Gas analysis apparatus and gas analysis method

Abstract

The present invention is one adapted to correct the effect of a second gas component on a first gas component in real time even when the concentration of the second gas component as a coexistent component varies every moment, and includes: a first gas analysis part adapted to measure the concentration of the first gas component contained in sample gas; a second gas analysis part adapted to measure the concentration of the second gas component contained in the sample gas; a correction coefficient storage part adapted to store a correction coefficient for correcting the effect of the second gas component on the first gas component; and a concentration correction part adapted to correct the first gas component concentration on the basis of the correction coefficient, the second gas component concentration of calibration gas used for calibrating the first gas analysis part, and the second gas component concentration.

Claims

1. A gas analysis apparatus comprising: a first gas analysis part adapted to measure concentration of a first gas component in sample gas; a second gas analysis part adapted to measure concentration of a second gas component contained in the sample gas, wherein the first gas analysis part is calibrated with use of mixed gas containing known concentrations of the first gas component and the second gas component; a correction coefficient storage part adapted to store a correction coefficient for correcting error in measured concentration of the first gas component in the sample gas due to presence of the second gas component in the sample gas; and a concentration correction part adapted to correct, as the concentration of the second gas component in the sample gas varies, the error in the measured concentration of the first gas component on a basis of the correction coefficient, the known concentration of the second gas component in the mixed gas, and the measured concentration of the second gas component obtained by the second gas analysis part.

2. The gas analysis apparatus according to claim 1, wherein the concentration correction part corrects the error in the measured concentration of the first gas component further on a basis of a difference between the known concentration of the second gas component contained in the mixed gas and the measured concentration of the second gas component in the sample gas.

3. The gas analysis apparatus according to claim 1, wherein the correction coefficient indicates a relationship between a given concentration of the second gas component and a relative error of the concentration of the first gas component at the given concentration of the second gas component, the gas analysis apparatus further comprising a correction coefficient changing part adapted to shift and change the correction coefficient to make the concentration of the second gas component at which the relative error of the concentration of the first gas component is zero equal to the concentration of the second gas component of the mixed gas, wherein the concentration correction part corrects the error in the measured concentration of the first gas component with use of the correction coefficient after the change by the correction coefficient changing part and the measured concentration of the second gas component.

4. The gas analysis apparatus according to claim 1, wherein the first gas analysis part and the second gas analysis part include a detector using an NDIR method.

5. The gas analysis apparatus according to claim 1, wherein one of the first gas component and the second gas component is carbon monoxide (CO), and the other of the first gas component and the second gas component is carbon dioxide (CO2).

6. The gas analysis apparatus according to claim 1, wherein the sample gas is exhaust gas discharged from an internal combustion engine.

7. A gas analysis method using a first gas analysis part adapted to measure concentration of a first gas component in sample gas and a second gas analysis part adapted to measure concentration of a second gas component in the sample gas, the method comprising: correcting, as the concentration of the second gas component in the sample gas varies, error in the measured concentration of the first gas component obtained by the first gas analysis part on a basis of a correction coefficient, a known concentration of the second gas component contained in mixed gas used for calibrating the first gas analysis part, and the measured concentration of the second gas component obtained by the second gas analysis part, wherein the correction coefficient accounts for error in the measured concentration of the first gas component in the sample gas due to presence of the second gas component in the sample gas; and calibrating the first gas analysis part with mixed gas containing known concentrations of the first gas component and the second gas component.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a schematic diagram illustrating the configuration of a gas analysis apparatus according to the present invention;

(2) FIG. 2 is a functional configuration diagram of a calculation device in the same embodiment;

(3) FIG. 3 is a diagram illustrating relational expressions representing correction coefficients before and after change in the same embodiment; and

(4) FIG. 4 is a diagram illustrating the relative errors of CO concentration before and after correction in the same embodiment.

DESCRIPTION OF EMBODIMENTS

(5) In the following, one embodiment of a gas analysis apparatus according to the present invention will be described with reference to the drawings.

(6) <Apparatus Configuration>

(7) A gas analysis apparatus 100 of the present embodiment is one adapted to analyze multiple gas components contained in exhaust gas discharged from an exhaust gas source such as an engine. In the present embodiment, the gas analysis apparatus 100 is one that, using a non-dispersive infrared absorption method (NDIR method), simultaneously measures the multicomponent gas containing carbon monoxide (CO) as a first gas component and carbon dioxide (CO.sub.2) as a second gas component both contained in engine exhaust gas. The gas analysis apparatus 100 does not have to be one adapted to simultaneously measure the multicomponent gas but may be one using an optical absorption method other than the NDIR method, such as an FTIR method.

