What is a 4-20mA signal? Basic knowledge about industrial-grade liquid level transmitters.
n the field of industrial automation, the 4-20mA current loop is one of the most widely used standards for transmitting analog signals. Whether it’s for level, pressure, temperature, or flow transmitters, you’ll often see specifications like “Output signal: 4-20mA.” So, what exactly is a 4-20mA signal, and why is it commonly used in industrial-grade level transmitters? HojellyTek answers these questions for you today.
- What is a 4-20mA signal?
The 4-20mA signal is an analog current signal used to transmit the measured value of a sensor (such as the height of a liquid level) as a current magnitude to a controller, display instrument, or PLC (programmable logic controller).
4mA corresponds to the minimum range of the sensor (e.g., when the liquid level is 0%).
The 20mA corresponds to the maximum range of the sensor (e.g., a liquid level of 100%).
Any intermediate current value corresponds linearly to measurement values ranging from 0% to 100%.
For example, a level transmitter with a range of 0-5 meters that outputs 12 mA indicates that the current level is 2.5 meters (50%).
II. Why 4-20mA Instead of 0-20mA?
Choosing 4mA as the zero point, rather than 0mA, involves important engineering considerations:
| Feature | Explanation |
|---|---|
| Broken Wire Detection | If 0mA is used as the zero point, once the wire is disconnected, the receiving end will also receive 0mA, making it impossible to distinguish between “the actual liquid level being zero” and “a line fault.” Using 4mA, when the receiving end detects an electric current < 4mA (such as 0mA), it can determine that the line has been disconnected and trigger an alarm. |
| Sensor Power Supply | Many two-wire transmitters are powered directly by the signal loop (i.e., the current loop both transmits the signal and provides power). 4mA is sufficient to power the transmitter's internal circuitry, while 0mA would not. |
| Strong Anti-Interference | Current signals are less susceptible to electromagnetic interference than voltage signals, making them suitable for long-distance transmission (up to several kilometers). |
III. Why do industrial-grade level transmitters favor 4-20mA?
The demands placed on sensors in industrial environments are extremely stringent: high temperatures, high humidity, strong interference, long distances, and explosion protection. The 4-20mA signal has become the preferred choice for industrial-grade liquid level transmitters due to the following advantages:
Long transmission range: The current signal experiences minimal attenuation within the loop, allowing it to be transmitted easily over several kilometers.
High resistance to interference: In industrial settings, motors and variable frequency drives generate strong electromagnetic fields. The current loop effectively suppresses interference.
Two-wire system simplifies wiring: the power supply line and the signal line are combined, reducing cable costs and making installation simpler.
Essential Safety and Explosion Protection: When combined with safety barriers, it enables essential safety protection, making it suitable for use in hazardous environments such as oil and chemical industries.
International standard: PLCs, DCSs, and instruments all come standard with 4-20mA input modules, offering excellent compatibility.
IV. Ceshhome Industrial Level Transmitter Series
HojellyTek has been deeply involved in liquid level measurement for many years and offers a variety of industrial-grade liquid level transmitters that provide output signals of 4-20mA.
| Product Series | Measuring Principle | Features | Typical Applications |
|---|---|---|---|
| CSP-300 Submersible Level Transmitter | Piezoresistive | Corrosion-resistant, anti-clogging, with vented cable, range 0-200m optional | Deep wells, reservoirs, sewage tanks |
| CSU-500 Ultrasonic Level Transmitter | Ultrasonic | Non-contact measurement, anti-fouling, with temperature compensation, range 0-15m | Corrosive liquids, slurries, open channels |
| CSR-200 Radar Level Transmitter | FMCW | High accuracy ±1mm, unaffected by steam/foam, suitable for high temp/pressure | Chemical storage tanks, reactors |
V. How to Select and Use 4-20mA Level Transmitters
- Confirm Supply Voltage: Two-wire systems need loop power, typically 24V DC; ensure sufficient power capacity.
- Match Load Resistance: Receiving devices (e.g., PLC analog modules) have input impedance; ensure the transmitter can drive it.
- Take measures to protect against lightning and explosions: outdoor installations should be equipped with lightning protection devices; for areas of risk, use intrinsically safe explosion-proof designs.
- Periodic Calibration: Although industrial products are highly stable, annual calibration is recommended to maintain accuracy.
Conclusion
The 4-20mA signal may seem simple, but it is one of the cornerstones of industrial automation. Ceshhome industrial level transmitters strictly adhere to this standard, providing users with stable, reliable, and compatible measurement solutions. If you're looking for high-quality level transmitters, feel free to contact us!https://www.ceshhome.com/wp-admin/post.php?post=4964&action=edit
