Patent Publication Number: US-2022236225-A1

Title: Arithmetic device, arithmetic method, and gas detection system

Description:
TECHNICAL FIELD 
     The present invention relates to an arithmetic device, an arithmetic method, and a gas detection system relating to gas detection. 
     BACKGROUND ART 
     In order to distinguish an odor that is an aggregate of a plurality of gas components, an odor sensor in which adsorption films having a plurality of different chemical properties are arrayed into a multi-array sensor and an odor is patterned has been actively developed in recent years. Use of such an odor sensor makes it possible to distinguish between a pleasant odor and an unpleasant odor. The application of the odor sensor to the field that has been organoleptically evaluated by a person until now has begun to be examined, such as environment management in a room or a vehicle, process management in a factory, or environment monitoring for detecting an initial fire or an offensive odor that adversely affects a human body. 
     For example, a detection element including an adsorption film provided on a crystal oscillator is used as the odor sensor. Since the resonant frequency of the crystal oscillator decreases in proportion to the weight of the gas adsorbed on the adsorption film, the gas can be detected on the basis of the amount of change in the resonant frequency. Thus, a decrease in the sensitivity level due to the deterioration over time of the adsorption film that adsorbs odor substances affects the reliability of the odor sensor. The deterioration of the adsorption film due to the deterioration over time is mainly caused by the fact that odor components adsorbed at the time of using the sensor cannot be desorbed from the adsorption film, continue to stay on the adsorption film, and are thus deposited. 
     Patent Literature 1 discloses that, in order to reduce factors that may become detection errors before or after the detection of an odor by a detection unit, the inside of a container in which a detection element is contained is subjected to a refresh process by performing degassing or deodorization, and the detection unit is initialized. 
     Further, in order to remove odor components adsorbed on the adsorption film, there is also a method in which heat treatment is performed to refresh the adsorption film, for example, when the adsorption film is an organic film. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: WO 2017/145933 
     Disclosure of Invention 
     Technical Problem 
     In Patent Literature 1, although the detection unit is initialized by the refresh process to correct the deterioration over time of the detection element, there is a case where odor components that are hardly desorbed at normal temperature even by the refresh process are adsorbed on the adsorption film of the detection element. In such a case, it is the simplest to replace the detection unit, but there is a problem that labor and cost are required. 
     In addition, refreshing by heat treatment requires several hours, and gas detection cannot be performed during refreshing, resulting in poor time efficiency. In addition, thermal decomposition or thermal degradation of the adsorption film may occur due to heat treatment, which makes it difficult to perform gas detection with stable detection sensitivity. 
     In view of the above circumstances, it is an object of the present invention to provide an arithmetic device, an arithmetic method, and a gas detection system that are capable of easily correcting deterioration over time of a detection element. 
     Solution to Problem 
     In order to achieve the above object, an arithmetic device according to an embodiment of the present invention includes a calculation unit. 
     The calculation unit calculates a correction coefficient on the basis of a resonant frequency change amount associated with a humidity change of a detection element in a degraded state and a resonant frequency change amount associated with a humidity change of the detection element in an initial state and acquired in advance, the detection element causing a resonant frequency change by adsorption of gas, and corrects the resonant frequency change amount of the detection element in the degraded state by using the correction coefficient. 
     According to such a configuration of the present invention, the ratio of adsorption sites capable of adsorption in the adsorption sites of the adsorption film excluding adsorption sites occupied by odor components that are not desorbed can be estimated on the basis of the resonant frequency change amount associated with the humidity change of the detection element in each of the initial state and the degraded state. The ratio of the adsorption sites capable of adsorption is calculated as a correction coefficient, and the resonant frequency change amount is corrected using the correction coefficient, so that it is possible to convert the resonant frequency amount into a resonant frequency amount of substantially the same output level as that in the initial state. As a result, it is possible to detect the gas at substantially the same sensitivity level as that in the initial state, and it is possible to easily correct the deterioration over time of the detection element. 
     The arithmetic device may further include a storage unit that stores the resonant frequency change amount associated with the humidity change of the detection element in the initial state. 
     The storage unit may store the correction coefficient. 
     The detection element may include an oscillator, and an adsorption film that is provided on the oscillator and adsorbs the gas. The storage unit may store a resonant frequency change amount associated with a humidity change of each of a plurality of detection elements in an initial state, the plurality of detection elements having different types of the adsorption film. 
     The arithmetic device may further include a determination unit that determines a lifetime of the detection element on the basis of a difference between a resonant frequency detected from the detection element in the degraded state at which a zero-point gas has arrived, and a resonant frequency detected from the detection element in the initial state at which the zero-point gas has arrived, and acquired in advance. 
     In order to achieve the above object, an arithmetic method according to an embodiment of the present invention includes: calculating a resonant frequency change amount associated with a humidity change of a detection element in a degraded state, the detection element causing a resonant frequency change by adsorption of gas; calculating a correction coefficient on the basis of the resonant frequency change amount associated with the humidity change of the detection element in the degraded state and a resonant frequency change amount associated with a humidity change of the detection element in an initial state and acquired in advance; and correcting the resonant frequency change amount of the detection element in the degraded state by using the correction coefficient. 
     The calculating the resonant frequency change amount associated with the humidity change of the detection element may be performed on the basis of resonant frequencies detected by the detection element at which a zero-point gas and a gas having a humidity different from a humidity of the zero-point gas have arrived. 
     The zero-point gas may be a dehumidified and deodorized gas. 
     A lifetime of the detection element may be determined on the basis of a difference between a resonant frequency detected from the detection element in the degraded state at which the zero-point gas has arrived, and a resonant frequency detected from the detection element in the initial state at which the zero-point gas has arrived, and acquired in advance. 
     In order to achieve the above object, a gas detection system according to an embodiment of the present invention includes a gas sensor and an arithmetic device. 
     The gas sensor includes a detection element that causes a resonant frequency change by adsorption of gas. 
     The arithmetic device includes a calculation unit that calculates a correction coefficient on the basis of a resonant frequency change amount associated with a humidity change of the detection element in a degraded state and a resonant frequency change amount associated with a humidity change of the detection element in an initial state and acquired in advance, and corrects the resonant frequency change amount of the detection element in the degraded state by using the correction coefficient. 
     The gas sensor may include a first flow path that includes dehumidification and deodorization means disposed therein and guides the detection target gas having passed through the dehumidification and deodorization means to the detection element, a second flow path that includes deodorization means disposed therein and guides the detection target gas having passed through the deodorization means to the detection element, and a third flow path directly guides the detection target gas to the detection element. 
     Advantageous Effects of Invention 
     As described above, according to the present invention, it is possible to easily correct deterioration over time of a detection element. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic diagram showing a configuration of a gas detection system according to a first embodiment of the present invention. 
         FIG. 2  is a front view of a QCM included in a gas sensor constituting a part of the gas detection system. 
