JPH05312708A - Distinguishing method of mixed gas - Google Patents

Abstract

PURPOSE:To obtain a distinguishing method of a mixed gas which can surely identify the kind and the concentration of a gas in the mixed gas. CONSTITUTION:A plurality of quartz oscillators each having a different kind of adsorption film 2 provided on the surface thereof are used. Each quartz oscillator is exposed to a gaseous sample. The saturating adsorption of each gas to be detected is predicted from the change with time of the resonant frequency with the use of the time constant of the gas for every quartz oscillator, so that the gas is identified from the saturating adsorption. Accordingly, the mixed gas can be identified correctly although it is conventionally difficult.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a mixed gas discrimination method for identifying the type and concentration of each gas to be detected in a mixed gas, and in particular, using a quartz oscillator having an adsorption film on the surface thereof, By determining the change in the resonance frequency of the crystal oscillator due to adsorption to the adsorption film, the type of each detected gas,
The present invention relates to a mixed gas discrimination method for identifying the concentration.

[0002]

2. Description of the Related Art In a fire alarm or a chemical sensor, it is necessary to detect and discriminate a small amount of a target gas with high sensitivity.
Conventionally, as a gas discrimination method in such applications,
There is a method of using a crystal oscillator provided with an adsorption film on the surface.
This method utilizes the fact that when the gas molecules to be detected are adsorbed on this adsorption film, the resonance frequency of the crystal oscillator changes in proportion to the mass change of the adsorption film. The type and concentration of the gas to be detected are identified. If the gas to be detected is contained in the mixed gas,
A plurality of crystal oscillators provided with different adsorption films are used, and identification is performed by a method such as pattern recognition from the value of each saturated adsorption amount.

[0003]

The above-mentioned conventional mixed gas discrimination method has the following drawbacks. That is,
In order to identify each gas to be detected in the mixed gas, the saturated adsorption amount of each sensor for the mixed gas whose type and concentration of the constituent gas are known must be prepared as a database, and each detected gas must be identified by pattern matching. I have to. However, since the pattern of a certain mixed gas and the saturated adsorption amount of each sensor for it does not correspond one-to-one, it is the current situation that the mixed gas cannot be accurately discriminated.

An object of the present invention is to provide a mixed gas discriminating method which can surely identify the type and concentration of the gas to be detected in the mixed gas.

[0005]

The mixed gas discrimination method of the present invention uses a plurality of crystal oscillators each having a different type of adsorption film on the surface thereof, and exposes each of the crystal oscillators to a gaseous sample, For each of the crystal oscillators, using the time constant of each detected gas, the saturated adsorption amount of each detected gas is estimated from the time change of the resonance frequency, and the detected gas is identified from each saturated adsorption amount. To do.

That is, the present invention uses a crystal oscillator provided with an adsorption film on the surface thereof, and obtains the change in the resonance frequency of the crystal oscillator due to the adsorption of the gas to be detected on the adsorption film, thereby In the mixed gas discrimination method for identifying the type of each gas to be detected, a plurality of crystal oscillators provided with adsorption films of different types respectively on the surface are used, and each crystal oscillator is exposed to a gaseous sample, Resonance frequency change for each crystal oscillator, approximated to the sum of the exponential function using each time constant obtained from the exponential function representing the time change of the resonance frequency for each gas to be detected, A mixed gas discrimination method characterized in that the saturated adsorption amount of each detected gas is estimated by obtaining the coefficient of each exponential function, and each detected gas is identified and quantified using the scatter diagram for a single gas. Is.

[0007]

The present invention has been made as a result of detailed examination of the adsorption process of the gas molecules to be detected on the adsorption film. Here, the operation of the present invention will be described by explaining the findings obtained by the present inventors.

Polychlorotrifluoroethylene (PCTF
The adsorption film was formed on the quartz oscillator by the high frequency sputtering of E), and the change of the resonance frequency of the quartz oscillator due to the adsorption of the gas molecules to be detected on the adsorption film was investigated. As described above, it is known that the resonance frequency of the crystal oscillator is proportional to the mass change of the adsorption film, that is, the mass of the adsorbed gas to be detected. As a result, when the gas to be detected is a normal organic compound such as various alcohols, aromatic compounds, and ketones, this adsorbed film in a state in which the gas to be detected is not adsorbed at all is exposed to the gas having a certain concentration. However, the time change m (t) of the resonance frequency is

[0009]

[Equation 1] Will be expressed as It is known that A in the above equation represents the saturated adsorption amount of the gas to be detected, and the time constant T is deeply related to the types of the adsorption film and the gas to be detected.

It has been found that the time change m (t) of the resonance frequency of the crystal oscillator with respect to the mixed gas (including n kinds of detected gases) is expressed by the sum of exponential functions as follows.

[0011]

[Equation 2] Here, since the time constant Ti of each detected gas does not change even in the mixed gas, the time constant Ti measured beforehand with respect to the standard sample of each detected gas is used, and Ai of the above equation, that is, each The saturated adsorption amount of the detected gas is A
Estimate under the condition that i ≧ 0.

