In general, a pressure transmitter is mainly composed of three parts: a pressure measuring element sensor (also known as a pressure sensor), a measurement circuit, and process connectors. It can convert the physical pressure parameters of gases, liquids, etc. sensed by pressure sensors into standard electrical signals (such as 4-20mADC, etc.) to supply secondary instruments such as indicator alarms, recorders, regulators, etc. for measurement, indication, and process adjustment.
working principle:
The two pressures of the measured medium in the capacitive pressure transmitter are introduced into the high and low pressure chambers, acting on the isolation diaphragms on both sides of the δ element (i.e. sensitive element), and transmitted to both sides of the measuring diaphragm through the isolation diaphragms and the filling liquid inside the element. A capacitive pressure transmitter is composed of a measuring diaphragm and electrodes on both sides of the insulating film, each forming a capacitor. When the pressure on both sides is not consistent, it causes displacement of the measuring diaphragm, which is proportional to the pressure difference. Therefore, the capacitance on both sides is unequal. Through oscillation and demodulation, it is converted into a signal proportional to the pressure. The working principle of capacitive pressure transmitters and capacitive absolute pressure transmitters is the same as that of differential pressure transmitters, except that the pressure in the low-pressure chamber is atmospheric pressure or vacuum. The A/D converter of the capacitive pressure transmitter converts the current of the demodulator into a digital signal, which is used by the microprocessor to determine the input pressure value. The microprocessor controls the operation of the transmitter. Additionally, it linearizes the sensor. Reset the measurement range. Engineering unit conversion, damping, square root, sensor fine-tuning and other calculations, as well as diagnosis and digital communication.
Structural analysis:
The pressure transmitter microprocessor has 16 byte program RAM and three 16 bit counters, one of which performs A/D conversion.
The D/A converter fine tunes the digital signal from the microprocessor that has been calibrated, and these data can be modified using transmitter software. The data is stored in EEPROM and remains intact even when powered off.
The digital communication line provides a connection interface for the transmitter to external devices such as the 205 smart communicator or control systems using the HART protocol. This circuit detects the digital signal superimposed on the 4-20mA signal and transmits the required information through the circuit. The type of communication is frequency shift keying (FSK) technology and is based on the BeII202 standard.
Analysis and processing:
The sensitive components of the capacitive pressure transmitter measurement part adopt a fully welded structure, and the electronic circuit part adopts wave soldering and connector installation methods. The overall structure is sturdy, durable, and has few faults. For the vast majority of users, if sensitive components are found to have malfunctioned, they generally cannot be repaired on their own and should contact the manufacturer to replace the entire component.
1、 Inspection of transmitter measurement section
Malfunctions in the measurement part of the transmitter can cause the transmitter to have no output or abnormal output, so the measurement sensitive components of the transmitter should be checked first.
1. Remove the flange and check for deformation, damage, and oil leakage in the isolation membrane of sensitive components.
2. Remove the compensation board without removing sensitive components, check the insulation resistance of the pins to the housing, and ensure that the insulation resistance is not less than 100M Ω when the voltage does not exceed 100V.
3. Connect the circuit and gas circuit. When the pressure signal reaches the upper limit of the range, turn off the gas source, and the output voltage and reading value should remain stable. If the output voltage drops, it indicates that the transmitter has a leak, and the leak location can be checked with soap water.
2、 Inspection of transmitter circuit section
1. Connect the power supply and check the status of the voltage signal at the output terminal of the transmitter. If there is no output voltage, first check if the power supply voltage is normal; Whether it meets the power supply requirements; Is there any wiring error between the power supply, transmitter, and load equipment. If there is no voltage or the polarity is reversed on the wiring terminals of the transmitter, it can cause the transmitter to output no voltage signal. If the above reasons are excluded, further inspection should be carried out to see if there is any damage to the components in the amplifier board circuit; Whether there is poor contact in the circuit board connectors can be determined by comparing the measured voltage of the normal instrument with the corresponding measured voltage of the faulty instrument. If necessary, the faulty amplifier board can be replaced. When inspecting flow type transmitters, special attention should be paid to taking anti-static measures for the J-type amplifier board.
2. Connect the power supply. After receiving the input pressure signal, if the transmitter output is too high (greater than 10VDC) or too low (less than 2.0VDC), and there is no response when changing the input pressure signal or adjusting the zero and range screws. For such faults, in addition to checking for any abnormalities in the sensitive components of the transmitter measurement part, the "oscillation control circuit part" on the transmitter amplifier board should be checked for normal operation. The normal peak voltage between high-frequency transformers T1-12 should be 25~35VP-P; The frequency is approximately 32kHz. Next, check the working condition of each operational amplifier on the amplifier board; Whether the components of each part are damaged or not. This type of malfunction requires replacement of the amplifier board.
3. The transmitter has strict requirements for circuit design and process assembly quality. In actual use, if there is a circuit failure, it is best to contact the manufacturer to replace the faulty circuit board after inspection and confirmation to ensure the stability and reliability of the instrument's long-term operation.
3、 On site fault inspection
The majority of faults that occur on construction sites are caused by improper use and installation methods, which can be summarized into several aspects.
1. One component (orifice plate, remote measurement connector, etc.) is blocked or installed incorrectly, and the pressure point is unreasonable.
2. Leakage or blockage of the pressure pipe, residual gas in the filling pipe or residual liquid in the filling pipe, sediment in the flange of the transmitter process, forming a measurement dead zone.
3. The transmitter wiring is incorrect, the power supply voltage is too high or too low, and there is poor contact between the indicator head and the instrument wiring terminal.
4. The installation did not strictly follow the technical requirements, and the installation method and on-site environment did not meet the technical requirements.
The above-mentioned faults can cause abnormal output or inaccurate measurement of the transmitter, but after careful inspection, strict use and installation according to technical requirements, and timely adoption of effective measures, the problems can be eliminated. For faults that cannot be handled, the transmitter should be sent to the laboratory or manufacturer for further inspection.