         FIG. 3  is a diagram showing a resonant frequency change with respect to the humidity of each QCM constituting a part of the gas sensor. 
         FIG. 4  is a diagram showing a resonant frequency change with respect to the humidity of each different QCM constituting a part of the gas sensor. 
         FIG. 5  is for describing a gas detection method in each QCM in the gas detection system. 
         FIG. 6  is a flowchart for describing an arithmetic method relating to gas detection in an arithmetic device of the gas detection system according to the first embodiment. 
         FIG. 7  is a flowchart for describing an arithmetic method relating to QCM lifetime determination in the arithmetic device of the gas detection system according to the first embodiment. 
         FIG. 8  is for describing the QCM lifetime determination. 
         FIG. 9  is a schematic diagram showing a configuration of a gas detection system according to a second embodiment of the present invention. 
         FIG. 10  is a flowchart for describing an arithmetic method relating to gas detection in an arithmetic device of the gas detection system according to the second embodiment. 
     
    
    
     MODE(S) FOR CARRYING OUT THE INVENTION 
     Hereinafter, embodiments of the present invention will be described with reference to the drawings. 
     In the following description, a QCM and an adsorption film, which are not yet subjected to odor detection, will be referred to as a QCM in an initial state and an adsorption film in an initial state, respectively. A QCM and an adsorption film after deterioration over time, which are subjected to odor detection and are obtained after a certain period of time has elapsed, will be referred to as a QCM in a degraded state and an adsorption film in a degraded state, respectively, for the purpose of convenience. 
     First Embodiment 
       FIG. 1  is a schematic diagram of a gas detection system  1  according to a first embodiment. 
     As shown in  FIG. 1 , the gas detection system  1  includes a gas sensor  2 , an arithmetic device  4 , and a display device  5 . 
     As shown in  FIG. 1 , the gas sensor  2  includes an intake port  21 , four flow paths of a first flow path  24 , a second flow path  25 , a third flow path  26 , and a fourth flow path  27 , and a fifth flow path  28  that is a single flow path into which the gas having passed through the four flow paths  24  to  27  flows. 
     The intake port  21  takes in detection target gas  32  from the outside. 
     The gas taken in from the intake port  21  can be guided to at least one of the four flow paths  24  to  27 . 
     Each of the flow paths  24  to  27  is configured to guide the detection target gas  23  to a multi-array sensor  6  disposed on the fifth flow path  28 . 
     It is possible to introduce the gas into each of the flow paths  24  to  27  mechanically at a constant flow rate using a fan, a pump, or the like (not shown). This makes it possible to cancel a disturbance factor caused by a pressure change due to a difference in flow rate or the like. Further, a valve (not shown) is provided in each of the flow paths  24  to  27 , and the presence or absence of introduction of gas from the outside to each of the flow paths  24  to  27  can be controlled by opening or closing the valve. 
     As shown in  FIG. 1 , the gas sensor  2  includes a dehumidification and deodorization filter  32 , a deodorization filter  33 , a temperature and humidity sensor  30 , the multi-array sensor  6 , a first frequency counter circuit  31   a , a second frequency counter circuit  31   b , a third frequency counter circuit  31   c , a fourth frequency counter circuit  31   d , a fifth frequency counter circuit  31   e , and a sixth frequency counter circuit  31   f.    
     If it is not necessary to distinguish between the first frequency counter circuit  31   a , the second frequency counter circuit  31   b , the third frequency counter circuit  31   c , the fourth frequency counter circuit  31   d , the fifth frequency counter circuit  31   e , and the sixth frequency counter circuit  31   f  in particular, they may be referred to as frequency counter circuit  31  for description. 
     The dehumidification and deodorization filter  32  serving as dehumidification and deodorization means removes the humidity and odor of the intake gas. The dehumidification and deodorization filter  32  is disposed on the first flow path  24 . The gas passing through the first flow path  24  is dehumidified and deodorized by the dehumidification and deodorization filter  32 . In such a manner, the first flow path  24  functions as a flow path capable of constantly guiding the gas, which is clean at a constant level, to the multi-sensor array  6 . The clean gas that has passed through the first flow path  24  can be used as a zero-point gas of the sensor. 
     If the dehumidification and deodorization filter  32  is provided in such a manner, the detection target gas  23  can be used as a zero-point gas without preparing a standard gas. 
     For the dehumidification and deodorization filter  32 , a material having a high humidity-removing effect for removing humidity and hydrophilic odor components, such as silica gel or molecular sieve, and a material having a high odor-adsorbing effect for removing odor components, such as activated carbon or zeolite, can be used in combination. 
     The deodorization filter  33  serving as deodorization means removes the odor of the intake gas. The deodorization filter  33  is disposed on the second flow path  25 . The gas passing through the second flow path  25  is deodorized by the deodorization filter  33 . In such a manner, the second flow path  25  functions as a flow path capable of guiding the gas, from which the odor components have been removed, to the multi-sensor array  6 . 
     For the deodorization filter  33 , a material having a high odor-adsorbing effect, such as activated carbon or zeolite, can be used to remove odor components. 
     The dehumidification and deodorization filter, the deodorization filter, or the like is not disposed on the third flow path  26 , and the detection target gas  23  taken in from the outside is directly introduced into the third flow path  26 . The third flow path  26  guides the detection target gas  23  for odor monitoring to the multi-array sensor  6  disposed on the fifth flow path  28 . 
     The temperature and humidity sensor  30  is for detecting the temperature and humidity of the detection target gas  23 . The temperature and humidity sensor  30  is disposed on the fourth flow path  27 . The temperature and humidity information detected by the temperature and humidity sensor  30  is output to the arithmetic device  4 . 
     The arithmetic device  4  corrects, on the basis of the temperature detected by the temperature and humidity sensor  30 , the resonant frequency detected by each of QCMs  10   a  to  10   f  so as to cancel the resonant frequency change due to temperature. This makes it possible to detect the resonant frequency change without the effect of temperature. Note that in the following description the description of the correction using the temperature detected by the temperature and humidity sensor  30  will be omitted. 
     Further, the arithmetic device  4  corrects, on the basis of the humidity detected by the temperature and humidity sensor  30 , the resonant frequency detected by each of the QCMs  10   a  to  10   f . This will be described in detail later. 
     For example, a digital temperature and humidity sensor (model number: SHT21) manufactured by Sensirion AG can be used for the temperature and humidity sensor  30 . 
     The multi-array sensor  6  includes a plurality of QCMs  10  as detection elements. In this embodiment, an example in which six QCMs are provided is given, but at least one QCM may suffice. 
