Patent Description:
In oil and gas industry, trace moisture sensors in natural gas are used, but variation of composition in background gases degrades the precision of the sensors. The composition variation takes place when the gas production condition is changed. Also, the composition of background gases is an important parameter for evaluating the heat value. Thus, the composition of background gases is usually measured by gas chromatography (GC), but GC is large and expensive apparatus.

As recited in non patent literature (NPL) <NUM>, a ball SAW sensor has been developed and applied to a trace moisture sensor. In the ball SAW sensor, the SAW excited on a spherical surface with a specific condition may be naturally collimated, and multiple roundtrips along the equator of the ball can be realized. Thus, the ball sensor based on the multiple-roundtrips effect of the SAW may provide high performance, such as high sensitivity and wide sensing range. Further, in NPL <NUM>, SAW attenuation due to propagation at the boundary between a solid and a monatomic gas is described. Further, in NPL <NUM>, frequency dependence of SAW attenuation by a gas is described. Further, documents <CIT>, <CIT>, <CIT> and the paper of <NPL>), disclose gas analysis by means of a ball SAW sensor driven at two different frequencies, wherein a sensitive layer is deposited on the ball so that a collimated SAW beam passes through it.

In a new technology for hydrogen transport, hydrogen is injected into natural gas and carried in natural gas pipeline system, as reported in NPL <NUM>. In the technology, rapid hydrogen concentration measurement is needed in a concentration range up to <NUM> %. Though sound velocity measurement, thermal conductivity measurement, and infra-red spectroscopy are used for analysis of natural gases, fast and precise hydrogen gas sensor is not available.

In view of the above problems, an object of the present invention is to provide a system, a method and a computer program product for gas analysis, which facilitate a high precision measurement of gas species and concentration in a short time.

A first aspect of the present invention inheres in a system for gas analysis, as defined in claim <NUM>. Preferred features are defined in the dependent claims.

A second aspect of the present invention inheres in a method for gas analysis, as defined in claim <NUM>. Preferred features are defined in the dependent claims.

A third aspect of the present invention inheres in a computer program product, as defined in claim <NUM>.

According to the present invention, it is possible to provide the system, the method and the computer program product for gas analysis, which facilitate a high precision measurement of gas species and concentration in a short time.

In the descriptions of the following drawings, the same or similar reference numerals are assigned to the same or similar portions. However, the drawings are diagrammatic, and attention should be paid to a fact that the relations between thicknesses and plan view dimensions, the configuration of the apparatus and the like differ from the actual data. Thus, the specific thicknesses and dimensions should be judged by considering the following descriptions. Also, even between the mutual drawings, the portions in which the relations and rates between the mutual dimensions are different are naturally included. Also, the first and second embodiments as described below exemplify the apparatuses and methods for embodying the technical ideas of the present invention, and in the technical ideas of the present invention, the materials, shapes, structures, arrangements and the like of configuration parts are not limited to the followings.

In the following description, α, β, γ, Δ and ρ represent Greek alphabet characters, respectively. And, the "horizontal" direction or the "vertical" direction is simply assigned for convenience of explanation and does not limit the technical spirit of the present invention. Therefore, for example, when the plane of paper is rotated <NUM> degrees, the "horizontal" direction is changed to the "vertical" direction and the "vertical" direction is changed to the "horizontal" direction. When the plane of paper is rotated <NUM> degrees, the "left" side is changed to the "right" side and the "right" side is changed to the "left" side. Therefore, various changes can be added to the technical ideas of the present invention, within the technical scope prescribed by claims.

As illustrated in <FIG>, a system for gas analysis, or a gas analyzer, pertaining to an embodiment of the present invention includes a sensor unit <NUM>, a temperature controller <NUM>, a gas supply unit <NUM>, a velocity measurement unit <NUM> and a signal processing unit <NUM>. The sensor unit <NUM>, as illustrated in <FIG>, has a SAW sensor <NUM>, which is a ball SAW sensor, embedded in a tubular sensor cell <NUM>, wherein the tubular sensor cell <NUM> is fixed on a plate-shaped adapter <NUM> disposed on a block-shaped holder <NUM>. As the SAW sensor <NUM> has spherical shape, the inner structure of the sensor cell <NUM> has a concave configuration for mounting a lower portion of the SAW sensor <NUM> in a tubular topology of the sensor cell <NUM>. An electrode-holder base <NUM> is fixed on the sensor cell <NUM>, such that the bottom of the electrode-holder base <NUM> is inserted in an inner wall of a window, which is vertically cut at the top wall of the sensor cell <NUM>. An opening of a canal, which penetrates vertically through the bottom of the electrode-holder base <NUM>, partially covers an upper portion of the SAW sensor <NUM>. Furthermore, a top of the electrode-holder base <NUM> is capped by a sensor-cell cap <NUM>.

