4-20 mA Current Loop
Process control 4–20 mA loops
In industrial process control, analog 4–20 mA current loops are commonly used for electronic signalling, with the two values of 4 & 20 mA representing 0–100% of the range of measurement or control. These loops are used both for carrying sensor information from field instrumentation, and carrying control signals to the process modulating devices, such as a valve.
The key advantages of the current loop are:
- The loop can often power the remote device, with power supplied by the controller, thus removing need for power cabling. Many instrumentation manufacturers produce 4–20 mA sensors which are "loop powered".
- The "live" or "elevated" zero of 4 mA allows powering of the device even with no process signal output from the field transmitter.
- The accuracy of the signal is not affected by voltage drop in the interconnecting wiring.
- It has high noise immunity as it is low impedance circuit usually through twisted pair conductors.
- It is self-monitoring; currents less than 3.8 mA or more than 20.5 mA are taken to indicate a fault.
- It can be carried over long cables up to the limit of the resistance for the voltage used.
- In line displays can be inserted and powered by the loop, as long as total allowable loop resistance is not exceeded.
- Easy conversion to voltage using a resistor.
- Loop powered "I to P" (current to pressure) converters can convert the 4–20 mA signal to a 3–15 psi pneumatic output for control valves, allowing easy integration of 4–20 mA signals into existing pneumatic plant.
PI-SPI-8AI and a 3 Wire Transmitter
PI-SPI-8AI and a 2 Wire Transmitter
Field instrumentation measurements are such as pressure, temperature, level, flow, pH or other process variables. A current loop can also be used to control a valve positioner or other output actuator. Since input terminals of instruments may have one side of the current loop input tied to the chassis ground (earth), analog isolators may be required when connecting several instruments in series.
The relationship between current value and process variable measurement is set by calibration, which assigns different ranges of engineering units to the span between 4 and 20 mA. The mapping between engineering units and current can be inverted, so that 4 mA represents the maximum and 20 mA the minimum.