JP5769167B2 - Gas molecular weight measuring device - Google Patents

Description

  The present invention relates to a gas molecular weight measuring apparatus. In particular, it is possible to measure the molecular weight of gas in real time, to be able to downsize because it does not have an evacuation system, and to be able to cope with the measurement pressure range from atmospheric pressure to reduced pressure. The present invention relates to a molecular weight measuring apparatus.

Several methods are known for measuring the molecular weight of a gas.
As a method using a mass spectrometer, there is a measurement method in which a gas is introduced into a vacuum to be ionized to fly in an electric field, and ions are separated and detected based on a difference in mass to charge ratio (m / e) (Patent Document 1). ).
When the gas to be measured can be regarded as close to an ideal gas, the molecular weight can be calculated by measuring volume / temperature / pressure using the gas equation of state. For example, when oxygen, nitrogen, hydrogen, helium, etc. have a molar volume of about 10 atm or less near room temperature, the deviation from the ideal gas is 1% or less, and shows properties close to the ideal gas.
A concentration measurement method is shown as an application of gas measurement using a vibrator. In Patent Document 2, two parameters that change depending on the viscosity of a gas are measured using one crystal resonator. Then, for a two-component mixed gas whose constituent gas is known and whose concentration is unknown, the concentration can be measured from the two measured parameters when there is a monotonous relationship between the concentration and the viscosity. In Patent Document 3, when a pressure gauge sensitive to both gas pressure and viscosity and a pressure gauge sensitive only to pressure are used, two pressure indications are given when there is a monotonous relationship between concentration and viscosity. It shows that the concentration of the two-component mixed gas can be measured from the value.

JP 2007-192692 A JP 2005-241355 A (Patent No. 4266850) JP 2001-330543 A (Patent No. 3336384)

In the method using a mass spectrometer, there is a problem that an exhaust pump for creating a vacuum is required and the apparatus volume is increased.
In the method using the gas equation of state, there is a problem that the gas needs to be close to an ideal gas and there is a limit to the gas that can be measured. In addition, since it is necessary to fill the container with the gas to be measured, there is a problem that it is difficult to use it in applications that require real-time measurement or in-situ measurement, such as measurement of the gas to be measured flowing down the pipe.
As a result of diligent research, it became clear that a physical property called viscosity can be measured by performing a detailed analysis of two parameters that change depending on the viscosity of a gas using a single crystal unit. The problem that the amount of physical properties cannot be obtained by this method has also been clarified. For this reason, when measuring the concentration of a two-component gas mixture using a vibrator, the concentration cannot be determined uniquely even if the viscosity is measured for a gas mixture that does not have a monotonous relationship between viscosity and concentration. There has been a problem that a plurality of calculated concentrations exist. Therefore, in order to measure the concentration of the mixed gas, a method for measuring the molecular weight instead of the viscosity value is required.
In measurement using a vibrator, the frequency of resonance vibration changes due to temperature fluctuations of the vibrator, which hinders gas measurement. However, there is a problem that the temperature of the vibrator is easily influenced by the temperature of the gas at atmospheric pressure.
In the measurement of gas by the vibrator, the vibrator is exposed to the gas, and if the gas contains fine particles, the fine particles may adhere to the vibrator and affect the measurement.
The present invention solves the above-mentioned problems and can measure the molecular weight of gas in real time, can be downsized because it does not have an evacuation system, and the measurement pressure region is from atmospheric pressure or higher. An object of the present invention is to provide a molecular weight measuring apparatus characterized by being capable of handling pressure reduction, performing molecular weight measurement regardless of temperature fluctuation of gas, and the above.

In order to achieve the above object, a gas molecular weight measuring apparatus of the present invention includes a measurement chamber viewed in a gas to be measured, a vibrator installed in the measurement chamber, a vibrator, and excitation parameters of the vibrator. In the apparatus for measuring the molecular weight of a gas, comprising: an excitation measuring unit for measuring the pressure; a pressure measuring element for measuring the pressure of the gas on which the vibrator is placed; and a temperature measuring element for measuring the temperature of the vibrator. An arithmetic unit that calculates the molecular weight of the gas from the excitation parameter measured by the excitation measuring unit, the pressure measured by the pressure measuring unit, and the temperature measured by the temperature measuring unit is provided.
In addition, the gas molecular weight measuring apparatus of the present invention includes a measurement chamber viewed in a gas to be measured, a vibrator installed in the measurement chamber, an excitation measurement unit that excites the vibrator and measures excitation parameters of the vibrator. And a pressure measuring element for measuring the pressure of the gas in which the vibrator is placed, and a device for measuring the molecular weight of the gas having a constant temperature portion for making the temperature of the vibrator constant, measured by the excitation measuring section. A calculation unit is provided that calculates the molecular weight of the gas from the excitation parameter, the pressure measured by the pressure gauge, and the temperature.
In the gas molecular weight measuring apparatus according to the present invention, the excitation parameter uses a resonance frequency of a vibrator and a resistance value obtained from a current flowing through the vibrator and an applied voltage.
Further, the present invention is characterized in that, in the gas molecular weight measuring apparatus, the gas to be measured is guided to the measurement chamber through a filter that transmits the gas to be measured.
Further, in the gas molecular weight measuring apparatus according to the present invention, the calculation of the gas molecular weight in the calculation unit first obtains a product value of the gas molecular weight and the gas pressure using an excitation parameter, and then The molecular weight of the gas is obtained by dividing the product value by the pressure of the gas measured by the pressure gauge.

