JPH01285840A - Gas sensor - Google Patents

Abstract

PURPOSE:To accurately detect even the concn. of gas having a wide absorption spectrum band, by measuring transmissivity difference using two oscillation modes. CONSTITUTION:In measuring gas having large transmissivity difference and a wide wavelength band in absorption spectrum such as c4H10, a laser current is set so as to extend over two oscillation modes M2, M3 oscillated by an oscillation control circuit 20. The difference (h) between the transmissivity K2 in the oscillation mode M2 and the transmissivity K3 in the oscillation mode M3 is measured by a signal processing circuit 21 to calculate the concn. proportional to said transmissivity difference (h). By this method, the transmissivity difference (h) can be accurately measured. Subsequently, the signal corresponding to the concn. calculated by the circuit 21 is supplied to a divider 13 to calculate the ratio of the signal on the side of gas 4 to be measure to the signal on the side of a reference gas cell 9 and the concn. of the gas is displayed on a display device 14.

Description

[Detailed Description of the Invention] [Summary] This invention relates to a gas sensor that measures the absorption spectrum of a gas to be measured in the atmosphere using measurement light from a semiconductor laser, and detects gas 11 degrees based on this absorption spectrum. With the aim of accurately detecting the concentration of gas species with a wide wavelength band with a large transmittance difference in This is different from the configuration in which the gas concentration of the gas to be measured is detected from the difference between the two.

[Industrial application field]

The present invention relates to a gas sensor that measures the absorption spectrum of a gas to be measured in the atmosphere using measurement light from a semiconductor laser, and detects the gas concentration based on this absorption spectrum.

In recent years, the management of harmful gases has been strictly regulated due to pollution prevention and the like, and the concentration of harmful gases has been measured in the field of such management. As an example of a gas sensor, a wavelength-tunable infrared laser diode (semiconductor laser) is used to project a laser beam with a continuously changing wavelength onto a gas to be measured, and obtain an absorption spectrum of the wavelength of the laser beam that is unique to that gas. There is a method that measures gas 8 degrees by measuring the difference in transmittance.

In this case, some gases have a wide wavelength band with a large transmittance difference, but the temperature of these gases can be measured as accurately as other gases. is necessary.

(Prior Art) As a gas sensor for measuring gas S degree using an infrared laser diode, the present applicant has previously proposed, for example, Japanese Patent Application No.
-105090 (Japanese Unexamined Patent Publication No. 62-261032) discloses a device that can stably measure concentration even with changes in the absolute temperature of the environment, and Japanese Patent Application No. 59-105881 (Unexamined Japanese Patent Application No. 60-60
No. 49038), we proposed a device that can measure concentration with high precision by eliminating the influence of speed fluctuations of a light chopper that intermittently blocks laser light and projects it onto the gas.

All of these methods measure the absorption spectrum of a gas to be measured in the atmosphere using measurement light, and measure the gas concentration based on this absorption spectrum.

FIG. 5 shows a configuration diagram of a general gas sensor υ. In the figure, the emitted light from a semiconductor laser 1 is made into parallel light by a lens 2, passes through a half mirror 3, a trace amount of gas to be measured 4 in the atmosphere or in a cell, and is focused by a lens 5 onto an infrared sensor 6. The light is then photoelectrically converted and input to the signal processing circuit 7. As shown in FIG. 6, the semiconductor laser 1 has multiple oscillation modes M1... by changing the laser current. M2゜M3. The wavelength can be continuously swept in ..., and the absorption spectra 1 to 1 of the measured gas as shown in
can be measured. The signal processing circuit J3 measures the difference between the maximum transmittance point and the minimum transmittance point at wavelength λ1, and outputs a temperature proportional to this difference.

On the other hand, the light reflected by the half mirror 3 is reflected by the mirror 8, and passes through the reference gas cell 9 (a capsule filled with a gas of the same type as the gas to be measured 4 and a known concentration in advance),
The light is focused by the lens 10 onto the infrared sensor 11, photoelectrically converted there, and input to the signal processing circuit 12 having the same function as the signal processing circuit 7. In the signal processing circuit 12, a signal for the reference gas cell 9 is calculated.

Each output of the signal processing circuits 7 and 12 is supplied to a divider 13, where the signals of the four gases to be measured are divided into reference gas cells 9II.
The ratio to the signal of 4 is subtracted, and the concentration is displayed on the display 14.

[Problem to be solved by the invention]

As shown in FIG. 7, the conventional measurement method focuses on one mode (for example, M
2) was used for measurement. In this case, gases such as CHa (methane gas) and H2O (water vapor) have a large transmittance difference in their absorption spectra and a narrow wavelength band (so-called narrow absorption spectrum width), so one oscillation -[-
Measuring can be carried out satisfactorily with only M2.