(8) Note that CO.sub.2 as the second gas component exerts a coexistent effect on CO as the first gas component in the NDIR method. That is, the infrared absorption spectrum of CO as the first gas component broadens because wavenumbers are shifted by the intermolecular interaction of CO.sub.2.

(9) Specifically, as illustrated in FIG. 1, the gas analysis apparatus 100 includes: a first gas analysis part 2 adapted to continuously measure the concentration of CO contained in the engine exhaust gas; a second gas analysis part 3 adapted to continuously measure the concentration of CO.sub.2 contained in the engine exhaust gas; and a calculation device 4 adapted to acquire outputs from the respective analysis parts 2 and 3 and calculate the concentrations of CO and CO.sub.2 contained in the engine exhaust gas.

(10) The first gas analysis part 2 and the second gas analysis part 3 are ones including NDIR detectors, and configured using a single shared cell. Specifically, the analysis parts 2 and 3 include: the measurement cell 10 into/from which the engine exhaust gas is introduced/led out; an infrared ray irradiation part 11 adapted to irradiate the measurement cell 10 with infrared light, such as an infrared light source; and the infrared detectors 12 adapted to detect infrared rays having passed through the measurement cell 10.

(11) The infrared detectors 12 in the present embodiment are pyroelectric infrared detectors, and include a detector 12a for CO measurement and a detector 12b for CO.sub.2 measurement. In addition, the infrared detectors 12 also include a detector 12c for hydrocarbon (HC) measurement and a detector 12d for a comparison signal. Between the respective detectors 12a to 12d and the measurement cell 10, optical filters 13a to 13d are provided, and the respective optical filters 13a to 13d have different transmission characteristics, and correspond to absorption wavelengths of CO, CO.sub.2, and HC, and a reference wavelength at which any of them does not cause absorption. Note that as the infrared detectors 12, in addition to the pyroelectric infrared detectors, pneumatic cell infrared detectors, detectors using lead selenide, thermopile detectors, or the like can be used.

(12) The calculation device 4 is a dedicated or general-purpose computer including a CPU, a memory, an input/output interface, an AD converter, and the like, and in accordance with an analysis program stored in the memory, calculates the CO concentration, the CO.sub.2 concentration, and HC concentration.

(13) Specifically, the calculation device 4 is one adapted to acquire output signals (light intensity signals) from the infrared detectors 12 constituting the first and second gas analyzers 2 and 3, and using absorption spectra obtained from the light intensity signals from the respective detectors 12a to 12d, calculate the CO concentration, CO.sub.2 concentration, and HC concentration.

(14) In addition, the calculation device 4 has a function of correcting the coexistent effect of the CO.sub.2 as the second gas component on the CO as the first gas component, and in accordance with the analysis program stored in the memory, as illustrated in FIG. 2, fulfills functions as a concentration calculation part 40, a correction coefficient storage part 41, a correction coefficient changing part 42, a concentration correction part 43, and the like.

(15) The concentration calculation part 40 is one adapted to calculate the CO concentration, CO.sub.2 concentration, and HC concentration using the absorption spectra obtained from the light intensity signals from the respective detectors 12a to 12 d.

(16) The correction coefficient storage part 41 is one adapted to store a correction coefficient for correcting the coexistent effect of the second gas component on the first gas component. Correction coefficient data indicating the correction coefficient is preliminarily inputted to the correction coefficient storage part 41 before product shipment or before product operation.

(17) Note that as illustrated in FIG. 3, the correction coefficient is one indicating the relationship between CO.sub.2 concentration and the relative error of CO concentration at the CO.sub.2 concentration. The saving format of the correction coefficient may be a function format indicating the relationship, or a table format such as a lookup table.

(18) Also, the relative error of CO concentration at the CO.sub.2 concentration refers to a ratio of an error to CO concentration (exact value) in the absence of the coexistent effect of CO.sub.2. The error is represented by the difference between the CO concentration in the absence of the coexistent effect of CO.sub.2 and the CO concentration in the presence of the coexistent effect of CO.sub.2.
Relative error=([CO concentration_.sub.under coexistent effect][CO concentration_.sub.not under coexistent effect])/[CO concentration_.sub.not under coexistent effect]100%

(19) The correction coefficient changing part 42 is one adapted to change the correction coefficient on the basis of second gas component concentration of calibration gas used for calibrating the first gas analysis part 2. Note that the infrared detectors 12 including the first gas analysis part 2 in the present embodiment are calibrated using mixed gas of a known concentration of CO and a known concentration of CO.sub.2 as the calibration gas. Also, data on the second gas component concentration of the calibration gas used for calibrating the first gas analysis part 2 is stored in the correction coefficient storage part 41 or another data storage part.

