JPH0220934B2 - - Google Patents

Description

[Detailed Description of the Invention] The present invention provides 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 in LNG tankers, LNG tanks, and even in coal mine tunnels. Regarding.

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 the risk of 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 A method and device for measuring methane gas concentration based on the band having the lowest transmission loss of optical fiber was proposed in Japanese Patent Application No. 57-166836. This measurement method is applicable to 1.6 μm band or
Light in the 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 in this wavelength range, and the measurement cell absorbs 1.666 μm of methane gas.
m or light after absorption at 1.331μm
The light is sent to a bandpass filter using an optical fiber with low transmission loss in the wavelength range of 1.6 μm or 1.3 μm, and the light is separated into the absorption wavelength of the methane gas and other wavelengths, and the light of these two wavelengths is separated. The intensity ratio of these lights is determined by sending each light to a photodetector, thereby measuring the methane gas concentration in the measurement cell.

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 correctly indicates the 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,
It is highly reliable even under severe measurement conditions, can perform real-time measurements, can perform extremely remote monitoring and measurement, has no risk of causing accidents, and can determine the presence or absence of interference with hydrocarbon gases other than methane. The object of the present invention is to provide a method for measuring methane gas concentration and an apparatus therefor.

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 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,
Further measurements confirmed that the characteristic absorption band of CO ₂ was almost absent.

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 the characteristic absorption bands of methane gas at wavelengths of 1.666 μm and 1.331 μm are used, methane gas in a remote location can be almost unaffected by coexisting H ₂ O and CO ₂ . It can be measured with high accuracy without any problems, and the absorbance ratio at 1.666 μm and the absorbance ratio at 1.331 μm can be determined simultaneously.
By comparing the methane concentration measured at 1.666 μm and the methane concentration measured at 1.331 μm, it is possible to determine whether there is interference 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 bulbs, xenon lamps, and heating wires can be used, but they are easy to handle, durable, and low in consumption. Semiconductor laser diodes (LDs), light emitting diodes (LEDs), etc. are used in terms of power, etc.

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, LEDs have a low output, but their 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 sufficient depending on the detection range of the target gas. Available. However, the LED
When used 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 bandpass 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 1.6661μmA, 1.6666μm
This is 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 band pass filters with a half width of 2 nm at 1.6656 μm and B and 1.6656 μmC. 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 filter A with a center wavelength of 1.6661 μm gives the highest absorption ratio.

In addition, Figure 6 shows the extinction ratio of methane gas using three types of bandpass filters with a center wavelength of 1.6661 μm and a half-width of 1.5 nmE, 2.0 nmF, and 2.5 nmG under the same conditions as shown in Figure 5. It's what I asked for. As a result, for example, 3 Torr of methane gas in the air (corresponding to a concentration of about 6% of the lower explosive limit).
Since it shows the value obtained by taking the logarithm of the number, in the case of 3Torr, the position of log3=0.447 on the horizontal axis scale indicates 3Torr. ), it is sufficient to use a filter with a half-width of 2.5 nmG and measure the extinction ratio of approximately 1.5%, that is, the decrease in light intensity. (however,
From Figure 6, it can be seen that the E filter has the highest sensitivity, but the one with a narrow half-width value is somewhat expensive, and the G filter can also be used satisfactorily.
I selected the G filter. ) 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. For bandpass filters, the center wavelength
A material with a width of 1.6661 μm 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,
By measuring the methane gas concentration at 1.666 μm and 1.331 μm simultaneously, interference with hydrocarbon gases other than methane gas can be detected. 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 since the molecular extinction coefficient at 1.666 μm and the molecular extinction coefficient at 1.331 μm are different, If a hydrocarbon gas other than methane is mixed in, the methane concentration determined at 1.666 μm and the methane concentration determined at 1.331 μm will not match, and this discrepancy may indicate that the methane concentration determined at 1.331 μm is caused by a hydrocarbon gas other than methane. Obstruction can be confirmed.

