JP4840257B2 - Signal processing device for VOC analyzer - Google Patents

Description

  The present invention relates to a signal processing apparatus for processing an output signal of a VOC analyzer, particularly a flame ionization type VOC analyzer that analyzes a sample gas based on a calibration curve created using zero gas and span gas.

  Air pollution related to suspended particulate matter and photochemical oxidants is still serious, and there are still concerns about the effects of suspended particulate matter on human health, and many health hazards due to photochemical oxidants have been reported. . One of the substances involved in the generation of these suspended particulate matter and photochemical oxidant is volatile organic compounds (VOC). VOC is a general term for organic compounds that are volatile and become gaseous in the atmosphere, and include a wide variety of substances such as toluene, xylene, and ethyl acetate. In order to suppress this VOC emission, in Japan, not only regulation of hydrocarbon emission from automobiles but also regulation of VOC emission and scattering from fixed sources such as factories are being promoted.

  Specifically, regulations are particularly imposed on facilities installed in factories and business establishments that emit a large amount of VOCs that cause air pollution. In such a situation, the importance of measuring TVOC, which is the total amount of VOC, is increasing.

One of such TVOC measuring apparatuses is a flame ionization type VOC analyzer (hereinafter referred to as “FID type VOC analyzer”). The FID-type VOC analyzer measures the hydrocarbon concentration in the sample gas based on the amount of ions generated by burning the sample gas with a hydrogen flame. Notification No. 61 of the Ministry of the Environment (June 2005) 10th) is a VOC analyzer.
FIG. 4 is a diagram showing a schematic configuration of a conventional FID type VOC analyzer. For example, sample gas such as room air is introduced into the flame ionization detector (FID) 12 through the introduction flow path 10 by a pump (not shown).

  The output of the hydrogen flame ionization detector 12 is an ionic current generated by burning organic substances in the hydrogen flame. After being converted into a voltage signal by the electrometer 14, the signal processing unit passes through the A / D converter 16. 18 is input. A calibration curve is stored in the signal processing unit 18, and the hydrocarbon concentration in the sample gas is obtained based on the calibration curve and the output from the flame ionization detector 12.

  The calibration curve is created using a zero gas not containing hydrocarbons and a span gas having a known hydrocarbon concentration. A switching valve 20 is provided in the introduction flow path 10, and the type of gas introduced into the detector 12 is switched by the switching valve 20 to one of sample gas, zero gas, and span gas. The switching valve 20 is controlled by an analysis control unit 22.

In general analyzers including the above-mentioned FID type VOC analyzers, there are noises with a relatively high frequency such as intrinsic noise generated in electronic circuits such as detectors and A / D converters, and external noise. To do. Therefore, an appropriate filter process for removing high-frequency noise is performed in the signal processing unit. For example, Patent Document 1 discloses a method for determining a cutoff frequency in accordance with chromatographic analysis conditions such as column dimensions and column oven temperature when filtering an output signal of a detector in a chromatographic signal processing apparatus. Is described.
Japanese Patent No. 3780634

  Generally, when the cutoff frequency of the filter process is lowered, noise is reduced and detection accuracy is improved. However, if the cut-off frequency is lowered too much, it takes time until the detection signal is stabilized.

  The problem to be solved by the present invention is to provide a signal processing apparatus for a VOC analyzer that can obtain an accurate hydrocarbon concentration by performing an appropriate filtering process and that can shorten the signal processing time. It is.

In order to solve the above-mentioned problems, the present invention provides a method for combusting hydrocarbons contained in a sample gas with a hydrogen flame, and detecting the amount of ions generated by a detector to detect the total volatile content contained in the sample gas. In a signal processing apparatus for a VOC analyzer that measures the amount of organic compounds,
a) a filter processing unit that cuts off a high-frequency region of the output signal of the detector at either a first frequency or a second frequency higher than the first frequency;
b) A calibration curve is created based on a processing signal in the filter processing unit when zero gas and span gas are introduced into the detector, and the filter processing when sample gas is introduced into the calibration curve and the detector. A signal processing unit for detecting a hydrocarbon concentration contained in the sample gas from a processing signal in the unit;
c) When the type of gas introduced into the detector is switched, the cutoff frequency of the filter processing unit is switched from the first frequency to the second frequency, and when a predetermined time elapses after the gas type is switched. Frequency switching means for switching the cutoff frequency of the filter processing unit from the second frequency to the first frequency;
It is characterized by providing.