The SAW sensor <NUM> is connected to a rod-shaped external electrode <NUM> through a contact pin 35a along a vertical direction via the canal at the bottom of the electrode-holder base <NUM>. The external electrode <NUM> is held in a hollow space of a vertically aligned cylindrical electrode holder <NUM>, the bottom of which is inserted in an inner portion of the sensor-cell cap <NUM>. A sensing gas containing in a background gas, for example, a humid gas, is introduced into the sensor cell <NUM> through a horizontally aligned tubing <NUM> with a gas flow rate v, so that the humid gas can touch the surface of the SAW sensor <NUM>. The gas flow rate v is typically <NUM>/min to <NUM>/min.

As illustrated in <FIG>, the SAW sensor <NUM> may have a sensor electrode <NUM> and a sensitive film <NUM> arranged in predetermined areas on the surface of a homogeneous piezoelectric ball <NUM>. As a three-dimensional base body, the piezoelectric ball <NUM> provides a homogeneous material sphere, on which a circular orbital band for propagating a SAW can be defined. The sensor electrode <NUM> generates a collimated beam <NUM> of the SAW, which includes a fundamental wave of a first frequency and a harmonic wave of a second frequency, propagates repeatedly through the circular orbital path defined on the piezoelectric ball <NUM> while passing through the sensitive film <NUM> deposited on the orbital path. The sensitive film <NUM> can be formed on almost the entire surface of the orbital band, which defines the orbital path on the three-dimensional base body. Because the sensitive film <NUM> is configured to react with specific gas molecules, the sensitive film <NUM> adsorbs water vapor in the sensing gas-to-be-measured.

For the piezoelectric ball <NUM>, a crystal sphere, such as quartz, langasite (La<NUM>Ga<NUM>SiO<NUM>), lithium niobate (LiNbO<NUM>), lithium tantalate (LiTaO<NUM>), piezoelectric ceramics (PZT), bismuth germanium oxide (Bi<NUM>GeO<NUM>) and the like, may be used. For the sensitive film <NUM>, a silica (SiOx) film and the like may be used. The sensor electrode <NUM> may be deposited in an opening of the sensitive film <NUM>, the opening exposes a part of the surface of the piezoelectric ball <NUM>, in a configuration such that the opening is formed on a part of the equator of the homogeneous piezoelectric ball <NUM>. For the sensor electrode <NUM>, an interdigital electrode (IDT) using a chromium (Cr) film and the like may be used as an electroacoustic transducer. In the case of a sphere of single crystal such as the homogeneous piezoelectric ball <NUM>, a SAW orbiting route is limited to a specific orbital band having a constant width, depending on type of crystal material. The width of the orbital band may be increased or decreased depending on anisotropy of the crystal.

There are no diffraction losses during roundtrips around the piezoelectric ball <NUM>, and only propagation loss due to material attenuation. The collimated beam <NUM> is scheduled to propagate many turns passing through the sensitive film <NUM>, which is configured to adsorb water molecules. Because the adsorbed water molecules change the propagation characteristic of the SAW, the changes due to adsorbed water molecules in the humid gas on the sensitive film <NUM> can be integrated every turn through the multiple roundtrips. Thus, even though the sensitive film <NUM> may be so thin as to adsorb the small amount of the water vapor, measurement accuracy of gas analysis may be increased.

The suitable relationship between the first frequency f<NUM> of the fundamental wave and the second frequency f<NUM> of the harmonic wave shall be represented by f<NUM> = nf<NUM>, where n = <NUM> or <NUM>. That is, in the system for gas analysis pertaining to the embodiment of the present invention, the harmonic wave is the third-order harmonic wave or the fifth-order harmonic wave. Thus, when the first frequency f<NUM> is <NUM>, the second frequency f<NUM> is <NUM> for the third-order harmonic wave or <NUM> for the fifth-order harmonic wave. Appropriate range of the first frequency f<NUM> for the piezoelectric ball <NUM> of <NUM> millimeters diameter may be from <NUM> to <NUM>, and the most suitable first frequency f<NUM> may be <NUM>. The first frequency f1 is inversely proportional to the diameter of the piezoelectric ball <NUM>.

For example, the SAW sensor <NUM> may be fabricated as described below. A pattern of an IDT of about <NUM> nanometers thick Cr film is deposited on a surface of a quartz ball having a diameter of <NUM> millimeters. The IDT has a pair of bus bars, and a plurality of electrode fingers extending from the bas bars, respectively. The electrode fingers overlap each other with a cross width Wc, and each electrode finger has a width Wf and a periodicity P. The cross width Wc, the width Wf and the periodicity P are designed as <NUM> micrometers, <NUM> micrometers and <NUM> micrometers, respectively, for the natural collimation of <NUM> SAW (refer to NPL <NUM>).

