus  200  operating in its unfiltered mode. The ventilator  233  is off. Convective flows of unfiltered ambient air  213 ,  214  reach the measuring chamber  210  through the passageways  211 ,  212 . 
       FIG. 3B  illustrates the gas sensing apparatus  200  operating in its filtered mode. The ventilator  233  is on, causing a flow of ambient air  235  in the duct  230  to pass the filter  234  and reach the measuring chamber  210  as filtered air  237 , leaving the measuring chamber  210  via passageway  212  which now acts as an output. 
     Preferably, the control device  240  switches the ventilator  233  on and off periodically, such as to periodically alternate between the filtered mode and the unfiltered mode. 
     In the schematical layout of  FIGS. 3A and 3B , the first passageway  211  is shown between the duct  230  and the measuring chamber  210 . In such a case, the design should be such that a portion  236  of the flow  235  generated in the duct  230  is blown out through the first passageway  211  in order to prevent unfiltered ambient air from reaching the measuring chamber  210 .  FIG. 3C  illustrates a layout where this issue does hardly play a role. 
     In the above, it has been mentioned that, in the unfiltered mode, unfiltered ambient air may reach the sensor convectively, i.e. with the ventilator off. Alternatively, it is possible that the gas sensing apparatus  200  comprises a second ventilator for, in the unfiltered mode, causing a flow of unfiltered ambient air to enter the measuring chamber  210  through one passageway  211  and leave the measuring chamber through the other passageway  212 , or vice versa. 
     In the above, switching between the filtered operational mode and the unfiltered operational mode is done by switching the ventilator  233  on or off. It is also possible to obtain such switching by selectively operating the ventilator  233  in one direction or an opposite direction, as illustrated in  FIG. 4 . The duct  230  is shown to have two input/output openings  231 ,  232  with the sensor  220 , ventilator  233  and filter  234  being arranged between said openings. In the upper half of the figure, the ventilator  233  is operated to generate an airflow from the right to the left, so that the sensor  220  is upstream of the filter  234 : the airflow reaches the sensor  220  before reaching the filter  234 . This is the unfiltered mode, wherein the sensor  220  is exposed to unfiltered air and outputs the unfiltered output signal Smu. In the lower half of the figure, the ventilator  233  is operated to generate an airflow from the left to the right, so that the sensor  220  is downstream of the filter  234 : the airflow reaches the filter  234  before reaching the sensor  220 . This is the filtered mode, wherein the sensor  220  is exposed to filtered air and outputs the filtered output signal Smf. 
     It is noted that the relative position of the ventilator  233  is not essential: it may be located between opening  231  and the filter  234 , between opening  232  and the sensor  220 , or between filter  234  and sensor  220 . It is further noted that, instead of a bi-directional ventilator, two mono-directional ventilators can be used, arranged in mutually opposite orientation. 
     In a further elaboration, illustrated in  FIG. 5 , apparatus  300  has two different filters  234  and  334  arranged on opposite sides of the sensor  220 . Again, the relative position of the ventilator  233  is not essential. The second filter  334  has filtering characteristics differing from the first filter  234  in that it does not filter the target gas. More particularly, the second filter  334  is capable of filtering a specific gas or group of gases or class of gases, and the first filter  234  is capable of filtering the same specific gas or group of gases or class of gases, respectively, as well as the target gas. 
     In the lower half of  FIG. 5 , the situation is equivalent to the situation of  FIG. 4  (lower half): the second filter  334  is downstream of the sensor  220  and has no influence on the sensor output signal Smf 1 . In the upper half of the figure, with the reverse flow direction, the second filter  334  is upstream of the sensor  220 , so that the airflow reaches the second filter  334  before reaching the sensor  220 ; the sensor output signal in this case is indicated as Smf 2 . In both cases, the sensor  220  is exposed to filtered gas. In both cases, said specific gas or group of gases or class of gases, respectively, has been removed from the original ambient gas. In the case of the lower half of the figure, with the first filter  234  upstream of the sensor  220 , also the target gas has been removed. Consequently, a difference signal SΔ′=Smf 2 −Smf 1  is proportional to the target gas concentration in the filtered gas and therefore proportional to the target gas concentration in the unfiltered ambient gas. This embodiment is advantageous in cases where it is desirable to protect the sensor  220  against the influence of certain gases. 