  In the above configuration, the frequency switching means switches the cutoff frequency of the filter processing unit from the first frequency to the second frequency when the type of gas introduced into the detector is switched, It is also possible to configure so that the cutoff frequency of the filter processing unit is switched from the second frequency to the first frequency when the rate of change of the output signal becomes a predetermined value or less.

  When the type of gas introduced into the detector is switched, the level of the output signal of the detector fluctuates and then stabilizes at a level corresponding to the type of gas after switching. The measurement of the hydrocarbon concentration is carried out by processing the output signal of the detector after being stabilized in this way. In the signal processing apparatus for a VOC analyzer of the present invention, the output of the detector is increased by increasing the cutoff frequency of the filter processing unit until a predetermined time has elapsed after the type of gas introduced into the detector is switched. The time until the signal stabilizes to a constant value can be shortened. On the other hand, after the detector output signal is stabilized, the cutoff frequency is lowered, so that noise contained in the signal can be cut and the S / N ratio can be improved. For this reason, analysis time can be shortened, raising the measurement precision of hydrocarbon concentration. In addition, the amount of gas used for analysis can be reduced by shortening the analysis time.

  Several embodiments in which the present invention is applied to a flame ionization type (FID type) VOC analyzer will be described below. The feature of the present invention is that the cutoff frequency of the filter processing of the output signal of the detector in a predetermined period after switching the type of gas introduced into the flame ionization detector in the FID type VOC analyzer is set in other periods. That is, it is larger than the cutoff frequency.

FIG. 1 is a schematic configuration diagram of a FID type VOC analyzer according to a first embodiment of the present invention. The same parts as those in FIG.
In the FID type VOC analyzer of the present embodiment, the signal processing unit 18 is provided with first and second IIR (recursive) digital filters 31 and 32 in parallel, and the digital filters 31 and 32 are provided with an A / D converter 16. The digital signal from is input. The first and second digital filters 31 and 32 both remove high-frequency noise contained in the digital signal from the A / D converter 16, but the cut-off frequency of the first digital filter 31 is greater. It is set to a value higher than the cutoff frequency of the second digital filter 32. A switch 36 controlled by a signal processing control unit 34 is provided on the output side of the first and second digital filters 31, 32. The first and second digital filters 31, 32 are controlled according to the switching operation of the switch 36. One of the outputs of 32 is an output of the signal processing unit 18.

  The analysis control unit 22 outputs a synchronization signal to the signal processing control unit 34 when the switching valve 20 is switched to switch the flow path. Upon receiving the synchronization signal from the analysis control unit 22, the signal processing control unit 34 switches the switch 36 from S1 to S2 for a predetermined time, and then switches from S2 to S1. As a result, the filtering process is performed using the second digital filter 32 having a high cutoff frequency for a certain time after the type of gas introduced into the detector 12 is switched, and thereafter the first digital having a low cutoff frequency is used. Filter processing is performed using the filter 31. The length of the “certain time” in which the filtering process is performed using the second digital filter 32 is preset based on the time from when the type of gas is switched until the output signal of the detector 12 becomes stable. Yes, and stored in the signal processing unit 18.

The reason for changing the cut-off frequency in this way is as follows.
If the cut-off frequency is lowered, noise can be surely cut off and an accurate hydrocarbon concentration can be measured, but the detection signal is also attenuated. For this reason, it takes time until the output signal is stabilized, and the analysis time becomes long.
FIG. 2 is a diagram schematically showing the output signal of the flame ionization detector before and after the gas type switching. In FIG. 2, T1 indicates a period during which zero gas is introduced, and T2 indicates a period during which span gas or sample gas is introduced.

