Current-loop technology has been used for analog sensors for the past 4 decades to transmit important process data to the control system, whether that system is a DCS (distributed control system) a PLC (programmable logic controller), or a single-loop controller.
Current-loop data transmission is simple and cost-effective. Only a small amount of current (4 to 20 milliamps to be exact) is required over a single pair of wires for each current loop sensor. One 2-amp, 24 Volt DC power supply can “drive” dozens of sensors.
For current-loop analog sensors, the lowest measurable process value is called the Lower Range Limit, or LRL. The analog sensor will output 4 milliamps at this 0% reading. The highest measurable process value is called the Upper Range Limit, or URL. The analog sensor will output 20 milliamps at this 100% reading.