Abstract: In many industrial equipments, pressure transmitter is an important component to control industrial process and pressure change. The pressure transmitter is used to measure the liquid level, density and pressure of liquid, gas or steam, and then convert the pressure signal into a 4 ~ 20mA DC signal for output. Pressure transmitters are divided into capacitive pressure transmitters, diffused silicon pressure transmitters, ceramic pressure transmitters and strain pressure transmitters.

Keywords: pressure transmitter; Intelligent transmitter; EJA transmitter; Rosemount pressure transmitter

Text:

I. Introduction

Pressure transmitter is a field instrument that is in direct contact with the measured medium, and often works in high and low temperature environments such as corrosion, vibration and impact. Widely used in pressure measurement and field control in petroleum, chemical industry, electric power, steel, light industry and other industries.

The development of pressure transmitter has gone through four stages:

(1) Early pressure transmitters all used the principle of large displacement, such as mercury float differential pressure gauge and diaphragm differential pressure transmitter, which were all mass-produced. These transmitters have low accuracy and large volume.

(2) In 1950s, there was a force-balanced differential pressure transmitter with slightly higher accuracy, but the feedback force was small, the structure was complex, and the reliability, stability and vibration resistance were poor.

(3) In the mid-1970s, with the appearance of new technologies, new materials and new processes, especially the rapid development of electronic technology, displacement transmitters with small volume and simple structure appeared.

(4) With the rapid development of science and technology in 1990s, these transmitters have high measurement accuracy, and the gradually developed intelligent digital signal transmission is more conducive to data acquisition.

So far, there are different types of pressure transmitters, such as capacitive transmitter, diffused silicon piezoresistive transmitter, differential inductive transmitter and ceramic capacitive transmitter.

Second, several kinds of pressure transmitter

1. Diffusion silicon pressure transmitter

In the mid-1990s, Icsensors Company and Nova Company of the United States produced a new type of diffused silicon pressure sensor and developed a high-precision, low-repeatability, corrosion-resistant diffused silicon pressure transmitter by using two cutting-edge technologies of silicon fine etching and silicon wafer superposition. From 65438 to 0993, Changsha Mining Research Institute developed SBP800 pressure transmitter with high cost performance, which was widely used in dozens of large and medium-sized enterprises such as Shougang and Changling Refinery.

The process pressure is transferred to the diffusion silicon diaphragm through the isolation diaphragm and sealing silicone oil, and the atmospheric pressure at the reference end acts on the other end of the diaphragm. In this way, the pressure difference between the two sides of the diaphragm produces a pressure field, which makes one part of the diaphragm compressed and the other part stretched. Two strain resistors are set in the compression zone and the tension zone respectively to feel the resistance change caused by pressure, so as to convert the pressure signal into an electrical signal diagram. This SBP800 pressure transmitter can measure any liquid or gas medium carried by 3 16 steel.

2. Capacitive diaphragm pressure transmitter

Capacitive diaphragm absolute pressure transmitter has a history of 20 years since it was born in 1980s. Because of its high precision, corrosion resistance, pollution resistance and good stability, it is recognized as an ideal instrument for detecting low vacuum pressure at home and abroad. M.K.S Company of the United States is the main manufacturer of capacitive membrane pressure transmitter in the world. Up to now, its annual sales have reached hundreds of millions of dollars, involving all fields of civil industry and playing a unique role in the aerospace industry, nuclear industry and other military fields.

This kind of pressure transmitter is made by using the principle that the elastic diaphragm produces pressure change under the action of pressure difference, which causes capacitance conversion. It consists of a detection part and a conversion circuit. The detection part has two cavities, namely a vacuum cavity and a detection cavity. The vacuum chamber is a fully sealed structure, which is formed by exhausting for a long time after the mass spectrometer passes the leak detection, and finally sealing the exhaust pipe, and is equipped with a getter to remove residual gas and maintain a high vacuum for a long time. The fixed polar plate is located in the vacuum cavity and leads out of the cavity from the polar plate. The detection diaphragm is placed between the high vacuum vacuum chamber and the detection chamber connected with the low vacuum system to be detected. The detection diaphragm is a movable plate, which forms a flat capacitor with the fixed plate and has a certain capacitance. The measured low vacuum pressure enters the detection cavity through the detection hole, and the detection diaphragm flexes, changing its distance from the fixed plate, and the capacitance value also changes. Different low vacuum pressure values determine different capacitance values. Finally, the capacitance signal is transmitted to the circuit conversion part, and the circuit conversion part outputs the standard voltage or current signal by transforming, classifying and amplifying the capacitance signal. This standard electrical signal comes from the capacitance signal, which is proportional to the vacuum pressure. The vacuum precooling testing machine adopts CPCD 100Z capacitive membrane pressure transmitter to measure the vacuum chamber pressure. The signal is output to the panel through the microprocessor. Experiments show that the transmitter has fast response, stability and reliability, and can be used continuously, which fully meets the requirements of pressure measurement in the test.

