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​7 Points To Fully Understand The Pressure Transmitter
Jan 02, 2019

7 points to fully understand the pressure transmitter

I What is a pressure transmitter?

A pressure transmitter is a device that converts pressure into a pneumatic or electric signal to stop control and remote transmission.

It can convert the physical pressure parameters such as gas and liquid sensed by the sensor of the load cell into a standard electrical signal (such as 4~20mADC, etc.), and supply the secondary instrument such as the alarm device, the recorder, the conditioner, etc. to stop measuring. Instructions and process conditioning.

Pressure transmitter is the most commonly used sensor in industrial theory. It is widely used in various industrial self-control environments, touching water conservancy and hydropower, railway transportation, intelligent building, consumer automation, aerospace, military, petrochemical, oil well, electric power, ship. , machine tools, pipelines and many other industries.

II What is stress?

Physical stress refers to the force exerted on the contact surface of two objects, or the vertical force of a gas on the appearance of solids and liquids, or the vertical force of a liquid on the appearance of a solid. In the habit, in the mechanics and most engineering disciplines, the word "stress" is synonymous with the pressure in physics.

The pressure of a solid appearance is usually the result of elastic deformation, which is generally a contact force. The pressure on the surface of liquids and gases is usually the result of gravity and molecular motion.

The direction of action of the pressure is usually perpendicular to the contact surface of the object. If the direction of action of the pressure is observed to be not perpendicular to the contact surface, it is usually the result of a combination of pressure and friction.

7 points to fully understand the pressure transmitter

III pressure transmitter development history

The development of pressure transmitters has gone through four stages:

1. Early pressure transmitters used large displacement working principles, such as mercury float differential pressure gauges and bellows type differential pressure transmitters that have been consumed in large quantities. These transmitters are low in accuracy and cumbersome.

2. In the 1950s, there was a force-balanced differential pressure transmitter with a slightly higher precision, but the reaction force was small, the structure was complicated, and the reliability, stability and vibration resistance were poor.

In the mid-1970s, with the emergence of new technologies, new materials, and new technologies, especially the rapid development of electronic technology, the displacement transmitters with small size and simple structure were presented.

In the 4th and 90s, science and technology developed rapidly. These transmitters have high measurement accuracy and gradually carry out digital signal transmission to facilitate data acquisition.

Pressure transmitters have been developed to date, such as capacitive transmitters, diffused silicon piezoresistive transmitters, differential inductive transmitters, and ceramic capacitive transmitters.

In the 1990s, the fieldbus technology developed rapidly, and the industrial process control system gradually moved toward the fieldbus control system with two-way communication and intelligent instrument control. This has resulted in a new generation of intelligent pressure transmitters. Their main characteristics are as follows:

1. Self-compensation functions such as nonlinearity, temperature error, response time, noise, and insertion inductance.

2. Self-diagnosis function If the self-test is stopped when the power is turned on, the operation check is completed during the work.

3. The function of two-way communication between the micro-processor and the underlying sensor constitutes a closed-loop working system.

4. Information storage and memory functions.

5, digital output.

Based on the above functions, the accuracy, stability, repeatability and reliability of the intelligent pressure transmitter have been improved and improved. The two-way communication can complete the state of computer software control and remote setting range.

IV The principle of measuring pressure in pressure transmission

The electrical components that the pressure transmitter feels pressure are usually resistance strain gauges, which are sensitive devices that convert the pressure on the device under test into an electrical signal. The most widely used resistance strain gauges are metal resistance strain gauges and semiconductor strain gauges. The metal resistance strain gauge has two kinds of filament strain gauges and metal foil strain gauges. Usually, the strain gauge is tightly bonded to the mechanical strain matrix through a special adhesive. When the stress of the matrix changes, the strain gauges are also deformed together, so that the resistance of the strain gauge is changed, thereby adding The voltage on the resistance changes. The measurement principle of the pressure transmitter is divided into the following three categories:

4.1 resistance strain gauge pressure transmitter

(1) A basic overview. Resistance strain gauge pressure transmitter large signal output, stainless steel construction, with high linearity, small hysteresis error, good temperature performance, stable operation, wide range, corrosion resistance, etc., mainly used in national defense and industrial automation. Part of its important component is a strain gauge that converts strain changes on the device under test into an electrical signal.