The IDT on the quartz ball having <NUM> millimeters diameter can generate <NUM> SAW as a fundamental wave and <NUM> SAW as a third-order harmonic wave. Then a silica film is synthesized by using a sol-gel method and coated on the surface of the quartz ball as follows: <NUM> grams of tetraethoxysilane (TEOS), <NUM> grams of isopropanol (IPA), and <NUM> grams of <NUM>. 1N hydrochloric acid (HCl) are mixed and stirred by sonication (<NUM>, <NUM>, <NUM>, <NUM> minutes). TEOS is polymerized by hydrolysis and resulted in SiOx. After sonication, the mixture is diluted with IPA and <NUM> mass% SiOx solution is obtained. The surface of propagation route of SAW is coated with the SiOx solution using a spin coating. Condition of the spin coating is <NUM> rpm for <NUM> seconds. The thickness of SiOx film is confirmed as <NUM> nanometers from measurement using interference microscope.

An RF voltage is applied to the sensor electrode <NUM> via an electrode pad (not illustrated) arranged around the north-pole, which is a top of the piezoelectric ball <NUM> in <FIG>, using the contact pin 35a attached on the bottom of the external electrode <NUM>. Another electrode pad (not illustrated) arranged around the south-pole, which is a bottom of the piezoelectric ball <NUM> in <FIG>, is in contact with the grounded sensor cell <NUM>.

In the above description, a ball SAW sensor is used as the SAW sensor <NUM>. In an alternative arrangement, not claimed, a planar SAW sensor 2a illustrated in <FIG> may be used. The planar SAW sensor 2a have an input electrode 22a, a sensitive film 23a and an output electrode 22b, which are arranged in predetermined areas on the surface of a homogeneous piezoelectric substrate 20a.

As illustrated in <FIG>, the temperature controller <NUM> is connected to a Peltier element <NUM>, which is held in a lower portion of the holder <NUM> at a position just below the SAW sensor <NUM>, and a thermistor <NUM> is inserted in the holder <NUM> at a side position of the holder <NUM>. Furthermore, the temperature controller <NUM> is connected to the thermistor <NUM>. The Peltier element <NUM> is used for heating and cooling the SAW sensor <NUM> in the sensor cell <NUM> through the adapter <NUM>. The thermistor <NUM> is used for detecting a monitoring temperature Tth of the holder <NUM>. The temperature controller <NUM> controls the Peltier element <NUM> by using the monitoring temperature Tth. As illustrated in <FIG>, the thermistor <NUM> cannot be directly inserted into the sensor cell <NUM> to prevent leakage of gases through the sensor cell <NUM>. Note that, although the thermistor <NUM> is used for detecting the monitoring temperature Tth in the first embodiment, but other thermometers, such as a thermocouple and the like, may be used.

The signal processing unit <NUM>, as illustrated in <FIG>, includes a signal generator and a signal receiver (hereinafter the set of the signal generator and the signal receiver is referred as the "signal generator/receiver") <NUM> and a waveform data processor <NUM>. The signal generator/receiver <NUM> includes a signal generator 42a, and a signal receiver 42b. As illustrated in <FIG>, the waveform data processor <NUM> includes a communication module (communication logical circuit) <NUM>, a calculation module (calculation logical circuit) <NUM>, a comparison module (comparison logical circuit) <NUM>, and a memory unit <NUM> for logical hardware resources of a computer system. The communication module <NUM> of the waveform data processor <NUM> sends a predetermined "set temperature" or a control temperature for the Peltier element <NUM> to the temperature controller <NUM>, which are illustrated in <FIG> and <FIG>. And, the communication module <NUM> sends instructions for flowing a gas into the sensor cell <NUM> to the sensor unit <NUM> and the gas supply unit <NUM> illustrated in <FIG> and <FIG>. Moreover, the communication module <NUM> sends instructions for measuring a sound velocity in the gas supplied from the gas supply unit <NUM> to the velocity measurement unit <NUM>. In addition, when the velocity measurement of the gas is not necessary, the gas may be directly supplied to the sensor unit <NUM> from the gas supply unit <NUM>.

Moreover, the communication module <NUM> sends instructions to the signal generator/receiver <NUM> so that the signal generator 42a illustrated in <FIG> transmits an exciting burst signal to the sensor electrode <NUM> of the SAW sensor <NUM> and the sensor electrode <NUM> can excite the collimated beam <NUM> of a SAW propagating around the piezoelectric ball <NUM> illustrated in <FIG>. And furthermore, the communication module <NUM> sends instructions to the signal generator/receiver <NUM> so that the signal receiver 42b illustrated in <FIG> can receive returned burst signals of the collimated beam <NUM> through the sensor electrode <NUM> after the collimated beam <NUM> has propagated a predetermined number of turns around the piezoelectric ball <NUM> illustrated in <FIG>. As illustrated in <FIG>, the signal generator/receiver <NUM> transmits waveform data of the returned burst signals to the waveform data processor <NUM>.

