JP2012239690A - Oxygen meter using ultrasonic flowmeter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an oxygen meter using an ultrasonic flowmeter that precisely determines the mixing ratio of mixed gas containing oxygen.SOLUTION: The oxygen meter includes: a pair of ultrasonic wave transmitter and receiver SU, SD arranged oppositely to each other upstream and downstream of the measuring tube flow path 1 of an ultrasonic flowmeter; a temperature sensor ST and a humidity sensor SH to measure the temperature and humidity in the measuring tube flow path 1; a transmission time measuring unit for measuring the ultrasonic wave transmission time tD from SU to SD and the ultrasonic wave transmission time tU from SD to SU; a temperature/humidity corrector for converting the sum of the tU and tD into a value tC at a specified temperature and humidity; a gas transmission time measuring unit for calculating the time tG when the micro wave is transmitted only in a gas by subtracting the time Δt when the micro wave is transmitted in a thing other than in a gas from the tC; a sound velocity calculator for calculating the sound velocity C of the mixed gas using the tG, etc.; an instrument for preliminarily inputting elements of two kinds of gases to be measured; and an instrument for calculating the concentrations of the two kinds of gases in the mixed gas from the calculated sound velocity C.

Description

  The present invention uses an ultrasonic flow meter incorporated in a system in which a mixed gas of various gases used in the field of medicine, particularly in a ventilator, flows, and the gas concentration (mixing ratio) of each gas in the mixed gas. In particular, the present invention relates to an oxygen concentration meter using an ultrasonic flow meter capable of calculating an oxygen concentration.

  In general, a ventilator is a medical device used from a newborn to an elderly person, especially for a patient with a high urgency in which breathing is unsatisfactory or a patient with a lung disease. In such a ventilator, air of various oxygen concentrations, that is, a mixed gas of nitrogen and oxygen is supplied for patient inhalation. At that time, it is desirable that not only the flow rate of the mixed gas but also the oxygen concentration can be managed at the same time. The ultrasonic flowmeter measures the flow rate of the flowing gas, but the ultrasonic flowmeter also has a function that can measure the sound speed of the fluid, and if the sound speed of the gas is known, the molecular weight of the gas can be known. The ratio or oxygen concentration may be known. The present invention aims to simultaneously calculate the flow rate and the mixing ratio (gas concentration) of the mixed gas flowing through the tube for the ventilator using the ultrasonic flow meter, and the oxygen concentration using the ultrasonic flow meter. Regarding the total.

  There is the following relationship between the molecular weight M of a mixed gas of two kinds of gases (for example, oxygen and nitrogen) and the sound velocity C of the gas. Here, γ is the specific heat ratio of the mixed gas, R is the gas constant, and T is the temperature (° K). Therefore, if the sound velocity C of the gas is known, the molecular weight M of the mixed gas is obtained.

  On the other hand, when the molecular weight of each gas is MA and MB, and the mixing ratio (concentration) is αA and αB, they satisfy the following relational expression with the molecular weight of the mixed gas.

Therefore, if the mixed gas is oxygen and nitrogen, the molecular weight of each gas is known to be MA = 32 and MB = 28. Therefore, if the molecular weight M of the mixed gas is determined from the speed of sound, the mixing of each gas A ratio is required. That is, the oxygen concentration αA is obtained.
Next, the sound velocity C of the gas can be calculated from the distance L between the ultrasonic sensors and the time for which the ultrasonic wave propagates. However, since gas flows in the measurement tube of the ultrasonic flowmeter, the propagation time tU when the ultrasonic wave is propagated in the upstream direction is slightly longer than the propagation time when there is no flow. On the other hand, the propagation time tD when the ultrasonic wave is propagated in the downstream direction is slightly shorter than the propagation time when there is no flow. However, since the increment and decrement of these times are almost equal, taking the sum of tU and tD gives the round-trip propagation time in the absence of flow. Therefore, in the ultrasonic flowmeter, the sound velocity of gas is obtained by the following equation.

JP 2002-306603 A

  Ultrasonic flowmeters that are supposed to be able to measure gas concentration already exist, but they are not so popular yet. According to the analysis, it is found that when the gas concentration is obtained from the sound velocity of the gas, an accuracy of about 0.05% or less is necessary. It seems that the problem was how accurately the value of sound velocity could be obtained. On the other hand, there is an apparatus that can measure the flow rate while being an ultrasonic oximeter. This is often provided with a lead-in tube for oxygen concentration measurement, and the main tube is not measured. Further, in order to accurately determine the oxygen concentration, only a minute flow rate can be measured, and the response is very slow, from several seconds to 1 minute. Again, the problem is how to obtain the sound speed value with high accuracy.