Compared to conventional mass spectrometers, the present invention does not require an evacuation system, can be made smaller and less expensive, and does not require ionization of the gas to be measured. There is no need for.
In addition, it can be measured without being affected by pressure even if it is installed in a place with pressure fluctuations. As a result, measurement at places with large pressure fluctuations is possible. For example, in pipes where pressure fluctuations are likely to occur due to flow fluctuations. Gas can be measured.
Furthermore, in the present invention, the following three technical points are noted in order to realize gas measurement by the vibrator.
One of them is measurement control of the temperature of the vibrator. It has been clarified that the frequency of the resonant vibration of the vibrator changes with temperature, and if this is ignored, the molecular weight cannot be measured accurately. Since vibrators excited in a gas are easily affected by the temperature of the gas, either a means to measure the vibrator temperature and compensate for frequency fluctuations due to temperature or to keep the vibrator temperature constant is required. In the present invention, these are realized.
Another is prevention of adhesion of fine particles. In the measurement of the gas by the vibrator, the vibrator is exposed to the gas, and if fine particles are contained in the gas, the fine particles may adhere to the vibrator and affect the measurement. Therefore, the structure is such that the flowing gas passes through the filter before it touches the vibrator to prevent contamination.
Another role of the filter is gas heat exchange. The gas exchanges heat with the filter when passing through the filter, and after passing through the filter, the temperature of the gas becomes equal to the temperature of the filter. Therefore, if the temperature of the filter is kept constant, even if there is a temperature fluctuation in the flowing gas, the temperature of the vibrator is not affected. If the filter does not have a constant temperature mechanism, it can be considered as the temperature of the vibrator by measuring the temperature of the filter.

A vibrator used in the present invention and a driving method thereof. The block diagram of Example 1 which is one Embodiment of the gas molecular weight measuring apparatus of this invention. The block diagram of Example 2 which is one Embodiment of the gas molecular weight measuring apparatus of this invention. The block diagram of Example 3 which is one Embodiment of the gas molecular weight measuring apparatus of this invention. The figure which shows the example of molecular weight measurement by the gas molecular weight measuring apparatus of this invention.

When the vibrator is placed in the gas to be measured for resonance vibration, the vibrator receives an external force that tries to prevent vibration from the surrounding gas. The magnitude of the external force depends on the gas pressure, viscosity, and molecular weight. Therefore, information about the gas can be obtained by measuring the external force. The external force received by the vibrator can be measured by measuring an index (excitation parameter) indicating the state of the vibrator.
As a method for exciting the vibrator, a vibrator is manufactured using a magnetic material, an AC magnetic field is applied from the outside, and the frequency is made to coincide with the resonance frequency of the vibrator, or a vibration is produced using a piezoelectric material. There is a method in which a child is formed, electrodes are formed on the front and back surfaces of the vibrator, an alternating electric field is applied, and the frequency is made to coincide with the resonance frequency of the vibrator to perform excitation.
Materials with a piezoelectric effect include barium titanate (BaTiO ₃ ), lead zirconate titanate (PZT), polyvinylidene fluoride (PVDF), potassium dihydrogen phosphate (KDP), aluminum nitride (AlN), quartz (quartz) (SiO ₂ ) is known and can be used as an electric field excitation type vibrator. However, in order to vibrate in the gas, it is necessary to select one that is chemically stable with respect to the gas to be measured. In addition, a material that is stable against moisture is still preferable in consideration of handling in the atmosphere. In order to form an electrode, an electrode material (such as Au or Cr) that can be stably fixed is desirable.
For the formation of the vibrator, if a micro electro mechanical systems (MEMS) technique is used, a micro or an arbitrary shape can be manufactured. Shapes include cantilever beams with one side fixed, those with both sides fixed, and tuning fork type.

