JPH09318481A - Calibrating method of remote seal-type differential pressure and pressure transmitter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enable measurement with high accuracy even by a diaphragm with small pore size. SOLUTION: Each of reference static pressure, reference differential pressure, and reference temperature is impressed on the main body of detection by a few points in a constant temperature bath and then corrected so that the sensor outputs at the time of impress may agree with the value of static pressure, value of differential pressure, and temperature. These output characteristics are stored as data in a write-only memory. The expansion amount of a filler liquid is obtained. The compliances of a liquid-contact diaphragm, center diaphragm and seal diaphragm are obtained in advance, and the efficiency of propagation is obtained from these compliances and stored in the memory of a correcting circuit. The output is corrected by multiplying the output from the write-only memory according to the real differential pressure by the reciprocal of the propagation efficiency at the time of measurement.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for calibrating a remote seal type differential pressure / pressure transmitter which corrects an output varying with temperature and improves the performance.

[0002]

2. Description of the Related Art In process measurement, when measuring pressure or liquid level, a differential pressure / pressure transmitter directly connected to a flange of a tank or the like is used. In such a differential pressure / pressure transmitter, the pressure receiver and the transmitter main body are connected by a capillary tube filled with a filled liquid, and the process fluid pressure is received by the liquid contact diaphragm of the pressure receiver, and the displacement of this liquid contact diaphragm is received. Since it is guided to the transmitter body through the filled liquid, it is usually a remote seal type differential pressure / pressure transmitter (eg, JP-A-55-108947, JP-A-62-62).
No. 88931, Japanese Utility Model Laid-Open No. 63-33437, etc.)
I am calling.

FIG. 3 is a sectional view showing a conventional example of a remote seal type differential pressure transmitter. In the figure, a remote seal type differential pressure transmitter 1 includes a transmitter main body 2, two pressure receivers 3 and 3 having the same structure, a transmitter main body 2 and each pressure receiver 3 and 3.
And two capillary tubes 5 and 5 respectively connecting the two. The transmitter main body 2 includes a transmitter body 9 in which a filling liquid 13 is filled, a center diaphragm 6, seal diaphragms 8 and 8, and a head portion 12 having a semiconductor pressure sensor 11 built therein. The center diaphragm 6 located at the center divides the inner chamber 7 of the transmitter body 9 into two chambers 7a and 7b. The seal diaphragms 8 and 8 respectively cover the respective side surfaces of the transmitter body 9 and further cover the outer sides thereof with covers 10 and 10.
The seal diaphragms 8 and 8 and the chambers 7a and 7b communicate with each other through passages a and a, and also communicate with the upper and lower chambers of the semiconductor pressure sensor 11. The seal diaphragm 8 composes an overpressure protection mechanism that prevents the semiconductor pressure sensor 11 from being broken by bottoming on the side surface of the transmitter body 9 when an excessive pressure difference higher than the measurement pressure is applied.

The pressure receiver 3 has a detector body 17 connected to a flange of a tank or the like. Detector body 17
A liquid contacting diaphragm 15 is provided on one side surface, and a sealed liquid passage 18 is formed inside. Filled liquid passage 18
Has one end communicating with a diaphragm chamber 16 provided on the back side of the liquid contact diaphragm 15, and the other end connected to one end of the capillary tube 5. An enclosed liquid 20 is enclosed in the capillary tube 5, the diaphragm chamber 16 and the enclosed liquid passage 18. An end portion of the capillary tube 5 opposite to the pressure receiver 3 side communicates with a chamber 21 formed between the seal diaphragm 8 and the cover 10 of the detector body 2. In addition, a small amount of the filling liquid 20 is filled in the cover 10.
In the above description, the differential pressure transmitter is described, but the same can be said for the pressure transmitter. Therefore, hereinafter, both transmitters are generically referred to as a differential pressure / pressure transmitter.

In the remote seal type differential pressure transmitter 1 having such a structure, when the low pressure side and high pressure side measured pressures PL and PH are applied to the liquid contact diaphragms 15 of the respective pressure receivers 3, respectively. Is displaced, and this displacement is transmitted to each seal diaphragm 8 via the filled liquid 20 to move the filled liquid 13 in the chambers 7a and 7b. Therefore, the center diaphragm 6 has a differential pressure (PH-P
L), and a pressure balanced with this displacement is applied to the semiconductor pressure sensor 11 via the filled liquid 13. Therefore, the semiconductor pressure sensor 11 is distorted according to the differential pressure, and the differential pressure is measured by converting the amount of strain into an electric signal and extracting the electric signal.

