Patent Description:
Earlier piezoelectric gas sensors, such as planar SAW sensors, utilize the propagation property that amplitude and phase of the exited SAW change when passing through a sensitive film in which elastic characteristics are changed by adsorbing gas molecules. However, diffraction occurs when waves of a finite width are propagating, and the SAW on the planar SAW sensor is attenuated by diffraction loss. Therefore, because of the diffraction loss, there is a limit to the propagation distance of the SAW, and measurement accuracy of gas concentration is limited.

As recited in non-patent literatures (NPLs) <NUM> and <NUM>, a ball SAW sensor (hereinafter called "ball sensor") has been developed and applied to a trace moisture sensor. In the ball 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 this effect may provide high performance, such as high sensitivity and wide sensing range.

Since the sensitivity of the piezoelectric gas sensor also depends on temperature of the sensor, measured gas concentration is disturbed when the sensor temperature is largely changed. However, it is not easy to measure the sensor temperature, when it is not possible to insert a thermometer into the sensor cell. The ball sensor also has the same problem. Patent literature (PL) <NUM> discloses an electrical signal processing device in which narrowband frequency filtering is applied to a cyclic waveform in a delay-line-type-SAW sensor capable of transmitting and receiving a plurality of frequencies, two frequencies (f1, f2) (f2 > f1) are extracted, undersampling is applied to the two frequencies at a frequency lower than two times f1, and the obtained aliasing is used to determine the delay time.

In view of the above problems, an object of the present invention is to provide a system and a method for measuring a gas concentration, which can simultaneously measure the ball temperature of the ball sensor and the gas concentration, with high sensitivity and reliability even under varying temperature.

A first aspect of the present invention inheres in a system for measuring a gas concentration, as defined in claim <NUM>.

A second aspect of the present invention inheres in a method for measuring a gas concentration, as defined in claim <NUM>.

According to the present invention, it is possible to provide the system and the method for measuring gas concentration, which can simultaneously measure the ball temperature of the ball sensor and the gas concentration, with high sensitivity and reliability even under varying temperature.

First and second embodiments of the present invention will be described below with reference to the drawings. 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, 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> and <FIG>, a system for measuring water concentration pertaining to a first embodiment of the present invention includes a sensor unit <NUM>, a temperature controller <NUM>, and a signal processing unit <NUM>. The sensor unit <NUM> has a ball sensor <NUM> embedded in a tubular sensor cell <NUM>, which is fixed on a plate-shaped adapter <NUM> disposed on a block-shaped holder <NUM>. As the ball sensor <NUM> has spherical shape, with a tubular configuration, the inner structure of the sensor cell <NUM> has a concave configuration for mounting a lower portion of the ball sensor <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 tubular 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 ball sensor <NUM>. Furthermore, the electrode-holder base <NUM> is capped by a sensor-cell cap <NUM>.

The ball 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 gas-containing trace-moisture or "the target gas-to-be-measured" is introduced into the sensor cell <NUM> through a horizontally aligned tubing <NUM> with a gas flow rate v, so that the target gas-to-be-measured can touch the surface of the ball sensor <NUM>. The gas flow rate v is typically <NUM>/min to <NUM>/min.

As illustrated in <FIG>, the ball sensor <NUM> may have a sensor electrode <NUM> and a sensitive film <NUM>, which are 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 target 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 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 water concentration 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 measuring water concentration pertaining to the first 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 f<NUM> is inversely proportional to the diameter of the piezoelectric ball <NUM>.

For example, the ball 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. As illustrated in <FIG>, the IDT has a pair of bus bars 25a, 25b, and a plurality of electrode fingers 26a, 26b extending from the bas bars 25a, 25b, respectively. The electrode fingers 26a, 26b overlap each other with a cross width Wc, and each electrode finger 26a, 26b 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>).

This 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 (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 (bottom of the piezoelectric ball <NUM> in <FIG>) is in contact with the grounded sensor cell <NUM>.

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 ball sensor <NUM>, and a thermistor <NUM> is inserted in the holder <NUM> at a side position of the holder <NUM>. Furthermore, a temperature controller <NUM> is connected to the thermistor <NUM>. The Peltier element <NUM> is used for heating and cooling the ball 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 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, as illustrated in <FIG>. The communication module <NUM> of the waveform data processor <NUM> sends a predetermined "set temperature" or a control temperature of the Peltier element <NUM> to the temperature controller <NUM> and instructions for flowing a gas into the sensor cell <NUM> to the sensor unit <NUM>.