The value of the sound velocity of gas depends not only on the molecular weight of gas but also on temperature and humidity. The gas mixture supplied to the patient with a ventilator is often humidified to provide sufficient humidity. Therefore, in order to obtain the sound speed of gas with high accuracy, it is necessary to know the accurate values of temperature and humidity, and use them to perform temperature correction and humidity correction. In particular, the mathematical formula (1) for obtaining the molecular weight of the gas from the value of the sound velocity of the gas is for the dry gas, and the humidity must be considered separately. Humidity correction is essential, but there are very few examples of performing humidity correction with an ultrasonic flowmeter (Patent Document 1).
However, although the invention disclosed in Patent Document 1 takes into account humidity correction, there is a problem in the accuracy of ultrasonic propagation time.

  Also, even if the temperature changes slightly, the sound velocity of the gas changes. Therefore, when trying to obtain the gas concentration from the value, it is necessary to obtain a highly accurate value of the temperature and perform temperature correction with that value. . Although it is best to know the value of the temperature in the measurement pipe of the ultrasonic flowmeter, it is not desirable to project an inclusion such as a sensor inside the measurement pipe. However, this increases the pressure loss that is the basis of the patient's respiratory resistance and thus detracts from the maximum features of the ultrasonic flowmeter. Therefore, we would like to install temperature and humidity sensors along the inner wall of the tube.

  Furthermore, the ultrasonic propagation time needs to be determined with high accuracy, but the diameter of industrial industrial ultrasonic flowmeters is generally as large as 50 mm or more, and the fine structure of the sensor has been a problem until now. There was no need to do. However, the diameter of ultrasonic flowmeters for ventilators is often as small as 20 mm or less, and it is necessary to consider the fine structure of the sensor.

  The ultrasonic wave radiated from the transducer surface of the ultrasonic sensor not only passes through the gas, but also passes through a solid portion such as a front layer provided on the front surface of the transducer. The ultrasonic propagation time that is normally measured includes propagation times other than this gas. Even if the propagation time is extremely small, an error occurs if the sound velocity of the gas is obtained with this propagation time. In the conventional ultrasonic flowmeter for gas, the error due to the propagation time in this portion is so small that it can be ignored. However, when the concentration of the gas is obtained, it is no longer a negligible amount. Therefore, it is necessary to delete the total propagation time and obtain the speed of sound using the propagation time passing only in the gas.

  The present invention has been proposed in view of the above circumstances, and provides an oxygen concentration meter using an ultrasonic flowmeter that can accurately measure the oxygen concentration together with the flow rate using an ultrasonic transducer that measures the flow rate with a main pipe. The purpose is to do.

  In order to achieve the above object, the present invention relates to an oxygen concentration meter using an ultrasonic flow meter incorporated in a system in which two kinds of mixed gas containing oxygen flow, and a) a measurement tube flow path of the ultrasonic flow meter. A pair of ultrasonic transducers SU and SD arranged to face the upstream side and the downstream side of b, and b) a temperature sensor ST and a humidity sensor SH for measuring the temperature and humidity in the measurement tube flow path, C) an ultrasonic wave propagation time tD from when the ultrasonic wave is radiated from the upstream ultrasonic transducer SU to the measurement tube flow path until the ultrasonic wave reaches the downstream ultrasonic transducer SD, and Propagation for measuring an ultrasonic propagation time tU from when an ultrasonic wave is emitted from the downstream ultrasonic transducer SD to the measurement tube flow path until the ultrasonic wave reaches the upstream ultrasonic transducer SU. A time measuring means; and d) a sum t of measured propagation times tU and tD. = TU + tD, temperature / humidity correction means for converting this to a value tC at a specific temperature and humidity, and e) subtracting the time Δt for propagating other than gas from the propagation time tC corrected for temperature / humidity in d) Then, using the gas propagation time measuring means for calculating the time tG for propagation only in the gas, and f) the propagation time tG calculated in the above e), the sound velocity C of the mixed gas flowing through the measurement pipe flow path is calculated. Sound speed calculating means, g) specification input means for previously inputting specifications (name, molecular weight, number of constituent atoms, etc.) of two kinds of gases to be measured, and h) calculated sound speed C of the mixed gas From the gas concentration calculation means for calculating the gas concentration (mixing ratio) of the two types of gas in the mixed gas, the humidity of each gas concentration in the two types of mixed gas includes humidity However, it can be measured The

  In the present invention, instead of the temperature sensor ST or the humidity sensor SH, the temperature or humidity value may be replaced with a means for automatically or manually inputting a measured value in another existing device. Features.

  In the present invention, there is provided means for automatically or manually inputting a value separately measured by another means in parallel as the propagation time Δt other than the gas.

  In the present invention, when the gas to be measured is a gas in which three or more kinds of gases are mixed, the mixing ratio of the two kinds of gases is unknown, and the names and molecular weights of the other gases are unknown. When the number of constituent atoms and the mixing ratio are known or when separately measured values are available, a means for inputting them in advance and a means for calculating the temperature / humidity correcting means and the gas concentrations (mixing ratios) of two unknown gases Information is obtained, and the gas concentrations (mixing ratios) of these two kinds of gases are obtained.

