Pressure transmitter measurement principle and classification

In modern industrial automation control systems, pressure transmitters are used in the section where pressure monitoring is required to collect signals or serve as feedback signals to monitor and adjust the process, so as to achieve safe and orderly management and control. Because of the different characteristics and application conditions of the process medium, we must combine the measurement principle of the pressure transmitter and the on-site usage to analyze the cause of the pressure transmitter failure, find practical and effective solutions, and achieve from selection to installation, From application to maintenance rationality, efficient problem solving.

Basic introduction of pressure transmitter

Pressure transmitter is the most commonly used sensor in industrial practice. It is widely used in various industrial automation environments, involving water conservancy and hydropower, railway transportation, intelligent buildings, production automation, aerospace, military, petrochemical, oil wells, electric power, ships , machine tools, pipelines and many other industries.

There are two types of pressure transmitters: electric and pneumatic. The electric unified output signal is DC signal such as 0~10mA, 4~20mA or 1~5V. The pneumatic unified output signal is a gas pressure of 20-100Pa.

Pressure transmitters can be divided into force (torque) balance type, capacitive type, inductive type, strain type and frequency type according to different conversion principles. The following briefly introduces the principle, structure, Knowledge of use, maintenance and calibration.

The main function of the pressure transmitter is to transmit the pressure signal to the electronic equipment, and then display the pressure on the computer. The magnitude of the current has a linear relationship, generally a proportional relationship. Therefore, the voltage or current output by the transmitter increases as the pressure increases, thereby obtaining a relationship between pressure and voltage or current. The two pressures of the measured medium of the pressure transmitter are passed into the high and low pressure chambers , The pressure of the low pressure chamber adopts atmospheric pressure or vacuum, which acts on the isolation diaphragm on both sides of the delta element (ie, the sensitive element), and is transmitted to both sides of the measuring diaphragm through the isolation diaphragm and the filling liquid in the element.

The pressure transmitter is composed of a measuring diaphragm and electrodes on both sides of the insulating sheet to form a capacitor. When the pressure on both sides is inconsistent, the measuring diaphragm is displaced, and the displacement is proportional to the pressure difference, so the capacitances on both sides are not equal, through the oscillation and demodulation links.

Classification of pressure transmitters

The pressure transmitter uses sensors, demodulators, current detectors, amplifiers, etc. to convert the pressure change signal into a standardized signal that the controller can read. The current signal is mainly 4~20mA DC, and there are also 1~5 VDC. voltage signal or communication signal, etc. The types of pressure transmitters can be simply divided into:

(1) Piezoresistive transmitters are mainly composed of semiconductor piezoresistive materials under pressure. Due to the piezoresistive effect of the piezoresistor, the resistivity changes, so that the bridge generates a voltage signal proportional to the pressure. This can measure changes in pressure.

(2) Capacitive transmitter, which uses the measuring diaphragm and the electrodes on the insulating sheets on both sides to form a capacitor. The two pressures of the measured medium pass through the high and low voltage chambers and act on the isolation diaphragms on both sides of the sensitive element. The diaphragm will be displaced according to the pressure, and the capacitance on both sides will change. This capacitance change will be converted into a readable signal by the controller through oscillation and demodulation.

(3) Inductive pressure transmitters are generally composed of pointer pressure gauges and electronic remote transmission components. When the pressure causes the spring tube of the pressure gauge to deform and displace, the iron core in the inductance coil will also be displaced, and the inductance will change, and this change value will be converted into a DC current/voltage signal.

(4) The strain-type pressure transmitter is also measured by converting pressure into resistance value. The two strain gauges are glued to the strain tube along the axial and radial directions with special adhesives. When the strain tube is under pressure, the shape of the resistance strain gauge will change, which will affect the resistance value, and the pressure. The difference between the resistive measurement principle is that the strain transmitter will cause the shape of the strained material to change. In fact, in different application conditions, there are other pressure transmitters with various measurement methods, which will not be described here.