Transmitter is an important part of the automatic control system. Pressure transmitters are mainly used for the measurement and control of pressure parameters in industrial processes. They are widely used in petroleum, chemical, electric power and other fields. There are many types of pressure transmitters, and there are different classification methods according to different working principles and uses. Pressure transmitters are mainly used for remote display and control of pressure. They often work under high temperature, low pressure, corrosion, vibration and other environments, and the probability of failure is greater. Therefore, the influence factors and common failures of pressure transmitters are studied. It is very meaningful.
1. Common types of pressure transmitters
A pressure transmitter is an instrument that can convert a pressure variable 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 transmitter. The piezoresistive transmitter applies pressure to the front surface of the diaphragm. Under pressure, the diaphragm will deform to a certain extent. 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 will generate a corresponding voltage signal, which is proportional to the excitation voltage.
(2) Piezoelectric transmitter. The piezoelectric transmitter is developed by using the positive piezoelectric effect. The positive high voltage electric effect is to apply a certain external force to the electrolyte to deform it. With different negative charges, when the external force stops acting, the electrolyte returns to an uncharged state. The polarity of the charge will change as the direction of the applied force changes. When an electric field is applied in the polarity direction of the electrolyte, the electrolyte will also deform, and the deformation of the electrolyte will disappear after withdrawal, which is the reverse piezoelectric effect.
(3) Strain type transmitter. Special adhesives are used to bond the strain gauges together to generate mechanical strain. When the force of the body changes, the resistance strain gauge will also undergo a certain deformation, which will affect the resistance value and the voltage on the resistor. changes happened. However, in this case, the resistance change is small, usually a strain bridge is formed, which becomes larger under the action of the instrument amplifier, and finally transmitted to the processing line display or actuator.
(4) Capacitive transmitter. Capacitive transmitters are divided into two types: electric and pneumatic. The standardized input signal of the former is a direct current signal, and the output signal of the latter is gas pressure. The two pressures of the medium to be measured are input into the high and low pressure chambers respectively, and act 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 produce displacement, and the current on both sides is different. Under the action of oscillation and adjustment, a current, voltage or digital output signal is formed.
2. Working principle of pressure transmitter
The module circuit, display meter head, load cell sensor, watch case, etc. together constitute the pressure transmitter. The pressure difference from the pressure pipes on both sides will act on the diaphragm measured by the transmitter sensor. The measuring element can convert the received pressure signal into a standard current and voltage signal, and send the signal to the alarm, recorder and Adjuster, and then make a second measurement on it.
3. Common faults of pressure transmitters
It is inevitable that pressure transmitters will have various faults during use. There are many influencing factors, such as filling liquid leakage between the inner diaphragm of the pressure transmitter and the sensing element, zero point and span shift, and unstable output This will cause the pressure transmitter to decrease in accuracy or even be damaged; weather factors will also affect the transmitter. For example, lightning strikes will damage the circuit of the transmitter’s capsule and cause communication failure; humid environments will damage the transmitter; Unreasonable range selection will cause irreversible deformation of pressure-sensitive components. The following introduces several common fault types:
(1) Line failure. When there is a line failure, the computer displays an abnormal value. Open the transmitter junction box to check whether there is a virtual connection, short circuit or disconnection in the line. Use methods such as power supply measurement, insulation and resistance measurement to troubleshoot.
(2) Frequency conversion interference. When wiring, various signal lines interfere with each other, especially when the power line and the signal line are in the same pipeline, the interference effect is more serious. In this case, the transmitter will not communicate or even cause errors. This kind of wrong operation can be avoided by increasing the distance between the instrument cable and the power cable tray.
(3) Failure of the impulse tube. Pressure pipe failures usually include three types of faults: pressure pipe blockage, pressure pipe leakage, and pressure pipe effusion. Pressure pipe blockage is generally caused by delayed discharge or dirty or sticky media; pressure pipe leaks There are many accessories such as the transmitter and the shut-off valve, which increase the leakage point; the pressure pipe effusion is usually caused by the unreasonable gas pressure method or the wrong pressure pipe installation. The pressure pipe effusion will affect measurement accuracy.
(4) Electrical signal transmission failure. If the pressure transmitter is used or maintained improperly, it is easy to cause electrical signal transmission failure. For example, to save time, place the transmitter near the device under test, which causes the signal transmission distance to be too far, and the signal will interfere or attenuate. When necessary, increase the cross-sectional area of the cable.
4. Troubleshooting of pressure transmitter
4.1 The output signal is zero
When the pressure transmitter has zero pressure, it can be dealt with from the following aspects: first check whether there is pressure in the pipeline, whether the instrument is normally powered, then check whether there is a reverse polarity of the power supply, and finally check the electronics Circuit board, pressure sensitive film head, transmitter power supply voltage, etc.
4.2 No response when pressure is applied
If there is no response when pressure is applied, check whether the valve of the pressure pipe is normal, whether the jumper switch of the transmitter protection function is normal, whether the pressure pipe is blocked, check the zero point and range of the white feeder, and replace the sensor film head.
4.3 Deviation of pressure variable reading
When the pressure reading of the pressure transmitter is obviously high or low, first check whether there is leakage in the pressure pipe, then check the valve on the pressure pipe, and fine-tune the sensor. If there is still a problem, replace it with a new one. Sensing film head.
4.4 Unstable pressure variable reading
This problem can be checked by isolating external sources of interference, checking whether the pressure guiding tube is leaking, whether there is debris in the pipeline, clothing or deformation when checking the isolation diaphragm, and checking the pressure-sensitive membrane head.
At present, pressure sensors are widely used. Whether it is a domestic transmitter or an imported transmitter, various failures will occur due to the working environment, improper operation or our own reasons during use. Therefore, we must not only follow the standard specifications in the manual. Operation, but also to master the correct fault judgment, maintenance and repair knowledge, so as to ensure the service life and measurement accuracy of the pressure sensor.