Differential pressure transmitter is a crucial instrument in industrial process control, used to measure the pressure difference between two points and convert it into a standard signal output. The core workflow can be divided into three stages: pressure sensing, mechanical conversion, and electronic signal transmission.
1、 Pressure perception and mechanical conversion
The core sensing element of differential pressure transmitters is usually a diaphragm or bellows. It has two pressure chambers inside, high (H) and low (L), which respectively introduce the two pressures P1 and P2 that need to be compared.
When the pressure on both sides is unequal, the pressure difference (Δ P=P1-P2) will act on the sensitive diaphragm, causing it to undergo slight deformation displacement. This deformation is directly proportional to the differential pressure value Δ P, that is, the larger the differential pressure, the greater the deformation displacement. This process completes the conversion of invisible pressure differentials into measurable mechanical displacements.
2、 Signal detection and conversion mechanism
The small displacement of the membrane needs to be accurately detected and converted. Traditional and modern transmitters use different mechanisms:
Capacitive principle (mainstream technology): The membrane itself serves as the active electrode of the capacitor, and its fixed electrodes on both sides form two capacitors with it. When the diaphragm is displaced due to differential pressure, it will change the capacitance values of two capacitors (one increases and the other decreases). The detection circuit accurately measures this capacitance difference, which directly corresponds to the input differential pressure value.
Strain gauge principle: The displacement of the diaphragm will be transmitted to a metal strain gauge through the filling liquid, causing it to deform. The resistance value of the strain gauge pasted on the strain gauge changes accordingly, and then the resistance change is converted into a voltage signal through a Wheatstone bridge circuit.
3、 Electronic signal transmission and output
The electrical signal (voltage or capacitance change) obtained through the detection circuit is very weak and nonlinear. It will be sent to the signal conditioning circuit for amplification, linearization, and temperature compensation to eliminate the influence of environmental interference.
Finally, the processed signal is converted by a microprocessor controlled circuit into an industry standard 4-20mADC analog signal or a digital signal that complies with specific protocols such as HART and Profibus PA. This standard signal has an accurate linear relationship with the measured differential pressure and can be transmitted remotely to control systems, recorders, or display instruments to achieve precise monitoring and control of process pressure.
In summary, differential pressure transmitters achieve precise conversion from physical quantities to standardized information through a chain reaction of "pressure difference → mechanical displacement → electrical signal change → standard signal", becoming the "sensing nerve" in the field of industrial automation.