     The multi-array sensor  6  includes a first QCM sensor element (hereinafter, referred to as a first QCM)  10   a  as a first detection element, a second QCM sensor element (hereinafter, referred to as a second QCM)  10   b  as a second detection element, a third QCM sensor element (hereinafter, referred to as a third QCM)  10   c  as a third detection element, a fourth QCM sensor element (hereinafter, referred to as a fourth QCM)  10   d  as a fourth detection element, a fifth QCM sensor element (hereinafter, referred to as a fifth QCM)  10   e  as a fifth detection element, and a sixth QCM sensor element (hereinafter, referred to as a sixth QCM)  10   f  as a sixth detection element. 
     The first QCM  10   a , the second QCM  10   b , the third QCM  10   c , the fourth QCM  10   d , the fifth QCM  10   e , and the sixth QCM  10   f  each have a configuration including a crystal oscillator as an oscillator, and an adsorption film for adsorbing a specific gas, which is provided on the crystal oscillator. The first QCM  10   a , the second QCM  10   b , the third QCM  10   c , the fourth QCM  10   d , the fifth QCM  10   e , and the sixth QCM  10   f  have the same basic structure and are different only in the type of the adsorption film. Hereinafter, if it is not necessary to distinguish between the first QCM  10   a , the second QCM  10   b , the third QCM  10   c , the fourth QCM  10   d , the fifth QCM  10   e , and the sixth QCM  10   f , they may be referred to as QCM(s)  10 . 
     As shown in  FIG. 2 , the QCM  10  includes a crystal oscillator  13 , an electrodes  11 , an adsorption film  12 , a lead land  16 A, a lead land  16 B, a lead  14 A, a lead  14 B, a pin terminal  19 A, a pin terminal  19 B, and a holder  18 . The crystal oscillator  13  is an AT-cut quartz plate. 
     Since the resonant frequency of the crystal oscillator  13  of the QCM  10  decreases in proportion to the weight of the gas adsorbed on the adsorption film  12 , it is possible to calculate a resonant frequency change amount for each QCM, and detect, on the basis of such a calculation result, whether or not the detection target gas contains a gas to be detected on the adsorption film  12 . 
     In this embodiment, the crystal oscillator is used as the detection element for the resonant frequency, but the crystal oscillator is not limited thereto. For example, a ceramic oscillator, a surface acoustic wave element, a cantilever, a diaphragm, or the like may be used other than the crystal oscillator, and it can be applied as long as it can detect a physical change such as a weight increase or an expansion stress increase due to gas adsorption of the adsorption film and can convert the physical change into an electrical signal. 
     Hereinafter, the adsorption film provided to the first QCM  10   a  will be referred to as a first adsorption film  12   a , the adsorption film provided to the second QCM  10   b  will be referred to as a second adsorption film  12   b , the adsorption film provided to the third QCM  10   c  will be referred to as a third adsorption film  12   c , the adsorption film provided to the fourth QCM  10   d  will be referred to as a fourth adsorption film  12   d , the adsorption film provided to the fifth QCM  10   e  will be referred to as a fifth adsorption film  12   e , and the adsorption film provided to the sixth QCM  10   f  will be referred to as a sixth adsorption film  12   f.    
     The electrode  11  is formed on each of both surfaces of the crystal oscillator  13 , and the adsorption film  12  is formed on the electrode  11  formed on one surface of the crystal oscillator  13 . The lead land  16 A is formed integrally with the electrode  11  formed on one surface, and the lead land  16 B is formed integrally with the electrode  11  formed on the other surface. 
     The lead  14 A and the lead  14 B are made of a metallic spring material and are disposed parallel to each other. 
     One end of the lead  14 A is electrically connected to the electrode  11  formed on one surface through the lead land  16 A, and the other end thereof is connected to the pin terminal  19 A. One end of the lead  14 B is electrically connected to the electrode  11  formed on the other surface through the lead land  16 B, and the other end thereof is connected to the pin terminal  19 B. 
     The pin terminal  19 A and the pin terminal  19 B are supported by the holder  18  provided on the substrate, and the crystal oscillator  13  is supported vibratingly by the holder  18 . 
     The pin terminals  19 A and  19 B of the QCM  10  are connected to an oscillator circuit (not shown), and a drive voltage is applied to the QCM  10 . When a drive voltage is applied to the QCM  10 , the crystal oscillator  13  oscillates at a particular resonant frequency. 
     The adsorption film  12  adsorbs the gas and thus changes the mass, and the oscillation frequency of the crystal oscillator  13  decreases in accordance with the amount of adsorption. In such a manner, in the QCM  10 , the gas detection is performed with the weight change due to gas adsorption being as a resonant frequency change. In addition, the gas concentration can be quantified in accordance with the amount of change in frequency. 
     The first QCM  10   a , the second QCM  10   b , the third QCM  10   c , the fourth QCM  10   d , the fifth QCM  10   e , and the sixth QCM  10   f  are respectively connected to the first frequency counter circuit  31   a , the second frequency counter circuit  31   b , the third frequency counter circuit  31   c , the fourth frequency counter circuit  31   d , the fifth frequency counter circuit  31   e , and the sixth frequency counter circuit  31   f , each of which is a resonant frequency measurement unit. 
     The frequency counter circuit  31  measures a resonant frequency of the adsorption film  12  of the QCM  10 . The electrical signal of the resonant frequency measured by each of the frequency counter circuits  31   a  to  31   f  is output to the arithmetic device  4 . 
     Each of the first adsorption film  12   a , the second adsorption film  12   b , the third adsorption film  12   c , the fourth adsorption film  12   d , the fifth adsorption film  12   e , and the sixth adsorption film  12   f  is of a different type. 
     In this embodiment, the first adsorption film  12   a  is an adsorption film made of a fluorine-containing polymer material (product name: FS-2040 (manufactured by Fluoro Technology)) and selectively adsorbs a lipophilic gas. 
     The second adsorption film  12   b  is an adsorption film made of a fluorine-containing amphipathic polymer material (product name: FS-6130 (manufactured by Fluoro Technology)) and selectively adsorbs a hydrophilic gas and a lipophilic gas. 
     The third adsorption film  12   c  is an adsorption film made of an oily hydrocarbon material Squalene (manufactured by Fujifilm Wako Pure Chemical Corporation) and selectively adsorbs a hydrophilic gas and a lipophilic gas. 
     The fourth adsorption film  12   d  is an adsorption film made of Cellulose Acetate Propionate (CAP-482, manufactured by Eastman) and selectively adsorbs a hydrophilic gas. 
     The fifth adsorption film  12   e  is an adsorption film made of Poly(3, 4-ethylenedioxythiophene) (product name: PEDOT687316, manufactured by Ardrich) and selectively adsorbs a hydrophilic gas. 
     The sixth adsorption film  12   f  is an adsorption film made of cellulose, Cellulose Acetate Butyrate (CAB-553, manufactured by Eastman) and selectively adsorbs a hydrophilic gas. 
     (A), (B), and (C) of  FIG. 3  are diagrams for describing resonant frequency changes associated with the humidity change of the gas detected by the first QCM  10   a , the second QCM  10   b , and the third QCM  10   c , respectively. 
