JPH0220935B2 - - Google Patents

Description

[Detailed Description of the Invention] The present invention relates to a methane gas concentration measuring method and a measuring device suitable for measuring the concentration of methane gas in locations where measurement points are far apart, such as LNG tankers, LNG tanks, and even coal mine shafts. .

Methane gas is extremely important as a fuel gas, and is contained in large amounts in natural gas and the like.
Particularly in recent years, as city gas has become more caloric, natural gas has been increasingly used as city gas.
Therefore, in order to prevent gas explosions caused by city gas leaks, there is an urgent need to develop a safety system that can reliably and quickly detect methane gas leaks in specific areas such as underground malls and high-rise buildings, and issue warnings. has been done.

Furthermore, methane gas is the main component of coal mine gas generated in coal mines, and a similar system is required to prevent gas explosions caused by coal mine gas or coal dust explosions triggered by these gases.

However, conventionally used methane gas sensors such as catalytic combustion type, thermal conduction type, and semiconductor type have insufficient gas selectivity and response due to their operating principles, and are sensitive to surrounding coexisting gases, temperature, and humidity. There were complaints about reliability.
Therefore, it is unsuitable for mining sites where measurement conditions are severe, and real-time measurement is also difficult. Moreover,
In the case of remote monitoring and telemetry, electrical signals are sent and received, so there are problems that cannot be ignored, such as false alarms caused by electromagnetic induction and accidents caused by cable damage.

In order to solve such problems, the present inventor first discovered that methane gas has characteristic absorption in the 1.6 μm band and 1.3 μm band, and that light in the 1.6 μm band and 1.3 μm band is We proposed a method for measuring methane gas concentration based on the band having the lowest transmission loss in optical fibers, and a device for the same, in Japanese Patent Application Nos. 166836-1983 and 86770-1988. In these measurement methods, light in at least one wavelength range of the 1.6 μm band and 1.3 μm band is transmitted to a measurement cell where atmospheric gas flows in and out through an optical fiber with low transmission loss, and the measurement cell collects methane gas. is the characteristic absorption wavelength of
The light after absorption at at least one wavelength of 1.666 μm and 1.331 μm is transmitted through an optical fiber with low transmission loss in at least one wavelength range of 1.6 μm band and 1.3 μm band. The light is sent to a transmission filter, separated into light at the absorption wavelength of the methane gas (measurement wavelength) and one light at another wavelength (reference wavelength), and the light at the measurement wavelength and the reference wavelength is sent to a photodetector, respectively. The intensity ratio of these lights is determined, and the methane gas concentration in the measurement cell is thereby measured.

However, although this measurement method solves the above-mentioned problems, there are cases in which it is difficult to determine whether the obtained measured value indicates a more reliable methane gas concentration.

In other words, when a small amount of hydrocarbon gas other than methane, such as propane, is mixed in the gas to be measured, these hydrocarbon gases have a diameter of around 1.6 μm or
Since it has a characteristic absorption near 1.3 μm, the obtained measurement value indicates that there are no hydrocarbon gases other than methane in the gas being measured, and that the concentration of only methane gas is being measured, or that the concentration of the above hydrocarbon gas is being measured. There were times when gas was mixed in and it was difficult to determine whether the concentration of a mixture with a hydrocarbon gas other than methane was being measured.

This invention was made in view of the above circumstances,
This method attempts to increase reliability by taking multiple pieces of light at a reference wavelength and taking the ratio of multiple pieces of light at the measurement wavelength.It is highly reliable even under severe measurement conditions and can perform real-time measurements. It is an object of the present invention to provide a method and apparatus for measuring methane gas concentration, which can perform extremely remote monitoring and measurement, is free from the risk of inducing accidents, and can determine the presence or absence of interference with hydrocarbon gases other than methane. This is the purpose.

Hereinafter, the present invention will be explained in detail with reference to the drawings.

