A 4-20 mA analog output is a universally recognized industry standard for relaying electrical signals (as currents) over long distances. For example, many sensing instruments such as pressure transmitters and transducers use a 4-20 mA output as a simple and reliable method to convey information from the field to a system control room.
If you want to learn more about this type of output, and the benefits of the 2-wire connection option vs. a 3-wire option, you've come to the right place. I have been a product leader at Ashcroft for almost two decades and have extensive experience in pressure and temperature measurement. During that time, I've been asked a lot of questions about how things work and the benefits of selecting one measurement device over another. We created our Understanding the Basics series to give those who are new to the industry a framework of information to use as a valuable point of reference when needed.
In this article, you will learn about 4-20 mA analog outputs, including where they are used, how they work, and the advantages and challenges of using the 2-wire vs. a 3-wire in your application. When you're finished, you'll know if this type of output is right for you.
What is a 2-wire 4-20 mA analog output?
Pressure sensors such as transmitters and transducers use a 2-wire 4-20 mA output to convert pressure readings into electrical signals that range between 4 mA (which typically represents the lowest end of the measured range) and 20 mA (representing the highest end). These instruments operate with two wires that power the device and carry the output signal to provide a simple and reliable method for conveying information about variables such as temperature, pressure, or flow.
Figure 1. 2-wire 4-20 mA output illustration.
Advantages of the 2-wire 4-20 mA analog output?
The 2-wire 4-20 mA analog output is a reliable communication standard in industrial applications and provides several key benefits over other methods.
Simple and cost-effective installation and verification. By using only two wires to connect the pressure sensor and control system, this configuration simplifies installation, making it a practical choice for many applications. With a zero pressure point at 4 mA output allows for system verification without having to initiate the entire process.
Enhanced signal reliability and immunity to interference: The current loop configuration ensures the 4-20 mA signal remains strong and resistant to electromagnetic noise, ensuring accurate measurements even in challenging environments.
Extended signal reach without compromising signal quality: The current loop design allows for signals to be sent over long distances without degradation low impedance cables, reducing voltage drops. This enables the 4-20 mA output to cover several thousand feet without compromising signal quality.
Compatible with loop-powered devices: Loop-powered instruments draw power from the current loop, eliminating the need for extra power supplies and simplifying installation.
When would you have to use a 3-wire output?
Although the 2-wire output option is common for most applications, there are instances when a 3-wire output is required.
While this configuration may seem more complex than a 2-wire system, it offers the advantage of higher resistance loads and the ability to operate on measuring instruments with higher input impedance. This can be particularly useful in certain applications where precise measurements and signal integrity are paramount.
The 3-wire output has a separate power, output, and ground wire, each serving a specific purpose in the transmission of signals. The external power supply required for a 3-wire setup provides the necessary energy to drive the sensor and ensure accurate readings.
Figure 2. 3-wire 4-20 mA output illustration.
Applications and industries for 4-20 mA output signals
- Industrial Processes: Equipment manufacturing, chemical processing, food and beverage, power generation, distribution facilities, HVAC/R systems, health and safety products and others all use the 4-20 mA analog output to ensure reliable data transmission from sensors to control systems.
- Oil and Gas: Due to the demanding nature of oil and gas operations, safety and efficiency are crucial. Using a 4-20 mA output to transmit critical measurements from sensors in oil rigs, refineries, and pipelines ensures real-time data transfer to control system operators.
- Water and Wastewater: Effective water resource management is crucial for sustainability. In water and wastewater treatment plants, the 4-20 mA analog output plays a vital role in monitoring variables to help maintain optimal treatment processes and ensure regulatory compliance.
Ready to learn more?
Now that you understand the basics of a 2-wire 4-20 mA analog output, and how it differs from the 3-wire output option, you may be interested in learning more about selecting the best pressure sensor for your applications. The following articles can help get you started.
- The Importance of Installing Analog Pressure Gauges with Pressure Transmitters
- Pressure Transducer or Pressure Transmitter? The answer depends on the output
- Choosing the Right Pressure Sensor: 5 Factors to Consider
Also, feel free to contact one of our industry experts and get your questions answered. In the meantime, download our guide to avoid common mistakes when selecting a pressure sensor.
