JPH0678924B2 - Mass flow meter - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a mass flow meter, and more particularly to a Coriolis force type mass flow meter whose main component is an axisymmetric curved conduit, in which the density of a fluid to be measured is changed. The present invention relates to a mass flowmeter that does not cause an error.

Prior Art When a fluid of unit mass m flows at a flow velocity V in a conduit,
When this fluid is oscillated at an angular velocity ω, the Coriolis of the direction in which the flow tube fluid is proportional to the mass flow rate of the fluid, and which corresponds to the vector product of the flow direction of the fluid and the vector direction of the angular velocity vibrating the fluid It is known that force is generated, and a mass flowmeter utilizing this principle is disclosed in, for example, Japanese Patent Laid-Open No. 54-5257.
It is known in Japanese Patent Publication No. 0. As shown in FIG. 2, this flow meter has a U-shaped conduit (curved conduit) 1 fixedly supported by a supporting member 2 symmetrically with respect to an axis XX ', and one end of a reciprocating member 3 extending in the direction of the axis XX' is supported. It is fixed to the member 2.
At the other end of the reciprocating member 3, there is an axis Y connecting the fixing points of the conduit 1.
An electromagnetic coil 4 is provided which vibrates at a frequency substantially equal to the natural frequency of the conduit 1 around Y '. The magnet 4 is attached to the conduit 1 via the holding plate 6 on the axis of the electromagnetic coil 4, and is absorbed and repulsed in the ZZ 'direction by a driving source (not shown). As a result, the conduit 1 moves around the axis YY'. Driven. As a result, Coriolis force acts on the liquid flowing in the Q direction, and a rotational force is generated around the XX 'axis. From the change in the amount of light in the photodetector 7 that is shielded by the shielding plates 8 that are symmetrically fixed to both arms of the conduit 1, a time difference for both arms of the conduit 1 to pass through the neutral plane is obtained. Seeking the power of.

In the above U-shaped curved conduit, the Coriolis force is obtained as the time difference between the reference plane of the conduit and each arm of the conduit. In the method, the torsion moment about the X axis and the torsion angle are proportional to each other. In the range, the vibration frequency term, which is one of the proportional terms of the Coriolis force, is eliminated, and the mass flow rate is obtained as a function of the time difference only.

Problems in the Related Art In the above-described related art, by vibrating in a tuning fork shape by the conduit 1 and the reciprocating member 3, which are selected as the substrate common to the supporting member 2 so that the natural frequencies are substantially equal, Attempts have been made to minimize the electromagnetic drive energy by the coil 4 and the magnet 5, and generally the displacement of the conduit 1 due to the Coriolis force, which is extremely small relative to the vibration amplitude when vibrating at such a resonance frequency, By using a curved conduit such as a U-shape, it is possible to increase the rotational moment about the XX 'axis and to obtain a substantially large displacement amount. As a result, highly sensitive displacement detection was possible and a highly accurate mass flow rate could be obtained. As is clear from the principle, the Coriolis force in such a vibration system is generated at a frequency equal to the vibration frequency of the conduit 1 around the YY 'axis. In the conventional technique, X is more than the natural frequency of the conduit 1 around YY ′.
Although the natural vibration around the X ′ axis is large, ideally the vibration / amplitude characteristics around the XX ′ axis where the force of the orientation is generated is that the amplitude is constant even if the vibration frequency changes, This means that the amount of displacement due to the force of the orientation does not change even if the vibration frequency changes. However, in reality, when the vibration frequency changes due to the vibration characteristics around the XX 'axis, different displacement amounts are detected. Since the natural frequency of the conduit 1 changes depending on the density of the fluid to be measured, the mass flow rate also slightly changes. For a mass flow meter that measures the mass flow rate without being affected by the density of the fluid, this is a serious problem in seeking accuracy improvement.

