The zero elevation also gives you the ability to use the voltage drop this has to create to power the transmitter, thus allwoing for 2 wire transmitters.
The signals could just as easily start at 0 milliamps. One of the main reasons for choosing a non-zero value was that it gives a means of error checking called a "Live Zero." If the loop reads 4 milliamps then a zero measured value is
detected, but a valid current loop exists. If the same loop reads 0 milliamps then an open loops exists, and something is wrong.
There is no technical reason why 0 ma cannot be used IF you are using a field powered device (usually called 4 wire transmitters). If however
you are using loop powered devices (usually called 2 wire transmitters), the device itself needs to obtain power to operate, even with a zero indication. The power available in that 4 ma or 1 V offset is sufficient to provide this function, and still retain the open loop detection as well.
The reason is that if there is a line break, the current drops to zero in a 4-20 mADC system, while in a 0-20 mADC system this alarm feature does not exist.
Walt Boyes
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Apart from the wire break detection, another reason for 4-20 mA is that two-wire devices that power themselves from the 4-20 mA need the guaranteed 4 mA to operate. I.e. the internal workings of this type of device is that about 4 mA is used to power the circuitry, and about 0-16 mA is shunted past, thus adding up to the 4-20 mA signal.