The transmitter is an important part of the automatic control system. The pressure transmitter is mainly used for the measurement and control of pressure parameters in industrial processes. It is widely used in petroleum, chemical, electric power and other fields. There are many types of pressure transmitters, and there are different classification methods depending on the working principle and use. The 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 the probability of failure is large. Therefore, the factors affecting the pressure transmitter and common faults are studied. It is very meaningful.
1, the common type of pressure transmitter
A pressure transmitter is a meter that converts a pressure variable into a standard output signal. There is a certain functional relationship between the pressure variable and the output signal. Pressure transmitters can be divided into the following types according to different working principles:
(1) Piezoresistive transmitter. The piezoresistive transmitter applies pressure to the front surface of the diaphragm. Under pressure, the diaphragm will undergo a certain deformation. The back of the pressure-sensitive diaphragm is printed with a thick film resistor to form a Wheatstone bridge. Under the piezoresistive effect, the bridge generates a corresponding voltage signal that is proportional to the excitation voltage.
(2) Piezoelectric transmitter. Piezoelectric transmitters are developed using the positive piezoelectric effect. The positive high-voltage effect is to apply a certain external force to the electrolyte to deform it. Polarization occurs inside the electrolyte and it will produce positive on both surfaces. Negatively different charges, when the external force stops, the electrolyte returns to the uncharged state. The polarity of the charge changes as the direction of the force changes. When an electric field is applied in the polarity direction of the electrolyte, the electrolyte is also deformed, and the deformation of the electrolyte disappears after the withdrawal, which is the inverse piezoelectric effect.
(3) Strain gauge transmitter. Special strains are used to bond the strain gauges together to produce mechanical strain. When the force is changed, the strain gauges will also deform, which will affect the resistance value and the voltage on the resistor. changes happened. However, the change in resistance is small in this case. Usually, the strain bridge is formed, which becomes larger under the action of the instrumentation amplifier, and finally transmitted to the processing line display or actuator.
(4) Capacitive transmitter. Capacitive transmitters are divided into electric and pneumatic types. The former’s standardized input signal is DC signal, and the latter output signal is gas pressure. The two pressures of the medium to be tested are input into the high and low pressure chambers respectively, and act on the isolating diaphragms on both sides of the sensitive component. The measuring diaphragm and the electrodes on the insulating sheets on both sides form a capacitor. When the pressures on both sides are different, the module will generate displacement, and the currents on both sides are different. Under the action of oscillation and regulation, a current, voltage or digital output signal is formed.
2, the working principle of the pressure transmitter
The module circuit, the display head, the load cell sensor, the case, and the like together constitute a pressure transmitter. The pressure difference from the pressure guiding tubes on both sides acts on the diaphragm measured by the transmitter sensor. The measuring component can convert the received pressure signal into a standard current and voltage signal and send the signal to the alarm, recorder and The regulator is then subjected to a second measurement.
3. Common faults of pressure transmitters
Pressure transmitters inevitably have various faults during use, and there are many influencing factors, such as filling liquid leakage between the diaphragm and the sensing element in the pressure transmitter, zero point and span shift, and unstable output. Will cause the pressure transmitter to drop in accuracy or even damage; weather factors will also affect the transmitter, such as lightning damage will damage the circuit of the transmitter bellows, resulting in communication failure; wet environment will damage the line; transmitter Unreasonable range selection can cause irreversible deformation of the pressure sensitive component. Here are a few common types of failures:
(1) Line failure. When there is a line fault, the computer displays an abnormal value, and the transmitter junction box is opened to check whether the line has a virtual connection, short circuit or disconnection. The fault is detected by measuring the power supply, shaking the insulation and measuring the resistance.
(2) Frequency conversion interference. When wiring is performed, various signal lines interfere with each other, especially when the power line and the signal line are in the same pipeline, and the interference is more serious. In this case, the transmitter does not communicate or even causes an error. This type of erroneous operation can be avoided by increasing the distance between the instrument cable and the power cable tray.
(3) The impulse tube is faulty. There are three types of failures of the impulse piping, such as clogging of the pressure piping, air leakage of the pressure piping, and effusion of the pressure piping. The pressure clogging is generally caused by untimely discharge or dirty or sticky medium; The transmitter is connected to the power, the shut-off valve and other accessories are more, and the leakage point is increased; the effluent of the impulse tube is usually caused by the unreasonable pressure of the gas or the installation of the pressure tube, and the pressure of the pressure tube will affect measurement accuracy.
(4) Electrical signal transmission failure. If the pressure transmitter is improperly used or maintained, it may easily cause the transmission of electrical signals. For example, in order to save time and place the transmitter near the device under test, the signal transmission distance is too far, and the signal may be interfered or attenuated. Whenever necessary, increase the cross-sectional area of the cable.
4, pressure transmitter troubleshooting
4.1 Output signal is zero
When the pressure transmitter has zero pressure, it can be processed from the following aspects: first check whether there is pressure in the pipeline, whether the instrument is normally powered, and then check whether there is a reverse polarity of the power supply, and finally check the electrons. Circuit board, sensible film head, transmitter power supply voltage, etc.
4.2 Adding pressure without reaction
If the pressure is not reactive, check whether the valve of the pressure tube is normal, whether the transmitter protection function jumper switch is normal, whether the pressure tube is blocked, check the white point and range of the white feeder, and replace the sensor head.
4.3 Pressure variable reading deviation
When the pressure transmitter appears to have a high or low pressure reading, first check the pressure piping for leaks. Check the valve on the pressure tube to fine tune the sensor. If there is still a problem, replace it with a new one. Sensor head.
4.4 Pressure variable reading is unstable
This problem can be checked by isolating external sources of interference, checking whether the pressure guiding tube is leaking, whether there is any debris in the pipeline, clothing or deformation when inspecting the diaphragm, and checking the pressure sensitive membrane head.
At present, the pressure sensor is widely used. Whether it is a domestic transmitter or an imported transmitter, various faults may occur due to the working environment, improper operation of the human body or its own reasons during use. Therefore, we must not only follow the standard specifications of the manual. Operation, but also to master the correct fault diagnosis, maintenance and maintenance knowledge, in order to ensure the life of the pressure sensor and measurement accuracy.