Such a remote seal type differential pressure transmitter 1
In, most of the enclosed liquid 20 is enclosed in the capillary tube 5. The enclosed liquid 20 expands and contracts due to the temperature change, and the liquid contact diaphragm 15 changes accordingly. In order for the oscillator to continue to operate normally, the wetted diaphragm 15 needs to have a spring property. The stress, which is an index of springiness, is expressed by the following equation. σmax ≧ σ = k1 tΔV / D ⁴ (1) where σmax is the allowable stress (limit having springiness), σ
Is the generated stress, k1 is a constant (depending on the shape of the diaphragm), t is the plate thickness of the wetted diaphragm, ΔV is the displacement volume of the wetted diaphragm, and D is the diameter of the wetted diaphragm. Here, σ must not exceed σ max.

The displacement volume ΔV of the wetted diaphragm is expressed by the following equation. ΔV = α (TS-TF) V (2) where α is the coefficient of thermal expansion of the filled liquid, TS is the ambient temperature of the transmitter, TF is the temperature when the filled liquid is filled, and V is all filled. It is the liquid volume.

As described above, the remote seal type differential pressure transmitter 1 has a total of five diaphragms of three types. The pressure guided to the oscillator is transmitted to the sealed liquid through these diaphragms and finally to the semiconductor pressure sensor 11. When the wetted diaphragm is displaced at that time, the pressure σ generated inside the diaphragm causes the pressure to increase. Loss of. Due to this pressure loss, the differential pressure received by the sensor 11 becomes smaller than the actual differential pressure of the measurement target. The ratio of the differential pressure received by the sensor 11 and the actual differential pressure is hereinafter referred to as transmission efficiency.
As the stress σ increases, the transmission efficiency η decreases, causing a span shift in the oscillator output. FIG. 4 shows the relationship between the generated stress σ and the transmission efficiency η. The transmission efficiency η is expressed by the following equation. η = C ^-1 / (2CB ^-1 + 2CS ^-1 + C ^-1 ) (3) where η is the transmission efficiency, C is the compliance of the center diaphragm, CB is the compliance of the seal diaphragm, and CS is the wetted diaphragm. Compliance. As shown in FIG. 4, CS is the stress σ generated by the displacement of the diaphragm when the diameter of the diaphragm becomes smaller.
Becomes larger, the transmission efficiency η becomes smaller. The compliance C of the center diaphragm and the compliance CB of the seal diaphragm can be regarded as substantially constant in design.

Since the stress σ that affects the transmission efficiency η varies depending on the temperature as shown by the above equations (1) and (2), the transmitter causes a span shift even when the temperature changes. Since the temperature characteristic is an important factor for performing stable process measurement, it is essential to improve its performance as a transmitter.

Therefore, conventionally, the transmission efficiency of the seal diaphragm was considered to be constant, and only the transmitter main body 2 was corrected by characterization. Characterization is a method in which the output characteristics for each oscillator are captured as data and the most suitable correction is performed for each one.

As described above, conventionally, the calibration is performed only by the transmitter main body 2, but the reason is as follows. When the capillary tube 5 and the pressure receiving unit 3 are added, the error from the calibrated value is mostly due to the change in the transmission efficiency η that occurs depending on the degree of softness of the liquid contact diaphragm 15. Therefore,
As shown in the equation (1), if the diameter of the wetted diaphragm 15 is large, the change of the stress σ is small and the change of the transmission efficiency η is small, so that there is little influence on the output of the transmitter, and the calibration of the transmitter main body 2 is sufficient. Is.

[0012]

However, since a flange having a smaller diameter has recently been used, there is a demand for downsizing of the oscillator. for that reason,
If the wetted diaphragm 15 is made small, the change in transmission efficiency becomes large as is apparent from FIG. 4, and the output of the transmitter is span-shifted, which makes it impossible to perform highly accurate measurement. For example, in the case of a remote seal type transmitter with a 11 / 2B flange, the diameter D of the wetted diaphragm must be halved compared to a remote seal type transmitter with a 3B flange, but only D is 1/2. If so, the generated stress σ becomes 16 times as large as that of the 3B remote seal type transmitter, the change in transmission efficiency becomes large, and stable measurement cannot be performed.