The calculation module <NUM> of the waveform data processor <NUM> calculates a gas parameter by using first and second attenuations in amplitudes of the SAWs of the first and second frequencies, respectively, using the waveform data of the returned burst signals. The comparison module <NUM> of the waveform data processor <NUM> compares the calculated gas parameter with data of gas parameters for various gases in order to determine gas species. The memory unit <NUM> of the waveform data processor <NUM> stores a program for driving the waveform data processor <NUM> to implement processing of the waveform data for calculating the gas parameter. Also, the memory unit <NUM> stores the data of gas parameters for various gases, and data obtained during the calculation and analysis of the gas during the operation of the waveform data processor <NUM>.

The waveform data processor <NUM> may be a part of central processing unit (CPU) of a general purpose computer system, such as a personal computer (PC) and the like. The waveform data processor <NUM> may include an arithmetic logic unit (ALU) that performs arithmetic and logic operations, a plurality of registers that supply operands to the ALU and store the results of ALU operations, and a control unit that orchestrates the fetching (from memory) and execution of instructions by directing the coordinated operations of the ALU. The communication module <NUM>, the calculation module <NUM>, and the comparison module <NUM> implementing the ALU may be discrete hardware resources such as logical circuit blocks or the electronic circuitry contained on a single integrated circuit (IC) chip, or alternatively, may be provided by virtually equivalent logical functions achieved by software, using the CPU of the general purpose computer system.

In addition, the program for the waveform data processor <NUM> for the gas analysis is not limited to being stored in the memory unit <NUM> installed in the waveform data processor <NUM>. For example, the program may be stored in an external memory. Moreover, the program may be stored in a computer readable medium. By reading the computer readable medium in the memory unit <NUM> of the computer system, which includes the waveform data processor <NUM>, the waveform data processor <NUM> implements coordinated operations for the gas analysis, in accordance with a sequence of instructions recited in the program. Here, the "computer readable medium" refers to a recording medium or a storage medium, such as an external memory unit of a computer, a semiconductor memory, a magnetic disk, an optical disk, a magneto optical disk, and a magnetic tape, on which the program can be recorded.

In NPL <NUM>, leaky attenuation coefficient αL of SAW is given by <MAT> where f is a frequency, ρs is a density of the piezoelectric ball <NUM>, Vs is a SAW velocity of the piezoelectric ball <NUM>, ρ is a density of the gas and KG is a compressibility of the gas. Substituting known relations <MAT> into Eq. (<NUM>), it becomes <MAT> where M is a molecular weight of the gas, P is a pressure of the gas, R is a gas constant, T is a temperature and γ is the heat capacity ratio which is ratio of the specific heat at constant pressure to the specific heat at constant volume of the gas.

In a SAW sensor shown in <FIG>, burst of two frequencies f<NUM> and f<NUM> is transmitted and attenuations α<NUM> and α<NUM> at each frequency is measured. Then a leakage factor ΔαL is defined as <MAT> where the superscript "u" is an index to describe the frequency dependence of the attenuation by the sensing gas, which is <NUM> or more and <NUM> or less, and l is the SAW propagation length.

A model is constructed for the calculation purpose with <MAT> <MAT> where F<NUM> = f<NUM>/f<NUM>, F<NUM> = f<NUM>/f<NUM>, f<NUM> is a reference frequency, and with the propagation length l of the SAW, <MAT> is an attenuation caused by a leakage to the background gas at frequency f<NUM>, the superscript "z" is a frequency dependence index of leaky attenuation αL*, a<NUM>(w) is a loss by the sensing gas, w is a concentration of the sensing gas, a<NUM> is a device loss due to scattering at the sensor electrode <NUM>, etc., the superscript "y" is a frequency dependence index of the device loss. The index z is normally equal to <NUM> according to NPL2, as in Eq. (<NUM>) and Eq. (<NUM>), and αL*= αL is assumed as described later. However, the index z may be <NUM> or more and <NUM> or less. It is noted that the concept and process of modifications to Eq. (<NUM>) and following equations of Eq. (<NUM>) in the case of using z other than z = <NUM> is obvious to a person having skills in the technical field.

It is noted that [γM]<NUM>/<NUM> in Eq. (<NUM>) is an important parameter describing property of the background gas, and thus it is defined as a gas parameter, here. Examples of gas parameters G, or "reference gas parameters", for typical light gasses, each of which is calculated by molecular weight M and heat capacity ratio γ of each gas, are listed in a table of <FIG>. It is noted that the order of magnitude of the gas parameter is not identical to that of molecular weight, nor that of heat capacity ratio.