Moreover, the communication module <NUM> sends instructions to the signal generator/receiver <NUM> so that the signal generator/receiver <NUM> transmits a burst signal to the sensor electrode <NUM> of the ball sensor <NUM> so that the sensor electrode <NUM> can excite the collimated beam <NUM> of a SAW propagating around the piezoelectric ball <NUM>, and receives 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>. The signal generator/receiver <NUM> transmits waveform data of the burst signals to the waveform data processor <NUM>.

The calculation module <NUM> of the waveform data processor <NUM> calculates the water concentration w and the ball temperature TB by using first and second relative changes in delay times of the first and second frequencies, respectively, using the waveform data of the burst signals. The comparison module <NUM> of the waveform data processor <NUM> compares the calculated ball temperature TB with the value of the previously measured ball temperature TB in order to determine whether the measurement has been implemented in thermal equilibrium. The memory unit <NUM> of the waveform data processor <NUM> stores a program for allowing the waveform data processor <NUM> to implement processing of the waveform data for calculating the water concentration w and the ball temperature TB. Also, the memory unit <NUM> stores the set temperature of the Peltier element <NUM>, the calculated ball temperature TB, the previously measured ball temperature TB, and data obtained during the calculation and analysis thereof during the operation of the waveform data processor <NUM>.

The waveform data processor <NUM> may be 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 measuring the water concentration is not limited to being stored in the memory unit <NUM> which is 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 measuring water concentration, 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.

The principle of measurement executed in the waveform data processor <NUM> will be described as follows, representing a first relative changes in delay time (DTC) by the Greek-alphabet as Delta-ti, and a second relative DTC by the Greek-alphabet as Delta-t<NUM>, as a macroscopic change in the value of a variable is represented by Greek-letter Delta in mathematics or science. Delta-t<NUM> is defined as Delta-Taui/Taui at the first frequency f<NUM> and Delta-t<NUM> is defined as Delta-Tau<NUM>/Tau<NUM> at the second frequency f<NUM>. Here, the Greek-alphabets Tau<NUM> and Tau<NUM> are delay times of the SAW at the first and second frequencies f<NUM> and f<NUM>, respectively, during propagating a predetermined number of turns without moisture adsorbed on the sensitive film <NUM>, and Delta-Tau<NUM> and Delta-Tau<NUM> are delay time changes of the delay times Tau<NUM> and Tau<NUM> due to both the water concentration and the ball temperature change. Each of delay times Tau<NUM> and Tau<NUM> at each turn is obtained as a zero cross time closest to the maximum magnitude of a real part of wavelet transform of the received burst signals at the turns (refer to NPL <NUM>).

The first and second relative changes Delta-t<NUM>, Delta-t<NUM> are given by: <MAT> <MAT> where B(TB) is a sensitivity factor, w is water concentration, G(w) is a function of the water concentration, TB is the ball temperature of the ball sensor <NUM>, TREF is a reference temperature, and A<NUM> and A<NUM> are temperature coefficients at frequencies f<NUM> and f<NUM>, respectively.

(<NUM>) and (<NUM>), a first objective change Delta-twin delay time due to gas concentration w is given by: <MAT> and, a second objective change Delta-tr in delay time due to the ball temperature (temperature term) TB is given by: <MAT> here A<NUM> and A<NUM> are temperature coefficients at the first and second frequencies f<NUM>, f<NUM>, respectively, and C=A<NUM>/A<NUM> is temperature coefficient ratio. The water concentration w and the ball temperature TB can be simultaneously obtained by Eqs. (<NUM>) and (<NUM>), respectively.

Test measurements have been implemented using the fundamental wave and the third-order harmonic wave of the SAW, that is, f<NUM> = 3f<NUM>, and without a gas flow. Each procedure of the test measurements will be described with reference to the flowchart illustrated in <FIG>. In step S100, the signal generator/receiver 42a transmits the burst signal to the ball sensor <NUM>, so as to exite the collimated beam <NUM> of the SAW. In step S101, after the collimated beam <NUM> has propagated a predetermined number of turns around the ball sensor <NUM>, the signal generator/receiver <NUM> receives the burst signals of the collimated beam <NUM> through the ball sensor <NUM>. Waveform data of the burst signals is transmitted to the waveform data processor <NUM>.

