Pressure transmitters are essential components of industrial automation & process control because they monitor and regulate pressure levels in pipes, tanks, and other equipment. A 4-20 mA analog signal is a popular and dependable way to transfer pressure values from the field to the control system.
How 4-20 mA Signals Works?
Pressure transmitters tend to be built to measure a specified pressure range & convert it to an electrical signal. This signal, which ranges between 4 and 20 milliamps (mA), is routed to a Programmable Logic Controller (PLC) (or) other control system inputs.
| Pressure (bar) | Output Signal (mA) |
|---|---|
| 0 | 4 mA |
| 5 | 12 mA |
| 10 | 20 mA |
Assume a pressure transmitter calibrated from 0 to 10 bar. It generates this particular analog output:
This analog signal is subsequently read by PLC’s analog input module. The PLC interprets current value, scales it to corresponding pressure utilizing internal logic or programming, and delivers the data to the HMI or SCADA system for display and monitoring.
Why use 4-20 mA signals?
The automation sector has widely adopted the 4-20 mA standard for a variety of reasons:
Noise Immunity
Current signals, unlike voltage signals, are far more resistant to electrical noise and signal degradation over extended distances. This makes 4-20 mA excellent for industrial settings where electromagnetic interference (EMI) is prevalent.
Live Zero (4 mA vs 0 mA)
The usage of 4 mA to denote the zero point (i.e., 0 bar) enables quick discovery of problems such as:
Broken Wires
Sensor Failure
Power Loss
If the signal falls below 4 mA (e.g., 0 mA), the PLC can detect a fault and initiate an alert or system action.
Industry Standard
4-20 mA signaling is a long-standing industrial standard. It is supported by almost every automation equipment manufacturer, making integration & compatibility simple.
Simple & Robust
The 4-20 mA loop is a simple two-wire device that powers the transmitter & transmits the signal, minimizing wiring complexity and increasing reliability. Its durability and dependability have made it a popular choice in process sectors such as oil and gas, chemical, water treatment, & manufacturing.
How PLC interprets Signal?
Inside the PLC, the analog input module accepts the current signal and converts it to a digital value using Analog-to-Digital Conversion (ADC). The scaling mechanism within the PLC converts the 4-20 mA range to the actual process value (0-10 bar). This scaled value is then utilized to regulate, monitor, trend, or trigger alarms.
Ex:
4 mA = 0% = 0 bar
12 mA = 50% = 5 bar
20 mA = 100% = 10 bar
The PLC utilizes this scaled data to sustain process parameters, initiate safety routines, and display real-time pressure data on HMI/SCADA.
The use of 4-20 mA analog signals for pressure transmitters in PLC systems is still one of the most dependable and cost-effective techniques of process measurement. The 4-20 mA current loop is a fundamental component in industrial instrumentation & control because to its outstanding noise immunity, built-in defect detection via the live zero, and widespread acceptance throughout the automation industry.