JP2816992B2 - Acoustic density meter - Google Patents

Description

The present invention relates to an apparatus for measuring the density of a gas and a liquid, and particularly to an apparatus for measuring the density of a fluid from a pressure gradient fluctuation generated when an oscillating acceleration is applied to a fluid to be measured. Concerned.

Conventional methods for measuring fluid density include the following.
As a laboratory method, a method of measuring the weight of a fluid to be measured having a fixed volume, a method of utilizing buoyancy by a fluid such as a float and a gas balance. As a method suitable for the purpose of measuring the density momentarily in a process line or the like, a method utilizing the fact that the resonance frequency of a tube filled with a fluid to be measured or a vibrating piece immersed in the fluid to be measured changes with the fluid density, A method that utilizes the fact that the torque generated in one of the two impellers when it rotates is changed by the density of the fluid around the impeller, a method that measures the liquid column pressure at a fixed height, and a gamma ray There is a method utilizing the fact that the ratio of absorption changes depending on the density of the substance.

Apart from the conventional method described above, the L-shaped pitot tube is vibrated to give an oscillating acceleration to the fluid in the tube, and at that time, the pressure fluctuation occurring at the root of the pitot tube is detected and the density of the fluid in the tube is detected. There is a way to measure However, this method requires a special driving device for oscillating the L-shaped pitot tube at an acceleration of a known magnitude, and the measuring device has a complicated structure. Also, if pressure fluctuation in the same phase as the acceleration applied to the fluid is present as external noise, an error occurs in the density measurement.

The present invention also detects a pressure change when an oscillating acceleration is applied to a fluid, but instead of vibrating the entire pipe containing the fluid as in the above-described method, only the fluid in a stationary pipe is used. Is to be excited. Therefore, in the present invention, a special drive device is not required, and ordinary inexpensive devices such as an acoustic speaker and a Roots blower type pump can be used to excite the fluid, and the structure of the device is also simplified. It is possible. Also, in the case where external noise having the same phase as the acceleration applied to the fluid to be measured is present, the present invention detects the fluctuation of the pressure gradient and is not affected. In principle, the above method utilizes the fact that pressure fluctuation proportional to the density of the fluid occurs at the root of the pipe due to the inertial force when the fluid column inside the pitot tube is accelerated as a single object. On the other hand, the present invention utilizes the pressure gradient fluctuation caused by the fluid moving with acceleration in a stationary pipe, and the pressure gradient fluctuation is governed by the equation of motion of the fluid (Navier-Stokes equation). There is a difference. In summary, an object of the present invention is to realize a density meter which is hardly affected by the temperature, pressure and viscosity of a fluid to be measured, has a simple structure and is easy to handle, based on a new operating principle different from the conventional method. It is.

Hereinafter, the configuration of the present invention will be described with reference to FIG. 1 corresponding to the embodiment. In FIG. 1, reference numeral 1 denotes a pipe through which a fluid to be measured having a density ρ flows, and 2 denotes a measuring pipe connected to 1 and filled with the fluid to be measured. 3 and 3 'are intervals The pressure is led to the differential pressure transducer 7 by a pressure guiding tube connected to the measuring pipe 2 at a distance. Reference numeral 4 denotes a back pipe connected to the end of the measurement pipe 2 and connected to the pipe 1. Reference numeral 5 denotes a speaker fixed to the measuring tube 2, which is driven by the amplifier 8 and vibrates the fluid in 2. The back tube 4 is provided to make the pressure before and after the diaphragm of the speaker 5 substantially equal. When the pressure in the tube 1 is almost equal to the atmospheric pressure, the back tube 4 is not necessarily provided. Further, it may be connected to 1 on the upstream side of the measuring tube 2. The differential pressure detector 7 is used for detecting a pressure gradient in the measurement tube 2 as described later. 6 is a speed pick-up for detecting the vibration speed of the diaphragm of the speaker 5, 9 is a subtractor, and 10 is a sine wave sinωt having a constant frequency.
Is a sine wave oscillator. 5, 6, 8 and 9 constitute a speed servo system, and the diaphragm of the speaker 5
It oscillates at a constant amplitude speed with the same waveform as the output signal sinωt of the sine wave oscillator 10. When the frequency of the oscillator 10 is set to a low frequency such that the wavelength of the sound caused by the vibration of the fluid to be measured is sufficiently longer than the entire length of the measuring tube 2, the fluid in the unit 2 vibrates integrally in the tube axis direction. The vibration speed u is also sinω
A signal u _o sin ωt having the same homologous waveform as t and a constant amplitude is obtained. 11
Is a phase shifter, which advances the phase of 10 output signals by 90 °, and cosω
Output t. 12 and 12 'are multipliers, and 13 and 13' are low-pass filters. The output signal of the differential pressure converter 7 is
Is multiplied by the output signal sinωt sine wave oscillator 10 by a multiplier 12, the output of 12 becomes a signal E _x is smoothed by the low pass filter 13. Similarly, the output signal cosωt output signal and the phase shifter 11 of the differential pressure transducer 7, 'is multiplied by 13' 12 becomes smoothed by the signal E _y by. In short, 12, 12 ', 13, 13' are constituted the synchronous rectifier circuit, of the output signal of the differential pressure transducer 7, signals E _x and cosωt in phase representing the magnitude of the component of sinωt and phase outputs a signal E _y representing the magnitude of the component of, and serves to remove a noise component. 14 is a subtractor and 15 is an indicating instrument.

