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Common faults and solutions of pressure transmitter are discussed

Post: 2020-02-29

As an important part of automatic control system, pressure transmitter is mainly used for measuring and controlling pressure parameters in industrial process.Pressure transmitter of various types, according to the working principle, the use of different classification.Pressure transmitter is mainly used for remote display and control of pressure. It often works in high temperature, low pressure, corrosion, vibration and other environments and has a high probability of failure. Therefore, it is of great significance to study the influencing factors and common failures of pressure transmitters.

1. Common types of pressure transmitters

Pressure transmitter is an instrument that can convert pressure variable into standard output signal. There is a certain functional relationship between pressure variable and output signal.Pressure transmitters can be divided into the following types according to different working principles:

(1) piezoresistive transmitter.Piezoresistive transmitter applies pressure to the front surface of the diaphragm, under which certain deformation of the diaphragm will occur. The back of the pressure-sensitive diaphragm is printed with thick film resistance, thus forming a wheatstone bridge. Under the piezoresistive effect, the bridge will generate the corresponding voltage signal, which is in direct proportion to the excitation voltage.

(2) piezoelectric transmitter.Piezoelectric transducer is developed using the direct piezoelectric effect, is high voltage effect is to a certain external force ACTS on the deformation of electrolyte to make it happen, electrolyte polarization phenomenon appear internally, and in its two different surface will produce positive and negative charge, when the stop forces of electrolyte back to a state of no charge.The polarity of the charge changes with the direction of the action.When an electric field is applied in the polar direction of the electrolyte, the electrolyte will also be deformed, and the deformation of the electrolyte will disappear after being removed, which is the inverse piezoelectric effect.

(3) strain transmitter.Special adhesive is used to bond the strain gauge together to generate mechanical strain. When the force on the body changes, the resistance strain gauge will also be deformed to some extent, which will affect the resistance value and the voltage on the resistance.However, in this case, the change of resistance value is small. Normally, the strain bridge is formed, which becomes larger under the action of instrument amplifier and is finally transferred to the processing line display or actuator.

(4) capacitive transmitter.Capacitive transmitters are divided into electric and pneumatic, the former standardized input signal for dc signal, the latter output signal for gas pressure.The two kinds of pressure of the medium to be tested are input into the high and low pressure Chambers respectively and act on the isolation diaphragm on both sides of the sensitive element.The electrodes on the insulation plates on both sides of the diaphragm are used to measure the capacitor. When the pressure on both sides is different, the module will produce displacement and the current on both sides is different. Under the action of oscillation and regulation, the current, voltage or digital output signal will be formed.

Ii. Working principle of pressure transmitter

The module circuit, the display head, the load cell sensor, the case and so on together constitute the pressure transmitter.The pressure difference between the two pressure tubes ACTS on the diaphragm measured by the transducer sensor. The measuring element can convert the received pressure signal into a standard current and voltage signal, and send the signal to the alarm device, logger and regulator for secondary measurement.

Common faults of pressure transmitter

Various faults are inevitable in the use of pressure transmitters, and there are many influencing factors, such as the leakage of filling liquid between the internal spacer of pressure transmitters and the sensor element, zero point and range deviation, unstable output, etc., which will lead to the decrease or even damage of the accuracy of pressure transmitters.Weather factors can also affect the transmitter, such as lightning strike will damage the circuit of the transmitter membrane box, resulting in no communication;Damp conditions can damage wiring;The improper selection of the transducer range results in the irreversible deformation of the sensor.The following are some common fault types:

(1) line fault.When there is a line fault, the computer shows the value is abnormal, open the transmitter junction box, check whether there is any virtual connection, short connection or broken connection of the line, in the use of measuring power supply, shaking insulation and resistance and other methods to troubleshoot the fault.

(2) frequency conversion interference.When wiring, various signal lines interfere with each other, especially in the power line and signal line string in the same pipe produced more serious interference, in this case will lead to transmitter communication or even error.This error can be avoided by increasing the distance between the meter cable and the power cable tray.

(3) failure of pressure lead pipe.The failure of the lead pipe is usually divided into three fault types: the plug of the lead pipe, the leakage of the lead pipe and the liquid accumulation of the lead pipe.Pressure pipe leakage is connected by the transmitter, cut-off valve and other accessories, increasing the leakage point;Liquid accumulation in pressure pipette is usually caused by unreasonable gas extraction or wrong installation of pressure pipette, which will affect the measurement accuracy.

(4) transmission failure of electrical signals.If the pressure transmitter is used or maintained improperly, it can easily lead to the transmission failure of electrical signals. For example, in order to save time, the transmitter is placed near the equipment under test, resulting in too long signal transmission distance, and the signal will be interfered or attenuated. At this time, the cross-sectional area of the cable should be increased according to the need.

Four, pressure transmitter fault treatment

(1) the output signal is zero

When the pressure transmitter appears the phenomenon of zero pressure, can be processed from the following aspects: first check whether there is pressure in the pipeline, whether the instrument is normal power supply, then check whether there is the phenomenon of reverse polarity of the power supply, and finally check the electronic circuit board, sensing film head, transmitter power voltage.

(2) no response to pressure

If there is no response to the added pressure, check whether the valve of the pressure tube is normal, whether the jumper switch of the transmitter protection function is normal, check whether the pressure tube is blocked, check the zero point and range of the white feeder, and replace the sensor film head.

(3) deviation of pressure variable reading

When the pressure reading of the pressure transmitter is obviously too high or too low, first check whether there is leakage in the pressure collection pipeline, then check the valve on the pressure collection pipeline, fine tune the sensor, if there is still a problem, replace the sensor film head.

(5) unstable reading of pressure variable

This problem can be checked by isolating external sources of interference, checking for leakage of pressure guide tubes, whether there is any debris in the pipeline, checking for clothing or deformation of the isolation diaphragm, and checking for pressure sensitive membrane and other methods.