JP2002243613A - Density-measuring method and device using coriolis mass flowmeter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To facilitate handling by simplifying calibration at customer site and to reduce time and cost required for calibration at the customer sites. SOLUTION: Density is measured through the use of a Coriolis mass flowmeter for measuring mass flow. The density of a fluid to be measured is obtained by detecting the oscillation period or oscillation frequency of a flow tube, through which the fluid to be measured flows, and using a function with at least two coefficients from the detected oscillation period or the oscillation frequency. At this time, at least two coefficients are obtained through the use of the oscillation period or oscillation frequency measured, by making only one known calibration fluid flow and a value determined commonly for each type of product, and the commonly determined value is corrected by variation factors based on individual products.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method and an apparatus for measuring density using a Coriolis mass flow meter.

[0002]

BACKGROUND OF THE INVENTION It is known to use Coriolis mass flow meters to measure the density of unknown fluids. Coriolis mass flow meter supports both ends of the flow tube through which the fluid to be measured flows, and when the flow tube vibrates around the support point in a direction perpendicular to the flow direction of the flow tube,
This is based on the fact that the Coriolis force acting on a flow tube is proportional to the mass flow rate, and is known per se.

[0003] On the other hand, a vibratory density meter measures the density of a fluid to be measured by utilizing the fact that the resonance frequency of a flow tube through which a fluid flows changes according to a change in density. Since such a vibration type densitometer has the same main configuration as the Coriolis mass flow meter, it is configured using a Coriolis mass flow meter for measuring the mass flow rate of the fluid to be measured, and the mass flow rate is measured. At the same time, measuring the density has been conventionally performed. By measuring the period or frequency at which the flow tube of the Coriolis mass flowmeter resonates and vibrates, the density of the fluid can be measured.

[0004] However, the resonance frequency of the flow tube varies not only with the density of the flowing fluid alone, but also with the change in the density of the entire flow tube containing the fluid. Further, the spring constant of the flow tube varies with the temperature. Considering that the resonance frequency changes, as a preparation for measuring the density of the unknown fluid, knowing the relationship between the measured resonance frequency and the density of the fluid to be measured for each device used for measurement, i.e., calibration Is required.

Japanese Patent Publication No. 6-63958 discloses such a technique. This technique determines the period at which an unknown fluid oscillates in one of the two flow tubes of a Coriolis mass flow meter and squares this period. Next, the cycle of the density of the unknown fluid squared and the two known fluids that flowed through the Coriolis mass flow meter during calibration,
For example, it is determined as a linear function of the square of the cycle of air and water.

[0006] However, a density measuring device using a Coriolis mass flowmeter is shipped after being manufactured in a factory,
When the measuring point is at the customer site, it is assembled integrally with the pipe through which the fluid to be measured flows, and measures the density of the fluid to be measured.
When calibrating using water and air in such a condition, remove the density measuring device from the piping, completely remove the fluid, dry thoroughly, then calibrate first with air and then with water. become. As described above, there is a problem that it takes a very long time and cost to re-calibrate the density measuring device installed in the pipe.

[0007]

SUMMARY OF THE INVENTION Accordingly, the present invention solves such a problem and, when shipped from a factory and calibrated at a customer, simplifies calibration at the customer and facilitates handling. It is another object of the present invention to reduce the time and cost required for calibration at a customer site.

[0008]

According to the present invention, there is provided a density measuring method for measuring a density using a Coriolis mass flow meter.
By having a flow tube through which two fluids to be measured flow, a pair of vibration detection sensors are installed at symmetrical positions on both the left and right sides with respect to the mounting position of the drive device, and by detecting a phase difference proportional to the Coriolis force The density is measured using a Coriolis mass flow meter that measures the mass flow rate. The density of the fluid to be measured is determined by detecting a vibration cycle or a vibration frequency of the flow tube through which the fluid to be measured flows, and using a function having at least two coefficients from the detected vibration cycle or the vibration frequency. At this time, the vibration cycle or vibration frequency measured by flowing only one known calibration fluid and the at least two coefficients are obtained by using a value that is commonly determined for each product type, and the common and fixed values are determined. Is corrected by the variation factor based on each product.

