JP2011027653A - Vibration type densitometer - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a vibration type densitometer capable of removing the effect on a density indication value in a case that the temperature distribution in a cell is changed by a change in the outside temperature and a change in the temperature of a sample. <P>SOLUTION: A sample to be measured is introduced into a measuring cell 1 through a sampling tube 16 and the inherent vibration cycle T<SB>SAMP</SB>of the measuring cell 1 is measured and the temperature t<SB>CELL</SB>and joint temperature t<SB>JOINT</SB>of the measuring cell are also obtained on the basis of the outputs of thermistors 14 and 15. Then, at the temperature t<SB>CELL</SB>of the measuring cell=20.00°C and the joint temperature t<SB>JOINT</SB>=20.01°C, 20.01°C is input to the fitting formula read from a memory part 25 to calculate the vibration frequency T<SB>AIR</SB>,<SB>20.01°C</SB><SP>2</SP>and T<SB>WATER</SB>,<SB>20.01°C</SB><SP>2</SP>and the values of the calibration parameters K<SB>120.01°C</SB>and K<SB>220.01°C</SB>at the temperature of 20.01°C are calculated from the calculated T<SB>AIR</SB>,<SB>20.01°C</SB><SP>2</SP>and T<SB>WATER</SB>,<SB>20.01°C</SB><SP>2</SP>. The density ρ<SB>SAMP</SB>of the sample to be measured is calculated according to ρ<SB>SAMP</SB>=K<SB>120.01°C</SB>×T<SB>SAMP</SB><SP>2</SP>+K<SB>220.01°C</SB>to be displayed on a display part 24. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a vibration type density meter, and more particularly to a vibration type density meter having a function of removing the influence of a change in environmental temperature on a measurement cell (vibrator).

  The vibration type density meter is a device that vibrates the measurement cell containing the sample to be measured, and calculates and outputs the density of the sample to be measured from the measured natural vibration period. For example, the density of various samples such as concentration management of soft drinks It is used for measurement.

  This vibration-type density meter has, for example, a glass U-shaped measurement cell, a permanent magnet is fixed to the tip of the measurement cell, and a drive head and a detection unit are built in a position facing the permanent magnet. Is arranged. At the time of density measurement, a sample to be measured is introduced into the measurement cell, and the measurement cell is vibrated by applying a driving current to the driving coil of the measurement head and applying an electromagnetic force to the permanent magnet. The density of the sample to be measured is obtained from the natural vibration period.

The vibration type density meter can be represented by a spring constant k and a mass m, as shown in FIG. In this vibration system, when the natural vibration period is T,
T = √ (m / k)
That is,
T ² = m / k
It is.
The mass m is divided into the mass m _glass of the glass tube and the mass m sample of the _sample to be measured.
T ² = (m _sample + m _glass ) / k
Therefore, the mass m _sample of the _{sample to} be measured is
m _sample = k · T ² −m _glass (1)
It becomes.

Density _{[rho sample} of the above (1) sample to be measured from the equation, the capacitance of the measuring cell and _{V cell,}
ρ _sample = m _sample / V _cell = k · T ² / V _cell -m _glass / V _cell
It becomes. Here, when k / V _cell is K ₁ and −m _glass / V _cell is K ₂ ,
ρ _sample = K ₁ · T ² + K ₂ (2)
Can be expressed as

Next, a method for calculating the density of the sample to be measured will be described.
When T _WATER and T _AIR are natural vibration periods of two kinds of substances having a known density, for example, pure water (ρ _WATER ) and air (ρ _AIR ) measured by a measuring cell,
ρ _WATER = K ₁・ T _WATER ²
+ K ₂ (3)
ρ _AIR = K ₁ · T _AIR ² + K ₂ (4)
From the above equations (3) and (4), the calibration parameters K ₁ and K ₂ are
K ₁ = (ρ _WATER −ρ _AIR ) / (T _WATER ² −T _AIR ² ) (5)
K ₂ = −T _AIR ² · (ρ _WATER −ρ _AIR ) / (T _WATER ² −T _AIR ² ) + ρ _AIR (6)
It becomes.
Since the densities ρ _WATER and ρ _AIR at the measured temperatures of pure water and air are known, the calibration parameters K ₁ and K ₂ are calculated from the vibration periods T _WATER and T _AIR measured for pure water and air. The density ρ _sample of the sample to be measured can be obtained from the natural vibration period T obtained by measuring the sample to be measured by the above equation (2).

