JP3432415B2 - Vibrating tube density meter - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a vibrating tube densitometer for measuring the density of a fluid to be measured in a vibrating tube, and more particularly to a vibrating tube densitometer having optical vibration detecting means.

[0002]

2. Description of the Related Art The measurement of the density of a fluid is very important to know the properties of the fluid, and various density meters have been used. A vibration tube densitometer is one of the densitometers.
The vibrating tube densitometer measures the density of the fluid to be measured by utilizing the fact that the natural frequency of the vibrating tube containing the fluid to be measured depends on its mass.

For example, the magazine “Rev. Sci. Inst”
rum. , Vol. 55, No. 4, April 19
84 P. 589-593 ", such a vibrating tube densitometer is shown. In this vibrating tube densitometer, a drive wire and a pickup wire are attached to the vibrating tube, and these wires are arranged in a constant magnetic field by a permanent magnet. Then, by vibrating the vibrating tube by the drive current to the drive wire and feedback controlling the drive current according to the current generated in the pickup wire, the vibration frequency of the vibrating tube becomes the resonance frequency (natural frequency). Match. Then, the density is obtained by measuring the resonance frequency.

Further, Japanese Patent Laid-Open No. 5-142134 discloses such a vibrating tube densitometer. In the vibrating tube densitometer of this publication, a magnetic body is attached to the vibrating tube, and the vibrating tube is vibrated by the drive coil. Then, the pickup coil detects the current generated based on the vibration of the vibrating tube. Then, by feeding back the detected current to the drive coil (feedback control),
The vibrating tube is vibrated at the natural frequency, and the natural frequency is obtained.

In such a vibrating tube densitometer, the density of the fluid can be measured while the fluid to be measured is contained in the vibrating tube.
Therefore, it is widely used for density measurement in various process lines. In particular, density measurement of various fluids in the supercritical state is very important for knowing the properties of the fluid in the supercritical state, and there is a great demand for measuring this. It is considered that the vibrating tube densitometer is suitable for the density measurement of such a fluid because the fluid to be measured can be measured by the densitometer under high temperature and high pressure.

[0006]

In the conventional vibrating tube densitometer, the frequency of the vibrating tube is controlled by feedback control. Therefore, when the density of the fluid to be measured changes, it takes a considerable time to adjust the vibration of the vibrating tube to the natural frequency, and there is a problem that the followability is poor. Moreover, in the conventional vibrating tube densitometer, a counter is used to measure the frequency. That is, the frequency is measured by measuring the period from the number of zero crosses of the detected current and the time interval therebetween. Therefore, it takes time to measure the frequency itself, and it is difficult to measure the frequency in the transient state of the fluid.

The present invention has been made in view of the above problems, and an object of the present invention is to provide a vibrating tube densitometer having improved followability.

[0008]

SUMMARY OF THE INVENTION The present invention is a vibrating tube densitometer for measuring the density of a fluid to be measured in a vibrating tube, wherein the vibrating tube is impacted or vibrated, or vibrated within a predetermined range.
Vibrating means for sweeping and vibrating the frequency, and optical vibration detecting means for irradiating the vibrating tube with light rays to receive the reflected light and detecting the vibration of the vibrating tube based on the received reflected light, Resonant frequency of the vibrating tube based on the detected vibration of the vibrating tube
Is obtained, and the density of the fluid to be measured is measured based on this resonance frequency .

As described above, according to the present invention, the vibration frequency of the vibrating tube is detected by the optical vibration detecting means based on the reflected wave at the vibrating tube. Therefore, the vibrating tube need only reflect light, and it is not necessary to attach an extra member to the vibrating tube. Therefore, it is easy to perform accurate density measurement. Moreover, since it is not necessary to install an extra member (for example, a permanent magnet or the like) near the vibrating tube, the vibrating tube becomes compact and can be easily housed in a constant temperature bath or the like.