  In the FID type VOC analyzer, zero gas and span gas are sequentially introduced into the detector 12, and a calibration curve is created based on the respective outputs. On the other hand, when measuring the hydrocarbon concentration in the sample gas, the zero gas is introduced into the detector 12 for initialization, and then the sample gas is introduced. That is, zero gas and span gas / sample gas are alternately introduced into the detector 12. Thus, when the type of gas introduced into the detector 12 is switched between zero gas and span gas or sample gas, the level of the output signal of the detector 12 changes suddenly (see the period indicated by reference numeral A1 in FIG. 2). . Since the accurate value of the hydrocarbon concentration cannot be measured while the signal level changes suddenly, the hydrocarbon concentration is based on the output signal after the signal level is stabilized (see the period indicated by A2 in FIG. 2). Is measured.

  Therefore, in the present embodiment, in the period A1 in which the output signal changes suddenly, the filter processing cutoff frequency is increased to shorten the stabilization time, and in the period A2 after stabilization, the filter processing cutoff frequency is set. Lowered to ensure that noise is cut off. Thereby, the measurement time can be shortened while increasing the measurement accuracy of the hydrocarbon concentration. In FIG. 2, a solid line a and a broken line b indicate the difference in output waveform when the cut-off frequency is switched and when the cut-off frequency is not switched.

  In addition, it is possible to switch the cut-off frequency by providing one digital filter and changing the filter coefficient, but in this case, the output signal may vibrate when the filter coefficient changes. In contrast, in this embodiment, two digital filters 31 and 32 are provided in parallel, and the output of one of the filters is used as the output of the signal processing unit 18. For this reason, the coefficients of the digital filters 31 and 32 do not change, and the output of the signal processing unit 18 does not vibrate even when the cutoff frequency is switched.

FIG. 3 is a schematic configuration diagram of an FID type VOC analyzer according to the second embodiment of the present invention. In addition, the same code | symbol is attached | subjected to the same part as a 1st Example, and detailed description is abbreviate | omitted.
In the second embodiment, the output signal of the A / D converter 16 is sent to one of the first digital filter 31 and the second digital filter 32 and also sent to the change rate calculation unit 40. A threshold value is input to a digital filter to which the output signal of the A / D converter 16 is not input. Which of the output of the A / D converter 16 or the threshold value is input to the first and second digital filters 31 and 32 is switched by switches 41 and 42.

The output of the change rate calculation unit 40 is input to the comparator 44. The comparator 44 switches the output of the signal processing unit 18 according to the relationship between the threshold value and the output of the change rate calculation unit 40.
In the above configuration, the signal processing control unit 34 that has received the synchronization signal from the analysis control unit 22 switches the switch 36 from S1 to S2, switches the switch 41 from S3 to S4, and switches the switch 42 from S6 to S5. As a result, the output of the A / D converter 16 is input to the second digital filter 32, and the threshold value is input to the first digital filter 31. Further, the output of the second digital filter 32 becomes the output of the signal processing unit 18.

  Then, the first digital filter 31 calculates the output of the first digital filter 31 on the assumption that the threshold has been input from infinite time. Thereby, since the output of the first digital filter 31 does not change until the input of the first digital filter 31 is switched to the output of the A / D converter 16, the calculation in the first digital filter 31 can be stopped. That is, the two digital filters 31 and 32 need not always be operated.

  On the other hand, when the output rate of the change rate calculation unit 40, that is, the change rate of the output of the A / D converter 16, falls below the threshold value, the comparator 44 switches the output of the signal processing unit 18 to the first digital filter 31 and the signal processing unit. The input of the digital filter 31 selected as the output of 18 is switched to the output of the A / D converter 16. Further, the threshold value is input to the second digital filter 31 to which the output of the A / D converter 16 has been input. With such a configuration, the first digital filter 31 is forcibly initialized, and the output of the signal processing unit 18 can be stabilized in a short time.