(2) Working principle. Adsorbed on the matrix data, the strain resistance of the metal resistance strain gauge changes with the mechanical deformation, which is commonly known as the resistance strain effect. Under normal circumstances, the strain gage is tightly bonded to the mechanical strain matrix by a special adhesive. After the stress of the matrix changes, the resistance strain gauge is also deformed together, so that the resistance of the strain gauge occurs. Modifications, such that the voltage applied to the resistor changes. The strain gauges have a small change in resistance when subjected to force. Normal strain gauges form a strain bridge and are stopped by a subsequent instrumentation amplifier and then transmitted to the disposal circuit (usually A/D conversion and CPU) Visualization or execution mechanism. In the application, the range of resistance should be paid special attention, the resistance value is too small, the required driving current is too large, and the heating will cause the temperature of the strain gauge to be too high, resulting in too much change in the resistance of the strain gauge. The zero drift is obvious, and the zeroing circuit is too complicated. The resistance is too large, the impedance is too high, and the electromagnetic interference against the outside can be reduced.

7 points to fully understand the pressure transmitter

4.2 diffused silicon pressure transmitter

(1) A basic overview. The pressure detecting component of the diffused silicon pressure transmitter adopts an imported diffused silicon or ceramic core, and the sensor signal is converted into a 0-10 mA or 4-20 mA unified output signal by a high performance electronic amplifier. This pressure transmitter can be replaced

It is a traditional remote pressure gauge, Hall element, differential transmitter, and has DDZ-II and DDZ-III transmitter performance. The diffused silicon pressure transmission has powerful application performance, and can be used not only with various types of moving coil indicator, digital pressure gauge, electronic potentiometer, but also with various automatic conditioning systems or computer systems. (2) Working principle.

When the pressure signal of the medium acts on the sensor, the pressure sensor converts the pressure signal into an electrical signal, which is amplified by differential amplification and output amplifier, and finally converted into a linear relationship with the liquid pressure of the measured medium by the V/A voltage current. The 4-20mA specification current output signal.

4.3 ceramic pressure transmitter

(1) A basic overview.

The high elasticity, corrosion resistance, wear resistance, impact resistance and vibration of ceramic materials are recognized. The ceramic's operating temperature range is between -40 and 135 ° C, with high precision and high stability. Electrical insulation level > 2kV, not only the output signal is strong, but also can maintain long-term stability. Such high characteristics and low price advantages will prompt ceramic sensors to become the future direction.

(2) Working principle.

The pressure of the corrosion-resistant ceramic pressure transmitter acts directly on the front surface of the ceramic diaphragm, causing the ceramic diaphragm to produce a slight deformation, and the thick film resistor is printed on its opposite side to form a Wheatstone bridge. The piezoresistive effect of the varistor allows the bridge to produce a highly linear voltage signal proportional to the pressure, which is proportional to the encouraged voltage. The 2.0/3.0/3.3mV/V specification signal is based on the pressure range. Calibrated, it is compatible with strain gauge sensors. The sensor is laser calibrated for high temperature stability and time stability. The sensor comes with temperature compensation from 0 to 70 ° C and is in direct contact with most media.

V pressure transmitter structure

The main components are the sensor module and the electronics housing.

The sensor module includes an oil-filled sensor system (isolator, oil-filled system and sensor) and sensor electronics. The sensor electronics are housed within the sensor module and include - a temperature sensor (resistive test detector), a storage module, and a capacitance/digital signal converter (C/D converter). The electronic signal from the sensor module is transmitted to the output electronics in the electronics housing. The electronics housing includes an output electronics board (microprocessor, storage module, digital/imitation signal converter or D/A converter).

Since the design pressure of the 3151SMT transmitter is suitable for the isolation diaphragm, the capacitance signal is changed when the oil deviates from the center film. The capacitive signal is then converted to a digital signal in a C/D converter. The microprocessor then takes the signal from the resistive temperature detector and C/D converter and calculates the correct transmission output. This signal is then sent to a D/A converter that converts the signal back to the analog signal and superimposes the HART signal on the 4-20 mA output.

VI pressure transmitter application

Pressure transmitter is a commonly used sensor in industrial theory. It is widely used in various industrial self-control environments, touching water conservancy and hydropower, railway transportation, intelligent building, consumer automation, aerospace, military, petrochemical, oil well, electric power, ship. , machine tools, pipelines and many other industries. The pressure transmitter studio must directly contact the measured medium, often operating in high temperature, low temperature, corrosion, vibration, shock and other environments. Whether it can operate normally at the work site depends not only on the quality of the product, but also on optimization. Engineering design, reasonable model configuration, and proper device maintenance. Pressure transmitters are key components in the fluid industry. Pressure transmitters are mainly used in the following areas:

1 Petroleum, petrochemical, chemical, and throttling installation, providing accurate flow measurement and control. It can measure the pressure and liquid level of pipes and tanks. 2 Electric power, city gas, and other company businesses are demanding high-stability and high-precision measurement. 3 Pulp and paper, used to request places such as chemical resistant liquids, corrosion resistant liquids. 4 Steel, non-ferrous metals, ceramics, used for furnace pressure measurement and other places requiring high stability, high-precision measurement, etc., used for stable measurement (temperature, humidity, etc.) conditions to request stable measurement. 5 Machinery, shipbuilding, used to strictly control the location where high-precision conditions require stable measurement.