Subtracting Eq. (<NUM>) from Eq. (<NUM>) multiplied by (F<NUM>/F<NUM>)u, the leakage factor ΔαL is related to the losses by <MAT> As a special case, we define F<NUM>=3F<NUM>, F<NUM>=<NUM> and the loss to be a viscoelastic loss with u=<NUM> (refer to NPL <NUM>). Then, <MAT> From Eq. (<NUM>), <MAT> Using second equation of Eq. (<NUM>) and Eq. (<NUM>), the gas parameter is given by <MAT>.

Coefficient A and term d caused by device loss can be determined by calibration. To determine A and d, the leakage factor ΔαL is first measured at T<NUM> and P<NUM> for a gas having a gas parameter G<NUM> and secondly at T<NUM> and P<NUM> for a gas having a gas parameter G<NUM>. Thus, G<NUM>=A(T<NUM><NUM>/<NUM>/P<NUM>)(ΔαL,<NUM>-d) and G<NUM>= A(T<NUM><NUM>/<NUM>/ P<NUM>)(ΔαL,<NUM> - d) giving <MAT> and
In the second measurement, all parameters (T<NUM>, P<NUM>, G<NUM>) do not have to be changed from (T<NUM>, P<NUM>, G<NUM>). Different gas species can be measured at the same temperature and the same pressure, that is, calibration-condition (T<NUM>=T<NUM>, P<NUM>=P<NUM>, G<NUM>≠G<NUM>) or only the pressure is changed, that is, calibration-condition (T<NUM>=T<NUM>, P<NUM> #P<NUM>, G<NUM>=G<NUM>). In an environment with constant temperature T or pressure P, calibration can be made by <MAT> with <MAT>.

Since the leaky attenuation is proportional to the first frequency F<NUM> and the second frequency F<NUM> when z is equal to <NUM> in Eqs. (<NUM>) and (<NUM>), it will be cancelled in a viscoelastic factor ΔαV defined and given by <MAT> In a special case of F<NUM>=3F<NUM>, F<NUM>=<NUM>, <MAT>.

Test measurements for gas analysis of background gases have been executed using a humid gas in which trace moisture as a sensing gas has been mixed in various background gases. The gas supply unit <NUM> used for the test measurement, as illustrated in <FIG>, includes a plurality of gas sources 52a, 52b, 52c, 52d, a gas controller <NUM> and a moisture generator <NUM>. Each of the gas sources 52a, 52b, 52c, 52d includes a gas container of a background gas and a flow controller for controlling a flow rate of the background gas, and supplies the background gas to the sensor unit <NUM> through the gas-switching valve <NUM> and the moisture generator <NUM>. Note that, in <FIG>, the velocity measurement unit <NUM> is omitted for convenience of explanation. In the test measurement, for example, the gas source 52a supplies a nitrogen (N<NUM>) gas, the gas source 52b supplies an argon (Ar) gas, the gas source 52c supplies a methane (CH<NUM>) gas, and the gas source 52d supplies air. The gas-switching valve <NUM> switches to select the background gas from the gas sources 52a, 52b, 52c, 52d by instructions of the communication module <NUM> of the waveform data processor <NUM>. The moisture generator <NUM> generates the trace moisture as the sensing gas in the background gas at a predetermined concentration by instructions of the communication module <NUM> illustrated in <FIG>. Thus, the humid gas at a predetermined frost point, or a predetermined water concentration can be supplied to the sensor unit <NUM>.

In the test measurement, the fundamental wave and the third-order harmonic wave of the SAW, that is, f<NUM> = 3f<NUM>, has been used. Each procedure of the test measurements will be described with reference to the flowchart illustrated in <FIG>. In addition, in the test measurement, the background gas has been assigned as a "target gas" in the humid gas.

In step S100, the gas supply unit <NUM> supplies the humid gas with the background gas, which is selected from the gas sources 52a to 52d, into the sensor unit <NUM>. In step S101, the signal generator 42a of the signal generator/receiver <NUM> transmits the burst signal to the SAW sensor <NUM>, so as to excite the collimated beam <NUM> of the SAW as illustrated in <FIG>. In step S102, after the collimated beam <NUM> has propagated a predetermined number of turns around the ball sensor <NUM>, the signal receiver 42b of the signal generator/receiver <NUM> receives the returned burst signals of the collimated beam <NUM> through the ball sensor <NUM>. Waveform data of the returned burst signals is transmitted to the waveform data processor <NUM>.