     In a further elaboration, illustrated in  FIG. 6 , the duct  230  of apparatus  400  has one or more openings  401  allowing ambient gas  403  to directly reach the sensor  220  without being filtered. The one or more openings  401  is/are provided with controllable closure means  402  controlled by the control device  240  at its output  242 . With the controllable closure means  402  in the closed state, the situation is basically identical to the situation of apparatus  300  in  FIG. 5  and the target gas concentration can be inferred from the signal difference SΔ′=Smf 2 −Smf 1 . The control device  240  now has the additional option of switching the ventilator  233  off and opening the closure means  402  in order to allow ambient gas  403  to directly reach the sensor  220 , for instance through convection. The sensor  220  now provides a measuring signal Smu derived from unfiltered ambient gas, and it is possible to calculate a difference signal SΔ″=Smu−Smf 2  proportional to the concentration of said specific gas or group of gases or class of gases, respectively. In other words, apparatus  400  is selective to two different gases or two different groups of gases independently of each other, and these can be inferred by the apparatus  400  from the signal differences SΔ′=Smf 2 −Smf 1  and SΔ″=Smu−Smf 2 , respectively. It will be clear that in this embodiment the gas sensor  220  should have sufficient sensitivity to both different gases or to both different groups of gases in order to achieve selectivity to both different gases or to both different groups of gases, independently of each other, at the respective concentrations thereof in the ambient air. 
     In a further elaboration, illustrated in  FIG. 7 , the apparatus  400  has a series arrangement of first filter  234  and first ventilator  233  arranged on one side of the sensor  220 , wherein the relative order of the first filter  234  and first ventilator  233  is not essential, and has a series arrangement of second filter  334  and a second ventilator  533  arranged on the opposite side of the sensor  220 , wherein the relative order of the second filter  334  and second ventilator  533  is not essential. Both ventilators may be bi-directional ventilators, but that is not essential. The control device  240  again has the option of switching off both ventilators  233 ,  533 : the situation then is equivalent to the situation of  FIG. 6 , where ambient gas  403  is allowed to directly reach the sensor  220  through convection. The control device  240  now has the additional option of switching on both ventilators  233 ,  533  to generate a first forced airflow  404  from opening  401  towards first filter  234  and a second forced airflow  405  from opening  401  towards second filter  334 . Consequently, ambient gas is actively forced to pass the sensor  220 , and the unfiltered measuring signal Smu is obtained without being dependent on convection. 
     With reference to  FIG. 5 , it is noted that the first filtered output signal Smf 1  can now be obtained by closing the closure means  402  and operating one or both ventilators  233 ,  533  to create an airflow from the left to the right in the figure, so that effectively the situation is equivalent to the situation of the lower half of  FIG. 5 , and it is further noted that the second filtered output signal Smf 2  can now be obtained by closing the closure means  402  and operating one or both ventilators  233 ,  533  to create an airflow from the right to the left in the figure, so that effectively the situation is equivalent to the situation of the upper half of  FIG. 5 . Alternatively, however, it is possible to obtain the first filtered output signal Smf 1  by opening the closure means  402 , switching off the second ventilator  533  and operating the first ventilator  233  to create an airflow from the left to the right in the figure, exiting via the opening  401 , and it is possible to obtain the second filtered output signal Smf 2  by opening the closure means  402 , switching off the first ventilator  233  and operating the second ventilator  533  to create an airflow from the right to the left in the figure, exiting via the opening  401 . 
     In the above description with reference to  FIGS. 4-7 , the sensor  220  is described as being arranged in a “duct”  230  having input/output openings  231 ,  232  at opposite sides of the sensor. However, with reference to  FIGS. 3A-D , it is also possible to describe such an arrangement as being a combination of two ducts, each having one input/output opening communicating with the ambient atmosphere and an opposite input/output opening communicating with a location where the sensor is positioned, such a location being referred to as measuring chamber. In the following description with reference to  FIG. 8 , the latter wording will be used for the sake of convenience. 
     Apparatus  600  of  FIG. 8  is a further elaboration of apparatus  500  of  FIG. 7 , illustrating that it is possible to effectively have multiple apparatuses  500  sharing one common sensor  220  arranged in a measuring chamber  210 . The figure shows an embodiment with six ducts  610 - 660 , each duct having a first opening  611 - 661  communicating with the ambient atmosphere and an opposite opening  612 - 662  communicating with the measuring chamber  210 . In each duct  610 - 660 , a series arrangement of a filter  613 - 663  and a bi-directional ventilator  614 - 664  (or other type of flow generator) is arranged. The ventilators are controlled by a control device not shown in this figure for the sake of simplicity. With reference to  FIGS. 6 and 7 , it is noted that this apparatus  600  may also comprise an opening for allowing ambient gas to reach the measuring chamber  210 , either through convection (as in  FIG. 6 ) or through suction by operating all ventilators (as in  FIG. 7 ). 