In step S102, the waveform data processor <NUM> calculates the first and second relative changes Delta-ti, Delta-t<NUM> of the first and second frequencies f<NUM>, f<NUM>, respectively, using the waveform data. Then, the first and second objective changes Delta-tW, Delta-tT due to the water concentration w and the ball temperature TB, respectively, are calculated using the first and second relative changes Delta-ti, Delta-t<NUM>. In step S103, the waveform data processor <NUM> calculates the ball temperature TB by Eq. (<NUM>) using the second objective change Delta-tr. In step S104, a temperature change Delta-T of the ball temperature TB from the previous measurement cycle is compared with a threshold value Delta-Tc that is a criterion of thermal equilibrium. In the test measurements, the threshold value Delta-Tc is temporarily set as <NUM>, the condition Delta-T < Delta-Tc is always satisfied for each measurement cycle of <NUM> seconds. In step S105, the gas concentration w is calculated by Eq. (<NUM>).

As a result of the test measurements, the temperature coefficient ratio C has been determined as C=<NUM> by least square fitting of the second relative change Delta-t<NUM> against the first relative change Delta-tr. Further, as illustrated in <FIG>, the second objective change Delta-tT has been plotted as a function of the ball temperature TB, by changing the set temperature of the Peltier element <NUM>. Here, the ball temperature TB has assumed to be identical to the monitoring temperature Tth of the holder <NUM> when the gas flow rate v is zero. From Eq. (<NUM>), the temperature coefficient A<NUM> can be defined by the slope of the fitting line, and the reference temperature TREF can be defined by a particular ball temperature where the second objective change Delta-tT is zero. Thus, the temperature coefficient A<NUM> and the reference temperature TREF can be determined as - <NUM> ppm/°C, and <NUM>.

Substituting the temperature coefficient A<NUM> and the reference temperature TREF into Eq.(<NUM>), ball temperature TB can be obtained as; <MAT>.

The error of other ball temperatures calculated using Eq. (<NUM>) has been evaluated to be less than <NUM> %. As mentioned above, according to the first embodiment, the ball temperature may be measured with high sensitivity and reliability.

In order to evaluate the effect of heat capacity of the sensor cell <NUM>, the ball temperature TB calculated by Eq. (<NUM>) has been compared with the monitoring temperature Tth measured by the thermistor <NUM>. As illustrated in <FIG>, when the specific set temperature of the Peltier element <NUM> has been changed from <NUM> to <NUM>, the ball temperature TB has been delayed by about <NUM> minute from the monitoring temperature Tth and has not reached <NUM> even after three minutes. This phenomenon is due to a large heat capacity of the adapter <NUM> made of stainless steel plate. Therefore, it is necessary to measure the water concentration with thermal equilibrium for precise measurement using viscoelastic property of the sensitive film <NUM>. In order to avoid an error caused by the non-equilibrium phenomenon, it is desirable to set the threshold value Delta-Tc to a smaller value, for example <NUM> or less. On the contrary, it is desirable to set the threshold value Delta-Tc to a larger value, for example <NUM> or more, to continue measurement without interruption.

As illustrated in <FIG>, the water concentration w in a nitrogen (N<NUM>) gas flow has been changed by the sequence of <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM> ppbv, where each water concentration w has been evaluated using the cavity ring down spectroscopy (CRDS) (refer to <NPL>). At the same time, the set temperature of the Peltier element <NUM> has been changed between <NUM> and <NUM>. The first objective change Delta-tw due to water concentration w and the ball temperature TB have been measured as illustrated in <FIG> and <FIG>. As illustrated in <FIG>, the ball temperature TB has precisely reproduced the temperature setting, and not disturbed by the changes of the water concentration w, illustrating validity of Eq. (<NUM>) or (<NUM>). The first objective change Delta-tw has been changed with the water concentration w and also with the ball temperature TB.

Using the first objective change Delta-tw in the Table illustrated in <FIG>, the right hand side term in Eq. (<NUM>) may be evaluated as; <MAT> where a = -<NUM>'L10-<NUM>, Delta-e = <NUM> (eV), kB = <NUM>'L10-<NUM> eV/K (Boltzmann Constant) and <MAT>.