In step S103, the waveform data processor <NUM> measures a first attenuation α<NUM> of a first burst signal having the first frequency f<NUM> and a second attenuation α<NUM> of a second burst signal having the second frequency f<NUM>. In step S104, the waveform data processor <NUM> calculates the target gas parameter G of the target gas using the leaky attenuation coefficient αL and the leakage factor ΔαL, which is derived by the first and second attenuations α<NUM> and α<NUM> using Eqs. (<NUM>) and (<NUM>). Then, in Step S105, the waveform data processor <NUM> estimates a gas species of the target gas by comparing the measured gas parameter with the true gas parameters, or the reference gas parameters, which are calculated by the physical-property data of gases. In addition, the waveform data processor <NUM> measures the viscoelastic factor ΔαV of the target gas using Eq. (<NUM>) so as to calculate a concentration of the sensing gas.

An example of calibration for the coefficient B and the term d in Eqs. (<NUM>) and (<NUM>) will be described below. In the calibration procedure, a humid gas having frost point of -<NUM> or water concentration of <NUM> ppmv has been used with background gases of Air, N<NUM>, Ar and CH<NUM> which have been supplied from the gas source 52a to 52d of the gas supply unit <NUM> illustrated in <FIG>. In the humid gas, the background gas has been changed in order of Air, N<NUM>, Ar and CH<NUM>, and the first and second attenuations α<NUM> and α<NUM> have been measured, as illustrated in <FIG>. Then, the leakage factor ΔαL has been measured using the first and second attenuations α<NUM> and α<NUM>, as plotted in <FIG>. In the measurement, the propagation length l of the SAW has been set to be l=<NUM>. In the case of the calibration using N<NUM> and Ar, the relation between gas parameters G and the leakage factors ΔαL, which have been calculated using the known values of gas parameters G for N<NUM> and Ar, has been plotted as closed circles in <FIG>. Extrapolating a straight line connecting two closed circles extending to G=<NUM> with M=<NUM>, the coefficient B and the term d caused by device loss in Eqs. (<NUM>) and (<NUM>) have been calibrated as B=<NUM> and d=<NUM> dB/m.

Assuming that the first background gas and the fourth background gas in <FIG> are unknown, let the first and the fourth background gases, which are air and CH<NUM>, be target gases X2 and X1, respectively. Then, the target gases X2 and X1 have been estimated by using the gas analyzer pertaining to the embodiment of the present invention as illustrated in <FIG>.

Using Eq. (<NUM>) with the calibrated parameter B, measured gas parameters G*, or target gas parameters G*, of the target gases X1 and X2 have been measured, as <NUM> and <NUM> as listed in a table of <FIG>. Comparing with the gas parameters G listed in the table illustrated in <FIG>, which will be denoted by the "true gas parameters" below, it has been understood that the values of the measured gas parameters G* of the target gases X1 and X2 have been most close to the true gas parameter <NUM> of CH<NUM> and the true gas parameter <NUM> of Air. Thus, the target gases X1 and X2 can be estimated to be CH<NUM> and Air, respectively. The open circles, which indicates the measured gas parameters G* of the target gases X1, X2, are close to the calibration straight line illustrated in <FIG> and the measurement error from the true gas parameters G has been evaluated as -<NUM> % and <NUM> %, respectively, as listed in the table illustrated in <FIG>.

In a humid gas having another humidity of frost point of -<NUM> and water concentration of <NUM> ppmv, the background gases X3, X4, X5, X6 have been used as target gases. The background gas has been changed in order of X6, X3, X5 and X4, as illustrated in <FIG>, and the leakage factor ΔαL has been measured as illustrated in <FIG>. Since the measured gas parameters G* of the background gases X3, X4, X5, X6, listed in a table of <FIG>, are very close to the true gas parameters G of <NUM>, <NUM>, <NUM> and <NUM>, which correspond to N<NUM>, Ar, CH<NUM> and Air, the target gases can be estimated to be X3=N<NUM>, X4=Ar, X5=CH<NUM> and X6=Air, respectively. The measured gas parameters G* has been very close to an extrapolated straight line in <FIG>, and the measurement error has been less than <NUM>% as illustrated in the table illustrated in <FIG>.

The measured gas parameters G* and the true gas parameters G are summarized in a graph illustrated in <FIG>. The agreement between the measured gas parameters G* and the true gas parameters G is significant. The result of the graph of <FIG>, which indicates that the measurement of the background gas is not disturbed by changing the humidity as the sensing gas, provides the accuracy of the gas analyzer according to the embodiment of the present invention.

