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4-20 mA Instruments in Industries | Fieldbus Training
Instrumentation Tools
Overview
This video introduces the fundamental concept of 4-20 mA signaling in industrial instrumentation. It showcases various field instruments like level transmitters, control valves, temperature transmitters, and pressure transmitters, explaining how they measure physical parameters and convert them into a 4-20 mA electrical signal. The video highlights the advantages of two-wire, 4-20 mA instruments, such as cost-effectiveness and simplified installation, and briefly mentions HART protocol for configuration. It emphasizes the widespread use of this signaling standard across industries for reliable data transmission to control systems like DCS.
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Chapters
- The 4-20 mA signal is a standard analog electrical signal used in industrial automation.
- It represents a measured physical value (like temperature, pressure, or level) as a current within the range of 4 to 20 milliamperes.
- This signal is transmitted from field instruments to control systems like Distributed Control Systems (DCS).
Understanding the 4-20 mA standard is crucial because it's the most common language for industrial instruments to communicate measurements, forming the backbone of process control.
A level transmitter measures the liquid level in a tank and outputs a 4-20 mA signal proportional to that level.
- Control systems send a 4-20 mA signal to a control valve's positioner.
- The positioner interprets this current signal to regulate the flow of instrument air to the valve actuator.
- The actuator, in turn, opens or closes the valve to a specific percentage based on the air pressure, effectively controlling the process flow.
This demonstrates how a control signal is translated into a physical action, allowing automated adjustments to industrial processes.
If the DCS sends a 12 mA signal (which is 50% of the 4-20 mA range), the valve positioner ensures the valve opens to 50%.
- Temperature transmitters (TT) measure temperature using thermocouples and send a 4-20 mA signal.
- Differential pressure (DP) transmitters measure pressure differences to infer flow rates, also using the 4-20 mA standard.
- All these instruments are designed to operate reliably within the 4-20 mA framework.
Familiarity with these common instrument types and their signaling methods is essential for anyone working with industrial control systems.
A temperature transmitter (TT) with a tag name like 'TT101' receives input from a thermocouple and transmits its reading as a 4-20 mA signal.
- Many 4-20 mA instruments are 'two-wire' devices, meaning a single cable carries both power to the instrument and the output signal.
- This design simplifies installation and reduces wiring costs compared to older three- or four-wire systems.
- The single cable connects to the instrument, providing power and transmitting the 4-20 mA signal back to the control system.
Understanding the two-wire concept highlights the practical engineering considerations that make 4-20 mA systems efficient and cost-effective in large industrial settings.
A flow transmitter is connected by a single blue cable, which supplies its power and carries its 4-20 mA output signal.
- Instruments are configured using HART communicators, allowing adjustments based on design specifications or datasheets.
- Signals from field instruments are typically routed through junction boxes before reaching the main control room.
- The HART protocol allows for digital communication alongside the analog 4-20 mA signal for advanced diagnostics and configuration.
This explains the practical steps involved in setting up and managing industrial instruments, ensuring they function correctly and can be diagnosed remotely.
A DP flow transmitter's 4-20 mA signal travels via a branch cable to a junction box, and then a single cable runs from the junction box to the control room.
Key takeaways
- The 4-20 mA current loop is the industry standard for transmitting process measurements reliably.
- This signaling method uses a current range (4-20 mA) to represent physical variables, with 4 mA often indicating a zero or minimum value and 20 mA a maximum.
- Two-wire instruments simplify installation and reduce costs by using a single cable for both power and signal transmission.
- Control valves use the 4-20 mA signal to precisely adjust their opening or closing based on commands from a control system.
- Instruments like temperature, pressure, and flow transmitters all commonly utilize the 4-20 mA standard.
- HART protocol enables digital communication for configuration and diagnostics on top of the analog 4-20 mA signal.
- Understanding the signal path, from field instrument to junction box to control room, is key to industrial automation.
Key terms
4-20 mA signalAnalog signalCurrent loopLevel transmitterControl valveValve positionerDCS (Distributed Control System)Temperature transmitter (TT)Differential pressure (DP) transmitterTwo-wire instrumentHART protocolJunction box
Test your understanding
- What is the primary function of a 4-20 mA signal in industrial settings?
- How does a control valve positioner use the 4-20 mA signal to adjust the valve's position?
- Why are two-wire instruments considered advantageous for industrial installations?
- What role does a junction box play in the signal path of a 4-20 mA instrument?
- How does the HART protocol enhance the functionality of 4-20 mA instruments?