How to Choose Between 4-20mA and RS485 Signal Transmission Sensors

 Complete Guide to Temperature & Humidity Transmitters and Signal Transmission Technologies 

As industrial automation and intelligence advance, temperature and humidity transmitters play a pivotal role in industrial production, agricultural greenhouses, smart buildings, and scientific research. They convert environmental temperature and humidity data into collectible electrical signals for processing by control systems, remote monitoring platforms, or IoT ecosystems.

 The most common industrial signal transmission methods are 4-20mA analog current output and RS485 digital communication. Each has unique strengths in anti-interference capability, transmission distance, accuracy, installation complexity, and cost. This article comprehensively analyzes both technologies—from principles and applications to selection guidelines and FAQs—to provide a clear decision-making framework. 

 4-20mA Signal and Its Advantages 

 1. Why Use 4-20mA Current Signal?

The 4-20mA current loop is one of the most classic industrial sensor outputs, offering: 

- Strong Anti-Interference: Current signals are unaffected by wire length or resistance changes, ensuring stability even in EMI-heavy environments.  

- Two-Wire Power Supply: 4mA serves as the sensor’s standby current; signal wires double as power lines, simplifying wiring.  

- Open-Circuit Detection: Non-zero lower limit (4mA) enables instant fault detection if the loop breaks.  

- Intrinsically Safe: Low power in the loop prevents sparks in flammable environments. 

> Tip: 4-20mA is ideal for industrial environment monitoring, greenhouse control, and explosion-proof zones. 

 2. 4-20mA Signal Transmission Distance 

Distance depends on: 

- Supply Voltage (Ue): Higher voltage = longer range  

- Sensor Minimum Operating Voltage (Umin): Must support full load (20mA)  

- Load Resistance (RL): Current-to-voltage conversion resistor  

- Cable Resistance (Rline): Thicker, shorter cables = better performance  

Calculation Example: 

Given:  

- Ue = 24V  

- I = 20mA  

- RL = 250Ω  

- Umin = 12V  

In theory, over 10km is possible. In practice, limit to a few hundred meters to 1km for accuracy and reliability in industrial settings. 

 3. Limitations of Voltage Signal Transmission 

DC voltage signals decay with cable length and are highly susceptible to interference, making them unsuitable for long-distance industrial use. 

> GB/T 3369.2-2008 Analog Signal Specification explicitly states: Voltage signals are not recommended for long-distance industrial transmission.

 RS485 Communication and Its Features 

 1. What is RS485?

RS485 is an industrial-standard differential serial communication protocol supporting half-duplex or full-duplex transmission. It enables multi-drop bus topology, connecting temperature/humidity data to PLC, DCS, or IoT platforms. 

 2. Advantages of RS485 Sensors

- Long-Distance Communication: Reliable over 1,000+ meters  

- Multi-Node Support: Up to 32 devices per bus (expandable with repeaters)  

- High Digital Accuracy: No degradation from resistance or voltage drop  

- Easy Integration: Seamless with industrial IoT, smart buildings, and PLC systems 

 3. Disadvantages of RS485

- Complex configuration (requires Modbus RTU/ASCII or similar protocols)  

- Needs gateway or data parser, increasing cost  

- Usually requires separate power supply (not two-wire) 

> Tip: RS485 is best for remote multi-point monitoring, industrial IoT, and high-precision applications. 

 4-20mA vs RS485 — Technical Comparison 

Selection Recommendations 

Pro Tips:

- 4-20mA: Minimize connectors to prevent signal loss  

- RS485: Use shielded twisted pair + termination resistors for stability 

 NiuBoL Temperature & Humidity Transmitter Application Cases 

 1. Industrial Environment Monitoring

- Scenario: Workshop temperature/humidity control for process stability  

- Signal Choice: 4-20mA – simple wiring, sufficient anti-interference   

 2. Precision Agriculture Greenhouse

- Scenario: Real-time monitoring for irrigation & ventilation  

- Signal Choice: RS485 – multi-sensor bus, cloud data upload   

 3. Smart Building & BMS

- Scenario: HVAC system integration & remote monitoring  

- Signal Choice: RS485 – easy integration with building automation   

 4. Scientific Research Lab

- Scenario: Long-term high-precision data logging  

- Signal Choice: RS485 or Dual Output (4-20mA + RS485)   

> Tip: Some NiuBoL transmitters offer dual output, supporting both local analog control and remote digital acquisition. 

 FAQ — Frequently Asked Questions

 1. How far can 4-20mA transmit outdoors?  

   Theoretically over 10km; practically 300m–1km for stability. 

2. How many sensors can an RS485 bus support?  

   Standard: 32 nodes; expandable with repeaters. 

3. Why is the 4-20mA lower limit 4mA, not 0mA?  

   4mA powers the sensor; enables open-loop fault detection. 

4. Can RS485 fully replace 4-20mA?  

   Possible, but consider system compatibility, protocol parsing, and cost. 

5. Precautions for long-distance 4-20mA wiring?  

   Ensure sufficient voltage, use proper gauge, minimize joints, avoid EMI sources. 

6. How to improve RS485 anti-interference?  

   Use shielded twisted pair, termination resistors, and isolators if needed. 

7. Does ambient temperature affect transmitter accuracy?  

   High-end models include temperature compensation; impact is minimal. 

8. Are both signals suitable for explosion-proof areas?  

   4-20mA is inherently safe; RS485 requires intrinsically safe barriers. 

9. Can both signals be used simultaneously?  

   Yes—NiuBoL dual-output models support local analog + remote digital. 

10. Does cable material affect transmission distance?  

    Copper has low resistance → longer range; aluminum requires larger gauge or shorter runs. 

11. Recommended RS485 protocol?  

    Modbus RTU – widely supported by PLCs, DCS, and IoT platforms. 

12. Is periodic calibration required?  

    For high-precision industrial/scientific use: calibrate every 6–12 months.  

Conclusion 

Temperature and humidity transmitters are essential for environmental monitoring in industry, agriculture, and smart buildings. 

- 4-20mA: Ideal for single-point, short-distance, simple, and explosion-proof applications  

- RS485: Perfect for long-distance, multi-point, high-precision, and IoT-integrated systems 

NiuBoL offers a full range of transmitters supporting 4-20mA, RS485, and dual-output models to meet diverse needs—delivering reliable, accurate, and remote-capable environmental monitoring.

Call to Action: Contact us today for a customized transmitter selection plan or technical datasheet tailored to your application.