JP5295028B2 - Vibrating density meter - Google Patents

Description

  The present invention relates to a vibration type density meter, and more particularly to a vibration type density meter having a function of correcting a drift when a temperature of a measurement cell (vibrator) is changed, that is, a change in one direction of a vibration cycle.

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

  When measuring fluid with such a vibratory density meter, alcohol is often measured at 15 ° C, petroleum is measured at 50 ° C / 60 ° C / 70 ° C, etc., and other fluids are often measured at 20 ° C. In some cases, it is necessary to change the temperature of the measurement cell. Also, when measuring the temperature characteristics of the density of a specific sample, it is necessary to change the temperature of the measurement cell.

However, when the temperature of the measurement cell is changed as described above, even if the temperature of the sample reaches the set temperature, drift of the vibration cycle, that is, a phenomenon in which the vibration cycle slowly changes in one direction for a long time has been confirmed. . This is thought to be due to the change in physical properties due to the unique properties of glass. Even when the power is turned on, drift occurs if the set cell temperature and the cell temperature at power on are different. .
Since the change that affects the measurement accuracy of a high-precision vibratory densimeter with 5 to 6 significant digits continues for about 10 hours, about 10 hours have passed since the temperature of the measurement cell reached the set temperature. It was necessary to make measurements after the drift had stabilized.

  In order to compensate for drift when changing temperature, a reference cell that is in thermal contact with the measurement cell is provided, and the cell temperature is changed by comparing the resonance vibration of the measurement cell and the reference cell. It has been proposed to compensate for deviations in measurement results of measurement cells caused by drift due to (see, for example, Patent Document 1).

JP-A-5-215659

As described above, in order to eliminate the effect of drift when changing the temperature of the measurement cell, measurement must be performed after waiting for the drift to stabilize. There was a problem that a long waiting time was required.
For this reason, as described above, it has been proposed to prepare a reference cell in addition to the measurement cell and cancel the influence of drift at the time of temperature change. In addition, there is a problem that the vibration driving circuit and the receiving circuit become complicated.

  The present invention was devised to solve the above-mentioned problems, and without using a reference cell, the waiting time required after the cell temperature change can be shortened, and the sample density measurement value immediately after the cell temperature change is repeated. An object of the present invention is to provide a vibratory densimeter capable of improving the performance.

The vibration type density meter of the invention according to claim 1 is a vibration type density meter provided with a control device that calculates the density of the sample based on the vibration period of the measurement cell filled with the sample, wherein the control device comprises: before changing the measuring cell temperature, the temperature after the change,-out based on the elapsed time from the temperature change, vibration cycle of T _SAMP measured sample, the elapsed time after the temperature change X, a temperature around the temperature change Δt When the difference is taken, the corrected vibration period T _{COMP is obtained} by T _COMP = T _SAMP / f (X, Δt) using the correction function f (X, Δt) .

According to the vibration type density meter according to the present invention, when the measurement cell temperature decreases, T _COMP = T _SAMP / f (X, Δt), for example, T _SAMP / {β ₁ · Δt · (logX−1) +1} In addition, when the measured cell temperature rises, the vibration period measured by T _COMP = T _SAMP / {β ₂ · Δt · logX + 1} is corrected, so that drift after changing the cell temperature can be compensated without using a reference cell. The waiting time required after the cell temperature change can be shortened, and the repeatability of the sample density measurement value immediately after the cell temperature change can be improved.

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 a graph which shows the influence on the density measured value of a pure water at the time of changing a cell temperature to 0 degreeC after keeping a cell temperature at 20 degreeC for 12 hours. It is a graph which shows the influence on the density measured value of the pure water at the time of changing a cell temperature to 10 degreeC after keeping a cell temperature at 30 degreeC for 12 hours. It is a graph which shows the influence on the density measured value of the pure water at the time of changing a cell temperature to 10 degreeC after keeping a cell temperature at 20 degreeC for 12 hours. It is a graph which shows the influence on the density measured value of a pure water at the time of changing a cell temperature to 20 degreeC after keeping a cell temperature at 30 degreeC for 12 hours. It is a graph of the measured value for 12 hours of the influence on the density of pure water at the time of changing a cell temperature to 30 degreeC after keeping a cell temperature at 0 degreeC for 12 hours. It is a graph which shows the influence on the density measured value of the pure water at the time of changing a cell temperature to 30 degreeC after keeping a cell temperature at 20 degreeC for 1 hour. It is a graph which shows the state of drift of the measurement vibration period at the time of temperature fall. It is the graph which displayed the time axis of the graph of FIG. 9 logarithmically. It is a graph which shows the change with respect to temperature difference (DELTA) t of the inclination A of the graph of FIG. It is a graph which shows the state of drift of the measurement vibration period at the time of a temperature rise. It is the graph which displayed the time axis of the graph of FIG. 12 logarithmically. It is a graph which shows the change with respect to temperature difference (DELTA) t of the inclination A of the graph of FIG. It is a flowchart which shows an effect | action when performing a drift countermeasure routine.