Since the calibration parameters K ₁ and K ₂ calculated as described above are constants determined based on the density and natural vibration period of two specific substances under a certain reference temperature t ₀ , the reference temperature t _When the density of the sample to be measured is calculated based on the natural vibration period obtained by measuring the sample to be measured using a measurement cell having a temperature different from ₀ , an error from the true value occurs.

Therefore, the temperature of the measurement cell is maintained at the reference temperature t ₀ using temperature control means such as a heat insulating material or a thermo module, and the natural vibration period when the temperature of the measurement cell reaches the reference temperature t ₀ is measured. (For example, refer to Patent Document 1).

Japanese Patent Laid-Open No. 6-58862

The vibration density meter as described above has a short measurement time, requires a small number of samples, and is highly accurate. However, there is a temperature effect on the measurement cell due to changes in the outside air temperature or sample temperature. Even if the indicated value is constant, when the environmental temperature changes, the temperature distribution in the original measurement cell changes, and the measured vibration period changes. That is, the temperature distribution does not exist in a completely insulated system (measurement cell), but in a normal apparatus, the heat insulation on the sample injection side is insufficient, and a large temperature is required from the center of the measurement cell to the sample injection side. Distribution exists.
For this reason, even if the indicated value of the measurement cell thermometer is constant, when the environmental temperature changes, the temperature distribution in the original measurement cell changes, and the vibration period and density indication value change.

  The present invention was devised to solve the above problems, and is a vibration type that can remove the influence on the density indication value when the temperature distribution in the cell changes due to external temperature change or sample temperature change. The purpose is to provide a density meter.

In the vibration system of FIG. 4, if the representative temperature of the spring constant k and the mass m is always constant, the vibration cycle does not change, but if the representative temperature of the spring constant or the mass m changes, the vibration cycle also changes. When temperature control is performed at one point of the measurement cell, temperature control is performed at the representative temperature of the mass m that has the greatest influence on the vibration cycle. In this case, the representative temperature of the spring constant k is affected by the outside air temperature.
As a method for removing this influence, temperature control at two points for temperature control of the representative temperature of the spring constant k or correction of the vibration period obtained from the relationship between the temperature and the spring constant k can be considered. Since temperature control at a point is difficult in terms of structure, the present invention corrects the vibration period.

  As shown in FIG. 5, in the vibrator of the vibratory density meter, since the representative temperature of the mass m is known to be about 1/3 from the tip, the temperature of this portion is measured with a thermistor and measured. Perform cell temperature control. On the other hand, in the vibrator of the vibration type density meter, the representative temperature of the spring constant k is near the node. Therefore, the temperature of this part is measured with a thermistor and used for temperature correction of the vibration period.

That is, in the above equation (2), the calibration parameter K ₂ is the mass of the glass tube / the capacity of the measuring cell, and thus is hardly affected by the representative temperature of the spring constant k and the representative temperature of the mass m, but the calibration parameter K ₁ Varies depending on the representative temperature of the spring constant k. In order to remove this influence, the calibration parameters K ₁ and K ₂ are corrected using the representative temperature of the spring constant k.

In other words, the spring is constant k to determine the representative temperature from calibration parameters K _1, K ₂ which is the right way, the calibration from the representative temperature of the mass m is far (during calibration of the set temperature) parameters K _1, K _In order to correct the problem that has determined ₂ , the representative temperature of the spring constant k, that is, the temperature near the node of the measurement cell, is always measured, and thereby the calibration parameters K ₁ and K ₂ are determined.
However, the value of the calibration parameters _K 1, _{K 2,} the temperature and the calibration parameter _K 1 previously prepared, _{K 2} tables or extraction from a function for calculating the calibration parameters _K 1, _{K 2} of the temperature, or ,calculate.

As described above, the largest factor of the vibration period change when the environmental temperature changes is firstly the temperature change of the sample in the cell due to the change of the cell internal temperature, and secondly the calibration by the change of the node temperature of the measurement cell. Regarding the changes in the parameters K ₁ and K ₂ , according to the vibration type density meter of the present invention, the temperature near the node of the measurement cell is measured by setting the representative point of the cell portion to a constant temperature, and thereby the calibration parameter K _1. since determining the K _2, it is possible to eliminate the influence of the external temperature changes, density indication of when the cell temperature distribution is changed by the sample temperature change.

It is a figure which shows the structure of the sensor part of the vibration type density meter of this invention. It is a conceptual diagram of the vibration type density meter which attached the sensor part of FIG. It is the graph which plotted the square of the natural vibration period which measured air and pure water at various temperatures with respect to temperature. It is the figure which modeled the vibration system of the vibration type density meter. It is a schematic diagram of the cell of a vibration type density meter.

