Analysis and processing of capacitive pressure transmitters

1. Inspection of the measuring part of the transmitter.

Faults in the measurement part of the transmitter will cause the transmitter to have no output or abnormal output, so the transmitter should be checked first. Measurement-sensitive parts of the transmitter.

1.Remove the flange and check the diaphragm of the sensitive parts for deformation, breakage and oil leakage.

2.Remove the compensation plate, do not take out the sensitive parts, check the insulation resistance of the plug-in shell, and the insulation resistance should not be less than 100m9 when the voltage does not exceed 100V.

3.When the pressure signal is the upper limit of the range, the air source should be turned off, and the output voltage and reading value should be stable. If the output voltage drops, the transmitter is leaking and can be checked with soapy water.

Second, the inspection of the circuit part of the transmitter.

1.Turn on the power supply and check the status of the voltage signal at the output of the transmitter after the input pressure signal is given. If there is no output voltage, you should first check whether the power supply voltage is normal; whether it meets the power supply requirements; Whether there is a wiring error between the power supply and the transmitter and load equipment.

If the transmitter terminals. No voltage on the transmitter or reverse polarity can cause the transmitter to output no voltage signal. If the above reasons are ruled out, the components in the amplifier board circuit should be further checked for damage; If there is no defective contact phenomenon of the circuit board connector, the method of comparing the measurement voltage of the normal instrument with the corresponding measurement voltage of the faulty instrument can be adopted to determine the fault point, and the faulty amplifier board can be replaced if necessary.

When inspecting the flow type transmitter, special attention should be paid to the anti-static measures taken for the J-type amplifier board.

2.After the power is turned on, if the output of the transmitter is too high (greater than 10VDC), or the output is too low (less than), and the output is not responsive when changing the input pressure signal and adjusting the zero point and range screws. For this kind of fault, in addition to checking whether there is any abnormality in the sensitive parts of the transmitter measurement part, you should check whether the "oscillation control circuit part" on the transmitter amplifier board is working properly.

The normal peak voltage between high-frequency transformer T1-12 should be 25 35VP-P; The frequency is about 32kHz. Secondly, check the working status of each operational amplifier on the amplifier board; Whether there is any damage to the components of each part, etc. Such failures require the amplifier board to be replaced.

Abstract:Capacitive pressure transmitter is a kind of capacitive sensor, the pressure is converted into a pneumatic signal or electric signal for control and remote transmission, it is composed of capacitive load cell sensor (also known as capacitive pressure sensor), measurement circuit and process connector. When working, the two pressures of the measured medium act on the isolation diaphragm on both sides of the sensitive element, and when the pressure is inconsistent, the measuring diaphragm produces displacement, so the capacitance on both sides is unequal, and it is converted into a signal proportional to the pressure through the oscillation and demodulation link.

Let's take a look at the working principle of capacitive pressure transmitters. 1. What is a capacitive pressure transmitter?

Pressure transmitter is a kind of equipment that converts pressure into pneumatic signals or electric signals for control and remote transmission, it has many types, and capacitive pressure transmitter is one of them.

The so-called capacitive pressure transmitter is a pressure transmitter using a capacitive sensor, which is an intelligent pressure transmitter with very high accuracy, and its measurement accuracy is very high, which can be used in many industries.

2. What is the composition of a capacitive pressure transmitter?

A capacitive pressure transmitter is mainly composed of three parts: a capacitive load cell sensor (also known as a capacitive pressure sensor), a measurement circuit and a process connector. The following is a structural analysis of a capacitive pressure transmitter:

The pressure transmitter microprocessor has 16 bytes of program RAM in it and three 16-bit counters, one of which performs a D conversion. A d converter fine-tunes the data from the microprocessor and corrected digital signal, which can be modified with the transmitter software. The data is stored in EEPROM, and even if the power is off, it is kept intact.

The digital communication line provides a connection interface for the transmitter to external devices such as the Type 205 Smart Communicator or a control system using the HART protocol. This line detects digital signals superimposed on 4-20mA signals and transmits the required information through the loop.

3. The working principle of capacitive pressure transmitter.

The principle of capacitive pressure transmitter is similar to that of ordinary pressure transmitter, and the specific working principle is:

The two pressures of the measured medium are passed into the high and low two pressure chambers, act on the isolation diaphragm on both sides of the δ element (i.e., the sensitive element), and are transmitted to both sides of the measuring diaphragm through the separator and the filling liquid in the element, when the pressure on both sides is inconsistent, the measuring diaphragm produces displacement, and its displacement is proportional to the pressure difference, so the capacitance on both sides is not equal, and it is converted into a signal proportional to the pressure through the oscillation and demodulation link. The AD converter of a capacitive pressure transmitter converts the current from the demodulator into a digital signal, the value of which is used by the microprocessor to determine the input pressure value. The microprocessor controls the operation of the transmitter.