  In the present invention, the temperature sensor ST and the humidity sensor SH are embedded in the tube wall of the ultrasonic flowmeter.

  Further, in the present invention, the theory at the specific temperature and humidity is obtained from the value tC obtained by correcting the propagation time Δt other than the gas to the value at the predetermined specific temperature and humidity of the measured ultrasonic round-trip propagation time tR. It is characterized by subtracting a typical propagation time tT, that is, by obtaining Δt = tC−tT.

  Since this invention consists of an above-described structure, there can exist an effect which is demonstrated below.

In the present invention, in an oxygen concentration meter using an ultrasonic flow meter incorporated in a system in which two kinds of mixed gas containing oxygen flow, a) on the upstream side and the downstream side of the measurement pipe channel of the ultrasonic flow meter A pair of ultrasonic transducers SU and SD arranged opposite to each other; b) a temperature sensor ST and a humidity sensor SH for measuring the temperature and humidity in the measurement pipe flow path; and c) an upstream ultrasonic wave. The ultrasonic wave propagation time tD from when the ultrasonic wave is emitted from the transducer SU to the measurement tube flow path until the ultrasonic wave reaches the downstream ultrasonic transducer SD, and the downstream ultrasonic transducer A propagation time measuring means for measuring an ultrasonic propagation time tU from when an ultrasonic wave is emitted from the SD to the measurement tube flow path until the ultrasonic wave reaches the upstream ultrasonic transducer SU; d) For the sum of measured propagation times tU, tD tR = tU + tD A temperature / humidity correction means for converting this to a value tC at a specific temperature and humidity; and e) subtracting the time Δt for propagating other than gas from the propagation time tC corrected for temperature / humidity in d) above, A gas propagation time measuring means for calculating a propagation time tG; and f) a sound speed calculating means for calculating the sound velocity C of the mixed gas flowing through the measurement pipe channel using the propagation time tG calculated in e). g) Specification input means for inputting in advance specifications (name, molecular weight, number of constituent atoms, etc.) of two kinds of gases to be measured, and h) from the calculated sound velocity C of the mixed gas, By comprising gas concentration calculation means for calculating the gas concentration (mixing ratio) of two types of gas, the gas concentration of each gas in the two types of mixed gas can be measured even when humidity is included. Accurate ultrasonic propagation time can be calculated, Using this value, the sound velocity of gas can be obtained with high accuracy, and the mixing ratio of the mixed gas can be obtained with high accuracy.
In addition, since the sound speed is obtained by subtracting the propagation time other than the gas from the temperature / humidity corrected value of the ultrasonic propagation time, the sound speed of the gas can be obtained with high accuracy, and the mixing ratio of the mixed gas can be obtained with high accuracy. it can.

  Further, in the present invention, instead of the temperature sensor ST or the humidity sensor SH, as a temperature or humidity value, it is replaced with a means for automatically or manually inputting a measured value in another existing device. The installation cost of a temperature sensor etc. can be reduced.

  Further, in the present invention, since a value separately measured by other means at the same time as the propagation time Δt other than the gas is provided either automatically or manually, the sound velocity of the gas is further improved with high accuracy. It is calculated | required and the mixing ratio of mixed gas can be calculated | required accurately.

  In the present invention, when the gas to be measured is a gas in which three or more kinds of gases are mixed, the mixing ratio of the two kinds of gases is unknown, and the names and molecular weights of the other gases are unknown. When the number of constituent atoms and the mixing ratio are known or when separately measured values are available, a means for inputting them in advance and a means for calculating the temperature / humidity correcting means and the gas concentrations (mixing ratios) of two unknown gases Information is obtained, and the gas concentrations (mixing ratios) of these two gases are determined, so the mixing ratio is unknown for the two gases, and the name, molecular weight, number of constituent atoms and mixing ratio of the other gases are unknown. When known or separately measured values can be obtained, the mixing ratio of the two unknown gases can be obtained by inputting them in advance.

  In the present invention, since the temperature sensor ST and the humidity sensor SH are embedded in the tube wall of the ultrasonic flowmeter, the temperature in the tube can be measured with high accuracy. In addition, since it is embedded in the pipe wall, no pressure loss is generated. Thereby, highly accurate temperature correction becomes possible, and the mixing ratio of the mixed gas can be accurately obtained by obtaining the sound velocity of the sound gas with high precision. Moreover, the humidity in the pipe can be measured with high accuracy. Thereby, even in the humidified gas mixture, humidity correction with high accuracy is possible, and by obtaining the sound speed of the gas with high accuracy, the mixture ratio of the gas mixture can be obtained with high accuracy.

  Further, in the present invention, the theory at the specific temperature and humidity is obtained from the value tC obtained by correcting the propagation time Δt other than the gas to the value at the predetermined specific temperature and humidity of the measured ultrasonic round-trip propagation time tR. Therefore, it is possible to subtract the propagation time other than the gas from the value obtained by correcting the ultrasonic propagation time measured from time to time by the temperature and humidity, and subtracting the typical propagation time tT, that is, Δt = tC−tT. Is obtained with higher accuracy, and the mixing ratio of the mixed gas can be obtained with high accuracy.