From this knowledge, the saturated adsorption amount of each gas to be detected can be estimated by considering the time constant of the change of the resonance frequency of each gas to be detected, and the method of the present invention can discriminate the mixed gas which has been difficult in the past. It turns out that it will be possible.

[0013]

Embodiments of the present invention will now be described with reference to the drawings. FIG. 1 is a block diagram showing the configuration of an example of a gas discriminating apparatus used for carrying out the gas discriminating method of the present invention.

A plurality of crystal oscillators 3 are arranged in a sensor cell 1 in which a gas sample flows from a gas generator 8, and different gas adsorption films 2 are provided on the surface of each crystal oscillator 3. In addition, an oscillator circuit 4 is provided for each crystal oscillator 3.
Is provided, and the oscillation circuit 4 oscillates at the resonance frequency of the corresponding crystal oscillator 3. Each oscillator circuit 4
Are connected to the inputs of the analog switch 5 provided in common. This analog switch 5 operates in response to a signal from a computer 7 which will be described later to generate each oscillation circuit
For selecting one of them, the output of which is connected to the input of the frequency counter 6. The output of the frequency counter 6 is connected to the computer 7. The computer 7 switches the analog switch 5 at regular time intervals, tracks changes in the oscillation frequency of each oscillation circuit 4 measured by the frequency counter 6, and detects changes in the resonance frequency of each crystal oscillator 3 from each detected gas. The saturated adsorption amount of each gas to be detected is estimated using the time constant of
The type of the gas to be detected is identified. A display device 9 is connected to the computer 7. The gas adsorption film 2 can be formed by sputtering targeting graphite, polychlorotrifluoroethylene, or the like to cover the surface of the crystal oscillator 3.

Next, the operation of this gas discriminating apparatus will be described. With each oscillating circuit 4 in the operating state, the computer 7 sequentially switches the analog switch 5 at fixed time intervals, for example, at 0.12 second intervals. If the number of the crystal oscillators 3 is k, it becomes possible to take in data for a specific crystal oscillator 3 every 0.12 × k seconds. Considering that the adsorption time to the gas adsorption film 2 is generally several minutes, if k is up to about 10, it can be regarded as simultaneous measurement even if the quartz oscillators 3 are sequentially measured.

A gas sample containing a gas to be detected is sent out from the gas generator 8 into the sensor cell 1, and the gas inside the sensor cell 1 is replaced with the gas sample within a negligible time compared to the measurement time. Then, the gas molecules to be detected start to be adsorbed on the gas adsorption film 2 of each crystal oscillator 3, the resonance frequency of each crystal oscillator 3 is shifted according to the adsorption amount, and each oscillation circuit 4
The oscillation frequency of shifts. This oscillation frequency shift is proportional to the mass of the gas to be detected adsorbed on the gas adsorption film 2 of each crystal oscillator 3, but is detected as a change in frequency measured by the frequency counter 6, and each crystal oscillator 3
The data is sampled by the computer 7 every time. Then, the computer 7 tracks the time change of the frequency shift for each crystal oscillator 3, and uses the time constant based on the measurement performed on the standard sample in advance to calculate the saturated adsorption amount of each detected gas by the least square method. presume.

After that, the computer 7 calculates the saturated adsorption amount obtained from the measurement performed on the standard sample based on the scatter diagram obtained by the principal component analysis which is one of the multivariate analysis. The type of the detected gas is discriminated from the amount. After the detection gas is discriminated, the concentration of the detection gas is estimated from the estimated saturated adsorption amount based on the saturated adsorption amount obtained from the measurement performed on the standard sample.

These processes will be described with reference to the flowchart of FIG.

First, the saturated adsorption amount and time constant of each standard sample are obtained from the sensor response to each standard sample. A scatter plot is created from the obtained saturated adsorption amount by principal component analysis.

Next, the mixed gas to be measured is introduced into the sensor cell, and the sensor response to the mixed gas is obtained. Here, using the time constant of each standard sample, Ai in the sum of exponential functions is estimated by the method of least squares. It is determined whether Ai is greater than the threshold value di. When Ai ≦ di, the gas i is not discriminated. When Ai> di, plot Ai in the scatter plot previously created and determine if it is in the group of gas i. If the plot is not in a group, gas i
Will be absent, and gas i if present in the group
The presence of is determined. Gas A obtained from Ai thus obtained
The concentration of can be estimated.

Next, the results of examining the effectiveness of the present invention based on this embodiment will be described. The number of crystal resonators 3 is 6, and the gas adsorption film 2 is formed on the surface of each crystal resonator 3 by high frequency sputtering using graphite, polychlorotrifluoroethylene, polyethylene, or polytetrafluoroethylene targets. Was used. Each adsorption film is as follows. Adsorption membrane 1 Polychlorotrifluoroethylene 2 Mixture of polyethylene and polytetrafluoroethylene 3 Graphite 4 Mixture of polyethylene and graphite 5 Polychlorotrifluoroethylene (changed production conditions) 6 Polychlorotrifluoroethylene (changed production conditions) ) And, as the mixed gas, methanol (1800 pp
m) and acetone (200 ppm) mixed gas 1 and methanol (400 ppm) and benzene (1100 ppm) mixed gas 2 were used.