3. Ceramic thick film pressure transmitter

Ceramic thick film pressure transmitter uses the force sensitive effect of ceramic thick film resistor. Using ceramic thick-film pressure chip as elastic element and thick-film resistor printed and sintered on ceramic diaphragm as sensitive resistor, the pressure signal is directly converted into standard current signal through precise compensation technology, resistance adjustment technology and signal processing technology, which is connected to industrial instrument or computer control system to realize automatic detection and control of production process.

Ceramic thick film pressure transmitter is mainly composed of ceramic thick film pressure sensor and microprocessor. The sensor is used to measure the pressure change. When the pressure acts on the sensor, the resistance value of the sensor changes, which is detected by the bridge circuit on the sensor chip, converted into digital signals by the A/D converter and sent to the microprocessor. Microprocessor is the core component of signal processing, which has the functions of linear operation, correction, fault diagnosis and communication. The sensor data memory can store the correction coefficient, and the microprocessor uses the data information in the memory to produce high-precision output with excellent characteristics after calculation and processing. This kind of transmitter can directly introduce various media including corrosive and non-corrosive gases and liquids into the ceramic diaphragm in the production process, without complicated isolation technology, so it is low in price. The elastomer of pressure transmitter adopts high alumina porcelain with extremely stable physical and chemical properties, which has no creep and plastic deformation after long-term work, and its linearity and hysteresis performance are obviously superior to other types of pressure transmitters.

4. Ceramic capacitive pressure transmitter

Ceramic capacitive pressure transmitter adopts dry ceramic capacitive sensor, which has no intermediate liquid and high technical performance. The main products on the market are the products of German E+H company and American Kavlico company. In the mid-1990s, Harbin Institute of Technology produced PTM 120 pressure transmitter by using the ceramic capacitance sensor of Kavlico Company in the United States. Its performance is stable and the test data is accurate. Widely used in pressure measurement and field control in petroleum, chemical industry, electric power, steel, light industry and other industries.

The working principle of ceramic capacitive pressure transmitter is different from other capacitive transmitters. The medium pressure directly acts on the ceramic diaphragm, which causes the displacement of the measuring diaphragm. The capacitance change caused by diaphragm displacement has a certain linear relationship with the input pressure, which is detected, amplified and output by electronic components.

Third, intelligent pressure transmitter

In the 1990s, the field bus technology rose rapidly, and the industrial process control system gradually developed to the field bus control system with two-way communication and intelligent instrument control. Therefore, a new generation of intelligent pressure transmitter came into being. Their main features are as follows.

(1) Self-compensation functions such as nonlinearity, temperature error, response time, noise and cross-induction.

(2) Self-diagnosis function, such as power-on self-test, can realize operation inspection at work.

(3) The bidirectional communication function between the microprocessor and the basic sensor constitutes a closed-loop working system.

(4) Information storage and memory function.

(5) Digital output.

Based on the above functions, the accuracy, stability, repeatability and reliability of the intelligent pressure transmitter are improved. Its two-way communication ability realizes computer software control and remote setting range.

There are two kinds of intelligent pressure transmitters: those with HART protocol and those with 482 or RS232 interface. HART protocol intelligent pressure transmitter is to superimpose a special frequency signal on the analog signal and realize analog and digital communication at the same time. The intelligent pressure transmitter with RS232 or 485 ports internally converts analog signals into A/D, and the analog signals are output by d/a after operation by a microprocessor. The RS232 interface is an asynchronous communication protocol interface, which is compatible with various communication protocols. Therefore, the latter is widely used.

Four. Representative products of pressure transmitter

(1)EJA transmitter

Technical specification of products

Application type, model, diaphragm range (KPa) and maximum working pressure (MPa)

Conventional installation of differential pressure and liquid level EJA110al0.5-103.5

L (liquid contact material code is "S") 0.5-1016

M 1- 100 16

H 5-500 16

V 0. 14- 14 MPa 16

Features:

The influence of static pressure is neglected.

When static pressure (working pressure) is applied, the deformation of two resonant beams with the same shape, size and material is the same, so the frequency change is the same, so the deviation is automatically cleared (the formula and graph are similar to the temperature effect).