     (A), (B), and (C) of  FIG. 4  are diagrams for describing resonant frequency changes associated with the humidity change of the gas detected by the fourth QCM  10   d , the fifth QCM  10   e , and the sixth QCM  10   f , respectively. 
     All the diagrams show an approximate line of the measured resonant frequency plotted with a changed humidity of the gas reaching each QCM. 
     As shown in  FIGS. 3 and 4 , since the types of the adsorption films  12  are different from each other, the resonance frequencies are different from each other. In any of the QCMs  10   a  to  10   f , however, the resonance frequencies substantially linearly vary with the humidity change of the gas. 
     The arithmetic device  4  includes an acquisition unit  41 , a calculation unit  42 , a determination unit  43 , an output unit  44 , a storage unit  45 , and a recognition unit  46 . 
     The storage unit  45  stores in advance the resonant frequency detected by each of the QCMs  10   a  to  10   f  in the initial state at which the zero-point gas has arrived, and the resonant frequency change amount associated with the humidity change. 
     The resonant frequency detected by the QCM  10  in the initial state is a resonant frequency detected when a clean gas (zero-point gas), which is obtained after outside gas passes through the first flow path  24  to be dehumidified and deodorized, arrives at the QCM  10  in the initial state having no film deterioration. Such a resonant frequency is acquired in advance for each of the QCMs  10   a  to  10   f  and stored. 
     The resonant frequency change amount associated with the humidity change is a resonant frequency change amount in the QCM  10  with respect to the change of a relative humidity of 1%. Such a resonant frequency change amount is calculated in advance on the basis of: a resonant frequency detected when the zero-point gas, which is obtained after outside gas passes through the first flow path  24 , arrives at the QCM  10  in the initial state; a resonant frequency detected when the deodorized gas, which is obtained after outside gas passes through the second flow path  25 , arrives at the QCM  10  in the initial state; and the humidity information detected by the temperature and humidity sensor  30  when outside gas passes through the fourth flow path  27 . Such a resonant frequency change amount is calculated in advance for each of the QCMs  10   a  to  10   f  and stored. 
     The storage unit  45  stores in time series a correction coefficient Rc calculated by the calculation unit  42 , which will be described later, in association with the calculated date and time information. The calculation of the correction coefficient Rc may be performed periodically or may be performed at any timing by the user. 
     Further, the storage unit  45  stores in advance reference detection patterns when various gases are detected by the multi-array sensor  6  for each different type of gas. 
     The recognition unit  46  compares a detection pattern corrected using the correction coefficient by the calculation unit  42 , which will be described later, with the reference detection pattern stored in advance in the storage unit  45 , and recognizes the presence or absence of gas and the type of gas by pattern recognition of machine learning. 
     The acquisition unit  41  acquires the resonant frequency of each of the QCMs  10   a  to  10   f  detected by each of the frequency counter circuits  31   a  to  31   f , and the temperature and humidity information detected by the temperature and humidity sensor  30 . 
     More specifically, the acquisition unit  41  acquires the resonant frequency detected by each of the QCMs  10   a  to  10   f  in the degraded state at which a so-called zero-point gas has arrived, the zero-point gas being obtained after the detection target gas  23  passes through the first flow path  24  to be dehumidified and deodorized. 
     The acquisition unit  41  acquires the resonant frequency detected by each of the QCMs  10   a  to  10   f  in the degraded state at which the deodorized gas has arrived, the deodorized gas being obtained after the detection target gas  23  passes through the second flow path  25 . 
     The acquisition unit  41  acquires the resonant frequency detected by each of the QCMs  0   a  to  10   f  in the degraded state at which the detection target gas  23  having passed through the third flow path  26  has arrived. 
     The acquisition unit  41  acquires the temperature and humidity information detected by the temperature and humidity sensor  30  on the fourth flow path  27 . 
     When calculating the correction coefficient Rc to be described later, the calculation unit  42  calculates the resonant frequency change amount of each of the QCMs  10   a  to  10   f  with respect to the change of a relative humidity of 1% on the basis of the resonant frequencies detected by the respective QCMs  10   a  to  10   f  at which the gas having passed through the first flow path  24  and the second flow path  25  has arrived, and the humidity information detected by the temperature and humidity sensor  30 . 
     When calculating the correction coefficient Rc, the calculation unit  42  acquires the resonant frequency change amount of each of the QCMs  10   a  to  10   f  in the initial state with respect to the humidity change, which is stored in the storage unit  45 . 
     The calculation unit  42  calculates, using the following equation, the ratio Rc between a resonant frequency change amount ΔF(a) of each of the QCMs  10   a  to  10   f  in the degraded state with respect to the humidity change and a resonant frequency change amount ΔF(0) of each of the QCMs  10   a  to  10   f  in the initial state with respect to the humidity change, which is stored in the storage unit  45 . The ratio Rc is used as a correction coefficient. 
         Rc=ΔF (0)/Δ F ( a )
 
     where ΔF(a) is a resonant frequency change (Hz) with respect to the change of a relative humidity of 1% in the adsorption film in the degraded state, and ΔF(0) is a resonant frequency change (Hz) with respect to the change of a relative humidity of 1% in the adsorption film in the initial state. 
     The calculation of the correction coefficient Rc is performed for each of the QCMs  10   a  to  10   f . The calculated correction coefficient Rc is stored in the storage unit  45 . 
     When not calculating the correction coefficient Rc, the calculation unit  42  acquires the latest correction coefficient Rc stored in the storage unit  45 . 
     The calculation unit  42  calculates the resonant frequency change amount on the basis of the resonant frequency detected by each of the QCMs  0   a  to  10   f  in the degraded state at which the detection target gas  23  having passed through the third flow path  26  has arrived. 
     Furthermore, the calculation unit  42  corrects, using the correction coefficient Rc, the resonant frequency change amount of each of the QCMs  0   a  to  10   f  in the degraded state at which the detection target gas  23  having passed through the third flow path  26  has arrived. 
     More specifically, the calculation unit  42  corrects the resonant frequency change amount by multiplying the value of the resonant frequency change amount of each of the QCMs  0   a  to  10   f  in the degraded state at which the detection target gas  23  having passed through the third flow path  26  has arrived, by the correction coefficient Rc. Thus, it is possible to correct the resonant frequency change amount to be an output value at substantially the same sensor sensitivity level as that in the initial state. A specific example will be described later with reference to  FIG. 5 . 
     As described above, the detection results in the respective QCMs  10   a  to  10   f , at which the gas having passed through the first flow path  24  and the second flow path  25  has arrived, are used for calculating the correction coefficient Rc. 
     The calculation of the correction coefficient Rc and the correction of the resonant frequency change amount using the correction coefficient Rc can be the step of calibrating the output value of the QCM in the degraded state to be an output value at substantially the same sensitivity level as that in the initial state. 
