Advantages and disadvantages of single-crystal silicon and capacitive serta diffused silicon pressure transmitters

The answer is based on Baidu Baike’s article on the differences between capacitive pressure transmitters and diffused silicon pressure transmitters. A capacitive transmitter has a sensing component with a variable capacitance, known as the “δ” chamber. This sensor is a completely sealed component. The process pressure and differential pressure are transmitted to the sensing diaphragm via the isolation diaphragm and the filling silicone oil, causing displacement; the capacitance difference between the sensing diaphragm and the two capacitor plates is converted by electronic components into a two-wire output signal of (4–20) mA. The electronic amplification circuit consists of a demodulator, an oscillator, an oscillation control amplifier, a current detector, a current control amplifier, a current limit controller, a reference voltage, a voltage regulator, and other components. They detect the capacitive signal to control the oscillation frequency, which is then converted into a current output. Performance Advantages: 1. Can achieve extremely low pressures. 2. Strong overload resistance. 3. High precision. Disadvantages: Lacks advantages in high-pressure measurement; requires sophisticated sensor packaging processes. Working Principle: The instrument converts temperature signals into electrical signals using temperature sensors, then the pre-amplifier amplifies and filters these electrical signals before sending them to the CPU’s A/D conversion module for conversion from analog to digital values. Finally, the CPU performs data processing, displays it, and outputs PWM. Medium under test ---〉 Sensor ---〉 Electronic circuitry ---〉 Output signal. The pressure of the medium under test acts directly on the ceramic/diffused silicon diaphragm of the sensor, causing the diaphragm to undergo a slight displacement that is proportional to the pressure of the medium. Under normal operating conditions, the maximum displacement of the diaphragm does not exceed 0.025 millimeters. Once the electronic circuitry detects this displacement, it converts it into a standard industrial measurement signal corresponding to that pressure. Under overpressure, the diaphragm comes into direct contact with the rigid ceramic substrate/diffusion silicon. Since the gap between the diaphragm and the substrate is only 0.1 millimeters, the maximum displacement of the diaphragm under overpressure is also 0.1 millimeters. Thus, the structure ensures that the diaphragm does not undergo excessive deformation, giving this sensor excellent stability and high reliability. Main features of diffused silicon pressure transmitters: 1. High stability – better than 0.1% of full scale per year. 　　2. Small temperature offset – By eliminating the medium fluid in the sensing element, the sensor achieves high measurement accuracy. 3. Good reliability – The use of large-scale ICs ensures good circuit reliability and strong resistance to interference. 　　4. Wide applicability – The product is available in various models and with different process connection types, enabling it to be used with a wide range of media in industrial measurements. 　　5. Easy to install and maintain: It features a rational product design, is small in size and light in weight, allowing it to be installed in any location. The disadvantages are: the temperature stability drops significantly below 0°C; it cannot withstand dynamic pressures; the diaphragm is prone to damage. Differences between diffused silicon pressure transmitters and capacitive pressure transmitters: In the mid-1990s, the American companies Icscnsor and Nova utilized two cutting-edge technologies – silicon crystals and silicon wafers – to develop new types of diffused silicon pressure sensors, as well as diffused silicon pressure transmitters that feature high accuracy, low repeatability, and corrosion resistance. Process pressure is transmitted to the diffused silicon diaphragm through an isolation diaphragm and sealed silicone oil, while the pressure at the reference point (atmospheric pressure) acts on the other side of the diaphragm. In this way, the pressure difference on both sides of the diaphragm creates a pressure field that compresses a part of the diaphragm. In the other stretching section, there are two strain gauge elements in the compression zone and the stretching zone respectively, which detect changes in resistance caused by pressure, thereby converting pressure signals into electrical signals. Diffused silicon cannot function properly below zero degrees Celsius or above 85°C, nor is it suitable for high pressures of over 20 MPa. Capacitive diaphragm pressure transmitters, as well as capacitive diaphragm absolute pressure transmitters, have been around for over 20 years since their invention in the 1980s. Thanks to their high precision, corrosion resistance, resistance to contamination, and good stability, they are widely recognized both domestically and internationally as ideal instruments for measuring low-vacuum pressures. The U.S.-based company M.K.S is one of the leading manufacturers of capacitive pressure transmitters worldwide; its products are used in various fields of the civil industry, and they play a vital role in industries such as aerospace and nuclear energy. This type of pressure transmitter is designed based on the principle that an elastic diaphragm undergoes deformation under a pressure difference, which in turn causes a change in capacitance. It consists of a sensing section and a conversion circuit, as shown in Figure 7-34. The sensing section includes two chambers: a vacuum chamber and a sensing chamber. The vacuum chamber is of a fully sealed design; after passing leak detection using a mass spectrometer, it is formed by prolonged exhaust processes followed by sealing of the exhaust pipes. An odor eliminator is also provided to remove residual gases and maintain a high vacuum level over time. The fixed electrode plate is located within the vacuum chamber, with wires leading from this plate to outside the chamber. The detection diaphragm is placed between the high-vacuum vacuum chamber and the detection chamber connected to the low-vacuum system to be tested. This detection diaphragm acts as a movable electrode plate, and together with the fixed electrode plate it forms a parallel plate capacitor with a certain capacitance value. The low vacuum pressure under test enters the detection chamber through the detection hole, causing the detection diaphragm to bend and altering its distance from the fixed plate; as a result, the capacitance value changes as well. Different low vacuum pressure values result in different capacitance values. Finally, the capacitance signal is sent to the circuit conversion section, which processes this signal through steps such as transformation, conditioning, and amplification to produce a standard voltage or current signal that is proportional to the vacuum pressure. This type of pressure transmitter provides high-sensitivity differential pressure measurement with stable zero point. It has good dynamic response characteristics and strong adaptability, and is generally suitable for measuring static pressure.