This invention is based on the fact that optical fibers such as silica-based optical fibers, which have been developed in recent years for optical communications, have wavelengths
The transmission loss is extremely low at 1.0 to 1.8 μm, and there is characteristic absorption of methane gas around 1.3 μm and 1.6 μm within this wavelength range, and furthermore, at 1.3 μm and 1.6 μm, the transmission loss is extremely low.
This was done based on the knowledge that there is almost no absorption of water vapor (H ₂ O) and carbon dioxide (CO ₂ ) near the 1.6 μm characteristic absorption band of methane gas.

Figure 1 shows the wavelength of silica optical fiber from 0.6μm to 1.8μm.
3 is a graph showing transmission loss in a wavelength range of m. As is clear from this graph, the wavelength is 1.1~
At 1.7μm, the transmission loss is less than 1dB/Km, especially
It shows an ultra-low loss of 0.2dB/Km near 1.6μm. It can be seen that if such an ultra-low loss optical fiber is used as an optical transmission line, it is possible to measure the concentration of methane gas in a remote location by spectrophotometry.

Figures 2 and 3 show the characteristic absorption of methane gas, which is the subject of this invention, and the graph in Figure 2 shows the characteristic absorption of methane gas in the 1.33 μm band.
It can be seen that there is a strong absorption band at 1.331 μm. The graph in Figure 3 shows the characteristic absorption of methane gas in the 1.66 μm band, and it can be seen that there is a relatively strong and broad absorption band at 1.666 μm. In the vicinity of these two absorption bands, H ₂ O,
Another measurement confirmed that CO ₂ also has almost no characteristic absorption band.

However, as mentioned above, the 1.6μm band or 1.3μm band
It is known that there is a characteristic absorption band for hydrocarbon gases other than methane.

Therefore, for example, if a quartz-based optical fiber is used as an optical transmission line and at least one of the methane gas characteristic absorption wavelengths (measurement wavelengths) of wavelengths 1.666 μm and 1.331 μm is used, methane gas in a remote location can be coexisted with H ₂ O, It can be measured with high precision without being affected by CO ₂ , and in addition to the measurement wavelength, it is in the same wavelength band as the measurement wavelength, preferably in the vicinity of the measurement wavelength, and not at the measurement wavelength (the characteristic absorption wavelength of methane gas). ,
Also, select at least two or more reference wavelengths of light at wavelengths where almost no absorption of H ₂ O or CO ₂ occurs, that is, wavelengths excluding 1.350 to 1.393 μm, for example.
When you select the measurement wavelength of 1.331μm, use the two wavelengths before and after it, 1.30μm and 1.34μm, as the reference wavelength.
In addition, when 1.331 μm and 1.666 μm are selected as measurement wavelengths, 1.30 μm and 1.62 μm are selected as reference wavelengths. ratio or 1.331μ
By determining at least two or more ratios of the extinction ratio at m or at both wavelengths and comparing the methane concentration measured at 1.666 μm and the methane concentration measured at 1.331 μm, or, Since there are two or more reference wavelengths for one measurement wavelength (for example, 1.331 μm), multiple ratios of the light intensity at the measurement wavelength and the light intensity at the reference wavelength can be obtained.
By comparing a plurality of these, it is possible to know whether or not there is interference caused by other hydrocarbon gases.

Next, a light source that emits light in the near-infrared region with wavelengths of 1.3 μm and 1.6 μm will be described. As light sources in this wavelength range, conventionally well-known light sources such as gas mantles, Grover lamps, Nern lamps, tungsten lamps, xenon lamps, and heating wires can also be used, but they are easy to handle, durable,
Semiconductor laser diodes (LD), light emitting diodes (LED), etc. are used from the viewpoint of power consumption.

Although LDs can provide high output, their emission wavelength easily fluctuates depending on temperature and power supply voltage, and they are highly monochromatic, so advanced technology is required to use them for such purposes. On the other hand, although LED output is low, the emission spectrum is somewhat broad, so the wavelength stability is good, and it is easy to cover the characteristic absorption wavelength, making it easy to use and depending on the detection range of the target gas. Fully available. but,
When using an LED as a light source, the emission spectrum is broad, so wavelength selection is required. There are various types of wavelength selection, but here we decided to use an inexpensive band pass filter.