  FIG. 1 is a diagram showing the structure of a sensor unit of a vibration type densitometer according to the present invention. FIG. 1 (a) is a side view of the sensor unit, and FIG. 1 (b) 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 has a thermistor inserted in the glass tube, and measures the temperature of the sample. This sensor part is sealed after helium is injected into the inside 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 above-described sensor unit is attached. The sensor unit is housed in the heat insulating material 11 and a copper block 12 having a Peltier element (not shown) is provided. Control is performed 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. Further, as shown in the figure, a thermistor 14 is arranged in the temperature sensor 4 and measures the temperature near the tip of the measurement cell 1, that is, the temperature of the sample to be measured.
On the other hand, one open end of the measurement cell 1 is connected to a sampling tube 15 that introduces a fluid to be measured, and the other open end is connected to a drain tube 16 that discharges the measured fluid that has been measured.

  The control device 17 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. Furthermore, 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 measurement, first, a fluid to be measured is introduced into the measurement cell 1 through the sampling tube 15, a drive current is input to the drive coil of the measurement head 13 from the drive unit 22 of the control device 17, and an electromagnetic force is applied to the permanent magnet 2. As a result, the measurement cell 1 starts to vibrate.
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 is obtained.

Next, the control unit 21 calculates the density of the fluid to be measured based on the detected natural vibration period and displays it on the display unit 24. Hereinafter, a method for calculating the density of the sample to be measured from the natural vibration period will be described. .
When the natural vibration period of the measurement cell is T and the density of the fluid to be measured is ρ,
ρ = PT ² / 4π ² VM / V (1)
Can be obtained. Where P is the vibration constant of the vibration system, V is the volume of the fluid to be measured (volume of the measurement cell 1), and M is the mass of the measurement cell 1 and the permanent magnet 2.
Since each P, V, and M is a constant determined by the structure of the sensor unit, when P / 4π ² V = K ₁ and M / V = K ₂ ,
ρ = K ₁ T ² −K ₂ (2)
It can be expressed as

Here, when two natural substances having a known density, for example, pure water (ρ _WATER ) and air (ρ _AIR ) are measured in the measurement cell 1 are T _WATER and T _AIR ,
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 constants K ₁ and K ₂ are
K ₁ = (ρ _AIR −ρ _WATER ) / (T _AIR ² −T _WATER ² ) (5)
K ₂ = T _AIR ² · (ρ _AIR −ρ _WATER ) / (T _AIR ² −T _WATER ² ) −ρ _AIR (6)
It becomes.

From the above formulas (5) and (6), formula (2) is
ρ = ρ _AIR + (ρ _AIR −ρ _WATER ) * (T ² −T _AIR ² ) / (T _AIR ² −T _WATER ² ) (7)
Since the density ρ _WATER and ρ _AIR at a set temperature of pure water and air are already known, the measurement is performed from the vibration periods T _AIR and T _WATER that measure pure water and air, and the vibration period T that measures the fluid to be measured. The density ρ of the fluid can be calculated.

Next, correction of drift at the time of changing the cell temperature according to the present invention will be described.
FIGS. 3 to 8 show the influence on the density of pure water when the cell temperature is changed, that is, the difference between the time change of the measured value of the pure water and the value when the influence of drift disappears when the cell temperature is changed. It is the graph which illustrated the change with respect to the elapsed time from.
The graph of FIG. 3 is a measured value for 12 hours of the influence on the density of pure water when the cell temperature is changed to 0 ° C. after maintaining the cell temperature at 20 ° C. for 12 hours. The graph of FIG. It is a measured value for 12 hours of the influence on the density of pure water when the cell temperature is changed to 10 ° C. after maintaining the cell temperature at 30 ° C. for 12 hours.

