JP7473170B2 - Gas concentration measuring device - Google Patents

Description

The present invention relates to a gas concentration measurement device based on the non-dispersive infrared (NDIR) method, and in particular to a fluid modulation type gas concentration measurement device.

NDIR gas concentration measuring devices are now widely used in many industrial fields for a variety of applications. Early devices were large and complex in structure, so their uses were limited. Eventually, in tandem with the development of the electronics industry, the old gas-filled infrared detectors were replaced with infrared detection mechanisms using solid-state photoreceivers and interference filters, and the uses of NDIR gas concentration measuring devices expanded dramatically due to their simplified structure, smaller size, and longer life.

NDIR gas concentration measurement devices typically use infrared light that has passed through an optical path that does not contain the gas being measured, or infrared light with a wavelength that is not absorbed by the gas being measured, as a reference light, and detect the gas concentration from the difference or ratio between the measurement light and the reference light, thereby compensating for changes in the light source, etc.

However, the difference or ratio between the measurement light and the reference light changes due to factors other than the concentration of the gas being measured, and this appears as zero drift in the gas concentration measurement device. Zero drift is a weakness common to all absorption measurement methods, not just NDIR, and was an important issue that needed to be improved.
Examples of conventional techniques for reducing zero drift are shown in Japanese Patent Application Laid-Open No. 2003-233993 and Japanese Patent Application Laid-Open No. 2003-233994.

The former is a method of signal modulation using an optical chopper rotated by a motor. Multiple measurement cells and reference cells are arranged alternately on a circumference, and the gas concentration is determined by comparing the sum of the measurement light with the sum of the reference light. This technology aims to reduce zero drift by averaging out the changes that occur in the individual measurement light or reference light.

The latter, on the other hand, is a technology that aims to miniaturize the device and extend its lifespan while achieving the same effect as the former in terms of reducing zero drift by replacing the motor and optical chopper with multiple light-emitting parts (light sources) that can emit light individually using silicon micromachining.

Moreover, Patent Document 3 describes an example of a technique for solving the problem of zero drift more fundamentally.
A mechanism is provided that changes (modulates) the concentration of the gas introduced into the measurement cell at a constant frequency and at a constant rate, separate from changes in gas concentration at the gas sampling point, and by detecting only the light receiving signal of the same modulation frequency, factors other than gas concentration are eliminated. In particular, this has the advantage that zero drift is essentially eliminated, and in the same document this is referred to as the fluid modulation method.

Figure 1 of Patent Document 3 shows an example of a fluid modulation method in which a pump and an electromagnetic valve are used to alternately introduce a sample gas and a reference gas (zero gas) into a measurement cell at a constant frequency, and Figure 5(b) of Patent Document 3 explains the characteristics of the received light signal.

In FIG. 5(b) of Patent Document 3, when the sample gas does not contain the component to be measured, the received light signal remains in a no-signal state, and therefore no zero drift occurs.
On the other hand, when the sample gas contains a component to be measured, a light receiving signal having a level corresponding to the gas concentration is generated at the modulation frequency, and the gas concentration is measured.

As a more practical configuration, as shown in Figure 3 of Patent Document 3, there is one that adds a means for supplying a calibration gas suitable for the fluid modulation method. Because it switches between the sample gas and the reference gas (zero gas), this is sometimes called the "gas switching method" and has been in practical use for many years, particularly in the field of automobile exhaust gas measuring instruments.

Among NDIR gas concentration measurement devices using the fluid modulation method, there are cases where the gas concentration (density) in the measurement cell is modulated by a different method.
For reference, referring to Fig. 4 of Patent Document 3, the reference gas in the figure is not used, and a two-way solenoid valve that opens and closes at a constant modulation frequency is used instead of the three-way solenoid valve 4. By switching the gas pressure in the measurement cell, i.e., the gas density, between normal pressure and a constant negative pressure, a light receiving signal similar to that of the gas switching method is obtained, enabling stable gas measurement without generating zero drift.

This method is called the "pressure modulation method" and has been in practical use for many years in applications where a reference gas (zero gas) such as clean air cannot be used, such as in the field of gas leak detection.

Figure 3 attached to this specification shows the output waveform of a photoreceiver using light intensity modulation according to the prior art, and Figure 4 shows the output waveform of a photoreceiver using fluid modulation according to the prior art. In NDIR devices, the measurement light is modulated to ensure S/N ratios. For example, devices using a light chopper rotated by a motor, or devices using a light intensity modulation method that periodically switches the measurement light on and off by blinking the light source are common.

There are a wide variety of NDIR optical systems, including single beam/multi-beam, single wavelength/multi-wavelength, and many other types in practical use, but there is a relationship between the target gas concentration and the detection signal, as shown in Figure 3.

The output waveforms in FIGS. 3 and 4 are shown as signals that have been approximated to sine waves by filter processing that selectively amplifies the signal components at the modulation frequency.
3 is a diagram for explaining in more detail the waveform obtained by light intensity modulation, similar to the output waveform shown in FIG. 5(a) of Patent Document 3, and shows the output waveform at a modulation frequency of 1 Hz. The output waveform by light intensity modulation has a maximum amplitude in zero gas, which does not contain the target gas, and the amplitude decreases as the gas concentration increases.

In FIG. 3, the notation "FS gas" indicates gas with the maximum concentration (full-scale concentration), and the notation "1/2FS gas" indicates gas with 1/2 the maximum concentration.
The signal amplitude value in light intensity modulation fluctuates due to factors other than gas concentration (such as temperature effects and changes over time in all related parts and materials), and this appears as zero drift, which is a major factor determining the limits of equipment performance.