     The calculation of the correction coefficient Rc in the calibration step does not have to be performed for each gas detection in the gas sensor  2 , and may be configured to be performed automatically every predetermined period, for example, every month, or performed by a user at any timing. 
     Subsequently, the correction coefficient Rc in each of the QCMs  10   a  to  10   f  calculated in the calibration step is stored in the storage unit  45 , and the correction of the resonant frequency change amount is performed using the correction coefficient Rc most recently calculated and stored in the storage unit  45  until the next correction coefficient Rc is calculated. 
     Thus, in the gas detection by the gas detection system  1  in a period in which the calculation of the correction coefficient Rc is not performed, it is not necessary to detect the gas having passed through the first flow path  24  and the second flow path  25 . As described above, the first flow path  24  and the second flow path  25  can be used for the calibration step. 
     The determination unit  43  determines the lifetime of the QCM  10  on the basis of the difference between the resonant frequency detected by the QCM  10  in the degraded state at which the zero-point gas having passed through the first flow path  24  has arrived, and the resonant frequency detected by the QCM  10  in the initial state at which the zero-point gas having passed through the first flow path  24  has arrived, which is acquired in advance and stored in the storage unit  45 . 
     When the adsorption film  12  reaches the end of its lifetime due to deterioration over time, the QCM  10  hardly has stable oscillation characteristics. In such a state, even if the calculation of the resonant frequency change amount using the correction coefficient is performed, it is difficult to sufficiently correct the resonant frequency change amount. 
     In this embodiment, when the difference between the resonant frequency detected by the QCM  10  in the degraded state at which the zero-point gas having passed through the first flow path  24  has arrived, and the resonant frequency detected by the QCM  10  in the initial state at which the zero-point gas having passed through the first flow path  24  has arrived, which is acquired in advance and stored in the storage unit  45 , exceeds a threshold value, it is possible to determine that the QCM  10  has reached the end of its lifetime. The threshold value used for the lifetime determination varies depending on the resonant frequency band of the QCM and is individually set according to the type of the adsorption film  12 . 
     Here, the difference between the resonant frequency detected by the QCM  10  in the degraded state at which the zero-point gas has arrived and the resonant frequency detected by the QCM  10  in the initial state at which the zero-point gas has arrived is a frequency accumulated due to deterioration over time. From such a difference in resonant frequency, a negative resistance of the QCM  10  in the degraded state can be estimated. If the negative resistance of the oscillator circuit is sufficiently larger than an equivalent series resonant resistance of the crystal oscillator, stable oscillation characteristics can be obtained, otherwise stable oscillation characteristics cannot be obtained. 
       FIG. 8  shows, as an example, the relationship between the negative resistance and the resonant frequency of the QCMs including two different types of absorption films and having a resonant frequency of 32 MHz. Circle plots and diamond plots show the QCMs respectively including different types of absorption films. 
     As shown in  FIG. 8 , the negative resistance varies linearly with the change in resonant frequency. In the example shown in  FIG. 8 , when the negative resistance of the QCM exceeds 500Ω, the QCM fails to stably oscillate. Assuming that the end of the lifetime comes when the negative resistance exceeds 500Ω, if the difference between the resonant frequencies in the degraded state and in the initial state is larger than 200,000 Hz, which is the threshold value, the QCM is determined to have reached the end of the lifetime. 
     In such a manner, in the QCM associated with resonant vibration, the resonant frequency at which oscillation cannot be stably performed can be estimated from the negative resistance, and the lifetime of the QCM can be determined from the estimated negative resistance. Thus, if it is determined that the end of the lifetime has come, it is possible to promptly replace the QCM or the gas sensor, maintain substantially the same level as the sensitivity level in the initial state from the start of use of the QCM or the gas sensor to the replacement thereof, and stably perform the gas detection. 
     The output unit  44  outputs, to the display device  5 , the resonant frequency change amounts of the respective QCMs  10   a  to  10   f  calculated by the calculation unit  42  using the correction coefficients, the discrimination result of the detected gas recognized by the recognition unit  46 , the quantitative analysis result of the detected gas, and the life determination result determined by the determination unit  43 . 
     The display device  5  includes a display unit and displays, on the display unit, the corrected resonant frequency change amounts of the respective QCMs  10   a  to  10   f  output from the arithmetic device  4 , the discrimination result of the detected gas, the gas concentration, the lifetime determination result, etc. The user can understand the type and concentration of the detected gas and the replacement timing of the QCM or the gas sensor by checking the display unit. 
     Next, the arithmetic method relating to the calculation of the correction coefficient performed by the arithmetic device  4  and the calculation of the resonant frequency change amount using the correction coefficient will be described with reference to the flowchart of  FIG. 6 . 
     As shown in  FIG. 6 , the acquisition unit  41  acquires the electric signal of the resonant frequency detected by each of the QCMs  10   a  to  10   f  in the degraded state at which the gas having passed through the first flow path  24  has arrived, which is output from each of the frequency counter circuits  31   a  to  31   f  (Sa). 
     Next, the acquisition unit  41  acquires the electric signal of the resonant frequency detected by each of the QCMs  10   a  to  10   f  in the degraded state at which the gas having passed through the second flow path  25  has arrived, which is output from each of the frequency counter circuits  31   a  to  31   f  (St 2 ). 
     Next, the acquisition unit  41  acquires humidity information detected by the temperature and humidity sensor  30  (St 3 ). 
     Next, the calculation unit  42  calculates the resonant frequency change amount ΔF(a) of each of the QCMs  10   a  to  10   f  in the degraded state with respect to the change of a relative humidity of 1% by using the resonant frequency serving as a detection result of the gas passing through the first flow path  24 , which is obtained in St 1 , the resonant frequency serving as a detection result of the gas passing through the second flow path  25 , which is acquired in St 2 , and the humidity information detected by the temperature and humidity sensor  30  (St 4 ). 
     The gas having passed through the first flow path  24  and the gas having passed through the second flow path  25  are both the deodorized gases from which odor components are removed. The gas having passed through the first flow path  24  is a dehumidified gas from which moisture is removed, and the gas having passed through the second flow path  25  is a gas that is not dehumidified, and both of them are different in humidity. Therefore, it is possible to calculate the resonant frequency change amount of each of the QCMs  10   a  to  10   f  in the degraded state with respect to the change of a relative humidity of 1% on the basis of the resonant frequency obtained in St 1 , the resonant frequency obtained in St 2 , and the humidity information obtained in St 3 . 
     Next, the calculation unit  42  acquires the resonant frequency change amount ΔF(0) of each of the QCMs  10   a  to  10   f  in the initial state with respect to the humidity change, which is stored in advance in the storage unit  45  (St 5 ). 
     Next, the ratio Rc between the resonant frequency change amount ΔF(a) calculated in St 4  and the resonant frequency change amount ΔF(0) acquired in St 5  is calculated as a correction coefficient (St 6 ). The correction coefficient Rc is calculated for each of the QCMs  10   a  to  10   f.    