Substituting Eqs. (<NUM>) and (<NUM>) into Eq.(<NUM>), the water concentration w can be obtained as; <MAT> where TB is given by Eq. (<NUM>). As illustrated in <FIG>, the water concentration w has almost correctly reproduced the set value in the sequence. Therefore, according to the first embodiment, it is possible to achieve the concentration measurement even with varying temperature.

<FIG> illustrates the transition of the first objective change Delta-tw and the ball temperature TB when the water concentration w has been changed from <NUM> to <NUM> ppbv. As illustrated in <FIG>, the first DTC Delta-tw has illustrated rather complex behavior due to the changes of water concentration w and the ball temperature TB. However, as illustrated in <FIG>, the water concentration w has almost correctly reproduced the set value.

Therefore, reliability of the concentration measurement even with varying temperature has been confirmed. However, the variation of the water concentration w near temperature jump where the ball temperature TB drastically changes between <NUM> and <NUM> is a subject matter to be solved for improvement of accuracy. The temperature jump may occur when the temperature change Delta-T of the ball temperature TB from the previous measurement cycle is larger than <NUM>. Although the variation of the water concentration w might be due to adsorption and/or desorption of water in the sensor cell <NUM> and the tubing <NUM>, the variation of the water concentration w may occur when the temperature change Delta-T of the ball temperature TB is too large.

To solve the problem of the variation of the water concentration w near the temperature jump, the other test measurement has been implemented with the threshold value Delta-Tc of <NUM>. The water concentration w in the N<NUM> gas flow has been changed by the sequence of <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM> ppbv, evaluated using the CRDS. At the same time, the ball temperature TB has been changed between <NUM> and <NUM> every <NUM> minutes by using the Peltier element <NUM>.

As illustrated in <FIG>, the measured ball temperature TB has reproduced the temperature setting. As illustrated in <FIG>, it is known that the ball temperature TB has been kept at around <NUM> for about five minutes, changed from <NUM> to <NUM> in about ten minutes, and kept at around <NUM> for about five minutes. The first objective change Delta-tw due to the water concentration w has been changed with the water concentration w and also with the ball temperature TB.

The threshold value Delta-Tc has been set <NUM> in step S104 of the flow chart illustrated in <FIG>, and the equilibrium condition Delta-T < Delta-Tc has been satisfied in the duration where the ball temperature TB has been kept at around <NUM> or <NUM> for about five minutes. In this case, the right hand side term of Eq. (<NUM>) may be evaluated as Eqs. (<NUM>) and (<NUM>) with a = -<NUM>'LI0-<NUM>, Delta-e = <NUM> (eV). Substituting Eqs. (<NUM>) and (<NUM>) with a = -<NUM>'LI0-<NUM>, Delta-e = <NUM> (eV) into Eq.(<NUM>), the water concentration w can be obtained using Eq. (<NUM>). As illustrated in <FIG> and <FIG>, the variation of the measured water concentration w near the temperature jump can be decreased compared with that in <FIG> and <FIG>.

<FIG> shows also comparison between the set concentration values, and measured concentration values averaged over <NUM> minutes. As illustrated in <FIG>, it is understood that the agreement between set and measured concentration values is remarkable. Therefore, according to the first embodiment, it is possible to achieve reliability of the concentration measurement even under varying temperature.

A measurement method of the water concentration according to the first embodiment will be described with reference to the flowchart illustrated in <FIG>. First, the communication module <NUM> of the waveform data processor <NUM> transmits a specific set temperature of the Peltier element <NUM> to the temperature controller <NUM> illustrated in <FIG>. The ball temperature is controlled by the Peltier element <NUM> with the specific set temperature, and the thermistor <NUM> inserted in the holder <NUM> monitors the temperature of the holder <NUM>. In accordance with the instruction sent from the communication module <NUM>, a gas containing water vaper is flowed into the sensor cell <NUM> through the tubing <NUM>.

In step S100, in accordance with the instruction sent from the communication module <NUM>, a burst signal is transmitted to the sensor electrode <NUM> from the signal generator/receiver <NUM>, so as to exite the collimated beam <NUM> of the SAW. As illustrated in <FIG>, the collimated beam <NUM> propagates repeatedly through the orbital path on the piezoelectric ball <NUM> while passing through the sensitive film <NUM> allocated on the orbital path.