Further, the present invention is characterized in that the optical vibration detecting means detects the frequency by irradiating a laser beam and detecting a Doppler shift in the reflected light. By detecting the Doppler shift of the laser light in this way, the speed of the vibrating tube can be instantaneously measured, and the frequency can be detected in a relatively short time based on the speed. Therefore, the frequency can be measured immediately, and the followability of the measurement is improved. Therefore, even when the density of the fluid to be measured changes rapidly, it is possible to follow this. This makes it possible to follow the phase change of the fluid to be measured.

Further, the present invention is characterized in that the vibrating means includes a piezoelectric element and transmits the vibration of the piezoelectric element to a vibrating tube. By using the piezoelectric vibrating element, various vibration modes (shock wave, sine wave, rectangular wave, etc.) and vibration frequencies can be easily changed.

[0012]

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention (hereinafter referred to as embodiments) will be described below with reference to the drawings.

FIG. 1 is a diagram showing the overall configuration of a vibrating tube densitometer according to the embodiment. The constant flow rate pump 10 is composed of a syringe pump and quantitatively pumps the liquid to be measured. The discharge end of the constant flow rate pump 10 is connected to one end of the preheating unit 14 via the supply pipe 12, and the fluid to be measured has the preheating unit 1
Sent to 4. The other end of the preheating section 14 is connected to one end of the vibration tube 16.

The vibrating tube 16 is formed in a U shape as a whole, and an end portion (base portion) 16a on the side where the two U-shaped tubes are parallel to each other is fixed to a fixing portion 17. The fixing portion 17 is composed of, for example, two plate-shaped members joined by bolts, and sandwiches and fixes the vibrating tube 16. The vibrating tube 16 is bent at an intermediate portion thereof so that the U-shaped tip portion 16b faces the base portion 16a. Therefore, the tip portion 16b of the vibrating tube 16 is a free end. In such a vibrating tube 16, since there is a bent portion between the fixed end fixed by the fixing portion 17 and the free end of the U-shaped tip, this acts as a spring,
The vibration at the tip of the vibrating tube 16 depends on the mass of the vibrating tube 16 (including the fluid to be measured), and vibration suitable for density measurement is obtained.

The other end of the vibrating pipe 16 is connected to a discharge pipe 18, and the discharge pipe 18 is connected to a pressure control unit 22 after passing through a cooling unit 20. This pressure control unit 22
Maintains the pressure in the system at a predetermined pressure. Therefore, the constant flow rate pump 10 determines the flow rate of the fluid to be measured in the vibrating tube 16, and the pressure control unit 22 determines the pressure of the fluid to be measured in the vibrating tube. The pressure control unit 22 can also be composed of a syringe pump.

The preheating section 14 and the vibrating tube 16 are housed in a constant temperature bath 24, and the temperature of the fluid to be measured in the vibrating tube 16 can be maintained at a predetermined temperature. The cooling unit 20 is for reducing the temperature of the fluid to be measured flowing into the pressure control unit 22. The fixed portion 17 fixes the base portion of the vibration tube 16 to the constant temperature bath 24. The preheating unit 14 does not have to be housed in the same constant temperature bath 24 as the vibrating tube 16, but it is also preferable to provide another preheating tank and house the preheating unit 14 therein.

A thermometer 26 is connected to a portion of the preheating section 14 immediately before being connected to the vibrating tube 16.
The temperature of the fluid to be measured introduced into the is measured. By controlling the flow rate of the constant flow pump 10 and the temperature of the constant temperature bath 24 according to this measured value, the temperature of the fluid to be measured can be maintained at an appropriate level. In particular, when a preheating tank is provided separately, by controlling the heating state in the preheating tank,
The temperature can be controlled more preferably.

Further, the discharge pipe 18 is provided with a pressure gauge 28 so that the pressure in the vibrating pipe 16 can be measured. Therefore, by controlling the pressure controller 22 according to the detection result of the pressure gauge 28, the pressure of the fluid to be measured in the vibrating tube 16 can be accurately controlled.