FIG. 1 is a cross-sectional view of an essential part showing an example of a measurement tube part in an oximeter using an ultrasonic flowmeter according to the present invention. FIG. 2 is a block diagram for explaining the operation of the oximeter. FIG. 3 is an explanatory diagram showing a main signal flow in the oximeter. FIG. 4 is an explanatory diagram relating to a propagation time Δt other than gas in the oximeter. FIG. 5 is an enlarged cross-sectional view of a main part showing an example in which a temperature sensor and a humidity sensor used in the oximeter are embedded in a tube wall.

  The oxygen concentration meter using the ultrasonic flowmeter of the present invention is an ultrasonic flowmeter, which accurately obtains the sound velocity of the gas flowing through the pipe of the flowmeter, and obtains the mixture ratio (gas concentration) of the mixed gas from the value. It was. For this purpose, the temperature and humidity in the pipe of the flowmeter are accurately obtained, and the temperature and humidity correction are performed using the values, and the time Δt for propagating other than gas is subtracted from the measured ultrasonic propagation time, The sound speed was calculated from this value.

  Hereinafter, the present invention will be described in detail with reference to the drawings illustrating an embodiment. FIG. 1 is a cross-sectional view of an essential part showing an example of a measurement tube portion in an oximeter using an ultrasonic flowmeter according to the present invention. Here, the measurement pipe flow path 1 is a straight pipe having a diameter D, and a pair of ultrasonic transducers SU2 and ultrasonic transmission / reception waves are disposed at positions opposed to the upstream portion 1a and the downstream portion 1b with the flow channel interposed therebetween. A device SD3 is arranged. The interval between the ultrasonic transducers 2 and 3 is L, and the measurement tube 4 and the measurement tube flow path 1 to which the ultrasonic transducers 2 and 3 are attached intersect at an angle θ. A temperature sensor ST5 and a humidity sensor SH6 are embedded in the inner wall of the measurement tube flow path in the vicinity of the measurement tube 4.

  That is, the oxygen concentration meter using the ultrasonic flow meter of the present invention is an oxygen concentration meter using an ultrasonic flow meter incorporated in a system in which two kinds of mixed gas containing oxygen flow. A) Ultrasonic flow rate A pair of ultrasonic transducers SU2 and SD3 disposed opposite the upstream side 1a and the downstream side 1b of the measuring tube channel 1 of the meter, and b) for measuring the temperature and humidity in the measuring tube channel Temperature sensor ST5 and humidity sensor SH6, and c) after radiating ultrasonic waves from the upstream ultrasonic transducer SU2 to the measurement tube flow path 1, the ultrasonic waves reach the downstream ultrasonic transducer SD3. Until the ultrasonic wave reaches the upstream ultrasonic transducer SU3 after the ultrasonic wave is radiated from the downstream ultrasonic transducer SD2 to the measurement tube flow path 1. Time measurement to measure the ultrasonic propagation time tU Stage 7 and d) temperature humidity correction means 8 for converting the measured propagation time tU, tD sum tR = tU + tD into a value tC at a specific temperature and humidity, and e) at d) above The gas propagation time measuring means 9 for calculating the time tG for propagating only in the gas by subtracting the time Δt for propagating other than the gas from the temperature / humidity corrected propagation time tC, and f) the propagation time calculated in e) above. a sonic speed calculation means 10 for calculating the sonic speed C of the mixed gas flowing through the measuring tube flow path using tG or the like, and g) specifications of two kinds of gases to be measured (name, molecular weight, number of constituent atoms, etc.) And a gas concentration calculation means 12 for calculating the gas concentrations (mixing ratios) of two kinds of gases in the mixed gas from the calculated sound velocity C of the mixed gas. ing.

As the material of the measurement tube channel 1, an appropriate material such as metal or plastic can be used. The ultrasonic transducers S _U and S _D have a structure in which a diaphragm 13 made of a piezoelectric material is fixed to the front wall of the case 14. The front surface 14a of the diaphragm 14 has a function of an acoustic matching layer to compensate for the large difference in acoustic impedance between the vibrator and the load (gas) in addition to the function as a protective plate. . Therefore, various materials such as plastics, metals, ceramics and the like are selected according to the design concept of the vibrator. The lead wires 15 of the ultrasonic transducers 2 and 3 are taken from the back and connected to a flow rate measuring device.

  FIG. 2 is a block diagram for explaining the operation of the oximeter using the ultrasonic flowmeter of the present invention. Various command signals are output from the control unit 16 to operate each part of the apparatus. First, the main signal flow is shown in FIGS. 2 and 3 (main signal flow). For example, the control unit 16 transmits the i-th driving trigger signal (iD) to operate the transmission circuit TD17 to transmit / receive ultrasonic waves. Driving voltage input is applied to the wave generator SU2, and ultrasonic waves are radiated to the measuring tube flow path 1. This ultrasonic wave is received by the ultrasonic transducer SD3, converted into an electric signal, and input to the receiving circuit RD18.