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 emitted by this light source 1 and
The light in the 1.6 μm 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 line. 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 μm.
Km to 10 Km is acceptable. Light from the quartz optical fiber 3 is sent into the measurement cell 4 via an optical coupler 4b. This measurement cell 4 has optical couplers 4b and 4 at both ends of a cylindrical body 4a, respectively.
b' is provided, and the cylindrical body 4a is formed of porous sintered metal or plastic foam with an open pore structure so as to allow the natural flow in and out of the measurement gas. Further, the optical path length of the measurement cell 4 (the distance between the optical couplers 4b and 4b') is not particularly limited, but is often set to 50 to 100 cm, for example. Furthermore, when the concentration of methane gas is low, a well-known multi-optical path type absorption cell can also be used. The light emitted from the measurement cell 4 passes through an optical coupler 4b' and is connected to a low transmission loss optical fiber, for example, a quartz optical fiber 5.
sent to. Similarly, this quartz optical fiber 5 is also of 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 split into two beam splitters.
It is divided into two luminous fluxes. 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; a third beam 12 and a fourth beam splitter 11 are sent. The light beam is divided into 13 beams. The third beam 12 is sent to the second bandpass filter 14, and the fourth beam 13 is
The signals are respectively sent to a third band pass filter 15.

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. And the first
The center wavelength of the filter 9 is set to 1.6661 μm, and the center wavelength of the second filter 14 is set to 1.3312 μm, or the reverse combination thereof is used. Also,
The center wavelength of the third filter 15 is set to, for example, 1.62 μm or 1.30 μm, which does not exhibit characteristic absorption of moisture or carbon dioxide at wavelengths other than the absorption wavelength of methane. Below, the center wavelength of the first filter 9 is
1.6661 μm, the center wavelength of the second filter 14 is
1.3312μm, the center wavelength of the third filter 15 is
This will be explained assuming that it is 1.62 μm. As a result, the first filter 9 and the second filter 14
The transmitted light becomes Gaussian-shaped light with a transmission wavelength distribution centered at 1.6661 μm or 1.3312 μm, whose intensity has decreased due to absorption of methane gas, and
The light transmitted through the filter 15 has a Gaussian wavelength distribution with a center wavelength of 1.62 μm, which is unrelated to absorption of methane gas. These lights are transmitted to first, second, and third photodetectors 16, each of which is composed of an avalanche photodiode (APD) or a photodiode (PD) (for example, a Ge semiconductor).
17, 18, converted into an electrical signal, amplified by amplifiers 19, 20, 21, and then sent to an arithmetic processing unit 22 composed of a microcomputer or the like. The arithmetic processing unit 22 divides the electrical signal detected and converted by the first photodetector 16 by the electrical signal detected and converted by the third photodetector 18, and calculates methane at a wavelength of 1.6661 μm. Absorption ratio A
At the same time, the electrical signal detected and converted by the second photodetector 17 is
Absorption ratio of methane at a wavelength of 1.3312 μm divided by the electrical signal detected and converted by the third photodetector 18
This is to find A′.

In addition, using this measuring device in advance, we measured the wavelength of 1.6661μ for several standard methane gases with different concentrations.
The absorption ratios B and B' at m and wavelength of 1.3312 μm are determined respectively. Then, by inputting the relationship between the concentration of methane gas as the standard gas and the determined absorption ratios at wavelengths of 1.6661 μm and 1.3312 μm into the arithmetic processing unit 22, the concentration of methane gas in the gas existing in the measurement cell 4 can be determined. can be determined from the extinction ratios A and A'. 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. In addition, if there is a difference greater than a predetermined value between the two, the measurement cell 4
The gas inside contains a hydrocarbon gas other than methane, or the part of the measuring device after the optical coupler 6; beam splitter 7, 11, band transmission filter 9, 14, 15, photodetector 16. ,17,1
8. This means that an abnormality has occurred in the amplifiers 19, 20, and 21, so a message to that effect is shown on the display 23. Note that a test light source is provided between the optical coupler 6 and the first beam splitter 7, and when the above abnormality occurs, the light from the optical coupler 6 is blocked, and the test light source is made to emit light to complete the measuring device. If it is possible to determine whether there is an abnormality in itself, it will be possible to confirm interference caused by hydrocarbon gases other than methane.

