3–15 psi pneumatic output for control valves, allowing easy integration of 4–20 mA signals into existing pneumatic plant. Field instrumentation measurements include 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. 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 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 The 4–20 mA convention was born in the 1950s out of the earlier highly successful 3–15 psi pneumatic control signal standard, when electronics became cheap and reliable enough to emulate the older standard electrically. The 3–15 psi standard had the same features of being able to power some remote devices, and have a "live" zero. However the 4–20 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 psi devices. As the operation of pneumatic valves over motorised valves has many cost and reliability advantages, pneumatic actuation is still an industry