FIG. 5 is a diagram showing temperature characteristics of a small-diameter transmitter. As is clear from this figure, the conventional small-diameter transmitter has a large shift (error) because the transmission efficiency fluctuates greatly.

Therefore, in the case of a small-sized oscillator, it is necessary to calibrate the transmitter with the transmitter assembled, but it is necessary to put the entire transmitter in a thermostatic chamber, so a large thermostatic chamber is required. Becomes Further, if the constant temperature chamber is large, it takes a long time to achieve a stable environment even if the temperature is changed for calibration, and it takes a long time for calibration.

The present invention has been made to solve the above-mentioned conventional problems, and an object of the present invention is to transmit differential pressure / pressure that enables highly accurate measurement even with a small-diameter diaphragm. It is to provide a method for calibrating a vessel.

[0016]

In order to achieve the above object, the present invention is directed to a liquid-contacting diaphragm made of a single substance,
The compliance of the center diaphragm and the seal diaphragm is calculated, and the transmission efficiency is calculated from the compliance of the wetted diaphragm, the center diaphragm and the seal diaphragm and stored in the memory in the correction circuit. Pressure, reference differential pressure and reference temperature are applied to the seal diaphragm at several points, and the sensor output at this time is corrected to match the static pressure value, differential pressure value and temperature, and the output characteristics are written as data only. Stored in a memory, the detector main body is taken out from the constant temperature bath, the detector main body and the pressure receiver are connected by a capillary tube, and the total volume of the filled liquid in the capillary tube,
Obtain the expansion amount of the enclosed liquid from the thermal expansion coefficient of the enclosed liquid and the compression coefficient of the enclosed liquid, and correct the output by multiplying the output from the write-only memory according to the actual differential pressure by the reciprocal of the transfer efficiency during measurement. It is characterized by performing.

In the present invention, the characterization of only the detector main body is performed, and the output at this time is corrected and stored in the memory. In addition, the transmission efficiency is calculated from the compliance of the wetted diaphragm, the center diaphragm, and the seal diaphragm that consist of a single body that is obtained in advance,
Since the output difference is corrected by multiplying the actual differential pressure by the reciprocal of the transmission efficiency, the shift (error) is small even if the transmitter has a small diameter. Further, since only the transmitter main body needs to be placed in the constant temperature bath for characterization, the size of the constant temperature bath can be small.

[0018]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described in detail below. The remote seal type differential pressure / pressure transmitter calibration method according to the present invention comprises the following steps.

Step 1 Characterization of the main body of the detector The main body of the detector is placed in a thermostatic chamber, and a standard static pressure, a standard differential pressure, and a standard temperature are applied to the main body of the detector at several points. The pressure value, the differential pressure value, and the temperature are corrected so that they match, and the output characteristic is stored as data in the write-only memory.

Step 2 The volume ΔVc that moves when the pressure on the liquid contact diaphragm of the pressure receiver is changed by ΔP is measured, and the compliance Cs of the liquid contact diaphragm is determined from this. Cs = ΔVc / ΔP (4) Cs is as shown in FIG. 1 when the total movement amount Vc is plotted on the horizontal axis.

Step 3 The detector main body is taken out from the constant temperature bath, the detector main body and the pressure receiver are connected by a capillary tube, and an enclosure liquid is enclosed.

Step 4 The expansion amount of the filled liquid is obtained from the total volume of the filled liquid, the thermal expansion coefficient of the filled liquid, and the compression coefficient of the filled liquid. The volume ΔV of displacement of the liquid contact diaphragm is proportional to the expansion of the enclosed liquid. That is, ΔV = (1 + αΔT + βΔP) × V (5) where: V is the total volume of the enclosed liquid enclosed in the capillary tube, V≈kl (k; constant, l; length of the capillary tube) , Α is a thermal expansion coefficient of the enclosed liquid, β is a compression coefficient, ΔT is a temperature change amount, and ΔP is a static pressure change amount.