  The graph of FIG. 5 is a measurement value of 12 hours of the influence on the density of pure water when the cell temperature is changed to 10 ° C. after maintaining the cell temperature at 20 ° C. for 12 hours. Is a measured value for 12 hours of the influence on the density of pure water when the cell temperature is changed to 20 ° C. after maintaining the cell temperature at 30 ° C. for 12 hours. Furthermore, the graph of FIG. 7 is a measurement value of 12 hours of the influence on the density of pure water when the cell temperature is changed to 30 ° C. after maintaining the cell temperature at 0 ° C. for 12 hours. This is a 12-hour measurement of the effect on the density of pure water when the cell temperature is changed to 30 ° C. after keeping the cell temperature at 20 ° C. for 1 hour.

3 to 8, ε is the difference between the time change of the measured value of the density of pure water when the cell temperature is lowered or raised and the value when the effect of drift is eliminated, and t is the cell temperature. As shown in the figure, the measured value t of the cell temperature reaches the set value immediately after the cell temperature change, but the measured value of the density of pure water is about 10 hours when the cell temperature falls. When the temperature rises, drift occurs for about one hour.
As described above, the effect of the drift due to the cell temperature change differs depending on the direction of the temperature change, has a large effect when the temperature decreases, and has a small effect when the temperature increases.

In the present invention, as a method for correcting the influence of drift due to cell temperature change, the change in the vibration period of the measurement cell is corrected by a correction formula. Hereinafter, a method for creating this correction formula will be described.
First, even if there is no drift due to changes in cell temperature, that is, when the raw vibration period is changed to a measurement state, correction is made with a ratio so that it can be used without changing the calibration factor. Then, a correction formula f _Δt (X) for calculating the corrected vibration period T _COMP is created. That is, if the sample measurement period is T _SAMP ,
T _COMP = T _SAMP / f _Δt (X) (8)
It becomes.

On the other hand, if the vibration period after X time during temperature fall and _{T X,} Tx is _{T X = a · (logX-} 1) + b ··· (9)
When the temperature drops, the influence of drift disappears 10 hours after the cell temperature is changed. Therefore, when the above equation (9) is divided by the reference value T _{10 based} on the vibration period T ₁₀ after 10 hours, the vibration Expression (8) of the ratio of period change
f _Δt (X) = T _X / T ₁₀ = A · (logX−1) + B (10)
It becomes. However, A = a / T ₁₀ and B = b / T ₁₀ .

FIG. 9 shows a value obtained by standardizing the measured value T _X , which is a time change of the vibration cycle of pure water, by T ₁₀ when the temperature is lowered by changing the temperature after changing the cell temperature variously. The graph was plotted against the axis, and it was found that the same graph was obtained if the temperature difference before and after the temperature change was the same regardless of the cell temperature before the temperature change. When the time axis of the graph of FIG. 9 is logarithmically displayed, it is as shown in FIG. 10. When the change of the slope A with respect to the temperature difference Δt in each temperature difference of FIG. 10 is graphed, it is as shown in FIG. The slope A is proportional to the temperature difference Δt before and after the cell temperature change. Therefore, A = a / T ₁₀ = β ₁ · Δt, and when X = 10 is input, the virtual vibration period T ₁₀ after 10 hours as a reference is obtained, so b = T ₁₀ and B = b / T ₁₀ = 1. The slope β ₁ calculated based on the result of measuring the vibration period of pure water by changing the temperature difference before and after the temperature change by the manufactured density measuring device was 0.0000007.

Therefore, f _Δt (X) when the temperature difference Δt is −10 ° C., the temperature difference Δt is −20 ° C., the temperature difference Δt is −30 ° C., and the temperature difference Δt is −40 ° C. is
f ₋₁₀ (X) = T _X / T ₁₀ = −0.0000070 · (logX−1) +1
f ₋₂₀ (X) = T _X / T ₁₀ = −0.0000140 · (logX−1) +1
f ₋₃₀ (X) = T _X / T ₁₀ = −0.0000210 · (logX−1) +1
f ₋₄₀ (X) = T _X / T ₁₀ = −0.0000280 · (logX−1) +1
It becomes.