A vibration transmission member 30a of the actuator 30 is arranged near the fixed portion 17 of the vibration tube 16. The vibration transmission member 30a is pressed against the vibration pipe 16 from above, and the vibration is transmitted to the vibration pipe 16 by vibrating the vibration transmission member 30a. As described above, in the present embodiment, the vibration transmitting member 30a and the vibration pipe 16 are brought into contact with each other by the elasticity of the vibration pipe 16. The actuator 30 has a piezoelectric element, and the vibration of the piezoelectric element is transmitted to the vibration transmitting member 30a.
Output to.

A function generator 32 is connected to the actuator 30. The function generator 32 supplies a predetermined AC voltage to the actuator 30, thereby driving the piezoelectric element to vibrate the vibration transmitting member 30a. The function generator 32 changes the frequency of the output AC voltage within a predetermined range. That is, the actuator 32 vibrates the vibrating tube 16 according to the frequency of the voltage generated by the function generator 32.

The U-shaped tip of the vibrating tube 16 is irradiated with laser light from the laser Doppler vibrometer 40, and the reflected light reflected by the surface of the vibrating tube 16 is received by the laser Doppler vibrometer 40. It has become so. Then, the reflected light undergoes a Doppler shift according to the speed (vibration) of the laser reflection portion of the vibration tube 16. Therefore, the laser Doppler vibrometer 40 calculates, based on the wavelength of the received reflected light,
Detect the speed of the vibrating tube. That is, a signal regarding the time change of the speed (a signal regarding vibration) is output. The reflected surface of the vibrating tube 16 is reflected by the laser Doppler vibrometer 4
It is also preferable to polish it to a mirror surface or flatten it so that it can easily enter 0. The velocity signal may be integrated and a signal regarding the displacement may be output.

The signal about the vibration detected by the laser Doppler vibrometer 40 is supplied as an electric signal to the FFT analyzer 42, where the frequency analysis is performed and the intensity signal on the frequency axis is obtained. Here, the FFT analyzer 42 can analyze in a wide frequency range. Therefore, it is easy to detect harmonics that are three or five times the actual frequency. The analysis of such harmonics can improve the accuracy of detecting the peak of the natural frequency. That is, in the FFT analyzer 42,
Since the resolution on the frequency axis is basically the same, 3 times,
As a result of the analysis at the frequency of 5 times, the accuracy of the peak frequency analysis can be improved.

Then, the function generator 32
By sweeping the vibration frequency within a predetermined range, the vibration frequency of the actuator 30 is changed by the vibration tube 16
Resonates when it matches the natural frequency of. Therefore, F
In the FT analyzer 42, a large peak occurs in the natural frequency at the time of resonance. Therefore, the natural frequency of the vibrating tube 16 can be obtained by obtaining the peak frequency from the frequency analysis result. The natural frequency of the vibrating tube 16 is a function of the density of the fluid inside the vibrating tube 16,
The fluid density can be obtained from the detected natural frequency.

As described above, in the vibrating tube densitometer of the present embodiment, the laser Doppler vibrometer 40 is used, and the natural frequency of the vibrating tube 16 is obtained from the frequency analysis of the output. The amount of data (time) for vibration required for frequency analysis is
Although it depends on the frequency analysis range (frequency range), it is not so large. Therefore, the frequency can be obtained relatively early, and the density can be measured by following the change of the fluid to be measured in the vibrating tube 16. Furthermore, it is possible to follow the phase change of the fluid to be measured in the vibration tube 16.

Further, in this embodiment, the FFT analyzer 42 and the function generator 32 are connected. Therefore, the FFT analyzer 42 recognizes the frequency of the electric signal output from the function generator 32. Therefore, the function generator 32
It is also possible to perform feedback control on the frequency of the electric signal generated in the above-mentioned manner based on the signal from the laser Doppler vibrometer 40. For example, in the initial peak search, FF
Frequency range to be analyzed by the T analyzer 42 (frequency sweep range of the function generator 32)
A wider peak makes it easier to find peaks. Then, when the peak is found, the frequency range is set in the vicinity of the peak, so that the resolution can be increased. Further, in a state where the density of the fluid to be measured changes rapidly such that the phase changes, the sweep range of the frequency in the function generator 32 is widened, and the frequency range to be analyzed by the FFT analyzer 42 is widened correspondingly. It is preferable to keep it.