When the fluid in the measurement tube 2 is moving in the direction of the tube axis at the speed u, the following equation of motion is established.

ρ∂u / ∂t = -∂p / ∂x (1) where t represents time, p represents pressure, and x represents distance in the tube axis direction. Because it is u = u _o sinωt as described above, the acceleration ∂u / ∂t fluid is equal to? U _o cos .omega.t. On the other hand, the frequency of the vibration velocity is such a low frequency that the wavelength is sufficiently longer than the entire length of the measuring tube 2, so that the axial pressure gradients in 2 are equal everywhere. Therefore, the distance in the measuring tube 2 Let Δp be the differential pressure measured at a distance of Holds, and the differential pressure Δp is proportional to the pressure gradient Δp / Δx. From equations (1) and (2) And the differential pressure Δp becomes a cosine wave signal. Therefore, the signal E _y becomes a signal proportional to the amplitude of Δp, Because the size of the? U _o is constant, E _y is the signal representing the magnitude of density of the fluid [rho. On the other hand, the signal _Ex is zero. In Figure 1, rather than entering directly the E _y to indicating instrument, than is input to the indicating instrument in After Hikisa' signals E _x, as the density of large fluid viscosity can be accurately measured To do that.

Next, the case where the viscosity of the fluid is large and the influence thereof appears will be described. Now, assuming that the measuring pipe 2 is a pipe having a circular cross section, the differential pressure Δp when there is an influence of viscosity is obtained from the equation of motion. Here, γ = (2μ / ρω) ^1/2 / a μ is obtained as the viscosity coefficient, and a is obtained as the inner radius of the tube 2. That is, a component proportional to the vibration velocity (sin ωt component) appears in Δp, and a component proportional to the acceleration (conωt component) increases as compared with the case where there is no viscosity. As can be seen from equation (4), the signal E _y becomes (1 + gamma) times the magnitude in the absence of viscosity, the signal E _x is a gamma multiple of the signal E _y in the absence of viscosity. Therefore, by subtracting E _x from E _y by the subtractor 14, the correct density can be measured even in the case of a highly viscous fluid. Even if the measuring tube 2 does not have a circular cross section, since the same relational expression as the expression (4) holds (the magnitude of γ is different), the density measurement can be performed with the same configuration. As can be seen from equation (4), when the radius a of the measuring tube 2 is increased and the angular frequency ω of the sine wave oscillator 10 is increased, the influence of the viscosity is reduced.
If appropriate a and ω are selected, the effect of the viscosity can be almost neglected except for a fluid having a particularly high viscosity. In this case, directly input signals E _y to an indicating instrument 15, can be configured density system.

In the embodiment of FIG. 1, since the shape of the measuring tube 2 is simple, even when the density of the fluid flowing through the main tube 1 changes,
The fluid in the measuring tube is replaced within a relatively short time and has the same density as the fluid in the main tube. However, the required time varies depending on the speed of the flow in the main pipe 1 and the like. The embodiment of FIG. 2 solves this disadvantage. In FIG. 2, reference numeral 16 denotes a sampling pump which sucks in a fixed volume of fluid from the main pipe 1 through the measuring pipe 2 and discharges the fluid through the back pipe 4 to 1. Thus, when the fluid density in the main tube changes, the fluid density in the measuring tube becomes equal to the density in the main tube for a certain short time. Here, a pump that suctions and discharges fluid in a pulsating manner is used. Then, the velocity of the fluid in the measuring pipe 2 in the axial direction also pulsates, and the fluid receives an oscillating acceleration. For example, when the Roots blower is rotated at a constant speed, the flow velocity in the measurement tube 2 pulsates, and the pulsation waveform is determined by the shape and the rotation speed of the Roots blower. That is, in this embodiment, sampling of the fluid to be measured and applying acceleration to the fluid to be measured are performed by one sampling pump. Reference numeral 17 denotes a power supply for driving the sampling pump. The sampling pump is driven under the condition that the wavelength of the sound in the fluid to be measured due to the pulsation is sufficiently longer than the entire length of the measuring tube 2. Reference numeral 18 denotes a pressure transducer which detects the pressure in the measuring pipe 2 guided by the pressure guiding pipe 3. Fluctuation of the pressure gradient in the axial direction occurs in the measurement tube 2 due to the pulsation of the fluid. However, since the frequency of the pulsation is low, the pressure gradient is 2.
Equal everywhere in the world. On the other hand, at the pipe end where the measuring pipe 2 is connected to the main pipe 1, the pressure is constant. Therefore, from the pipe end Measuring the pressure trajectory fluctuations at points just apart by a pressure transducer is equivalent to measuring the fluctuations of the pressure gradient in the measuring tube 2. Note that the pressure detector 18 may be attached to the back tube 4 instead of the measurement tube 2. However, in this case, the polarity of the signal is inverted. Reference numeral 19 denotes a synchronous signal generator which outputs a sine wave having a constant amplitude and the same phase as the fundamental wave of the acceleration waveform of the fluid. 12 is a multiplier, 13 is a low-pass filter, and 15 is an indicating instrument.

