Editing Current loop (section)

==Process control 4–20 mA loops==
[[File:Analogue control loop evolution.png|thumb|Showing the evolution of analogue control loop signalling from the pneumatic era to the electronic era]]
[[File:Smart current loop positioner.png|thumb|Example of current loops used for sensing and control transmission. Specific example of a smart valve positioner is shown.]]
In industrial [[process control]], [[analog signal|analog]] 4–20&nbsp;mA current loops are commonly used for electronic signalling, with the two values of 4 and 20&nbsp;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&nbsp;mA sensors which are "loop powered".
* The "live" or "elevated" zero of 4&nbsp;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&nbsp;mA or more than 20.5&nbsp;mA are taken to indicate a fault.<ref>NAMUR standard NE 043 "Standardisation of the Signal Level for the Failure Information of Digital Transmitters".</ref>
* It can be carried over long cables up to the limit of the resistance for the voltage used.
* Inline 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&nbsp;mA signal to a 3–15&nbsp;psi pneumatic output for control valves, allowing easy integration of 4–20&nbsp;mA signals into existing pneumatic plant.

Field instrumentation measurements include [[pressure]], [[temperature]], level, [[Volumetric flow rate|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&nbsp;mA. The mapping between engineering units and current can be inverted, so that 4&nbsp;mA represents the maximum and 20&nbsp;mA the minimum.

===Active and passive devices===
Depending on the source of current for the loop, devices may be classified as ''active'' (supplying or "sourcing" power) or ''passive'' (relying on or "sinking" loop power). For example, a [[chart recorder]] may provide loop power to a pressure transmitter. The pressure transmitter modulates the current on the loop to send the signal to the strip chart recorder, but does not in itself supply power to the loop and so is passive. Another loop may contain two passive chart recorders, a passive pressure transmitter, and a 24&nbsp;V battery (the battery is the active device). Note that a ''4-wire'' instrument has a power-supply input separate from the current loop.

Panel mount displays and chart recorders are commonly termed "indicator devices" or "process monitors". Several passive indicator devices may be connected in series, but a loop must have only one transmitter device and only one power source (active device).

===Evolution of analogue control signals===
[[Image:Pl control valve.jpg|thumb|right|Control valve with pneumatic diaphragm actuator and "smart" 4–20&nbsp;mA positioner which will also feed back the actual valve position and status over the current loop]]

The 4–20&nbsp;mA convention was born in the 1950s out of the earlier highly successful 3–15&nbsp;psi pneumatic control signal standard, when electronics became cheap and reliable enough to emulate the older standard electrically. The 3–15&nbsp;psi standard had the same features of being able to power some remote devices, and have a "live" zero. However, the 4–20&nbsp;mA standard was better suited to the electronic controllers being developed at the time.

The transition was gradual and has extended into the 21st century, due to the huge installed base of 3–15&nbsp;psi devices. As the operation of pneumatic valves over motorised valves has many cost and reliability advantages, pneumatic actuation is still an industry standard. To allow the construction of hybrid systems, where the 4–20&nbsp;mA is generated by the controller, but allows the use of pneumatic valves, a range of current to pressure (I to P) converters are available from manufacturers. These are usually local to the control valve and convert 4–20&nbsp;mA to 3–15&nbsp;psi (or 0.2–1.0&nbsp;bar). This signal is then fed to the valve actuator or, more commonly, a pneumatic positioner. The positioner is a dedicated controller which has a mechanical linkage to the actuator movement. This ensures that problems of friction are overcome and the valve control element moves to the desired position. It also allows the use of higher air pressures for valve actuation.

With the development of cheap industrial micro-processors, "smart" valve positioners have become available since the mid-1980s and are very popular for new installations. These include an I to P converter, plus valve position and condition monitoring. These latter are fed back over the current loop to the controller, using protocols such as [[Highway Addressable Remote Transducer Protocol|HART]].