The frequency change (ΔF) of the vibrator can be selected as the excitation parameter. ΔF is given as a change amount with respect to the frequency (F ₀ ) of the vibrator in vacuum. This ΔF is proportional to the molecular weight near atmospheric pressure, and has a property proportional to the square root of the product of the molecular weight and the viscosity coefficient in the vacuum region where the pressure is the molecular flow region.
There is a resistance value as another excitation parameter. The resistance value can be obtained as the ratio of the current flowing to the applied voltage when an AC voltage equal to the resonance frequency of the vibrator is applied to the vibrator when a material having a piezoelectric effect is selected. ΔR is given as a change amount with respect to the resistance value (R ₀ ) of the vibrator in vacuum. ΔR has a property proportional to the viscosity coefficient of gas near atmospheric pressure, and proportional to the square root of the product of molecular weight and viscosity coefficient in a vacuum region where the pressure is a molecular flow region.
An oscillator is manufactured by selecting quartz as a material having a piezoelectric effect, an AC voltage is applied to the oscillator, and an excitation measurement unit in which the frequency is equal to the resonance frequency of the oscillator is manufactured. Excitation parameters ΔF and ΔR As a result of earnest research such as development of a method for simultaneously measuring the molecular weight M, it was found that the relationship between the molecular weight M and the excitation parameters ΔF and ΔR can be approximated by the following equation 1.

Here, M is the molecular weight of the gas, P is the pressure of the gas, and K ₁ , K ₂ , and K ₃ are constants.
Here, the pressure range is 1 Pa or more and 2 MPa or less, preferably 10 Pa or more and 1 MPa or less, and most preferably 100 Pa or more and 0.13 MPa or less.
The constants K ₁ , K ₂ , and K ₃ can be obtained by measuring excitation parameters in advance using a known gas such as high-purity oxygen gas.
From Equation 1, it can be seen that the product of M and P is obtained from measurement of excitation parameters ΔF and ΔR of the transducer sensor. Therefore, in the present invention, the molecular weight M is obtained from Equation 1 by measuring the pressure P using a pressure gauge.
When the temperature of the vibrator fluctuates, the excitation parameter, particularly ΔF, is affected even if the pressure and gas molecular weight are the same.
Therefore, in the present invention, by evaluating the temperature characteristic ΔF (T) of ΔF in advance, it is possible to take a method of obtaining a value (ΔF ₂₅ ) at 25 ° C., for example, from the temperature of the vibrator. The temperature compensation range can be, for example, 15 ° C. to 35 ° C. The vibrator has various temperature characteristics depending on the material, shape, etc., but an easy-to-handle vibrator can approximate the frequency compensation amount by a simple function such as a linear function or a quadratic function of temperature.
In another aspect of the present invention, a method of keeping the temperature of the vibrator constant is adopted in order to omit the procedure by temperature compensation. It is easy to handle the temperature slightly higher than room temperature. The temperature to be constant is any temperature between 25 to 50 ° C, preferably any temperature between 30 to 45 ° C, more preferably 45 ° C.
The average pore size of the filter is preferably in the range of 10 microns to 0.001 microns. The filter material is preferably a ceramic such as stainless steel or alumina.

Example 1
An example in which a quartz substrate is cut out to produce a vibrator is shown in FIG. The width, length, and thickness of the vibrator can be arbitrarily processed. For example, the length may be 1 mm, the width may be 0.1 mm, and the thickness may be 0.05 mm. Au electrodes for applying alternating current are vapor-deposited on both surfaces of the vibrator. The AC voltage to be applied is matched with the resonance frequency of the vibrator. Therefore, the vibrator is incorporated in a part of the feedback circuit of the oscillation circuit. Although the shape of the vibrator is a single beam type in FIG. 1, it is only necessary to have a vibration part in a part such as a tuning fork type beam or a beam that is fixed at both ends, and is not limited to the shape of the vibrator shown in FIG. Absent.
FIG. 2 is a configuration diagram of Example 1, which is an example for measuring the molecular weight of gas flowing down a pipe.
Part of the flowing gas is guided through a filter to the measurement chamber with the vibrator.
The vibrator cooperates with the oscillator so that an AC voltage is applied from an external drive / measurement circuit and its frequency automatically matches the resonance frequency of the vibrator. Further, an excitation parameter measuring device for measuring the applied voltage, the flowing current, and the frequency is provided inside. The amplitude adjuster controls the AC voltage to be applied and continues stable vibration.
A pressure gauge is placed in the measurement chamber to measure the pressure of the gas around the vibrator. The location of the pressure gauge need only be at a position where the pressure of the vibrator can be measured, and need not be in the measurement chamber.
The measurement chamber is kept at a constant temperature of 35 ° C. by the vibrator constant temperature unit.
The filter installed between the gas pipe and the measurement chamber has an average pore diameter of 0.01 microns. The heat of the gas is exchanged with this filter, and the temperature of the gas flowing into the measurement chamber is set to 35 ° C.
The molecular weight calculator obtains the product of molecular weight and pressure from the excitation parameters (frequency and resistance value) from the drive / measurement unit, and then obtains and outputs the molecular weight from the pressure gauge value.