A density measuring apparatus for measuring a density using a Coriolis mass flow meter according to the present invention includes a circuit for detecting a vibration cycle or a vibration frequency of a flow tube through which a fluid to be measured flows, a detected vibration cycle or a vibration frequency. And a density calculator for calculating the density of the fluid to be measured using a function having at least two coefficients. In the density calculation unit, for calibration processing for obtaining at least two coefficients,
A vibration cycle or a vibration frequency measured by flowing only one known calibration fluid, and the at least two coefficients are obtained by using a value commonly determined for each product type, and the commonly determined value is calculated by Correct by the variation factor based on each product.

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described based on examples. FIG. 1 is a diagram exemplifying a schematic overall configuration of a density measuring device using a Coriolis mass flow meter of the present invention. Coriolis mass flowmeters are known in various flow tube shapes such as straight pipes and curved pipes, or in various flow tube configurations such as single pipes, two parallel pipes, and inner / outer double pipes.
Any Coriolis mass flow meter can be used as the density measuring device of the present invention. The flow tube of the Coriolis mass flow meter is driven in resonance by a driving device provided at the center thereof, and a pair of vibration detection sensors
It is installed at symmetrical positions on both the left and right sides with respect to the mounting position of the drive device, and detects a phase difference proportional to the Coriolis force.

The sensors A and B shown in FIG. 1 represent a pair of vibration detecting sensors. The signals A and B having a sine wave shape detected by the sensors A and B and amplified as necessary are input to the phase difference detection unit 1. Here, the phase difference between the signal A and the signal B is detected using a method known per se. The mass flow rate calculator 2 calculates the mass flow rate from the detected phase difference by a known method in a Coriolis mass flow meter.

On the other hand, the signal B from the sensor B (or the signal A from the sensor A) is input to the period or frequency detection circuit 3, where the period or frequency of the signal B is detected. This is then input to the density calculator 4 where the density is calculated from the square of the period or frequency. At this time, according to the characteristics of the present invention, the density was confirmed at the factory before shipment. The density of the fluid to be measured is determined based on the variation factor and the coefficient determined using only one liquid for calibration, for example, water.

Hereinafter, the determination of the density of the fluid to be measured will be described in more detail by taking a curved tube type flow tube as an example. The relationship between the tube vibration period T of a curved tube and the fluid density ρ is as follows: A and B are constants specific to the density measuring device (having values specific to the device used), ρ = AT ² + B ( It is known that 1) can be expressed as In short, if the A and B unique to the device can be known, the fluid density ρ can be obtained by measuring the tube vibration period T.

A and B are defined according to the prior art.
The fluid density ρ can be determined by flowing a known fluid, for example, air and water, and measuring the oscillation period T. Air (hereinafter, represented by a suffix a) and water (hereinafter, represented by a suffix w)
Ρa = ATa ² + B (2) where ρa = ATw ² + B (3), where ρa is the density of ρa and ρw is the density of the tube and Ta and Tw are the vibration cycles of the tube, respectively. And from these two equations, A,
B can be obtained. The fluid density ρ can be obtained by measuring the tube vibration period T of the unknown fluid using the equation (1) in which the obtained A and B are substituted. In other words, from equations (1) to (3), the following equation, which is a relational equation between the tube vibration period T of the unknown fluid and the density ρ, can be derived.

Ρ = (ρa−ρw) (T ² −Tw ² ) / (Ta ² −Tw ² ) + ρw (4) As described above, the density ρ can be obtained by measuring the tube vibration period T. The premise is that the tube vibration periods Ta and Tw of the air and water, and the densities ρa and ρw
You need to know the relationship.

In order to simplify this, the equation (4) is further developed, and the density factor KD defined as the equation (5) is introduced, and the equation (4) is changed to the equation (6). Can be rewritten. ^{KD = Tw 2 / (Ta 2} -Tw 2) (5) ρ = KDρw '(T 2 / Tw 2 -1) + ρw where ρw' = (ρa -ρw) ( 6) Equation (5), (6) From the cycle T of the tube vibration, f =
When rewritten to a frequency f having a relationship of 1 / T, equations (5) ′ and (6) ′ are obtained. KD = fa ² / (fw ² −fa ² ) (5) ′ ρ = KDρw ′ (fw ² / f ² −1) + ρw where ρw ′ = (ρa−ρw) (6) ′ By determining the average value of the experimental values for each type (type) of the density measuring device, it is considered that the calibration of each product is performed using only the vibration frequency fw (or the vibration period Tw) measured using water. Can be

