HK1142126B - Ultrasonic gas concentration measuring method and device using the same - Google Patents

Description

Ultrasonic gas concentration measuring method and apparatus using the same

Technical Field

The invention relates to a method and a device for measuring the concentration of sample gas by using ultrasonic waves. More particularly, the present invention relates to a method suitable for measuring the oxygen concentration in an oxygen-concentrated gas delivered from an oxygen concentrator used for medical purposes, for example, and an apparatus using the method.

Background

As is well known, the propagation speed of an ultrasonic wave propagating through a sample gas is expressed as a function of the concentration and temperature of the sample gas. When the average molecular weight of the sample gas is M and the temperature is T [ K ], the propagation velocity of ultrasonic waves C [ M/sec ] in the sample gas can be represented by the following formula (1).

[ formula (1) ]

Wherein k and R are constants (k represents the ratio of constant volume molar specific heat to constant pressure molar specific heat, and R represents a gas constant). That is, the average molecular weight M of the sample gas can be determined by measuring the ultrasonic propagation velocity C [ M/sec ] in the sample gas and the temperature T [ k ] of the sample gas. If the sample gas is a gas composed of two components of oxygen and nitrogen, for example, k is 1.4. When the molecular weight of oxygen is 32, the molecular weight of nitrogen is 28, and oxygen 100 × P [% ] (0 ≦ P ≦ 1) and nitrogen 100 × (1-P) [% ], the average molecular weight M of the sample gas can be written as M ═ 32P +28(1-P), and the oxygen concentration P can be determined from the measured average molecular weight M.

In addition, when the propagation velocity of the ultrasonic wave in the sample gas is C [ m/sec ]]The flow rate of the sample gas is V [ m/sec ]]When the ultrasonic wave is transmitted forward to the flow of the sample gas, the propagation velocity V1[ m/sec ] of the ultrasonic wave is measured]Available V₁The ultrasonic propagation velocity V measured when the ultrasonic wave is transmitted in the reverse direction is represented by C + V₂[m/sec]Available V₂Expressed as C-V, the flow velocity V [ m/sec ] of the sample gas]Can be obtained by the following formula (2).

[ formula (2) ]

The flow velocity V [ m/sec ] of the sample gas determined as described above]Inner cross-sectional area (m) of pipe flowing with sample gas²]Multiplying the flow rates to determine the flow rate [ m ] of the sample gas³/sec]. Furthermore, [ L/min ] can be easily adopted when volume conversion and time conversion are performed]The flow rate was determined.

Various methods and apparatuses have been proposed for measuring the concentration and flow rate of a sample gas based on the propagation velocity or propagation time of an ultrasonic wave propagating through the sample gas, using this principle. For example, Japanese patent laid-open No. 6-213877 describes an apparatus in which: the apparatus is configured to measure the concentration and flow rate of the sample gas by arranging two ultrasonic transducers so as to face each other in a pipe through which the sample gas flows, and measuring the propagation time of the ultrasonic wave propagating between the ultrasonic transducers. Further, Japanese patent laid-open Nos. 7-209265 and 8-233718 disclose an apparatus comprising: a device for measuring the concentration of a sample gas by measuring the propagation speed or propagation time of an ultrasonic wave propagating in a sensing region by means of reflection of the wave using an ultrasonic transducer.

In order to accurately measure the concentration of the sample gas using the propagation velocity of the ultrasonic wave and the like as described above, it is necessary to know an accurate propagation velocity of the ultrasonic wave in consideration of the influence factors in the pipe through which the sample gas flows.

As a method of treating respiratory diseases such as asthma, pulmonary emphysema and chronic bronchitis, there is an oxygen supply therapy in which a patient inhales oxygen or oxygen enriched (oxygen enriched) air, and as the oxygen supply source, there is a pressure swing adsorption type oxygen concentration device capable of concentrating about 21% of oxygen present in the air at a high concentration and supplying the oxygen to the user. The pressure swing adsorption type oxygen concentration device described above is a device in which: as an adsorbent which selectively adsorbs nitrogen compared with oxygen, an apparatus is used which uses an adsorption bed packed with molecular sieve zeolite of 5A type, 13X type, Li-X type, MD-X type, or the like, and supplies compressed air to the adsorption bed by a compressor to adsorb nitrogen under pressurized conditions, and takes out unadsorbed oxygen as an oxygen-concentrated gas. The apparatus generally comprises two or more adsorption beds, and oxygen is continuously generated by switching the following two steps in sequence: a pressure adsorption step of adsorbing nitrogen to the adsorbent in an adsorption bed to produce non-adsorbed oxygen; and a desorption regeneration step of depressurizing the other adsorption bed, discharging the adsorbed nitrogen and regenerating it.