Step 5 The compliance Cs of the wetted diaphragm is determined by replacing Vc in step 2 with ΔV in step 3. Also, the compliance of the center diaphragm and the seal diaphragm should be obtained in advance. Next, the transmission efficiency η is obtained from the compliance of the liquid contact diaphragm, the center diaphragm and the seal diaphragm.
The transmission efficiency η can be obtained by knowing the temperature and static pressure at the time of measurement. Η is known because the temperature and static pressure are always known in the calculations used to calibrate the detector body. Note that step 2 and step 5 can be performed prior to step 1 described above.

Step 6 The output from the write-only memory corresponding to the actual differential pressure at the time of measurement is multiplied by the reciprocal of the transfer efficiency for correction, and the value is used as the output of the oscillator.

By doing so, highly accurate measurement can be performed even with a small diameter liquid contact diaphragm having a large change in transmission efficiency.

FIG. 2 shows the temperature characteristics of a small-diameter transmitter.
As is clear from this figure, compared with the conventional small-diameter oscillator shown in FIG. 5, it is possible to suppress the shift (error) caused by the variation due to the characteristics of the wetted diaphragm.

[0027]

As described above, the differential pressure according to the present invention
The calibration method of the pressure transmitter is to calculate the compliance of the liquid contact diaphragm, center diaphragm and seal diaphragm, each consisting of a single unit, and calculate the transmission efficiency from the compliance of the liquid contact diaphragm, center diaphragm and seal diaphragm and store it in the memory in the correction circuit. Memorize it, install the detector body in a thermostatic chamber, apply the reference static pressure, reference differential pressure, and reference temperature to the seal diaphragm at several points.At this time, the sensor output and static pressure value, differential pressure value,
Correct the temperature so that they match, store the output characteristics as data in a write-only memory, take out the detector body from the thermostatic chamber, connect the detector body and the pressure receiver with a capillary tube, and Of the filled liquid, the thermal expansion coefficient of the filled liquid, and the compression coefficient of the filled liquid, the expansion amount of the filled liquid is determined, and the transfer efficiency of the transfer efficiency is output to the output from the write-only memory according to the actual differential pressure during measurement. Since the output is corrected by multiplying it by the reciprocal, it is possible to perform highly accurate measurement even with a small diameter wetted diaphragm and improve the performance. Further, since only the detector body needs to be put in the constant temperature bath for characterization, the size of the constant temperature bath can be small and the stable temperature can be maintained in a short time.

[Brief description of drawings]

FIG. 1 is a diagram showing a relationship between a compliance Cs of a liquid contact diaphragm and a total movement amount of an enclosed liquid.

FIG. 2 is a diagram showing a temperature characteristic of a small-diameter transmitter adapted to perform correction according to the present invention.

FIG. 3 is a cross-sectional view of a conventional remote seal type differential pressure transmitter.

FIG. 4 is a diagram showing a relationship between stress and transmission efficiency.

FIG. 5 is a diagram showing temperature characteristics of a conventional small-diameter transmitter.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Differential pressure transmitter, 2 ... Transmitter main body, 3 ... Pressure receiver, 5 ... Capillary tube, 6 ... Center diaphragm, 8 ... Seal diaphragm, 13 ... Fill liquid, 15 ... Wetted diaphragm, 16 ... Diaphragm chamber, 18 … Filled liquid passage, 20
... filled liquid.

Claims (1)

[Claims] 1. A compliance of a liquid contact diaphragm, a center diaphragm and a seal diaphragm, each of which is composed of a single element, is calculated, and a transmission efficiency is calculated from the compliance of the liquid contact diaphragm, the center diaphragm and the seal diaphragm and stored in a memory in a correction circuit, Mount the detector body in a thermostatic chamber and apply reference static pressure, reference differential pressure and reference temperature to the seal diaphragm at several points,
The sensor output at this time is corrected to match the static pressure value, differential pressure value, and temperature, and the output characteristics are stored as data in a write-only memory, and the detector body is taken out from the thermostatic chamber and the detector body is removed. And the pressure receiver are connected by a capillary tube, and the expansion amount of the filled liquid is calculated from the total volume of the filled liquid in the capillary tube, the thermal expansion coefficient of the filled liquid, and the compression coefficient of the filled liquid. A method for calibrating a remote seal type differential pressure / pressure transmitter, characterized in that the output from the write-only memory is multiplied by the reciprocal of the transfer efficiency to perform output correction.