FIG. 2 shows the results of calculating the air density for different types of curved tube Coriolis mass flowmeters. The horizontal axis indicates the type of the curved tube type Coriolis mass flow meter having different sizes, and the number indicates the diameter (mm) of the pipe to which the flow meter is attached. The vertical axis indicates the error of the density of the air obtained by this calculation in a method in which the density is measured by calibrating only water using the above KD. The error is in the range of -0.06 to +0.06, as shown in FIG. 2, and it cannot be said that sufficient accuracy has been obtained.

A major factor that could not obtain sufficient accuracy is that the KD value obtained as an average value is not actually a constant value but varies from product to product, and this variation cannot be ignored. Things. Therefore, based on the above recognition, the present invention requires that the calibration of each product be performed using only fw (or Tw) measured using water, and that an experimental value be determined in advance for each type (type) of the density measuring device. Is performed as described above in that the value of KD determined as the average value is used, but the present invention additionally includes
The KD value obtained as the average value is corrected based on the variation obtained for each product.

Hereinafter, this will be further described. KD
The variation in the value of fa is, as seen in equation (5) ', fa
, Fw are varied. Therefore, when K and M are introduced as the variation coefficients, the vibration frequency of each product can be expressed as Kfakd and Mfwkd. Note that kd added as a subscript is f
This shows that a and fw were obtained by averaging the experimental values. Using this, equation (5) ′ can be rewritten as equation (7).

KD ′ = (Kfakd) ² / {(Mfwkd) ² − (Kfakd) ² } (7) This can be further rewritten as Expression (8). KD ′ = fakd ² / ｛((M / K) · fwkd) ² −fakd ² ｝ (8) When M / K = η, KD ′ becomes KD shown in equation (5) ′.
And the quadratic expression of η. Of course, the n-th order equation may be used in this approximation.

KD ′ = KD × (Pη ² + Qη + R) (9) In other words, the KD value previously determined as the average value of the experimental values for each type (type) of the density measuring device is individually determined for each product. This corresponds to correction based on the obtained variation factor (Pη ² + Qη + R).

The variation factor (Pη ² + Qη + R) can be calculated by obtaining coefficients P, Q, R and η. First, as for η, as described above, η = M /
It is defined as K, and K and M are introduced as variation coefficients of fakd and fwkd.
For example, the value of K can be determined as K = actual value / average of drive frequency of each product by measuring the drive frequency at the time of complete assembly of each product. Further, the value of M can be determined as M = actual value / average value of drive frequency of each product when calibration is performed using water.

Next, determination of the coefficients P, Q, and R will be described. From equation (9), (Pη ² + Qη + R) is equal to KD ′
It can be seen that it is obtained by the calculation of / KD. Therefore,
By dividing KD ′ in equation (7) by KD in equation (5) ′ and setting M / K = η, equation (10) is obtained.

KD ′ / KD = (fwkd ² −fakd ² ) / ｛(η · fwkd) ² −fakd ² ｝ (10) Equation (10) is a function of η, which is (Pη ²
+ Qη + R), the coefficients P, Q, and R can be determined. This determination can be usually obtained by quadratic regression by changing η in the region around 1.000 in Equation (10) by a well-known method using computer software.

In this manner, the variation factor of each product, that is, the value of (Pη ² + Qη + R) can be determined.

FIG. 3 shows the result of calculation under the same conditions as in FIG. 2 except that the variation factor is considered. The horizontal axis shows the type of curved tube type Coriolis mass flow meter, and the vertical axis shows KD 'in consideration of the above variation coefficient.
Is a method in which the density is measured by calibrating only water by using the above, and the error of the density of the air obtained by this calculation is displayed. This error indicates that very good accuracy is obtained, as can be seen by comparing with the case shown in FIG.

In consideration of the influence of the temperature of the flow tube, the oscillation period T is a function of the temperature t, so that k (= √
(1 + αt, α: temperature correction coefficient of Young's modulus), kw (=
Assuming that √ (1 + αtw), equation (6) can be rewritten as follows. ρ = KDρw ′ (k ² T ² / kw ² Tw ² −1) + ρw (11) Thus, when calculating the density ρ from the vibration period T,
It can be corrected by temperature. Further, since the value of KD itself is affected by the temperature, it is desirable to determine the value of KD at a constant temperature, for example, 20 ° C.