Since the flow velocity in the measuring tube 2 is determined by the shape and the operating speed of the sampling pump 16, the acceleration waveform of the fluid is also determined,
The magnitude of the pulsation is constant. The equation of motion (1) also holds for the fluid in the measuring tube 2.
Therefore, the output of the pressure transducer 18 has a waveform similar to the acceleration waveform, and the magnitude of the fluctuation is proportional to the density of the fluid to be measured. By multiplying the outputs of the pressure transducer 18 of the synchronizing signal generator 19, by smoothing, the signal E _o which is proportional to the magnitude of the fundamental wave component of the pressure gradient change is obtained. The size of the fundamental wave of the pressure gradient change is proportional to the magnitude of the overall pressure gradient variations, signal E _o is proportional to the density of the fluid to be measured. Here, the reason why the density is obtained from the fundamental wave component of the pressure gradient using the synchronous detection is to remove noise caused by the flow in the main pipe 1 and the like. When the signal-to-noise ratio is relatively large, it is not always necessary to perform synchronous detection, and a signal proportional to the density can be obtained by passing the output of the pressure transducer through a band-pass filter and rectifying the output. . Further, the polarity is inverted at an appropriate phase in synchronization with the rotation of the sampling pump by another synchronization signal generator, ±
Synchronous detection may be performed by generating a binary signal of 1, or correlation detection may be performed by generating an acceleration waveform itself.

As is apparent from the principle described above, when a pump such as a Roots blower is connected to the main pipe 1 through which the fluid flows and the flow has a pulsation, the measuring pipes 2 and 4 are used.
It is not necessary to measure the pressure gradient of the main pipe directly,
You can know the density. Further, when the viscosity of the fluid is large, the influence of the viscosity can be eliminated by using a signal component proportional to the pulsation speed of the fluid, that is, a signal orthogonal to the acceleration component, as in the embodiment of FIG. .

As described above, according to the present invention, the density is obtained by knowing the oscillating acceleration and the pressure gradient fluctuation generated along with the vibrational acceleration, based on the equation of motion that is established between the density, the acceleration, and the pressure gradient of the fluid. In order to apply acceleration to the fluid, a speaker is used in the embodiment of FIG. 1, and a sampling pump is used in the embodiment of FIG. In this way, means for applying acceleration to the fluid, means for detecting the pressure gradient,
Various methods can be considered for an electronic circuit or the like that performs signal processing, and the essence of the present invention is not affected by these specific methods. It goes without saying that the application is not limited to the measurement of the density of the fluid flowing in the pipe. The operating principle of the present invention is based on the equation of motion, which is one of the most basic separation methods. For this reason, the operation principle of the present invention is valid regardless of temperature, pressure, and the like. Therefore, it is possible to measure the density without being affected by these conditions. The viscosity of the fluid is related to the equation of motion, but since the relationship is clearly understood, the influence can be easily eliminated as described above. Further, the density meter according to the present invention has an advantage that the structure is simple, it can be realized compactly, and the handling is simple. Furthermore, it is a feature of the present invention that dust in the fluid is not affected even if it adheres to the movable part.

[Brief description of the drawings]

FIG. 1 shows an embodiment of the present invention, and FIG. 2 shows another embodiment of the present invention. 1 ... pipe through which fluid flows, 2 ... measurement pipe, 3 3 '... pressure guide pipe, 4 ... back pipe, 5 ... speaker, 6 ... speed pickup, 7 ... differential pressure transducer, 8 ... ... Amplifier, 9 ... Subtractor, 10 ... Sine wave oscillator, 11 ... Phase shifter, 12, 12 '...
... Multipliers, 13, 13 '... Low-pass filters, 14 ... Subtractors, 15 ... Indicating instruments, 16 ... Sampling pumps, 17
... power supply, 18 ... pressure transducer, 19 ... synchronization signal generator.

────────────────────────────────────────────────── ─── Continuing from the front page (56) References Report from the Institute of Space and Astronautics, The University of Tokyo, 17, [1 (A)] (1981) Yasushi Ishii, P. 139-153, New Measurement of Vibrating Pitot-Tube-Mass Flow Law (58) Field surveyed (Int.Cl. ⁶ , DB name) G01N 9/00-9/36 JICST file (JOIS)

Claims (1)

(57) [Claims] 1. A stationary tube filled with a fluid, means for applying an oscillating acceleration to the fluid in the tube, and a detector for measuring a change in a pressure gradient in the tube. A density meter, wherein the density of the fluid in the pipe is known from the measured pressure gradient and the acceleration.