1A and 1B are diagrams showing the structure of a sensor unit of a vibration type density meter according to the present invention. FIG. 1A is a side view of the sensor unit, and FIG. 2B is a top view of the sensor unit. This sensor part is composed of a measurement cell 1, a permanent magnet 2, holders 3a and 3b, a temperature sensor 4 and a glass outer cylinder 5.
The measurement cell 1 is a thin U-shaped tube made of glass having a wall thickness of about 0.2 mm, and a thin plate 2 of a permanent magnet is fixed to the tip of the tube with an adhesive. Further, the base end portion of the measurement cell 1 is fixed to holders 3a and 3b, and the holders 3a and 3b are fixed by projections 6 provided on the outer cylinder 5 as shown in FIG. The temperature sensor 4 is a glass tube in which a thermistor is inserted, and measures the temperature of the sample and the temperature of the node of the measurement cell 1. This sensor part is sealed after helium is injected into it through the helium inlet 7 after assembly.

  On the other hand, FIG. 2 is a conceptual diagram of the vibration type density meter to which the sensor unit of FIG. 1 is attached. The sensor unit is housed in the heat insulating material 11 and has a copper block 12 having a Peltier element (not shown). Is controlled so that the temperature of the sample to be measured in the sensor unit, that is, the measurement cell 1 is maintained at the set temperature. In addition, a measurement head 13 incorporating a drive coil and a detection coil is disposed at a position facing the permanent magnet 2 fixed to the tip of the measurement cell 1.

As shown in the figure, two thermistors 14 and 15 are arranged in the glass tube of the temperature sensor 4, and the thermistor 14 measures the temperature near the tip of the measurement cell 1, and the thermistor 15 is the root of the measurement cell 1. Measure the temperature in the vicinity, that is, the node temperature.
On the other hand, one open end of the measurement cell 1 is connected to a sampling tube 16 that introduces a fluid to be measured, and the other open end is connected to a drain tube 17 that discharges the measured fluid that has been measured.

  The control device 18 includes a control unit 21, a drive unit 22, a detection unit 23, a display unit 24, and a storage unit 25. The temperature detection output of the thermistor 14 is input to the control unit 21, and the temperature of the thermistor 14 is measured and set. The Peltier element of the copper block 12 is controlled so that the temperature is reached. The drive unit 22 supplies a drive current to the drive coil of the measurement head 13, and the detection unit 23 detects the output of the detection coil of the measurement head 13 to detect the vibration cycle of the measurement cell 1. Further, the control unit 21 displays the measurement setting screen and the density measurement value on the display unit 24, and stores the measurement condition set by the user, the detected vibration cycle, and the like in the storage unit 25. In addition, the storage unit 25 is provided with a table that stores density data of air and pure water at various temperatures.

In the present invention, the calibration parameter is calculated using the environmental temperature correction, that is, the node temperature of the measurement cell. Hereinafter, a method for calculating the calibration parameter at various temperatures will be described.
First, the vibration periods T _WATER and T _AIR of pure water and air at 5 ° C., 20 ° C., 50 ° C., and 70 ° C. are measured with a vibrating _densimeter .
FIG. 3 is a plot of the relationship between the square of the vibration period of pure water and air measured and the temperature, where ■ is the vibration period when pure water (WATER) is measured, and ◆ is the vibration period when air (AIR) is measured. The curve in FIG. 3 is obtained by fitting the relationship between the square of the vibration period and the temperature with a quadratic function as shown in the following equations (7) and (8). This function is stored in the storage unit 25 of the control device 18 for use.
T _{AIR, t} ² = a _AIR · t ² + b _AIR · t + c _AIR (7)
T _{WATER, t} ² = a _WATER · t ² + b _WATER · t + c _WATER (8)

When calculating the density, etc., by inputting a predetermined temperature in the above equations (7) and (8), the vibration period of air and pure water at that temperature can be obtained. By substituting the value into (6), the calibration parameters K _1t and K _2t at each temperature can be obtained by the following equations.
K _1t = (ρ _WATER− ρ _AIR ) / (T _{WATER, t} ² −T _{AIR, t} ² ) (9)
K _2t = −T _{AIR, t} ² · (ρ _WATER −ρ _AIR ) / (T _{WATER, t} ² −T _{AIR, t} ² ) + ρ _AIR (10)

