JPS6139609B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a vibrating density meter that utilizes the fact that the resonance frequency of a mechanical vibrator changes depending on the density of the fluid surrounding the mechanical vibrator.

FIG. 1 is a block diagram showing an example of a conventionally known density meter of this type. This device was manufactured by the Special Publication Publication No. 51-16794.
The device described in the above publication includes a cylindrical resonator (mechanical resonator) 1 in which a fluid to be measured passes through inside and outside, and a cylindrical resonator 1 which is excited to vibrate the cylindrical resonator 1. A driving means 2 for vibrating at a resonant frequency, a detecting means 3 for detecting the frequency of the cylindrical resonator 1, an amplifier 4 which inputs a signal from the detecting means 3 and gives an excitation signal to the driving means 2, It also includes a monitoring device 5 that monitors the frequency.

A densitometer with such a configuration has the feature that an output signal related to density can be obtained as a frequency signal, but there is a limit to its accuracy because it is affected by the viscosity of the fluid to be measured. Although it is possible to improve accuracy by inserting a viscosity measuring means into the fluid to be measured and correcting the density signal using the viscosity signal from the viscosity measuring means, the overall configuration becomes complicated.

Here, the present invention aims to realize a density meter with high measurement accuracy that eliminates the influence of the viscosity of the fluid to be measured without using a viscometer or the like.

The present invention is characterized in that the Q or gain of a mechanical vibrator is measured and calculated as the resonance frequency.

FIG. 2 is a block diagram of the vibrating density meter according to the present invention. In the figure, 1 generally shows a mechanical vibrator, 2 is an excitation means for vibrating the mechanical vibrator, 3 is a vibration detection means for detecting the vibration of the mechanical vibrator, and 4 is an oscillation circuit. Oscillation circuit 4
inputs the signal from the vibration detection means 3, and its output is applied to the vibration means 2, forming a self-excited oscillation loop including the vibrator 1.
is vibrated at a resonant frequency. 5 is the oscillation circuit 4
6 is a gain measurement circuit that is coupled to the oscillation circuit 4 and measures the Q or gain of the mechanical resonator 1; 7 inputs the signal from the counter 5 and the signal from the gain measurement circuit 6; 8 is a display circuit.

FIG. 3 is a sectional view showing an example of the mechanical vibrator 1 shown in the block diagram of FIG. 2. As shown in FIG. Here, a cylindrical resonator is used, and one end is provided with a flange 11, the other end is provided with a flange 12, and a bottom portion 13 that closes the other end. A through hole 14 is formed in the flange 12 on the other end side, through which the fluid to be measured that has entered through the opening on the one end side flows out. The excitation means 2 and the vibration detection means 3 are connected to the flange 12
It is attached to the stepped part of the
PZT is used. 16 and 17 are covers with the same diameter. The cover 16 is connected to the flange portions 11 and 12 at both ends and covers the outside of the cylindrical resonator 1. The cover 17 is connected to the flange portion 12 at one end and is used for excitation. It covers the lead wires 21 and 31 connected to the means 2 and the vibration detecting means 3. A chamber 15 formed between the cover 16 and the cylindrical resonator 1
The inside is filled with gas at atmospheric pressure or constant pressure (including vacuum).

The operation of the apparatus configured in this way will be explained next. First, the relationship between the fluid density ρ inside the cylindrical resonator 1 and the resonance frequency is expressed by the following equation.

However, o: resonance frequency when the cylindrical resonator 1 is placed in a vacuum α: density of the cylindrical resonator 1 with density sensitivity,
Determined by wall thickness, radius, etc.

Therefore, by measuring the resonance frequency, the fluid density ρ can be determined.

By the way, equation (1) does not take into account the viscosity η of the fluid. Now, assuming that the viscosity of the fluid increases, a transverse wave is generated, and in addition to the fluid component in the normal direction of the vibrator 1, the fluid component in the tangential direction also vibrates, and the equivalent mass of the vibrator increases. Therefore, this increase appears as an indication error due to viscosity.

Here, the mass increase L′ due to viscosity and the oscillator 1
Q of is obtained from the equation of shear motion as follows.

First, in a Newtonian fluid, the shear force Y is (2)
It can be expressed by the formula.

Y=η・∂v/∂y (2) However, v: speed η: Viscosity coefficient From the balance of forces, equations (3) and (4) are obtained.

Y・dA=ρ・∂v/∂t・(dx dy pz) (3) ∴ρ∂v/∂t=η(∂ ² v/∂y ² ) (4) However, dA=dx・dz ρ: Density The solution to equation (4) can be expressed as equation (5).

However, ω: angular frequency From equations (5) and (2), the input impedance Zi of the vibrator 1 is as follows.

Equation (7) can be written as equation (8).