The source answer working principle of the capacitive compression combustion shed force transmitter is ()aParallel plate capacitors.

b.Cylindrical capacitors.

Correct Answer: Pize A

There are many types of sensors used in general differential pressure transmitters, and many kinds of technologies have been updated over the decades; For example, differential pressure transmitters, using diffusion silicon as differential pressure sensors, are already very backward technologies. Capacitor differential pressure box is the most used technology, the typical is Rosemount's differential pressure transmitter, the technology it uses is capacitor technology, this technology is also a relatively old technology, but it can be said that this technology is the most enduring technology at present. Other world-renowned brands of differential pressure sensors have emerged with new technologies such as silicon resonance, such as Japan's EJA.

In terms of performance, both capacitive and others have their own advantages and disadvantages, and in terms of performance, they are comparable! But technology belongs to technology, you also have to take into account the impact of factors such as the manufacturer's production process on product quality, so it is best to choose a regular and powerful manufacturer's differential pressure sensor!

In addition, the sensors used in general low-end pressure transmitters (different from differential pressure transmitters) are commonly used: diffusion silicon sensors, ceramic piezoresistive sensors, ceramic capacitive sensors, monocrystalline silicon sensors, etc. This kind of sensor can only measure gauge pressure or absolute pressure, they also have their own shortcomings, and the sensors used in different occasions are also different, for example, the general small-range pressure transmitter needs to use a ceramic capacitive sensor, and the stability and accuracy will be higher than others; However, due to the limitation of the production process, the general ultra-large range is usually more ceramic. For diffusion silicon sensors, generally oil-filled diffusion silicon sensors are more suitable for temperature compensation and other technical improvements, and are also outstanding in terms of stability and overload capacity.

Filling the differential pressure sensor with silicone oil or inert gas is usually the "patent" of capacitive sensors, of course, the sensors of other technologies are also filled with inert liquid or inert gas, and its function is to act evenly on the pressure sensing diaphragm!

With capacitance as the sensitive element, when the differential voltage acts on the sensitive element, the capacitance changes, and then after the conversion circuit, it becomes a transmitter of electrical signal, the input signal of the capacitive differential pressure transmitter is the differential pressure signal, and the output is a voltage signal of 1 5 volts or a current signal of 4 20 milliamperes.

Analog features.

High precision. The measuring range and zero point are continuously adjustable externally.

Good stability and performance.

Positive migration can reach 500%, negative migration can reach 600% Second-line system.

Adjustable damping and overpressure resistant.

Solids sensor design.

There are no mechanical moving parts and the amount of maintenance is low.

Light weight ( The whole series has a unified structure and strong interchangeability.

Miniaturization (166 mm overall height).

Diaphragm materials for contact media are optional.

Unilateral overpressure resistance.

Low-pressure cast aluminum alloy housing.

Smart Features:

Superior measurement performance for pressure, differential pressure, level, flow measurement Digital Accuracy: +(

Analog accuracy: +(

Full Performance: +(

Stability: 60 months.

Turndown ratio: 100:1

Measurement rate: Miniaturized (all stainless steel flange, easy to install.)

The process connection is compatible with other products for optimal measurements.

The world's only sensor with an H-alloy sheath (patented technology) achieves excellent thermal and cold stability.

Smart transmitter with 16-bit computer.

Standard 4-20mA with HART-based digital signals, remote control Upgrades to fieldbus and field-based technologies are supported.

Analog: 2-wire 4 20 madc digital side burn word: 2-wire 4 20 madc and superimposed with HART protocol-compliant process variable digital signals.

Power supply: Supply voltage Load limit: The maximum load resistance rmax of the circuit depends on the supply voltage.

HART communication requires that the load resistance is not less than 250 and not higher than 600 Communication distance: the maximum communication distance is that multi-station communication can be connected to up to 15 intelligent transmitters. The output displays:

Four-digit digital display, five-digit LCD display, analog type. Explosion-proof form: intrinsically safe type:

exiaii ct5;Flameproof type. Zero migration range: The zero point can be migrated positively or negatively at will, but the upper and lower limits of the measurement range and the measuring range must not exceed the limit value.

Temperature range: Ambient temperature: -40 85 (-20 70 with LCD indicator meter) Measuring medium temperature:

Start within 5 seconds Capacity change: less than Damping adjustment: Time Changshu seconds Fault alarm:

In the event of a sensor or circuit failure, the automatic diagnostic function excitation chamber is automatically output or Configuration Protection: In order to prevent the transmitter's configuration data from being tampered with, the stored data can be locked via software or a protection switch.