Why do we not use a 5-20 mA signal instead of 4-20 mA?
The 4-20 mA current loop is used in process control and instrumentation for several reasons and one of the reasons that a 5-20 mA signal is not frequently used instead is associated with signal integrity and fault finding. Here’s a breakdown of the key factors:Here’s a breakdown of the key factors:
Zero Signal Distinction:
That is why the choice is made in favor of the 4-20 mA range, which clearly defines the boundary between zero and a failure indication. The value 4 mA indicates the minimum or ‘’zero signal’’, and any reading below 4 mA will imply a problem within the system. This aspect makes it easier to pin point a problem if there is and thus isolate and diagnose it effectively.
Signal Span and Resolution:
The fact that the output current is 4-20 mA means that there is a constant 16 mA signal span (20-4 mA) throughout the range. This offers relatively high resolution and susceptibilities for a vast spectrum of industrial applications. A 5-20 mA signal would have a small span of 15 mA and therefore may not give much accuracy if used to measure certain quantities.
Signal Integrity:
Since the 4-20 mA loop signal is in current, it is possible to transmit currents over long distances with moderate loses of accuracy. The 4 mA baseline guarantees that any voltage drop that might occur because of the wiring resistance does not influence the measurement. A 5-20 mA system can be thought of as less robust than a system running at 4-20 mA , where signal loss or interference is of concern.
Standardization:
The 4-20 mA signal is the industry standard and is very frequently used. Many of the instruments, controls, and measuring instruments are developed based on some standard with this in mind to interconnect ease and use across the system.
Fail-Safe Operation:
The 4mA minimum signal produces a fail-safe feature that is constructed in the circuit. There is an option to select a sensor with the signal less than 4 mA, in case a sensor fails, or there is a wiring problem, the dropping of a signal might be an indication of the problem, while using 5-20 mA it might not be easily identified.
Why Not 5-20 mA?
Switching to a 5-20 mA range would diminish these advantages:
- Fault Detection Limitations: A 5-20 mA signal does not allow for easy differentiation between zero and fault conditions as effectively as 4-20 mA.
- Reduction in Signal Span: The usable signal span would be reduced to 15 mA, decreasing resolution and sensitivity.
- Increased Power Requirement: Two-wire devices might require more power, complicating the power loop dynamics.