(20) For example, when the correction coefficient is one indicating that the relative error of CO concentration at a CO.sub.2 concentration of 0 (zero) is zero, the correction coefficient changing part 42 changes the correction coefficient in the following manner using the CO.sub.2 concentration of the calibration gas used for calibrating the first gas analysis part 2 as a parameter.

(21) When mixed gas of a known concentration of CO and a known concentration (e.g., 10% vol) of CO.sub.2 is used for calibrating the first gas analysis part 2 as the calibration gas, the calibration is performed such that the relative error becomes zero at the known CO.sub.2 concentration (10% vol). Accordingly, the correction coefficient changing part 42 changes the correction coefficient on the basis of the difference between the CO.sub.2 concentration (0% vol) at which the relative error of CO concentration is zero under the condition of the concentration coefficient and the CO.sub.2 concentration (10% vol) of the calibration gas used for calibrating the first gas analysis part 2. That is, the correction coefficient changing part 42 shifts and changes the correction coefficient such that at the CO.sub.2 concentration of the calibration gas used for the calibration, the relative error of the CO concentration becomes zero (see FIG. 3). Note that the CO.sub.2 concentration (0% vol) used for changing the correction coefficient, at which the relative error is zero, may be the CO.sub.2 concentration (0% vol) at which the relative error is within a predetermined range. The predetermined range is one selected by a user.

(22) For example, when under the condition of a correction coefficient before change, the relative error of CO at a CO.sub.2 concentration of 0% vol is 0%, and the relative error of CO at a CO.sub.2 concentration of 10% vol is 2.65%, under the condition of a correction coefficient after the change, the relative error of CO at a CO.sub.2 concentration of 0% vol is 2.65%, and the relative error of CO at a CO.sub.2 concentration of 10% vol is 0%.

(23) The concentration correction part 43 is one adapted to, using the correction coefficient after the change by the correction coefficient changing part 42 and the CO.sub.2 concentration calculated by the concentration calculation part 40 using the light intensity signals from the second gas analysis part 3, correct the CO concentration calculated by the concentration calculation part 40 using the light intensity signals from the first gas analysis part 2.

(24) Specifically, the calculation device 4 corrects the CO concentration on the basis of the functions of the correction coefficient changing part 42 and concentration correction part 43 in accordance with the following expression.
C(CO)_corr=C(CO)/(1+f(C(C.sub.2)))

(25) Here, f(C(CO.sub.2)) represents a function (the correction coefficient after the correction) indicating the relative error of the CO concentration, and
f(C(CO.sub.2)=K.sub.1C(CO.sub.2)C(CO.sub.2)+K.sub.2C(CO.sub.2){K.sub.1C(C.sub.2.sub._span)C(CO.sub.2.sub._span)+K.sub.2C(CO.sub.2.sub._span)}

(26) C(CO)_corr: CO concentration [% vol] at the time of actual measurement after the correction

(27) C(CO): the CO concentration [% vol] at the time of actual measurement before the correction

(28) C(CO.sub.2): the CO.sub.2 concentration [% vol] at the time of actual measurement

(29) C(CO.sub.2 .sub._span): Coexistent CO.sub.2 concentration [% vol] at the time of span calibration of the CO meter

(30) K.sub.1, K.sub.2: Coefficients obtainable by experiment (in the present embodiment, coefficients when the relationship between CO.sub.2 concentration and the relative error of CO concentration is approximated to a quadratic curve).

(31) FIG. 4 illustrates the relative errors of CO concentration before and after correction obtained when the span calibration of the NDIR detectors were performed using mixed gas having a CO.sub.2 concentration of 14% vol as the calibration gas.

(32) As can be seen from FIG. 4, it turns out that correcting the coexistent effect in accordance with the present embodiment allows the relative error of CO concentration after the correction to fall within a range of 1%.

Effects of the Present Embodiment

(33) The gas analysis apparatus 100 according to the present embodiment configured as described above corrects CO concentration obtained by the first gas analysis part 2 on the basis of a correction coefficient after change by the correction coefficient changing part 42 and CO.sub.2 concentration at the time of actual measurement obtained by the second gas analysis part 3, and can therefore accurately correct the coexistent effect of CO.sub.2 on CO in real time even when the concentration of CO.sub.2 as a coexistent component varies. Note that when using the infrared detectors 12, an interference effect may be caused by the superposition between a CO.sub.2 absorption spectrum and a CO absorption spectrum; however, the interference effect is small as compared with the above-described coexistence effect. In order to more accurately measure CO concentration, the interference effect of CO.sub.2 on CO may be further corrected in addition to the present embodiment. The interference effect can be corrected using a similar method to that for the coexistence effect by storing a correction coefficient for interference effect correction in the correction coefficient storage part 41.