However, gases such as 04HIG (butane gas) have a large number of atoms in the molecule and a large molecular group. For this reason, as shown in Fig. 8, the width of the wavelength band with a large transmittance difference in the absorption spectrum becomes wide (the so-called absorption spectrum width is wide), so there is a problem that measurement cannot be performed using only one oscillation mode M2. Ta.

SUMMARY OF THE INVENTION An object of the present invention is to provide a gas sensor that can accurately detect the concentration of a gas species having a wide wavelength band with a large transmittance difference in its absorption spectrum.

[Means for Solving the Problems] FIG. 1 shows a transmittance characteristic diagram for explaining the present invention. The present invention provides two oscillation modes M2. M
The configuration uses two wavelengths spanning 3.3 degrees, and detects 8 degrees of the gas to be measured from the difference of 2 and 3 points in transmittance at each wavelength.

[For n]

As shown in Figure 1, the gas to be measured has a wide wavelength band with a large transmittance difference (the absorption spectrum is wide).
- M2. 2 and K3 are measured for each transmittance with M3 (-), the difference is measured, and the concentration of the gas to be measured is detected from the difference.

In this case, even gas species such as CaH+a, which have a wide absorption range, can be detected accurately.

(Example) FIG. 2 shows a configuration diagram of an example of the gas sensor of the present invention,
In the same figure, parts that have the same functions as those in FIG. 5 are given the same numbers and their explanations are omitted. is 2
The laser current is set so that it oscillates over one mode. 21 is a signal processing circuit that measures the difference in transmittance in two oscillation modes.For example, in FIG.
4I-ItOW- gas J, sea urchin absorption spectra where the transmittance difference is large is measured (at this time, the laser current is set in the oscillation mode control circuit 20 so as to span the two oscillation modes M2 and M3). In this case, if we reverse the relationship with the oscillation wavelength as shown in Fig. 3 (A>), we can see that in the conventional example b, the radar current is set so that only the oscillation mode M2 occurs, whereas in the present example In the invention, the laser current is set so as to span two oscillation modes M2 and M3, and in the signal processing circuit 21 shown in FIG. 2, as shown in FIG. The difference between the transmittance in the oscillation mode M3 and the transmittance in the oscillation mode M3 is measured, and the density proportional to this transmittance difference is calculated.

In this way, two oscillation modes M2. Since M3 is used to measure the transmittance of 2 and 3 at two wavelengths, the wavelength band where the transmittance difference is large is wide (absorption spectrum) like C4HIOW gas. ) When measuring gas, the difference in transmittance can be accurately measured 3. The signal corresponding to the concentration calculated by the signal processing circuit 21 is sent to the divider 1.
Here, as in the conventional example, the ratio of the signal on the gas to be measured 4 side to the signal on the reference gas cell 9 side is calculated by 4, and the concentration is displayed on the display 14.

In fact, as shown in FIG. 4<A), the laser current is periodically cut off and allowed to flow. In this case, as shown in Fig. 4 (B), the transmittance of the gas cell to be measured is measured by cutting off the laser current and measuring the sensor output B, while setting the laser current across the two oscillation modes. Measure the sensor υ output At. Here, the signal processing circuit 21.1
By calculating A/B in step 2, it is possible to prevent the influence of contamination of the window of the gas cell to be measured.

〔Effect of the invention〕

As explained above, according to the present invention, since the transmittance difference is measured using two oscillation modes, it is possible to accurately detect the concentration of a gas species having a wide absorption spectrum, such as CaH+O.

[Brief explanation of the drawing]

Fig. 1 is a transmittance characteristic diagram for explaining the present invention, Fig. 2 is a configuration diagram of an embodiment of the present invention, and Fig. 3 shows the relationship between laser current and oscillation wavelength in the present invention and the conventional method. Figure 4 is a diagram explaining the laser current and sensor output horn according to the present invention, Figure 5 is a configuration diagram of a general gas sensor, Figure 6 is a diagram showing the relationship between laser current and oscillation wavelength, Figure 7 shows that concentration can be detected with one oscillation mode (
Transmittance characteristic diagram, FIG. 8 is a transmittance special diagram showing that concentration cannot be detected in one oscillation mode. In the figure, 1 is a semiconductor laser, 4 is a gas to be measured, 9 is a reference gas cell, 12.21 is a signal processing circuit, 13 is a divider, 14 is a display, 20 is an oscillation mode control circuit, M2. M3 indicates the oscillation mode, K2 and 3 indicate the transmittance, and h indicates the transmittance difference. 7% /% "+ku"
of

Claims (1)

[Scope of Claims] A gas sensor that measures the absorption spectrum of a gas to be measured in the atmosphere using measurement light from a semiconductor laser, and detects the gas concentration of the gas to be measured based on the absorption spectrum, comprising: Oscillation mode (M_2, M_3
), and the gas concentration of the gas to be measured is detected from the difference (h) in transmittance (K_2, K_3) at each of the two wavelengths. gas sensor.