Common faults and handling methods of capacitive pressure transmitters

When it comes to capacitive pressure transmitters, many may feel a bit unfamiliar. However, if we mention Rosemount transmitters, many people are likely to be more familiar. In fact, Rosemount transmitters are capacitive pressure transmitters. The sensitive components of the measurement part of the capacitive pressure transmitter use a fully welded structure, while the electronic circuit part uses wave soldering and connector installation methods, making the overall structure robust, durable, and with few faults. However, even the best pressure transmitters may inevitably experience various faults over time. Today, the technical editor from Shaanxi Huibo Electromechanical will introduceCommon faults and handling methods of capacitive pressure transmitters.

　　1. Inspection of the measurement part of the capacitive pressure transmitter
Analysis: Faults occurring in the measurement part of the transmitter will cause no output or abnormal output from the transmitter, so the sensitive measurement components of the transmitter should be checked first.
Handling method:
1. Remove the flange and check if the sensitive component isolation diaphragm is deformed, damaged, or leaking oil.
2. Remove the compensation plate without taking out the sensitive component, and check the insulation resistance of the pins against the housing. The insulation resistance should not be less than 100MΩ when the voltage does not exceed 100V.
3. Connect the circuit and gas path. When the pressure signal is at the upper limit of the range, close the gas source. The output voltage and reading should remain stable. If the output voltage drops, it indicates a leak in the transmitter, which can be checked with soapy water to locate the leak.
　　2. Inspection of the circuit part of the capacitive pressure transmitter
1. Turn on the power, and after providing the input pressure signal, check the status of the voltage signal at the output terminal of the transmitter.
If there is no output voltage, first check whether the power supply voltage is normal; whether it meets the power supply requirements; and whether there are any wiring errors between the power supply, transmitter, and load equipment.
If there is no voltage at the transmitter terminal or if the polarity is reversed, it can also cause the transmitter to have no voltage signal output.
If the above reasons are excluded, further check whether there are any damaged components in the amplifier board circuit; check for poor contact in the circuit board connectors. A method of comparing the measured voltage of a normal instrument with that of the faulty instrument can be used to determine the fault point. If necessary, the faulty amplifier board can be replaced. When checking flow-type transmitters, special attention should be paid to taking anti-static measures for the J-type amplifier board.
2. Turn on the power, and after providing 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 and adjusting the zero point and range screws.
For this type of fault, in addition to checking for abnormalities in the sensitive components of the transmitter's measurement part, the operation of the 'oscillation control circuit' on the transmitter's amplifier board should also be checked.
The normal peak voltage between high-frequency transformer T1-12 should be 25 to 35VP-P; the frequency is about 32kHz. Next, check the operating status of each operational amplifier on the amplifier board; check for any damaged components, etc. This type of fault requires replacing the amplifier board.
3. The transmitter has very strict requirements on circuit design and assembly quality. In actual use, for any circuit faults that occur, it is best to contact the manufacturer to replace the faulty circuit board after inspection and confirmation, to ensure the long-term stability and reliability of the instrument.
　　3. On-site fault inspection of capacitive pressure transmitters
Most faults that occur at the construction site are caused by improper use and installation methods, which can be summarized into the following aspects.
1. Primary components (orifice plates, remote measurement joints, etc.) are blocked or improperly installed, and the pressure tapping points are unreasonable.
2. The pressure guiding tube is leaking or blocked, there is residual gas in the filling tube or residual liquid in the gas filling tube, and there are deposits in the transmitter process flange, forming a measurement dead zone.
3. Incorrect wiring of the transmitter, power supply voltage being too high or too low, and poor contact at the connection between the indicator head and the instrument terminal.
4. Installation was not carried out strictly according to technical requirements, and the installation method and site environment do not meet technical requirements.
The faults mentioned above can cause abnormal output or inaccurate measurement from the transmitter. However, with careful inspection, strict adherence to technical requirements for use and installation, and timely effective measures, the issues can be resolved. For faults that cannot be handled, the transmitter should be sent to a laboratory or manufacturer for further inspection.