In order to fundamentally solve the zero drift problem of NDIR equipment, a method was developed that modulates the gas concentration (or gas density) at a constant period instead of modulating the amount of light. This is the NDIR with the fluid modulation method shown in Patent Document 3.

Figure 4 is a diagram explaining in more detail the waveform obtained by fluid modulation, similar to the output waveform shown in Figure 5(b) of Patent Document 3, and shows the output waveform when the concentration or density of the gas to be measured is modulated at a modulation frequency of 1 Hz. The output waveform by fluid modulation has no signal in zero gas that does not contain the target gas, and the amplitude increases as the gas concentration increases.

In Figure 4, "FS gas" indicates gas with maximum concentration (full-scale concentration), and "1/2FS gas" indicates gas with half the maximum concentration. In a fluid modulation type NDIR, the amount of light incident on the light source is kept constant, and only when infrared absorption occurs due to the target gas is the degree of absorption modulated at a constant rate to generate an output waveform (AC signal component). Therefore, for zero gas, there is a completely no-signal state (AC signal component = zero), and zero drift is theoretically eliminated, and this state can be maintained for a long period of time.

However, conventional fluid modulation methods use a solenoid valve and a pump, and the switching frequency is set to ensure that the change in gas concentration in the measurement light path (measurement cell) caused by switching the solenoid valve takes time to become almost constant. As a result, the practical modulation frequency is limited to about 1 to 2 Hz, and the responsiveness of the gas concentration measurement device achieved is about 5 to 10 seconds as a 90% response time, including the signal averaging processing time. Fluid modulation cannot be applied to applications that require faster response than this.

Japanese Patent Publication No. 03-047700 Patent No. 3347264 Patent Publication No. 2965507

NDIR gas concentration measurement devices using fluid modulation have the advantage of not generating zero drift, but the configuration of the gas concentration measurement device is not complete with just the optical system, and a dedicated ventilation circuit is required, which can limit usage. As for the gas switching method, it is limited to applications where a reference gas (zero gas) is available, so if fluid modulation is applied to other applications, the pressure modulation method will be selected.

Fluid modulation methods have a common issue related to the use of pumps and solenoid valves. In conventional fluid modulation, the modulation frequency is set to obtain a highly reproducible light receiving signal by ensuring time for the gas concentration or gas pressure in the measurement cell to reach a nearly constant value each time the solenoid valve is switched. For this reason, taking into account the volume of the ventilation line including the measurement cell, the modulation frequency is limited to about 1 to 2 Hz, and the responsiveness of the gas concentration measurement device has been proven to be about 5 to 10 seconds as a 90% response time, including the signal averaging processing time. Fluid modulation cannot be applied to applications that require faster response than this.

The second issue is that fluid modulation cannot be applied to applications where the gas pressure at the sampling point of the gas to be measured is different from atmospheric pressure (normal pressure), such as in-line measurement of gas concentration in piping at a gas-handling plant. In order to apply zero-drift-free fluid modulation to in-line measurements, a structure that is not easily affected by the gas pressure at the gas sampling point and a technology that modulates the gas density in the measurement cell at a constant frequency and constant ratio are required.

In order to solve the above problems, the present invention provides a gas concentration measuring device comprising: a vibration chamber that forms a vibration space; a main passage that is a path through which the sample gas moves between the vibration space and a sample gas atmosphere that is an atmosphere of a sample gas containing a gas of a target compound to be measured; a vibration device that causes pressure fluctuations in the sample gas in the vibration space at a predetermined drive frequency; and a light quantity measuring device that irradiates the sample gas that has been subjected to the pressure fluctuations with measurement light to transmit the sample gas, detects the amount of light in the measurement light that has transmitted through the sample gas and is absorbed by the target compound to be measured, and converts the amount of light into an electrical signal, and the gas concentration measuring device determines the magnitude of a frequency component that coincides with the drive frequency among frequency components contained in the electrical signal detected and converted by the light quantity measuring device.
The present invention also provides a gas concentration measuring device having a pressure sensor for detecting the pressure of the sample gas subjected to the pressure fluctuation.
The present invention also provides a gas concentration measuring device, wherein the gas concentration is a ratio between a volume of the gas of the compound to be measured contained in the sample gas and a volume of the sample gas.
The present invention also provides a gas concentration measuring device, wherein the vibration device is controlled so as to keep constant the vibration amplitude of the pressure fluctuation detected by the pressure sensor.
The present invention also provides a gas concentration measurement device that determines the gas concentration using an electrical signal in which a ratio of signal components that fluctuate at the drive frequency to signal components that fluctuate at other frequencies is reduced among signal components contained in the electrical signal.
The present invention also relates to a gas concentration measurement device that is provided with a vibration plate whose surface is exposed to the vibration space, the vibration device having a transmission device that transmits a force that vibrates the vibration plate, and the vibration device vibrates the vibration plate at the drive frequency to fluctuate the pressure of the sample gas.
The present invention also provides a gas concentration measuring device having a shaft whose tip is attached to the vibration plate, the vibration device reciprocatingly moving the shaft to vibrate the vibration plate.
The present invention also provides a gas concentration measurement device comprising a back-side space in which the back surface of the vibration plate is exposed, a back-side flow passage which is a path through which the sample gas moves between the back-side space and the sample gas atmosphere, a force-receiving magnet having an N pole and an S pole, one of which is directed toward the vibration space and the other is directed toward the back-side space, a first force-feeding magnet which is an electromagnet which forms a first magnetic pole at a position facing the force-receiving magnet across the vibration space, and a second force-feeding magnet which is an electromagnet which forms a second magnetic pole at a position facing the force-receiving magnet across the back-side space, wherein the vibration device vibrates the vibration plate by alternately reversing the polarity of the first magnetic pole and the second magnetic pole while maintaining the same polarity.
The present invention also provides a gas concentration measuring device in which the sample gas moving between the vibration space and the sample gas atmosphere passes through a ventilation resistor having a flow conductance smaller than the flow conductance of the main passage.