Other Embodiments

(34) Note that the present embodiment is not limited to the above-described embodiment.

(35) For example, the above-described embodiment is one such that the calculation device 4 includes the correction coefficient changing part 42 and the correction coefficient changing part 42 changes a correction coefficient. However, without changing a correction coefficient, the concentration correction part 43 may correct first gas component concentration using as a parameter the difference between second gas component concentration at which the relative error of the first gas component concentration is zero under the condition of the correction coefficient and second gas component concentration of a calibration gas used for calibrating the first gas analysis part 2. That is, the concentration correction part 43 may be adapted to correct the first gas component concentration obtained by the first gas analysis part 2 on the basis of the unchanged correction coefficient and second gas component concentration obtained by the second gas analysis part 3, and further correct the corrected first gas component concentration on the basis of the difference between the second gas component concentration at which the relative error of the first gas component concentration is zero under the condition of the correction coefficient and the second gas component concentration of the calibration gas used for calibrating the first gas analysis part 2.

(36) For example, when the first gas component concentration obtained by the first gas analysis part 2 is 5% vol and the second gas component concentration obtained by the second gas analysis part 3 is 6% vol, the concentration correction part 43 obtains a relative error using the correction coefficient (e.g., the correction coefficient before the change in the above-described embodiment). In this case, the relative error is approximately +2%. Then, the concentration correction part corrects the first gas component concentration (5% vol) using the relative error (approximately 2%). Subsequently, the concentration correction part 43 further corrects the corrected first gas component concentration (approximately 4.9%) using a change in relative error (a shift amount: 2.65%) under the condition of the correction coefficient shifted on the basis of the difference between the second gas component concentration (0% vol) at which the relative error of the first gas component concentration is zero under the condition of the correction coefficient and the second gas component concentration (e.g., 10% vol) of the calibration gas used for calibrating the first gas analysis part 2. In this case, the further corrected first gas component concentration is approximately 5.37% vol.

(37) Also, the above-described embodiment is one adapted to analyze the engine exhaust gas, but besides may be adapted to analyze sample gas such as environmental gas.

(38) Further, in the above-described embodiment, the first gas analysis part 2 and the second gas analysis part 3 are configured using the single cell, but may be respectively configured as single component meters.

(39) In addition, the above-described embodiment is one adapted to correct the coexistent effect of CO.sub.2 on CO between the two components (CO and CO.sub.2). However, the present invention may be one adapted to correct the coexistent effect of CO on CO.sub.2, correct the coexistent effect between other two components (e.g., two components selected from CO.sub.2, H.sub.2O, HC, NO, SO.sub.2 or the like), or correct the coexistent effect among three components or more (three components or more selected from CO, CO.sub.2, H.sub.2O, HC, NO, SO.sub.2 or the like).

(40) Still in addition, the correction coefficient in the above-described embodiment indicates that at a CO.sub.2 concentration of 0% vol, the relative error of CO is 0%, but may be one indicating that at another CO.sub.2 concentration, the relative error of CO is 0%.

(41) Yet in addition, the present invention may be one such that without obtaining a correction coefficient after change by the correction coefficient changing part, a correction coefficient preliminarily shifted on the basis of second gas component concentration of a calibration gas is stored in the correction coefficient storage part 41, and the concentration correction part uses the changed correction coefficient stored in the correction coefficient storage part 41.

(42) The gas analysis apparatus may be configured as a vehicle-mounted type that is mounted in a vehicle, and analyzes exhaust gas discharged from an exhaust pipe of the vehicle in running. In the vehicle-mounted gas analysis apparatus as well, calibration is performed using mixed gas of known concentrations of CO, CO.sub.2, propane, and the like as calibration gas. Also, the gas analysis apparatus may be adapted to include a moisture meter and correct CO concentration or CO.sub.2 concentration using H.sub.2O concentration obtained by the moisture meter.

(43) The above-described embodiment is one adapted to correct the coexistent effect of CO.sub.2 on CO, but may be one adapted to correct an interference effect instead of the coexistent effect. The interference effect can also be corrected using a similar method to that for the coexistence effect by storing a correction coefficient for interference effect correction in the correction coefficient storage part 41.

(44) Besides, it goes without saying that the present invention is not limited to any of the above-described embodiment and variations, but can be variously modified without departing from the scope thereof.

REFERENCE SIGNS LIST

(45) 100: Gas analysis apparatus 2: First gas analysis part 3: Second gas analysis part 4: Calculation device 41: Correction coefficient storage part 42: Correction coefficient changing part 43: Concentration correction part