JPH09304278A - Infrared gas analyser - Google Patents

Abstract

PROBLEM TO BE SOLVED: To achieve the reduction of cost, high sensitivity, excellent reproducibility and the enhancement of stability by providing a quartz filter with a specific thickness before or behind a measuring filter. SOLUTION: A quartz filter with a thickness of 1mm in conventional technique is replaced with a quartz filter with a thickness of 3mm. By this constitution, the absorbancy of infrared rays of quartz is shifted toward a short wavelength and the main absorbing band of carbon dioxide is contained in the absorbing region thereof and the interference of carbon dioxide can be reduced as compared with a case using a filter composed of a PTFE plate. Therefore, it becomes unnecessary to use the PTFE plate and the quantity of infrared rays in the vicinity of 3.4μm being the main absorbing band of hydrocarbon increases. Further, the signal quantity of an HC detector 6 increases by about 30% and S/N is enhanced and cost is reduced. Further, the unstableness accompanying the strain of the PTFE plate at a time of high temp. and the deterioration of reproducibility is eliminated.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to an infrared gas analyzer for measuring harmful components such as hydrocarbons contained in automobile exhaust gas, combustion exhaust gas and the like.

[0002]

2. Description of the Related Art As a gas analyzer for measuring harmful components such as hydrocarbons contained in automobile exhaust gas and combustion exhaust gas, there is a non-dispersive infrared gas analyzer. This analyzer passes infrared rays emitted from an infrared light source through a measurement gas, measures the amount of absorption of infrared rays peculiar to the components contained in the gas, and analyzes the amount of the components.

FIG. 2 shows the structure of a conventional infrared analyzer for hydrocarbons. This analyzer is called a single beam system and has one infrared beam. The infrared light emitted from the infrared light source 1 is converted into an infrared beam by a mirror, an intermittent beam by a chopper 2, a quartz filter 3 having a thickness of 1 mm, and a measurement cell 4 (FIG. Then, it simply passes through cell 4) and has a thickness of 1 mm of tetrafluoroethylene (hereinafter, P
The hydrocarbon detector 6 (in FIG. 2, HFE in FIG. 2) is filled with n-hexane through a filter composed of a plate 5.
C detector 6). This HC detector 6
An AC signal having an amplitude corresponding to the absorption amount of infrared rays of a specific wavelength in the cell 4 and having the chopping frequency of the chopper 2 is output, and this AC signal is amplified by the amplifier 7 and becomes the final output.

The atmosphere to be analyzed contains a considerable concentration of water vapor and carbon dioxide gas, and the infrared absorption bands by both are as shown in FIG. 4 (a) to (e) show infrared absorption spectra of n-hexane and n-heptane among hydrocarbons, water vapor, carbon dioxide, quartz and PTFE, respectively. In the lower side, the wave number per 1 cm of infrared rays is shown, in the upper side, the corresponding wavelength is shown in μm unit, and the vertical axis shows the infrared transmittance. As can be seen from FIGS. 4 (a) and 4 (b), the infrared absorption band of water vapor largely overlaps with the infrared absorption band of hydrocarbons, forming a strong interference component, and accurate analysis cannot be performed unless this interference is compensated. . As a means for compensating for this water vapor interference, one is a hydrocarbon detector 6,
The structure shown in FIG. 3 is used. That is, this detector
Front expansion chamber through which infrared rays that have passed through cell 4 first pass
61 and a rear expansion chamber 62 that passes through later, and a microflow sensor 63 that detects the flow of gas flowing due to the pressure difference between the two chambers in the passage that connects the two chambers. The amount of absorption in the cell 4 is obtained from the gas flow caused by the pressure difference caused by the difference, and the gas component is analyzed. Therefore, the size and shape are determined so that the pressure change in the front expansion chamber 61 and the pressure change in the rear expansion chamber 62 that will pass therethrough that are caused by the absorption by the water vapor, which is an interference component, are almost the same, and the water vapor interference is compensated. To do. However, this measure alone cannot completely compensate for the absorption of water vapor over a wide wavelength range.
As a means of using, a 1 mm thick quartz filter is used, and infrared rays on the long wavelength side are cut off to minimize the overlapping portion between the base absorption of hydrocarbons and the absorption of water vapor, thereby improving the accuracy of compensation. [See FIG. 4 (d)].

Even if the water vapor interference is compensated in this manner, the carbon dioxide gas interference is not sufficiently compensated. In particular, the main absorption band near 4.2 μm has a very large absorption coefficient and remains as an interference component, which becomes a problem. A PTFE plate 5 is used for the compensation, and removes infrared rays in this wavelength region before reaching the HC detector 6 (see FIGS. 4C and 4E).

However, when trying to remove the interference of the main absorption band of carbon dioxide gas by the PTFE plate 5, as shown in FIG. 4 (e), the absorption in the vicinity of 4.2 μm of PTFE is not sharp, and it is different from other wavelength regions. Since the absorption ratios of the two are close to each other, in order to eliminate the interference of carbon dioxide gas, the infrared rays of other wavelengths are also largely absorbed at the same time, the total amount of infrared rays in the absorption band of hydrocarbons is reduced, and the sensitivity is low. It has the drawback of becoming Further, since the PTFE plate 5 has a large coefficient of thermal expansion, it has a drawback that the plate is distorted when the temperature changes and becomes high, resulting in poor reproducibility and stability.

