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Since introducing the first analog sensor or transducer, engineers have been seeking a way to transmit analog signals. The old standby, the 420 milliamp (mA) instrumentation loop, fills that gap, and is also unaffected by wire resistances and voltage-supply variations. Things haven\'t always been this way, though. Engineers had trouble transmitting an electrical signal without introducing errors. Simple voltage variation didn\'t work; changes in wire length and resistance seriously upset readings. Prior to the 4-20 mA loop, engineers attempted two solutions. One, the pulse-duration modulation system, used a pulse with its length proportional to the analog value. Among the problems encountered were high initial expense and ongoing maintenance. Another solution used a tone with a frequency proportional to the analog value. This solution involved expensive and drift-prone tone transmitters at one end of the wire and tone receivers at the other end. Some of these rare systems still exist today. At least four elements make up the 4-20 mA loop: one transmitter; one loop power supply; loop wires to connect components in series like tree-ornament lights; and at least one receiver. The transmitter You can place or mount the transmitter on the pipe, tank wall, or wherever it can sense the realworld analog variable, such as flow rate, pressure, or temperature. The transmitter converts this process variable into a 4-20 mA current. The 4-mA lower limit is the starting point of 0% and the 20-mA upper limit is full-scale reading of 100%. If you read 0 mA, you know there\'s a problem, like a broken wire or some other loop failure. Most transmitters allow you to calibrate span (the 4-20 mA range) and the zero or offset (04 mA range).