The reason why the above configuration is adopted in this embodiment is as follows.
When the switching valve 20 is switched, the gas in the introduction flow path 10 is not immediately switched, and the gas before switching and the gas after switching are mixed for a while. Thus, the length of the period in which the gas before and after the switching is mixed varies depending on the type of gas (adsorbability), the length of the introduction flow path 10, the temperature, the humidity, and the like.
On the other hand, in the present embodiment, the cutoff frequency is increased until the rate of change of the signal input to the signal processing unit 18 becomes equal to or lower than the threshold after the type of gas introduced into the detector 12 is switched, and the change is made. The cut-off frequency is configured to be lowered when the rate falls below a certain value. For this reason, the cut-off frequency can be switched at an appropriate timing regardless of the type of gas, temperature, humidity, and the like.

In addition, the said Example is an example, Comprising: The following changes and corrections can be performed.
The time from when the cut-off frequency is lowered by switching the type of gas introduced into the detector 12 to when the cut-off frequency is raised may vary depending on the type of gas before and after the change.
In the above embodiment, two digital filters having different cut-off frequencies are provided, and the cut-off frequency in the filter processing is switched by using one of these filters as the output of the signal processing unit. A cut-off frequency may be switched by providing one variable digital filter.
In addition to the above, it is obvious that modifications, additions, and modifications as appropriate within the scope of the present invention are included in the scope of the claims of the present application.

1 is a schematic configuration diagram of a FID type VOC analyzer and a signal processing device thereof showing a first embodiment of the present invention. The wave form diagram of an output signal when the kind of gas introduce | transduced into a detector is switched. FIG. 1 is a view corresponding to FIG. 1 showing a second embodiment of the present invention. The schematic block diagram of the conventional FID type VOC analyzer and its signal processing apparatus.

Explanation of symbols

10 ... Introduction channel 12 ... Hydrogen flame ionization detector (FID)
DESCRIPTION OF SYMBOLS 14 ... Electrometer 16 ... A / D converter 18 ... Signal processing part 20 ... Switching valve 22 ... Analysis control part 31 ... 1st digital filter 32 ... 2nd digital filter 34 ... Signal processing control part 36, 41, 42 ... Switch 40 ... Change rate calculation unit 44 ... Comparator

Claims (2)

Signal processing for a VOC analyzer that measures the total amount of volatile organic compounds contained in the sample gas by burning hydrocarbons contained in the sample gas with a hydrogen flame and detecting the amount of ions generated by a detector In the device
a) a filter processing unit that cuts off a high-frequency region of the output signal of the detector at either a first frequency or a second frequency higher than the first frequency;
b) A calibration curve is created based on a processing signal in the filter processing unit when zero gas and span gas are introduced into the detector, and the filter processing when sample gas is introduced into the calibration curve and the detector. A signal processing unit for detecting a hydrocarbon concentration contained in the sample gas from a processing signal in the unit;
c) When the type of gas introduced into the detector is switched, the cutoff frequency of the filter processing unit is switched from the first frequency to the second frequency, and when a predetermined time elapses after the gas type is switched. Frequency switching means for switching the cutoff frequency of the filter processing unit from the second frequency to the first frequency;
A signal processing apparatus for a VOC analyzer, comprising: Signal processing for a VOC analyzer that measures the total amount of volatile organic compounds contained in the sample gas by burning hydrocarbons contained in the sample gas with a hydrogen flame and detecting the amount of ions generated by a detector In the device
a) a filter processing unit that cuts off a high-frequency region of the output signal of the detector at either a first frequency or a second frequency higher than the first frequency;
b) A calibration curve is created based on a processing signal in the filter processing unit when zero gas and span gas are introduced into the detector, and the filter processing when sample gas is introduced into the calibration curve and the detector. A signal processing unit for detecting a hydrocarbon concentration contained in the sample gas from a processing signal in the unit;
c) When the type of gas introduced into the detector is switched, the cutoff frequency of the filter processing unit is switched from the first frequency to the second frequency, and the rate of change of the output signal of the detector is a predetermined value or less. The frequency switching means for switching the cutoff frequency of the filter processing unit from the second frequency to the first frequency,
A signal processing apparatus for a VOC analyzer, comprising:

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