The electric signal output from the function generator 32 is preferably a sine wave signal. With this electric signal, the vibration of the piezoelectric vibrator of the actuator 30 can be made smooth, and the vibration tube 16
It is possible to prevent the generation of unnecessary higher harmonic components. Furthermore, it is advisable to smoothly change the transmission frequency with a sine wave. That is, in the sweep frequency range, the vibration frequency is temporally changed in accordance with the sine wave, so that the vibration frequency of the excitation by the actuator 30 can be appropriately swept.

The vibrating tube 16 is made of stainless steel (SU).
A φ1 / 8 inch tube (quenching process) of S316) or the like can be used, but an appropriate one can be adopted according to other temperature, pressure, and the like. For example, Hastelloy (Ha
stelloy), Inconel, platinum rhodium,
Titanium or the like can be used. As the wall thickness of the vibrating tube 16 is smaller than the diameter, the ratio of the weight of the fluid to be measured to the total weight is larger, and the sensitivity of density measurement can be increased. Considering these conditions, an appropriate material, pipe diameter, and wall thickness can be selected according to the measurement target, measurement conditions, and the like.

The vibration transmitting member 30a of the actuator 30 is in direct contact with the vibrating tube 16 in the constant temperature bath 24. Therefore, the vibration transmitting member 30a also needs to be made of a material that is resistant to high temperatures, and it is preferable that it is difficult to transfer heat to the piezoelectric vibrating element. The vibration transmitting member 30a can be made of, for example, ceramic. Further, when it is made of metal such as stainless steel, it is preferable to attach a cooling device using water or the like.

Further, in the above embodiment, the actuator 30 using the piezoelectric element is used as the vibrating means. However, the vibrating means is not limited to this,
A crank driven by a motor may be used. Alternatively, the vibration tube 16 may be impacted with a hammer or an air gun, and the vibration of the vibration tube 16 thereafter may be measured. That is, if the vibrating tube 16 is vibrated by one forced displacement, the vibration frequency of the vibrating tube 16 becomes the natural frequency. Therefore, the natural frequency can be easily detected by the direct frequency analysis without sweeping the vibration frequency and analyzing the frequency change of the vibration amplitude.

Next, actual measurement by such a vibrating tube densitometer will be described. First, the mass (density: ρ) of the vibrating tube 16 and its resonance frequency f have the relationship of the following expression (1).

Ρ = (k / f ² −1) / w (1) Here, k and w are device constants, which are functions of the temperature and pressure of the tube.

Therefore, vibration measurement is performed by using at least two fluids of known density, and a calibration curve is created. For example, the vibration is measured by filling the inside of the vibrating tube 16 with a substance having a known density such as methanol, water, or nitrogen gas, or by evacuating the inside. Thus, the device constants k and w are obtained and the calibration curve is obtained.

Then, the fluid to be measured is circulated through the vibrating tube densitometer and the natural frequency (resonance frequency) is detected.
The density of the fluid to be measured is measured by the equation (1). The calibration curve is preferably performed at the temperature and pressure at which the fluid to be measured is desired to be measured, but it is not necessary to create the calibration curve at all temperature and pressure. For example, if the device constants k and w are obtained as a function of temperature and pressure in a predetermined range, they can be used for density measurement of the fluid to be measured in that range. Here, it is preferable that the FFT analyzer 42 be provided with a function of storing the calibration curve and calculating and outputting the density, or another data processing device may perform this processing.