Here, the transmission width of a band-pass filter is generally wide, on the order of 1 to several nm, and if the spectral line of the object to be measured is narrower than this width, it is disadvantageous in terms of efficiency. However, in cases where the width is quite wide, such as 1.331 μm or 1.666 μm for methane gas, we will specifically consider below that even using such a band pass filter will be sufficient to improve the detection efficiency of the entire measurement system. I found it.

Figure 4 shows the intensity distribution of light after passing through the filter using a band-pass filter with a center wavelength of 1.6661 μm and a half-value width of 2 nm and a Gaussian distribution type transmission characteristic.The solid line shows the intensity distribution of light after passing through this filter. The dotted line represents the case where methane gas is not present in the measurement cell at a pressure of 20 Torr. It can be understood that the extinction ratio due to methane gas can be determined by dividing the difference in area between these two curves by the area surrounded by the dotted line.

In Figure 5, the center wavelength is 1.6661μm (A), 1.6666μm
(B) and 1.6656 μm (C) A graph showing the absorption ratio of the 1.666 μm absorption spectrum line of methane gas measured by changing the methane concentration using three types of bandpass filters with a half-width of 2 nm. It is something. The pressure of the mixed gas of methane gas and air was set to 1 atm, and the partial pressure (Torr) of the methane gas therein was varied. As is clear from the graph, if the center wavelength of the filter is different, the absorption ratio will change even if the partial pressure of methane gas is the same, and it can be seen that the filter (A) with a center wavelength of 1.6661 μm gives the highest absorption ratio.

In addition, Figure 6 shows three types of band pass filters with a center wavelength of 1.6661 μm and a half width of 1.5 nm (E), 2.0 nm (F), and 2.5 nm (G), which are the same as those shown in Figure 5. The absorbance ratio of methane gas was determined using the following conditions. This results in, for example, 3 Torr of methane gas in the air (equivalent to a concentration of approximately 6% of the lower explosive limit).
It can be seen that in order to detect this, it is sufficient to use a filter with a half width of 2.5 nm (G) and measure the extinction ratio of about 1.5%, that is, the decrease in light intensity. (However, the 6th
From the figure, it can be seen that the filter (E) has the highest sensitivity, but the filter with a narrow half-width is somewhat expensive.
In addition, since the filter (G) can also be used satisfactorily, I selected the filter (G). ) Furthermore, a similar study was conducted for city gas, which contains methane gas.
FIG. 7 is a graph when a mixture of city gas and air containing 20% methane gas is used as a sample, and the absorption ratio is measured by varying the amount of city gas in the mixture. Center wavelength 1.6661μ for band pass filter
m, and a half width of 2.0 nm is used.

From the above study results, we found that it is possible to quantify methane gas concentration by using a small LED as a light source and a band pass filter for wavelength selection. Also,
The characteristic absorption wavelength of methane gas is 1.666 μm.
By measuring the methane gas concentration from the extinction ratio of at least one measurement wavelength of 1.331 μm and a plurality of reference wavelengths, it is possible to detect interference with hydrocarbon gases other than methane gas. In other words, some hydrocarbon gases other than methane have characteristic absorption bands in the 1.6 μm band and 1.3 μm band, but because the molecular extinction coefficient at 1.666 μm and the molecular extinction coefficient at 1.331 μm are different, Also, by taking multiple reference wavelengths, the molecular extinction coefficients of hydrocarbon gases differ, so if hydrocarbon gases other than methane are mixed in the measured gas,
The methane concentration determined at 1.666 μm and the methane concentration determined at 1.331 μm no longer match, or the absorption ratio of light at one measurement wavelength is different from that at the reference wavelength, so the methane concentration is not the same. This discrepancy confirms interference by hydrocarbon gases other than methane.