The pulsating flow type gas concentration measuring device of the present invention can utilize an infrared absorption signal synchronized with the gas concentration pulsation generated by transmitting the pressure pulsation of a specific frequency generated in the chamber to the measurement optical path, and can realize zero drift free, which is an advantage of the fluid modulation method, and at the same time, can achieve a high speed response of 5 seconds or less as a 90% response time. Also, if the diaphragm is magnetically driven, it becomes easy to ensure the airtightness of the chamber, and it can be used as an in-line gas concentration measuring device.
Furthermore, by replacing the solenoid valve and pump that were essential in the conventional fluid modulation method with a small pulsating flow generating unit, it is expected that the fluid modulation type gas concentration measurement device can be made smaller and less expensive.

An example of the gas concentration measuring device of the present invention Another Example of the Gas Concentration Measuring Device of the Present Invention Graph showing a light quantity signal waveform of a conventional gas concentration measuring device using a light quantity modulation method. Graph showing the light quantity signal waveform of a conventional fluid modulation type gas concentration measuring device Graph showing the waveform of a pressure signal from a gas concentration measuring device of the present invention. Graph showing the relationship between gas concentration and output signal waveform of the gas concentration measuring device of the present invention. Graph showing the relationship between gas concentration and the relative value of the output signal of the gas concentration measuring device of the present invention. FIG. 1 is a diagram for explaining a method of using the gas concentration measuring device of the present invention.

Figure 8(a) is a diagram for explaining a gas concentration measuring device 41a according to an example of the present invention, and Figure 8(b) is a diagram for explaining a gas concentration measuring device 41b according to another example of the present invention.

8(a) and 8(b), reference numerals 40a and 40b denote a vehicle fuel supply system using liquefied gas tanks 46a and 46b.
In this fuel supply system 40a, 40b, liquefied gas stored in liquefied gas tanks 46a, 46b is vaporized by carburetors 47a, 47b, and the resulting fuel gas flows through supply pipes 20a, 20b and is introduced into injectors 48a, 48b, supplied from the injectors 48a, 48b to engines 49a, 49b, where it is combusted together with the intake air to operate the engines 49a, 49b and run the vehicle.

The liquefied gas stored in the liquefied gas tanks 46a, 46b is consumed through natural evaporation even if it is not consumed by the engines 49a, 49b.
Among the compounds contained in liquefied gas, those with high vapor pressure tend to evaporate easily. Therefore, when fuel gas generated by natural evaporation is released into the atmosphere through a release valve for safety reasons, the component ratio of the liquefied gas changes over time.

Therefore, it is necessary to periodically measure the gas concentration of the gas to be measured in the fuel gas obtained from the liquefied gas tanks 46a, 46b and grasp the degree of deterioration due to evaporation.
The inside of a gas concentration measuring device 41a according to one example of the present invention is shown in FIG. 1, and the inside of a gas concentration measuring device 41b according to another example is shown in FIG.

1 and 2, the gas concentration measuring devices 41a and 41b respectively include vibration chambers 44a and 44b, introduction pipes 7a and 7b, vibration devices 43a and 43b, and light quantity measuring devices 17a and 17b.
Fuel gas flows inside the supply pipes 20a, 20b, and therefore fuel gas atmospheres 21a, 21b are formed inside the supply pipes 20a, 20b. The fuel gas contains a combustible gas whose concentration is to be measured, and this combustible gas is the measurement target gas whose concentration is measured by the gas concentration measuring devices 41a, 41b.

The supply pipes 20a, 20b are provided with main valves 13a, 13b. During regular inspection, one end of the introduction pipes 7a, 7b is connected to the main valves 13a, 13b, and then the main valves 13a, 13b are switched so that fuel gas is introduced from the fuel gas atmospheres 21a, 21b inside the supply pipes 20a, 20b into the introduction pipes 7a, 7b.
The vibration chambers 44a, 44b are connected to the other ends of the introduction pipes 7a, 7b.

The vibration chambers 44a, 44b have vibration containers 4a, 4b and vibration plates (also called diaphragms) 3a, 3b, and the vibration plates 3a, 3b are arranged facing the bottom surfaces 27a, 27b of the vibration containers 4a, 4b, but spaced apart from the bottom surfaces 27a, 27b. The periphery of the vibration plates 3a, 3b is fixed in close contact with the side walls of the vibration containers 4a, 4b.

The vibration plates 3a, 3b close the openings of the vibration containers 4a, 4b, and closed vibration spaces 8a, 8b are formed between the vibration plates 3a, 3b and the bottom surfaces 27a, 27b of the vibration containers 4a, 4b.
Through holes 12a, 12b are provided in the bottom surfaces 27a, 27b or wall surfaces of the vibration containers 4a, 4b, and the fuel gas introduced from the fuel gas atmospheres 21a, 21b to the introduction pipes 7a, 7b is introduced into the vibration spaces 8a, 8b through the through holes 12a, 12b.