     The six ducts  610 - 660  together define three pairs of ducts  610 ,  640 ;  620 ,  650 ;  630 ,  660 . The ducts of each pair are arranged in such a way that their respective second openings  612 ,  642 ;  622 ,  652 ;  632 ,  662  are located on opposite sides of the measuring chamber  210 . The control device is designed such as to selectively operate one of said pairs of ducts, with the other pairs being out of operation. Then, such a selected pair of ducts behaves like the embodiment  300  discussed with reference to  FIG. 5 .  FIG. 8  illustrates this for the ducts  610  and  640 : by appropriate control of the corresponding ventilators  614 ,  644 , ambient air flows from opening  611  via measuring chamber  210  towards opening  641 , as shown by arrows, or in the opposite direction, so the sensor  220  is exposed to air selectively filtered by either filter  613  or filter  643 , respectively. Likewise, the ducts  620  and  650  form an associated pair: by appropriate control of the corresponding ventilators  624 ,  654 , ambient air flows from opening  621  via measuring chamber  210  towards opening  651 , or in the opposite direction, so the sensor  220  is exposed to air selectively filtered by either filter  623  or filter  653 , respectively. Likewise, the ducts  630  and  660  form an associated pair: by appropriate control of the corresponding ventilators  634 ,  664 , ambient air flows from opening  631  via measuring chamber  210  towards opening  661 , or in the opposite direction, so the sensor  220  is exposed to air selectively filtered by either filter  633  or filter  663 , respectively. 
     It should be clear that the same type of operation applies if the number of such pairs is equal to 2 or equal to 4 or more. 
     It is further noted that  FIG. 8  shows all the ducts as being in open communication with the ambient atmosphere. However, in order to avoid an undesired airflow through any of the inactive ducts, each duct is preferably equipped with a controllable closure device, for instance a valve, controlled by the control device, which controls the closure devices such that the closure devices of the inactive ducts are always closed and the closure devices of the active ducts are always open. 
     In each pair of associated ducts (for instance  610 ,  640 ), the corresponding pair of filters (for instance  613 ,  643 ) is designed in the same way as in the apparatus  300  described with reference to  FIG. 5 . Thus, one filter of this pair of filters (for instance  613 ) is capable of filtering a specific gas or group of gases or class of gases, and the other filter of this pair of filters (for instance  643 ) is capable of filtering the same specific gas or group of gases or class of gases, respectively, as well as the target gas. When comparing different pairs of associated ducts with each other, the design of the corresponding pairs of filters differs because either the target gas of one pair of filters differs from the target gas of the other pair of filters, or the specific gas or group of gases or class of gases of one pair of filters differs from the specific gas or group of gases or class of gases of the other pair of filters, or both. Thus, by suitable activation of the several ventilators, it is possible to obtain individual information on the concentration of multiple gases separately while using only one sensor, provided that the sensor has non-zero sensitivity towards each of the said multiple gases at their respective concentrations in the ambient air. 
     In the apparatus  600  as illustrated in  FIG. 8 , each duct is provided with an associated ventilator. Selecting the path which the gas flow takes in the apparatus, and thus selecting which filter is upstream of the sensor, is done by suitable control of the ventilators. However, alternative embodiments having the same functionality may have fewer ventilators.  FIG. 9  illustrates an alternative apparatus  700  wherein each duct  610 - 660  is provided with a controllable closure device  615 - 665 , for instance implemented as a shutter or a valve, controlled by the control device (not shown in this figure), such that the closure devices  625 ,  635 ,  655 ,  665  of the inactive ducts  620 ,  630 ,  650 ,  660  are always closed and the closure devices  615 ,  645  of the active ducts  610 ,  640  are always open. The apparatus  700  has a common duct  710  leading to/from the measuring chamber  210 , and a ventilator  714  arranged in the common duct  710 . In order to allow gas to flow in two opposite directions, the ventilator  714  may be a bi-directional ventilator, or it is possible to use a second ventilator  714 ′, as shown. 
     Each duct in a pair of associated ducts is always coupled to one end or to the other end of the common duct  710 , either via a first manifold  701  or a second manifold  702 . With the ventilator  714  operating continuously, selecting the path which the gas flow takes in the apparatus, and thus selecting which filter  613  is upstream of the sensor  220 , is done by suitable control of the closure devices. 
     An important advantage of the above embodiments is that the set-up is relatively simple and robust and, in view of the fact that only a single gas sensor is needed, they do not suffer from signal biasing problems. 
     The present invention also provides an apparatus with two (or more) sensors. An apparatus with two sensors involves the problem that it is difficult to ensure that the sensor responses of two different sensors positioned at two different locations areidentical. However, such an apparatus offers the advantage that it is possible to provide a continuous result in real time, thereby enabling it to quickly note rapidly changing pollution conditions, and that it is actually possible to perform two measurements on the same air sample. 