In general, pressure transmitters are primarily used to measure the pressure and level of the media. In our industrial consumption, applications are very common. Jiangsu Meiant special professional pressure transmitter adopts imported high quality diffused silicon and ceramic pressure sensors as sensitive components, using special integrated modules, precise temperature, zero point, full range and nonlinear compensation to complete liquid, gas, steam, etc. Accurate measurement and transmission of media pressure changes. Complete the ideal monitoring of pressure medium changes in various places in industrial and mining enterprises, research institutes and other departments.

7 points to fully understand the pressure transmitter

VII Selection of pressure transmitter

1. According to the type of pressure to be measured

Pressure types mainly include gauge pressure, absolute pressure, and differential pressure. Gauge pressure refers to the pressure based on the atmosphere, less than or greater than atmospheric pressure; absolute pressure refers to the absolute pressure zero as the benchmark, higher than the absolute pressure; differential pressure refers to the difference between the two pressures.

2. According to the measured pressure and air stroke

Under normal conditions, the actual measurement pressure is selected as 80% of the measurement range.

Think about the maximum pressure of the system. In general, the pressure transmitter pressure range should reach a maximum of 1.5 times the maximum system pressure. Some water pressure and process control, with pressure spikes or continuous pulses. These spikes may reach 5 times or even 10 times the "maximum" pressure, which may cause damage to the transmitter. Continuous high-voltage pulses that approach or exceed the transmitter's maximum rated pressure will shorten the life of the transmitter. However, the rated pressure of the progressive transmitter will sacrifice the resolution of the transmitter. Capacitors can be used in the system to attenuate spikes, which can slow down the sensor's response.

Pressure transmitters are typically designed to accept maximum pressure in 200 million cycles without degrading performance. A compromised process plan can be found between system performance and transmitter life when selecting a transmitter.

3. Depending on the measured medium

According to the different measurement medium, it can be divided into dry gas, gas liquid, strong corrosive liquid, viscous liquid, high temperature gas liquid, etc. According to different media, the correct selection of the medium is beneficial to prolong the service life of the transmitter.

4. Depending on the maximum overload of the system

The maximum overload of the system should be less than the overload maintenance limit of the transmitter, otherwise it will affect the operating life of the transmitter and damage the transmitter. Usually the pressure transmitter's safe overload pressure is twice the full scale.

5. Accuracy level based on demand

The measurement error of the transmitter is stopped according to the accuracy level. Different accuracy corresponds to different fundamental error limits (expressed as the percentage of full-scale output). In practical application, the selection is based on the control request of the measurement error and the use of the economic criteria.

6. According to the system operating temperature range

The measured medium temperature should be within the working temperature range of the transmitter. If it is used for over-temperature, it will produce a large measurement error and affect the service life of the transmitter. During the consumption of the pressure transmitter, the temperature will stop. Measure and compensate to ensure that the measurement error due to temperature is within the accuracy level request. In places with high temperatures, you can think about selecting high-temperature pressure transmitters or taking auxiliary cooling measures such as condensers and radiators.

7 points to fully understand the pressure transmitter

7. According to the measurement medium and contact material compatibility

In some measurement places, the measuring medium is corrosive. In this case, it is necessary to select materials compatible with the measuring medium or stop special process disposal to ensure that the transmitter is not damaged.

8. According to the pressure interface

Usually the thread connection (M20×1.5) is the standard interface.

9. According to the power supply and output signal

Usually the pressure transmitter is powered by a DC power supply and offers a variety of output signal options, including 4 to 20 mA. DC; 0~5V. DC, 1 ~ 5V. DC, 0 ~ 10mA. DC, etc., can have 232 or 485 digital output.

10. According to the working environment of the site and other

Whether vibration, electromagnetic interference, etc. can exist, relevant information should be provided for selection in order to take appropriate measures. In the selection, other electrical connections, etc. can also be considered based on the detailed situation.

Huaheng Instrument, pressure transmitter expert, supply better and more stable pressure transmitter!