What is 4-20 mA?
A 4–20 mA signal is an analogue electrical output used by fixed gas detectors to communicate the concentration of gases to controllers, PLCs, or building management systems. It is one of the most widely used methods for transmitting gas measurements in industrial environments because of its reliability and simplicity.
What Does 4–20 mA Mean?
A 4–20 mA signal works by converting the gas concentration detected by a fixed gas sensor into a proportional electrical current. When the sensor measures the gas level, it adjusts the current in the loop accordingly. The current always stays within the range of 4 to 20 milliamps, where 4 mA represents zero gas detected and 20 mA represents the maximum concentration for which the detector is calibrated. Any value between these two points corresponds proportionally to the actual gas level, allowing the controller to interpret the reading accurately in real time.
The signal travels through a two- or three-wire loop from the gas detector to the controller or building management system. Because it is a current-based signal, it is highly resistant to electrical interference and voltage drops, even over long cable runs, ensuring that the readings remain reliable.
Why does it start at 4 and not 0?
In a 4–20 mA current loop, 4 mA represents zero gas detected, while 20 mA represents the maximum calibrated level. The reason the scale starts at 4 mA instead of 0 mA is to allow fault detection.
If the signal were allowed to start at 0 mA, it would be impossible to distinguish between a genuine zero gas reading and a problem in the system, such as:
A wiring break or disconnection
A sensor or detector malfunction
A power failure in the loop
By starting at 4 mA, any current below this level immediately signals a fault. This ensures that operators and control systems can quickly identify issues rather than assuming a zero reading is safe.
This design makes the 4–20 mA loop safer and more reliable, providing continuous monitoring while also giving a simple method to detect errors in the system.
Gas Detection Using 4-20 mA
Crowcon Xgard Bright Fixed Gas Detector
The Crowcon Xgard Bright fixed gas detector uses a 4–20 mA analogue output to transmit real-time gas concentration readings to a controller, PLC, or building management system. As the detector measures flammable, toxic, or oxygen gas levels, it converts the measurement into a proportional current: 4 mA represents zero gas, while 20 mA represents the maximum calibrated level.
For more information on this product, please visit: Crowcon Xgard Bright Fixed Gas Detector | Rockall Safety
Honeywell XCD Fixed Gas Detector
The Sensepoint XCD is a fixed‑point gas detector/transmitter designed for flammable, toxic, and oxygen‑related hazards. It’s built with a 3‑wire current‑loop output and supports a selectable 4–20 mA analogue signal.
For more information on this product, please visit: Honeywell Sensepoint XCD Fixed Gas Detector | Rockall Safety