Excellent one-way overvoltage characteristics

The liquid-receiving diaphragm and the capsule body adopt the original corrugated processing technology. When the external pressure increases to a certain value, the liquid-receiving diaphragm can completely contact with the capsule body, and the pressure transmitted by the silicone oil to the sensor will not increase with the increase of external force, thus protecting the sensor.

Flexible installation

Can be directly installed without a bracket;

For normal use, three valve groups are not needed.

Comprehensive evaluation:

Using MEMS technology, two H-shaped resonant beams with the same shape, size and material are fabricated at the center and edge of the surface of monocrystalline silicon wafer, and the resonant beams oscillate at high frequency in a self-excited oscillation circuit (as shown in figure 1). When the pressures on the upper and lower surfaces of the monocrystalline silicon wafer are not equal, deformation will occur, which will cause the frequency of the central resonant beam to decrease due to compressive force and the frequency of the edge resonance to increase due to tensile force (as shown in Figure 2).

The difference signal of two frequencies is directly sent to CPU for data processing, and then (1) is converted into 4-20mA output signal through D/A, and the digital signal of Brain or Hart is superimposed during communication. (2) Direct output of digital signals conforming to the TM standard of Fieldbus Foundation.

(2) Rosemount pressure transmitter

First, product overview:

Rosemount's 305 1C pressure transmitter sets a new standard for pressure measurement technology. It has unparalleled operational performance, flexible CoPlanarTM platform and can be upgraded. The performance index of the new 305 1C pressure transmitter ensures the accuracy and stability under different working conditions.

Working principle: When working, the isolation diaphragm and filling liquid on the high and low pressure sides transfer the process pressure to the filling liquid, and then the filling liquid transfers the pressure to the sensing diaphragm in the sensor center. The sensing diaphragm is a stretched elastic element, and its displacement changes with the pressure (for GP gauge pressure transmitter, the atmospheric pressure is the same as the pressure applied to the low-voltage side of the sensing diaphragm). AP absolute pressure transmitter, the low voltage side always keeps a reference pressure. The maximum displacement of the sensing diaphragm is 0.004 inch (0. 1 mm), and the displacement is directly proportional to the pressure. Capacitor plates at both sides detect the position of the sensing diaphragm. The capacitance difference between the sensing diaphragm and the capacitor plate is converted into corresponding current, voltage or digital HART (high-speed addressable remote transmitter data high-speed channel) output signal.

Application of 305 1 transmitter

The performance index of the new model 305 1C ensures the accuracy and stability under different working conditions. The flexible CoPlanarTM platform design can not only provide the best solution for your current application requirements, but also fully meet your future technical requirements through PlantWeb factory control network and field bus technology.

Second, the technical parameters:

Overall performance: 0. 15%

Accuracy: 0.075%

Differential pressure: the calibration range is from 0.5 inch water column to 2000 psi.

Gauge pressure: the calibration range is from 2.5 inches of water column to 2000 psi.

Absolute pressure: the calibration range is from 0. 167psia to 4000psia.

Process isolation diaphragm: stainless steel, Hastelloy CR, Monel R, Tantalum (CD, CG only) and gold-plated Monel.

Compact design, strong and light, easy to install.

V. Development trend of pressure transmitter

At present, the research field of pressure transmitter in countries all over the world is very extensive, almost permeating all industries, but in summary, there are the following trends:

(1) Intelligentization Due to the appearance of integration, some microprocessors can be added to the integrated circuit to make the transmitter have the functions of automatic compensation, communication, self-diagnosis and logical judgment.

(2) More and more integrated pressure transmitters are integrated with other measuring transmitters to form a measurement and control system. Integrated system can improve the speed and efficiency of process control and factory automation.

(3) Miniaturization At present, there is an increasing demand for small pressure transmitters in the market. This small pressure transmitter can work in extremely harsh environment, and it needs little maintenance and has little impact on the surrounding environment. It can be placed in various important organs of human body to collect data without affecting people's normal life. For example, the transmitter with a range of 2 ~ 500 psi produced by Entran Company in the United States has a diameter of only 1.27mm, which can be placed in human blood vessels without great influence on blood circulation.

(4) The design and manufacture of standardized transmitters have formed certain industry standards. Such as ISO international quality system; American ANSI and ASTM standards, Russian ггOCT and Japanese J IS standards.

(5) Another development trend of extensive pressure transmitter is to expand from mechanical industry to other fields, such as automobile parts, medical instruments and energy and environmental control systems.