  The output of the receiving circuit RD18 becomes a received pulse signal via the switch 19 and the comparator 20, and is input to the time measuring gate generator of the propagation time measuring means 7. The time measurement gate generator uses the trigger signal for driving the transmission circuit TD17 output from the control unit 16 and the received pulse signal output from the comparator 20 to transmit the ultrasonic wave from i to when it is received. A gate pulse having a length corresponding to the first ultrasonic propagation time tD (i) is generated and sent to the counter. The counter converts the ultrasonic propagation time tD (i) into a digital numerical value and inputs it to the calculator of the temperature / humidity correction means 8.

  Next, after a predetermined period T from the drive trigger signal of the previous control unit 16, the control unit 16 again transmits the i-th drive trigger signal (iU) to operate the transmission circuit TU21 and the ultrasonic transducer SD3. In addition, an input of a driving voltage is applied and ultrasonic waves are radiated to the measuring tube channel 1. This ultrasonic wave is received by the ultrasonic transducer SU2, converted into an electric signal, and input to the receiving circuit RU22. The output of the receiving circuit SU is a received pulse signal via the switch 19 and the comparator 20, and is input to the time measuring gate generator of the propagation time measuring means 7. The timing gate generator uses the trigger signal for driving the transmission circuit TU21 output from the control unit 16 and the received pulse signal output from the comparator 20 to transmit the ultrasonic wave from when it is transmitted to when it is received. A gate pulse having a length corresponding to the acoustic wave propagation time tU (i) is generated and sent to a counter that is also included in the propagation time measuring means 7. The counter converts the ultrasonic propagation time tU into a digital numerical value and inputs it to the calculator of the temperature / humidity correction means 8.

  In this apparatus, as described above, the ultrasonic propagation times tU and tD are measured while being alternately repeated at regular intervals T, and tR = tU (i) + tD (i) or tR = tU (i) + tD. (I + 1) is obtained, and these values are input to the arithmetic unit which is the temperature / humidity correction means 8. The arithmetic unit always uses the new value of tR to calculate the following equation and correct the temperature / humidity of the ultrasonic round-trip propagation time tR at that time.

  Here, tC is a propagation time corrected for temperature and humidity, and td30 represents a propagation time in a gas having a relative humidity of 0% and a temperature of 30 ° C. R and θ represent the relative humidity r% and the temperature θ ° C. when tU and tD are measured, respectively. PS (θ) and H (θ) represent the saturated water vapor pressure and the atmospheric pressure at the temperature θ ° C., respectively. That is, by calculating with the equation (5), the ultrasonic propagation time tR = tU + tD measured at an arbitrary temperature / humidity is converted into a value at a relative humidity of 0% and a temperature of 30 ° C.

  However, the temperature and relative humidity values at this time are values read from the outputs of the temperature sensor ST5 and the humidity sensor SH6 embedded in the measurement tube 4 (see FIG. 1) by the temperature / humidity correction means 8 (see FIG. 2). The correction calculation described above is performed using the digital value.

  As the values of temperature and relative humidity, it is best to use values measured by the temperature sensor ST5 and the humidity sensor SH6 embedded in the measurement tube 4, but the temperature of the environment where the ultrasonic flowmeter is used. Alternatively, when the humidity is well controlled or measured near the measuring tube 4 with almost the same accuracy, temperature and humidity correction can be performed using those values. Such numerical values are manually or automatically input to the temperature / humidity input means 23 (see FIG. 2), and the numerical values converted from the temperature / humidity input means 23 into digital numerical values are transferred to the temperature / humidity correction means 8. Sent and used for temperature and humidity correction calculation.

  The value of the ultrasonic reciprocating propagation time tC corrected for temperature and humidity is sent to the time calculating means for propagating only in the gas, and the in-gas propagation time calculating means 24 (see FIG. 2). The temperature / humidity corrected propagation time tC includes a propagation time Δt other than gas. In general, an ultrasonic transducer (see FIG. 1) has a case front surface 14a (protective plate or acoustic matching layer) interposed between a piezoelectric vibrator (vibrating plate) 13 and a gas. The time during which the ultrasonic wave passes through the front surface 14a is a typical element constituting the propagation time Δt other than gas. When the sound velocity C of the gas is obtained, it is obtained by subtracting this from the propagation time tC. That is, the time tG for propagating only in the gas is obtained by the following equation.

The propagation time Δt other than gas is generally different depending on the ultrasonic transducers 2 and 3. Therefore, a method of obtaining in advance at the time of shipment and inputting as a constant to the arithmetic unit is taken.
The propagation time Δt other than gas is obtained by obtaining an ultrasonic round-trip propagation time, for example, td30, under conditions of temperature and relative humidity predetermined in the factory, and subtracting the theoretical propagation time tT under the same conditions from this. It is done. That is, it is calculated by the following formula. Basically, once this value is obtained, the same value can be used for calculation thereafter.