     Next, the acquisition unit  41  acquires the resonant frequency detected by each of the QCMs  10   a  to  10   f  in the degraded state at which the gas having passed through the third flow path  26  has arrived, which is output from each of the frequency counter circuits  31   a  to  31   f  (St 7 ). 
     Next, the resonant frequency change amount of each of the QCMs  10   a  to  10   f  in the degraded state is calculated using the resonant frequency acquired in St 7 , and the above resonant frequency change amount is multiplied by the correction coefficient Rc calculated in St 6 , so that the resonant frequency change amount is corrected (St 8 ). 
     The resonant frequency change amount calculated and corrected using the correction coefficient Rc is substantially the same as the value detected at the same sensitivity level as in the gas sensor  2  in the initial state. 
     Hereinafter, the fact that the resonant frequency change amount calculated using the correction coefficient Rc substantially coincides with the resonant frequency change amount when detected by the gas sensor  2  in the initial state will be described with reference to  FIG. 5 . 
     In  FIG. 5 , the correction coefficient Rc is obtained by the ratio between the resonant frequency change amount ΔF(0) associated with the humidity fluctuation in the initial state and the resonant frequency change amount ΔF(a) associated with the humidity fluctuation after deterioration over time (degraded state). 
     The resonant frequency change amount of ethanol (100 ppm) in the initial state is a resonant frequency change amount of the QCM  10  in the initial state when ethanol is detected. The resonant frequency change amount of ethanol (100 ppm) after deterioration over time (degraded state) is a resonant frequency change amount of the QCM  10  in the degraded state when ethanol is detected. 
     The resonant frequency change amount after correction is a value obtained by multiplying the resonant frequency change amount of ethanol (100 ppm) after deterioration over time (degraded state) by the correction coefficient Rc. 
     The output coincidence rate indicates the coincidence rate of the corrected resonant frequency change amount to the resonant frequency change amount of the QCM  10  in the initial state when ethanol is detected. 
     Here, in the adsorption film  12  of the QCM  10  in the degraded state, some of the adsorption sites are occupied by the odor components that are not desorbed, and the adsorption is allowed only on the empty adsorption sites. For this reason, the resonant frequency change amount due to adsorption in the QCM  10  in the degraded state is smaller than that in the QCM  10  in the initial state. 
     The resonant frequency change associated with the humidity change in each QCM is substantially linear as shown in  FIGS. 3 and 4  described above. It is possible to obtain the resonant frequency change amount of the QCM with respect to the change of a relative humidity of 1% from the resonant frequencies detected by the QCMs at which at least two gases having different humidities have arrived. 
     Therefore, the ratio of adsorption sites capable of adsorption on the surface of the adsorption film can be estimated from the ratio between the resonant frequency change amount of the QCM in the initial state with respect to the change of a relative humidity of 1% and the resonant frequency change amount of the QCM in the degraded state with respect to the change of a relative humidity of 1%. Subsequently, using such a ratio, the resonant frequency change amount detected by the QCM in the degraded state is corrected to perform calibration, so that it is possible to convert it into the resonant frequency change amount detected at substantially the same sensitivity level as that of the QCM in the initial state. 
     As shown in  FIG. 5 , in the first QCM  10   a , a value (resonant frequency change amount after correction) obtained by correcting the resonant frequency change amount detected by the first QCM  10   a  in the degraded state, 9 Hz, using the correction coefficient Rc is 22 Hz. This value is substantially the same as the resonant frequency change amount of ethanol by the first QCM  10   a  in the initial state, 20 Hz. The coincidence rate of the output values is 108%. 
     Similarly, as shown in  FIG. 5 , in the second QCM  10   b , a value (resonant frequency change amount after correction) obtained by correcting the resonant frequency change amount detected by the second QCM  10   b  in the degraded state, 55 Hz, using the correction coefficient Rc is 72 Hz. This value is substantially the same as the resonant frequency change amount of ethanol by the second QCM  10   b  in the initial state, 70 Hz. The coincidence rate of the output values is 103%. 
     In the third QCM  10   c , a value (resonant frequency change amount after correction) obtained by correcting the resonant frequency change amount detected by the third QCM  10   c in the degraded state,  95 Hz, using the correction coefficient Rc is 191 Hz. This value is substantially the same as the resonant frequency change amount of ethanol by the third QCM  10   c  in the initial state, 200 Hz. The coincidence rate of the output values is 95%. 
     In the fourth QCM  10   d , a value (resonant frequency change amount after correction) obtained by correcting the resonant frequency change amount detected by the fourth QCM  10   d  in the degraded state, 145 Hz, using the correction coefficient Rc is 202 Hz. This value is substantially the same as the resonant frequency change amount of ethanol by the fourth QCM  10   d  in the initial state, 210 Hz. The coincidence rate of the output values is 96%. 
     In the fifth QCM  10   e , a value (resonant frequency change amount after correction) obtained by correcting the resonant frequency change amount detected by the fifth QCM  10   e  in the degraded state, 176 Hz, using the correction coefficient Rc is 363 Hz. This value is substantially the same as the resonant frequency change amount of ethanol by the fifth QCM  10   e  in the initial state, 350 Hz. The coincidence rate of the output values is 104%. 
     In the sixth QCM  10   f , a value (resonant frequency change amount after correction) obtained by correcting the resonant frequency change amount detected by the sixth QCM  10   f  in the degraded state, 64 Hz, using the correction coefficient Rc is 118 Hz. This value is substantially the same as the resonant frequency change amount of ethanol by the sixth QCM  10   f  in the initial state, 120 Hz. The coincidence rate of the output values is 99%. 
     In such a manner, the resonant frequency change amount corrected with the correction coefficient Rc is a value detected at substantially the same sensitivity level as that of the QCM  10  in the initial state. 
     Further, as described above, when the absorbent film  12  reaches the end of its lifetime due to deterioration over time, the QCM  10  fails to stably oscillate. Such a determination on the end of lifetime that the QCM  10  fails to stably oscillate can be performed as described below, and it is possible to predict the lifetime of the gas sensor. Hereinafter, the description will be given with reference to  FIG. 7 . 
       FIG. 7  is a flowchart for describing an arithmetic method relating to the QCM lifetime determination. 
     As shown in  FIG. 7 , the acquisition unit  41  acquires the resonant frequency of the QCM  10  in the degraded state at which the zero-point gas has arrived, which is detected by the frequency counter circuit  31  (Sal). 
     Next, the determination unit  43  acquires the resonant frequency of the QCM  10  in the initial state at which the zero-point gas has arrived, which is stored in the storage unit  46  in advance (St 12 ). 
     Next, the determination unit  43  determines the lifetime of the QCM  11  on the basis of the difference between the resonant frequency of the QCM  10  in the degraded state acquired in St 11  and the resonant frequency of the QCM  10  in the initial state acquired in St 12  (St 13 ). Specifically, when the calculated difference is larger than a threshold value, it is determined that the QCM  10  has reached the end of the lifetime. When the difference is equal to or smaller than the threshold value, it is determined that the QCM  10  has not yet reached the end of the lifetime. 