In the pressure swing adsorption type oxygen concentration device, the pressure adsorption step and the desorption regeneration step in the pipeline are switched to continuously generate oxygen, and the supply flow rate of oxygen can also be switched at any time, so that the pressure of the sample gas in the pipeline fluctuates. However, in general, since a change in the propagation velocity of the ultrasonic wave due to the pressure is not considered at all, it is a factor that deteriorates the accuracy of the sample gas concentration measurement value.

Disclosure of Invention

The invention aims to provide an ultrasonic sample gas concentration measuring method and a device using the method, which can derive a coefficient for correcting a propagation speed along with the pressure fluctuation of a sample gas and accurately measure the concentration of the sample gas in the pressure.

The present inventors have conducted intensive studies to achieve the above object, and as a result, have found that: the pressure in the pipe through which the sample gas flows is changed at various temperatures, the propagation velocity correction coefficient is calculated, and the propagation velocity is corrected by expressing the propagation velocity correction coefficient as a function of the temperature, whereby the sample gas concentration can be accurately measured.

That is, the present invention provides an ultrasonic gas concentration measurement device, comprising: the device includes: two ultrasonic transducers which are disposed in a pipe through which a sample gas flows, the two ultrasonic transducers being opposed to each other and which transmit and receive ultrasonic waves; temperature sensor and pressure sensor still include: and a concentration calculation means for calculating the concentration of the sample gas based on the propagation velocity correction coefficient based on the pressure of the sample gas.

Further, the present invention provides an ultrasonic gas concentration measuring apparatus, wherein the concentration calculating means includes: the concentration calculation means includes means for correcting a propagation time until the ultrasonic transducer receives the ultrasonic wave, using a propagation velocity correction coefficient corresponding to a measured temperature and a measured pressure of the sample gas, the propagation time being calculated by: and a means for correcting the propagation velocity C until the ultrasonic transducer receives the ultrasonic wave, by the following equation (3).

[ formula (3) ]

Wherein k represents a ratio of a constant volume molar specific heat to a constant pressure molar specific heat, R represents a gas constant, T represents a sample gas measurement temperature, M represents an average molecular weight of the sample gas, P represents a measurement pressure of the sample gas, and B (T) and C (T) represent propagation velocity correction coefficients.

Furthermore, the present invention provides an ultrasonic gas concentration measurement method for measuring a concentration in a sample gas based on a propagation time until ultrasonic waves transmitted from ultrasonic transducers that are disposed opposite to each other in a pipe through which the sample gas flows and that transmit and receive the ultrasonic waves are received by the ultrasonic transducers, the measurement method comprising: the method of measuring is characterized in that the propagation time until the ultrasonic transducer receives the ultrasonic wave is corrected based on a propagation velocity correction coefficient corresponding to the temperature and pressure of the sample gas, and in particular: the propagation velocity C until the ultrasonic transducer receives the ultrasonic wave is corrected by equation (3).

Further, the present invention provides an ultrasonic concentration measurement method, comprising: propagation velocity correction coefficients (B (Ta), C (Ta)) at a temperature Ta are obtained from ultrasonic propagation velocities of a plurality of sample gases at different pressures at the temperature Ta, and the propagation velocities are corrected based on a function of the temperature T of the propagation velocity correction coefficients.

Drawings

Fig. 1 is a schematic diagram showing the configuration of an ultrasonic oxygen concentration measurement device according to the present invention.

Fig. 2 shows the relationship between the oxygen concentration value and the pressure in a conventional ultrasonic oxygen concentration measuring apparatus.

Fig. 3 shows a relationship between the propagation velocity correction coefficient and temperature.

Fig. 4 shows the relationship between the oxygen concentration value and the pressure in the ultrasonic oxygen concentration measuring apparatus using the propagation velocity correction coefficient according to the present invention.

Fig. 5 shows an experimental apparatus for determining a propagation velocity correction coefficient according to the embodiment.