  However, when the propagation time Δt other than gas is easily affected by the external environment such as temperature and humidity, it is necessary to measure this and use an appropriate value appropriately. For this purpose, the apparatus can also be provided with an input means 25 (see FIG. 2) for a non-gas propagation time Δt that allows manual or automatic input of a non-gas propagation time Δt.

The value of the time tG propagating only in the gas is sent to the next sound speed calculation means 10 (see FIG. 2) for the sound speed C. The sound velocity C of gas is calculated by the following formula.
Here, L is a linear distance between the ultrasonic transducers SU2 and SD3, and is usually a distance between the front surfaces 14a of the protective plates interposed between the piezoelectric vibrator (vibrating plate 13) and the gas. This value is known from the design value, and is previously input as a constant into the arithmetic unit.

The value of the gas sound velocity C obtained above is sent to the next gas concentration calculation means 12. In the gas concentration calculation means 12, first, the molecular weight M of the gas is calculated by the following equation.
Here, γ is the specific heat ratio of the gas. When the gas is a mixed gas of nitrogen and oxygen, γ = 1.4. R is a gas constant, and R = 8314. T is the temperature of the gas. In this case, absolute temperature (° K) is used.

  However, when the gas contains carbon dioxide (trimolecular gas) or helium gas (monomolecular gas), the specific heat ratio γ is not 1.4 but an appropriate value depending on the mixing ratio. Such information is input in advance as specifications (name, molecular weight, number of constituent atoms, etc.) of each gas and used in the calculation of equation (9). This is provided as an input means for gas specifications.

  Next, the gas concentration (mixing ratio) of the gas is calculated from the obtained molecular weight M of the mixed gas using the following equation. Now, assuming that the molecular weights of the two types of gases A and B are MA and MB, respectively, and the gas concentrations are αA and αB, the following equations are established.

  Here, if the gas concentration to be obtained is that of gas A, the gas concentration αA is obtained by the following equation.

  Therefore, if the gas mixture is composed of nitrogen and oxygen and it is desired to obtain the gas concentration α of oxygen now, since MA = 32 and MB = 28, it can be calculated by the following equation.

  When the gas concentration is output as data, it is output by data output means. As the data format, digital numerical data and data such as analog voltage or current are appropriately selected depending on the purpose.

  The ultrasonic propagation time measured values tU and tD and their sum tR described above are used to obtain the temperature / humidity corrected propagation time tC and gas sound velocity C by using the measured values each time. Although described, of course, using an average value of each of a plurality of data is a method that is usually conceivable.

Further, the temperature / humidity correction has been described as being performed on tR, which is the sum of tU and tD. However, the temperature / humidity correction is performed on each value of tU and tD first, and then the temperature / humidity correction is performed. The effect is the same even if tR is obtained as the sum of tU and tD.
In addition, it has been described that the propagation time tG in the gas is obtained by subtracting the propagation time Δt other than the gas from the ultrasonic reciprocation propagation time tC corrected for the temperature and humidity. Even if the propagation time tG in the gas is obtained by subtracting from the acoustic round-trip propagation time tR and correcting the temperature and humidity for the value, it has almost the same effect.

(About propagation time Δt other than gas)
Here, a supplementary explanation will be given regarding the propagation time Δt other than the gas. In the above description, Δt is described as if it takes a positive value. However, in practice, it depends on the structure of the ultrasonic transducer and may take a negative value. FIG. 4 shows the structure of two typical types of ultrasonic transducers and examples of their installation, and this will be described in detail.

First, the case where the ultrasonic transducer has the structure shown in FIG. A pair of ultrasonic transducers are installed facing each other. When high-accuracy measurement is required, the ultrasonic propagation distance L is generally defined as the distance L between steps prepared with high accuracy in the measurement tube. Although the thickness l ₁ of the front layer is actually slightly different between the two transducers, it may be equal in the description here.

  When the sound velocity in the gas is C and the sound velocity other than the gas, that is, the sound velocity passing through the front surface layer is C ′, the ultrasonic round-trip propagation time t is expressed by the following equation.

  Here, the theoretical propagation time tT in the gas is expressed by the first term of the above equation. Therefore, the propagation time Δt other than the gas is expressed by the following equation and is obtained as a positive value.

Here, L is a value that is accurately controlled in the manufacturing process, but l ₁ and C ′ are somewhat uncertain values. In particular, in the case where the front layer is a material having a large variation in individual materials such as plastics, C ′ has a large uncertainty. Therefore, Δt is a numerical value that collectively handles uncertain portions in the propagation time.
However, the situation is different when the ultrasonic transducer has the structure shown in FIG. Describe that. Again, the distance between the ultrasonic transducers is L. The thickness of the front layer is l ₁ + l ₂ , and the portion of l ₂ protrudes inward from the step that is the reference of the measurement tube. Therefore, the ultrasonic round-trip propagation time t is expressed by the following equation.