Therefore, the drift correction equation for cell temperature change is as follows when the temperature difference Δt is −10 ° C. difference:
T _COMP = T _SAMP / f ₋₁₀ (X) = T _SAMP /{−0.0000070·(logX−1)+1}
When the temperature difference Δt is −20 ° C.,
T _COMP = T _SAMP / f ₋₂₀ (X) = T _SAMP /{−0.0000140·(logX−1)+1}
When the temperature difference Δt is −30 ° C.,
T _COMP = T _SAMP / f ₋₃₀ (X) = T _SAMP /{−0.0000210·(logX−1)+1}
When the temperature difference Δt is −40 ° C.,
T _COMP = T _SAMP / f ₋₄₀ (X) = T _SAMP /{−0.0000280·(logX−1)+1}
It becomes.

On the other hand, assuming that the vibration period after X hours when the temperature rises is Tx, Tx is Tx = a · logX + b (11)
When the temperature rises, the influence of drift due to the cell temperature change disappears in 1 hour after the cell temperature change, so the above equation (11) is divided by the reference value T _{1 based} on the vibration period T ₁ after 1 hour. And the ratio of the vibration period change (8)
f _Δt (X) = T _X / T ₁ = A · log _X + B (12)
It becomes. However, A = a / T ₁ and B = b / T ₁ .

FIG. 12 shows a value obtained by standardizing the measured value T _X , which is a time change of the vibration cycle of pure water, by T ₁ when the temperature is raised by changing the temperature after changing the cell temperature variously. It was graphed with respect to the axis, and it was found that the same graph was obtained if the temperature difference before and after the temperature change was the same regardless of the cell temperature before the temperature change, as in the case of the temperature drop. When the time axis of the graph of FIG. 12 is logarithmically displayed, it is as shown in FIG. 13. When the change of the slope A with respect to the temperature difference Δt in each temperature difference of FIG. 13 is graphed, as shown in FIG. The slope A is proportional to the temperature difference Δt before and after the cell temperature change. Therefore, A = a / T ₁ = β ₂ · Δt, and when X = 1 is input, the virtual vibration period T ₁ after 1 hour as a reference is obtained, so b = T ₁ and B = b / T ₁ = 1. And the inclination (beta) ₂ computed based on the result of having measured the vibration period of the pure water by changing the temperature difference before and after temperature change with the manufactured density measuring apparatus was 0.0000005.

Therefore, f _Δt (X) when the temperature difference Δt is + 10 ° C., the temperature difference Δt is + 20 ° C., the temperature difference Δt is + 30 ° C., and the temperature difference Δt is + 40 ° C.
f ₊₁₀ (X) = T _X / T ₁ = + 0.0000050 · logX + 1
f ₊₂₀ (X) = T _X / T ₁ = + 0.0000100 · logX + 1
f ₊₃₀ (X) = T _X / T ₁ = + 0.0000150 · logX + 1
f ₊₄₀ (X) = T _X / T ₁ = + 0.0000200 · logX + 1
It becomes.

Therefore, the drift correction equation for the cell temperature change is calculated when the temperature difference Δt is + 10 ° C.
T _COMP = T _SAMP / f ₊₁₀ (X) = T _SAMP /{+0.0000050·logX+1}
When the temperature difference Δt is + 20 ° C difference,
T _COMP = T _SAMP / f ₊₂₀ (X) = T _SAMP /{+0.0000100·logX+1}
When the temperature difference Δt is + 30 ° C difference,
T _COMP = T _SAMP / f ₊₃₀ (X) = T _SAMP /{+0.0000150·logX+1}
When the temperature difference Δt is + 40 ° C,
T _COMP = T _SAMP / f ₊₄₀ (X) = T _SAMP /{+0.0000200·logX+1}
It becomes.

Next, the procedure for executing the drift countermeasure routine will be described with reference to the flowchart of FIG.
When the drift countermeasure routine is activated, the control unit 21 of the control device 17 first determines whether or not the drift influence countermeasure routine at the time of temperature change has been activated (step 101), and the drift influence countermeasure routine at the time of temperature change. If it is determined that the cell setting temperature has been activated, the cell setting temperature before the change (t1) and the temperature after the change (t2) are acquired and stored in the storage unit 25 (step 102).

On the other hand, if it is determined in step 101 that the drift effect countermeasure routine at the time of temperature change is not started, that is, if it is determined that the drift effect countermeasure routine at the time of power ON is started, the control unit 21 detects the temperature immediately after the cell power is turned on. (T1) The cell set temperature (t2) is acquired and stored in the storage unit 25 (step 103).
After acquiring the temperature, the control unit 21 calculates temperature difference Δt = t2−t1 and determines whether the temperature difference Δt is positive or negative by determining whether the temperature difference Δt is positive or negative (step 104).