In this way, the insides of the inlet pipes 7a, 7b form main passages 9a, 9b that move the fuel gas between the fuel gas atmospheres 21a, 21b and the vibration spaces 8a, 8b, and the vibration spaces 8a, 8b are filled with fuel gas.
The vibration devices 43a and 43b include driving devices 1a and 1b and transmission devices 2a and 2b.

The driving devices 1a and 1b are power sources that vibrate the diaphragms 3a and 3b, and the transmission devices 2a and 2b transmit the forces generated by the driving devices 1a and 1b to the diaphragms 3a and 3b.
The surfaces of the vibrating plates 3a, 3b are exposed to the vibration spaces 8a, 8b and are in contact with the fuel gas filling the vibration spaces 8a, 8b. As described below, when the vibrating plates 3a, 3b are vibrated in a direction perpendicular to the surfaces of the vibrating plates 3a, 3b by the force generated by the driving devices 1a, 1b and transmitted by the transmission devices 2a, 2b, the pressure of the fuel gas located in the vibration spaces 8a, 8b vibrates.

To explain this vibration, in the gas concentration measuring device 41a of FIG. 1, first, of both surfaces of the diaphragm 3a, the back surface opposite to the front surface exposed to the vibration space 8a is exposed to the atmosphere.
In this example, the driving device 1a has a motor 34, the transmission device 2a has a shaft (rod) 25, and the motor 34 and the shaft 25 are disposed on the rear side of the diaphragm 3a.

The base portion of shaft 25 is attached to a motor 34, and the rotational motion of motor 34 is converted into reciprocating motion of shaft 25. As a result, operation of motor 34 causes shaft 25 to reciprocate in a direction parallel to the extension direction of shaft 25.
The tip portion of the shaft 25 is fixed to the diaphragm 3a. Here, the diaphragm 3a has a circular shape, and the tip of the transmission device 2a is attached to its center position.

The part of the vibration plate 3a where the force is transmitted from the transmission device 2a is called the force receiving part 30a. The force receiving part 30a is the part of the vibration plate 3a where the tip of the shaft 25 is attached. When the drive device 1a starts to operate, the periphery of the vibration plate 3a is fixed to the side walls of the vibration containers 4a and 4b, and the force receiving part 30a is repeatedly pressed and pulled alternately.

In the gas concentration measuring devices 41a and 41b in Figures 1 and 2, the vibration plates 3a and 3b are made of a flexible material, and if the state in which the vibration plates 3a and 3b are flat is called the central state, when the force receiving points 30a and 30b are repeatedly pressed and pulled alternately, the force receiving points 30a and 30b move alternately in a direction approaching the bottom surfaces 27a and 27b and in a direction away from the bottom surfaces 27a and 27b, with the position of the force receiving points 30a and 30b in the central state as the center (reference numbers 30b and 27b will be described later).

As a result of the force receiving points 30a, 30b moving in a direction approaching the bottom surfaces 27a, 27b, the diaphragms 3a, 3b bulge convexly on the vibration spaces 8a, 8b side and become concave on the opposite side, reducing the volume of the vibration spaces 8a, 8b.
Conversely, when the force receiving points 30a, 30b move in a direction away from the bottom surfaces 27a, 27b, the diaphragms 3a, 3b become concave on the vibration spaces 8a, 8b side and convex on the opposite side, increasing the volume of the vibration spaces 8a, 8b.

The vibration devices 43a, 43b vibrate the vibration plates 3a, 3b by reciprocating the force receiving points 30a, 30b so that the distance between the force receiving points 30a, 30b and the bottom surfaces 27a, 27b of the vibration containers 4a, 4b increases and decreases by an equal distance from the distance in the central state, and as a result, the volume of the vibration spaces 8a, 8b repeatedly increases and decreases by an equal amount.

The vibration devices 43a and 43b are connected to the main control devices 18a and 18b, and the drive devices 1a and 1b vibrate the vibration plates 3a and 3b at a predetermined drive frequency input from the main control devices 18a and 18b. Therefore, the volume of the vibration spaces 8a and 8b repeatedly increases and decreases by an equal amount at the drive frequency.

If the flow conductance of part or all of the main passages 9a, 9b is small, even if the volume of the vibration spaces 8a, 8b fluctuates, the amount of fuel gas that can flow in and out of the vibration spaces 8a, 8b is limited, and as a result, pressure vibrations of the drive frequency are generated in the vibration spaces 8a, 8b.

In a gas concentration measuring device 41b in FIG. 2, a driving device 1b has a power supply device 35, and a transmission device 2b has a force receiving magnet 26, a first force supplying magnet 22, and a second force supplying magnet 24.
A rear-side container 5 is disposed on the rear side of the vibration plate 3b. The periphery of the vibration plate 3b is fixed in close contact with the side wall of the rear-side container 5 so as to close the opening of the rear-side container 5, and a closed rear-side space 38 is formed between the vibration plate 3b and the bottom surface 28 of the rear-side container 5.

The supply pipe 20b is provided with a sub-valve 33, and during regular inspections, one end of the sub-pipe 37 is connected to the sub-valve 33. The other end of the sub-pipe 37 is connected to a sub-through hole 32 provided in the bottom or side wall of the rear-side container 5, and the fuel gas introduced into the sub-pipe 37 is supplied to the rear-side space 38 through the sub-through hole 32, filling the rear-side space 38 with fuel gas.