Further, in the vibrating tube densitometer of this embodiment, the reflected light of the vibrating tube 16 is used for vibration detection. Therefore, the frequency can be detected in a non-contact manner, and there is no need to attach a detection cable or the like, and accurate vibration can be detected. In addition, the device can be downsized as a whole, and it can be easily incorporated into an existing device, thus increasing versatility. Furthermore, since no permanent magnet is used, it is possible to perform measurement even at a temperature above the Curie point, which is suitable for measurement at high temperature.
Also, the pressure of the fluid to be measured can be made high by selecting an appropriate strength for the vibrating tube 16 and the like. Therefore, the vibrating tube densitometer of this embodiment is suitable for measuring the density of a high-temperature and high-pressure fluid such as a supercritical solvent.

"Laser Doppler Vibrometer" FIG. 2 shows a schematic configuration of the laser Doppler vibrometer 40. In this way, the laser light from the laser light source 50 is applied to the vibrating tube 16 via the acousto-optic modulator 52 and the lens 54.
The acousto-optic modulator 52 allows the light reaching the lens 54 to pass through without being modulated. The light reflected by the vibrating tube 16 is received by the light receiving element 58 via the lens 54 and the beam splitter 56. The remaining light passing through the acousto-optic modulator 52 is modulated and is incident on the beam splitter 56 via the reflecting mirror 62 as reference light having a constant frequency shift. Therefore, the interference light between the reference light, which has been subjected to the constant frequency shift by the acousto-optic modulator 52, and the reflected light, which has undergone the Doppler shift in the vibrating tube 16, receives the light receiving element 58.
Is received by. Therefore, the beat frequency of the interference light is an FM modulated wave centered on the shift frequency by the acousto-optic modulator 52. By converting this into an electric signal by the light receiving element 58 and performing FM demodulation in the converter 60, a voltage signal proportional to the vibration speed of the vibration tube 16 can be obtained. Here, since the present laser Doppler vibrometer 40 uses the reference light that is shifted by the predetermined frequency by the acousto-optic modulator 52 as described above, it is possible to obtain a velocity signal having a positive or negative sign. The signal is output as a signal indicating the vibration of the vibration tube 16. Such a laser Doppler vibrometer is available, for example, from Ono Sokki Co., Ltd. LASERVIBROME.
There is TER LV-1300 (trade name). It should be noted that it is also possible to use a displacement meter that detects the displacement that integrates the velocity, and in this case, the frequency of the displacement exhibiting the maximum resonance state may be detected.

[0037]

EXAMPLES The measurement performed by the apparatus of the embodiment will be described. First, methanol (CH ₃ OH) and water (H ₂
O) was adopted as a calibrator and the measurement was performed. Using the function generator 32, set the frequency to 0-25
The results of FIGS. 3A and 3B were obtained by the FFT analyzer 42 when the sweep was performed in the range of 0 Hz. Temperature 20
These are the data at ° C and atmospheric pressure.

From FIGS. 3A and 3B, the resonance frequency of water was 219.7320 Hz, and the resonance frequency of methanol was 220.8667 Hz. The density of water and methanol at this time, from the literature,
It is 0.997 g / cc and 0.792 g / cc.

By substituting this value into the above equation (1), k = 50814.9 and w = 0.052616 are obtained. Therefore, the density ρ in this vibrating tube densitometer
And the resonance frequency f are expressed by the following equation (2).

Ρ = (50814.9 / f ² −1) /0.052616 (2) Using this as a calibration curve, measure the fluid to be measured with the vibrating tube 16
It can be distributed to and its density can be measured.

The resonance frequency of n-amyl alcohol (C ₅ H ₁₁ OH) was determined to be 220.801, which gave a calculated density (measured density) of 0.804. The literature value density of n-amyl alcohol is 0.810.
The difference is 0.006, which means that the measurement is performed with considerably high accuracy.