What is shown in FIG. 8 is an example of a methane gas measuring device constructed based on the above findings. Reference numeral 1 in the figure is a light source made of a light emitting diode (LED). The 1.3 μm band and 1.6 μm band emitted by this light source 1
Light including at least one wavelength band of the m-band is sent through an optical coupler 2 to an optical fiber with low transmission loss, such as a quartz optical fiber 3, which is an optical transmission path. The silica-based optical fiber 3 has transmission characteristics as shown in FIG. 1, and has an extremely low loss of 1.1 to 1.7 .mu.m.Therefore, its length may be approximately several kilometers to 10 kilometers. Light from the quartz optical fiber 3 is sent into the measurement cell 4 via an optical coupler 4b. This measurement cell 4 is provided with optical couplers 4b and 4b' at both ends of a cylindrical body 4a, and the cylinder 4a is made of porous sintered metal to allow natural inflow and outflow of measurement gas. It is made of plastic foam with a continuous pore structure. This is a cell in which light of a specific wavelength is absorbed while the light is transmitted from optical coupler 4b to 4b' within the cell. Also, the optical path length of this measurement cell 4 (optical couplers 4b, 4
The distance between b' is not particularly limited, but is often set to 50 to 100 cm, for example. Also,
When the concentration of methane gas is low, a well-known multi-optical absorption cell can also be used. The light emitted from the measurement cell 4 is sent to a low transmission loss optical fiber, for example, a quartz optical fiber 5, via an optical coupler 4b'.
Similarly, this silica-based optical fiber 5 is one with low loss. The light that has passed through the optical fiber 5 is sent from an optical coupler 6 to a first beam splitter 7 composed of a half mirror, where it is first divided into two beams. The first beam 8 is sent to a first bandpass filter 9 and the second beam 10 is sent to a second beam splitter 11, where two further beams are formed: a third beam 12 and a fourth beam splitter 11. The light beam is divided into 13 beams. The third beam 12 is sent to a second bandpass filter 14, and the fourth beam 13 is sent to a third bandpass filter 15, respectively.

These filters 9, 14, and 15 are all interference filters that utilize the light interference effect of thin films, and multilayer film interference filters are preferably used.
The transmittance at the center wavelength is as high as possible, and the half-value width is
A narrow one of 1.0 to 2.0 nm is desirable. The center wavelength of the first filter 9 is 1.3312 μm,
The center wavelength of the second filter 14 is a wavelength other than the absorption wavelength of methane, and is 1.30 μm, which shows almost no characteristic absorption even in H ₂ O and CO ₂ .

Further, the third filter 15 is also for the reference wavelength, and its center wavelength is 1.34 μm. As a result, the light transmitted through the first filter 9 has a wavelength of 1.3312 μm whose intensity has been reduced due to absorption by methane gas.
The transmission wavelength distribution centered on 1.331 μm becomes Gaussian-shaped light, and the light transmitted through the second and third filters 14 and 15 has no relation to the absorption of methane gas, and moreover, Also has a center wavelength, and the wavelength distribution becomes Gaussian-shaped light.
These lights are transmitted through the first, second, and
The signals are sent to third photodetectors 16, 17, and 18, converted into electrical signals, amplified by amplifiers 19, 20, and 21, and then sent to a signal processing device 22 comprised of a microcomputer and the like.