The first force-feeding magnet 22 is positioned outside the vibration space 8b in a position facing the bottom plate of the vibration container 4b, and the second force-feeding magnet 24 is positioned outside the back side space 38 in a position facing the bottom plate of the back side container 5.
The force-receiving magnet 26 is provided at the central position of the vibration plate 3b, and is located between the first force-feeding magnet 22 and the second force-feeding magnet 24, and is arranged so that the center of the first force-feeding magnet 22, the center of the force-feeding magnet 26, and the center of the second force-feeding magnet 24 are aligned in a straight line.

Here, the force-receiving magnet 26 is a permanent magnet, and the first force-feeding magnet 22 and the second force-feeding magnet 24 are electromagnets. When current is supplied from the power supply unit 35, the first force-feeding magnet 22 and the second force-feeding magnet 24 become magnets and generate magnetic force.

The first force-feeding magnet 22 is arranged so that, when generating magnetic force, one of the N and S poles is formed in a position facing the vibration plate 3b, and the other magnetic pole is formed in a position facing the opposite side to the vibration plate 3b.
In addition, the second force-feeding magnet 24 is arranged so that, when the second force-feeding magnet 24 generates a magnetic force, one of the N pole and S pole is formed in a position facing the vibration plate 3b, and the other magnetic pole is formed in a position facing the opposite side to the vibration plate 3b.

The power supply unit 35 passes current through the first force-feeding magnet 22 and the second force-feeding magnet 24 in a direction that generates magnetic poles of the same polarity between the magnetic pole formed at a position facing the diaphragm 3b of the first force-feeding magnet 22 and the magnetic pole formed at a position facing the diaphragm 3b of the second force-feeding magnet 24.
In addition, the power supply unit 35 reverses the direction of the current flowing through the first force-feeding magnet 22 and the direction of the current flowing through the second force-feeding magnet 24 together so that the polarity of the magnetic pole formed in the position facing the vibration plate 3b is reversed.

The force-receiving magnet 26 is fixed to the diaphragm 3b with one of its north and south poles facing the vibration space 8b and the other facing the back space 38. When the force-receiving magnet 26 is attracted by the first force-feeding magnet 22, it is repelled by the second force-feeding magnet 24, and when it is repelled by the first force-feeding magnet 22, it is attracted by the second force-feeding magnet 24.

The part of the vibration plate 3b where the force-receiving magnet 26 is provided is called the force-receiving part 30b. Due to the attraction and repulsion between the first force-receiving magnet 22 and the second force-receiving magnet 24, the force-receiving part 30b moves alternately in a direction approaching and away from the bottom surface 27b of the vibration container 4b, with the position of the force-receiving part 30b in the central state as the center, and the vibration plate 3b vibrates in a direction perpendicular to the surface, and the volume of the vibration space 8b vibrates.

The transmission device 2b is moved back and forth by the vibration device 43b so that the distance between the force receiving point 30b and the bottom surface 27b of the vibration container 4b increases and decreases by an equal distance from the distance when the vibration plate 3b is in the central state, and as a result, the volume of the vibration space 8b repeatedly increases and decreases by an equal amount.

The vibration device 43b is connected to a main control device 18b, and the drive device 1b vibrates the diaphragm 3b at a drive frequency input from the main control device 18b.
If the flow conductance of part or all of the main passage 9b, which moves the fuel gas between the fuel gas atmosphere 21b and the vibration space 8b, is small, even if the volume of the vibration space 8b fluctuates, the amount of fuel gas that can flow in and out of the vibration space 8b is limited, and as a result, pressure vibrations of the driving frequency are generated in the vibration space 8b.

Specifically, a sinusoidal voltage with a frequency of 40 Hz is applied to the first force-feeding magnet 22 and the second force-feeding magnet 24 with the same polarity, causing the diaphragm 3b to vibrate at a frequency of 40 Hz, generating pressure vibrations with a frequency of 40 Hz.

As explained above, the gas concentration measuring devices 41a and 41b generate pressure vibrations in the vibration spaces 8a and 8b that vibrate at the drive frequency. In the gas concentration measuring device 41a in FIG. 1, however, there may be a pressure difference between the front and back surfaces of the vibration plate 3a. When the pressure in the vibration space 8a is the same as the atmospheric pressure, the vibration center of the vibration plate 3a is in a flat state, but when the pressure in the vibration space 8a is significantly higher than the atmospheric pressure, the vibration plate 3a bulges toward the atmosphere (back surface).

In the gas concentration measuring device 41b in FIG. 2, the inside of the auxiliary pipe 37 forms a back side flow passage 29 that connects the back side space 38 and the fuel gas atmosphere 21b, and the average pressure of the vibration space 8b is the same as the average pressure of the back side space 38. Therefore, the vibration plate 3b vibrates around the central state.

Next, the inlet pipes 7a and 7b of the gas concentration measuring devices 41a and 41b in Figures 1 and 2 are provided with ventilation resistors 6a and 6b, and the fuel gas moving between the fuel gas atmospheres 21a and 21b and the vibration spaces 8a and 8b passes through the ventilation resistors 6a and 6b.