In step S101, after the collimated beam <NUM> has propagated a predetermined number of turns, for example <NUM> turns, around the piezoelectric ball <NUM>, the signal generator/receiver <NUM> receives burst signals of the collimated beam <NUM> through the sensor electrode <NUM>. Through the communication module <NUM>, waveform data of the burst signals is transmitted to the waveform data processor <NUM> illustrated in <FIG>.

In step S102, the calculation module <NUM> of the waveform data processor <NUM> calculates the first and second relative changes Delta-ti, Delta-t<NUM> of the first and second frequencies f<NUM>, f<NUM>, respectively, by the waveform data of the burst signals. Then, the first and second objective changes Delta-tw, Delta-tT due to the water concentration w and the ball temperature TB, respectively, are calculated using the first and second relative changes Delta-t<NUM>, Delta-t<NUM>.

In step S103, the calculation module <NUM> of the waveform data processor <NUM> calculates the ball temperature TB using the second objective change Delta-tT.

In step S104, a temperature change Delta-T of the ball temperature TB from the previous measurement cycle is compared with a threshold value Delta-Tc by the comparison module <NUM> of the waveform data processor <NUM>. In the concentration measurement according to the first embodiment, the threshold value Delta-Tc is set as <NUM>.

When the temperature change Delta-T is equal to or smaller than the threshold value Delta-Tc, in step S105, the gas concentration w is calculated by the calculation module <NUM> and recorded in the memory unit <NUM> as a new measured value. On the other hand, when the temperature change Delta-T is larger than the threshold value Delta-Tc, it is determined that the thermal equilibrium needed for precise measurement of viscoelastic property of the sensitive film is not realized. Thus, the water concentration w in the previous cycle is still effective, and processing returns to step S100, so as to start a next cycle of the measurement.

In the measurement method according to the first embodiment, the water concentration w and the ball temperature TB of the ball sensor <NUM> can be simultaneously measured with high sensitivity and reliability even under varying temperature.

As illustrated in <FIG>, the change of the ball temperature TB is rather slow and delayed by about <NUM> minute compared with the monitoring temperature Tth of the thermistor <NUM> in the first embodiment. Moreover, the ball temperature TB may not reach the desired set temperature even after three minutes. One of the reasons is the large heat capacity of the adapter <NUM> made of stainless steel plate. Moreover, in order to monitor the ball temperature TB, the thermistor <NUM> should be installed as close to the ball sensor <NUM> as possible. However, since leakage of moisture from the outside air should be avoided, the thermistor <NUM> cannot be installed in the sensor cell <NUM>.

As mentioned in the first embodiment, the ball temperature TB is available using the delay times Tau<NUM> and Tau<NUM> of the SAW and the relative changes Delta-Tau<NUM> and Delta-Tau<NUM> of the delay times Tau<NUM> and Tau<NUM>. More specifically, the ball temperature TB calculated by the waveform data processor <NUM> may be used to control the Peltier element <NUM> instead of the monitoring temperature Tth by the thermistor <NUM>. Thus, the ball sensor <NUM> itself may be used as a precise thermometer to monitor the ball temperature TB.

Consequently, performance of the temperature control process may be ideal and the response of the ball temperature TB can be significantly faster. The temperature control requires the use of the ball temperature TB as a control signal, and when the ball temperature TB is used as the control signal, the performance of the system for measuring water concentration pertaining to the first embodiment can be improved, compared with the configuration, in which the commercial temperature controller is used. This improvement is realized using an apparatus illustrated in <FIG>.

In a second embodiment of the present invention, as illustrated in <FIG>, the temperature controller <NUM> includes a command interpreter <NUM> for receiving a specific set temperature of the Peltier element <NUM> and a calculated ball temperature TB from the waveform data processor <NUM>. The second embodiment differs from the first embodiment in that the command interpreter <NUM> is provided in the waveform data processor <NUM>. Other configurations are almost same as in the first embodiment, so duplicated descriptions are omitted.