In the arithmetic processing unit 22, the electrical signal detected by the first photodetector 16 and the second photodetector 1 are processed.
The electrical signal detected in step 7 is compared and the wavelength
The extinction ratio A of methane is determined from the intensity ratio of light at a wavelength of 1.3312 μm and a wavelength of 1.30 μm, and calculation processing is performed using the relationship between the absorption ratio A and the methane gas concentration determined in advance using standard methane gas. The measured concentration of methane gas in the gas present at 1.3312 μm is determined. At the same time, the electrical signal detected by the first photodetector 16 and the third photodetector 18
The electrical signal detected at
The absorption ratio A' of methane is determined from the intensity ratio of light at wavelengths of 1.3312 μm and 1.34 μm, and the measured concentration is determined in the same manner. These two measured concentrations are further compared with each other, and if they are the same within the error range, the result is displayed on the display 23 as the methane gas concentration of the gas within the measurement cell 4. Further, if there is a difference of more than a predetermined value between the two, the gas in the measurement cell 4 contains a hydrocarbon gas other than methane, or the part of the measurement device after the optical coupler 6; Beam splitter 7,
11. This means that an abnormality has occurred in the bandpass filters 9, 14, 15, the photodetectors 16, 17, 18 and the amplifiers 19, 20, 21, so a message to that effect is displayed on the display 23. Note that the optical coupler 6 and the first
A test light source is provided between the beam splitter 7 and the test light source to block the light from the optical coupler 6 in the event of an abnormality, and to make the test light source emit light so that an abnormality in the measuring device itself can be determined. For example, interference by hydrocarbon gases other than methane can be confirmed.

FIG. 9 shows another example of the measuring device of the present invention, in which the same components as those shown in FIG. 8 are given the same reference numerals and their explanation will be omitted. In this example, the light exiting the measurement cell 4 passes through a low-loss optical fiber 5 such as a silica-based optical fiber, and is divided into three beams by an optical branch 24, each of which is connected to an optical coupler 25, 26. , 27 through the filter 28, the first filter 9, the second filter 14,
The electrical signals sent to the third filter 15, the first photodetector 16, and the second photodetector 17 are sent to the amplifier 29, and the electrical signals from the first photodetector 16 and the second photodetector 17 are sent to the
The difference from the previous example is that the electrical signals from the and third photodetector 18 are sent to the amplifier 30. In this example, the photodetectors 16 and 1 are detected by the chopper 28.
The electrical signals from 7 and 18 become alternating current, which has the advantage of being easy to amplify.

Note that the example is not limited to the above example, and the light from the light source 1 is divided into a plurality of lights by an optical branch path, and these lights are sent to a plurality of measurement cells 4 through separate silica-based optical fibers 3, so that the light can be measured at a plurality of points. It can also be configured to measure methane gas at the same time.

As explained above, according to the methane gas concentration measuring method and measuring device of the present invention, the characteristic absorption band of methane gas includes the lowest loss wavelength region of optical fiber and the absorption band of CO ₂ and H ₂ O. We selected at least one or more absorption bands of the 1.33 μm band and 1.66 μm band, which almost never exist, and used a low-loss optical fiber as the optical transmission line and a small and inexpensive band pass filter for wavelength selection. Methane gas is determined by determining the extinction ratio using one or more μm wavelength bands and at least two or more reference wavelengths.
The measurement cell can be installed at an extremely remote location, without being subjected to electromagnetic induction or short-circuiting when the cable breaks, and is therefore useful for measuring methane gas concentration in tunnel gas in coal mines and in large areas such as underground shopping malls. This is suitable for cases where multiple measurement cells are installed in a region and central monitoring is performed at one location. Furthermore, it is highly accurate because it is hardly affected by H ₂ O and CO ₂ present in the measurement gas. Furthermore, the measurement wavelength is 1.331 μm.
By using 1.666 μm or both and using multiple reference wavelengths, the methane gas concentration can be determined individually. By comparing these measured values, it is possible to determine the concentration of methane gas in the gas being measured. It is possible to know whether hydrocarbon-based gas is mixed or not, and also to know if there is an abnormality in the measuring device itself, and furthermore, the reliability of the measured values themselves can be improved. Also, since it is a spectrophotometric method,
Real-time measurement is possible, making it possible to quickly respond to changes in methane concentration. Furthermore, since a bandpass filter is used for wavelength selection, the device can be made smaller and less expensive. Furthermore, high-sensitivity detection of methane below the explosion limit can be achieved even by using small-sized, low-power, low-output light emitting diodes that do not require cooling or the like.

