Industrial measurements often are converted to analog currents (not voltages) by process transmitters and sent on a pair of wires. The almost-universal current range is 4-20 mAdc; that is, 4 milliamperes DC at the low end of the range (0 %) and 20 mA at full scale (100 %). For example, a temperature range of 50-250oC would translate to 4 mA at 50o, 12 mA at 150o, 20 mA at 250o. If you’re interested, here’s a link to an article on why 4-20mA is used.
Photo courtesy of JMS Southeast.
Normally, one pair of wires would supply power to the transmitter while a second pair would carry the 4-20 mA signal. Two wire, or loop-powered, transmitters, though, use the same wires for both, eliminating the need for separate power wiring. The transmitter is wired in a series loop between a DC power supply (usually 24Vdc) and the readout instrument or instruments. It controls the current in the loop to the desired level and “steals” its power from the current flowing through it. This works only because the loop current always is at least 4 mA, never zero. The transmitter is located near the sensor or point of measurement. The power and readout(s) usually are a distance away, in a control room or cabinet.
Two wire transmitters are widely used with thermocouples and RTDS (resistance thermometers) for temperature measurement. Transmitters also are available for use with strain gauges or load cells, tachometers, potentiometers and others. For temperature, transmitters such as the one pictured above can be enclosed inside a probe’s connection head, where the sensor wires are connected to the field wiring.