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 branching path 4, each of which is connected to an optical coupler 25, 26. , 27, through the chopper 28, to the first filter 9, second filter 14, and third filter 15, and from the first photodetector 16 and the third photodetector 18. The signal is sent to the amplifier 29, and the electrical signals from the second photodetector 17 and the third photodetector 18 are sent to the amplifier 3.
The difference from the previous example is that it is sent to 0. 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 split 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, and the light is transmitted 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 absorber of methane gas has the lowest loss wavelength region of the optical fiber and the absorption band of CO ₂ and H ₂ O. We selected 1.33 μm and 1.66 μm, which are almost non-existent, and used a low-loss optical fiber as the optical transmission line and a small and inexpensive band pass filter for wavelength selection. Because it performs quantitative determination, the measurement cell can be installed at an extremely remote location, and it is not subject to electromagnetic induction or short-circuit accidents due to cable breakage. It is suitable for concentration measurement and when multiple measurement cells are installed in a wide area such as an underground mall for centralized monitoring 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, since the methane gas concentration is determined separately at two absorption wavelengths, by comparing these two measured values, it can be determined whether hydrocarbon gases other than methane are mixed in the measured gas. It is possible to know the abnormality of the measuring device itself, and furthermore, the reliability of the measured values themselves can be improved. In addition, since it is an absorption photometry 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 a small-sized, low-power, low-output light emitting diode that does 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. 1... Light source, 3... Quartz-based optical fiber, 4...
Measurement cell, 5... quartz optical fiber, 6... optical coupler, 7... first beam splitter, 9... first band pass filter, 11... second beam splitter, 14... ...Second band pass filter, 1
5...Third band pass filter, 16, 17, 1
8...Photodetector, 19,20,21...Amplifier,
22...Arithmetic processing unit, 23...Display device, 24...
...Light branch, 28... Chiyotupa.

Claims (1)

[Claims] 1. Light including the wavelength ranges of 1.6 μm band and 1.3 μm band is simultaneously transmitted through an optical fiber with small transmission loss in these wavelength ranges to a measurement cell into which atmospheric gas flows in and out, and this measurement 1.666 μm of methane gas in the cell
and 1.6 μm after absorption at 1.331 μm
It is sent to a band-pass filter by an optical fiber with low transmission loss in the m-band and 1.3μm wavelength range,
The light is separated into light having the absorption wavelength of the two methane gases and one light having the other wavelength, and the intensity ratio between the absorption wavelength of the one methane gas and the other wavelength, and the absorption wavelength of the other methane gas and the light are separated. The intensity ratios with other wavelengths are determined, and the methane gas concentration in the measurement cell is measured from these intensity ratios.The methane gas concentrations obtained from the two intensity ratios are compared, and interference gases other than methane gas are determined. A method for measuring methane gas concentration, which is characterized by detecting the presence or absence of methane gas. 2. A light emitting source that emits light in the 1.6 μm and 1.3 μm wavelength ranges, an optical fiber that transmits this light and has low transmission loss in these wavelength ranges, a measurement cell through which atmospheric gas flows in and out, and a measurement cell. of methane gas
a bandpass filter that separates the light absorbed at 1.666 μm and 1.331 μm into light at the two absorption wavelengths of methane gas and one light at another wavelength; a first photodetector that detects and converts it into an electrical signal; a second photodetector that detects the light with the absorption wavelength of 1.331 μm and converts it into an electrical signal; and a second photodetector that detects the light with the other wavelengths. a third photodetector for converting into an electrical signal, and calculating the ratio of the electrical signal of each of the first photodetector and the second photodetector to the electrical signal of the third photodetector; A methane gas concentration measuring device comprising an arithmetic processing device that calculates the respective methane gas concentrations and compares the two methane gas concentrations to determine the presence or absence of an interfering gas other than methane gas.