Means for Solving the Problems To solve the above-mentioned problems, the present applicant has found that even when the densities of the fluids to be measured are different, when the fluids of different densities flow around the XX ′ axis of the curved conduit 1. When the ratio of the natural frequency ω _T and the natural frequency ω b around the YY ′ axis has a constant value even if the fluid density changes, it is experimentally shown that an accurate mass flow rate that is not affected by the density can be obtained. I confirmed.

Example In FIG. 2, assuming that the mass of the conduit 1 through which the fluid of density ρ is flowing is M, the spring constant about the Y axis is Ky, the moment of inertia about the X axis is J, and the torsion spring constant is Kθ, respectively. The natural frequencies ωy and ωθ of Can be expressed as

The mass of the conduit 1 and the mass of the fluid existing in the conduit 1 are respectively mp
And ml and the moment of inertia are jp and jl, the equation (1) is It is represented by. In the equation (2), the spring constants Ky and Kθ around the Y axis and the X axis of the conduit 1 are ωy within a certain range.
When the condition that the ratio between ω and ωθ is constant is obtained, the ratio of the mass mp of the conduit 1 to the moment of inertia jp is calculated as follows:
Is equal to the ratio of the moment of inertia jl, Is to satisfy the formula. Since the left side of equation (3) is determined by the fluid, change the mass mp or moment of inertia jp of the conduit on the right side to match equation (3). For example,
Increase the mass of the conduit 1 by adding a long mass m on the X axis that does not affect the moment of inertia jp of the conduit 1 around the X axis at the intersection of the conduit 1 and the X axis, or parallel to the Y axis. The equation (3) is satisfied by adding an equal mass to both arms of the conduit 1 intersecting with the line to increase the inertia moment jp. Usually X in conduit 1
This method will be described because it is advantageous to add the mass m on the shaft because the number of parts is small. For example, when the fluids to be measured are water and air, the bending natural frequency and the torsional natural frequency when water and air are introduced into the flowmeter are measured. The bending natural frequencies of water and air are ω y _H and ω, respectively.
Let y _A and the torsional natural frequencies be ω θ _H and ω θ _A , respectively. Since the spring constant Ky around the YY ′ of the conduit can be easily measured, 1. For example, as shown in FIG. 1, the stainless steel thin plate 10 may be welded with an electron beam or the like so as to have a mass of Δm obtained by the equation (4).

Effect As described above, according to the present invention, the bending natural frequency and the torsional natural frequency when conducting fluids having different densities are measured, and further, the spring constant of the flow meter curved conduit is measured to determine by the formula (4). It is possible to make the ratio of the natural frequencies of bending and torsion constant even if the liquid density changes by a simple method of adding the added mass that is obtained. There is no error and a highly accurate mass flow meter can be provided.

[Brief description of drawings]

FIG. 1 is a diagram for explaining an example of the present invention, and FIG. 2 is a diagram for explaining an example of a conventional technique. 1 ... Curved conduit, 2 ... Support member, 3 ... Reciprocating member, 4 ... Coil, 5
… Magnets, 7, 8… Photodetectors, shielding plates, 10… Additional mass.

Claims (2)

[Claims] 1. A curved conduit, which is supported between two points on the Y axis and is axially symmetric with respect to the X axis passing through the center between the two points and orthogonal to the Y axis, is provided with an axis parallel to the Y axis or the X axis. In a mass flowmeter which applies a sine vibration as an axis and determines a mass flow rate from a Coriolis force generated around an X axis, a predetermined mass is added to the curved conduit to obtain a natural frequency about the Y axis of the curved conduit and an X axis. A mass flowmeter, characterized in that a ratio with a natural frequency around is a constant value even if a density of a fluid to be measured flowing through the curved conduit changes. 2. A weight is integrally fixed to the curved conduit on the X-axis, and a mass Δm of the weight is applied to the curved conduit at a density ρ _1.
And the Y-axis of the curved conduit when flowing a fluid of ρ ₂ ,
The natural frequencies around the X axis are ωy ₁ , ωy ₁ and ωθ ₁ , ω
When θ ₂ and the spring constant around the Y axis is Ky, The mass flowmeter according to claim (1), characterized in that