During measurement, the waveform data processor <NUM> sets a temperature in the temperature controller <NUM> for controlling the Peltier element <NUM>. The signal generator/receiver <NUM> transmits a burst signal to the ball sensor <NUM>, and receives burst signals of the collimated beam <NUM> after the collimated beam <NUM> has propagated a predetermined number of turns around the piezoelectric ball <NUM>. Subsequently, the signal generator/receiver <NUM> sends the waveform data of the burst signals to the waveform data processor <NUM>. The waveform data processor <NUM> applies a signal processing to the waveform data using Eqs. (<NUM>) and (<NUM>), so as to obtain a ball temperature TB as a calculated temperature. The calculated ball temperature TB is sent to the command interpreter <NUM> using the Recommended Standard <NUM> version C (RS232C) communication protocol defined by the Electronic Industries Association (EIA).

When the calculated ball temperature TB is lower or higher than the set temperature, the temperature controller <NUM> sends a heating or cooling current to the Peltier element <NUM> in accordance with the proportional-integral-differential (PID) control algorithm. In the second embodiment, since the ball temperature TB is used as a control signal for controlling the Peltier element <NUM>, the response of the ball temperature TB may be significantly faster compared with using the monitoring temperature Tth monitored by the thermistor <NUM>.

The response time has been evaluated by the time required to reach the specific set temperature and to be stabilized within the temperature range of ±<NUM> from the set temperature. <FIG> illustrate the time changes of the ball temperature TB controlled by the Peltier element <NUM> when changing the specific set temperature of the Peltier element <NUM> from <NUM> to <NUM> at time t = <NUM>, and from <NUM> to <NUM> at time t = <NUM>. <FIG> illustrates the temperature control according to the first embodiment, in which the monitoring temperature Tth by the thermistor <NUM> has been used as a control signal. <FIG> illustrates the temperature control according to the second embodiment, in which the calculated ball temperature TB by the waveform data processor <NUM> has been used as a control signal.

As illustrated in <FIG>, the response time is about <NUM> when the specific set temperature has been changed from <NUM> to <NUM>, and about <NUM> when the specific set temperature has been changed from <NUM> to <NUM>, respectively. As illustrated in <FIG>, the response time is about <NUM> when the specific set temperature has been changed from <NUM> to <NUM>, and about <NUM> when the specific set temperature has been changed from <NUM> to <NUM>, respectively. Thus, it is understood from <FIG>, that the response of the ball temperature TB using the ball temperature TB as a control signal may be significantly faster compared with using the monitoring temperature Tth monitored by the thermistor <NUM>.

As mentioned above, the present invention has been described on the basis of the first and second embodiments. However, the discussions and drawings that configure a part of this disclosure should not be understood to limit the present invention. From this disclosure, various variations, implementations and operational techniques would be evident for one skilled in the art.

In the first and second embodiments, the temperature control unit <NUM> is used for controlling temperature of the ball sensor <NUM>. However, when measurement is implemented at room temperature or in the temperature controlled chamber, the temperature control for the ball sensor <NUM> is not always necessary. In such cases, the measurement system may include the sensor unit <NUM> and the signal processing unit <NUM>.

Claim 1:
A system for measuring a gas concentration, comprising:
a ball sensor (<NUM>) having:
a piezoelectric ball (<NUM>),
a sensor electrode (<NUM>) configured to generate a collimated beam (<NUM>) of a surface acoustic wave including a fundamental wave of a first frequency and a harmonic wave of a second frequency, which propagates through an orbital path on the piezoelectric ball (<NUM>), and
a sensitive film (<NUM>) deposited on the piezoelectric ball (<NUM>), configured to adsorb a target gas, the sensitive film (<NUM>) is arranged in a position where the collimated beam (<NUM>) of the surface acoustic wave passes through; and
a signal processing unit (<NUM>) having:
a signal generator configured to transmit a burst signal to the sensor electrode (<NUM>) so as to excite the collimated beam (<NUM>) propagating around the piezoelectric ball (<NUM>),
a signal receiver configured to receive 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>), and
a waveform data processor (<NUM>) configured to calculate the gas concentration of the target gas and the ball temperature by first and second relative changes in delay times of the first and second frequencies, respectively, using waveform data of the burst signals,
wherein, when calculating the gas concentration and the ball temperature, the waveform data processor (<NUM>) calculates first and second objective changes in delay times by the first and second relative changes, the first objective change due to the gas concentration, the second objective change due to the ball temperature.