Detailed Description

Hereinafter, embodiments of the ultrasonic gas concentration flow rate measurement method according to the present invention will be described. In this embodiment, an apparatus for measuring the oxygen concentration of a sample gas composed of three components of oxygen, nitrogen and argon, or two components of oxygen and nitrogen is shown. The sample gas that can be measured by the present invention is not limited to the sample gas composed of oxygen, nitrogen, and argon described in the present embodiment, and is also applicable to a gas composed of other molecules.

Fig. 1 shows a structure of an ultrasonic type oxygen concentration/flow rate measuring mechanism. The ultrasonic oxygen concentration measuring mechanism includes: a switch 4 which is disposed in the pipe 1 through which the sample gas flows, with 2 ultrasonic transducers 2 (a first ultrasonic transducer 2a and a second ultrasonic transducer 2b) facing each other, and which switches transmission and reception of the ultrasonic transducers 2; a driver 5 for transmitting an ultrasonic transmission pulse to the ultrasonic transducer 2; a zero-cross (zero-cross) time detection circuit 6 for detecting a zero-cross time of the ultrasonic wave reception waveform; a microcomputer 7 for calculating the concentration and flow rate of the sample gas; a temperature sensor 3 for measuring the temperature of the sample gas in the pipe 1; a pressure sensor 8 for measuring the pressure in the pipe 1; and a nonvolatile memory 9 for storing the propagation velocity correction coefficient. The display 10 displays the measured concentration of the sample gas. The temperature sensor and the pressure sensor may be disposed at the center on the ultrasonic propagation path as long as the flow of the sample gas is not disturbed.

The method for measuring the concentration of the sample gas using the above-described apparatus structure will be described below. As is well known, the propagation speed of an ultrasonic wave propagating through a sample gas is expressed as a function of the concentration and temperature of the sample gas. That is, assuming that the average molecular weight of the sample gas is M and the temperature is T [ K ], the propagation velocity of ultrasonic waves C [ M/sec ] in the sample gas can be represented by the above formula (1). As shown in the formula (1), the method can measure the correct oxygen concentration regardless of whether the sample gas flowing through the pipe is affected by the pressure or when the pressure in the pipe is zero, but cannot accurately derive the measured value of the oxygen concentration when the pressure is present in the pipe.

FIG. 2 shows the results of the oxygen concentration in the sample gas derived by the measurement method described in JP-A-2004-317459 and using a conventional algorithm that does not take the pressure into consideration. As shown in fig. 2, since the propagation velocity of the ultrasonic wave is correlated with the pressure, it is known that the output value of the oxygen concentration decreases as the pressure increases.

It is generally known that the ultrasonic wave propagation speed C is related to pressure, expressed as a function of temperature, which can be represented by equation (3).

[ formula (3) ]

Wherein, P [ N/m ]²]The output value of the pressure sensor incorporated in the ultrasonic sample gas concentration measurement device of the present invention is shown. B (T) and C (T) m³/mol]The ultrasonic propagation velocity correction coefficient is shown.

In this embodiment, the part before the item b (t) is used, and is represented by the following formula (4).

[ formula (4) ]

A method of calculating the propagation velocity correction factor is presented. As shown in fig. 5, a plurality of ultrasonic sample gas concentration measuring devices 13a and 13b are disposed inside a temperature-adjustable device 14, for example, a thermostatic chamber. The ultrasonic sample gas concentration measuring devices 13a and 13b are connected to each other through a flow rate adjuster 11 from a gas cylinder (gas bomb)16 provided outside. The length of the pipe body 12 extending into the thermostatic bath of the ultrasonic sample gas concentration measuring devices 13a and 13b is further increased in order to stabilize the temperature so that the sample gas temperature matches the set temperature of the thermostatic bath. A pipe body extending from the thermostatic bath 14 is connected to the pressure regulating valve 15, so that the pressure value can be adjusted.

P is prepared by using the pressure regulating valve 15₁、P₂Under two pressure conditions, a stable temperature T is determined₁The propagation speeds of ultrasonic waves C1 and C2, whereby the average molecular weight M of the sample gas₁、M₂Can be obtained from the following formulas (5) and (6). Here, the temperature conditions at the two pressure levels may also not be identical.

[ formula (5) ]

[ formula (6) ]

When the sample gas is the same in actual measurement, M1 becomes M2, and the propagation velocity correction coefficient b (ta) can be calculated.

Other temperature T₂、T₃Propagation velocity correction coefficient B of equal magnitude₂、B₃Can also be determined by the same method, including the temperature T₁Propagation velocity correction coefficient B (ta) of B₁In the interior, FIG. 3 plots the propagation velocity correction factor (B)₁～B₃) And temperature (T)₁～T₃) The relationship between them.