If it is determined in step 104 that the temperature has dropped, the control unit 21 uses the temperature difference Δt to change the gradient A ₁ of the temperature drop correction equation to A ₁ = β ₁ · Δt = 0.0000007 · Δt
And is stored in the storage unit 25 (step 105).
Next, the control unit 21 displays a screen for prompting the user to perform air / pure water calibration after changing the cell temperature on the display unit 24 (step 106).

When the user performs air / pure water calibration, the vibration period T _{X · AIR} at the time of air measurement, the elapsed time t _AIR after the temperature change at that time, the vibration period T _{X · WATER} at the time of pure water measurement, The elapsed time t _WATER after the temperature change is stored in the storage unit 25, and the vibration periods T _{10 · AIR} and T _{10 · WATER} at 10 hours after the temperature change are calculated by the following formula, and this value is Stored in the storage unit 25 as a calibration factor.
T _{10 · AIR} = T _{X · AIR} / {A ₁ · (logt _AIR −1) +1}
T _{10 · WATER} = T _{X · WATER} / {A ₁ · (logt _WATER -1) + 1}

Thereafter, the control unit 21 determines whether or not the measurement of the fluid to be measured has been executed (step 107). When the measurement of the fluid to be measured is executed, the vibration period T _{X · SAMP of the} fluid to be measured, The elapsed time t _SAMP after the temperature change during measurement fluid measurement is stored in the storage unit 25 (step 108).

Next, the control unit 21 determines whether or not the measurement time of the fluid to be measured is within 10 hours after the temperature change of the measurement cell by determining whether or not the elapsed time t _SAMP is within 10 hours (step 109). ). When it is determined that the measurement time of the fluid to be measured is within 10 hours after the temperature change of the measurement cell, the control unit 21 sets the measured vibration period _{TX / SAMP} to the value _{T10 / SAMP} 10 hours after the temperature change. Correction is performed using an equation (step 110).
T _{10 · SAMP} = T _{X · SAMP} / {A ₁ · (logt _SAMP −1) +1}

When it is determined in step 109 that the elapsed time t _SAMP exceeds 10 hours, the control unit 21 corrects the measured vibration period T _{X · SAMP} to a value T _{10 · SAMP} 10 hours after the temperature change ( Step 111). However, at this time, processing is performed with t _SAMP = 10. That is,
_{T 10 · SAMP = T X ·} SAMP / {A 1 · (log10-1) +1} = T X · SAMP
Thus, the vibration cycle is not corrected.

After correcting the vibration period, the control unit 21 is based on the above equation (7).
ρ = ρ _AIR + (ρ _AIR −ρ _WATER ) * (T _{10 · SAMP} ² −T _{10 · AIR} ² ) / (T _{10 · AIR} ² −T _{10 · WATER} ² )
To calculate the density ρ of the fluid to be measured and display it on the display unit 24 (step 112). Since the density ρ _WATER and ρ _AIR of the pure water and air at the measurement temperature are known, the correction values T _{10 · WATER} , T _{10 · AIR} and the fluid under measurement are measured for the pure water and air. The density ρ of the fluid to be measured can be calculated from the corrected oscillation period T _{10 · SAMP} .

On the other hand, when it is determined in step 104 that the temperature has risen, the control unit 21 uses the temperature difference Δt to change the gradient A ₂ of the temperature rise correction equation to A ₂ = β ₂ · Δt = 0.00000000 · Δt
And is stored in the storage unit 25 (step 113).
Next, similarly to the above, the control unit 21 displays a screen for prompting the user to perform air / pure water calibration after changing the cell temperature on the display unit 24 (step 114).

When the user performs air / pure water calibration, the vibration period T _{X · AIR} at the time of air measurement, the elapsed time t _AIR after the temperature change at that time, the vibration period T _{X · WATER} at the time of pure water measurement, The elapsed time t _WATER after the temperature change is stored in the storage unit 25, and the vibration periods T _{1 · AIR} and T _{1 · WATER} at 1 hour after the temperature change are calculated by the following formula, and this value is Stored in the storage unit 25 as a calibration factor.
T _{1 · AIR} = T _{X · AIR} / {A ₂ · log (t _AIR ) +1}
T _{1 · WATER} = T _{X · WATER} / {A ₂ · log (t _WATER ) +1}

Thereafter, the control unit 21 determines whether or not measurement of the fluid to be measured has been executed (step 115). When measurement of the fluid to be measured is executed, the vibration period T _{X · SAMP of the} fluid to be measured, The elapsed time t _SAMP after the temperature change during measurement fluid measurement is stored in the storage unit 25 (step 116).