Next, the operation when the density of the sample to be measured is measured at 20 ° C. will be described.
As described above, the control unit 18 controls the Peltier element of the copper block 12 so that the temperature detection output of the thermistor 14 is 20 ° C., and introduces the sample to be measured into the measurement cell 1 through the sampling tube 16. . Then, a driving current is input to the driving coil of the measuring head 13 from the driving unit 22 of the control device 18 to apply an electromagnetic force to the permanent magnet 2, thereby causing the measuring cell 1 to start vibrating.
The detection coil of the measurement head 13 detects the vibration at this time, and a detection signal is input to the detection unit 23, and a drive signal synchronized with this vibration cycle is continuously input to the drive coil of the measurement head 13, thereby allowing the measurement cell to 1 is vibrated at a constant period, and the natural vibration period T _SAMP is measured. At this time, the temperature t _CELL and the node temperature t _{JOINT of} the measurement cell are also acquired from the outputs of the thermistors 14 and 15.

For example, when the temperature t _{CELL of the} measurement cell is 20.00 ° C. and the node temperature t _JOINT = 20.01 ° C., the density of the sample to be measured is calculated as follows.
The control unit 21 inputs 20.01 ° C. to the fitting equations of the above formulas (7) and (8) read from the storage unit 25 and calculates _{TAIR, 20.01 ° C.} ² , T _{WATER, 20.01 ° C.} ² and calculates this. ^{_{T AIR, 20.01 ℃ 2, T}} WATER, formula (9) _{20.01 ° C.} ^2, by substituting (10), determines the value of the calibration parameter _{K 120.01 ℃, K 220.01 ℃} at temperature 20.01 ° C. .

Then, as the calibration parameters, the above K _{120.01 ° C.,} using the values of K _{220.01 ° C.,} as the vibration period, by using the above T _SAMP, the control unit 21, the density of the sample to be measured ρ _SAMP is calculated by the following formula and displayed on the display unit 24.
ρ _SAMP = K _{120.01 ° C} × T _SAMP ² + K _{220.01 ° C}
As described above, the density of the sample to be measured is calculated using the calibration parameter calculated based on the node temperature of the measurement cell. Therefore, even if the temperature distribution in the cell changes due to external temperature change or sample temperature change, the density indication value The influence on can be removed.

In the above embodiment, the relationship between the square of the vibration period of pure water and air and the temperature is fitted by a quadratic function, and the square of the vibration period of pure water and air at the desired temperature is calculated from this function. However, by changing the temperature of the vibratory densimeter in various ways, we obtained a lot of vibration period data of air and pure water at various temperatures, and obtained a table of the relationship between the square of the vibration period of pure water and air and the temperature. It is also possible to store in the storage unit 25 as a table and extract the square of the vibration cycle of pure water and air at a desired temperature from this table.
In the above embodiment, the relationship between the square of the vibration period of pure water and air and the temperature is fitted by a quadratic function, but the number of temperature measurement points for measuring the vibration period of pure water and air is increased. Thus, it is also possible to fit the relationship between the square of the vibration period of pure water and air and the temperature with a polynomial such as a cubic or quartic equation.

  Furthermore, in the above embodiment, the vibration type density meter provided with the measurement head including the drive coil and the detection coil arranged to face the permanent magnet attached to the tip of the measurement cell has been described as an example. The vibration type density meter of the present invention can also be applied to other density meters such as the type of vibration density meter detected by the above.

DESCRIPTION OF SYMBOLS 1 Measurement cell 2 Permanent magnet 3a, 3b Holder 4 Temperature sensor 5 Outer cylinder 6 Protrusion 7 Helium inlet 11 Heat insulating material 12 Copper block 13 Measurement head 14, 15 Thermistor 16 Sampling tube 17 Drain tube 18 Controller 21 Controller 22 Drive Unit 23 detection unit 24 display unit 25 storage unit

Claims (2)

A vibration type densimeter equipped with a control device for calculating the density of the sample to be measured from the vibration period of the measurement cell containing the sample to be measured and the calibration parameter,
A temperature sensor arranged near the base of the measurement cell is provided, and the control device calculates the calibration parameter at the measured temperature of the measurement cell detected by the temperature sensor and calculates the density of the fluid to be measured when calculating the density. This is a vibration type density meter.   The control device obtains the vibration period of the two substances of known density at each temperature based on the relationship between the vibration period and the temperature of the two substances of known density at various temperatures. The vibration type density meter according to claim 1, wherein a calibration parameter is calculated.

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