       FIG. 10  illustrates an embodiment of a gas sensor apparatus  800  comprising an air duct  830  with an entrance  831  for allowing an airflow of ambient air  835  to enter the duct  830  and an exit  832  for allowing the airflow to be discharged. A target gas filter  834  is arranged in the air duct  830 . A first sensor  821  is arranged upstream of the filter  834 , between the entrance  831  and the filter  834 , and a second sensor  822  is arranged downstream of the filter  834 , between the filter  834  and the exit  832 . The first sensor  821  is subjected to unfiltered air upstream of the filter  834 ; therefore its measuring signal is indicated as Smu. The second sensor  822  is subjected to filtered air downstream of the filter  834 ; therefore its measuring signal is indicated as Smf. A calculation device  840  has a first input  841  receiving the measuring output signal Smu from the first sensor  821  and a second input  842  receiving the measuring output signal Smf from the second sensor  822 , and is thus capable of inferring the concentration of the target gas in the air from the signals Smu and Smf. 
     For driving the airflow  835 , it is possible to arrange within the duct  830  an airflow generator such as for instance a ventilator, as in the case of the above-described embodiments, and such a generator may be arranged upstream or downstream of the filter  834 . It is also possible to equip the apparatus with an external airflow generator, or any device capable of generating a pressure difference over the entrance  831  and output  832 . It is even possible to have the apparatus cooperate with another apparatus, for instance a stand-alone air cleaner, which would include a ventilator to pass air through its cleaner units and hence causes a pressure difference. 
     When the two sensors  821 ,  822  have identical characteristics, i.e. identical responses under otherwise identical conditions, their signals Smu and Smf can be directly compared, thus yielding a signal differential SΔ=Smu−Smf that is directly proportional to the target gas concentration in the unfiltered ambient air. However, it may be that the two sensors  821 ,  822  exhibit a (perhaps slowly drifting) offset signal with respect to each other, or even if they don&#39;t, it may be desirable to be able to check this.  FIG. 11  illustrates an embodiment of a gas sensing apparatus  900 , which is a further elaboration of the apparatus  800  of  FIG. 10 . At a position downstream of the filter  834  and upstream of the second sensor  822 , the duct  830  is provided with an opening  970  allowing the duct  830  to communicate with the ambient environment, which opening  970  is provided with a controllable valve or shutter or door  971 , controlled by the unit  840  which now not only functions as a calculating device but also as control device. The figure illustrates the controllable shutter  971  as a moveable slide, but other implementations are also possible. 
     When the shutter  971  is closed, the apparatus  900  is in effect equivalent to the apparatus  800  of  FIG. 10 . 
     When the shutter  971  is in its “open” position, both sensors  821 ,  822  are exposed to unfiltered ambient air. It is noted that this applies even in the absence of an external pressure difference across the duct  830 , or in the absence of an otherwise forced airflow, albeit that in such a case it may take somewhat longer for the unfiltered ambient air to reach the respective sensors. Preferably, the sensors  821 ,  822  are positioned close to the openings  831 ,  832 ,  970  to ensure quick and full exposure to unfiltered ambient air. 
     Due to the fact that sensors  821 ,  822  are now exposed to the same gas composition, and assuming that all other parameters at the two different sensing locations are mutually identical or do not have any significant influence on the sensor output signals, the two sensor output signals should ideally be mutually identical, and any difference Δ between these signals represents an offset. Without the cause of such offset being known, it is possible for the calculating portion of the unit  840  to compensate for the offset. 
     Thus, the apparatus  900  is capable of operating in a measuring mode and in a calibration mode, and the control device  840  is designed to regularly switch to the calibration mode. In the calibration mode, the control device  840  opens the shutter  971  and calculates the difference Δ between the two measuring signals Smu and Smf received from the two sensors  821 ,  822 , according to Δ=Smu−Smf. In the measuring mode, the control device  840  closes the shutter  971  and calculates a compensated measuring output signal SΔc of the apparatus  900  according to the formula
 
 SΔc=Smu−Smf−Δ 
 
which represents the concentration of the target gas in the unfiltered ambient air.