The ventilation resistors 6a, 6b have pores 31a, 31b, and the flow conductance is smaller in the parts of the main passages 9a, 9b where the ventilation resistors 6a, 6b are provided than in other parts of the main passages 9a, 9b. In other words, it is difficult for the fuel gas to flow through the places of the main passages 9a, 9b where the ventilation resistors 6a, 6b are provided.

Since the fuel gas atmospheres 21a, 21b and the vibration spaces 8a, 8b are connected by the main passages 9a, 9b, while the pressure of the fuel gas atmospheres 21a, 21b is maintained at a constant value, the average pressure of the vibration spaces 8a, 8b is equal to the pressure of the fuel gas atmospheres 21a, 21b. However, since it is difficult for the fuel gas to flow in and out of the vibration spaces 8a, 8b through the ventilation resistors 6a, 6b, the pressure fluctuation amount of the vibration spaces 8a, 8b becomes large when the volume of the vibration spaces 8a, 8b vibrates.

A ventilation resistor may also be provided in the secondary pipe 37 to make the magnitude of the flow conductance between the rear space 38 and the fuel gas atmosphere 21b equal to the magnitude of the flow conductance between the vibration space 8b and the fuel gas atmosphere 21b.

Of the main passages 9a, 9b, the portions between the vibration spaces 8a, 8b and the ventilation resistors 6a, 6b also vibrate together with the pressure vibrations of the vibration spaces 8a, 8b, and of the inlet pipes 7a, 7b, the portions between the vibration spaces 8a, 8b and the ventilation resistors 6a, 6b are provided with inlet side windows 36a, 36b and outlet side windows 39a, 39b.
The light quantity measuring devices 17a and 17b include light transmitters 11a and 11b, light receivers 15a and 15b, and optical filters 14a and 14b.

Outside the inlet pipes 7a and 7b, light transmitters 11a and 11b are arranged at locations facing the inlet side windows 36a and 36b, and light receivers 15a and 15b are arranged at locations facing the outlet side windows 39a and 39b.

The wavelength of light absorbed by a gas of a specific compound is called the absorption wavelength, and the absorption wavelength of the gas to be measured is known. The light transmitters 11a and 11b emit measurement light containing light of the absorption wavelength toward the entrance side windows 36a and 36b. In this example, incandescent light bulbs are used for the light transmitters 11a and 11b.
The inlet side windows 36a, 36b and the outlet side windows 39a, 39b are arranged facing each other, and the fuel gas filling the inside of the inlet pipes 7a, 7b is located between the inlet side windows 36a, 36b and the outlet side windows 39a, 39b.

The optical filters 14a, 14b are positioned at positions through which the measurement light emitted by the light transmitters 11a, 11b and received by the light receivers 15a, 15b passes, and when the measurement light passes through the optical filters 14a, 14b, light of wavelengths other than the absorption wavelength is attenuated.
Here, the optical filters 14a and 14b are infrared bandpass filters with a transmission center wavelength of 3.4 μm and a transmission wavelength width of 0.2 μm, and the optical filters 14a and 14b are disposed between the light receivers 15a and 15b and the outlet side windows 39a and 39b. The measurement light emitted by the light transmitters 11a and 11b passes through the inlet side windows 36a and 36b, enters the inside of the introduction tubes 7a and 7b, passes through the fuel gas located between the inlet side windows 36a and 36b and the outlet side windows 39a and 39b, passes through the outlet side windows 39a and 39b and the optical filters 14a and 14b, and is received by the light receivers 15a and 15b. The light receivers 15a and 15b use non-cooled quantum type PbSe light receiving elements.

The receivers 15a and 15b generate a light intensity signal indicating the amount of light received and output it as an electrical signal to the main controllers 18a and 18b. The light intensity signal output by the receivers 15a and 15b is the amount of light at the absorption wavelength of the gas to be measured.

Here, the light intensity signals output from the photodetectors 15a and 15b are input to the main controllers 18a and 18b via the amplifiers 19a and 19b, and the amplifiers 19a and 19b detect and amplify only the signal components (AC signal components) that oscillate at the drive frequency among the signal components contained in the light intensity signal, and output them to the main controllers 18a and 18b. As a result, only the signal that depends on the concentration of the gas to be measured and is synchronized with the drive frequency is output to the main controllers 18a and 18b, and the measurement accuracy is improved by being zero drift-free.

When the pressure of the fuel gas into which the measurement light is incident oscillates and decreases at the drive frequency, the density of the measurement target gas (density = mass/volume) also oscillates and decreases at the same rate, and an oscillating increase and decrease (AC signal component) of the drive frequency occurs in the light receiving signal at the absorption wavelength. The amplitude of this AC signal component increases and decreases depending on the concentration of the measurement target gas contained in the fuel gas atmosphere 21a, 21b, so the measurement target gas concentration can be measured from the value of the amplitude of this AC signal component. The amplitude of the AC signal component may be detected and used directly, or it may be converted into a DC signal after rectification and smoothing processing. Furthermore, the measurement accuracy is improved by detecting and using the amplitude of the AC signal component or an equivalent DC signal after processing with a filter circuit or software filter that selectively amplifies frequency components near the drive frequency.