Next, the control unit 21 determines whether or not the measured time of the fluid to be measured is within 1 hour after the temperature change of the measurement cell by determining whether or not the elapsed time t _SAMP is within 1 hour (step 117). ). When it is determined that the measurement time of the fluid to be measured is within one hour after the temperature change of the measurement cell, the control unit 21 sets the measured vibration period T _{X · SAMP} to the value T _{1 · SAMP} one hour after the temperature change. Correction is performed by the equation (step 118).
T _{1 · SAMP} = T _{X · SAMP} / {A ₂ · log (t _SAMP ) +1}

When it is determined in step 117 that the elapsed time t _SAMP exceeds 1 hour, the control unit 21 corrects the measured vibration cycle T _{X · SAMP} to a value T _{1 · SAMP} after 1 hour of temperature change ( Step 119). However, at this time, processing is performed with t _SAMP = 1. That is,
T _{1 · SAMP} = T _{X · SAMP} / {A ₂ · (log 1) +1} = T _{X · SAMP}
Thus, the vibration cycle is not corrected.

After correcting the vibration cycle, the control unit 21 performs the same as described above.
ρ = ρ _AIR + (ρ _AIR −ρ _WATER ) * (T _{1 · SAMP} ² −T _{1 · AIR} ² ) / (T _{1 · AIR} ² −T _{1 · WATER} ² )
Thus, the density of the fluid to be measured is calculated and displayed on the display unit 24 (step 112). Since the density ρ _WATER and ρ _AIR of the pure water and air at the measurement temperature are known, the vibration period correction values T _{1 · WATER} and T _{1 · AIR} and the fluid to be measured are measured. The density ρ of the fluid to be measured can be calculated from the corrected vibration period T _{1 · SAMP} .

As described above, T _COMP = T _SAMP / {β ₁ · Δt · (logX−1) +1} when the measurement cell temperature is lowered, and T _COMP = T _SAMP / {β ₂ · Since the vibration period is corrected by Δt · logX + 1}, the drift after the cell temperature change can be compensated without using the reference cell, and the waiting time required after the cell temperature change can be shortened. In FIGS. 3 to 8, comp is an error between the time change of the measured value of the density of pure water calculated using the vibration period subjected to the above correction and the value when the influence of drift disappears, It can be seen that the drift after the cell temperature change is compensated.

Note that the values of the slopes β ₁ and β ₂ , 0.0000007, and 0.0000005 used in the above examples are the measurement results of the used apparatus. If the structure and material of the density measuring apparatus are changed, it is necessary to recalculate them. There is.
In the above-described embodiment, β ₁ · Δt · (logX−1) +1 is adopted as f (X, Δt) when the measurement cell temperature is lowered, and β ₂ · Δt · is adopted when the measurement cell temperature is raised. Although logX + 1 is adopted, this function f (X, Δt) can be appropriately changed according to the time when the influence of drift at the time of temperature change of the measurement cell is eliminated. For example, β ₁ · Δt · (logX− [gamma]) + 1 ([gamma] is a constant value) and can be expressed by a polynomial instead of logX.

  Furthermore, in the above embodiment, calibration with air and pure water was performed at the time of calculating the density of the fluid to be measured. It is also possible to calculate the density without performing 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 Measuring head 14 Thermistor 15 Sampling tube 16 Drainage tube 17 Controller 21 Control part 22 Drive part 23 Detection unit 24 Display unit 25 Storage unit

Claims (1)

A vibratory densimeter equipped with a control device that calculates the density of a sample based on the vibration period of a measurement cell filled with the sample,
The control device, before the change of the measurement cell temperature, the temperature after the change,-out based on the elapsed time from the temperature change, vibration cycle of T _SAMP measured sample, the elapsed time after the temperature change X, a Δt When the temperature difference before and after the temperature change is used, a vibration equation characterized by _obtaining a corrected vibration period T _COMP by T _COMP = T _SAMP / f (X, Δt) using a correction function f (X, Δt). Density meter.

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