[Brief explanation of drawings]

Figure 1 is a graph showing the transmission loss of the silica optical fiber used in this invention, Figure 2 is the absorption spectrum of methane gas in the 1.33 μm band, Figure 3 is the spectrum of methane gas in the 1.66 μm band, and Figure 4 is the Gaussian spectrum. Figure 5 is a graph showing the intensity distribution of light transmitted through a distributed bandpass filter. Figure 6 is a graph showing the relationship between the concentration of methane gas and the absorption ratio when three types of band transmission filters with different half widths are used, and Figure 7 is a graph showing the relationship between the concentration of city gas in the air and the absorption ratio using a band transmission filter. 8 and 9 are graphs showing the relationship between methane gas concentration in city gas and FIG. 9, which are both schematic configuration diagrams showing examples of the methane gas measuring device of the present invention. DESCRIPTION OF SYMBOLS 1... Light source, 3... Quartz-based optical fiber, 4... Measurement cell, 5... Quartz-based optical fiber, 6... Optical coupler, 7...
First beam splitter, 9... First band pass filter, 11... Second beam splitter, 14...
Second band pass filter, 15... Third band pass filter, 16, 17, 18... Photodetector, 19,
20, 21...Amplifier, 22... Arithmetic processing unit, 23
...Indicator, 24...Optical branch path, 28...Chiyotsupa.

Claims (1)

[Scope of Claims] 1. A measurement cell in which atmospheric gas flows in and out of light including at least one wavelength band of 1.6 μm band and 1.3 μm band through an optical fiber with small transmission loss in these wavelength bands. The light after being transmitted to and absorbed by this measurement cell at at least one wavelength of 1.666 μm and 1.331 μm, which are the characteristic absorption wavelengths of methane gas, is
The absorbed light is sent to a bandpass filter through an optical fiber that has a small transmission loss in the wavelength band, and the measurement light of the absorption wavelength of the at least one methane gas and the absorption wavelength of this measurement light of only 1.666 μm are sent to the band pass filter. The absorption wavelength of the measurement light is 1.666μm and 1.666μm.
When the wavelength is 1.331 μm, the light is separated into at least one reference light of at least one wavelength from the 1.6 μm band and the 1.3 μm band, and the ratio of the intensity of the measurement light to the intensity of the reference light in the wavelength band to which this measurement light belongs is calculated. is determined, and the methane gas concentration in the measurement cell is measured from these intensity ratios, while the methane gas concentrations obtained from these intensity ratios are compared to detect the presence or absence of interfering gases other than methane gas. Methane gas concentration measurement method. 2. A light emitting source that emits light that includes at least one wavelength band of 1.6 μm band and 1.3 μm band, an optical fiber that transmits this light and has low transmission loss in these wavelength bands, and atmospheric gas flowing in and out of it. The measurement cell and the measurement cell measure 1.666 μm, which is the characteristic absorption wavelength of methane gas.
An optical fiber with low transmission loss in the wavelength band of the absorbed light for transmitting light absorbed at at least one wavelength of 1.331 μm, and a light for measuring the absorption wavelength of the at least one methane gas. When the absorption wavelength of this measurement light is only 1.666 μm, it is 1.6 μm.
From the m band, when only 1.331 μm, the 1.3 μm band is separated into reference light of at least two wavelengths,
The absorption wavelength of the above measurement light is 1.666μm and 1.331μm
In this case, there is a band-pass filter that separates reference light into at least one wavelength from each of the 1.6 μm band and 1.3 μm band, a detector that detects each of these lights and converts them into electrical signals, and a detector that detects each of these lights and converts them into electrical signals. The ratio of the electric signal to the electric signal of the reference light in the wavelength band to which this measurement light belongs is calculated to calculate the respective methane gas concentration, and these methane gas concentrations are compared to determine the presence or absence of interfering gases other than methane gas. A methane gas concentration measuring device comprising: an arithmetic processing device;