Further, a straight tube type tube will be examined. The relationship between the tube vibration cycle T of a straight tube type and the fluid density ρ is as follows: a and c are constants specific to the density measuring device, and β is a fixed value for each type (type) of the density measuring device. It is known that the average value can be expressed as follows: ρ = a + c / (1 / T + β) ² (12) Since this is basically the same as the case of the curved tube type, detailed description is omitted, but the following equations corresponding to equations (5) and (6) can be obtained. KD = (1 + βTakd) ² Twkd ² / ｛(1 + βTwkd) ² Takd ² − (1 + βTakd) ² Twkd ² ｝ (13) ρ = KDρw ′ ｛(1 + βTw) ² T ² / (1 + βT) ² Tw ² −1 14) ρw ′ = (ρa−ρw) (15) Thus, even when the temperature of the flow tube is affected, or even when the flow tube is a straight tube type, basically, equations (5) and (6) Since it is reduced, it is possible to sufficiently express the equations (5) ′ and (6) ′. That is, according to the formulas (5) ′ and (6) ′, according to the present invention, the value of KD is expressed as
An average value of the experimental values is determined in advance for each type (type) of the density measuring device, and the determined KD value is corrected based on the variation factor (Pη ² + Qη + R) obtained for each product. The calibration of each product is performed using only fw (or Tw) measured using water. Accordingly, the density ρ can be obtained by measuring the vibration frequency f (or period T) of the unknown fluid.

As described above, in the present invention, only one solution, for example, water is used for calibration, and calibration is simplified. The accuracy is also good as shown in FIG.

[0030]

The present invention can be calibrated using only water when shipped from a factory and calibrated at a customer site.
There is no need to use air for calibration as before,
Therefore, there is no need to dry the inside of the pipe. Calibration can be performed only by switching the fluid in the pipe from the fluid in use to water. As described above, there is an effect that the calibration at the customer can be simplified, the handling can be facilitated, and the time and cost required for the calibration at the customer can be reduced.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating a schematic overall configuration of a density measuring device using a Coriolis mass flow meter according to the present invention.

FIG. 2 is a diagram showing a comparative example in which the density of air is measured for comparison.

FIG. 3 is a diagram showing a test result of measuring the density of air according to the present invention.

[Explanation of symbols]

 Reference Signs List 1 phase difference detection unit 2 mass flow rate calculation unit 3 cycle or frequency detection circuit 4 density calculation unit

Claims (2)

[Claims] An apparatus has a flow tube through which at least one fluid to be measured flows in a resonance manner by a driving device provided at a central portion, and a pair of vibration detection sensors are provided on both left and right sides with respect to a mounting position of the driving device. In the density measurement method, which is installed at a symmetrical position and measures the mass flow rate by detecting the phase difference proportional to the Coriolis force, the density is measured using a Coriolis mass flowmeter, the oscillation cycle of the flow tube through which the fluid to be measured flows Or, a vibration frequency is detected, and at least 2
In order to obtain the density of the fluid to be measured using a function having two coefficients, a vibration cycle or a vibration frequency measured by flowing only one known calibration fluid and a value commonly determined for each product type are used. Calculating the at least two coefficients by using a Coriolis mass flowmeter, and correcting the commonly determined value by a variation factor based on each product. 2. A method according to claim 1, further comprising a flow tube through which at least one fluid to be measured flows, which is resonated by a driving device provided at a central portion, wherein a pair of vibration detecting sensors are provided on both left and right sides with respect to a mounting position of the driving device. In a density measuring device installed at a symmetrical position and measuring the mass flow rate by detecting a phase difference proportional to the Coriolis force, a density measurement using a Coriolis mass flowmeter, the oscillation cycle of the flow tube through which the fluid to be measured flows Or a circuit for detecting a vibration frequency, and at least 2
And a density calculator for determining the density of the fluid to be measured using a function having two coefficients. Using the vibration period or vibration frequency measured by flowing
A density measuring device using a Coriolis mass flowmeter, which determines two coefficients and corrects the commonly determined value by a variation factor based on each product.