Before using the gas concentration measuring devices 41a and 41b, a calibration curve is created in advance using a target gas (standard gas) of known concentration. This calibration curve determines the relationship of the output signal to the target gas concentration, and includes information on the linearity of the target gas concentration and output signal, and the magnitude of the output signal to the target gas concentration. Specifically, it is an approximation function that specifies the linearity and a sensitivity coefficient that is an index of gas sensitivity, and these information are stored in the amplifiers 19a and 19b as parameter values, and maintain a state in which the relationship between the target gas concentration and the output signal can be calculated. In the measurement, the main control devices 18a and 18b calculate the target gas concentration from the obtained output signal and display it on the display devices 23a and 23b. In addition, when the gas concentration measuring devices 41a and 41b are readjusted, usually only the sensitivity coefficient is recalculated and updated among the parameter values stored in the amplifiers 19a and 19b. Even if the characteristics of the light transmitters 11a and 11b or the light receivers 15a and 15b change during long-term use, the measurement accuracy can be ensured by performing readjustment. In addition, in addition to volumetric concentration, mass concentration (gas density) is also commonly used to measure the concentration of the gas to be measured.

Incidentally, the gas concentration is often expressed as a volume fraction (e.g., vol% or simply %), which is the ratio of the volume of the gas to the volume of all gas components contained in the fuel gas. However, depending on the application or measurement purpose of the gas concentration measuring device, mass concentration (e.g., mg/ ^m3 ) or substance amount concentration (e.g., mol/ ^m3 ) may also be used.

The fuel gas in the vibration spaces 8a, 8b and the fuel gas in the main passages 9a, 9b are gradually replaced with the fuel gas in the fuel gas atmospheres 21a, 21b due to the vibration of the vibration plates 3a, 3b, so the change in gas concentration in the fuel gas atmospheres 21a, 21b is reflected in the measurement results.

In this example, the measurement light passes through the fuel gas in the main passages 9a and 9b. However, in the present invention, it is sufficient that the measurement light passes through a known distance of the fuel gas whose pressure is oscillating at the drive frequency and is received by the photodetectors 15a and 15b.
Moreover, the pressure vibration is not limited to the pressure fluctuation caused by the vibration plates 3a and 3b, and for example, a piston may be used to cause the pressure vibration of the measurement gas.

The above ventilation resistors 6a and 6b are provided in the inlet pipes 7a and 7b, but the present invention is not limited to this, and it is also possible to use an inlet pipe that is thinner and has a flow conductance similar to that of the ventilation resistors 6a and 6b.

The gas concentration measuring devices 41a and 41b in Figures 1 and 2 also have pressure sensors 16a and 16b, and the pressures detected by the pressure sensors 16a and 16b are input to the main control devices 18a and 18b, which measure the magnitude of the amplitude of the pressure vibration of the fuel gas that transmits the measurement light, compare it with a target amplitude value stored in the main control devices 18a and 18b, and control the driving devices 1a and 1b so that the pressure of the fuel gas that transmits the measurement light vibrates at the target amplitude value.

In the gas concentration measuring device 41a of FIG. 1, the fuel gas comes into contact with the front side of the diaphragm 3a and the atmosphere comes into contact with the rear side, so that the gas concentration measuring device 41a is suitable for measuring when the fuel gas is at a pressure close to atmospheric pressure.
Specifically, when the pressure of the fuel gas is within approximately ±10% of atmospheric pressure, the vibration plate 3a can be vibrated relatively easily even when the back surface of the vibration plate 3a is exposed to atmospheric pressure, and therefore the vibration plate 3a can be driven by a shaft or the like directly connected to a vibration device installed in the atmospheric atmosphere.

In the gas concentration measuring device 41b in FIG. 2, fuel gas is in contact with both the front and back sides of the vibration plate 3b, and therefore it is suitable for measurements when the fuel gas pressure is significantly different from the atmospheric pressure. Specifically, it is suitable for a gas concentration measuring device that continuously measures the gas concentration in piping in a plant that handles fuel gas.

When determining the concentration of the target gas contained in the fuel gas as a volume fraction (e.g., vol% or simply %) as the gas concentration, even if there is no change in the volume fraction, if the total pressure of the fuel gas onto which the measurement light is irradiated changes, the mass concentration (e.g., mg/ ^m3 ) of the target gas changes, and therefore the signal component that vibrates at the drive frequency of the electrical signal output from the photodetectors 15a and 15b also changes, causing the value of the gas concentration to change.

The pressure signals obtained from the pressure sensors 16a and 16b provide the total pressure value of the fuel gas as well as pressure vibration control at the drive frequency of the fuel gas through which the measurement light passes. However, the total pressure value is an average pressure obtained by smoothing the pressure vibration. The main control devices 18a and 18b use the total pressure value of the fuel gas to compensate for changes in the volume fraction gas concentration. Specifically, a standard gas containing the components of the gas to be measured at known volume fractions is used, and the relationship between the total pressure of the standard gas and the gas concentration value provided by the gas concentration measuring devices 41a and 41b is obtained in advance and stored in the main control devices 18a and 18b as parameter values. In gas concentration measurement, the main control devices 18a and 18b calculate the compensation amount of the gas concentration for the momentary total pressure value from the stored parameter values, and compensate to the correct value as the volume fraction gas concentration.

In the above example, the ventilation resistors 6a and 6b have pores 31a and 31b, but porous filler may be placed in place of the pores 31a and 31b.

<Experimental Results>
In the gas concentration measuring devices 41a and 41b of the present invention, the distance between the inlet side windows 36a and 36b and the outlet side windows 39a and 39b was set so that the distance (effective absorption length) that the measuring light passes through the measuring gas was 1 mm, and air (zero gas: 0 vol%) and four types of mixed gas of air and butane gas (1 vol%, 2 vol%, 4 vol%, 5 vol%) were used, and the diaphragms 3a and 3b were vibrated at 40 Hz to generate pressure vibrations and detect the gas concentrations.