In equation (8), the first term is the decrease in Q due to the change in viscosity, and the second term is the increase in mass. Then, However, k: equivalent spring constant of the vibrator Qo: Q at η=0 From equations (9) and (10), equation (11) is obtained.

L'=k/ω2(1/Q-1/Qo) (11) As is clear from equation (11), if k and Qo are constant in equation (11), by measuring ω and Q, , the mass increase L′ can be determined.

The relationship between fluid density and resonance frequency is generally given by a quadratic equation, but is calculated using equation (12) in order to compensate for the apparent increase in mass due to viscosity.

ρ=A・1/ω2+B・1/ω+C−D/ω2(1/
Q-1/Qo) (12) However, A, B, C, D: Constants On the other hand, in order for a self-excited resonant system as shown in Figure 2 to maintain resonance, the consumed energy P and
The driving energy Po should be equal, and these are respectively shown by the following equations.

P=1/Q・E _M・2π (13) where _EM : internal storage energy Po=Vo ² /Z (14) where, Z: input impedance Vo: drive voltage Here, the input impedance Z is at the resonance point Therefore, it can be expressed by equation (15).

Z=R=1/ωCoQ (15) Also, the equivalent circuit of mechanical oscillator 1 is shown in Figure 4 with inductance Lo, mechanical resistance R, and capacitor.
If it is expressed by series connection of Co, the internal storage energy E _M can be expressed by equation (16).

E _M = 1/2Co・e ² (16) where e: vibration amplitude From equations (13) to (16), the gain of the oscillation circuit is
Av, this Av is as shown in equation (17).

Av=Vo/e=1/Q (17) By substituting equation (17) into equation (12), equation (18) is obtained.

ρ=A・1/ω2+B1/ω+C−D/ω2(Av−
Aro）(1 8) However, Avo: Gain when resonator 1 is placed in a vacuum.

In the block diagram of FIG. 2, a circuit gain measuring circuit 6 is coupled to an oscillation circuit 4, and for example, by measuring the input voltage e and output voltage Vo of this oscillation circuit 4, (17) The gain Av of resonator 1 is determined from the formula. The arithmetic circuit 7 has a counter 5
By inputting the frequency signal ω from the circuit and the gain signal Av from the circuit gain measuring circuit 6 and calculating the above equation (17), a density signal ρ from which the influence of the viscosity η has been removed is output. Display circuit 8 displays this density.

Note that the arithmetic circuit 7 is, for example, a microprocessor, in which constants A, A,
It is assumed that various data regarding B, C, D, Avo, etc. have been set, and a gain signal Av is applied as a digital signal via an A/D converter (not shown).

In this way, the device according to the present invention focuses on the phenomenon that when the viscosity of the fluid to be measured changes, the Q of the mechanical resonator 1 also changes, measures the Q or gain of the vibrator, and combines this signal with the resonant frequency signal. By calculating this, the influence of viscosity is removed, and it is possible to perform highly accurate density measurements without using a viscometer or the like.

In the above explanation, the gain measurement (Q measurement) is obtained by measuring the input voltage e and the output voltage Vo of the oscillation circuit 4, and calculating it using equation (17). When keeping constant, a signal related to the Q of the vibrator 1 can be obtained by simply measuring the input voltage e. Also, Q
A measuring device may be provided and the signal from the measuring device may be used. In addition, as a mechanical vibrator, the third
Other configurations than those shown in the figures may also be used.

FIG. 5 is a diagram showing an example of the experimental results using a glycerin aqueous solution (1 to 100 cp), in which (a) is when the present invention is applied and (b) is when the present invention is not applied.
The results are extremely satisfactory.

[Brief explanation of the drawing]

FIG. 1 is a configuration diagram showing an example of a conventionally known density meter, FIG. 2 is a configuration block diagram of an apparatus according to the present invention, FIG. 3 is a configuration sectional view showing an example of a mechanical vibrator in FIG. FIG. 4 is an equivalent circuit of the mechanical vibrator portion, and FIG. 5 is a diagram showing an example of experimental results. DESCRIPTION OF SYMBOLS 1... Mechanical vibrator, 2... Excitation means, 3... Vibration detection means, 4... Oscillation circuit, 5... Counter,
6...Gain measurement circuit, 7...Arithmetic circuit, 8...Display device.

Claims (1)

[Claims] 1. In a vibrating density meter that utilizes the fact that the resonant frequency of a mechanical vibrator changes depending on the fluid density around the mechanical vibrator, circuit means for obtaining a signal related to the Q of the mechanical vibrator; , a density signal in which the influence of the viscosity of the fluid to be measured is removed by inputting a signal related to the Q of the mechanical vibrator and a signal related to the resonance frequency of the mechanical vibrator and calculating both signals. A vibrating density meter characterized by comprising an arithmetic circuit that outputs .