Because of the different characteristics of the process media and the different application conditions, we need to combine the measurement principle of the pressure transmitter with the field usage to analyze the causes of pressure transmitter failures, find practical and effective solutions, and ensure the rationality of the process from selection to installation, from application to maintenance, so as to solve problems efficiently.
The measurement principle of pressure transmitters is not the focus of this article; only a brief introduction to their working principle and classification will be provided. Simply put, a pressure transmitter uses sensors, demodulators, current detectors, amplifiers, etc., to convert pressure change signals into standardized signals that a controller can read. These are primarily 4~20mA DC current signals, but also include 1~5VDC voltage signals or communication signals. Pressure transmitters can be broadly categorized as follows:
(1) Piezoresistive transmitters are mainly based on the semiconductor pressure-sensitive material. When pressure is applied, the resistivity of the pressure-sensitive resistor changes due to the piezoresistive effect, which causes the bridge to generate a voltage signal that is proportional to the pressure, thereby allowing the pressure change to be measured.
(2) Capacitive transmitters utilize the electrodes on the measuring diaphragm and the insulating sheets on both sides to form a capacitor. The two pressures of the measured medium are applied to the insulating diaphragms on both sides of the sensitive element through the high and low pressure chambers. The diaphragms will be displaced according to the different pressures, and the capacitance on both sides will change. This capacitance change value will be converted into a signal that can be read by the controller through oscillation and demodulation.
(3) Inductive pressure transmitters are generally composed of a pointer-type pressure gauge and an electronic remote transmission component. When the pressure causes the Bourdon tube of the pressure gauge to deform and displace, the iron core in the inductor coil will also displace, and the inductance will change. This change value is converted into a DC current/voltage signal.
(4) Strain gauge pressure transmitters also measure pressure by converting it into resistance. Two strain gauges are glued to the strain tube along the axial and radial directions respectively using a special adhesive. When the strain tube is under pressure, the shape of the strain gauges changes, thus affecting the resistance value. The difference between strain gauge and piezoresistive pressure transmitters is that strain gauge transmitters cause the shape of the strained material to change. In fact, there are many other pressure transmitters with different measurement methods depending on the application conditions, which will not be elaborated here.