(Example 2)
FIG. 3 is a configuration diagram of Example 2 in which a measurement chamber is provided in a gas flow path. The gas flows through the filter into the measurement chamber. The filter on the outlet side is for preventing the inflow of fine particles from the downstream side when there is a back flow. This structure can be part of the piping and is easy to incorporate, but it can be applied when the gas flow rate is low due to pressure loss due to the filter.

(Example 3)
FIG. 4 is a configuration diagram of the third embodiment in which the temperature of the vibrator is not constant, and the temperature of the vibrator is measured and temperature compensation is performed based on the temperature dependence of the vibration parameter. A temperature gauge is arranged near the vibrator and measures the temperature of the gas. Alternatively, the temperature of the vibrator is measured integrally with the vibrator. As a temperature measuring element, a resistance temperature sensor can be formed by vapor-depositing a metal having temperature dependency on the substrate of the vibrator shown in FIG.
The molecular weight calculator performs temperature compensation of the excitation parameter obtained based on the temperature of the vibrator.
A temperature characteristic ΔF (T) of ΔF is evaluated in advance and stored in the arithmetic unit, and a value (ΔF ₂₅ ) at 25 ° C. is obtained from the temperature of the vibrator. The temperature compensation range is 15 ° C to 35 ° C. A vibrator is used whose frequency compensation amount is expressed by a quadratic function of temperature.

  FIG. 5 is an example of results obtained by using the apparatus of Example 1 shown in FIG. 2, using argon, oxygen, nitrogen, and neon as gases and measuring the molecular weight while changing the pressure. Here, ΔF and ΔR were measured in advance using oxygen as a reference gas, and K1 to K3 of Equation 1 were obtained. The molecular weight other than oxygen is obtained by calculating from K1 to K3 and the measured values of ΔF and ΔR of each gas and normalizing the molecular weight of oxygen as M = 32.

Claims (4)

A measurement chamber seen in the gas to be measured, a vibrator installed in the measurement chamber,
An excitation measurement unit that excites the vibrator and measures the amount of change in the frequency of the vibrator, and the amount of change in the ratio between the applied current and the applied voltage when an AC voltage equal to the resonance frequency of the vibrator is applied to the vibrator ;
A pressure gauge for measuring the pressure of the gas in which the vibrator is placed;
In an apparatus for measuring the molecular weight of a gas provided with a temperature probe for measuring the temperature of the vibrator,
The amount of change in the frequency of the vibrator measured by the excitation measurement unit, the amount of change in the ratio of the current to the applied voltage when an AC voltage equal to the resonance frequency of the vibrator is applied to the vibrator, and the pressure gauge An apparatus for measuring the molecular weight of a gas , comprising: an arithmetic unit that calculates the molecular weight of the gas from the pressure and the temperature measured by the temperature probe. A measurement chamber seen in the gas to be measured, a vibrator installed in the measurement chamber,
An excitation measurement unit that excites the vibrator and measures the amount of change in the frequency of the vibrator, and the amount of change in the ratio between the applied current and the applied voltage when an AC voltage equal to the resonance frequency of the vibrator is applied to the vibrator ;
A pressure gauge for measuring the pressure of the gas in which the vibrator is placed;
In a device for measuring the molecular weight of a gas with a constant temperature chamber that keeps the temperature of the vibrator constant,
The amount of change in the frequency of the vibrator measured by the excitation measurement unit, the amount of change in the ratio of the current to the applied voltage when an AC voltage equal to the resonance frequency of the vibrator is applied to the vibrator, and the pressure gauge An apparatus for measuring a molecular weight of a gas , comprising a calculation unit for calculating the molecular weight of the gas from the pressure and the temperature. The gas molecular weight measuring apparatus according to claim 1 or 2, wherein the gas to be measured is guided to the measurement chamber through a filter that transmits the gas to be measured. The calculation of the molecular weight of the gas in the calculation unit is performed by first using the amount of change in the frequency of the vibrator and the amount of change in the ratio between the applied current and the applied voltage when an AC voltage equal to the resonance frequency of the vibrator is applied to the vibrator. The product of the molecular weight of the gas and the pressure of the gas is obtained, and the molecular weight of the gas is then obtained by dividing the product value by the gas pressure measured by the pressure gauge. The gas molecular weight measuring apparatus according to any one of 1 to 3 .

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