In the gas analyzer according to the embodiment of the present invention, it is also possible to measure a concentration of the sensing gas in the background gas with high precision using the viscoelastic factor ΔαV, even when composition of the background gas is changed. To verify that the viscoelastic factor ΔαV does not depend on the background composition but only on the moisture content, the leakage factor ΔαL evaluated using Eq. (<NUM>) and the viscoelastic factor ΔαV evaluated using Eq. (<NUM>) have been compared in a wide time range of about <NUM> hours using humid gases having the frost points of -<NUM>, -<NUM> and -<NUM>, which correspond to <NUM> ppmv, <NUM> ppmv and -<NUM> ppmv, as illustrated in <FIG> and <FIG>. Then, it is confirmed that the leakage factor ΔαL is independent of the moisture content, as illustrated in <FIG>. In contrast, as illustrated in <FIG>, the viscoelastic factor ΔαV is almost independent on the change in the background composition and represents only the change in moisture content. Thus, the viscoelastic factor ΔαVis useful for the moisture measurement under different background gases without time-consuming recalibration procedure for each background gas.

In the following explanation of the second example, each of M(bar), G(bar), γ(bar) , Cp(bar) and Cv(bar), etc. represents a symbol labeled with an horizontal over line, or an over bar on the top of the characters of M, γ, Cp and Cv, etc..

In a mix gas having a plurality of component gases, an average gas parameter G(bar) is given by <MAT>.

In the case of two background gases of XA and XB, G(bar) is derived as <MAT> where x is the concentration by mole percentage, or mol%, of gas XA, MA, CPA and CVA are molecular weight, specific heat at a constant pressure and specific heat at constant volume of the gas XA, and MB, CPB, CVB are molecular weight, specific heat at a constant pressure and specific heat at constant volume of the gas XB, respectively.

To verify Eq. (<NUM>), helium (He) as the gas XA has been mixed with N<NUM> as gas XB, where molecular weight, specific heat at a constant pressure, specific heat at constant volume, ratio of specific heats and gas parameter of He and N<NUM> are listed in <FIG>. The measurement sequence was performed in which He concentration has been increased from <NUM>% to <NUM>% with <NUM>% steps and decreased from <NUM>% to <NUM>% with <NUM>% steps. The measurement time of each step has been <NUM>.

The measured leakage factor αL in the measurement sequence is plotted against time in <FIG>. Then, the average gas parameter G(bar) has been calculated over the last ten minutes of each step, and plotted by open circles against the He concentration in <FIG>. The measured values of the average gas parameter G(bar) agree well with a calculated curve, illustrated by solid curve in <FIG>, using Eq. (<NUM>) with parameters listed in <FIG>. Note that the calculated curve of the average gas parameter G(bar) in <FIG> does not agree with a simple average of the average gas parameter <MAT> of He and N<NUM>, illustrated by dashed line in <FIG>.

The concentration of He can be measured by the average gas parameter G(bar) with using a calibration curve. The calibration curve may be calculated by replacing CPB with βCPB and CVB with βCVB in Eq. (<NUM>) where the β is an adjustable parameter. The replacement does not change the average gas parameter G(bar) at the molar fraction x=<NUM> or at the molar fraction x=<NUM> but changes the average gas parameter G(bar) in the intermediate range of the molar fraction x from <NUM> to <NUM>, that is, <NUM> mol% to <NUM> mol%. Though the adjustable parameter β has no physical meaning, the adjustable parameter β helps to improve the agreement between the experimental data and the calibration curve when the adjustable parameter β is set to <NUM>, as illustrated by dotted curve in <FIG>, which is slightly deformed and shifted from the solid curve.

Using the calibration curve, He concentration has been measured as illustrated in <FIG>, where set concentration has been increased in order from <NUM> mol% to <NUM> mol% by <NUM> mol% steps, decreased from <NUM> mol% to <NUM> mol% by <NUM> mol% steps, increased from <NUM> mol% to <NUM> mol% by <NUM> mol% steps and decreased from <NUM> mol% to <NUM> mol% by <NUM> mol% steps. The measured concentration has been compared with the set concentration and it has been confirmed that good agreement between the set concentration and measured concentration has been obtained as illustrated in <FIG>. The standard deviation of the measured concentration from the set concentration has been about <NUM>% in the measurement of He <NUM> mol% to <NUM> mol%. Thus, in the gas analyzer according to the embodiment of the present invention, it is possible to measure the concentration of the component gas in the mix gas with high precision.

Further, it is also possible to apply the gas analyzer according to the embodiment of the present invention for checking whether an interior of a glove box has been replaced with a purge-gas. For example, the glove box used for Li-ion batteries or for 3D printers of metal objects, it is required to replace air and moisture in the glove box with the purge-gas. For the purge-gas, an inert gas, such as argon, helium, N<NUM> and the like, or a mixture of inert gases may be preferably used to avoid unwanted chemical reactions with oxygen (O<NUM>) in the air and the moisture. While purging the air and the moisture by introducing the purge-gas into the glove box, the purge-gas and the air may be implemented by the mix gas and concentration of the purge-gas may increase with time. Thus, it is possible to measure the concentration of the purge-gas as a component gas, which is mixed with the air in the glove box, using the gas parameter G(bar) of Eq. (<NUM>). Also, it is expected to be precise enough for the measurement of the spatial distribution of the purge-gas in the glove box. In addition, concentration of the moisture in the glove box may be also measured as the sensing gas using the viscoelastic factor ΔαV of Eq. (<NUM>).