Here, l ₁ , l ₂ , and C ′ are somewhat uncertain values, and only L is a value that is managed with high accuracy. Therefore, also here, the theoretical propagation time tT in the gas is expressed by the first term of the above equation. The propagation time Δt other than gas is represented by the following equation, and is a numerical value that handles uncertain portions in the propagation time in a lump.

When the gas is air, the sound velocity in the gas is approximately C = 350 m / s, and the sound velocity in the front layer is approximately C ′ = 3000 m / s. Typical thickness of the front layer is l ₁ = 2 mm, l ₂ = 1 mm. When these numerical values are substituted and calculated, Δt = −7.4 μs, which is a negative value. This is because the sound velocity C in the gas and the sound velocity C ′ of the front layer are almost 10 times different.

(A mixture of three or more gases)
Here, when the gas to be measured is a gas in which three or more kinds of gases are mixed, the mixing ratio is unknown for the two kinds of gases, and the name, molecular weight, number of constituent atoms and mixing ratio for the other gases. When the measurement values are known or separately available, and they are input in advance, it will be described that the mixing ratio of two unknown gases can be obtained. Here, an example will be described.

  As an example, let us describe expiration in a relatively simple case. Now, if the gas inhaled by the patient by inspiration is a mixed gas of oxygen and nitrogen, and their concentrations and molecular weights are known as αA, αB, MA, and MB, respectively, the molecular weight M and sound velocity C of inspiration are As described above, it is expressed by the following equation. Where γ is a specific heat ratio, R is a gas constant, and T is an absolute temperature.

  For exhalation in a relatively simple case, it can be considered that a part of oxygen becomes carbon dioxide. Then, the nitrogen concentration does not change but the oxygen concentration changes to α′A, the carbon dioxide concentration becomes α′C, and the nitrogen concentration becomes α′B = αB. That is, exhaled breath is a mixed gas of three types of gas, and the concentration of nitrogen is known, but the concentrations of oxygen and carbon dioxide, which are the remaining two types of gas, are unknown.

  Assuming that the molecular weights of oxygen, carbon dioxide, and nitrogen are MA, MC, and MB, the molecular weight and sound speed of exhaled air are expressed by the following equations.

  Note that the specific heat ratio changes to γ '. The specific heat ratio of diatomic molecules such as nitrogen and oxygen is 1.4, and the specific heat ratio of triatomic molecules such as carbon dioxide is 1.333. The specific heat ratio of such a mixture of diatomic and triatomic molecules contributes to their concentration and is expressed by the following equation.

  Therefore, the equations (17) to (21) are expanded as follows.

  Here, the equations are organized as follows.

  Finally, the oxygen concentration in the exhalation can be obtained by the following equation using the value of k determined by the sound velocity C and temperature T of the exhalation, the gas constant R, the molecular weight of each gas, and the oxygen concentration in the inspiration.

  The procedure described above is almost the same if the number of unknown gases is limited to two, even if there are four or more types of mixed gases, or even if one atom molecule gas such as helium is mixed. Become a thing.

(Embedded temperature sensor, humidity sensor)
Here, a supplementary explanation will be given with reference to FIG. 5 that the temperature sensor and the humidity sensor are installed inside the measurement tube of the flow meter and embedded in the tube wall.

  FIG. 5 shows only the tube wall cross section at the top of the measurement tube of the ultrasonic flowmeter. This corresponds to only the upper part of FIG. In the center, an ultrasonic transducer and a branch pipe portion where it is installed are shown. In this figure, the temperature sensor 5 is arranged on the right side. The temperature sensor 5 is a thermistor. The feature of the installation method is almost completely embedded in a part of the tube wall, and the thermistor measures the temperature in the measuring tube through the thin plastic wall 1c of the tube wall. The feature is that there is no part protruding into the tube.

  In this figure, the humidity sensor 6 is installed on the left side of the ultrasonic transducer. This is a method of calculating the amount of water vapor by measuring the capacitance between electrodes with a comb-type electrode. Here, in order to make the sensitivity to water vapor sensitive, the front surface of the sensor is exposed to the gas in the tube, but the front surface is almost flush with the wall surface of the tube, and there is no part protruding into the tube. The measurement tube channel 1 has a structure that is not recessed.