In FIG. 3, the groups of acetone (∘), benzene (∘), and methanol (□) were prepared in advance as standard samples (100
It is obtained by the principal component analysis with the characteristic value obtained by normalizing the saturated adsorption amount obtained from the measurement performed for the concentration (ppm to 3000 ppm). The adsorption amount of each crystal oscillator 3 for the mixed gas 1 and the mixed gas 2 is standardized by the sum of the total adsorption amount of each crystal oscillator 3, and the result of the principal component analysis is shown in FIG. It is an x in the figure. Even if this scatter diagram is analyzed, it is difficult to determine that the mixed gas 1 is the mixed gas 1 of methanol and acetone and the mixed gas 2 is the mixed gas of methanol and benzene. The saturated adsorption amount of each detected gas is estimated by the least square method using the time constant of each detected gas from the change in the frequency shift of each crystal oscillator 3 with respect to the mixed gas 1, and the principal component analysis is shown in FIG. FIG. Since the saturated adsorption amounts of methanol and acetone are correctly estimated, they are plotted as x in the groups of methanol and acetone, respectively.
For benzene, an incorrect position is plotted with x, which means that mixed gas 1 does not contain benzene. As for the mixed gas 2, as shown in FIG. 6, x is plotted in the group of methanol and benzene, and methanol and benzene were discriminated. On the other hand, x is not plotted in the acetone group, and it can be seen that acetone not contained in the mixed gas 2 was not discriminated.

From the above results, when the saturated adsorption amount of the gas to be detected contained in the mixed gas is estimated using the time constant as in the method of the present invention and the principal component analysis is performed, the adsorption amount of the conventional mixed gas is calculated. It was found that the distribution of the analysis results was clearly divided according to the type of the gas to be detected, as compared with the one obtained by the principal component analysis, and that the method of the present invention could favorably discriminate the mixed gas.

[0024]

As described above, the present invention uses a plurality of crystal oscillators each having a different type of adsorption film on the surface thereof, and uses the time constant of each gas to be detected for each crystal oscillator. Since the saturated adsorption amount of each gas to be detected is estimated from the time change of the resonance frequency and each gas to be detected is identified, there is an effect that the mixed gas, which has been difficult in the past, can be accurately identified.

[Brief description of drawings]

FIG. 1 is a block diagram showing a configuration of an example of a gas discriminating apparatus used for carrying out a gas discriminating method of the present invention.

FIG. 2 is a flowchart illustrating a gas discrimination method of the present invention.

FIG. 3 is a scatter diagram obtained by principal component analysis using a standardized saturated adsorption amount obtained from measurement performed on a standard sample (concentration of 100 ppm to 3000 ppm) as a characteristic value.

FIG. 4 is a characteristic value obtained by normalizing the adsorption amount of each crystal oscillator 3 with respect to the mixed gas 1 and the mixed gas 2 by the sum of the total adsorption amount of each crystal oscillator 3 and obtained by the principal component analysis. FIG.

FIG. 5: The saturated adsorption amount of each detected gas is estimated by the least square method from the change in the frequency shift of each crystal oscillator 3 with respect to the mixed gas 1 by using the time constant of each detected gas, and the principal component analysis is performed. It is the obtained scatter diagram.

FIG. 6 is a graph showing the saturated adsorption amount of each detected gas estimated by the least squares method from the change in the frequency shift of each crystal oscillator 3 with respect to the mixed gas 2 by using the time constant of each detected gas. It is the obtained scatter diagram.

[Explanation of symbols]

 1 Sensor Cell 2 Gas Adsorption Film 3 Crystal Oscillator 4 Oscillation Circuit 5 Analog Switch 6 Frequency Counter 7 Computer 8 Gas Generator 9 Display Device

Claims (1)

[Claims] 1. A quartz oscillator having an adsorption film on the surface thereof is used, and a change in resonance frequency of the quartz oscillator due to adsorption of the gas to be detected to the adsorption film is obtained to obtain each of the substances to be detected in the mixed gas. In the mixed gas discrimination method for identifying the type of gas, a plurality of crystal oscillators provided with adsorption films of different types on the surface are used, each crystal oscillator is exposed to a gaseous sample, and each crystal oscillator is exposed. The time variation of the resonance frequency is approximated by the sum of the exponential functions using the time constants obtained from the exponential function representing the time variation of the resonance frequency for each gas to be detected. A method for determining a mixed gas, wherein the saturated adsorption amount of each detected gas is estimated by obtaining a coefficient, and each detected gas is identified and quantified using a scatter diagram in the case of a single gas.