Devices that can detect pressure values and provide remote transmission signals are collectively referred to as pressure transmitters. They are commonly used sensors in industrial automation process control, used to measure the pressure of gas or liquid, and convert this pressure value into a measurable and transmittable electrical signal. These electrical signals are usually standard current or voltage signals, such as 4-20mA current signals or 0-10V/0-5V voltage signals, to facilitate remote transmission, display, recording or control in industrial automation, process control, monitoring and measurement systems. It can meet the requirements of centralized detection and control of automation systems and is widely used in industrial production.
Composition of pressure transmitters
Pressure transmitters usually consist of a pressure sensitive element (such as strain gauges, capacitor diaphragms, piezoresistive chips, etc.) and a signal processing circuit. When the measured medium acts on the pressure sensitive element, it will cause some physical changes in the element (such as resistance changes, capacitance changes, resonant frequency changes, etc.), which are then converted into electrical signals. The signal processing circuit is responsible for amplifying, linearizing, temperature compensating, and other processing of this electrical signal to output a stable and accurate pressure value.
Types of pressure transmitters
The pressure transmitters have various structural forms, and are classified according to their working principles into strain gauge, piezoresistive, capacitive, piezoelectric, vibration frequency, etc. The following briefly introduces several commonly used pressure transmitters.
The strain gauge pressure transmitter has a large measurement range, the measured pressure can reach several hundred MPa, and has good dynamic performance, which is suitable for fast-changing pressure measurement. However, there are relatively obvious temperature drift and time drift. This instrument is mostly used for dynamic pressure detection required by general requirements; the piezoresistive pressure transmitter is manufactured according to the principle of piezoresistive effect. Its pressure sensitive element is a diffuse resistor made by integrated circuit technology on a semiconductor material substrate. When it is subjected to external force, the resistance value of the diffuse resistor changes due to the change of resistivity.
The main features of the piezoresistive pressure transmitter are small size and simple structure. The sensitivity coefficient of the diffuse resistor is dozens of times that of the metal strain gauge, and it can directly measure tiny pressure changes. In addition, the piezoresistive pressure transmitter also has good dynamic response and small hysteresis, and can be used to measure pulsating pressures of several kilohertz or even higher. This is a pressure transmitter that has developed rapidly and is widely used.
The capacitive pressure transmitter uses differential capacitance as the detection element, which mainly includes two parts: measurement and conversion amplification. The differential signal is mainly converted into a differential capacitance signal by using the central pressure-sensitive diaphragm and the two arc-shaped capacitor plates on the left and right. The central pressure-sensitive diaphragm forms capacitance with the two arc-shaped capacitor plates on the left and right. The differential pressure causes the central pressure-sensitive diaphragm to displace, so that the spacing between the movable electrode and the two fixed electrodes on the left and right is no longer equal, forming a differential capacitance. The relative change value of the differential capacitance is linearly related to the differential pressure. Since there is no mechanical transmission mechanism of a lever inside the entire capacitive differential pressure transmitter, it has the characteristics of high precision, high stability and high reliability. Its accuracy level can reach 0.2, and it is a type of transmitter commonly used in industry.
Piezoelectric pressure transmitter uses the piezoelectric effect of piezoelectric materials to convert the measured pressure into an electrical signal. It is a commonly used sensor in dynamic pressure detection and is not suitable for measuring slowly changing pressure and static pressure.
The vibration frequency pressure transmitter uses the relationship between the resonant frequency of the pressure-sensitive element itself and the pressure to detect pressure by measuring the change of the frequency signal. This type of sensor has various forms such as vibrating tube, vibrating string, vibrating membrane, quartz resonance, etc. This instrument is small in size, outputs frequency signal, has good repeatability, is vibration-resistant, has high accuracy, and is suitable for gas measurement.