(Explanation of symbols)
t: Used to generally indicate the ultrasonic propagation time.
tU: Ultrasonic propagation time from when an ultrasonic wave is emitted from the downstream ultrasonic transducer SD to the flow path until the ultrasonic wave reaches the upstream ultrasonic transducer SU. Is often indicated.
tD: Ultrasonic propagation time from when the ultrasonic wave is emitted from the upstream ultrasonic transducer SU to the flow path until the ultrasonic wave reaches the downstream ultrasonic transducer SD. Is often indicated.
tR: the sum of the measured propagation times tU and tD, and the ultrasonic round-trip propagation time.
tR = tU + tD
tC: a value obtained by correcting tR to a value at a specific temperature and humidity.
Δt is the time for propagation other than gas, and is corrected for temperature and humidity.
That is, the theoretical propagation time tT at the specific temperature and humidity is subtracted from tC,
Δt = tC−tT For example, when Δt is obtained at the time of shipment in a factory, the ultrasonic round-trip propagation time tR under the conditions of temperature and relative humidity predetermined in the factory for a mixed gas whose concentration has been adjusted in advance. From
TC = td30 is obtained from equation (5).
tT: is a theoretical propagation time. When the temperature is T ° K and the molecular weight is M in a dry gas, it is obtained by the following equation. tG: Time to propagate only in the gas, and tG = tC−Δt.

1 Measurement Tube Channel 1a Upstream Side 1b Downstream Side 2 Ultrasonic Transceiver S _U
3 Ultrasonic transducer S _D
4 Measuring tube 5 Temperature sensor S _T
6 Humidity sensor _SH
7 Propagation time measurement means 8 Temperature / humidity correction means 9 Gas propagation time measurement means 10 Sonic speed calculation means 11 Specification input means 12 Gas concentration calculation means 13 Diaphragm 14 Case 14a Front
15 Lead wire 16 Control unit 17 Transmitter circuit T _D
18 Receiver circuit R _D
19 switch 20 comparator 21 transmitter circuit T _U
22 Receiver circuit R _U
23 Temperature / humidity input means 24 Gas propagation time calculation means 25 Input means for propagation time Δt other than gas

Claims (6)

In an oxygen concentration meter using an ultrasonic flow meter incorporated in a system in which two types of mixed gas containing oxygen flow.
a) a pair of ultrasonic transducers SU and SD arranged to face the upstream side and the downstream side of the measurement pipe flow path of the ultrasonic flowmeter;
b) a temperature sensor ST and a humidity sensor SH for measuring the temperature and humidity in the measuring tube flow path;
c) Ultrasonic propagation time tD from when the ultrasonic wave is radiated from the upstream ultrasonic transducer SU to the measurement tube flow path until the ultrasonic wave reaches the downstream ultrasonic transducer SD, and the downstream Propagation time for measuring the ultrasonic wave propagation time tU from when the ultrasonic wave is emitted from the side ultrasonic transducer SD to the measurement tube flow path until the ultrasonic wave reaches the upstream ultrasonic transducer SU Measuring means;
d) a temperature / humidity correction means for converting the measured propagation time tU, tD sum tR = tU + tD into a value tC at a specific temperature and humidity;
e) a gas propagation time measuring means for subtracting the time Δt for propagating other than the gas from the propagation time tC corrected for temperature and humidity in d) to calculate the time tG for propagating only in the gas;
f) sound speed calculation means for calculating the sound speed C of the mixed gas flowing through the measurement pipe channel using the propagation time tG calculated in e), and the like;
g) Specification input means for inputting in advance specifications (name, molecular weight, number of constituent atoms, etc.) of two kinds of gases that are gases to be measured;
h) Gas concentration calculating means for calculating the gas concentrations (mixing ratios) of two kinds of gases in the mixed gas from the calculated sound velocity C of the mixed gas;
The oxygen concentration meter using the ultrasonic flowmeter characterized by being able to measure the gas concentration of each gas in two types of mixed gas by comprising.   2. Instead of the temperature sensor ST or the humidity sensor SH, a means for automatically or manually inputting a measured value in another existing apparatus as a temperature or humidity value is substituted. An oxygen concentration meter using the ultrasonic flowmeter described in 1.   The ultrasonic flowmeter according to claim 1, further comprising means for automatically or manually inputting a value separately measured by other means simultaneously as the propagation time Δt other than gas. Oxygen concentration meter using.   When the gas to be measured is a mixture of three or more gases, the mixing ratio is unknown for the two gases, and the name, molecular weight, number of constituent atoms and mixing ratio for the other gases Is provided with means to input them in advance when known or separately measured values are available, providing information to temperature / humidity correction means and means for calculating the gas concentration (mixing ratio) of two unknown gases, The oxygen concentration meter using the ultrasonic flowmeter according to any one of claims 1 to 3, wherein gas concentrations (mixing ratios) of two kinds of gases are obtained.   The oxygen concentration meter using the ultrasonic flowmeter according to claim 1, wherein the temperature sensor ST and the humidity sensor SH are embedded in a tube wall of the ultrasonic flowmeter.   The theoretical propagation time tT at this specific temperature and humidity is obtained from the value tC obtained by correcting the propagation time Δt other than the gas to a value obtained by correcting the measured ultrasonic round-trip propagation time tR at a predetermined specific temperature and humidity. The oxygen concentration meter using the ultrasonic flowmeter according to any one of claims 3 to 5, wherein the oxygen concentration meter is obtained by subtraction, that is, Δt = tC−tT.

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