With Mayer's relationship, Cpi = CVi + R, Eq. (<NUM>) is replaced by <MAT> The average sound velocity V(bar) is usually measured in gas analysis as, <MAT> In Eq. (<NUM>), independent quantity of molecular weight nor ratio of specific heat is not available. However, when the gas analyzer gives a gas parameter G(bar) in Eq. (<NUM>), the average molecular weight M(bar) and the average specific heat ratio γ(bar) are independently solved from Eq. (<NUM>) and Eq. (<NUM>) as <MAT> and <MAT> The average molecular weight M(bar) and the average specific heat ratio γ(bar) are useful for calculation of many physical/chemical property of the mix gas.

To obtain molar fraction xi (i=<NUM>, N) of each component gas, N independent equations are required.

In a special case of N=<NUM>, measurements for the average gas parameter G(bar) and the average sound velocity V(bar) results in, <MAT> <MAT> <MAT> Linear simultaneous Eqs. (<NUM>), (<NUM>) and (<NUM>) can be solved for molar fraction (x<NUM>, X2, X3).

When hydrogen is injected to a mix gas of natural gases, for example, methane and ethane, and x<NUM>=[H<NUM>], x<NUM>=[CH<NUM>], x<NUM>=[C<NUM>H<NUM>], mole fraction (x<NUM>, x<NUM>, x<NUM>) is solved by Eqs. (<NUM>) to (<NUM>).

When hydrogen is injected to methane, and x<NUM>=[H<NUM>], x<NUM>=[CH<NUM>], <MAT> <MAT> Then, molar fraction (x<NUM>, x<NUM>) is solved by Eqs. (<NUM>) and (<NUM>).

As illustrated in <FIG> and <FIG>, the gas analyzer according to the embodiment can measure a sound velocity of the gas by the velocity measurement unit <NUM> using an ultrasonic wave having a frequency in a range of <NUM> to <NUM>. The average sound velocity is expressed by average density ρ(bar) and average compressibility KG (bar), as <MAT> In such case, it is useful to express the average leaky attenuation coefficient αL(bar) as <MAT> similarly to Eq. (<NUM>). Then, from Eqs. (<NUM>) and (<NUM>), the average compressibility and the average density are solved as <MAT> <MAT> The average leaky attenuation coefficient αL(bar) is calculated by, <MAT> similarly to Eq. (<NUM>).

Claim 1:
A system for gas analysis, comprising:
a sensor (<NUM>) having:
a piezoelectric substrate which is a piezoelectric ball (<NUM>),
a sensor electrode (<NUM>) configured to generate a collimated beam (<NUM>) of a surface acoustic wave of first, f<NUM>, and second, f<NUM>, frequencies, which propagates on the piezoelectric ball (<NUM>), wherein f<NUM> = nf<NUM> and n is <NUM> or <NUM>, and
a sensitive film (<NUM>, 23a) configured to adsorb a sensing gas contained in a background gas, wherein the sensitive film (<NUM>, 23a) is deposited on the ball (<NUM>) in a position where the collimated beam (<NUM>) passes through; and
a signal processing unit (<NUM>) having:
a signal generator (42a) configured to transmit an exciting burst signal to the sensor electrode (<NUM>) so as to excite the collimated beam (<NUM>),
a signal receiver (42b) configured to receive first and second returned burst signals of the collimated beam (<NUM>) through the sensor electrode (<NUM>) after the collimated beam (<NUM>) has propagated on the piezoelectric ball (<NUM>) the first returned burst signal having the first frequency and the second returned burst signal having the second frequency; and
a data processor (<NUM>) configured to calculate a target gas parameter by a target leakage factor of the background gas and by a relation between reference gas parameters and reference leakage factors of reference gases, wherein the target leakage factor is provided by a first attenuation of the first returned burst signal and a second attenuation of the second returned burst signal using waveform data of the first and second returned burst signals , wherein the target leakage factor and the target gas parameter are given by, <MAT> <MAT> wherein
ΔαL is the target leakage factor and G is the target gas parameter, respectively,
f<NUM> and f<NUM> are the first and second frequencies, respectively,
α<NUM> and α<NUM> are the first and second attenuations, respectively,
u is a real number satisfying <NUM> ≦ u ≦ <NUM>, and l is a propagation length of the surface acoustic wave,
T and P are temperature and pressure of the background gas, respectively, and
A and d are a coefficient and a term caused by a loss of the sensor, respectively.