In this embodiment, the FFT analyzer 42 has 2048 channels. FF
The resolution of the T analyzer 42 is the target frequency range / number of channels, and in the above example, the resolution is 0.12207 Hz because the target frequency range is 0 to 250 Hz. On the other hand, actually, the above-described measurement can be performed with 3 Hz as the target vibration range. In this case, the frequency resolution is 3 Hz / 2048 = 0.00146 Hz. Therefore, the density difference = 0.997 g / cc-0.792
Multiplying this frequency resolution by g / cc = 0.205 g / cc gives a theoretical density resolution of approximately 2.6 × 10 ⁻⁴ g
It should be able to rise to about / cc. Further, theoretically, the apparatus of the present embodiment should be able to increase the resolution by increasing the resolution in the FFT analyzer 42.

[0043]

As described above, according to the present invention,
Since the optical vibration detecting means is used, it is possible to follow the change in density of the fluid to be measured, and it is possible to perform appropriate measurement by following the change in density.

[Brief description of drawings]

FIG. 1 is a diagram showing an overall configuration of a vibrating tube densitometer according to an embodiment.

FIG. 2 is a diagram showing a configuration of a laser Doppler vibrometer.

FIG. 3 is a diagram showing an example of frequency measurement.

[Explanation of symbols]

10 constant flow pump, 12 supply pipe, 14 preheating section, 1
6 vibration pipe, 16a base, 16b tip, 17 fixing part, 18 discharge pipe, 20 cooling part, 22 pressure control part,
24 constant temperature bath, 30 actuator, 30a vibration transmission member, 32 function generator, 40 laser Doppler vibrometer, 42 FFT analyzer.

Continuation of the front page (56) References JP-A-5-296910 (JP, A) JP-A-9-126851 (JP, A) JP-A-63-262526 (JP, A) Actual development Sho-53-14570 (JP , U) (58) Fields investigated (Int.Cl. ⁷ , DB name) G01N 9/00 G01F 1/00 G01H 13/00

Claims (6)

(57) [Claims] 1. A vibrating tube densitometer for measuring the density of a fluid to be measured in a vibrating tube, comprising: vibrating means for applying an impact to the vibrating tube to vibrate; And the optical vibration detection means for detecting the vibration of the vibrating tube based on the received reflected light, and the resonance frequency of the vibrating tube based on the detected free frequency of the vibrating tube after the impact is applied. A vibrating tube densimeter, which is characterized in that the density of a fluid to be measured is measured based on the resonance frequency. 2. The vibrating tube densitometer according to claim 1, wherein the optical vibration detecting means detects a vibration by irradiating a laser beam and detecting a Doppler shift in the reflected light. A vibrating tube densitometer. 3. The vibrating tube densitometer according to claim 1 or 2 , wherein the vibrating means includes a piezoelectric element, and transmits the vibration of the piezoelectric element to the vibrating tube. 4. A vibration densitometer according to any one of claims 1-3, wherein the vibrating tube is formed by U-shaped tube, fixing the two proximal consisting parallel tubes of the U-shaped tube Then, the middle portion is bent into a semicircular shape while keeping the parallel tubes in a parallel state, whereby the closed tip of the U-shaped tube is shaped to face the base side. Total. 5. A vibration Dokan, and vibration means for vibrating by sweeping the excitation frequency in a predetermined range, by irradiating a light beam to vibrating tube receives the reflected light, the received reflection light Based on the detected resonance frequency of the vibrating tube based on the peak frequency of the detected vibration of the vibrating tube, the optical vibration detecting means for detecting the vibration of the vibrating tube based on
A vibrating tube densitometer for measuring the density of a fluid to be measured , wherein the vibrating tube is formed by a U-shaped tube, the base side of the two parallel tubes of the U-shaped tube is fixed, and both parallel tubes are parallel. The vibrating tube densitometer, characterized in that the intermediate portion is bent into a semicircular shape while maintaining such a state, whereby the closed front end of the U-shaped tube faces the base side. 6. The vibration tight timepiece according to claim 1, wherein when detecting the resonance frequency of the vibrating tube, a harmonic of the fundamental vibration is detected to detect the fluid to be measured. A vibrating tube densitometer characterized by measuring density.