Here, the plotted points are approximated by a quadratic approximation curve, and thereby, as shown in the following equation (7), the propagation correction coefficient b (T) can be obtained as a function of the temperature (T). But the approximation curve may not be a quadratic curve.

[ formula (7) ]

B(T)＝αT²+βT+γ

The output values of the temperature sensor and the pressure sensor are substituted into equations (7) and (4), and the sample gas concentration is measured using the corrected ultrasonic propagation velocity, and the measurement results are shown in fig. 4. As shown in fig. 4, the oxygen concentration was not affected by the pressure fluctuation, and the concentration could be accurately measured.

By using the method of the present invention, it is possible to provide an ultrasonic sample gas concentration measurement method and a gas concentration measurement apparatus that can derive a coefficient for correcting a propagation velocity that varies with the pressure of a sample gas and accurately measure the concentration of the sample gas in the pressure.

Claims (2)

1. An ultrasonic gas concentration measuring method for measuring a concentration of a specific component gas in a sample gas based on a propagation time until ultrasonic waves transmitted from 2 ultrasonic transducers for transmitting and receiving ultrasonic waves, which are arranged to face each other in a pipe through which the sample gas flows, are received by ultrasonic transducers arranged on the opposite side, the method comprising the steps of:

measuring a propagation time until the ultrasonic waves transmitted by each of the two ultrasonic transducers are received by the other ultrasonic transducer;

measuring the gas temperature of the sample;

measuring the gas pressure of the sample, an

Correcting a propagation time until the ultrasonic wave is received by the ultrasonic transducer based on a propagation velocity correction coefficient corresponding to the temperature and pressure of the sample gas,

wherein the step of correcting the propagation time until the ultrasonic wave is received by the ultrasonic transducer is a step of correcting the propagation velocity (C) of the ultrasonic wave using the following equation,

wherein k represents a ratio of a constant volume molar specific heat to a constant pressure molar specific heat, R represents a gas constant, T represents a sample gas measurement temperature, M represents an average molecular weight of the sample gas, P represents a measurement pressure of the sample gas, and b (T) represents a propagation velocity correction coefficient; and

the step of correcting the propagation velocity of the ultrasonic wave is performed by obtaining a propagation velocity correction coefficient b (Ta) at a temperature Ta from the propagation velocity of the ultrasonic wave of the sample gas having two different types of pressure values at the temperature Ta, and correcting the propagation velocity based on a function of the temperature T of the propagation velocity correction coefficient.

2. An ultrasonic gas concentration measuring apparatus for measuring a gas concentration, comprising:

a duct extending along an axis for flowing a gas to be measured therethrough;

a first ultrasonic transducer disposed inside the pipe and configured to transmit and receive ultrasonic waves;

a second ultrasonic transducer disposed opposite to the first ultrasonic transducer inside the pipe, for transmitting and receiving ultrasonic waves;

a transmission/reception switch that controls switching between an ultrasonic transmission mode in which the ultrasonic transducer transmits ultrasonic waves and an ultrasonic reception mode in which the transmitted ultrasonic waves are received between the first and second ultrasonic transducers;

a temperature sensor disposed in the duct and measuring a temperature of the target gas; and

a pressure sensor disposed in the duct and measuring a pressure of the target gas; further comprising:

concentration calculation means for calculating the concentration of the target gas from the propagation velocity correction coefficient by using the propagation time until the ultrasonic waves transmitted from each of the two ultrasonic transducers are received by the other ultrasonic transducer, the output value of the temperature sensor, and the pressure value of the pressure sensor; and

the concentration calculating means is provided with means for correcting the propagation velocity (C) of the ultrasonic wave by the following formula,

wherein k represents a ratio of a constant volume molar specific heat to a constant pressure molar specific heat, R represents a gas constant, T represents a sample gas measurement temperature, M represents an average molecular weight of the sample gas, P represents a measurement pressure of the sample gas, and b (T) represents a propagation velocity correction coefficient; and

the means for correcting the propagation velocity of the ultrasonic wave obtains a propagation velocity correction coefficient B (Ta) at a temperature Ta from the ultrasonic propagation velocity of the sample gas having two different types of pressure values at the temperature Ta, and corrects the propagation velocity based on a function of the temperature T of the propagation velocity correction coefficient.