Transmitters are an important component of automatic control systems. Pressure transmitters are mainly used for the measurement and control of pressure parameters in industrial processes, and their applications are widespread in the petroleum, chemical, and power industries. There are many types of pressure transmitters, classified in different ways according to their working principles and uses. Pressure transmitters are mainly used for remote display and control of pressure. They often operate in environments with high temperature, low pressure, corrosion, and vibration, resulting in a higher probability of failure. Therefore, research on the influencing factors and common faults of pressure transmitters is very meaningful.
1. Common Types of Pressure Transmitters
A pressure transmitter is an instrument that can convert pressure variables into a standard output signal. There is a certain functional relationship between the pressure variable and the output signal. According to different working principles, pressure transmitters can be divided into the following types:
(1) Piezoresistive transmitters. Piezoresistive transmitters apply pressure to the front surface of a diaphragm, causing deformation. A thick-film resistor is printed on the back of the pressure-sensing diaphragm, forming a Wheatstone bridge. Under the piezoresistive effect, the bridge generates a voltage signal that is directly proportional to the excitation voltage.
(2) Piezoelectric transmitters. Piezoelectric transmitters utilize the positive piezoelectric effect. This effect involves applying external force to an electrolyte, causing deformation. Polarization occurs within the electrolyte, resulting in positive and negative charges on its two surfaces. When the external force is removed, the electrolyte returns to its uncharged state. The polarity of the charge changes with the direction of the applied force. Applying an electric field in the direction of the electrolyte's polarity also causes deformation; this deformation disappears upon removal. This is the inverse piezoelectric effect.
(3) Strain gauge transmitters. Special adhesives are used to bond strain gauges together, thereby generating mechanical strain. When the force on the machine changes, the resistance strain gauges will also deform to a certain extent, thus affecting the resistance value and changing the voltage across the resistor. However, the change in resistance value is relatively small in this case. Usually, they form a strain bridge, which is amplified by the instrumentation amplifier and finally transmitted to the processing circuit for display or execution.
(4) Capacitive Transmitter. Capacitive transmitters are divided into electric and pneumatic types. The standardized input signal of the former is a DC signal, and the output signal of the latter is a gas pressure. The two pressures of the measured medium are input into the high and low pressure chambers respectively, acting on the isolation diaphragms on both sides of the sensitive element. The measuring diaphragm and the electrodes on the insulating sheets on both sides form a capacitor. When the pressure on both sides is different, the module will be displaced, and the current on both sides will be different. Under the action of oscillation and adjustment, current, voltage or digital output signals are formed.
2. Working Principle of Pressure Transmitter
The pressure transmitter consists of a module circuit, display head, pressure sensing element sensor, and housing. The pressure difference from the two pressure guide tubes acts on the diaphragm measured by the transmitter sensor. The measuring element converts the received pressure signal into a standard current and voltage signal and sends the signal to the alarm, recorder, and regulator for secondary measurement.
3. Common Faults of Pressure Transmitters
Pressure transmitters inevitably experience various faults during use. Many factors can affect them, such as leakage of filling fluid between the internal partition and the sensing element, zero-point and range deviations, and unstable output, all of which can lead to decreased accuracy or even damage. Weather conditions also affect the transmitter; for example, lightning strikes can damage the diaphragm circuitry, causing communication failure. Humid environments can damage the wiring. Inappropriate range selection can cause irreversible deformation of the sensing element. Several common fault types are described below:
(1) Circuit Faults When a line fault occurs, the computer displays abnormal values. Open the transmitter junction box and check for loose connections, short circuits, or open circuits. Troubleshoot using methods such as measuring power supply, insulation, and resistance.
(2) Frequency Conversion Interference. During wiring, various signal lines interfere with each other, especially when power lines and signal lines are connected in the same conduit. This interference is more severe and can cause the transmitter to fail to communicate or even malfunction. This can be avoided by increasing the distance between the instrument cable and the power cable tray.
(3) Pressure Tap Fault. Pressure tap faults typically include three types: blockage, air leakage, and liquid accumulation. Blockage is usually caused by untimely drainage or dirty/sticky media. Air leakage is caused by numerous transmitter connections, shut-off valves, and other accessories, increasing the number of leakage points. Liquid accumulation is usually caused by improper gas pressure tapping or incorrect installation of the pressure tap, which affects measurement accuracy.
Electrical Signal Transmission Fault. Improper use or maintenance of pressure transmitters can easily lead to electrical signal transmission failures. For example, placing the transmitter near the device being tested to save time can cause the signal transmission distance to be too far, resulting in signal interference or attenuation. In such cases, the cross-sectional area of the cable should be increased as needed.
4. Pressure Transmitter Troubleshooting
4.1 Zero Output Signal
When the pressure transmitter displays zero pressure, the following steps can be taken: First, check if there is pressure in the pipeline and if the instrument is powered normally. Then, check if the power supply polarity is reversed. Finally, check the electronic circuit board, pressure sensor, and transmitter power supply voltage.
4.2 No Response When Pressure is Applied
If there is no response when pressure is applied, check if the valves on the pressure tapping pipe are functioning properly, if the transmitter's protection function jumper switch is working correctly, if the pressure tapping pipe is blocked, check the transmitter's zero point and range, and replace the pressure sensor.
4.3 Pressure Variable Reading Deviation
When the pressure transmitter displays a pressure reading that is significantly higher or lower than normal, first check if there is a leak in the pressure tapping pipe. Then, check the valves on the pressure tapping pipe and fine-tune the sensor. If the problem persists, replace the pressure sensor.
4.4 Unstable Pressure Variable Readings
This problem can be troubleshooted by isolating external interference sources, checking for leaks in the pressure-conducting pipe, checking for debris in the pipeline, inspecting the diaphragm for signs of wear or deformation, and checking the pressure-sensing diaphragm head.
5. Conclusion
Pressure sensors are widely used. Whether domestic or imported, various malfunctions can occur during use due to the working environment, improper operation, or inherent reasons. Therefore, we must not only operate according to the standard specifications in the instruction manual, but also master correct fault diagnosis, maintenance, and repair knowledge to ensure the service life and measurement accuracy of the pressure sensor.