     As described above, in this embodiment, the amount of adsorption sites capable of adsorption on the surface of the adsorption film can be estimated from the ratio between the resonant frequency change amount of the QCM in the initial state with respect to the change of a relative humidity of 1% and the resonant frequency change amount of the QCM in the degraded state with respect to the change of a relative humidity of 1%. Using such a ratio, it is possible to perform calibration of the output value of the gas sensor  2  by correcting the resonant frequency change amount detected by the QCM in the degraded state. Thus, it is possible to convert the resonant frequency change amount detected by the QCM in the degraded state into a resonant frequency change amount detected at substantially the same sensitivity level as that of the QCM in the initial state. This makes it possible to easily correct deterioration over time of the detection element and to perform detection at a stable sensitivity level. 
     Furthermore, it is possible to estimate the lifetime of the sensor, which cannot be corrected by the output correction using the correction coefficient Rc, from the degree of deterioration due to deterioration over time. Therefore, if it is determined that the end of the lifetime has come, it is possible to promptly replace the QCM or the gas sensor, maintain substantially the same level as the sensitivity level in the initial state from the start of use of the QCM or the gas sensor to the replacement thereof, and stably perform the gas detection. 
     Second Embodiment 
     In the first embodiment, an example has been described in which the first flow path including the dehumidification and deodorization filter and the second flow path including the deodorization filter are provided to obtain a correction coefficient for calibrating the output value of the gas sensor  2 , but the present invention is not limited thereto. The correction coefficient may be obtained by using at least two kinds of standard gases having different humidities without providing a dehumidification and deodorization filter or a deodorization filter. Hereinafter, description will be given with reference to  FIG. 9 . In the following description, the same constituent elements as those described above will be denoted by the same reference symbols, and description thereof will be omitted. 
       FIG. 9  is a schematic diagram of a gas detection system  101  according to a second embodiment. 
     As shown in  FIG. 9 , the gas detection system  101  includes a gas sensor  102 , an arithmetic device  4 , and a display device  5 . 
     As shown in  FIG. 9 , the gas sensor  102  includes an intake port  21 , a monitoring flow path  126 , a temperature and humidity detection flow path  127 , and a flow path  128  for a multi-array sensor that is a single flow path into which gas passing through the two flow paths  126  and  127  flows. 
     The intake port  21  takes in the detection target gas  32  from the outside. 
     The gas taken in from the intake port  21  can be guided to the two flow paths  126  and  127 . 
     Each of the flow paths  126  and  127  is configured such that the detection target gas  23  can be guided to the multi-array sensor  6  disposed on the flow path  128 . 
     It is possible to introduce the gas into each of the flow paths  126  and  127  mechanically at a constant flow rate using a fan, a pump, or the like (not shown). This makes it possible to cancel a disturbance factor caused by a pressure change due to a difference in flow rate or the like. Further, a valve (not shown) is provided in each flow path, and the presence or absence of introduction of gas from the outside to each flow path can be controlled by opening or closing the valve. 
     As shown in  FIG. 9 , the gas sensor  2  includes a dehumidification and deodorization filter  32 , a deodorization filter  33 , a temperature and humidity sensor  30 , the multi-array sensor  6 , a first frequency counter circuit  31   a , a second frequency counter circuit  31   b , a third frequency counter circuit  31   c , a fourth frequency counter circuit  31   d , a fifth frequency counter circuit  31   e , and a sixth frequency counter circuit  31   f.    
     In the gas detection system  101 , a correction coefficient Rc for calibrating the output value of the gas sensor  102  can be obtained by using a first standard gas and a second standard gas having no odor components and having different humidities. The first standard gas is a zero-point gas. The first and second standard gases do not contain odor components. 
     The storage unit  45  of the arithmetic device  4  stores in advance a resonant frequency detected by each of QCMs  10   a  to  10   f  in the initial state, and a resonant frequency change amount associated with the change of a relative humidity of 1%. 
     The resonant frequency detected by each of the QCMs  10   a  to  10   f  in the initial state is a resonant frequency detected when the first standard gas passes through the monitoring flow path  126  and arrives at each of the QCMs  10   a  to  10   f  in the initial state, and is stored in advance in the storage unit  45 . 
     The resonant frequency change amount associated with the humidity change is a resonant frequency change amount in each QCM  10  with respect to the change of a relative humidity of 1%. Such a resonant frequency change amount is calculated in advance on the basis of: a resonant frequency detected when the first standard gas having passed through the monitoring flow path  126  arrives at each of the QCMs  10   a  to  10   f  in the initial state; a resonant frequency detected when the second standard gas having passing through the monitoring flow path  126  arrives at each of the QCMs  10   a  to  10   f  in the initial state; and the humidity information detected by the temperature and humidity sensor  30  when the first and second standard gases pass through the temperature and humidity detection flow path  127 . Note that if the humidities of the first and second standard gases are known, the humidity information of the temperature and humidity sensor  30  may not be used. 
     The storage unit  45  stores in time series a correction coefficient Rc calculated by the calculation unit  42 , which will be described later. 
     Further, the storage unit  45  stores in advance reference detection patterns when various gases are detected by the multi-array sensor  6  for each different type of gas. 
     The configuration of the recognition unit  46  is the same as that of the first embodiment. 
     The acquisition unit  41  acquires the electric signal of the resonant frequency of each of the QCMs  10   a  to  10   f , which is output from each of the frequency counter circuits  31   a  to  31   f , and acquires the temperature and humidity information detected by the temperature and humidity sensor  30 . 
     More particularly, the acquisition unit  41  acquires the resonant frequency detected by each of the QCMs  10   a  to  10   f  in the degraded state at which each of the first standard gas and the second standard gas has arrived. 
     The acquisition unit  41  acquires the resonant frequency detected by each of the QCMs  10   a  to  10   f  in the degraded state at which the detection target gas  23  having passed through the monitor flow path  26  has arrived. 
     The acquisition unit  41  acquires the temperature and humidity information detected by the temperature and humidity sensor  30  on the temperature and humidity detection flow path  127 . 
     The calculation unit  42  calculates a resonant frequency change amount ΔF(a) of each QCM  10  in the degraded state with respect to the change of a relative humidity of 1% on the basis of the resonant frequencies detected when each of the first standard gas and the second standard gas having passed through the monitoring flow path  126  arrives at the QCMs  10   a  to  10   f  in the degraded state, and the humidity information detected by the temperature and humidity sensor  30  when the first and second standard gases pass through the temperature and humidity detection flow path  127 . Note that if the humidities of the first and second standard gases are known, the humidity information by the temperature and humidity sensor  30  may not be used. 