5 is a graph showing an example of a pressure signal when air (zero gas) is measured at a drive frequency of 40 Hz. For a mixed gas of air and butane gas, a pressure signal similar to that of air is obtained up to a concentration of 5 vol%. Here, a positive pressure sensor was used, so the absolute pressure of the measurement gas was kept at about 20 kPa and a positive pressure bias was applied to the measurement. The vertical axis of FIG. 5 shows the gauge pressure, and pressure oscillations were observed at gauge pressures in the range of ±10 kPa, with the absolute pressure of the measurement gas with a positive pressure bias (about 20 kPa) set as zero.
6 is a graph comparing the amplitude of each gas concentration when measuring 0 vol%, 1 vol%, 2 vol%, 4 vol%, and 5 vol% butane gas (mixed with air) by processing the AC signal components of the drive frequency obtained from the photodetectors 15a and 15b with a filter circuit that selectively amplifies frequencies near the drive frequency to obtain a waveform that resembles a sine wave, and then aligning the phase of the vibration. The vertical axis shows the output signal as a relative output.
The amplitude of the output waveform in FIG. 6 changes depending on the concentration of butane gas, and indicates that there is no signal (AC signal component=zero) when the gas concentration is 0 (zero gas).
FIG. 7 shows the amplitude of the output signal for each gas concentration as a relative output from the measurement results shown in FIG.

With pressure vibration of ±10 kPa and 40 Hz, gas replacement in the main passages 9 a, 9 b is completed almost instantaneously, so that even when performing averaging processing required for stabilizing the measurement results, a high-speed response of within 1 second is possible, as a 90% response time.
The gas concentration determined by the present invention is not limited to that of the target compound in the fuel gas, but can be determined for the target compound in a general sample gas.

1a, 1b... Drive device 2a, 2b... Transmission device 3a, 3b... Vibration plate (diaphragm)
4a, 4b... Vibration containers 6a, 6b... Ventilation resistors 7a, 7b... Inlet pipes 8a, 8b... Vibration spaces 9a, 9b... Main passages 16a, 16b... Pressure sensors 17a, 17b... Light quantity measuring devices 20a, 20b... Supply pipes 21a, 21b... Fuel gas atmosphere 22... First force feeding magnet 24... Second force feeding magnet 25... Shaft 26... Force receiving magnet 29... Rear side flow passage 38... Rear side spaces 41a, 41b... Gas concentration measuring devices 43a, 43b... Vibration devices 44a, 44b... Vibration chambers

Claims (8)

a vibration chamber forming a vibration space;
a main passage which is a path through which the sample gas moves between the vibration space and a sample gas atmosphere which is an atmosphere of a sample gas containing a gas of a compound to be measured;
a vibration device for subjecting the sample gas in the vibration space to pressure fluctuations at a predetermined driving frequency;
a light quantity measuring device that irradiates the sample gas subjected to the pressure fluctuation with a measurement light to transmit the sample gas, detects the quantity of light of a wavelength that is absorbed by the compound to be measured in the measurement light that has transmitted through the sample gas, and converts the quantity of light into an electrical signal;
determining a gas concentration from a magnitude of a frequency component that coincides with the drive frequency among frequency components included in the electrical signal detected and converted by the light quantity measuring device ;
a vibration plate having a surface exposed to the vibration space;
The vibration device has a transmission device that transmits a force that vibrates the vibration plate,
The vibrating device vibrates the vibration plate at the drive frequency to vary the pressure of the sample gas . The gas concentration measuring device according to claim 1, further comprising a pressure sensor for detecting the pressure of the sample gas subjected to the pressure fluctuation. The gas concentration measuring device according to claim 2, wherein the gas concentration is the ratio between the volume of the gas of the compound to be measured contained in the sample gas and the volume of the sample gas. The gas concentration measuring device according to claim 2, wherein the vibration device is controlled to keep the vibration amplitude of the pressure fluctuation detected by the pressure sensor constant. A gas concentration measuring device according to any one of claims 1 to 4, which calculates the gas concentration using an electrical signal in which the ratio of signal components fluctuating at the drive frequency to signal components fluctuating at other frequencies is reduced among the signal components contained in the electrical signal. a shaft having a tip attached to the diaphragm;
2. The gas concentration measuring device according to claim 1 , wherein the vibration device vibrates the vibration plate by reciprocating the shaft. a back surface side space in which the back surface of the diaphragm is exposed;
a backside flow passage which is a path through which the sample gas moves between the backside space and the sample gas atmosphere;
a force-receiving magnet having an N pole and an S pole, one of which is oriented toward the vibration space and the other of which is oriented toward the back surface space;
a first force supplying magnet which is an electromagnet forming a first magnetic pole at a position facing the force receiving magnet across the vibration space;
a second force supplying magnet which is an electromagnet forming a second magnetic pole at a position facing the force receiving magnet with the back side space therebetween;
having
2. The gas concentration measuring device according to claim 1 , wherein the vibration device vibrates the vibration plate by alternately inverting the polarity of the first magnetic pole and the second magnetic pole while maintaining the same polarity. 8. A gas concentration measuring device according to claim 1 , wherein the sample gas moving between the vibration space and the sample gas atmosphere passes through a ventilation resistor having a flow conductance smaller than the flow conductance of the main passage.

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