[0007]

This invention is based on the above-mentioned P
With the use of the TFE plate 5, the total amount of infrared rays in the absorption band of the component to be measured is reduced and the sensitivity is reduced, and the reproducibility and stability are deteriorated due to the distortion of the PTFE plate 5 due to the temperature change. It is an object of the present invention to solve the above problems, to provide an infrared analyzer that is low in cost, high in sensitivity, and excellent in reproducibility and stability.

[0008]

In order to solve this problem, in the present invention, before or after the measuring cell,
It is equipped with a quartz filter with a thickness of 2.5 mm to 5 mm.
Since the infrared filter corresponding to the main absorption band of carbon dioxide can be largely removed by providing the quartz filter thicker than that of the conventional technique, the interference of carbon dioxide can be reduced.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION In the present invention, infrared rays in the vicinity of 4.2 μm, which is the main absorption band of carbon dioxide, are significantly reduced by using a thick quartz plate as a filter, and the interference of carbon dioxide is reduced. . FIG. 1 is a conceptual diagram showing an embodiment of an infrared analyzer for hydrocarbons according to the present invention. The difference from the infrared analyzer for hydrocarbons according to the prior art (see FIG. 2) is that a quartz filter having a thickness of 1 mm in the prior art is used. Replace 3 with a thick quartz filter 31 with a thickness of 3 mm,
That is, the filter made of the PTFE plate 5 is eliminated.

The inventor of the present invention, when the thickness of the quartz is increased, as shown in FIG. 4 (d), the infrared absorption edge of the quartz is shifted to the short wavelength side, and the main absorption band of carbon dioxide is It was found that they are almost included in the absorption region, and further that in the case of quartz, the absorption in the vicinity of 3.4 μm, which is the main absorption band of hydrocarbons, is much smaller than that of the filter made of the PTFE plate 5, By using this, PT
At the same time as eliminating the filter composed of the FE plate 5, the sensitivity of hydrocarbons can be increased (the signal amount of the HC detector 6 is increased by 20 to 40%), and the S / N ratio is also improved. Since quartz has a very small coefficient of thermal expansion, it was possible to solve the problem associated with the large coefficient of thermal expansion of the PTFE plate 5.

The most effective thickness of quartz is 3 to 4 mm. If it is 2.5 mm or less, the effect is not sufficient, and if it exceeds 5 mm, it becomes difficult to use because of its structure. In the above examples, the single-beam infrared spectrometer for hydrocarbons has been described, but in this case, the effect is most prominent, and therefore it is shown as an example. In order to remove the interference of carbon dioxide gas, It is needless to say that the fact that quartz is effective is the same as in the double-beam infrared analyzer, and is also effective when analyzing components other than hydrocarbons.

[0012]

According to the present invention, by increasing the thickness of the quartz filter 3 used in the prior art,
It is possible to reduce the interference of carbon dioxide gas to a level equal to or more than that when a filter made of the PTFE plate 5 is used.
Eliminates the need to use Therefore, the amount of infrared light in the vicinity of 3.4 μm, which is the main absorption band of hydrocarbons, is increased, the signal amount of the detector is increased by about 30%, the S / N ratio is improved, and the cost is reduced. Furthermore, the instability and poor reproducibility associated with the distortion of the PTFE plate 5 at high temperatures can be solved.

In this way, it is possible to provide an infrared analyzer which is low in cost, high in sensitivity, and excellent in reproducibility and stability.

[Brief description of drawings]

FIG. 1 is a conceptual diagram showing an embodiment of an infrared analyzer according to the present invention.

FIG. 2 is a conceptual diagram showing the structure of an infrared analyzer according to the prior art.

FIG. 3 is a conceptual diagram showing the structure of an infrared detector used in a single beam infrared analyzer.

FIG. 4 Hydrocarbon, water vapor, carbon dioxide, quartz and PTF
Infrared absorption spectrum of E

[Explanation of symbols]

 1 Infrared Light Source 2 Chopper 3, 31 Quartz Filter 4 Cell 5 PTFE Plate 6 HC Detector 61 Front Expansion Chamber 62 Rear Expansion Chamber 63 Micro Flow Sensor 7 Amplifier

Claims (3)

[Claims] 1. An infrared gas analyzer comprising an infrared light source, a measurement cell that allows the infrared light from the infrared light source to pass therethrough and also flows an analysis gas, and a detector that detects the infrared light that has passed through the measurement cell, An infrared gas analyzer comprising a quartz filter having a thickness of 2.5 mm to 5 mm before or after the measuring cell. 2. The infrared gas analyzer according to claim 1, wherein the number of measurement cells is one, and the detector has a front expansion chamber and a rear expansion chamber, and detects a pressure difference between both chambers. An infrared gas analyzer, which is a detector having a structure including a sensor. 3. The infrared gas analyzer according to claim 1 or 2, wherein a hydrocarbon detector is provided as the detector.