     The calculation unit  42  acquires a resonant frequency change amount ΔF(0) of each of the QCMs  10   a  to  10   f  in the initial state with respect to the humidity change, which is stored in advance in the storage unit  45 . 
     The calculation unit  42  calculates, using the following equation, the ratio Rc between the resonant frequency change amount ΔF(a) of each of the QCMs  10   a  to  10   f  in the degraded state with respect to the humidity change and the resonant frequency change amount ΔF(0) of each of the QCMs  10   a  to  10   f  in the initial state with respect to the humidity change, which is stored in the storage unit  45 . The ratio Rc is used as a correction coefficient. 
         Rc=ΔF (0)/Δ F ( a )
 
     where ΔF(a) is a resonant frequency change (Hz) with respect to the change of a relative humidity of 1% in the QCM in the degraded state, and ΔF(0) is a resonant frequency change (Hz) with respect to the change of a relative humidity of 1% in the QCM in the initial state. 
     The calculation of the correction coefficient is performed for each of the QCMs  10   a  to  10   f.    
     The calculation unit  42  calculates the resonant frequency change amount on the basis of the resonant frequency detected by each of the QCMs  10   a  to  10   f  in the degraded state at which the detection target gas  23  having passed through the monitoring flow path  126  has arrived. 
     Furthermore, the calculation unit  42  corrects, using the correction coefficient Rc, the resonant frequency change amount of each of the QCMs  0   a  to  10   f  in the degraded state at which the detection target gas  23  having passed through the monitoring flow path  126  has arrived, and calculates the resonant frequency change amount converted into the sensitivity level in the initial state. 
     More specifically, the calculation unit  42  calculates the resonant frequency change amount by multiplying the value of the resonant frequency change amount of each of the QCMs  0   a  to  10   f  in the degraded state at which the detection target gas  23  having passed through the monitoring flow path  126  has arrived, by the correction coefficient Rc. Thus, it is possible to correct the resonant frequency change amount to be an output value at substantially the same sensitivity level as that in the initial state. 
     Next, the arithmetic method for the resonant frequency change amount associated with the humidity change, which is used to calculate the correction coefficient by the arithmetic device  4 , will be described with reference to the flowchart of  FIG. 10 . Such a calculation of the resonant frequency change amount is the same in the initial state and the degraded state. 
     As shown in  FIG. 10 , the acquisition unit  41  acquires the electric signal of the resonant frequency detected by each of the QCMs  10   a  to  10   f  at which the first standard gas having passed through the monitoring flow path  126  has arrived, which is output from each of the frequency counter circuits  31   a  to  31   f  (St 21 ). 
     Next, the acquisition unit  41  acquires the electric signal of the resonant frequency detected by each of the QCMs  10   a  to  10   f  at which the second standard gas having passed through the monitoring flow path  126  has arrived, which is output from each of the frequency counter circuits  31   a  to  31   f  (St 22 ). 
     Next, the acquisition unit  41  acquires the humidity information of each of the first and second standard gases detected by the temperature and humidity sensor  30  (St 23 ). 
     Next, the calculation unit  42  calculates the resonant frequency change amount of each of the QCMs  10   a  to  10   f  with respect to the change of a relative humidity of 1% by using the resonant frequency serving as a detection result of the first standard gas passing through the monitoring flow path  126 , which is acquired in St 21 , the resonant frequency serving as a detection result of the second standard gas passing through the monitoring flow path  126 , which is acquired in St 22 , and the humidity information detected by the temperature and humidity sensor  30 , which is acquired in St 23  (St 24 ). 
     In such a manner, the resonant frequency change amount of each of the QCMs  10   a  to  10   f  with respect to the change of a relative humidity of 1% for determining the correction coefficient Rc is calculated using the standard gases. The calculation of the correction coefficient, and the correction of the output level using the correction coefficient are similar to those in the first embodiment, and thus description thereof will be omitted here. 
     The calibration may be performed using the standard gases as in this embodiment. The detection of the gas can be performed at a stable sensitivity level as in the first embodiment. 
     Note that in the first embodiment the first flow path including the dehumidification and deodorization filter and the second flow path including the deodorization filter are provided, and thus it is unnecessary to prepare a standard gas in calculating the correction coefficient Rc. 
     In each of the above embodiments, the calibration is performed periodically or at any timing, and thus deterioration over time of the detection element is easily corrected, the sensitivity is constantly maintained at a constant level, and quantitative monitoring becomes possible. 
     Further, the sensor lifetime is estimated on the basis on the difference in resonant frequency between the QCM in the initial state and the QCM in the degraded state. Thus, the sensitivity is constantly maintained at a constant level, and quantitative monitoring becomes possible. 
     Further, since the gas sensor is constantly maintained at a constant level of sensitivity, it is possible to maintain the recognition accuracy of the presence or absence or the type of gas at a constant level. In other words, since the recognition unit  46  compares a detection pattern, which is corrected by the calculation unit  42  using the correction coefficient Rc, with the reference detection pattern, and determines the presence or absence or the type of gas by pattern recognition, it is possible to maintain the recognition accuracy of the presence or absence or the type of gas at a constant level. In such a manner, it is possible to suppress the deterioration of the sensitivity level of the QCM due to the deterioration over time from affecting the recognition accuracy in the pattern recognition. 
     Further, in each of the above embodiments, the process of calculating the resonant frequency change amount of the QCM in the degraded state with respect to the humidity change, and obtaining the correction coefficient Rc from the ratio between such a resonant frequency change amount and the resonant frequency change amount of the QCM in the initial state with respect to the humidity change can be performed in a few minutes. 
     Therefore, in this embodiment, the calibration can be performed in a much shorter time and the time efficiency is improved as compared with the case where the calibration is performed after performing refreshing by heat treatment required for several hours. 
     Note that the QCM may be refreshed by deodorization or heating of the flow path in which the QCM is disposed. Even in such a case, it is possible to detect gas at substantially the same sensitivity level as that in the initial state by obtaining the correction coefficient Rc and correcting the resonant frequency change as in this embodiment, even if odor components that cannot be removed by the refresh process remain on the adsorption film. 
     Of course the present invention is not limited to the above embodiments only and can be variously modified. 
     REFERENCE SIGNS LIST 
     
         
           1 ,  101  gas detection system 
           2 ,  102  gas sensor 
           4  arithmetic device 
           10   a  first QCM (detection element) 
           10   b  second QCM (detection element) 
           10   c  third QCM (detection element) 
           10   d  fourth QCM (detection element) 
           10   e  fifth QCM (detection element) 
           10   f  sixth QCM (detection element) 
           12  adsorption film 
           13  oscillator 
           23  detection target gas 
           24  first flow path 
           25  second flow path 
           26  third flow path 
           32  dehumidification and deodorization filter (dehumidification and deodorization means) 
           33  deodorization filter (deodorization means) 
           42  calculation unit 
           43  determination unit 
           44  storage unit