 
       FIG. 12  illustrates an apparatus  1000  which is a further elaboration of the apparatus  800  of  FIG. 10 , comprising a controllable bi-directional ventilator  833  arranged in the duct  830 , and controlled by the control device  840  at an output  843  thereof. With the ventilator operating as shown, the operation is equivalent to that of the apparatus  800  of  FIG. 10 : the first sensor  821  is upstream of the filter  834  and provides an unfiltered measuring signal while the second sensor  822  is downstream of the filter  834  and provides a filtered measuring signal. When the control device  840  operates the ventilator  833  in the opposite direction, the second sensor  822  is upstream of the filter  834  and provides an unfiltered measuring signal while the first sensor  821  is downstream of the filter  834  and provides a filtered measuring signal. Any offset between the two sensors can now be determined by comparing the two unfiltered measuring signals from the two sensors and/or by comparing the two filtered measuring signals from the two sensors. 
     It is noted that the principles of the invention as described above do not depend on the type of gas sensor. Basically, any known gas sensor can be used, or even future gas sensors will be useable. All embodiments as described are capable of automated operation without being dependent on human intervention. 
     Further, it is noted that the principles of the invention as described above do not depend on the type of target gas filter. However, it is preferred to use a filter type that combines high filter efficiency with low flow resistance and long filter lifetime. By way of example, the target gas filter structure may be a corrugated structure, a parallel-plate structure or a granular filter bed. Such filters are disclosed in U.S. Pat. No. 6,071,479 and allow for a much higher target gas filtration capacity than the sheet filters used by Environmental Sensors mentioned in the introduction. The corrugated structure and parallel-plate structure are preferably made from a fibrous hydrophilic paper material or from a hydrophilic glass-fiber material, which can readily be filled with an aqueous solution of the desired reagent species. After drying, the impregnated reactant species inside the filter remain hydrated in equilibrium with the ambient humidity and can subsequently absorb a target gas from the air. The granular filter is preferably composed from activated carbon, zeolites, activated alumina or any other porous granular material. These materials can also be readily impregnated. Impregnation of these porous materials leaves the width of the air passage channels inside the filter essentially unchanged. Thus, impregnation does not change the diffusive barrier properties of the filter structure with respect to (?) gaseous species. The height of these filters can be readily adjusted, thereby changing the amount of impregnant that can be comprised inside these filters and thus their effective lifetime. Various examples of impregnant compositions that are effective absorbers of (?) formaldehyde, acidic gases or alkaline gases are disclosed in U.S. Pat. No. 6,071,479. 
     As an example, in the case that the target gas is formaldehyde, an advantageous aqueous impregnant solution comprises KHCO 3  (2-20% w/w), K 2 CO 3  (1-20% w/w), Trishydroxymethyl-aminomethane (3-30% w/w), Kformate (2-20% w/w). 
     A more preferred impregnant solution comprises: 
     KHCO 3  (10% w/w) 
     K 2 CO 3  (5% w/w) 
     Trishydroxymethyl-aminomethane (5-25% w/w) 
     Kformate (5-10% w/w) 
     The KHCO 3  and K 2 CO 3  species are examples of alkaline impregnants that are capable of absorbing acidic gases such as HNO x , SO 2  and organic carboxylic acids from air. Tris-hydroxymethyl-aminomethane is the impregnant capable of absorbing formaldehyde from air. Thus, the filter comprising the more preferred impregnant solution mentioned above is capable of absorbing the class of gases comprising HNO x , SO 2  and organic carboxylic acids and the target gas formaldehyde. In the case that Tris-hydroxymethyl-aminomethane is omitted from the preferred impregnant solution, the filter is only capable of absorbing the class of gases comprising HNO x , SO 2  and organic carboxylic acids. 
     While the invention has been illustrated and described in detail in the drawings and foregoing description, it should be clear to a person skilled in the art that such illustration and description are to be considered illustrative or exemplary and not restrictive. The invention is not limited to the disclosed embodiments; rather, several variations and modifications are possible within the protective scope of the invention as defined in the appending claims. It is noted that, in daily practice, the phrase “ambient air” may relate to the mixture of nitrogen and oxygen that we breathe, but in the context of the present invention, the sensing apparatus is basically applicable in any type of gas atmosphere and the phrase “ambient gas” is used to indicate the gas atmosphere in which the apparatus is placed. 
     It is further noted that an apparatus according to the present invention with one gas sensor exposes the gas sensor to two different gas flows during two different time intervals, but the order of these two intervals is not essential. 
     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 elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. A single processor or other unit may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope. 
     In the above, the present invention has been explained with reference to block diagrams, which illustrate functional blocks of the device according to the present invention. It is to be understood that one or more of these functional blocks may be implemented in hardware, where the function of such (a) functional block(s) is performed by individual hardware components, but it is also possible that one or more of these functional blocks are implemented in software, so that the function of such (a) functional block(s) is performed by one or more program lines of a computer program or a programmable device such as a microprocessor, microcontroller, digital signal processor, etc.